Internal loading of phosphorus in streams described by a Sediment-Water Exchange Model for Phosphorus (SWEMP): From lab to field scale

The reaction of phosphorus (P) between sediments and water in streams strongly affects the surface water P concentrations. A new reactive transport model (SWEMP: Sediment-Water Exchange Model for Phosphorus) was developed to describe redox dependent P sorption in the sediment and vertical diffusive transport of solutes to the overlying stream. The model parameters were independently obtained to first predict P release in ten different sediment-water batch systems and in two flumes. Input parameters are the degree of P saturation of the sediment, its organic matter content, dissolved oxygen (DO) concentration and temperature. The dissolved P concentrations in the overlying waters ranged from 0.02 to 1.2 mg P L-1 in these systems and were correctly predicted by the model within, on average, a factor 1.3 (batch) or 1.1 (flume). The P flux from the sediment towards the overlying water increased with increasing sediment P:Fe ratio and respiration rates, and with decreasing DO and water pH. After validation of the model with experimental data, it was used to predict monthly P concentrations in Flemish rivers using the total P emission data, total discharge, average sediment properties and the monthly averaged water temperatures, DO concentrations and electric conductivity. The monthly average P concentrations oscillate annually between 0.24 and 0.73 mg P L-1 and predictions matched the long-term monitoring data within 10 % using only one adjustable parameter for the entire water system (N > 250,000). The model predicts that summer peaks in P are related to internal loading from the sediment under anoxic conditions rather than to emission-dilution effects, i.e. external input of P and/or its concentration at lower flow rates. This suggests that, surface water P concentrations can be lowered by enhanced DO in the water, the addition of Fe and Al rich binding agents to the sediments and by reducing P emissions.

Elevated internal phosphorus loading from shallow areas of eutrophic boreal lakes: Insights from porewater geochemistry

Internal phosphorus (P) loading is widely recognized as a major cause of lake eutrophication. One conventional paradigm states that the magnitude of internal loading through P diffusion is constrained by the presence of iron (Fe) oxides in surface sediments under oxic conditions near the sediment-water interface (SWI). However, biogeochemical P dynamics in Fe-rich sedimentary systems are still not fully understood, especially in eutrophic lakes where intensively coupled organic matter (OM) remineralization and reductive dissolution of Fe-bound P (Fe-P) exist concurrently. Here, we assess the diagenetic processes that govern sedimentary P cycling in two eutrophic Fe-rich lakes in southern Finland, Lake Hiidenvesi and Lake Kytäjärvi, using a combination of porewater and solid-phase analyses. Coupled reductive dissolution of Fe-P and OM remineralization controlled P regeneration in both lakes, with Fe-P acting as the dominant source for porewater P. Vivianite formation likely immobilized sedimentary P in the deepest basin of Hiidenvesi. Elevated P diffusion rates were observed at shallow sites under oxic bottom water conditions in summer in both lakes, stimulated by enhanced remineralization of both freshly- (mostly phytoplankton-origin) and earlier-deposited OM under elevated temperatures. Areas overlain by oxic bottom water contributed more benthic P fluxes to the water column compared to anoxic/hypoxic areas in both lakes during all sampling seasons. Our study suggests that in shallow eutrophic settings with high OM deposition and elevated temperatures, remineralization in upper sediments regenerates P efficiently enough to support a significant amount of P release to the water column even under sedimentary molar Fe/P ratios >20. We also discuss the implication of our findings for lake restoration strategies.

Dynamics and geochemical responses of dissolved metals (Mn and Cu) in a subtropical estuary, China

The research delved into the occurrence and dynamics of dissolved metals, specifically manganese (Mn) and copper (Cu), within the Jiulong River Estuary, South China, a medium-sized subtropical estuary. Our findings unveiled a nuanced seasonal and spatial variability of dissolved metals throughout the entire estuarine system. Notably, dissolved Mn concentrations peaked (~ 3.5 μM) in the upper estuary, diminishing sharply along the salinity gradient, with a modest rise in the middle estuary and outer Xiamen Bay. In the upper estuary, heightened concentrations of dissolved Mn occurred in spring due to augmented terrestrial particle inputs, followed by suboxically reductive releases; conversely, concentrations were low in summer, attributed to dilution from increased freshwater discharges and particle scavenging. In contrast, dissolved Cu exhibited differently, with elevated concentrations (29.2-37.5 nM) in the upper and middle estuaries, driven by reductive dissolution of Mn particles and chloride-induced ion exchanges, respectively. Concurrently, heightened inputs of nutrients and metals correlated with elevated phytoplankton productivity (indicated by chlorophyll a) in the upper and outer estuary regions. Our analysis underscored the sensitivity of dissolved metals to environmental parameters, including temperature, pH, and dissolved oxygen. The integration of compiled historical data underscored the dynamic nature of dissolved metals, particularly Cu, in response to geochemical processes.The elevated ion levels indicated intensified ion releases from particles and sediments, attributable to increased anthropogenic perturbation and climatic changes (e. g. ocean warming).

Phosphorus release and recovery by reductive dissolution of chemically precipitated phosphorus from simulated wastewater

Chemically mediated recovery of phosphorous (P) as vivianite from the sludges generated by chemical phosphorus removal (CPR) is a potential means of enhancing sustainability of wastewater treatment. This study marks an initial attempt to explore direct P release and recovery from lab synthetic Fe-P sludge via reductive dissolution using ascorbic acid (AA) under acidic conditions. The effects of AA/Fe molar ratio, age of Fe-P sludge and pH were examined to find the optimum conditions for Fe-P reductive solubilization and vivianite precipitation. The performance of the reductive, chelating, and acidic effects of AA toward Fe-P sludge were evaluated by comparison with hydroxylamine (reducing agent), oxalic acid (chelating agent), and inorganic acids (pH effect) including HNO3, HCl, and H2SO4. Full solubilization of Fe-P sludge and reduction of Fe3+ were observed at pH values 3 and 4 for two Fe/AA molar ratios of 1:2 and 1:4. Sludge age (up to 11 days) did not affect the reductive solubilization of Fe-P with AA addition. The reductive dissolution of Fe-P sludge with hydroxylamine was negligible, while both P (95 ± 2%) and Fe3+ (90 ± 1%) were solubilized through non-reductive dissolution by oxalic acid treatment at an Fe/oxalic acid molar ratio 1:2 and a pH 3. With sludge treatment with inorganic acids at pH 3, P and Fe release was very low (<10%) compared to AA and oxalic acid treatment. After full solubilization of Fe-P sludge by AA treatment at pH 3 it was possible to recover the phosphorus and iron as vivianite by simple pH adjustment to pH 7; P and Fe recoveries of 88 ± 2% and 90 ± 1% respectively were achieved in this manner. XRD analysis, Fe/P molar ratio measurements, and magnetic attraction confirmed vivianite formation. PHREEQC modeling showed a reasonable agreement with the measured release of P and Fe from Fe-P sludge and vivianite formation.

Facet-dependent electron transfer induces distinct arsenic reallocations on hematite

The interfacial electron transfer (ET) between electron shuttling compounds and iron (Fe) oxyhydroxides plays a crucial role in the reductive dissolution of Fe minerals and the fate of surface-bound arsenic (As). However, the impact of exposed facets of highly crystalline hematite on reductive dissolution and As immobilization is poorly understood. In this study, we systematically investigated the interfacial processes of the electron shuttling compound cysteine (Cys) on various facets of hematite and the reallocations of surface-bound As(III) or As(V) on the respective surfaces. Our results demonstrate that the ET process between Cys and hematite generates Fe(II) and leads to reductive dissolution, with more Fe(II) generated on {001} facets of exposed hematite nanoplates (HNPs). Reductive dissolution of hematite leads to significantly enhanced As(V) reallocations on hematite. Nevertheless, upon the addition of Cys, a raipd release of As(III) can be halted by its prompt re-adsorption, leaving the extent of As(III) immobilization on hematite unchanged throughout the course of reductive dissolution. This is due to that Fe(II) can form new precipitates with As(V), a process that is facet-dependent and influenced by water chemistry. Electrochemical analysis reveals that HNPs exhibit higher conductivity and ET ability, which is beneficial for reductive dissolution and As reallocations on hematite. These findings highlight the facet-dependent reallocations of As(III) and As(V) facilitated by electron shuttling compounds and have implications for the biogeochemical processes of As in soil and subsurface environments.

Distribution, geochemical behavior, and risk assessment of arsenic in different floodplain aquifers of middle Gangetic basin, India

The present study interprets the distribution and geochemical behavior of As in groundwaters of different regions along the floodplains of Ganga river (Varanasi, Ghazipur, Ballia), Ghaghara river (Lakhimpur Kheri, Gonda, Basti), and Rapti river (Balrampur, Shrawasti) in the middle Gangetic basin, India for risk assessment (non-carcinogenic and carcinogenic). The concentration of As in groundwaters of these floodplains ranged from 0.12 to 348 μg/L (mean 24 μg/L), with around ~ 37% of groundwater samples exceeding the WHO limit of 10 μg/L in drinking water. Highest As concentration (348 μg/L) was recorded in groundwater samples from Ballia (Ganga Floodplains), where 50% of the samples had As > 10 μg/L in groundwater. In the study area, a relatively higher mean concentration was recorded in deep wells (28.5 μg/L) compared to shallow wells (20 μg/L). Most of the high As-groundwaters were associated with the high Fe, bicarbonate and low nitrate and sulfate concentrations indicating the release of As via reductive dissolution of Fe oxyhydroxides. The saturation index values of the Fe minerals such as goethite, hematite, ferrihydrite, and siderite showed the oversaturation to near equilibrium in groundwater, suggesting that these mineral phases may act as source/sink of As in the aquifers of the study area. The health risk assessment results revealed that a large number of people in the study area were prone to carcinogenic and non-carcinogenic health risks due to daily consumption of As-polluted groundwater. The highest risks were estimated for the aquifers of Ganga floodplains, as indicated by their mean HQ (41.47) and CR (0.0142) values.

Effects of montmorillonite on the adsorption of Fe(II) by ferrihydrite and its phase transformation at different pH

Recently, there has been a clear understanding of the mechanism and influencing factors of ferrihydrite (Fh) phase transformation catalyzed by Fe(II); however, these factors mainly belong to environmental conditions and exogenous substances. And there is a lack of research on the effect of soil composition and structure on the phase transformation of Fh. Therefore, this study investigated the effects of montmorillonite (Mt) on the adsorption of Fe(II) and phase transformation of Fh under near-neutral pH. The initial rates ([Formula: see text]) of Elovich equation demonstrated the addition of Mt inhibited the adsorption of Fh but simultaneously accelerated the initial adsorption, thus increasing the adsorption of the system (e.g., 22.09-25.03 mg/g as increased Mt under pH 6.5) due to its high surface charge density. Increased pH enhances the surface charge density by promoting the deprotonation of the surface group (Fe-OH, Al-OH, and Si-OH) and consequently increases adsorption of Fe(II) (e.g., 17.97-22.09 mg/g as increased pH of pure Fh). Based on the previous method of extracting labile Fe(III), we found that pH promotes the initial formation of labile Fe(III) by increasing electron transfer and promoting recrystallization caused by bridging condensation, via increased -OH. Although Mt inhibits the adsorption of Fh, it promotes the formation of labile Fe(III) by increasing the system adsorption and bond with Fh. The results of the analysis of variance showed both pH and solid ratio influence significantly on the maximum adsorption (p = 6.81 × 10^-9 and 2.54 × 10^-3) and the conversion ratios of labile Fe(III) (p = 3.43 × 10^-24 and 9.16 × 10^-43).

Phosphorus and iron-oxide transport from a hydrologically isolated grassland hillslope

Dissolved reactive phosphorus (DRP) loss from agricultural soils can negatively affect water quality. Shallow subsurface pathways can dominate P losses in grassland soils, especially in wetter months when waterlogging is common. This study investigated the processes controlling intra- and inter-event and seasonal DRP losses from poorly drained permanent grassland hillslope plots. Temporal flow related water samples were taken from surface runoff and subsurface (in-field pipe) discharge, analysed, and related to the likelihood of anaerobic conditions and redoximorphic species including nitrate (NO₃^-) over time. Subsurface drainage accounted for 89% of total losses. Simple linear regression and correlation matrices showed positive relationships between DRP and iron and soil moisture deficit; and negative relationships between these three factors and NO₃^- concentrations in drainage. These data indicate that waterlogging and low NO₃^- concentrations control the release of P in drainage, potentially via reductive dissolution. The relationship between DRP and metal release was less obvious in surface runoff, as nutrients gathered from P-rich topsoil camoflaged redox reactions. The data suggest a threshold in NO₃^- concentrations that could exacerbate P losses, even in low P soils. Knowledge of how nutrients interact with soil drainage throughout the year can be used to better time soil N and P inputs via, for example, fertiliser or grazing to avoid to excessive P loss that could harm water quality.

Role of indigenous microorganisms and organics in the release of iron and trace elements from sediments impacted by iron mine tailings from failed Fundão dam

After Fundão Dam failure in 2015, most of Gualaxo do Norte River in Doce River Basin in Brazil became silted by iron mining tailings consisting mainly of fine-grained quartz, hematite, and goethite. Previous work pointed to the possibility of reductive dissolution of iron and manganese from tailings, leading to mobilization of iron, manganese and trace elements. Several microorganisms were shown to reduce Fe(III) to Fe(II) and Mn(III, IV) to Mn(II) "in vitro", but their roles in mobilization of Fe and trace elements from freshwater sediments are poorly understood. In this work, bottom sediments and water collected in Gualaxo do Norte River were used to build anoxic microcosms amended with acetate, glucose or yeast extract, in order to access if heterotrophic microorganisms, either fermenters or dissimilatory Fe reducers, could reduce Fe(III) from minerals in the sediments to soluble Fe(II), releasing trace elements. The Fe(II) concentrations were measured over time, and trace elements concentrations were evaluated at the end of the experiment. In addition, minerals and biopolymers in bottom sediments were quantified. Results showed that organic substrates, notably glucose, fuelled microbial reduction of iron minerals and release of Fe(II), Mn, Ba, Al and/or Zn from sediments. In general, higher concentrations of organic substrates elicited mobilization of larger amounts of Fe(II) and trace elements from sediments. The results point to the possibility of mobilization of huge amounts of iron and trace elements from sediments to water if excess biodegradable organic matter is released in rivers affected by iron mine tailings.

The degradation of dissolved organic matter in black and odorous water by humic substance-mediated Fe(II)/Fe(III) cycle under redox fluctuation

In nature, the hydroxyl radical (•OH) is produced during the anaerobic-aerobic transition when groundwater level fluctuates. In addition, the •OH is also detected in iron-bearing clay minerals and iron oxides during the redox process. Goethite is one of the most stable iron oxides involved in biogeochemical cycles. In this study, the coexisting humic acid (HA) enhanced the generation of Fe(II) during the iron reduction process and accelerated the generation of •OH in the redox process of goethite. The organic contaminants in black and odorous water were decomposed by constructing an iron-reducing bacteria-HA-Fe(II)/Fe(III) reaction system under anaerobic-aerobic alternation. The results demonstrated that in the anaerobic stage, HA could promote the reduction and dissolution of goethite through the complexation effect and electron shuttle mechanism, as well as significantly strengthening the iron reduction process in water. Under aerobic conditions, Fe(II) in the reaction system would activate O₂ to generate •O₂^-. The •OH, formed by Fe (II) and •O₂^- via Fenton reaction and Haber-Weiss mechanism, oxidized dissolved organic matter (DOM) in water. The characterization of DOM by three-dimensional fluorescence spectroscopy (3DEEM) indicated that after four redox fluctuations, the organic contaminants in water samples were effectively degraded. Generally, this study provides new approaches and insights into the biogeochemical cycling of Fe and C elements and water pollution remediation at the anoxic-anoxic interface.

Effects of low molecular weight organic acids with different functional groups on arsenate adsorption on birnessite

In an aquatic ecosystem, especially constructed wetlands receiving arsenic (As)-containing wastewater, the fate and mobility of As is influenced by manganese (Mn) oxides and organic matter. Although Mn oxides have been extensively investigated for As(V) adsorption, effects of low molecular weight organic acids (LMWOAs) with different functional groups on As(V) adsorption onto birnessite and underlying mechanisms remain elusive. In this study, LMWOAs with two carboxyl groups (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively) were used. Results showed that more As(V) was adsorbed on birnessite with the presence of LMWOA, indicating that the LMWOA promoted As(V) adsorption via birnessite-carboxyl-As(V) ternary complex. Before birnessite dissolution, TA and MA facilitated As(V) adsorption more efficiently than SA, indicating that hydroxyl group enhanced the coordination among carboxyl groups, As(V) and birnessite. However, within high TA/MA batches, As(V) concentrations decreased sharply and then gradually increased, but Mn(II) concentrations continuously increased, showing the initial reductive dissolution of birnessite promoted As adsorption, while further dissolution was conducive to As mobilization. This study identifies the mechanisms of As adsorption in the presence of LMWOAs and highlights the importance of functional groups in As fate and mobility in aqueous environments.

A sharp contrasting occurrence of iron-rich groundwater in the Pearl River Delta during the past dozen years (2006-2018): The genesis and mitigation effect

Fe-rich (>0.3 mg/L) groundwater is generally present in areas where organic matter-rich fluvial, lacustrine, or marine sedimentary environments occur. The Pearl River Delta (PRD) that marine sediments is common, where a large scale of Fe-rich groundwater was distributed but disappearing in recent decade. This study aims to investigate the change of Fe-rich groundwater in the PRD, and to discuss the genesis controlling Fe-rich groundwater in the PRD during the past dozen years. A total of 399 and 155 groundwater samples were collected and analyzed at 2006 and 2018, respectively. Results showed that Fe-rich groundwater of the PRD was from 19.3% at 2006 dropped to 1.3% at 2018. Fe-rich groundwater in coastal-alluvial aquifers was more than 2 times that in other aquifers at 2006. Both of anthropogenic and geogenic sources were contributed to the widely distribution of Fe-rich groundwater in the PRD at 2006. The infiltration of industrial wastewater and the irrigation of Fe-rich surface water were the major anthropogenic driving forces for the occurrence of Fe-rich groundwater in the PRD at 2006. The reductive dissolution of Fe minerals in aquifer sediments, associated with the degradation of organic matter in marine sediments and the sewage infiltration, was the main driving force for the enrichment of groundwater Fe in coastal-alluvial aquifers at 2006. The intrusion of sewage triggering the reductive dissolution of Fe minerals in terrestrial sediments and the reductive dissolution of Fe minerals in carbon-rich rocks induced by sewage leakages were the major driving forces for the occurrence of Fe-rich groundwater in alluvial-proluvial and fissured aquifers at 2006. All these driving forces were weaker or even not work at 2018 because of the large decrease of untreated wastewater discharge in the PRD during 2006-2018. Therefore, limiting untreated wastewater discharge is the first choice to improve the groundwater quality in urbanized areas.

Distinct roles of pH and organic ligands in the dissolution of goethite by cysteine

Electron shuttles such cysteine play an important role in Fe cycle and its availability in soils, while the roles of pH and organic ligands in this process are poorly understood. Herein, the reductive dissolution process of goethite by cysteine were explored in the presence of organic ligands. Our results showed that cysteine exhibited a strong reactivity towards goethite - a typical iron minerals in paddy soils with a rate constant ranging from 0.01 to 0.1 hr^-1. However, a large portion of Fe(II) appeared to be "structural species" retained on the surface. The decline of pH was favorable to generate more Fe(II) ions and enhancing tendency of Fe(II) release to solution. The decline of generation of Fe(II) by increasing pH was likely to be caused by a lower redox potential and the nature of cysteine pH-dependent adsorption towards goethite. Interestingly, the co-existence of oxalate and citrate ligands also enhanced the rate constant of Fe(II) release from 0.09 to 0.15 hr^-1; nevertheless, they negligibly affected the overall generation of Fe(II) in opposition to the pH effect. Further spectroscopic evidence demonstrated that two molecules of cysteine could form disulfide bonds (S-S) to generate cystine through oxidative dehydration, and subsequently, inducing electron transfer from cysteine to the structural Fe(III) on goethite; meanwhile, those organic ligands act as Fe(II) "strippers". The findings of this work provide new insights into the understanding of the different roles of pH and organic ligands on the generation and release of Fe induced by electron shuttles in soils.

Reductive release of Fe mineral-associated organic matter accelerated by oxalic acid

The properties and composition of soil dissolved organic matter (DOM) are highly affected by the adsorption and desorption of organic matter (OM) on soil minerals and heterotrophic microbial respiration. Organic acids (e.g., oxalic acid), components of root exudates, have been revealed to liberate organic matter (OM) by the dissolution of protective mineral phases and stimulate microbial degradation of OM. However, the effects of organic acids on the properties and composition of soil DOM molecules and the related mechanisms are still poorly understood. In this study, we conducted microcosm incubation experiments with and without oxalic acid addition, and aimed to elucidate the variations of DOM properties and composition, employing a combination of Fourier transform ion cyclotron resonance mass spectrometry, optical spectroscopy, and bacterial community composition analysis. Our results indicated that the released OM from the direct dissolution of protective mineral phases by oxalic acid further stimulated the microbial reductive release of Fe mineral-associated OM under anoxic conditions. Furthermore, the addition of oxalic acid enhanced the degradation of aliphatic compounds and lignins with low O/C ratios, and increased the accumulation of lignins with high O/C ratios, tannins, and condensed aromatics. Linking the bacterial community composition to DOM molecular properties and composition further suggested that the enhanced reductive release of Fe mineral-associated OM was highly related to the increased abundances of Proteobacteria and Actinobacteria. Overall, oxalic acid induced long-lasting impacts on soil DOM properties and composition under anoxic soil conditions in our study. We expect that our results will contribute to understanding the dynamics of soil DOM molecules in the environment.

Removal of heavy metals from contaminated paddy soils using chemical reductants coupled with dissolved organic carbon solutions

General acid washing is commonly used to treat heavy metal-contaminated soils, but it is sometimes difficult to achieve remediation aims in severely polluted soils. If we expose the surfaces of Fe oxide minerals to reductive dissolution during washing treatment, more of the metals initially adsorbed to these surfaces will be liberated, which may encourage the removal of heavy metals. Initially, the metal extraction capabilities of nine chemical reductants were compared in ten soil samples polluted by Cr, Cu, Zn, and Ni. Sodium dithionite (Na₂S₂O₄) and ferrous sulfate (FeSO₄) were screened for subsequent intensive research. In summary, the Na₂S₂O₄ solutions had higher Cr, Cu, and Zn removal rates than either the FeSO₄ or acid solution. Application of dissolved organic carbon (DOC) further increased the removal of heavy metals by complexation. About 15%, 86%, 32%, and 52% of the Cr, Cu, Zn, and Ni, respectively, were removed from the representative soil (M-2) by two-stage washing using 0.2 M Na₂S₂O₄ coupled with 1,500 mg L^-1 DOC solution at pH 2.0. Meanwhile, most soil fertility was preserved: ammonium nitrogen was increased 3.9 times; the increase in exchangeable potassium was 33%; and the reduction in available P was only 10%.

Arsenite oxidation and arsenic adsorption on birnessite in the absence and the presence of citrate or EDTA

Birnessite not only oxidizes arsenite into arsenate but also interacts with organic matter in various ways. However, effects of organic matter on interaction between As and birnessite remain unclear. This study investigated effects of citrate and EDTA (3.12 and 2.05 mM, respectively) on oxidation of As(III) (1.07 mM) and adsorption of As(V) (0.67 mM) on birnessite (5.19 mM as Mn) at near-neutral pH. We found that As(V) adsorption on birnessite was enhanced by citrate and EDTA, which resulted from the increase in active adsorption sites via dissolution of birnessite. In comparison with citrate batches, more As was adsorbed on birnessite in EDTA batches, where dissolved Mn was mainly presented as Mn(III)-EDTA complex. Citrate or EDTA-induced dissolution of birnessite did not decrease the As(III) oxidation rate in the initial stage where As(III) oxidation rate was rapid. Afterwards, As(III) oxidation was conspicuously suppressed in citrate-amended batches, which was mainly attributed to the decrease in adsorption sites by adsorption of citrate/Mn(II)-citrate complex. This suppression was enhanced by the increase in concentrations of dissolved Mn(II). Citrate inhibited As adsorption after As(III) oxidation due to the strong competitive adsorption of citrate/Mn(II)-citrate complex. However, the As(III) oxidation rate was increased in EDTA-amended batches in the late stage, which mainly derived from the increase in the active sites via birnessite dissolution. The strong complexation ability of EDTA led to formation of Mn(III)-EDTA complex. Arsenic adsorption was not affected due to the limited competitive adsorption of the complex on the solid. This work reveals the critical role of low molecular weight organic acids in geochemical behaviors of As and Mn in aqueous environment.

Indices of the dual roles of OM as electron donor and complexing compound involved in As and Fe mobilization in aquifer systems of the Datong Basin

Organic matter (OM) acts as a source of carbon and is strongly implicated in biogeochemical processes, such as metal complexation and redox reactions. To illustrate the effects of OM on As mobilization in aquifers, this study characterized fluorescence features and hydrochemical properties of OM in sediments and groundwater from an As-affected field site located in the Datong Basin. Fluorescence analysis showed sediment and groundwater OM are dominated by oxidized and reduced quinone-like compounds; shorter emission wavelengths observed in groundwater indicated more labile and protein-like organic substances than in sediments. Dissolved As concentrations were positively correlated with dissolved Fe and HCO₃^- concentrations in middle and deep groundwater, suggesting labile OM degradation promotes the release of As and Fe from sediments into the groundwater. This result also demonstrated more bioavailable OM occurs in groundwater and that labile OM degradation promotes As release. Grain size distribution results indicated sedimentary As, Fe and OM are associated with fine-grained fractions. Sedimentary As content was significantly and positively correlated with Fe₂O₃ and OM content, suggesting the potential existence of As-Fe-OM ternary complexation; this was further supported by the results of Fourier-transform infrared (FTIR) spectra and extraction experiments. In addition, the ratio of dissolved reduced quinone-like compounds to oxidized quinone-like compounds was positively correlated with both dissolved As and HCO₃^- concentrations, implying quinone-like compounds participate in the complexation and influence As mobilization. In the reducing environment, labile OM served as the electron donor to maintain microbial respiration and mediated reductive dissolution of Fe minerals. As-Fe-OM ternary complexation in sediments and microbial reduction have a potentially strong impact on As enrichment in groundwater, and therefore are important considerations for regulating As contamination.

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010-2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5-510 μg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 μg/L for the studied period of 2010-2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations.

In situ reductive dissolution to remove Iodine-129 from aquifer sediments

The use of an aqueous reductant (Na-dithionite) with pH buffer (K-carbonate, pH 12) was evaluated in this laboratory study as a potential remedial approach for removing Fe oxide associated iodine and enhancing pump-and-treat extraction from iodine-contaminated sediments in the unconfined aquifer in the 200 West Area of the Hanford Site. X-ray fluorescence data of untreated sediment indicated that iodine was largely associated with Fe (i.e., potentially incorporated into Fe oxides), but XANES data was inconclusive as to valence state. During groundwater leaching, aqueous and adsorbed iodine was quickly released, then additional iodine was slowly released potentially from slow dissolution of one or more surface phases. The Na-dithionite treatment removed greater iodine mass (2.9x) at a faster rate (1-4 orders of magnitude) compared to leaching with groundwater alone. Iron extractions for untreated and treated sediments showed a decrease in Fe(III)-oxides, which likely released iodine to aqueous solution. Solid phase inorganic carbon and aqueous Ca and Mg analysis further confirmed that significant calcite dissolution did not occur in these experiments meaning these phases did not release significant iodine. Although it was expected that, after treatment, ¹²⁷I concentrations would eventually be lower than untreated sediments, continued, elevated iodine concentrations for treated samples over 750 h were observed for leaching experiments. Stop flow events during 1-D column leaching suggested that some iodide precipitated within the first few pore volumes. Further, batch extraction experiments compared iodine-129/127 removal and showed that iodine-129 was more readily removed than iodine-127 suggesting that the two are present in different phases due to their different origins. Although significantly greater iodine is removed with treatment, the long-term leaching needs to be investigated further as it may limit dithionite treatment at the field scale.

Coupled dynamics of As-containing ferrihydrite transformation and As desorption/re-adsorption in presence of sulfide

This study investigated the coupled dynamics of the redox transformation of arsenic-containing ferrihydrite, and arsenate desorption and re-adsorption in presence of sulfide. Batch experiments, various microscopic and spectroscopic analyses collectively revealed that electrons from sulfide competitively transferred to ferrihydrite and no arsenate was reduced. The reductive dissolution of ferrihydrite by sulfide led to the quick formation of FeS that competitively decreased the availability of sulfide for its subsequent reduction of ferrihydrite. The quick formation of FeS was followed by a relatively slow transformation of ferrihydrite to magnetite and other Fe(II)-Fe(III) minerals that were primarily bound to the residual ferrihydrite surfaces. As a result of the preservation of As-containing ferrihydrite and surface covering by the secondary minerals, the majority (> 90%)of sorbed arsenate resided in the solid phase, and <10% of arsenate participated in the desorption process during the ferrihydrite dissolution and transformation. The desorption of arsenate was fast, and followed by the kinetic re-adsorption. The rate and extent of the re-adsorption was consistent with the dynamic transformation of the secondary minerals and their sorption affinity toward As. The results have a strong implication to understanding of As concentration changes during the redox transformation of As-containing minerals in groundwater systems.

Tartaric acid-induced photoreductive dissolution of schwertmannite loaded with As(III) and the release of adsorbed As(III)

Schwertmannite (SCH) has strong adsorption ability to As(III). However, there are few reports on the stability of SCH load with As(III) (SCH-As(III)). In this study, the effects of tartaric acid (TA), pH and coexisting ions including K⁺, Ca²⁺, Al³⁺ and CO₃^2- on the photoreductive dissolution of SCH- As(III) and the release of the adsorbed As (III) were investigated. The results showed that under UV irradiation TA could greatly enhance the release of total Fe and total As from SCH-As(III). Nevertheless, the total Fe and total As in the solution decreased when TA was consumed up. Compared to SCH, the reductive dissolution of SCH-As(III) was obviously suppressed. In the dark, TA could slowly enhance the dissolution of SCH-As(III), but its effect on the release of adsorbed As(III) was weak. Low pH was conducive to the release of iron and arsenic. Ca²⁺, K⁺, and CO₃^2- promoted the decrease of the dissolved total Fe in the later reaction. However, Al³⁺ inhibited the decrease of the dissolved total Fe and total As. The analyses of FTIR and XRD demonstrated that the mineralogical phase of SCH-As(III) after reaction changed. With light, the dissolved total Fe and total As existed mainly as Fe(II) and As(V), respectively. This is because Fe(II) was generated via ligand to metal charge transfer and As(III) was oxidized to As(V) by ·OH produced during the reaction. Thus, this study provides us with a comprehensive understanding of the stability of SCH-As(III) and the release of adsorbed As(III) in natural environments.

Fate and dynamics of metal precipitates arising from acid drainage discharges to a river system

Neutralisation of acid drainage creates metal-rich precipitates that may impact receiving water bodies. This study assessed the fate of over seven years of acid drainage discharges on the sediments of the lower River Murray (Australia), including the potential for periodic water anoxia to enhance risk via reductive dissolution of amorphous (Fe, Mn and co-precipitated and bound metal) oxide phases. With the exception of one site with restricted water exchange, elevated reducible/reactive metal(oid) (Fe, Ni, As, Co, Zn) concentrations were only observed in the localised wetland-riparian area within approximately 100 m of the discharges. Only a minor exceedance of national sediment quality guideline values occurred for Ni. In the main river channel, elevated reactive metal (Fe, Mn, Ni, Zn) concentrations were also only observed less than approximately 100 m from the drainage discharge point. This appears due to (a) rapid neutralisation of pH leading to metal precipitation and deposition in the localised discharge area, and/or (b) dilution of any metal precipitates entering the main channel with natural river sediments, and/or (c) flushing of precipitates downstream during higher flow conditions. The influence of deoxygenation on metal release was profound with large increases in the concentration of dissolved Fe, Mn, Zn, Ni, and As in the overlying water during laboratory experimental simulations. However, given in situ sediment metal contamination is very localised, it appears on a river reach scale that the acid drainage precipitates will not significantly contribute, over and above, the background release of these metals during these conditions.

Chemometric tool to study the mechanism of arsenic contamination in groundwater of Puducherry region, South East coast of India

To understand occurrence, distribution and source of arsenic, 175 groundwater samples from coastal aquifers of the Puducherry region were collected and analyzed for major ions and trace metals. The concentration of As in groundwater of study area ranges from not detectable - 28.88 μg/L during the post-monsoon and not detectable - 36.88 μg/L in the pre-monsoon. The desirable limit for As in groundwater is 10 μg/L as per World Health Organization and Bureau of Indian standard. About 13.64 and 11.50% of groundwater samples shows arsenic concentration higher than recommended limit. Hydrochemical facies which dominate during pre and post monsoon are Na-K-Cl-SO₄, Ca-Cl and Ca-Mg-Cl-SO₄type and Na-K-Cl-SO₄, mixedCa-Na-HCO₃, Ca-HCO₃ and mixed Ca-Mg-Cl type respectively. The Gibbs diagram suggested that rock-water interaction is major process controlling hydrochemistry of groundwater. From the Pourbaix diagram, it is inferred that H3AsO3 is the principal As species in groundwater. The PHREEQC modelling indicates supersaturation of ferric oxides and hydroxide mineral phases in aquifer system which on reductive dissolution releases arsenic into groundwater. Statistical analysis (Spearman Correlation and Principal Component Analysis) showed that reductive dissolution of As-bearing minerals and Fe-oxyhydroxides in the presence of organic matter is the major process contributing arsenic into groundwater. The relationship between As, K⁺ and HCO^-₃ indicates agricultural and competitive exchange process which is an additional contributor of arsenic in groundwater. The sources which act as a sink and responsible for the release of As into the groundwater are marine sediments enriched in As and Fe-bearing minerals and organic matter.

Effect of dissolved organic carbon from sludge, Rice straw and spent coffee ground biochar on the mobility of arsenic in soil

To date, studies on the mobility of arsenic (As) in soil amended with biochar have primarily relied on broad empirical observations, resulting in a gap between the behavior of As in amended soil and the chemical mechanisms controlling that behavior. This study focuses on the influence of abiotic factors in As mobility in As-contaminated soils amended with biochar. In order to understand the leaching of DOC and phosphate across a range of biomass feedstock and pyrolysis temperature, rice straw and granular sludge from an anaerobic digester were pyrolyzed at 300, 550, and 700 °C, and subjected to leaching studies by mixing air dried soil with 10 wt% of biochar at a soil: water ratio of 1:1(w/v). The concentration of DOC in the presence of granular sludge biochar and rice straw biochar increased from 190 mg L^-1 to 2605 mg L^-1 and 1192 mg L^-1, respectively, which considerable accelerated the mobilization of Fe and As. More specifically, DOC drove the reduction of Fe(III) to Fe(II). Our results suggest enhanced release of As via the reductive dissolution of iron oxides, including by the chelating-enhanced dissolution of Fe oxides, and competitive desorption by DOC and phosphate from biochar. The influence of DOC and phosphate was further evaluated using realistic application amounts (1, 3, and 5 wt%) of biochars derived from pyrolysis of granular sludge, rice straw and spent coffee ground at 300 and 550 °C. The results from these experiments further confirm that DOC is a key factor for influencing the mobility of As in the amendment of biochar to As-contaminated soil, which indicates that biochar having low levels of leachable carbon should be amended to As-contaminated soils, and with caution.

Simultaneous application of oxalic acid and dithionite for enhanced extraction of arsenic bound to amorphous and crystalline iron oxides

To extract As bound to amorphous and crystalline iron oxides, this study proposed simultaneous application of oxalic acid and dithionite, which was observed to induce synergistic effect and accomplish effective extraction of As bound to both iron oxides. However, the formation of arsenic sulfide decreased overall removal of As because the insoluble precipitate form of As remained as a residual fraction of As in soil. Therefore, stepwise addition of dithionite in the simultaneous application was applied to minimize the formation of secondary minerals and maximize the As extraction. As a result, 74% of As bound to amorphous iron oxides and 65% of As bound to crystalline iron oxides were removed. More importantly, the stepwise application of oxalic acid and dithionite was effective to reduce the bioaccessible concentration of As in the treated soil. Therefore, the proposed application could reduce the potential risk of contaminated soil to human health by extraction-based remedial action.

Redox buffering and de-coupling of arsenic and iron in reducing aquifers across the Red River Delta, Vietnam, and conceptual model of de-coupling processes

Analysis of over 500 groundwater samples from throughout the Red River Delta indicates de-coupling of dissolved arsenic (As) and dissolved iron (Fe). Sorting of all data along the redox potentials suggests re-adsorption of As released initially from Mn(IV)-oxyhydroxides and later from Fe(III)-oxyhydroxides on remaining ferric phases at moderate redox levels. A gradually decreasing specific surface area available for re-adsorption of As probably plays a role as a consequence of limited reactivity of more crystalline phases such as goethite and hematite. At low redox levels, concentrations of Fe and phosphate decrease, but As concentrations keep increasing and most As is present as As(III) with limited adsorption affinity. Based on the results of speciation modeling, the water is supersaturated with respect to siderite and vivianite. A general conceptual model of As and Fe behavior is presented, suggesting that coupled behavior is possible in two geochemical "windows", i.e., 1: between saturation of remaining adsorption sites and the onset of siderite and vivianite precipitation, and 2: after the beginning of secondary sulfide phases precipitation and during methanogenesis. The de-coupling of As from Fe is common and has been observed at many sites around the world where As is released as a consequence of redox processes, e.g., in Bangladesh, West Bengal and Assam in India, the Mekong Delta in Cambodia and Vietnam, and Taiwan. The presented general conceptual model of de-coupling processes can be applied to the interpretation of As and Fe data, and, thus, it can help in the preparation of a site conceptual model which is a necessary prerequisite for reactive transport modeling.

Reductive solubilization of arsenic in a mining-impacted river floodplain: Influence of soil properties and temperature

Mining activities have contaminated many riverine floodplains with arsenic (As). When floodplain soils become anoxic under water-saturated conditions, As can be released from the solid phase. Several microbially-driven As solubilization processes and numerous influential factors were recognized in the past. However, the interplay and relative importance of soil properties and the influence of environmental factors such as temperature remain poorly understood, especially considering the (co)variation of soil properties in a floodplain. We conducted anoxic microcosm experiments at 10, 17.5, and 25 °C using 65 representative soils from the mining-impacted Ogosta River floodplain in Bulgaria. To investigate the processes of As solubilization and its quantitative variation we followed the As and Fe redox dynamics in the solid and the dissolved phase and monitored a range of other solution parameters including pH, Eh, dissolved organic C, and dissolved Mn. We related soil properties to dissolved As observed after 20 days of microcosm incubation to identify key soil properties for As solubilization. Our results evidenced reductive dissolution of As-bearing Fe(III)-oxyhydroxides as the main cause for high solubilization. The availability of nutrients, most likely organic C as the source of energy for microorganisms, was found to limit this process. Following the vertical nutrient gradient common in vegetated soil, we observed several hundred μM dissolved As after 1-2 weeks for some topsoils (0-20 cm), while for subsoils (20-40 cm) with comparable total As levels only minor solubilization was observed. While high Mn contents were found to inhibit As solubilization, the opposite applied for higher temperature (Q₁₀ 2.3-6.1 for range 10-25 °C). Our results suggest that flooding of nutrient-rich surface layers might be more problematic than water-saturation of nutrient-poor subsoil layers, especially in summer floodings when soil temperature is higher than in winter or spring.

Formation, reactivity and aging of amorphous ferric oxides in the presence of model and membrane bioreactor derived organics

Iron salts are routinely dosed in wastewater treatment as a means of achieving effluent phosphorous concentration goals. The iron oxides that result from addition of iron salts partake in various reactions, including reductive dissolution and phosphate adsorption. The reactivity of these oxides is controlled by the conditions of formation and the processes, such as aggregation, that lead to a reduction in accessible surface sites following formation. The presence of organic compounds is expected to significantly impact these processes in a number of ways. In this study, amorphous ferric oxide (AFO) reactivity and aging was investigated following the addition of ferric iron (Fe(III)) to three solution systems: two synthetic buffered systems, either containing no organic or containing alginate, and a supernatant system containing soluble microbial products (SMPs) sourced from a membrane bioreactor (MBR). Reactivity of the Fe(III) phases in these systems at various times (1-60 min) following Fe(III) addition was quantified by determining the rate constants for ascorbate-mediated reductive dissolution over short (5 min) and long (60 min) dissolution periods and for a range (0.5-10 mM) of ascorbate concentrations. AFO particle size was monitored using dynamic light scattering during the aging and dissolution periods. In the presence of alginate, AFO particles appeared to be stabilized against aggregation. However, aging in the alginate system was remarkably similar to the inorganic system where aging is associated with aggregation. An aging mechanism involving restructuring within the alginate-AFO assemblage was proposed. In the presence of SMPs, a greater diversity of Fe(III) phases was evident with both a small labile pool of organically complexed Fe(III) and a polydisperse population of stabilized AFO particles present. The prevalence of low molecular weight organic molecules facilitated stabilization of the Fe(III) oxyhydroxides formed but subsequent aging observed in the alginate system did not occur. The reactivity of the Fe(III) in the supernatant system was maintained with little loss in reactivity over at least 24 h. The capacity of SMPs to maintain high reactivity of AFO has important implications in a reactor where Fe(III) phases encounter alternating redox conditions due to sludge recirculation, creating a cycle of reductive dissolution, oxidation and precipitation.

In-situ mobilization and transformation of iron oxides-adsorbed arsenate in natural groundwater

Although reductive dissolution of Fe(III) oxides has been well accepted for As mobilization in alluvial aquifers, the key factors controlling this process are poorly understood. Arsenic(V)-adsorbing ferrihydrite, goethite and hematite were used to examine in-situ mobilization and transformation of adsorbed As(V) and Fe(III) oxides. In the Hetao basin, seven wells with wide ranges of groundwater As were selected to host As(V)-Fe(III) oxides sand. During 80 d experiments, As was firstly desorbed and then released via reductive dissolution of iron oxide from ferrihydrite, while only desorption was observed from goethite/hematite sand. Desorbed As was predominantly controlled by groundwater HCO₃^- and DOC, while reductive dissolution-related As release was mainly regulated by ORP values, DOC and Fe(II) concentrations. Mineral transformation from ferrihydrite to lepidocrocite and goethite/or mackinawite would also contribute to As release. Arsenic species was transformed from As(V) to As(III) on ferrihydrite, but remained unchanged on goethite and hematite. Arsenic partition between As-Fe(III) oxide sand and real groundwater ranged between 0.012 and 0.102L/g. K_d-sand between As-goethite sand/As-hematite sand and groundwater fell within the ranges observed between sediments and groundwater. This study suggests that As desorption, reductive dissolution and mineral transformation of ferrihydrite would be the major processes controlling As mobility.

ARSENIC: A Review on Exposure Pathways, Accumulation, Mobility and Transmission into the Human Food Chain

This review deals with exposure pathways of arsenic (As), as well as its transfer and uptake processes from its source to the human body. It is proven fact that uptake of inorganic As for a long period can lead to chronic As poisoning and a variety of adverse health effects such as skin, lung and bladder cancer, in addition to cardiovascular diseases, diabetes and gastrointestinal symptoms. As exposure occurs primarily from consumption of potable water containing high amounts of inorganic As and also from consumption of crops cultivated in As contaminated agricultural fields-either naturally or anthropogenically through contaminated air or pesticides-or irrigated with As containing water. In this review, light is shed on the transfer mechanism of As through the food chain and the parameters that enhance mobility of As in the environment. Amounts of As accumulation in plants and the transfer mechanisms are also quite different. These differences in As accumulation, such as in leaves, stems, fruits and roots, are discussed in detail. Moreover, presence of As in some vegetables consumed is given by investigating recent research articles that deal with As concentrations, especially in edible parts. Some comparative data are also presented, concerning the level of concentration of As in rice during washing, cooking and processing stages.

Effect of biogeochemical interactions on bioaccessibility of arsenic in soils of a former smelter site in Republic of Korea

The total concentration-based regulations for soil remediation do not consider the possible changes in bioaccessibility of remaining arsenic (As) in soils due to biogeochemical interactions after remediation. This study used As-contaminated soil and pore water samples that were collected from the rice paddy and forest/farmland located in the vicinity of a former smelter site in Republic of Korea to elucidate the changes in As bioaccessibility due to biogeochemical interactions. Bioaccessibility and chemical forms of As in soils were determined by using an in vitro method and sequential extraction, respectively, and soil microbial community was evaluated. Bioaccessibility of As in the rice paddy soil samples was higher than that in the forest/farmland soil samples. This could be attributed to relatively higher dependence of bioaccessible As in the rice paddy soils on the soil concentration of iron (Fe), aluminum, or manganese, which could lead to greater changes in bioaccessible As via reductive dissolution. The strong linear relationship (R ² = 0.90, p value ≤0.001) between the pore water As and Fe concentrations, and the greater portion of bacterial species related to reductive dissolution of Fe oxides in the rice paddies can support the higher As bioaccessibility promoted by reductive dissolution. Therefore, it is necessary to consider the potential changes in the bioaccessible As due to biogeochemical interactions in remediation of As-contaminated soils, particularly when soils are likely to be reused under reductive dissolution-promoting conditions (e.g., flooded conditions).

Effects of microbially induced transformations and shift in bacterial community on arsenic mobility in arsenic-rich deep aquifer sediments

Elevated concentration of arsenic (As) prevailed in deep aquifers of Chianan Plain, Taiwan. Arsenic release in relation to microbially induced transformations and shift in bacterial communities in deep aquifer sediments of Budai, southwestern Taiwan were investigated using microcosm experiments and substrate amendments over 90 days of anaerobic incubation. The results revealed that As reduction was independent of Fe reduction and a modest rate of sedimentary As release into aqueous phase occurred at the expense of the native organic carbon. Addition of lactate resulted in a parallel increase in As(III) (3.7-fold), Fe(II) (6.2-fold) and Mn (3.5 fold) in aqueous phase compared to un-amended slurries and the enrichment of sequences related to mostly Bacillus, Flavisolibacter, and Geobacter spp, suggesting the important role of these bacteria in As enrichment through reductive dissolution of As-bearing Fe and Mn minerals. The increase in phosphate-extractable As in solid phase with concomitant rise in As in aqueous phase over the course of incubation further attested to the importance of reductive dissolution in promoting As release. Furthermore, the increase in arrA gene abundance with addition of labile carbon suggests that dissimilatory As reduction also may contribute to As enrichment in the water of the deep aquifer of Budai.

Mobilization of soil-borne arsenic by three common organic acids: Dosage and time effects

A batch experiment was conducted to investigate the mobilization of soil-borne arsenic by three common low-molecular-weight organic acids with a focus on dosage and time effects. The results show that oxalic acid behaved differently from citric acid and malic acid in terms of mobilizing As that was bound to iron compounds. At an equivalent molar concentration, reactions between oxalic acid and soil-borne Fe were kinetically more favourable, as compared to those between either citric acid or malic acid and the soil-borne Fe. It was found that reductive dissolution of soil-borne Fe played a more important role in liberating As, as compared to non-reductive reactions. Prior to the 7th day of the experiment, As mobility increased with increasing dose of oxalic acid while there was no significant difference (P > 0.05) in mobilized As among the treatments with different doses of citric acid or malic acid. The dosage effect on soil-borne As mobilization in the citric acid and malic acid treatments became clear only after the 7th day of the experiment. Soluble Ca present in the soils could cause re-immobilization of As by competing with solution-borne Fe for available organic ligands to form practically insoluble organic compounds of calcium (i.e. calcium oxalate). This resulted in transformation of highly soluble organic complexes of iron (i.e. iron oxalate complexes) into slightly soluble organic compounds of iron (i.e. iron oxalate) or free ferric ion, which then reacted with the solution-borne arsenate ions to form practically insoluble iron arsenates in the latter part of the experiment.

Contrasting distributions of groundwater arsenic and uranium in the western Hetao basin, Inner Mongolia: Implication for origins and fate controls

Although As concentrations have been investigated in shallow groundwater from the Hetao basin, China, less is known about U and As distributions in deep groundwater, which would help to better understand their origins and fate controls. Two hundred and ninety-nine groundwater samples, 122 sediment samples, and 14 rock samples were taken from the northwest portion of the Hetao basin, and analyzed for geochemical parameters. Results showed contrasting distributions of groundwater U and As, with high U and low As concentrations in the alluvial fans along the basin margins, and low U and high As concentrations downgradient in the flat plain. The probable sources of both As and U in groundwater were ultimately traced to the bedrocks in the local mountains (the Langshan Mountains). Chemical weathering of U-bearing rocks (schist, phyllite, and carbonate veins) released and mobilized U as UO2(CO3)2(2-) and UO2(CO3)3(4-) species in the alluvial fans under oxic conditions and suboxic conditions where reductions of Mn and NO3(-) were favorable (OSO), resulting in high groundwater U concentrations. Conversely, the recent weathering of As-bearing rocks (schist, phyllite, and sulfides) led to the formation of As-bearing Fe(III) (hydr)oxides in sediments, resulting in low groundwater As concentrations. Arsenic mobilization and U immobilization occurred in suboxic conditions where reduction of Fe(III) oxides was favorable and reducing conditions (SOR). Reduction of As-bearing Fe(III) (hydr)oxides, which were formed during palaeo-weathering and transported and deposited as Quaternary aquifer sediments, was believed to release As into groundwater. Reduction of U(VI) to U(IV) would lead to the formation of uraninite, and therefore remove U from groundwater. We conclude that the contrasting distributions of groundwater As and U present a challenge to ensuring safe drinking water in analogous areas, especially with high background values of U and As.

Inhibition of iron (III) minerals and acidification on the reductive dechlorination of trichloroethylene

Reductive dechlorination of chlorinated ethenes is inhibited by acidification and by the presence of Fe (III) as a competitive electron acceptor. Synergism between both factors on dechlorination is predicted as reductive dissolution of Fe (III) minerals is facilitated by acidification. This study was set-up to assess this synergism for two common aquifer Fe (III) minerals, goethite and ferrihydrite. Anaerobic microbial dechlorination of trichloroethylene (TCE) by KB-1 culture and formate as electron donor was investigated in anaerobic batch containers at different solution pH values (6.2-7.2) in sand coated with these Fe minerals and a sand only as control. In the absence of Fe, lowering substrate pH from 7.2 to 6.2 increased the time for 90% TCE degradation from 14±1d to 42±4d. At pH 7.2, goethite did not affect TCE degradation time while ferrihydrite increased the degradation time to 19±1d compared to the no Fe control. At pH 6.2, 90% degradation was at 78±1 (ferrihydrite) or 131±1d (goethite). Ferrous iron production in ferrihydrite treatment increased between pH 7.2 and 6.5 but decreased by further lowering pH to 6.2, likely due to reduced microbial activity. This study confirms that TCE is increasingly inhibited by the combined effect of acidification and bioavailable Fe (III), however no evidence was found for synergistic inhibition since Fe reduction did not increase as pH decreases. To the best of our knowledge, this is the first study where effect of pH and Fe (III) reduction on TCE was simultaneously tested. Acid Fe-rich aquifers need sufficient buffering and alkalinity to ensure swift degradation of chlorinated ethenes.

Effect of iron oxide reductive dissolution on the transformation and immobilization of arsenic in soils: New insights from X-ray photoelectron and X-ray absorption spectroscopy

The geochemical behavior and speciation of arsenic (As) in paddy soils is strongly controlled by soil redox conditions and the sequestration by soil iron oxyhydroxides. Hence, the effects of iron oxide reductive dissolution on the adsorption, transformation and precipitation of As(III) and As(V) in soils were investigated using batch experiments and synchrotron based techniques to gain a deeper understanding at both macroscopic and microscopic scales. The results of batch sorption experiments revealed that the sorption capacity of As(V) on anoxic soil was much higher than that on control soil. Synchrotron based X-ray fluorescence (μ-XRF) mapping studies indicated that As was heterogeneously distributed and was mainly associated with iron in the soil. X-ray absorption near edge structure (XANES), micro-X-ray absorption near edge structure (μ-XANES) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the primary speciation of As in the soil is As(V). These results further suggested that, when As(V) was introduced into the anoxic soil, the rapid coprecipitation of As(V) with ferric/ferrous ion prevented its reduction to As(III), and was the main mechanism controlling the immobilization of As. This research could improve the current understanding of soil As chemistry in paddy and wetland soils.

Evaluation of air sparging and vadose zone aeration for remediation of iron and manganese-impacted groundwater at a closed municipal landfill

High concentrations of iron (Fe(II)) and manganese (Mn(II)) reductively dissolved from soil minerals have been detected in groundwater monitoring wells near many municipal solid waste landfills. Air sparging and vadose zone aeration (VZA) were evaluated as remedial approaches at a closed, unlined municipal solid waste landfill in Florida, USA. The goal of aeration was to oxidize Fe and Mn to their respective immobile forms. VZA and shallow air sparging using a partially submerged well screen were employed with limited success (Phase 1); decreases in dissolved iron were observed in three of nine monitoring wells during shallow air sparging and in two of 17 wells at VZA locations. During Phase 2, where deeper air sparging was employed, dissolved iron levels decreased in a significantly greater number of monitoring wells surrounding injection points, however no radial pattern was observed. Additionally, in wells affected positively by air sparging (mean total iron (FeTOT) <4.2mg/L, after commencement of air sparging), rising manganese concentrations were observed, indicating that the redox potential of the groundwater moved from an iron-reducing to a manganese-reducing environment. The mean FeTOT concentration observed in affected monitoring wells throughout the study was 1.40 mg/L compared to a background of 15.38 mg/L, while the mean Mn concentration was 0.60 mg/L compared to a background level of 0.27 mg/L. Reference wells located beyond the influence of air sparging areas showed little variation in FeTOT and Mn, indicating the observed effects were the result of air injection activities at study locations and not a natural phenomenon. Air sparging was found effective in intercepting plumes of dissolved Fe surrounding municipal landfills, but the effect on dissolved Mn was contrary to the desired outcome of decreased Mn groundwater concentrations.