水中有机物对金属氧化物吸附剂去除水中磷、砷的影响

In natural ecosystems, the (bio)availability of arsenic and phosphorus is greatly controlled by their interactions with metal (hydr)oxides and organic matter. Humic substances (HS), encompassing humic acids (HA) and fulvic acids (FA), constitute the primary form of organic matter. In this study, batch adsorption experiments were conducted and integrated with the NOM-CD model to achieve a molecular-level understanding of HS on the competitive interactions among arsenite, arsenate, and phosphate on goethite surfaces. Results demonstrate that the NOM-CD model is a reliable tool for elucidating the underlying interaction mechanisms of HS, arsenic and phosphate onto goethite. Both HA and FA exhibit a considerable impact on the competitive interactions between arsenite, arsenate, and phosphate at the goethite-water interface, particularly under acidic conditions. At pH below 5.0, influences of HA and FA on the interactions in arsenite-arsenate-phosphate-goethite systems demonstrate notable similarities. The larger particle size of HA results in a greater morphological variation of adsorbed HA (ξ = 5-8), generating not only a stronger steric hindrance effect but also a stronger electrostatic repulsion between the adsorbed HA and oxyanions on goethite. Conversely, the smaller particle size and greater density of carboxylic groups of FA facilitate closer interactions with the goethite surface, thereby reducing the electrostatic potential of goethite even without morphological change (ξ = 1). At pH 5-7, a higher amount of adsorbed HA results in intensified competition between carboxylic groups and oxyanions in comparison with FA, and consequently a higher concentration of arsenite, arsenate, and phosphate in solution. At pH above 7.0, the influence of HS diminishes due to an increase in both hydroxide ions in solution and negative charges on goethite surfaces, reducing the adsorption of arsenate, and phosphate and HS. The NOM-CD model identifies electrostatic repulsions and steric effects as the principal determinants in the complex interactions between multi-oxyanions and HS within goethite systems.

Natural organic matter (NOM) and crystalline metal oxide nanoparticles are both prevalent in natural aquatic environment and their interactions have important environmental and biogeochemical implications. Here, we show that these interactions are significantly affected by an intrinsic property of metal oxide nanocrystals, the exposed facets. Both anatase (TiO2) and hematite (α-Fe2O3) nanocrystals, representing common engineered and naturally-occurring metal oxides, exhibited apparent facet-dependent adsorption of humic acid and fulvic acid. This facet-dependent binding was primarily driven by surface complexation between the NOM carboxyl groups and surficial metal atoms. Thus, the adsorption affinity of different-faceted nanocrystals was determined by the atomic arrangements of crystal facets that controlled the activity of metal atoms and consequently, the ligand exchange and binding configuration of the carboxyl groups in the first hydration shell of nanocrystals. Distinct facet-dependent fractionation patterns were observed during adsorption of NOM components, particularly the low-molecular-weight and photo-refractory constituents. The molecular fractionation of NOM between water and metal oxide nanoparticles was dictated by the combined effects of facet-dependent metal complexation, hydrophobic interaction and steric hindrance, and may significantly influence the NOM-driven processes occurring both in aqueous phases and at water-nanoparticle interfaces.

No abstract available

The release of phosphorus (P) and tungsten (W) from sediments can contribute to eutrophication and heavy metal contamination in water bodies, respectively. This study simultaneously investigated the seasonal variation characteristics of P and W in sediments in Meiliang Bay, China. The results indicated that seasonal variations in pH, dissolved oxygen (DO), and temperature (T) at the sediment-water interface influenced the P and W composition as well as their release from sediments. The diffusion fluxes of soluble reactive phosphorus (SRP) and soluble W (except in winter) were 0.145-2.881 mg⋅m-2⋅d-1 and 1.785-3.006 μg⋅m-2⋅d-1, respectively, indicating that the sediments served as a source of P and W. In autumn, the diffusion fluxes of SRP (2.881 mg⋅m-2⋅d-1) and soluble W (3.006 μg⋅m-2⋅d-1) were significantly higher than in winter (0.147 mg⋅m-2⋅d-1 and -0.048 μg⋅m-2⋅d-1, respectively). The concentration of SRP and soluble W in winter (0.20 mg⋅L-1 and 0.22 μg⋅L-1, respectively) were significantly lower than in autumn (1.57 mg⋅L-1 and 1.39 μg⋅L-1, respectively). These results suggest that under high temperatures and cyanobacteria degradation, sediments release more SRP, soluble W, Fe(Ⅱ), Mn, and dissolved organic matter (DOM). The significant positive correlations among SRP, soluble W, Fe, Mn, and DOM and their consistent trends in the top 20 mm of the sediment indicate that the main processes causing the release of P and W from sediments are competitive adsorption by DOM and redox reactions of Fe (III)/Mn (IV) oxyhydroxides. This study is of great practical significance for simultaneously addressing lake eutrophication and heavy metal pollution.

The environmental fate of the herbicide glyphosate (PMG) is determined by its favorable binding to metal (hydr)oxides, which is affected by environmental factors and the presence of competitors. A major competitor binding to metal (hydr)oxides is natural organic matter (NOM). This study investigated the competitive binding between humic acids (HA) and PMG on goethite with batch adsorption experiments, varying pH, ionic strength, and HA surface loading. HA strongly decreases PMG adsorption, increasing its solution concentration by multiple orders of magnitude. Interpretation of the competitive adsorption data with the NOM-CD model revealed that site and electrostatic competition insufficiently explain the competition. The model can be greatly improved by introducing steric hindrance as an additional mechanism, requiring only a single adjustable parameter. At low pH, HA maximizes its interaction with the surface while at high pH, the ligands tend to move outward. Our model reveals that steric hindrance is most significant in acidic conditions, while in alkaline conditions, competition is primarily controlled by electrostatics. The steric NOM-CD model provides excellent predictions of the behavior of PMG competing with HA and provides a tool to describe the key role of NOM in assessing the availability, mobility, and risk of PMG in the environment.

No abstract available

No abstract available

No abstract available

No abstract available

No abstract available

Among natural organic matter (NOM), oxyanions and metal (hydr)oxides, a complicated interaction exists in natural aquatic and terrestrial systems and in waste waters. Effects of seven types of NOM (four humic acids (HA), three fulvic acids (FA)) that vary in properties on the adsorption of oxyanions, including phosphate, arsenate and arsenite, at goethite-water interface were quantitatively studied. Results show that the adsorption of oxyanions to goethite is decreased by the presence of NOM, especially for phosphate and arsenate at low pH. In general, the effects of the three FA are similar, which are more effective than HA in reducing oxyanion adsorption at low pH (<6). Differences were observed between the four HA in their competition with oxyanions. The adsorption of phosphate, arsenate and arsenite in the presence of NOM are well described with both the NOM-CD (CD: Charge Distribution) and LCD (Ligand and Charge Distribution) model. The NOM-CD model is relatively simple to use, whereas the LCD model can better reveal different factors in the interaction, including the spatial distribution of adsorbed NOM on oxide surface. According to these two models: site density of carboxylic groups, protonation constant of carboxylic groups, and particle size of NOM are major properties of NOM determining its effect on oxyanion adsorption to oxides. At relatively low loadings, morphological change of adsorbed NOM takes place, and the degree of morphological change of adsorbed NOM depends on the particle size, site density of carboxylic groups and aromaticity of NOM. The influence of particle size on the interaction becomes more important at higher NOM loadings. The results suggested that the fixation or removal efficiency of phosphate, arsenate and arsenite with iron oxides (e.g. goethite) can be significantly decreased by the presence of NOM, especially when NOM rich in acidic and aromatic groups.

Mitigation measures are needed to prevent large loads of phosphate originating in agriculture from reaching surface waters. Iron-coated sand (ICS) is a residual product from drinking water production. ICS has a high phosphate adsorption capacity and can be placed around tile drains taking no extra space which increases the farmers' acceptance. The main concern regarding the use of ICS filters below groundwater level is that limited oxygen supply and high organic matter concentrations may lead to the reduction and dissolution of iron (hydr)oxides present and the release of previously adsorbed phosphate. This study aimed to investigate phosphate adsorption on ICS at the onset of iron reduction. First, it was investigated whether simultaneous metal reduction and phosphate adsorption were relevant at two field sites in the Netherlands that use ICS filters around tile drains. Second, the onset of microbially mediated reduction of ICS in drainage water was mimicked in complementary laboratory microcosm experiments by varying the intensity of reduction through controlling the oxygen availability and the concentration of degradable organic matter. After 3 years, ICS filters in the field removed phosphorus under low redox conditions. Over 45 days, the microbial reduction of manganese and iron oxides did not lead to phosphate release confirming field observations. Electron microscopy and X-ray absorption spectroscopy did not evince systematic structural or compositional changes, only under strongly reducing conditions did iron sulfides formed in small percentages in the outer layer of the iron coating. Our results suggest that detrimental effects only become relevant after long operation periods. This article is protected by copyright. All rights reserved.

Phosphorus (P) immobilization has potential for reducing diffuse P losses from legacy P soils to surface waters and for regenerating low-nutrient ecosystems with a high plant species richness. Here, P immobilization with iron oxide sludge application was investigated in a field trial on a noncalcareous sandy soil. The sludge applied is a water treatment residual produced from raw groundwater by Fe(II) oxidation. Siliceous ferrihydrite (Fh) is the major Fe oxide type in the sludge. The reactive surface area assessed with an adapted probe ion method is 211-304 m2 g-1 for the Fe oxides in the sludge, equivalent to a spherical particle diameter of ~6-8 nm. This size is much larger than the primary Fh particle size (~2 nm) observed with transmission electron microscopy. This can be attributed to aggregation initiated by silicate adsorption. The surface area of the indigenous metal oxide particles in the field trial soils is much higher (~1100 m2 g-1), pointing to the presence of ultra-small oxide particles (2.3 ± 0.4 nm). The initial soil surface area was 5.4 m2 g-1 and increased linearly with sludge application up to a maximum of 12.9 m2 g-1 when 27 g Fe oxides per kg soil was added. In case of a lower addition (~10-15 g Fe oxides per kg soil), a 10-fold reduction in the phosphate (P-PO4) concentration in 0.01 M CaCl2 soil extracts to 0.3 µM was possible. The adapted probe ion method is a valuable tool for quantifying changes in the soil surface area when amending soil with Fe oxide-containing materials. This information is important for mechanistically predicting the reduction in the P-PO4 solubility when such materials are used for immobilizing P in legacy P soils with a low P-PO4 adsorption capacity but with a high surface loading.

Using magnetite-based nanocomposite adsorbents to remove and recycle phosphate from wastewater is crucial for controlling eutrophication and ensuring the sustainable use of phosphorus resources. However, the weak structural stability between magnetite and adsorptive nanoparticles often reduces phosphate removal efficiency in real-world applications. This instability primarily results from the loss of adsorptive nanoparticles from the magnetite surfaces, particularly when metal oxide nanoparticles are used for phosphate removal and recycling. In this study, we present a top-down approach that involves lattice locking magnesium iron oxide nanoparticles to the magnetite core, preventing magnesium loss from the magnetite surfaces. These nanocomposites exhibit exceptional performance in both phosphate recycling and removal, with a maximum adsorption capacity of 101.8 mg P·g-1. Excellent adsorption performance is also observed even in the presence of competing anions at phosphate-to-competing ion molar ratios of 1:5, 1:25, and 1:100, as well as dissolved organic matter, across a broad pH range of 4 to 10. The adsorbent also demonstrated minimal magnesium release during regeneration and in acidic conditions. Microscopic and spectroscopic analyses reveal that surface deposition is the primary mechanism of phosphate removal in the magnesium-containing shells. The findings of this study address the current limitations of magnetite nanocomposites in phosphate removal, paving the way for the development of highly stable and sustainable nanocomposites for various chemical removal and recycling applications in wastewater treatment.

No abstract available

No abstract available

No abstract available

Iron oxide-mediated phosphate immobilization (e.g., goethite) in acidic soils severely constrains phosphorus bioavailability through mineral-water interfacial reactions, resulting in a significant agricultural bottleneck. Enhancing phosphorus availability is therefore essential to sustain crop yields while minimizing phosphorus leaching into aquatic systems. Biochar-derived dissolved organic matter (BDOM), recognized for enhancing nutrient accessibility and mediating redox-driven transformations of pollutants through electron-donating functional groups, remains poorly understood in its interactions with goethite-bound phosphate (Ge-P). This study elucidates the pH-dependent mechanisms of Ge-P release by BDOM, emphasizing the roles of aromatic nitrogen and highly unsaturated sulfur compounds. The results demonstrate that the addition of BDOM at pH 7.5 induces 55.6 % phosphate release from Ge-P through synergistic Fe(III) reductive dissolution and ligand competition. This release efficiency is 21.5 times higher than that at pH 6.0 and 3.09 times greater than with pH adjustment alone. Conversely, at pH 4.5, BDOM-associated phosphate undergoes irreversible adsorption onto Ge-P. Fluorescence spectroscopy identifies low-emission-wavelength humic-like and tryptophan-like compounds in BDOM as dominant contributors to phosphate mobilization. Fourier-transform ion cyclotron resonance mass spectrometry reveals pH-dependent intensification of interactions between BDOM components, especially aromatic nitrogen and highly unsaturated sulfur compounds, and Ge-P. Aromatic amino (Ar-NH2), carboxylic (-COOH), and thiol (-SH) moieties are further identified as dominant redox groups driving phosphate liberation via electron transfer and ligand displacement. This study proposes a biochar strategy to enhance mineral-bound phosphorus availability in acidic soils, synergizing fertilizer-efficient sustainable agriculture with water quality-protected environmental governance against eutrophication.

In aquifers, the sequestration and transformation of organic carbon are closely associated with soil iron oxides and can facilitate the release of iron ions from iron oxide minerals. There is a strong interaction between dissolved organic matter (DOM) and iron oxide minerals in aquifers, but the extent to which iron is activated by DOM exposure to active iron minerals in natural aquifers, the microscopic distribution of minerals on the surface, and the mechanisms involved in DOM molecular transformation are currently unclear. This study investigated the nonbiological reduction transformation and coupled adsorption of iron oxide minerals in aquifers containing DOM from both macro- and micro perspectives. The results of macroscopic dynamics experiments indicate that DOM can mediate soluble iron release during the reduction of iron oxide minerals, that pH strongly affects DOM removal, and that DOM is more efficiently degraded at low rather than high pH values, suggesting that a low pH is conducive to DOM adsorption and oxidation. Spherical aberration-corrected scanning transmission electron microscopy (SACTS) indicates that the reacted mineral surfaces are covered with large amounts of carbon and that dynamic agglomeration of iron, carbon, and oxygen occurs. At the nanoscale, three forms of DOM are found in the mineral surface agglomerates (on the surfaces, inside the surface agglomerates, and in the polymer pores). The microscopic organic carbon and iron mineral reaction patterns can form through oxidation reactions and selective adsorption effects. Fourier transform ion cyclotron resonance mass spectra indicate that both synergistic and antagonistic reactions occur between DOM and the minerals, that the release of iron is accompanied by DOM decomposition and humification, that large oxygen- and carbon-containing molecules are broken down into smaller oxygen- and carbon-containing compounds and that more molecules are produced through oxidation under acidic rather than alkaline conditions. These molecules provide adsorption sites for sediment, meaning that more iron can be released. Microscopic evidence for the release of iron was acquired. These results improve the understanding of the geochemical processes affecting iron in groundwater, the nonbiological transformation mechanisms that occur at the interfaces between natural iron minerals and organic matter, groundwater pollution control, and the environmental behavior of pollutants.

Arsenic (As) is a metalloid that can accumulate in eutrophic lakes and cause adverse health effects to people worldwide. However, the seasonal process and dynamic mechanism for As mobilization in eutrophic lake remains effectively unknown. Here we innovatively used the planar optodes (PO), high-resolution dialysis (HR-Peeper) combined with fluorescence excitation-emission matrix coupled with parallel factor (EEM-PARAFAC) analysis technologies. We synchronously investigate monthly O2, As, iron (Fe), manganese (Mn), and naturally occurring dissolved organic matter (DOM) changes in sediments of Lake Taihu at high resolution in field conditions. We find high As contamination from sediments with 61.88-327.07 μg m-2 d-1 release As fluxes during the algal bloom seasons from May to October 2021. Our results show that an increase in DOM, mainly for humic-like components, resulting in high electron transfer capacity (ETC), promoted the reductive dissolution of Fe and Mn oxides to release As. Partial least square-path modeling (PLS-PM) and random forest modeling analysis identified that Mn oxide reductive dissolution directly accelerated sediments As contamination, which is the crucial factor. Understanding crucial factor controlling As release is especially essential in areas of eutrophic lakes developing effective strategies to manage As-rich eutrophic lake sediments worldwide.

Declining macrophytes in eutrophic lakes are altering material cycling in sediments. However, the transformation of arsenic (As) in response to these changes remains poorly understood. In this study, high-resolution dialysis was used to measure dissolved As in sediments from macrophyte-dominated (MD) and algae-dominated (AD) zones across different seasons. The relationship between sedimentary As fractionation and environmental variations was analyzed, and the As transformation process was explored. Results showed that the shift from macrophyte- to algae-dominated zones enhanced As release in sediments. Dissolved As in pore water of AD peaked at 120.36 μg/L in summer, exhibiting the highest release intensity, while MD showed a notable As release profile in spring (34.92 μg/L). In spring, decomposition and acidification of macrophyte residues, along with organic matter (OM) complexation, promoted the release of adsorbed As in MD. In contrast, reduction and dissolution of iron (Fe) oxides, along with competition for adsorption sites by dissolved phosphorus (P), drove As release in AD during summer. The high humification and low redox potential in MD sediments in summer promoted As-S co-precipitation, leading to As sequestration instead of release, this contrasts with the common view that warmer temperatures favor As release from sediments. The conversion from macrophytes to algae in eutrophic lakes may exacerbate the risk of As release, warranting further investigation.

Humification of exogenous soil organic matter (ESOM) remodels the organic compositions and microbial communities of soil, thus exerting potential impacts on the biogeochemical transformation of iron (hydr)oxides and associated trace metals. Here, we conducted a 70-day incubation experiment to investigate how aerobic straw humification influenced the repartitioning of arsenic (As) associated with ferrihydrite in paddy soil. Results showed that the humification was characterized by rapid OM degradation (1-14 days) and subsequent slow maturation (14-70 days). During the degradation stage, considerable As (13.1 mg·L-1) was released into the aqueous phase, which was reimmobilized to the solid phase in the maturation stage. Meanwhile, the low-crystalline structural As/Fe was converted to a more stable species, with a subtle crystalline phase transformation. The generated highly unsaturated and phenolic compounds and enriched Enterobacter and Sphingomonas induced ferrihydrite (∼3.1%) and As(V) reduction, leading to As release during the degradation stage. In the maturation stage, carboxylic-rich alicyclic molecules facilitated the aqueous As reimmobilization. Throughout the humification process, organo-mineral complexes formed between OM and ferrihydrite via C-O-Fe bond contributed to the solid-phase As/Fe stabilization. Collectively, this work highlighted the ESOM humification-driven iron (hydr)oxide transformation and associated As redistribution, advancing our understanding of the coupled biogeochemical behaviors of C, Fe, and As in soil.

Arsenic-contaminated groundwater is widely used in agriculture. To meet the increasing demand for safe water in agriculture, an efficient and cost-effective method for As removal from groundwater is urgently needed. We hypothesized that Fe (oxyhydr)oxide (FeOOH) minerals precipitated in situ from indigenous Fe in groundwater may immobilize As, providing a solution for safely using As-contaminated groundwater in irrigation. To confirm this hypothesis and identify the controlling mechanisms, we comprehensively evaluated the transport, speciation changes, and immobilization of As and Fe in agricultural canals irrigated using As-contaminated groundwater. The efficiently removed As and Fe in the canals accumulated in shallow sediment rather than subsurface sediment. Linear combination fitting (LCF) analysis of X-ray absorption near edge spectroscopy (XANES) indicated that As(V) was the dominant As species, followed by As(III), and there was no FeAsO4 precipitate. Sequential extraction revealed higher contents of amorphous FeOOH and associated As in shallower sediment than in the subsurface layer. Stoichiometric molar ratio calculations, SEM‒EDS, FTIR, and fluorescence spectroscopy collectively demonstrated that the microbial reductive dissolution of amorphous FeOOH proceeded via reactive dissolved organic matter (DOM) consumption in subsurface anoxic porewater environment facilitating high labile As, whereas in surface sediment, the in situ-generated amorphous FeOOH was stable and strongly inhibited As release via adsorption. In summary, groundwater Fe2+ can efficiently precipitate in benthic surface sediment as abundant amorphous FeOOH, which immobilizes most of the dissolved As, protecting agricultural soil from contamination. This field research supports the critical roles of the phase and reactivity of in situ-generated FeOOH in As immobilization and provides new insight into the sustainable use of contaminated water.

The objective of this study was to investigate the effect of straw return on the formation of Fe-Mn colloids in arsenic-contaminated soils and its subsequent influence on arsenic behavior. It was observed that organic matter (SD) resulting from straw decomposition interacted with iron/manganese (hydr)oxides (Fe/Mn (hydr)oxides) present in the soil, leading to the formation of colloidal particles. These particles significantly influenced the fixation and release of arsenic. The experimental results indicated that an increase in SD content facilitated the formation of colloidal particles. The highest concentration of colloidal particles was observed at a C/Fe-Mn ratio of 2.2, which significantly reduced the bioavailability and mobility of arsenic in the soil. The increase in SD content also diminished the depositional attachment efficiency of SD/Fe-Mn, thereby enhancing its migration through the soil. The actual field soil-filled column experiments further demonstrated that the content of SD significantly influenced arsenic bioavailability and mobility. Specifically, at a C/Fe-Mn ratio of 2.2, the inhibition of arsenic migration and bioavailability was found to be 1.46 times more effective compared to a C/Fe-Mn ratio of 0.4. Therefore, the return of straw to the field represents an effective soil remediation strategy for mitigating the bioavailability of arsenic by modulating the C/Fe-Mn ratio. This approach offers a novel perspective on strategies for heavy metal remediation.

Groundwater arsenic contamination is governed by the coupled iron-sulfur-arsenic biogeochemical cycle, where microbial functional genes and organic matter transformation play central roles, though regional-scale mechanisms remain unclear. This study integrates hydrogeochemistry, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), metagenomic sequencing, and metagenome-assembled genomes (MAGs) to reveal microbially driven mechanisms of arsenic migration and transformation in the Datong Basin. The results indicate distinct zonation of arsenic, sulfur, and iron speciation along the groundwater flow path. Furthermore, dissolved organic matter (DOM) dominated by carboxyl-rich alicyclic molecules (CRAM) and aromatic compounds promotes arsenic release through chelation and electron transfer. Microbial community and functional gene analyses further reveal key zonation characteristics. In the recharge zone, genera such as Acinetobacter and Hydrogenophaga were predominant, with functional genes related to arsenite oxidation (aioA, aoxB) contributing to arsenic retention. In the transition zone, sulfate-reducing bacteria including Desulfovibrio became abundant, and sulfate reduction genes (CysND, CysH, CysJI) facilitated the formation of thioarsenates, leading to arsenic release. In the discharge zone, methylotrophic genera such as Methylocystis together with methanogens were enriched. The co-occurrence of the methane metabolism gene ackA and the arsenic reduction gene arsC suggested a potential coupling between methane-related metabolism and arsenic transformation under reducing conditions. This study elucidates iron-sulfur-arsenic coupling as a key mechanism governing arsenic biogeochemical cycling, providing a theoretical biogeochemical framework for understanding regional arsenic spatial heterogeneity.

Dissolved organic matter (DOM) is widely distributed in sediments, however its potential effects on heavy metals at mine area remain insufficiently investigated. This study focused the influence of DOM on the activation and release of arsenic (As) and thallium (Tl) from mine-impacted sediments. Results showed that DOM governs the spatial heterogeneity, mobilization dynamics, and associated health risks of heavy metals in mine sediments. As and Tl concentrations in mine stream sediments significantly exceeded local background levels, and their spatial distributions exhibited a strong positive correlation with DOM. With increasing of DOM concentrations and more labile, microbially derived components collectively elevated, the concentrations of activated As and Tl increased by 144 % and 221 %, respectively. DOM accounts for over 99 % of the spatial variance of activated As and Tl through reductive dissolution of iron oxides, release proton or direct complexation while its molecular composition governs As and Tl mobility and associated health risks. The protein-like and fulvic acid-like DOM increased both short-term hazards and long-term carcinogenic risks above safety thresholds. Fulvic acid-like DOM mobilized As, while protein-like DOM co-released As and Tl. Fulvic acid-like DOM uniquely enhanced As(III) mobility while most effectively suppressing Tl(III) solubility. This study clarifies the DOM for the release of As and Tl in sediments at mine area, providing a scientific basis for heavy metals risk assessment and control in mine.

Environmental fluctuations like alternating dry-wet (DW) and freeze-thaw (FT) events significantly affect the long-term stability of arsenic (As) immobilized by iron‑based materials in soils. However, achieving stable As immobilization under these fluctuating conditions remains a major challenge. Herein, we develop a resilient chrysotile-based Fe/Ti (TiFe-Chy) nanomaterial for As immobilization in soils under environmental fluctuations. Results show that FT cycling has a negligible effect on As immobilization by TiFe‑Chy, while DW alternation leads to a slight decline. Following 150 days of incubation, the long-term As immobilization rate of TiFe-Chy was higher than that of commercial layered double hydroxide under the DW and FT scenarios, respectively. Sequential extraction analysis indicates that TiFe‑Chy promotes the transformation of non-specifically and specifically adsorbed As into a more stable Fe oxide bound As fraction through complexation with its surface -OH groups, markedly reducing the risk of As re-release to groundwater. Notably, solid phase characterization confirms that the FT processes do not alter the chemical properties of the TiFe‑Chy. Moreover, the high crystallinity and structural stability of TiFe‑Chy effectively suppressed microbially mediated iron reductive dissolution under DW scenario, with a 98 % reduction compared to ferrihydrite, thereby enhancing its long-term As immobilization. These findings offer valuable insights into the design of resilient iron-based materials for sustainable heavy metal remediation in soils under environmental fluctuations.

Groundwater recharge is a viable solution to groundwater overexploitation. However, the injection of recharge water may break the dissolution balance and induce the release of trace elements especially arsenic (As), which has been identified in river deltas. Only a few studies have been conducted in inland basins with high As concentration, high pH, and low Eh. Aiming to analyze As release with groundwater recharge in inland high-As regions and determine the effects of coexisting ions in recharge water, this study established PHase Equilibria Calculation (PHREEQC) models using rainwater and groundwater data from three inland sedimentary basins with slow groundwater flow in semi-arid regions. The simulations fitted with the batch experiments, achieving an R-squared (R2) of 0.98. The coexisting ions in the recharge water significantly affected As release during recharge. Ca2+ inhibited the release of total arsenic (Total-As) by increasing the surface charge of iron oxides. NO3- inhibited Total-As release by promoting the conversion of trivalent As into pentavalent As. Conversely, HCO3- facilitated As release by competing with arsenate for adsorption sites. On the basis of the modeling and batch experiment results, Total-As release with groundwater recharge was predicted. The results indicated that the high Ca2+ concentration in the recharge water inhibited the As release by 83.5 %, which can be used as a strategy to control As release during groundwater recharge in high-As inland basins.

The presence of arsenic in groundwater and other drinking water sources presents a notable public health concern. Although the utilization of iron oxide nanomaterials as arsenic adsorbents has shown promising results in batch experiments, few have succeeded in using nanomaterials in filter setups. In this study, the performance of nanomaterials, supported on sand, was first compared for arsenic adsorption by conducting continuous flow experiments. Iron oxide nanoparticles (IONPs) were prepared with different synthetic methodologies to control the degree of agglomeration. IONPs were prepared by thermal decomposition or coprecipitation and compared with commercially available IONPs. Electron microscopy was used to characterize the degree of agglomeration of the pristine materials after deposition onto the sand. The column experiments showed that IONPs that presented less agglomeration and were well dispersed over the sand had a tendency to be released during water treatment. To overcome this implementation challenge, we proposed the use of clusters of iron oxide nanoparticles (cIONPs), synthesized by a solvothermal methodology, which was explored. An isotherm experiment was also conducted to determine the arsenic adsorption capacities of the iron oxide nanomaterials. cIONPs showed higher adsorption capacities (121.4 mg/g) than the other IONPs (11.1, 6.6, and 0.6 mg/g for thermal decomposition, coprecipitation, and commercially available IONPs, respectively), without the implementation issues presented by IONPs. Our results show that the use of clusters of nanoparticles of other compositions opens up the possibilities for multiple water remediation applications.

Various iron oxyhydroxide and oxide minerals commonly found in old cast iron pipe scale were shown to exhibit high and similar affinity for arsenate [As(V)] and orthophosphate (PO4) via adsorption, co-precipitation, and other factors. PO4 is a common drinking water corrosion inhibitor. This 7.5-year study examined the accumulation and release of As from an old cast iron pipe scale by changing initial As(V) (0, 75, or 180 µg/L as As) and initial PO4 (0 or 3 mg/L as PO4) levels in the simulated drinking water. The results showed that sites within the iron scale accumulated As with a large capacity and concentrated 27% of the total amount As in water into the scale during the 7.5-year study. When no PO4 was added, the As accumulation followed a linear regression model with an accumulation rate of 0.27/hr (R2 = 0.80, p < 0.001), and higher initial As level of 180 µg/L (vs 75 µg/L) resulted in 2.3-3 times larger As accumulation rate at 0.25 mg/day (vs 0.084-0.11 mg/day). As much as 44 µg/L As was released back to water following the changes in the initial As and PO4 concentrations in water. Addition of 3 mg/L PO4 caused a rapid increase in As release from iron scale that gradually dropped off with time while PO4 was incorporated into the scale and most PO4 remained tightly bound to certain iron scale sites. Proactive measures such as sampling for As in the distribution systems following PO4 corrosion control treatment changes would help identify exposure risks.

Algal-derived dissolved organic matter (ADOM) is a critical component of endogenous dissolved organic matter (DOM) in aquatic systems, distinguished by its characteristic fluorescence response and high photochemical reactivity. Hulun Lake, the fifth largest lake in China, has experienced recurrent algal blooms accompanied by elevated iron (Fe) concentrations, providing an ideal natural setting to investigate the coupled photochemical roles of ADOM and Fe. This study examines the dual photochemical effects, both upward (emission of gas-phase substance) and downward (release of dissolved or particulate matter), during iron-mediated photodegradation of ADOM. Using ADOM extracted from Hulun Lake algae, we conducted simulated photodegradation experiments to investigate how Fe influences reactive intermediates formation and ADOM degradation, and to elucidate the mechanisms governing greenhouse gases (GHGs) emissions and nutrient release, particularly phosphorus. Our results reveal that both Fe(III) and Fe(II) enhance hydroxyl radical (•OH) generation and ADOM mineralization. At low concentrations (0.3 mg·L-1), Fe(III) saturates in quenching triplet-state ADOM (3ADOM*), while Fe(II) exhibits concentration-dependent inhibition. Fe(III) primarily drives free radical generation via complexation with ADOM, while Fe(II) mainly promotes the Fenton reaction, highlighting distinct photochemical pathways. Notably, the release rate of dissolved inorganic phosphorus (DIP) showed a significant positive correlation with •OH concentration (p < 0.001), indicating •OH-mediated oxidative transformation of dissolved organic phosphorus (DOP) to DIP. At an ADOM level of 10 mgC·L-1, the directly released DIP exceeds the eutrophication threshold for surface water (>0.02 mg·L-1). Furthermore, both Fe ions and ADOM concentration significantly enhance photochemical GHGs emissions, dominated by CO2. The relative contributions of CH4 and N2O to global warming potential decreased initially before stabilizing under radical-driven reactions, a pattern linked to ADOM chemical structure. These findings provide a new perspective for in-depth understanding of the carbon-iron-phosphorus interaction and climatic effects, offering a scientific basis for ecological protection and sustainable management.

The influence of iron (oxyhydr)oxides on the transformation and migration of arsenic(As) has garnered significant attention. Previous work has largely focused on the transformation of iron oxides related to As fate at molecular and mechanistic levels. However, studies examining the interplay between As concentration and iron oxides transformation within complex soil system are sparse. This study investigates the transformation of iron oxides in soils with varying As concentration during microbial dissimilatory iron reduction (DIR), employing humic acid (HA) as electron shuttle and assesses the impact on As speciation transformation. Comparative analyses indicate that in soils with high As concentration (>1000 mg/kg), the secondary transformation of iron (oxyhydr)oxides to other forms, such as the conversion of ferrihydrite to goethite and lepidocrocite, or schwertmannite to goethite, is impeded. Consequently, the formation of goethite and lepidocrocite, which would typically re-stabilize As, is inhibited, leading to elevated release of As(III). On the other hand, an increase in magnetite formation in soils with low As concentration (<100 mg/kg) appears to re-stabilize As effectively. Furthermore, the formation of new secondary iron (oxyhydr)oxides in soils with As concentration <200 mg/kg enhances fraction F5, which subsequently contributes to the re-immobilization of As, sequestering it within the soil matrix. This process results in a lower release of As(III) from soils with As concentration below 200 mg/kg. These findings enhance the understanding of the interdependent relationship between the transformation of iron oxides and the fate of As in complex soil systems.

Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.

Oxidation of chromium (Cr)-bearing minerals by manganese (Mn) oxides is viewed as the dominant mechanism controlling geogenic production of Cr(VI) and its contamination of groundwater. This process may be modulated by other chemical constituents found in the natural environment, but such confounding factors have not been quantified. Here, we evaluated the mechanism of Cr(III) oxidation by mixed-valence Mn oxide in the presence of citric and gallic acids, two natural organic matter (NOM) constituents commonly found in the soil environment. Incubation experiments showed that each organic acid enhanced solubilization of Cr(III) and Mn over controls without organic addition but increasing organic acid concentration decreased production of Cr(VI), with approximately 8.5 times less Cr(VI) produced in the citric acid than gallic acid experiments. X-ray absorption spectroscopy showed that negligible Cr(VI) was present in solid-phase reaction products, regardless of treatment. Geochemical modeling revealed that in the citric acid experiments, unprotonated Cr(III)-citrate was the dominant organo-metallic complex in solution, while (CrOH)2+ distribution positively correlated with concentrations of Cr(VI) produced. Collectively, these results illustrate how NOM can modify expected chemical pathways driving Cr cycling, and such mechanistic information should be better integrated into models predicting Cr redox dynamics and availability in the environment.

The interaction of manganese (Mn) oxides with natural organic matter (NOM) can mobilize Mn, impacting groundwater quality. However, the formation of Mn colloids, critical determinants of Mn transport and aggregation, is often overlooked. To investigate Mn behavior and colloid formation upon C amendment, humic acid was reacted with Mn oxide suspensions at different C:Mn molar ratios (C:Mn = 0-15) over 200 h. The addition of organic carbon promoted the formation of highly stabilized and mixed-valence Mn colloids through reductive dissolution and complexation, with "aqueous" Mn (<450 nm, C:Mn = 15) release increasing by 56.3% from 0 to 200 h and colloidal Mn increasing by up to 31.9% (3-450 nm, C:Mn = 15, t = 200 h) compared to without carbon addition. Analysis of Mn oxidation state revealed that C-Mn colloids contained Mn in multiple oxidation states, and the percentage of Mn(II,III) relative to total Mn increased with increasing C concentrations. In surface water, the hydrodynamic diameter of both Mn and C-Mn colloids remained stable. In groundwater, C-Mn colloids (C:Mn = 3) remained stable (150 nm) over 30 days, while Mn colloids aggregated into larger particles. Analysis of natural surface and groundwaters identified a substantial fraction of Mn (up to 19.2 and 27.2%, respectively) existing in colloidal phases. These findings shed light on the intricate cycling of Mn among particulate, colloidal, and dissolved phases, which governs Mn fate and transport in the environment.

Although it is well known that phosphate retention in soils and sediments is strongly influenced by binding to secondary iron oxides, there have been relatively few studies examining its adsorption/desorption behavior on multicomponent particles of realistic natural complexity. In this study, natural Mn-rich limonite (LM), was used to prepare naturally complex Fe- and Mn-oxide composite materials to examine phosphate adsorption/desorption. To clarify the role of the Mn-oxides, results for the LM sample were compared to those for an acid treated version (LAT), in which the acid-extractable Mn-oxide fraction has been selectively eliminated while leaving the Fe-oxide fraction intact. The saturated adsorption capacity on LAT was almost double that on LM, suggesting that phosphate adsorption to the iron oxides is strongly occluded by the Mn-oxide fraction. This result is reinforced by the comparing the pH dependence and fits to adsorption isotherms, and by desorption experiments and STEM-EDS mapping showing that phosphate loading on Mn-oxides was limited. Hence, although the collective results confirm that phosphate uptake and strong binding is selectively controlled by the Fe-oxide fraction, our study reveals that the Mn-oxide fraction strongly interferes with this process. Therefore, phosphate uptake behavior on metal oxides cannot be predicted solely on the basis of the Fe-oxide fraction present, but instead must take into account the deleterious impacts of other intimately associated phases. For co-diagenetic Fe/Mn-oxide composites in particular, Mn-oxides appear to severely limit phosphate uptake on the Fe-oxide fraction, either by hindering access to binding sites on the Fe-oxide or by lowering their affinity for P.

Manganese (Mn) oxides can oxidize dissolved organic matter (DOM) and alter its chemical properties and microbial degradability, but the compound selectivity for oxidation and oxidative alterations remain to be determined. We applied ultrahigh mass spectrometry to catalog the macromolecular composition of Suwannee River fulvic acid (SRFA) before and after oxidation by a Mn oxide (δ-MnO2) at pH 4 or 6. Polycyclic aromatic hydrocarbons, polyphenols, and carbohydrates were more reactive in reducing δ-MnO2 than highly unsaturated and phenolic (HuPh) compounds and aliphatics, but highly abundant HuPh contributed the most (∼50%) to the overall reduction of δ-MnO2. On average, oxidized species had higher molecular weights, aromaticity, carbon unsaturation degree, nominal oxidation state of carbon, and oxygen and nitrogen contents but were lower in hydrogen content compared to unoxidized species. The oxidation decreased these molecular indices and oxygen and nitrogen contents but increased the hydrogen content, with stronger changes at the lower pH. This DOM oxidation on polar mineral surfaces was more selective but shared similar selectivity rules to adsorption. The abiotic oxidation resembles microbial oxidative degradation of organic matter, and Mn oxide-oxidizable carbon may be a useful index for detection and identification of labile organic carbon.

No abstract available

Rapid sand filters (RSFs) have shown potential for removing organic micropollutants (OMPs) from groundwater. However, the abiotic removal mechanisms are not well understood. In this study, we collect sand from two field RSFs that are operated in series. The sand from the primary filter abiotically removes 87.5% of salicylic acid, 81.4% of paracetamol, and 80.2% of benzotriazole, while the sand from the secondary filter only removes paracetamol (84.6%). The field collected sand is coated by a blend of iron oxides (FeOx) and manganese oxides (MnOx) combined with organic matter, phosphate, and calcium. FeOx adsorbs salicylic acid via bonding of carboxyl group with FeOx. The desorption of salicylic acid from field sand indicates that salicylic acid is not oxidized by FeOx. MnOx adsorbs paracetamol through electrostatic interactions, and further transforms it into p-benzoquinone imine through hydrolysis-oxidation. FeOx significantly adsorbs organic matter, calcium, and phosphate, which in turn influences OMP removal. Organic matter on field sand surfaces limits OMP removal by blocking sorption sites on the oxides. However, calcium and phosphate on field sand support benzotriazole removal via surface complexation and hydrogen bonding. This paper provides further insight into the abiotic removal mechanisms of OMPs in field RSFs.

The stability of organic matter-iron-phosphate (OM-Fe-P) association has an important impact on the migration and sequestration of organic carbon (OC) and P in the environment. Here, we examined the release characteristics of Fe, P and OM due to the abiotic reduction of OM-Fe-P associations by Na-dithionite. The associations were synthesized with algae-derived OM (AOM) and terrestrial humic acid (HA) through either adsorption onto iron (hydr)oxide or coprecipitation with Fe(III). Results indicated that OM and P adsorbed onto the associations were rapidly released, whereas coprecipitation yielded much lower release rates of Fe, P, and OM. The stronger inhibitory effect on reduction from coprecipitation can be explained by larger particles formed by coprecipitation and coprecipitation taking up more OC that had a passivation effect on the associations. The release rates of OM and P were lower in coprecipitates formed with HA than formed with AOM for a given OC/Fe ratio. This observation can be attributed to a patchy distribution of OC in AOM associated coprecipitates, which showed a weaker aggregation of OC with Fe and P. In contrast, the distribution of OC in HA-associated coprecipitates was more homogenous, enabling a stronger aggregation of OM with P and a greater passivation effect on P release. Our results revealed that OM sources, association formation pathways, and elemental stoichiometry collectively controlled the stability of OM-Fe-P associations.

The interaction of soil organic matter with mineral surfaces is a critical reaction involved in many ecosystem services, including stabilization of organic matter in the terrestrial carbon pool and bioavailability of plant nutrients. Using model organic acids typically present in soil solutions, this study couples laboratory adsorption studies with density functional theory (DFT) to provide physical insights into the nature of the chemical bonding between carboxylate functional groups and a model FeOOH cluster. Topological determination of electron density at bond critical points using quantum theory of atoms in molecules (QTAIM) analysis revealed that the presence of multiple bonding paths between the organic acid and the FeOOH cluster is essential in determining the competitive adsorption of organic acids and phosphate for FeOOH surface adsorption sites. The electron density and Laplacian parameter values from QTAIM indicated that the primary carboxylate - FeOOH bond was more ionic than covalent in nature. The experimental and computational results provide molecular-level evidence of the important role of electrostatic forces in the bonding between carboxylic acids with Fe-hydroxides. This knowledge may assist in the formulation of management studies to meet the challenges of maintaining ecosystems services in the face of a changing climate.

The retention and mobilization of phosphate in soils are closely associated with the adsorption of iron (hydr)oxides and root exudation of low-molecular-weight organic acids (LMWOAs). This study investigated the role of LMWOAs in phosphate mobilization under incubation and field conditions. LMWOAs-mediated iron (hydr)oxide transformation and phosphate adsorption experiments revealed that the presence of LMWOAs decreased the phosphate adsorption capacity of iron (hydr)oxides by up to ~74 % due to the competition effect, while LMWOAs-induced iron mineral transformation resulted in an approximately six-fold increase in phosphate retention by decreasing the crystallinity and increasing the surface reactivity. Root simulation in rhizobox experiments demonstrated that LMWOAs can alter the contents of different extractable phosphate species and iron components, leading to 10 % ~ 30 % decreases in available phosphate in the near root region of two tested soils. Field experiments showed that crop covering between mango tree rows promoted the exudation of LMWOAs from mango roots. In addition, crop covering increased the contents of total phosphate and available phosphate by 9.08 % ~ 61.20 % and 34.33 % ~ 147.33 % in the rhizosphere soils of mango trees, respectively. These findings bridge the microscale and field scale to understand the delicate LMWOAs-mediated balance between the retention and mobilization of phosphate on iron (hydr)oxide surface, thereby providing important implications for mitigating the low utilization efficiency of phosphate in iron-rich soils.

Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of δ-MnO2, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO2 molar ratios. Without the oxyanions, δ-MnO2 transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO2 ratios (0.09 and 0.13) and into δ-MnOOH and Mn3O4 with minor poorly crystallized α- and γ-MnOOH at high Mn(II)/MnO2 ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of δ-MnO2, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.

The adsorption of phosphorus (P) onto active soil surfaces plays a pivotal role in immobilizing P, thereby influencing soil fertility and the filter function of soil to protect freshwater systems from eutrophication. Competitive anions, such as organic matter (OM), significantly affect the strength of this P-binding, eventually controlling P mobility and release, but surprisingly, these processes are insufficiently understood at the molecular level. In this study, we provide a molecular-level perspective on the influence of OM on P binding at the goethite-water interface using a combined experimental-theoretical approach. By examining the impact of citric acid (CIT) and histidine (HIS) on the adsorption of orthophosphate (OP), glycerolphosphate (GP), and inositol hexaphosphate (IHP) through adsorption experiments and molecular dynamics simulations, we address fundamental questions regarding P binding trends, OM interaction with the goethite surface, and the effect of OM on P adsorption. Our findings reveal the complex nature of P adsorption on goethite surfaces, where the specific OM, treatment conditions (including covering the surface with OM or simultaneous co-adsorption), and initial concentrations collectively shape these interactions. P adsorbs on goethite with an order of GP < OP < IHP. Crucially, this trend remains consistent across all adsorption experiments, regardless of the presence or absence of OM, CIT, or HIS, and irrespective of the specific treatment method. Notably, OP is particularly susceptible to inhibition by OM, while adsorption of GP depends on specific OM treatments. Despite being less sensitive to OM, IHP shows reduced adsorption, especially at higher initial P concentrations. Of significance is the strong inhibitory effect of CIT, particularly evident when the surface is pre-covered, resulting in a substantial 70 % reduction in OP adsorption compared to bare goethite. The sequence of goethite binding affinity to P and OM underscores a higher affinity of CIT and HIS compared to OP and GP, suggesting that OM can effectively compete with both OP and GP and replace them at the surface. In contrast, the impact of OM on IHP adsorption appears insignificant, as IHP exhibits a higher affinity than both CIT and HIS towards the goethite surface. The coverage of goethite surfaces with OM results in the blocking of active sites and the generation of an unfavorable electric potential and field, inhibiting anion adsorption and consequently reducing P binding. It is noteworthy that electrostatic interactions predominantly contribute more to the binding of P and OM to the surface compared to dispersion interactions.

Minerals constitute a primary ecosystem control on organic C decomposition in soils, and therefore on greenhouse gas fluxes to the atmosphere. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides—collectively referred to as “oxides” hereafter—are prominent protectors of organic C against microbial decomposition through sorption and complexation reactions. However, the impacts of Mn oxides on organic C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly crystalline δ-MnO2, has a greater maximum sorption capacity for dissolved organic matter (DOM) derived from a deciduous forest composite Oi, Oe, and Oa horizon leachate (“O horizon leachate” hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy and scanning transmission X-ray microscopy–near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calculations reveal that DOM sorption onto HMO induces partial Mn reductive dissolution and Mn reduction of the residual HMO. X-ray photoelectron spectroscopy further shows increasing Mn(II) concentrations are correlated with increasing oxidized C (C=O) content (r = 0.78, P < 0.0006) on the DOM–HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissolution does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 102 μg C m−2), DOM desorption—assessed by 0.1 M NaH2PO4 extraction—is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 102 μg C m−2). No significant differences are observed in the impacts of HMO and goethite on the biodegradability of the DOM remaining in solution after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM–HMO complexes may be more vulnerable to reductive dissolution than DOM–goethite complexes.

Minerals, natural organic matter (NOM) and divalent manganese (Mn(II)) often co exist in suboxic/oxic environment. Multiple adsorption and oxidation processes occur in this ternary system, which are coupled to affect the fate of both OM and Mn therein and alter their chemical reactivity towards metals and other pollutants. However, the details about the coupling are poorly known although much has been gained for the binary systems. We determined the mutual influence of surface catalyzed Mn(II) oxidation and humic acid (HA) adsorption and oxidation in a Fe(III) oxide (goethite)-HA-Mn(II) system at pH 5 - 8. The presence of Mn(II) increased HA adsorption by 74% - 100% whereas HA greatly impaired the extent and rate of Mn(II) oxidation by O2 on goethite surfaces. The impacts were more pronounced at higher pH. Mn(II) oxidation produced β-MnOOH, γ-MnOOH and Mn3O4 which in turn oxidized HA, producing small organic acids. The presence of HA markedly altered the composition of Mn(II) oxidation products by preferentially inhibiting the formation of β-MnOOH, leading to increased proportions of γ-MnOOH and Mn(II) adsorbed on the HA-mineral assemblage. Non-conducting γ-Al2O3 exhibited similar but weaker effects than semi-conducting goethite in the above processes. Our results suggest that similar to Mn oxidizing microorganisms, mineral surfaces can drive the coupling of the Mn redox cycle with NOM oxidative degradation under suboxic/oxic and circumneutral/alkaline conditions.

Manganese oxides are considered as one of the effective oxides capable of oxidizing arsenite and reduce the toxicity of arsenic. Since low molecular weight organic acids (LMWOAs) commonly found in nature can act as reducing and chelating agents for manganese oxides, it is particularly important to investigate how these organic acids with different numbers of carboxyl groups like citrate and EDTA affect oxidation and adsorption of arsenic by manganese oxides. In this study, low As(V) adsorption on manganese oxide is slightly enhanced by citrate and EDTA, which results from the increase in active sites via reduction of manganese oxide by LMWOAs. However, citrate and EDTA have different effects on the oxidation of As(III). MnIII/II citrate autocatalytic cycle as a manganese-based redox system decreases As(III) oxidation rate, but EDTA does not yield autocatalysis, which slightly increases the oxidation rate of As(III). Reduction of manganese oxide by EDTA and chelation between Mn(II) and EDTA lead to exposure of more active sites. Our research highlights the different effects of low molecular weight organic acids on the reactions between arsenic and manganese oxide.

While the availability of arsenic (As) in soil is well known to be highly correlated with the presence of iron (Fe) oxides and humic acid (HA) in the soil, the relationship between Fe oxides and HA and As species in the soil is less well understood. In this study, As speciation in an unsaturated soil in the presence of external HA and green synthesized Fe oxide nanoparticles (FeNPs) showed that As(V) was mainly distributed to the specifically-bound (F2), amorphous and poorly-crystalline hydrous oxides of Fe, Al (F3) and the well-crystallized hydrous oxides of Fe and Al (F4). While As(III). This was the major component in unsaturated soil, and was mainly distributed to F4 and the residual fraction (F5). As bound to F3 and F5 was most sensitive to the addition of HA and FeNPs, while HA/FeNPs treatment increased the F3-bound As(V); however, it decreased the F5-bound As(III). Nonetheless the effect of HA on As is completely different to the HA/FeNPs treatment. The increase of As(V) in F3 resulted from F5-bound As(III) oxidation when treated by HA/FeNPs. Cyclic voltammetry confirmed that HA and Fe3+/Fe2+ redox enhanced As(III) oxidation, while FTIR revealed that HA-bound As(III) was the least available fraction in the soil. Finally, a mechanism involving a combination of HA and FeNPs was proposed for explaining the redistribution of As species in the soil.

Competition between phosphorus and arsenic limits the application of phosphate materials in soil remediation. However, it is possible to simultaneously stabilize arsenic and cationic metals by sensible use of phosphate's solubility. In this study, ball-milling and humic acid (HA) activated phosphate rock (PR) were used to stabilize cadmium (Cd) and arsenic (As) in soil. After 30 days of treatment with ball-milling and 5 % humic acid-activated phosphate rock (BMP-HA), the leaching concentrations of Cd and As in the soil decreased from 0.12 mg/L and 0.11 mg/L to 0.0086 mg/L and 0.019 mg/L, respectively. The availability of Cd and As was significantly reduced after BMP-HA treatment. The acid-soluble fraction of Cd decreased from 16.30 % to 1.22 %, indicating its transformation into more stable forms. The water-soluble and surface-adsorbed fraction of As decreased to 0.11 %, while the Ca-associated fraction of As increased from 25.87 % to 31.41 %. Ball-milling increased the specific surface area of PR, enhancing the adsorption and surface complexation of Cd. The addition of humic acid facilitated the dissolution of PR. However, the phosphate release rate in BMP-HA was insufficient to activate As. Meanwhile, the released Ca bound with As, further reducing its availability. Overall, BMP-HA proves to be an effective strategy for remediating cadmium and arsenic-contaminated soils.

Water pollution by toxic heavy metals poses a threat to the environment and human bodies. Herein, a novel hydrated ferric oxide nanoparticle (HFO) based hybrid adsorbent was fabricated for the removal of toxic Cu(II), Cd(II) and Pb(II) from water. HFOs were immobilized into a porous resin D-201, and then this nanocomposite HFO-D201 was coated with humic acid (HA) to enhance the binding sites of target metals. Both HFOs and HA contribute to the sequestration of heavy metals. The as-synthesized hybrid adsorbent HA-HFO-D201 exhibited excellent performance on the removal of Cu(II), Cd(II), and Pb(II) in a pH range of 3-9, while no Fe leaching was observed. The presence of natural organic matter (20 mg C/L) has limited influences on the adsorption, and more than 85% of the target metals can be removed after treatment. HA-HFO-D201 showed preferable adsorption toward Cu(II) and Pb(II) (1 mg/L) from the background Ca2+ solution at much higher concentrations (100 mg/L), while the retention of Cd(II) (1 mg/L) decreased to some extent. Fixed-bed column experiments exhibited that the treatment capacities of HA-HFO-D201 are 90 bed volumes (BV) for Cd(II), 410 BV for Pb(II) and > 800 BV for Cu(II) of simulated contaminated water to meet the WHO drinking water standard. Meanwhile, depleted HA-HFO-D201 can be readily regenerated by a chelating agent Na2EDTA for repeated use. The hybrid adsorbent HA-HFO-D201 has excellent potential to remove heavy metals in water treatment systems.

No abstract available

Arsenopyrite is a common metal sulfide mineral and weathers readily in the open environment, releases As, and pollutes the surrounding environment. Humic acid (HA) is ubiquitous in soils, sediments and waters, and contains various functional groups and complex with arsenic, iron and other metal ions that affect the weathering behavior of arsenopyrite. Because As, iron, and HA are redox-active compounds, electrochemical techniques, including polarization curves, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV), were used to fundamentally investigate the weathering process and mechanism of arsenopyrite over a wide range of environmental relevant conditions. Polarization curves showed higher HA concentrations (0-1000 mg•L-1), higher temperatures (5-35°C) or acidities (pH 1.0-7.0) promoted arsenopyrite weathering; there was a linear relationship between the corrosion current density (icorr), temperature (T) and acidity (pH): icorr = -3691.2/T + 13.942 and icorr = -0.2445pH + 2.2125, respectively. Arsenopyrite weathering readily occurred in the presence of HA as confirmed by its activation energy of 24.1 kJ•mol-1, and EIS measurements confirmed that the kinetics were controlled by surface reaction as confirmed by decreased double layer resistance. CV and surface characterization (FTIR and XPS) showed that arsenopyrite initially oxidized to S0, As(III) and Fe2+, then S0 and Fe2+ were ultimately converted into SO42- and Fe3+, while As(III) oxidized to As(V). Furthermore, the carboxyl (-COOH) and phenolic (-OH) of HA could bind with As(III)/(V) and Fe3+ via a ligand exchange mechanism forming As(III)/(V)-HA and As(III)/(V)-Fe-HA complexes that hinders the formation of FeAsO4 and decreases the bioavailability of As. Findings gained from this study are valuable for the understanding of the fate and transport of As in acidic conditions, and have powerful implications for the remediation and management of As-bearing sites affected by mining activities.

Arsenic (As) is a highly poisonous heavy metal with major environmental ramifications. Inorganic components such as zinc (Zn) and iron (Fe), as well as organic vermicompost, have been used as management solutions, with limited attempts of using them together. The current study involved preparing non-enriched vermicompost as well as six distinct Zn and Fe enriched vermicomposts and analyzing their chemical composition using the standard procedures. Organic fractions from these seven vermicompost and arsenic polluted soils of West Bengal, India were recovered and separated into humic (HA) and fulvic acid (FA) fractions. Potentiometric titrations, viscometric assays, and visible spectrophotometry were used to characterize the HA and FA samples. In aqueous phase the stability constant (log K) of the complexes formed with As indicates that stability of FA extracted from enriched vermicompost V4 (Zn and Fe sulphate @ 10% w/w dry weight basis of composting substrates before application of vermiworms) was maximum as 10.20 with a mole ratio (x) value of 1.36. Fourier-transform infrared (FT-IR) spectroscopy and Scanning Electron Microscopy (SEM) studies confirmed the complexation of As with HA/FA. The release isotherm of As from the HA/FA complexes in the presence of competitive oxy-anions was found to follow the order of sulphate > nitrate > phosphate.

No abstract available

Due to the close spatial proximity and strong reactivity, soil humic components inevitably participate in iron (Fe) (oxyhydr)oxide formation, holding significant importance in contaminant immobilization, carbon cycling, and nutrient availability. Yet, the regulatory role of different humic components involved in the initial formation of Fe (oxyhydr)oxides is still lacking. In this study, we identified the characteristic formation periods of ferrihydrite (Fh), the initial phase of Fe (oxyhydr)oxides, through real-time monitoring of solution pH and in situ observations of precipitated Fh nanoparticles in the absence and presence of different humic components. The kinetics of Fh formation were quantified at micrometer and nanometer scales using Raman spectroscopy (RS) and atomic force microscopy (AFM), respectively. Results indicated that the extension of induction time, retardation of phase occurrence, and inhibition of nucleation rates for Fh formation were all dependent on the specific humic component with an order of fulvic acid (FA) > humic acid (HA) > humin (HM). Nanoscale data analysis revealed that the thermodynamic barrier to Fh nucleation increased by maximizing the interfacial free energy (γ) of the reaction system. Through molecular bonding quantification, AFM-based dynamic force spectroscopy (DFS) measurements demonstrated a linear relationship between Gibbs free energies (ΔGb) of soil organic matter (SOM) binding to Fh and γ within the classical nucleation theory (CNT), linking heterogeneous nucleation barriers with organo-mineral bonding. This study is the first to provide in situ evidence of the inhibitory effects of soil humic components on the formation of Fe (oxyhydr)oxides and quantitatively establish that higher energy barriers to nucleation correlate with stronger organo-mineral bonding. This relationship suggests that good organic binders are good inhibitors for mineral formation, offering a novel perspective for predicting the formation and fate of soil minerals through the lens of organo-mineral binding free energies.

The fractionation of humic substances (HS) at the mineral and water interface can change the constituents and reactivity of HS, but there is still a lack of the understanding of the effects of HS fractionation on the binding characteristics of heavy metals to HS. In this study, the binding characteristics of five heavy metals (Cd, Cu, Ni, Pb, and Zn) to humic acid (HA) before and after adsorption by ferrihydrite were investigated by employing two-dimensional correlation spectroscopy (2D COS) integrated with synchronous fluorescence and Fourier transform infrared (FTIR) spectroscopies. 2D COS analyses of the fluorescence results indicated that the susceptibility of the fluorescence of humic-like fraction to heavy metals significantly decreased after the adsorption of HA by ferrihydrite, which may be due to the fact that humic-like components were preferentially adsorbed by ferrihydrite. However, the fractionation processes did not alter the metal binding sequence and affinity to different HA components. 2D COS analyses of the FTIR results suggested that fractionation processes decreased the susceptibility of COO- groups to heavy metals, and changed the metal binding sequence to polysaccharides C-O and aryl groups, with the exception of Pb. Furthermore, model calculations showed that the binding ability of heavy metals to both humic-like and fulvic-like fractions decreased after the adsorption of HA by ferrihydrite. The results of this study contribute to predicting heavy metal behavior in the environment.

Iron (hydr)oxide-natural organic matter (NOM) colloids, the dominant components of soil, usually occur in varied circumstances and may affect Hg transport and fate in soil. This study aims to reveal the Hg binding to preformed composites rather than only focusing on Hg retention by iron (hydr)oxides in the presence of NOM. Ferrihydrite-humic acid (FH-HA) is chosen as a representative composite, and the effect of the complexation method and FH morphology on Hg binding to various composites is evaluated. Three types of composites are developed: a dense coprecipitated composite (p-d-f), a gel-like adsorbed composite (a-g-f) and a dense adsorbed (a-d-f) composite. Batch sorption and stirred-flow kinetic tests together with surface property analysis and modern spectral analyses are carried out to explore the binding behavior of Hg to the three composites and clarify the interactions in the ternary systems of FH-HA-Hg. The results show that the Hg sorption isotherms all fit well with the Langmuir model, and the maximum sorption capacities follow the order a-g-f> a-d-f > p-d-f, implying that the adsorbed composite is more favorable than the coprecipitated composite for Hg binding and a gel morphology is more beneficial than a dense morphology. The stirred-flow experiments show that the adsorbed composite has a small advantage in Hg sorption compared to the coprecipitated composite and that the gel-like composite can adsorb more Hg at a faster rate than the dense composite. Both FH and HA participate in Hg sorption, and FH-HA-Hg complexes are speculated to form. These findings are helpful to better understand the mobility and fate of Hg in soils, as well as the associated dynamic model for predicting Hg behavior in the environment where the iron (hydr) oxide-NOM composites are pre-existed.

No abstract available

Activated carbon air cathode combined with iron anode oxidation-flocculation synergistic Arsenic (As) removal was a new groundwater purification technology with low energy consumption and high efficiency for groundwater with high As concentration. The presence of organic matter such as humic acid (HA) had ambiguous effects on formation of organic colloids in the system. The effects of the particle size distribution characteristics of these colloids on the formation characteristics of flocs and the efficiency of As purification was not clear. In this work, we used five different pore size alumina filter membranes to separate mixed phase solutions and studied the corresponding changes in iron and arsenic concentrations in the presence and absence of humic acid conditions. In the presence of HA, the arsenic concentration of < 0.05 µm particle size components was 1.01, 1.28, 3.07, 7.69, 2.85 and 1.24 times of that in the absence of HA. At the same time, the arsenic content in 0.05-0.1 µm and 0.1-0.45 µm particle size components was also higher than that in the system without HA, which revealed that the presence of HA hindered the flocculation behavior of As distribution to higher particle sizes in the early stage of the reaction. The presence of HA affected the flocculation rate of iron flocs from small to large particle size fractions and it had limited effect on the behavior of large-size flocs in adsorption of As. These results provide a theoretical basis for targeted, rapid, and low consumption synergistic removal of arsenic and organic compounds in high arsenic groundwater.

Arsenic (As) bio-availability in the soil is influenced by different organic and inorganic anions. In the present study, the effects of various competitive agents, including phosphate, citrate, oxalate, humic acid (HA), and fulvic acid (FA), on the adsorption of As in calcareous soils were investigated. The results revealed the presence of phosphate, citrate, and oxalate in soil has a significant impact on the arsenic retention (adsorption) in soil which increases the As bio-availability. The negative impact of the competing anions was increased at higher concentrations. The Double Site Langmuir (DSL) isotherm was best fitted to the adsorption data, which indicates that most of the As adsorbed on the low-energy surfaces (non-specific adsorption by oxides, clays, and clay-size calcite). Accordingly, in soil 1, the DSL predicted that, due to phosphate, citrate, and oxalate competition (at a concentration of 10 mM), the adsorption capacity of the high- and low-energy surfaces decreased from 86.2 to 33.5, 82.1 and 61.3 mg/kg and from 663 to 659, 335.8, and 303.5 mg/kg, respectively, Moreover, after addition of phosphate, citrate, and oxalate to the soil-As system, the Langmuir constant of high-energy surfaces decreased from 0.686 to 0.074, 0.261, and 0.301 L/mg, respectively. No regular trend was observed for the Langmuir constant of low-energy surfaces. Similarly, in soils 2, 3, and 4, the adsorption capacities of both high- and low-energy surfaces as well as the Langmuir constant of high-energy surfaces decreased by the addition of phosphate, citrate, and oxalate to the soil-As system. HA and FA did not have a significant effect on the As adsorption behavior. Phosphate, citrate, and oxalate, as interfering oxyanions, increased the As bio-availability in the calcareous soils by decreasing the As adsorption.

No abstract available

No abstract available

No abstract available

The adsorption and mobility of glyphosate (PMG) in soils, sediments, natural waters, and wastewater treatment sludge are controlled by small-sized metal (hydr)oxides and affected by competition with phosphate. In this study, the adsorption of PMG to ferrihydrite, a ubiquitous nano-sized iron (hydr)oxide, is measured by batch adsorption experiments at a wide concentration (∼0.02 - 0.6 mM) and pH range (∼4-10), in the absence and presence of phosphate. The adsorption data were interpreted using the charge distribution and multisite ion complexation (CD-MUSIC) model for ferrihydrite. Including phosphate as a competitor induces electrostatic changes on the surface potential, independently of glyphosate adsorption, which allowed us to accurately distinguish between the chemical affinity and electrostatic effects contributing to PMG adsorption to ferrihydrite. PMG binds primarily as a binuclear bidentate complex, of which the amino group may protonate (logKH=7.9). Only at low pH and high PMG surface loading, when binuclear bidentate binding sites become scarce, a monodentate complex with protonated amino and phosphonate groups becomes prominent. Phosphate effectively decreases PMG adsorption and contributes to its enhanced mobility. The resulting CD model provides a quantitative and mechanistic description of PMG adsorption to ferrihydrite, which can be used for improved predictions of the environmental fate of PMG and its water removal effectiveness.

The diversity of soil adsorbents for arsenic (As) and the often-overlooked influence of manganese (Mn) on As(III) oxidation impose challenges in predicting As adsorption in soils. This study uses Mössbauer spectroscopy, X-ray diffraction of oriented clay, and batch experiments to develop a kinetic coupled multi-surface complexation model that characterizes As adsorbents in natural soils and quantifies their contributions to As adsorption. The model integrates dynamic adsorption behaviors and Mn-oxide interactions with unified thermodynamic and kinetic parameters. The results indicate that As adsorption is governed by five primary adsorbents: poorly crystalline Fe oxides, well crystalline Fe oxides, Fe-rich clay, Fe-depletion clay, and organic carbon (OC). Fe oxides dominate As adsorption at low As concentrations. However, at higher As concentrations, soils from carbonate strata, with higher content of Fe-rich clay, exhibit stronger As adsorption capabilities than soils from Quaternary sediment strata. The enrichment in Fe-rich clay can enhance the resistance of adsorbed As to reduction processes affecting Fe oxides. Additionally, extensive redox cycles in paddy fields increase OC levels, enhancing their As adsorption compared to upland fields. This model framework provides novel insights into the intricate dynamics of As within soils and a versatile tool for predicting As adsorption across diverse soils.

Unreasonable storage of phosphate ore is becoming an important pathway causing phosphate pollution in the surrounding aquatic environment. However, there is little research on the influence of dissolved organic matter (DOM) in water on the fate of phosphate ore. Here, we collected phosphate ores from two phosphate mines along the coast of Tanglang River and studied the effects of DOM concentrations and pH on the release of soluble active phosphorus (SRP) and fluoride ion (F−) from phosphate ores using humic acid (HA) as the representative of DOM. Based on the analysis of ZP, FTIR, XPS, and SEM, the influence mechanism of HA was revealed. The results showed that HA efficiently promoted the release of SRP and F− from phosphate ore. With decreasing pH, the P release increased in both water and HA solutions in general. The beneficial influence of HA on the release of SRP and F− from phosphate ore was ascribed to the introduction of oxygen-containing functional groups by HA, which altered the surface properties and enhanced the dispersion stability of phosphate ore. These findings provided new insights into the dispersion behavior of phosphate ore, which is helpful in promoting the pollution control and management strategy of phosphate ore.

Iron species have essential influence on the environmental/geochemical behaviors of arsenic species in water and soil. Colloidal ferric hydroxide (CFH) induces photooxidation of arsenite (As(III)) to arsenate (As(V)) in water at neutral pH through surface complexation and ligand-to-metal charge transfer (LMCT). However, the effect of the co-existing natural organic matter (NOM) on the complexation-photolysis in this process has remained unclear. In the present work, the photooxidation of As(III) induced by CFH was investigated in the presence of various carboxylic acids and polyphenols as simple model compounds of NOM. Two different light sources of ultraviolet A (UVA) (λmax = 365 nm) and ultraviolet B (UVB) (λmax = 313 nm) were used for photooxidation treatment of the experimental ternary system and the control binary system respectively. The obtained results demonstrated that all investigated NOM inhibited the photooxidation of As(III) in the As(III)/CFH system at pH 7. Moreover, the correlation analysis between the pseudo-first order rate constant kobs and various property parameters of NOM showed that the stable constant for the complexation between Fe(III) and NOM (logKFe-NOM) as well as the molecular weight of NOM and the percentages of total acidity of NOM exhibited significant correlations. A simple quantitative structure-activity relationship (QSAR) model was established between kobs and these three parameters utilizing a multiple linear regression method, which can be employed to estimate the photooxidation efficiency of As(III) in the presence of ferric iron and NOM. Thus, the present work contributes to the understanding of the environmental interactions between NOM and iron.

&NA; Phosphorus is a necessary nutrient for the growth and survival of living beings. Nevertheless, an oversupply of phosphorus in wastewater results in eutrophication. Therefore, its removal from wastewater is important. However, coexisting components, such as anions, heavy metals, and organic matter, might inhibit the phosphate‐adsorption mechanism by competing for the active surface sites of the adsorbent. In this study, iron oxide nanoflakes (INFs) were fabricated on iron foil via anodization. The rate of phosphate adsorption from wastewater onto INFs in the presence of three different coexisting components—anions, heavy metals, and organic matter—was evaluated. The morphology of the INFs was analyzed by X‐ray diffraction, field emission scanning electron microscopy, energy dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy, and Fourier‐transform infrared spectroscopy. The phosphate adsorption equilibrium time using INFs was found to be 1 h. The Elovich model (R2 > 0.99) and the Langmuir model (R2 >0.95) respectively provided the best description of the adsorption kinetics and isotherm, suggesting the chemisorption nature of adsorption. The estimated adsorption capacity of the INFs was 21.5 mg‐P g–1. The effect of anions (chloride, sulfate, nitrate, and carbonate) and heavy metals (Cd, As, Cr, and Pb) was studied at three different molar ratios (0.5:1, 1:1, and 1.5:1). The effect of different types of organic matter, such as citric acid, humic acid, and oxalic acid at concentrations of 100 and 200 mg L–1, was also examined. In five regeneration cycles, the total amount of phosphate adsorbed and desorbed, and the recovery percentage were 6.51 mg‐P g–1, 5.16 mg‐P g–1, and 79.24%, respectively. HighlightsIron oxide nanoflakes were synthesized via an electrochemical anodization.Effect of time, phosphate concentration, and coexisting components were tested.Phosphate uptake process was followed by the Elovich model.The maximum phosphate adsorption capacity of INF was 21.5 mg‐P g–1.Effect of coexisting organic matter is higher followed by heavy metals and anions.

Iron (hydr)oxides and humic acid (HA) are important active components in soils and usually coexist in the environment. The effects of HA on the adsorption and subsequent immobilization of phosphate on iron (hydr)oxide surface are of great importance in studies of soil fertility and eutrophication. In this study, two types of goethite with different particle sizes were prepared to investigate the phosphate adsorption behaviors and complexation mechanisms in the absence or presence of HA by combining multiple characterization and modeling studies. The adsorption capacity of micro- (M-Goe) and nano-sized goethite (N-Goe) for phosphate was 2.02 and 2.04 μmol/m2, which decreased by ∼25% and ∼45% in the presence of 100 and 200 mg/L HA, respectively. Moreover, an increase in equilibrium phosphate concentration significantly decreased the adsorption amount of goethite for HA. Charge distribution-multisite surface complexation (CD-MUSIC) and natural organic matter-charge distribution (NOM-CD) modeling identified five phosphate complexes and their corresponding affinity constants (logKP). Among these phosphate complexes, FeOPO2OH, (FeO)2PO2, and (FeO)2POOH species were predominant complexes on the surface of both M-Goe and N-Goe across a wide range of pH and initial phosphate concentrations. The presence of HA had little effect on the coordination mode and logKP of phosphate on goethite surface. These results and the obtained model parameters shed new lights on the interfacial reactivity of phosphate at the goethite-water interface in the presence of HA, and may facilitate further prediction of the environmental fate of phosphate in soils and sediments.

The influence of effluent organic matter (EfOM) on phosphate polishing removal by adsorption plays an important role in determining the application potential of adsorbents. Molecular understanding of EfOM regarding its impact on adsorption is insufficient due to a lack of appropriate EfOM fractionation/characterization protocols, corresponding to a particular structure-function property of adsorbents. In this work, a combinative method of coupling DEAE/XAD fractionation with molecular characterization was proposed, targeting the versatile structure-function characters of nanocomposite, for investigating EfOM and its impact on phosphate removal by nanocomposite during long-term adsorption/regeneration runs. Zirconium-based polystyrene anion exchanger (HZO-201) was selected as a representative nanocomposite, featuring with porous networking matrix, positively charged surface and multiple adsorptive sites. The EfOM samples from three biologically treated sewage effluent sources were separated into fractions of negatively charged organic acid (OA) and hydrophobic-, transphilic-, hydrophilic-neutral/base (HPO-n/b, TPI-n/b and HPI-n/b). The combinative method effectively differentiated the charge, aromaticity, molecular weight and functionalities between the fractions, which corresponded to the multiple structural/surface characteristics of HZO-201 and favored the evaluation on the impact of the EfOM fractions. The extent of interference of the EfOM fractions on phosphate removal followed an order of OA > HPO-n/b > TPI-n/b > HPI-n/b. The OA fraction, characterized by negatively charged, aromatic functionalities and broad molecular weight distribution (1-5 kDa and 14 kDa), was recognized as the key interfering fraction, presumably due to its multiple adsorption pathways (i.e., ion exchange, π-π interactions and pore filling). Particularly, the low-molecular-weight OA moieties (1-4 kDa) progressively accumulated onto the nanocomposite via irreversible adsorption, causing a continuous phosphate-capacity loss by 32.70% over multiple cycles. We believe the combinative fractionation/characterization method may widely apply to complex water systems for identifying key influential organic matters in polishing treatment of various pollutants by adsorption.

No abstract available

Metal oxide-Carbon composites have been developed tailoring towards specific functionalities for removing pollutants from contaminated environmental systems. In this study, we synthesized a novel CaO-MgO hybrid carbon composite for removal of phosphate and humate by co-pyrolysis of dolomite and sawdust at various temperatures. Increasing of pyrolysis temperature to 900 °C generated a composite rich in carbon, CaO and MgO particles. Phosphate and humate can be removed efficiently by the synthesized composite with the initial solution in the range of pH 3.0-11.0. The phosphate adsorption was best fitted by pseudo-second-order kinetic model, while the humate adsorption followed the pseudo-second-order and the intra-particle diffusion kinetic models. The maximum adsorption capabilities quantified by the Langmuir isotherm model were up to 207 mg phosphorus (or 621 mg phosphate) and 469 mg humate per one-gram composite used, respectively. Characterization of composites after adsorption revealed the contributions of phosphate crystal deposition and electrostatic attraction on the phosphate uptake and involvement of π - π interaction in the humate adsorption. The prepared composite has great potential for recovering phosphorus from wastewater, and the phosphate sorbed composite can be employed as a promising phosphorus slow-releasing fertilizer for improving plant growth.

The speciation and fate of arsenic (As) in soil-water systems is a topic of great interest, in part due to growing awareness of As uptake into rice as an important human exposure pathway to As. Rice paddy and other wetland soils are rich in dissolved organic matter (DOM), leading to As/DOM ratios that are typically lower than those in groundwater aquifers or that have been used in many laboratory studies of As-DOM interactions. In this contribution, we evaluate arsenite (As(III)) binding to seven different DOM samples at As/DOM ratios relevant for wetland pore waters, and explore the chemical properties of the DOM samples associated with high levels of As(III)-DOM complexation. We integrate data from wet chemical analysis of DOM chemical properties, dialysis equilibrium experiments, and two-site ligand binding models to show that in some DOM samples, 15-60% of As(III) can be bound to DOM at environmentally-relevant As/DOM ratios of 0.0032-0.016 μmol As/mmol C. Binding decreases as the As(III)/DOM ratio increases. The organic sulfur (Sorg) content of the DOM samples was strongly correlated with levels of As(III)-DOM complexation and "strong" binding site densities, consistent with theories that thiols are strong binding ligands for As(III) in natural organic matter. Finally, a whole-cell E. coli biosensor assay was used to show that DOM samples most effective at complexing As(III) also led to decreased microbial As(III) uptake at low As/DOC ratios. This work demonstrates that naturally-occurring variations in the Sorg content of DOM has a significant impact on As(III) binding to DOM, and has implications for As(III) availability to microorganisms.

Mineral-associated organic matter involving iron oxyhydroxide minerals is important to the preservation and transformation of organic matter in soils and sediments. Largely lacking is a quantitative evaluation of different binding mechanisms in relation to the mineral surface charges. Here, with ferrihydrite, we investigated complexes with organic compounds of various charges and structures, including a ribonucleotide, a sugar, a phenolic acid, and amino acids with different side chains. After constructing model ferrihydrite nanoparticles using reported iron-oxygen coordination, we mapped theoretically the spatial distribution of positive and negative charges, corroborated experimentally by atomic force microscopy. With these variable charges due to protonation extent of surface hydroxyls, molecular dynamics simulations revealed binding mechanisms of organic moieties with opposite charges, confirmed experimentally by infrared spectroscopy. For electrostatic interactions, quantum mechanics-calculated energies determined the order of binding strength consistent with our adsorption data: ester-linked phosphate > protonated primary amine ≥ carboxylate attached to phenyl ring = carboxylate attached to alkyl group. Ligand exchange, which was more thermodynamically favorable than electrostatic interactions despite the energy barrier to the transition state, was driven by the stability of the product. We obtained a quantitative rationale for the binding of ribonucleotide phosphate through ligand exchange versus binding of carboxylate and amino groups through electrostatic interactions, thus informing mechanistic frameworks for mineral-organic associations.

The competitive adsorption between phosphate and dissolved organic carbon (DOC) has been reported in Andosols and Podzols. However, the published results on the competitive adsorption between P and DOC are unclear and sometimes contradictory. The competitive behaviour between phosphate and DOC may be quite different in the surface and subsurface soils. In this study, we used surface and subsurface soils with substantial Fe oxide contents from Wagga Wagga (Chromosol) and Tumbarumba (Ferrosol) in New South Wales (Australia) to evaluate the adsorption behaviour of phosphate and DOC. Adsorption data were fitted into linear initial mass (IM) isotherm. The results showed that both surface and subsurface soils from Tumbarumba had a greater phosphate adsorption capacity than the soils from Wagga Wagga. Phosphate adsorption was greater for the subsurface soil ( m = 0.72) than the surface soil ( m = 0.82) from Tumbarumba, while this trend for was opposite for Wagga Wagga soils, where phosphate adsorption capacity was greater for the surface ( m = 0.55) soil than the subsurface ( m = 0.37) soil. The DOC adsorption was greater in the subsurface soils than the surface soils from both sites. In the mixed solution of P and DOC, phosphate adsorption promoted DOC desorption in the surface and subsurface soils from Wagga Wagga and Tumbarumba. The results of this study have crucial implications on the sustainability of Fe-rich soils. The adsorption of phosphate promoted DOC desorption in these soils, which may lead to the destabilisation of OC and impair OC sequestration and therefore enhance the microbial decomposition of OC in these soils.

Natural organic matter (NOM) is a heterogeneous mixture, including humic acid (HA) and fulvic acid (FA), that competitively interacts with metal (hydr)oxides. Despite its environmental importance, this competition has not yet been measured extensively, and mechanistic modeling is lacking. The present work examined the competitive adsorption to goethite and the corresponding molecular fractionation of HA and FA using UV–vis spectroscopy, acid precipitation, and size exclusion chromatography (SEC). Our findings reveal that on a mass basis, FA particles effectively remove HA particles from the surface. This efficiency can be mainly attributed to an interfacial space limitation in which FA restricts HA adsorption, as evidenced by mechanistic modeling with the Consistent Competitive Ligand and Charge Distribution (LCDcc) approach for the heterogeneous adsorption of NOM. The adsorbed FA particles occupying part of the surface prevent HA from accessing the corresponding double-layer space, disproportionally reducing HA adsorption. This restriction leads to a high HA/FA mass exchange ratio (∼2.4 ± 0.6), consequently affecting the mobility and transport of oxyanions (arsenate and phosphate) in the environment. The difference in the partitioning of NOM is also relevant for soil carbon sequestration via the selective preservation of NOM by association with oxide minerals.

Lanthanum (hydr)oxide-based materials are attractive as highly efficient adsorbents for phosphate removal from both sewage and lake environment. However, dissolved organic carbon (DOC) coexists in the waters and exact information is still lacking on how DOC influence the phosphate adsorption process. In this study, competitive adsorption of phosphate and DOC on lanthanum modified zeolite (LMZ) was investigated using humic acid as the representative. In LMZ, lanthanum hydroxide was shown to be the active ingredient accounting for >98% of the binding sites of both phosphate and DOC. Without competition, the maximum adsorption capacity of phosphate and DOC estimated from the Langmuir isotherm model was 52.25 and 41.32 mg/g, respectively. When coexisted, DOC did not affect the adsorption of phosphate while phosphate reduced the adsorption of DOC by ~40%. In addition, preloading LMZ with DOC had little effect on phosphate adsorption while coating with phosphate substantially lowered DOC adsorption. Furthermore, phosphate can release most of the adsorbed DOC (>60%), while DOC can not replace adsorbed phosphate (<2%). The adsorption kinetics of both phosphate and DOC was best described by the psudo-second-order model (r2 > 0.999). The adsorption of both phosphate and DOC increased with decreasing pH or increasing ionic strength. We proposed that phosphate was competitive than DOC for the ligand exchange sites of singly-coordinated hydroxyls, but DOC can be solely adsorbed onto the uncharged hydroxyls via hydrogen bonding.

Organic and inorganic phosphonates often co-exist in municipal sewage, and it is a challenge to remove them simultaneously. Nanoparticle Fe3O4 (Fe3O4 NPs) has attracted significant attention due to its high adsorption activity, low cost, environmental friendliness, and magnetic separation. Herein, the adsorption performance and mechanism of hydroxyethylidene diphosphonic acid (HEDP) and orthophosphate (PO43−) onto Fe3O4 NPs were systematically investigated. When the dosages were 0.4 g/L, the removal efficiencies of HEDP and PO₄³− reached 96.3% and 95.1%, respectively. pH had no significant impact on the adsorption, whereas the presence of HCO3−/CO32− markedly suppressed the removal of HEDP and PO43−. The adsorption of HEDP and PO43− onto Fe3O4 NPs conformed to the pseudo-second-order kinetics and Langmuir isotherm models in single and binary P systems. HEDP consistently inhibited the removal of PO43− in the binary P system. The adsorption mechanisms were primarily driven by the combined effect of electrostatic attraction, hydrogen bonding, and coordination complexation. DFT molecular simulation showed higher adsorption energy between HEDP and Fe3O4 NPs, and the simulation outcomes were in excellent agreement with the experimental data. Although the adsorption of HEDP and PO43− was competitive, total phosphorus in the effluent of municipal sewage could still meet the discharge standard.

Sustainable remediation of toxic metal(loid)-contaminated paddy soils using biowastes is of great importance from both agricultural and environmental perspectives. The redox-mediated interactions between organic amendments, with multivariate sources (i.e., biogas slurry (BGS), rice husk-biochar (RH-BC), cow dung (CD)) and geochemical drivers may influence arsenic (As) mobilization under dynamically changing redox situations, such as in paddy soils. Here, we explored the impact of BGS, RH-BC, and CD on the mobilization pathway of As in a contaminated paddy soil under a wide range of soil redox potentials (E h: −252 mV to +512 mV), using an automated biogeochemical microcosm system. The partial least-squares-path model (PLS–PM) was used to identify geochemical drivers, which govern As mobilization under reduced and oxidized conditions. Results revealed that As mobilization in the unamended (control) soil was higher under reduced conditions (E h ≤ +100 mV; dissolved As = 4.2–9.2 mg L–1) than that in oxidized conditions (E h ≥ +100 mV; dissolved As = 3.8–6.7 mg L–1). With CD addition, the concentration of dissolved soil As decreased significantly by 19–62% at E h < 0 mV, followed by RH-BC (12–54%) and BGS (34–49%) compared to control. Temporal increase in pH under moderately reduced conditions (E h > +100 mV) led to a maximum decrease in dissolved As concentration with CD (36–77%), and it ranged from 18 to 75% and 29 to 49% for RH-BC and BGS, respectively, over control. These findings highlight that the addition of BGS, RH-BC, and CD, particularly CD, to As-contaminated paddy soil can decrease As mobilization under slightly reduced to oxidized conditions, which occur in the natural paddy soil-rice system. This research advanced our understanding to employ multivariate tools to identify the most potent organic amendment to immobilize As under paddy soil conditions.

Ferrihydrite, a poorly ordered metastable iron oxide, is closely associated with dissolved organic matter (DOM) in soils and sediments. Although sunlight-induced photoreductive dissolution of ferrihydrite via ligand-to-metal charge transfer (LMCT) has been extensively studied, its potential impacts on mineralogical transformation and environmental behaviors of coexisting contaminants remain largely unknown. Here, we systematically investigated the effects of environmental parameters (e.g., solution pH, pO2 level, arsenic speciation, and content) on ferrihydrite transformation with the representative DOM-oxalate under simulated solar irradiation. Results showed that the oxalate-mediated LMCT process synchronously initiated Fe(II) production and proton consumption, the latter of which facilitated interfacial electron transfer and atom exchange (IET-AEFh-Fe2+) processes among ferrihydrite and newly formed Fe(II). At pH 5.0-8.0, ferrihydrite was prone to transform into goethite due to sufficient Fe(II) (approximately 80-2700 μM) from LMCToxa and enough affinity of Fe(II) with mineral to trigger IET-AEFh-Fe2+, while it only underwent reductive dissolution at pH 3.0-5.0 or kept a quasi-steady state over pH 8.0. Increasing the pO2 level and arsenic content hampered the recrystallization of ferrihydrite by reducing Fe(II) duration or altering the surface property of ferrihydrite, whereas the presence of As(III/V) also led to the formation of lepidocrocite with As(V) being more prominent. Additionally, chemical extraction and As K-edge EXAFS spectroscopy revealed that As was consecutively incorporated into the structures of goethite and lepidocrocite in the form of As(V) regardless of primary As speciation. These findings shed novel insights into low-crystalline iron oxide transformation and element migration driven by sunlight in natural environments.

Jarosite is an important scavenger for arsenic (As) due to its strong adsorption capacity and ability to co-precipitate metal(loid)s in acid mine drainage (AMD) environments. When subjected to natural organic matter (NOM), metastable jarosite may undergo dissolution and transformation, affecting the mobility behavior of As. Therefore, the present study systematically explored the dissolution and transformation of jarosite, and the consequent redistribution of coprecipitated As(V) under anoxic condition in the presence of a common phenolic acid-gallic acid (GA). The results suggested that As(V) incorporating into the jarosite structure stabilized the mineral and inhibited the dissolution process. Jarosite persisted as the dominant mineral phase at pH 2.5 up to 60 d, though a large amount of structural Fe(III) was reduced by GA. However, at pH 5.5, jarosite mainly transformed to ferrohexahydrite (FeSO4·6H2O) with GA addition, while the principal end-product was goethite in GA-free system. The dissolution process enhanced As(V) mobilization into aqueous and surface-complexed phase at pH 2.5, while co-precipitated fraction of As(V) remained dominant under pH 5.5 condition. Result of XPS indicated that no reduction of As(V) occurred during the interaction between GA and As(V)-bearing jarosite, which would limit the toxicity to the environment. The reductive process involved that GA promoted the dissolution of jarosite via the synergistic effect of ligand and reduction, following by GA and release As(V) competing for active sites on mineral surface. The findings demonstrated that phenolic groups in NOM can exert great influence on the stability of jarosite and partitioning behavior of As(V).

The environmental fate of arsenic (As) relies substantially on its speciation, which occurs frequently coupled to the redox transformation of manganese. While trivalent manganese (Mn(III)), which is known for its high reactivity, is believed to play a role in As mobilization by iron (oxyhydr)oxides in dynamic aquifers, the exact roles and underlying mechanisms are still poorly understood. Using increasingly complex batch experiments that mimick As-affected aquifer conditions in combination with time-resolved characterization, we demonstrate that Mn(III)-NOM complexes play a crucial role in the manganese-mediated immobilization of As(III) by ferrihydrite and goethite. Under anaerobic condition, Mn(III)-fulvic acid (FA) rapidly oxidized 31.8% of aqueous As(III) and bound both As(III) and As(V). Furthermore, Mn(III)-FA exerted significantly different effects on the adsorption of As by ferrihydrite and goethite. Mn(III)-FA increased the adsorption of As by 6-16% due to the higher affinity of oxidation-produced As(V) for ferrihydrite under circumneutral conditions. In contrast, As adsorption by crystalline goethite was eventually inhibited due to the competitive effect of Mn(III)-FA. To summarize, our results reveal that Mn(III)-NOM complexes play dual roles in As retention by iron oxides, depending on the their crystallization. This highlights the importance of Mn(III) for the fate of As particularly in redox fluctuating groundwater environments.

In this work, a green and highly efficient functional material (FMHs) was developed through the precise optimization of Fe2+, Fe3+, Mn2+, and humic acid (HA) ratios. The stabilization efficacy of FMHs on the high-concentration Cd-Pb-Cu-Zn-As-Sb co-contaminated soil and its potential ecological impacts were comprehensively evaluated from multiple perspectives. After 60 days of soil remediation with 5 wt% FMHs, the extractable Cd, Pb, Cu, Zn, As, and Sb were reduced by 59.06 %, 99.83 %, 92.60 %, 92.43 %, 98.06 %, and 77.04 %, respectively. These reductions were primarily attributed to the FMHs-driven transformation of unstable or substable metal(loid) species into stable residual forms. Furthermore, FMHs application enhanced soil redox potential (Eh), electrical conductivity (EC), nitrogen content (TN, NH4+-N, NO3--N), and enzymatic activities (acid phosphatase, sucrase, cellulase, and catalase), while concurrently reducing soil pH, phosphorus, potassium content, and urease activity. Molecular ecological network analyses revealed that FMHs significantly reshaped soil bacterial diversity and microbial community structure, reducing the complexity of bacterial and fungal networks while fostering positive correlations. Bacterial community assembly was predominantly governed by stochastic processes, with environmental variables playing a pivotal role in shaping the assembly dynamics. In summary, this work provided new insights and technologies for the personalized design and fabrication of functional materials for the simultaneous stabilization and remediation of multi-metal(loid)s contaminated soils.

No abstract available

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Arsenic (As) contamination of groundwater is primarily driven by microbially mediated redox processes and the dynamic evolution of dissolved organic matter (DOM). The influence of cycled methanogenesis and methane oxidation processes on As species transformation in geogenic As-contaminated groundwater, however, remain mechanistically elusive. In this study, quantitative relationships among DOM molecular characteristics, microbial functional networks, and As speciation were established using sediment microcosm experiments, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and metagenomic sequencing. The results indicate that rates of methanogenesis and methane oxidation are regulated by thermodynamic properties of DOM. Labile DOM promoted As(III) mobilization at a rate of 1.04 μg kg⁻¹ d⁻¹ through methyl-related metabolism. Remarkably, enhanced methane oxidation further elevated the As(III) generation rate to 3.30 μg kg⁻¹ d⁻¹, underscoring the accelerating effect of methane cycling on As release. In contrast, humified DOM decoupled the geochemical linkage between iron and As. Microbial succession governed the redox transitions, as the proliferation of methanogens substantially increased methane production (up to 7.23 mg kg⁻¹ d⁻¹), while methanotrophs enhanced oxidation rates from 94.99 to 190.76 mg kg⁻¹ d⁻¹. This microbial progression coupled sulfate and As(V) reduction through the up-regulation of key functional genes (dsrAB, arsC). Energy conversion during DOM biodegradation governs As migration stages. These findings highlight the interactive constraints on As speciation dynamics by molecular characteristics of DOM and microbial functional networks during methane biotransformation processes in groundwater systems. This research provides new mechanistic insights into As biogeochemical cycling in geogenic contaminated groundwater.

No abstract available

Natural organic matter (NOM) colloids are frequently encountered at the anoxic-oxic interface in subsurface environments. Their surface-rich functional groups and redox capacity exert a significant influence on the fate and transport of Fe and Cd in aquatic systems. The present study demonstrated that stable Fe-HA-Cd colloids formed in both anoxic and oxic environments, with hydrodynamic diameters stabilized at 97.4-134.5 nm at an HA concentration of 64.3 mg C/L. The incorporation of Fe promoted the formation of Cd colloids on the surface of HA to a certain extent. However, the high concentration of Fe(II) (C/Fe <22.4) and Fe(III) (C/Fe<7.0) in both anoxic and oxic conditions inhibited the formation of Cd colloid by competitive adsorption and co-precipitation, respectively. Furthermore, the redox effect in the oxic transformation of Fe(II)-HAred-Cd(II) colloid led to the release of truly dissolved Cd from colloidal particles to the water. The aggregation kinetics and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory demonstrated that Fe-HA-Cd colloids reduced particle stability compared to HA-Cd(II) colloids. Additionally, the depolymerization behavior of Fe-HA-Cd colloids during aggregation exhibited variability under different conditions, particularly with regard to the time-dependent size effect. This study offers detailed data on the formation, oxidative transformations, and stability of Fe-HA-Cd colloids in anoxic-oxic environments rich in organic matter. The findings provide valuable insights into Cd partitioning and environmental behavior between particulate and dissolved states, essential for understanding Cd pollution and advancing effective remediation strategies.

Natural organic matter (NOM) containing Fe/Mn (hydr)oxides effectively stabilizes antimony (Sb) and arsenic (As) in soils. However, the specific type of NOM that limits the mobility of Fe/Mn (hydr)oxides and how NOM-Fe/Mn colloidal properties can be modulated for better Sb and As stabilization remains unclear. This study suggests that the degree of stabilization of the colloidal structure formed between NOM and Fe/Mn (hydr)oxides is crucial for Sb and As stabilization. It was found that straw-derived (SD), compared to humic acid (HA) with a high content of carboxyl groups, forms more stable colloidal structures with Fe/Mn (hydr)oxides. HA-Fe/Mn colloids show greater mobility and less deposition than SD-Fe/Mn colloids. In soil remediation simulations, SD-Fe/Mn colloids more effectively stabilized Sb and As. After 35 days, SD-Fe/Mn achieved nearly complete stabilization (100 %) of water-soluble and decarbonate-extracted bioavailable fractions at depths of 1-12 cm, with high rates for other fractions as well. Even at depths of 23-34 cm, SD-Fe/Mn outperformed HA-Fe/Mn, showing higher stabilization rates for Sb and As by 12.6 % and 20.4 %, respectively. Morphological analysis suggests that the stabilization of Sb and As by SD-Fe/Mn primarily involves adsorption onto or incorporation within the Fe/Mn (hydr)oxides. This study offers guidance for optimizing NOM-Fe/Mn for in situ stabilization of Sb and As, enhances the understanding of different types of NOM that affect the behavior of Sb and As soil contamination, and presents new perspectives for developing effective in situ remediation materials.

No abstract available

Phosphorus (P) removal from lake/drainage waters by novel adsorbents may be affected by competitive substances naturally present in the aqueous media. Up to date, the effect of interfering substances has been studied basically on simple matrices (single-factor effects) or by applying basic statistical approaches when using natural lake water. In this study, we determined major factors controlling P removal efficiency in 20 aquatic ecosystems in the southeast Spain by using linear mixed models (LMMs). Two non-magnetic -CFH-12® and Phoslock®- and two magnetic materials -hydrous lanthanum oxide loaded silica-coated magnetite (Fe-Si-La) and commercial zero-valent iron particles (FeHQ)- were tested to remove P at two adsorbent dosages. Results showed that the type of adsorbent, the adsorbent dosage and color of water (indicative of humic substances) are major factors controlling P removal efficiency. Differences in physico-chemical properties (i.e. surface charge or specific surface), composition and structure explain differences in maximum P adsorption capacity and performance of the adsorbents when competitive ions are present. The highest P removal efficiency, independently on whether the adsorbent dosage was low or high, were 85-100% for Phoslock and CFH-12®, 70-100% for Fe-Si-La and 0-15% for FeHQ. The low dosage of FeHQ, compared to previous studies, explained its low P removal efficiency. Although non-magnetic materials were the most efficient, magnetic adsorbents (especially Fe-Si-La) could be proposed for P removal as they can be recovered along with P and be reused, potentially making them more profitable in a long-term period.

Organic phosphonates have been widely used in various industries and are ubiquitous in wastewaters, and efficient removal of phosphonates is still a challenge for the conventional processes because of the severe interferences from the complex water constitutions. Herein, an Nd-based nanocomposite (HNdO@PsAX) was fabricated by immobilizing hydrated neodymium oxide (HNdO) nanoparticles inside a polystyrene anion exchanger (PsAX) to remove phosphonates from high-salinity aqueous media. Batch experiments demonstrated that HNdO@PsAX had an excellent adsorption capacity (∼90.5 mg P/g-Nd) towards a typical phosphonate (1-hydrox-yethylidene-1,1-diphosphonic acid, HEDP) from the background of 8 g/L NaCl, whereas negligible HEDP adsorption was achieved by PsAX. Attractively, various coexisting substances (humic acid, phosphate, citrate, EDTA, metal ligands, and anions) exerted negligible effects on the HEDP adsorption by HNdO@PsAX under high salinity. FT-IR and XPS analyses revealed that the inner-sphere complexation between HEDP and the immobilized HNdO nanoparticles is responsible for HEDP adsorption. Fixed-bed experiments further verified that HNdO@PsAX was capable of successively treating more than 4500 bed volumes (BV) of a synthetic high-salinity wastewater (1.0 mg P/L of HEDP), whereas only ∼2 BV of effective treatment capacity was received by PsAX. The exhausted HNdO@PsAX was amenable to a complete regeneration by a binary NaOHNaCl solution without significant loss in capacity. The capability in removing other organic phosphonates and treating a real electroplating wastewater by HNdO@PsAX was further validated. Generally, HNdO@PsAX exhibited a great potential in efficiently removing phosphonates from high-salinity wastewater.

The paper considers the results of long-term studies on some chemical elements’ migration (Al, Fe, Ti, Mn, Cu, Zn, Pb, N, P, Si) in the «bottom sediments – water» system of surface water bodies under the effect of different aquatic environment factors. The greatest effect is made by water bodies’ oxygen regime, pH, and presence of dissolved organic substances, in particular humic substances. The migration of manganese, iron, inorganic nitrogen and phosphorus from bottom sediments is controlled mostly by oxygen regime. Migration of these chemical elements significantly increases, when there is oxygen deficiency and anaerobic conditions are formed in the bottom water layer. It has been observed in both natural and experimental conditions. Man- ganese concentration increases in bottom water in 25–50 times, iron – in 1.3–13, inorganic nitrogen – in 5.3–19.3, and inorganic phosphorus – in 2.8–23 times. The dissolved oxygen concentration hardly has any effect on the migration of aluminium, titanium, copper, zinc, lead, and silicon from bottom sediments into the water. The chemical elements’ migration is significantly affected by a decrease in pH of water contacting with bottom sediments and the presence of humic substances in it. High humic substances concentrations promote a reduction in water pH and oxygen content, which is consumed for their oxidation. A case study of several water bodies illustrates the cumulative effect of water pH lowering, anaerobic conditions at the solid and liquid phases’ interface, as well as complexation with humic substances on chemical elements’ migration from bottom sediments. Experimental modeling has shown that the metal migration from bottom sediments occurs both due to their labile fractions and complex compounds with dissolved organic matter, especially with humic substances of low molecular weight (≤2.0 kDa). The share of the metal labile fraction gets higher, when water pH decreases. Under recent climate change, the probability of water’s secondary pollution with different chemicals increases significantly in summer. This is mainly caused by oxygen deficiency, water pH lowering, and reducing conditions at the «bottom sediments – water» interface with hydrogen sulphide being formed. This is especially true for highly eutrophic water bodies subject to human impact. The aquatic environment toxicity can get considerably higher due to the migration of chemicals with strong toxic properties from bottom sediments, as well as labile metal fractions, marked by higher bioavailability for hydrobionts.

Dissolved organic matter (DOM), a ubiquitous and active ingredient, is extensively involved in the transformation and migration of environmental pollutants in aquatic ecosystems. However, its chemical composition in acid mine drainage (AMD)-impacted rivers remains poorly characterized, hindering our understanding of its role in the biogeochemistry of key elements in contaminated fluvial environments. Here, we investigated the concentration of dissolved organic carbon (DOC) and spectroscopic and molecular characteristics of DOM in a headwater river contaminated with polymetallic mine-derived AMD in southern China. Terrestrial humic-like (C1) and typically groundwater-supplied aromatic protein/tyrosine-like (C2) substances which were partially from AMD, were identified as the predominant fluorescent components in the river water. Notably, tryptophan-like (C3) substances originating from tailings pond spills were only occasionally detected in the river. Although DOM biogeochemical transformations and degradation occurred in the lateral soil-water riparian interface and longitudinal in-stream transport processes, the molecular compositions identified by FT-ICR MS showed a core set of molecular formulae in the lignin/saturated compound/tannin region of the van Krevelen diagram of the water samples across the rivers. The complexation of DOM with typical metals in AMD was investigated using fluorescence quenching experiments. The results showed that the highest binding ability of Fe(III) to C2 followed by C1, with both detected in the experimental water samples. Mg(II) and Ca(II) strengthened the binding of DOM-Fe(III) when the ferric/DOM ratio was low, while Cu(II) weakened the binding of DOM-Fe(III) due to competition. Ca(II) inhibited the binding of Fe(III) to C1 but promoted the binding of the complex to C2 when both Cu(II) and Mg(II) were present. Since DOM-Fe(III) complexation was associated with the cotransport of AMD-derived metals/metalloids in diverse aqueous environments with multiple co-existing ions (typically Ca(II) input for remediation), our study on the composition of DOM and its complexation with metals can contribute to managing and remediating AMD-impacted rivers.

No abstract available

Re-wetting of drained fens can release phosphorus, introducing a eutrophication risk for associated aquatic ecosystems. Characterisation of the different forms of organic and inorganic bound phosphorus in the peat is an important step towards the development of tools for assessing the level of risk attached to individual re-wetting projects. In the work reported here, a sequential extraction (fractionation) method was used to distinguish the following P binding forms: 1. labile P, detected by NH4Cl extraction; 2. redox-sensitive P, detected by Na2S2O4/NaHCO3 extraction; 3. P adsorbed to metal oxides, detected by HCl extraction; 4. P bound to humic substances, detected by NaOH extraction; and 5. organic and refractory bound P, detected using H2SO4 and H2O2. Special attention was paid to the degree of decomposition (DPD) of the peat, and metal concentrations were measured in selected fractions. Higher P concentrations were found in completely humified than in little humified peat for all fractions except the NH4Cl (labile P) fraction, where P content increased as DPD decreased. As only 1% of total phosphorus (TP) was present as labile P, the results indicate that the decisive horizons for nutrient release after re-wetting are those that are completely humified due to pedogenetic changes. The principal metal sorption partner for P was Fe.

No abstract available

No abstract available