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

Co-occurrence of arsenic and fluoride in groundwater of Guide basin in China: Genesis, mobility and enrichment mechanism

Endemic arsenic poisoning and fluorosis caused by primary high arsenic (As) and high fluoride (F-) groundwater have become one of the most serious environmental geological problems faced by the international society. High As and high F- groundwater exists in Neogene confined aquifers in Guide basin, with concentrations of 355 μg/L and 5.67 mg/L, respectively, and showing a co-occurrence phenomenon of As and F- in the groundwater. This poses a double threat to the health of tens of thousands of local residents. In this study, based on the systematic collection of groundwater and borehole sediment samples, analysis of hydrochemistry and isotope indexes, combined with laboratory tests, purpose of this study is to reveal the migration rule and co-enrichment mechanism of As and F- in aquifers, and finally establish a hydrogeochemical conceptual model of the enrichment process of As and F-. The main conclusions are as follows: hydrochemical type of unconfined and confined groundwater in Guide basin is Ca-Na-HCO3 and Na-Cl-HCO3 type, respectively. Main minerals in sediments are quartz and plagioclase. Concentrations of As and F- are lower in unconfined groundwater, but higher in confined groundwater, and which show a gradual increasing trend along the groundwater flow path. The mineralization of natural organic matter in confined aquifer causes iron and manganese oxide minerals containing As to dissolve gradually, which leads to the gradual release of As into groundwater. Large amount of HCO3- produced by mineralization of organic matter precipitate with Ca2+ in groundwater, resulting in reduction of Ca2+ content, promoting the dissolution of fluoride-containing minerals such as fluorite (CaF2), and continuously releasing F- into groundwater. Meanwhile, competitive adsorption reactions in confined aquifers causes more As and F- to be released from mineral surface into groundwater, which gradually migrate and accumulate along groundwater flow. Finally, it is established that a conceptual model for the formation of high As and F- groundwater in the confined aquifer of Guide basin. The research results not only help to improve our understanding of the formation and evolution of groundwater with high As and F- with similar geological background, but also provide scientific basis for rational development and utilization of groundwater, and prevention and control of chronic As and F- poisoning in local and similar areas.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

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.

New insights into arsenate removal during siderite oxidation by dissolved oxygen

Nowadays, arsenic (As) pollution in aquatic environments severely threatens the health of human beings. Although it has been known that siderite is capable of As adsorption and dissolved oxygen (DO) enhances the adsorption, effects of DO concentrations on As(V) adsorption onto siderite remain elusive. In this study, As(V) removal was investigated by synthesized siderite from aqueous solutions with different DO concentrations. Arsenic(V) adsorption kinetics were conformed to the pseudo-second-order model. As(V) adsorption onto siderite was enhanced in the presence of dissolved oxygen, but the excess DO concentration did not increase As(V) adsorption since Fe(III) oxides were coated onto the pristine siderite surface, preventing the mineral from further oxidation. With the increase in DO concentration, the rate of Fe(II) oxidation decreased, which was the kinetic-limited step during As(V) removal by siderite with the presence of DO. The theoretically generated Fe(III) was stoichiometrically proportional to the consumed oxygen. Microscopic characteristics by means of XRD, SEM, TEM, FTIR and XPS indicated that the adsorption was dominated by the chemical process via the As(V) complexation with siderite and co-precipitation with produced Fe(III) oxides. This study reveals the mechanisms of As(V) adsorption during siderite oxidation under different DO concentrations and emphasizes the importance of siderite oxidation in As(V) fate in aqueous systems.

Quantifying sulfidization and non-sulfidization in long-term in-situ microbial colonized As(V)-ferrihydrite coated sand columns: Insights into As mobility

Sulfide-induced reduction (sulfidization) of arsenic (As)-bearing Fe(III) (oxyhydro)oxides may lead to As mobilization in aquifer systems. However, little is known about the relative contributions of sulfidization and non-sulfidization of Fe(III) (oxyhydro)oxides reduction to As mobilization. To address this issue, high As groundwater with low sulfide (LS) and high sulfide (HS) concentrations were pumped through As(V)-bearing ferrihydrite-coated sand columns (LS-column and HS-column, respectively) being settled within wells in the western Hetao Basin, China. Sulfidization of As(V)-bearing ferrihydrite was evidenced by the increase in dissolved Fe(II) and the presence of solid Fe(II) and elemental sulfur (S⁰) in both the columns. A conceptual model was built using accumulated S⁰ and Fe(II) produced in the columns to calculate the proportions of sulfidization-induced Fe(III) (oxyhydro)oxide reduction and non-sulfidization-induced Fe(III) (oxyhydro)oxide reduction. Fe(III) reduction via sulfidization occurred preferentially in the inlet ends (LS-column, 31 %; HS-column, 86 %), while Fe(III) reduction via non-sulfidization processes predominated in the outlet ends (LS-column, 96 %; HS-column, 86 %), and was attributed to the metabolism of genera associated with Fe(III) reduction (including Shewanella, Ferribacterium, and Desulfuromonas). Arsenic was mobilized in the columns via sulfidization and non-sulfidization processes. More As was released from the solid of the HS-column than that of the LS-column due to the higher intensity of sulfidization in the presence of higher concentrations of dissolved S(-II). Overall, this study highlights the sulfidization of As-bearing Fe(III) (oxyhydro)oxides as an important As-mobilizing pathway in complex As-Fe-S bio-hydrogeochemical networks.

Mobilization of arsenic from As-containing iron minerals under irrigation: Effects of exogenous substances, redox condition, and intermittent flow

Irrigation activities can cause strong geochemical and hydrological fluctuations in the unsaturated zone, and affect arsenic (As) migration and transformation. The As geochemical cycle in the unsaturated zone is coupled with that of iron minerals through sorption-desorption, coprecipitation and redox processes. Dynamic batch experiments and wetting-drying cycling column experiments were conducted to evaluate As mobilization behaviors under the effects of exogenous substances, redox condition and intermittent flow. Our results show that As release under exogenous substances carried by irrigation (e.g., phosphate, carbonate, fulvic acid, humic acid, etc.) followed three trends with the types of exogenous inputs. Inorganic anions and organic matter resulted in opposite trends of arsenate release in different redox conditions. In anoxic environments, As(V) release was favored by the addition of phosphate and carbonate, while in oxic environments, the mobilization of As(V) was promoted by the addition of fulvic acid (FA). Further, intermittent irrigation promoted the reductive dissolution of Fe oxides and the mobilization of As. The addition of humic acid (HA) resulted in the mobilization of arsenate as As-Fe-HA ternary complexes. The mechanism of arsenic mobilization under irrigation has importance for prevention of arsenic exposure through soil to food chain transfer in typical high arsenic farmland.

Mechanistic and modeling insights into the immobilization of Cd and organic carbon during abiotic transformation of ferrihydrite induced by Fe(II)

Iron (Fe) oxides and fulvic acid (FA) are the key components affecting the fate of cadmium (Cd) in soil. The presence of FA influences Fe mineral transformation, and FA may complicate phase transformation and dynamic behavior of Cd. How varying Fe minerals and FA affect Cd immobilization during the ferrihydrite transformation induced by various Fe(II) concentrations, however, is still lack of quantitative understanding. In this study, we built a model for Cd species quantification during phase transformation based on mechanistic insights obtained from batch experiments. Spectroscopic analysis showed that Fe(II) concentrations affected secondary Fe minerals formation under the condition of co-existence of Cd and FA, and ultimately changed the distribution of Cd and FA. Microscopic analysis revealed that besides surface adsorption, part of Cd was sequestrated by magnetite, whereas FA was able to diffuse into lepidocrocite defects. The model revealed that adsorbed Cd was mainly controlled by FA and ferrihydrite, and direct complexation of Cd by FA had a strong impact on the continuous change in Cd at lower Fe(II) concentration. The results contribute to an in-depth understanding of the mobility of Cd in the environment and provide a method for quantifying the dynamic behavior of heavy metals in multi-reactant systems.

Delineating the controlling mechanisms of arsenic release into groundwater and its associated health risks in the Southern Loess Plateau, China

The mechanisms controlling arsenic (As) enrichment and mobilization associated with human health risk assessment of groundwater in the Longdong Basin, located in the southern part of the Loess Plateau, China, have been yet unexplained. This uncertainty is partly attributed to a poor understanding of groundwater arsenic management. To address this problem, this study investigated the occurrence and spatial distribution of As in unconfined groundwater (UG) and confined groundwater (CG) in the study area, integrated Self-Organizing Maps (SOM) and geochemical modeling to elucidate the mechanisms controlling As release and mobilization in groundwater, and conducted a health risk assessment of groundwater As. The results showed that 13.6% of UG samples (n = 66) and 22.4% of CG samples (n = 98) exceeded the WHO guideline limit of As (10 μg/L). The detailed hydrogeochemical studies showed that As-enrichment groundwater is dominated by Cl-Na type, and Gaillardet diagram indicated that evaporites weathering may contribute to As mobilization in CG. The SOM analysis combined with Spearman's correlation coefficient quantified the negative correlation between As and redox potential, dissolved oxygen, SO₄^2-, NO₃^-, and the positive correlation between As and HCO₃^-, Mn in UG. In CG, As is positively correlated to pH and negatively to electrical conductivity, SO₄^2-, Fe and Mn. The saturation indices of the mineral phases indicates an insignificant relationship between As and Fe. We conclude that under oxidizing conditions, evaporative controls and the desorption of Fe-oxides under alkaline and high salinity conditions are the dominant mechanisms controlling As release and mobilization in groundwater. In addition, exposure to groundwater As through drinking water posed potential risk of carcinogenic and non-carcinogenic effects on children and adults. This study contributes to groundwater As management and sustainable safe groundwater supply.

Distributions and interactions of dissolved organic matter and heavy metals in shallow groundwater in Guanzhong basin of China

Groundwater as the primary source of fresh water particularly in semi-arid regions is heavily threatened by various pollutants such as dissolved organic matter (DOM) and heavy metals due to anthropogenic activities. In this study, 113 shallow groundwater samples were collected from Guanzhong basin of China to explore spatial distributions and interactions of DOM and heavy metals (Fe, Mn and Cu). Fluorescence excitation-emission spectrophotometry with parallel factor analysis showed that DOM in groundwater mainly contained three humic-like and two protein-like substances with an average dissolved organic carbon (DOC) concentration of 12.85 mg L^-1. Average Mn and Cu concentrations in groundwater were 19.92 μg L^-1 and 7.05 μg L^-1 with an increasing trend from west to east, whereas Fe concentration in central regions was much higher (34.23 μg L^-1). Structural equation modeling analysis indicated that DOM in groundwater could be significantly affected by surface water, and heavy metals were influenced by urbanization. Moreover, DOM could strongly influence the bioavailability, migration, and transformation of Mn in groundwater. These findings would be beneficial for the effective utilization and protection of groundwater resources.

The influence of redox conditions on aqueous-solid partitioning of arsenic and selenium in a closed coal ash impoundment

A closed coal ash impoundment case study characterized the effects of field redox conditions on arsenic and selenium partitioning through monitoring of porewater and subsurface gas in conjunction with geochemical speciation modeling. When disposed coal ash materials and porewater were recovered for testing, oxidation led to lower arsenic and higher selenium concentrations in leaching test extracts compared to porewater measurements. Multiple lines of evidence suggest multiple mechanisms of arsenic retention are plausible and the concurrent presence of several redox processes and conditions (e.g., methanogenesis, sulfate reduction, and Fe(III)-reduction) controlled by spatial gradients and dis-equilibrium. Geochemical speciation modeling indicated that, under reducing field conditions, selenium was immobilized through the formation of insoluble precipitates Se(0) or FeSe while arsenic partitioning was affected by a progression of reactions including changes in arsenic speciation, reduction in adsorption due to dissolution and recrystallization of hydrous ferric oxides, and precipitation of arsenic sulfide minerals.

Determination of As species distribution and variation with time in extracted groundwater samples by on-site species separation method

It is challenging to dependably keep the native distribution of arsenic (As) species before sample analysis in the laboratory. The on-site separation method can avoid sample contamination and species change in the process of sample collection and transportation from field to laboratory. In this study, As species distribution and variation of the extracted groundwater was first analyzed by an on-site species separation method in Jianghan Plain, China. Our study illustrated that: 1) high-As groundwater generally existed under mildly reducing conditions (Eh < 200 mV), weak alkaline conditions (pH < 7.2), elevated concentrations of dissolved Fe(II) and S(-II), and high proportions of As (III); 2) As species in the groundwater changed dramatically at room temperature in 36 hours post extraction (HPE). Fe-sulfide and Fe oxides minerals, which adsorbed As (V), were the main reasons influencing the As species concentration; 3) Acidification and strong complexing agents cannot preserve As species effectively. The average proportion of As (III) in the wells, where groundwater samples from the depth of 25 m exceed 10 μg L^-1 As, can be reduced by 61% and 63% after HCl and EDTA were added, respectively. Accurate assessment of concentrations and distribution variation of As species in groundwater can guide the removal of As and the safe use of water resources, especially in drought areas relying on drinking well water.

Unraveling biogeochemical complexity through better integration of experiments and modeling

The evolution of groundwater quality in natural and contaminated aquifers is affected by complex interactions between physical transport and biogeochemical reactions. Identifying and quantifying the processes that control the overall system behavior is the key driver for experimentation and monitoring. However, we argue that, in contrast to other disciplines in earth sciences, process-based computer models are currently vastly underutilized in the quest for understanding subsurface biogeochemistry. Such models provide an essential avenue for quantitatively testing hypothetical combinations of interacting, complex physical and chemical processes. If a particular conceptual model, and its numerical counterpart, cannot adequately reproduce observed experimental data, its underlying hypothesis must be rejected. This quantitative process of hypothesis testing and falsification is central to scientific discovery. We provide a perspective on how closer interactions between experimentalists and numerical modelers would enhance this scientific process, and discuss the potential limitations that are currently holding us back. We also propose a data-model nexus involving a greater use of numerical process-based models for a more rigorous analysis of experimental observations while also generating the basis for a systematic improvement in the design of future experiments.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Hydrogeochemical and health risk evaluation of arsenic in shallow and deep aquifers along the different floodplains of Punjab, Pakistan

The current study delineated the distribution, (hydro)geochemical behavior and health risk of arsenic (As) in shallow (depth < 35 m; handpumps and electric pumps) and deep (depth > 35 m; tube wells) aquifers in five areas along the Indus River (Bhakar, Kallur Kot), Jhelum River (Jhelum) and Chenab River (Hafizabad, Gujranwala) floodplains of Punjab, Pakistan. Relatively, greater As concentration was observed in deep wells (mean: 24.3 µg L^-1) compared to shallow wells (19.4 µg L^-1), with groundwater As spanning 0.1-121.7 µg L^-1 (n = 133) in three floodplains. Groundwater from Hafizabad (Chenab River floodplain) possessed the highest As (121.7 µg L^-1), Na⁺ (180 mg L^-1), Ca²⁺ (95 mg L^-1), Cl^- (101 mg L^-1) and SO₄^2- (1353 mg L^-1) concentrations. Arsenic health risk modeling indicated the potential carcinogenic (value > 10^-4) and non-carcinogenic (hazard quotient > 1.0) risks for groundwater of all areas, with the utmost risk estimated for Chenab floodplain and deep aquifers. Positive saturation index values for Fe oxide mineral phases may suggest their potential role in As mobilization/release in these aquifer environments. This study provides critically-important and baseline knowledge for a widespread groundwater As examination along these three floodplains, which is vital for launching suitable As mitigation and remediation programs to reduce the potential health risk.

Arsenic removal from natural groundwater using 'green rust': Solid phase stability and contaminant fate

Arsenic (As) contamination in groundwater remains a pressing global challenge. In this study, we evaluated the potential of green rust (GR), a redox-active iron phase frequently occurring in anoxic environments, to treat As contamination at a former wood preservation site. We performed long-term batch experiments by exposing synthetic GR sulfate (GR_SO4) to As-free and As-spiked (6 mg L^-1) natural groundwater at both 25 and 4 °C. At 25 °C, GR_SO4 was metastable in As-free groundwater and transformed to GR_CO3, and then fully to magnetite within 120 days; however, GR_SO4 stability increased 7-fold by lowering the temperature to 4 °C, and 8-fold by adding As to the groundwater at 25 °C. Highest GR_SO4 stability was observed when As was added to the groundwater at 4 °C. This stabilizing effect is explained by GR solubility being lowered by adsorbed As and/or lower temperatures, inhibiting partial GR dissolution required for transformation to GR_CO3, and ultimately to magnetite. Despite these mineral transformations, all added As was removed from As-spiked samples within 120 days at 25 °C, while uptake was 2 times slower at 4 °C. Overall, we have successfully documented that GR is an important mineral substrate for As immobilization in anoxic subsurface environments.

Arsenic immobilization by in-situ iron coating for managed aquifer rehabilitation

A long-lasting challenge in eliminating the worldwide impact of geogenic arsenic (As)-contaminated groundwater is the development of efficient, in-situ treatment technologies that are applicable in decentralized and rural areas. Here we present a managed aquifer rehabilitation (MAR) approach based on the in-situ creation of Fe-oxide scavengers for remediating As-contaminated groundwater. The Fe-oxide coatings on sediment surfaces were generated via periodic injection of Fe²⁺ and ClO^- solutions into an As-affected sandy aquifer at the Datong Basin, northern China for 25 days. This treatment prompted the buildup of weakly alkaline/circumneutral and oxidizing conditions to enhance As(III) oxidation in the target aquifer. Dissolved As concentrations decreased from the initial average 78.0 to 9.8 μg/L over the 25-d amendment. Sediment imaging by scanning electron microscope-X-ray energy dispersive spectroscopy confirms the deposition of Fe-rich precipitates on sediment surfaces with the simultaneous retention of As, and high density electrical tomography suggests the occurrence of such a process throughout the target zone. Further X-ray diffraction analysis and sequential chemical extraction reveal that the neo-formed Fe minerals comprised both poorly crystalline (e.g., ferrihydrite) and better crystalline (e.g., goethite) Fe oxides. The process-based reactive-transport modeling for the variations of As species in the treated groundwater supports that the new Fe-oxide minerals, most probably goethite, acted as efficient removers of aqueous As. The low As level of ∼10 μg/L was maintained during the following 215-d monitoring, demonstrating the long effectiveness of the MAR approach. This study highlights the feasibility of As immobilization by manipulating in-situ Fe-oxide coating on sandy sediments at the pilot scale. The MAR technology may be applicable for As-affected aquifers with controlled oxidizing conditions in the Datong Basin and likely other high-As regions with similar hydrogeochemical settings.

Iron-based subsurface arsenic removal (SAR): Results of a long-term pilot-scale test in Vietnam

The principle of subsurface arsenic removal (SAR) from groundwater is based on oxidation and adsorption reactions by infiltrating oxygen into the anoxic aquifer and the immobilization of arsenic (As) onto freshly formed iron (Fe)-(hydr)oxides. In this study, a pilot-scale plant for SAR has been subject to long term testing in the Mekong Delta, Vietnam. Initial concentrations of Fe (8.4 ± 1.3 mg L^-1) and As (81 ± 8 μg L^-1) in the exploited groundwater were successfully lowered to below the WHO guideline value limits for drinking water of 0.3 mg L^-1 and 10 μg L^-1, respectively. Adsorption and co-precipitation of As with Fe-(hydr)oxides could be identified as the principal mechanism responsible for the As removal from groundwater, demonstrating the feasibility of SAR as a low-cost and zero-waste solution over a period of two years. However, naturally occurring geochemical reducing conditions and high ammonium levels in the groundwater delayed the removal of manganese (Mn). An additional post-treatment filtration for Mn-removal was temporarily used to comply with the Vietnamese drinking water standard until a Mn-mitigation was achieved by the SAR process. In contrast to most As-remediation technologies, SAR appears to be a long-term, sustainable treatment option with the salient advantage of negligible production of toxic waste, which with ex-situ processes require additionally management costs.

Hausmannite as potential As(V) filter. Macroscopic and spectroscopic study of As(V) adsorption and desorption by citric acid

Arsenic (As) is a toxic element that leads the list of human health threats and is one of the priority contaminants in soil and water. In order to remove As(V) and/or reduce its mobility, filters and amendments with high affinity for As(V) adsorption are used in drinking water treatment or directly applied to the soil, thereby promoting its immobilization. Hausmannite and hematite were compared by in-situ Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopy and batch experiments for evaluating As(V) adsorption and sequential desorption by citrate. The pH and contact time were used as variables. Hausmanite adsorbed more As(V) than hematite. As(V) was adsorbed on the mineral surface of simultaneously inner- and outer-sphere species. Inner-sphere bidentate complex form preferentially at high pH, early adsorption time and low surface loading, while the monodentate species should be responsible to increase total As(V) adsorption at low pH, later adsorption kinetics and higher As(V) surface loading. Citrate was effective in causing As(V) desorption at higher citric acid concentrations and higher pH values. After a long time of incubation, the neogenesis of a manganite by hausmnannite oxidation was observed. Concomitantly, less As(V) was desorbed by citrate desorption, even in the presence of high citric acid concentrations. Hausmannite was an efficient mineral for As(V) removal and immobilization.

Arsenic release from pyrite ash waste over an active hydrogeological system and its effects on water quality

Arsenic is a potentially toxic element of concern for environmental compartments, and it is a frequent pollutant in many abandoned industrial sites. In this study, geochemical and hydrogeological tools were used to determine the long-term effects of As-rich pyrite ash disposal (83,000 m³ as estimated by geostatistical tools) in a brownfield located over a quaternary alluvial aquifer. Throughout the site, soil pollution and water table oscillation led to leachates in the form of both run-off and infiltration waters, thereby reducing (ground)water quality (e.g. pH, electrical conductivity) and, in particular, increasing the concentration of arsenic (average approx. 4000 μg/l for one hydrological year). By means of laboratory and in situ measurements, the main mechanisms through which the sulphide remaining in the pyrite ash leaches were identified. In addition, to evaluate the effects of the polluted groundwater on the nearby main river, a mathematical approach using the Domenico analytical groundwater transport model revealed potential concentrations of 49 μg/l of arsenic in the junction between the study aquifer and the river, equivalent to an annual quantity of 49 kg of this element.

Transport of arsenic loaded by ferric humate colloid in saturated porous media

The transport behavior of arsenic (As(V)) loaded by ferric humate (HA-Fe) colloid, denoted as HA-Fe/As(V), moving in a saturated quartz sand column, was tested in the laboratory under varying pH values, ionic strengths, and HA and Fe(III) content. The time-fractional advection-dispersion equation (fADE) model was then employed to analyze the observed migration of HA-Fe/As(V). Results showed that the stability of the HA-Fe colloid exhibited an upward trend with an increasing pH and HA content. An increasing HA content led to a decrease in the particle size of the HA-Fe colloid. However, the effect of Fe(III) concentration on colloidal particle size exhibited the opposite phenomenon. The ability of the HA-Fe colloid to load As(V) gradually increased with the increase of the Fe(III) concentration. During the co-transport of the HA-Fe/As(V) colloid, transport of As(V) was promoted with increasing pH, increasing HA and Fe(III) content, and decreasing ionic strength in the saturated porous medium. The transport behavior of As(V) can be well fitted by the fADE model. The model analysis revealed that sub-diffusion of As(V) was weakened in the HA-Fe/As(V) colloid with high HA content. Sub-diffusion of As(V) in the low pH colloid was stronger than that of the high-pH colloid, and the molecular diffusion and mechanical dispersion were more weakened in the high-pH colloid than that of the low-pH colloid. When observing varying ionic strengths, As(V) exhibited stronger sub-diffusion in the HA-Fe/As(V) colloid with a higher ionic strength. As for the Fe(III) content, transport of As(V) was mainly affected by sub-diffusion in the HA-Fe/As(V) colloid with a low Fe(III) content. These findings provided direct and necessary insights into the effects of the HA-Fe colloid on the migration of As(V) throughout saturated porous media under different hydrochemical conditions found in natural environments.

Colorimetric and electrochemical arsenate assays by exploiting the peroxidase-like activity of FeOOH nanorods

The authors describe an electrochemical and an optical method for the determination of As(V) by using iron oxyhydroxide (FeOOH) nanorods that display peroxidase-mimicking activity. The nanorods catalyze the oxidation of substrate ABTS by H₂O₂ to form a green product with an absorption maximum at 418 nm. If, however, As(V) is electrostatically adsorbed on the nanorods, the oxidation is gradually inhibited. A colorimetric assay was worked out based on these findings. Response is linear in the 0 to 8 ppb and 8 to 200 ppb As(V) concentration range, and the detection limit is 0.1 ppb. Even higher sensitivity is achieved in an electrochemical method which is based on the excellent electrical conductivity of FeOOH nanorods. Electrochemical analysis of As(V) was achieved by first adsorbing As(V) on the nanorods. This inhibits the ABTS reduction current signal, best measured at a potential of 150 mV (vs. Ag/AgCl). The linear range extends from 0.04 to 200 ppb, and the detection limit is as low as 12 ppt. Graphical abstract Schematic representation of FeOOH nanorod-based colorimetric and electrochemical assays for arsenate (As(V)). As(V) adsorbed on FeOOH nanorods inhibits the peroxidase-mimicking activity of nanorods, and a colorimetric and electrochemical dual-signal assay was constructed to achieve sensitive determination of As(V).

Model-Based Interpretation of Groundwater Arsenic Mobility during in Situ Reductive Transformation of Ferrihydrite

Arsenic (As) release and mobility in groundwater is coupled to the iron (Fe) cycling and the associated transformation of Fe-oxides present in sediments. Recent in situ experiments have provided observations on arsenic mobilization and co-occurring reductive mineral transformation when placing As-loaded ferrihydrite-coated sand for 80 days in wells of an As-contaminated aquifer of Northern China. However, the complex temporal change in solid-associated arsenic and the multiple geochemical processes occurring when the flowing groundwater contacts the As-loaded ferrihydrite-coated sand hamper a detailed evaluation of the experimental data set. In this study, we develop a modeling approach that allows a quantitative interpretation of arsenic release and ferrihydrite transformation observed during the in situ experiments. The model accounts for the interplay of abiotic and biotic geochemical processes (i.e., surface complexation, reductive dissolution, formation of secondary iron minerals, and arsenic sequestration into the newly formed minerals) involved in the transformation of Fe-oxides and controlling arsenic mobility. The results show the capability of the proposed approach to reproduce the temporal trends of solid arsenic and ferrihydrite concentrations, as well as the spatial variability of mineral transformation, observed in different wells using a common set of surface complexation parameters and kinetic rate constants. The simulation outcomes allowed us to disentangle the specific contribution of the different mechanisms controlling the release of arsenic. It was possible to identify an initial rapid but minor release of As (13-23% of the initial surface concentration) due to desorption from ferrihydrite, as well as the reduction of adsorbed As(V) to As(III) upon contact with the flowing anoxic groundwater. Successively, reductive dissolution of ferrihydrite caused the decrease of the amount of the Fe mineral phase and led to a major depletion of solid-associated arsenic. The produced Fe(II) catalyzed the ferrihydrite conversion into more crystalline Fe(III) oxides (i.e., lepidocrocite and goethite) through Ostwald ripening, and resulted in the formation of siderite and mackinawite upon reaction with carbonates and sulfides naturally present in the groundwater. The model results also showed that, whereas the decrease in surface sites during reductive dissolution of ferrihydrite promoted arsenic mobilization, the mineral transformation limited As release through its sequestration into the newly formed secondary mineral phases.

Impact of microbial iron oxide reduction on the transport of diffusible tracers and non-diffusible nanoparticles in soils

In subsurface bioremediation, electron donor addition promotes microbial Fe(III)-oxide mineral reduction that could change soil pore structure, release colloids, and alter soil surface properties. These processes in turn may impact bioremediation rates and the ultimate fate of contaminants. Columns packed with water-stable, Fe-oxide-rich soil aggregates were infused with acetate-containing artificial groundwater and operated for 20 d or 60 d inside an anoxic chamber. Soluble Fe(II) and soil colloids were detected in the effluent within one week after initiation of the acetate addition, demonstrating Fe(III)-bioreduction and colloid formation. Diffusible Br^-, less diffusible 2,6-difluorobenzoate (DFBA), and non-diffusible silica-shelled silver nanoparticles (SSSNP) were used as tracers in transport experiments before and after the bioreduction. The transport of Br^- was not influenced by the bioreduction. DFBA showed earlier breakthrough and less tailing after the bioreduction, suggesting alterations in flow paths and soil surface chemistry during the 20-d bioreduction treatment. Similarly, the bioreduction increased the transport of SSSNP very significantly, with mass recovery increasing from 1.7% to 25.1%. Unexpectedly, the SSSNP was completely retained in the columns when the acetate injection was extended from 20 to 60 d, while the mass recovery of DFBA decreased from 89.1% to 84.1% and Br^- showed no change. The large change in the transport of SSSNP was attributed to soil aggregate breakdown and colloid release (causing mechanical straining of SSSNP) and the exposure of iron oxide surfaces previously unavailable within aggregate interiors (facilitating attachment of SSSNP). These results suggest a time-dependent fashion of microbial effect on the transport of diffusivity-varying tracers.

Recent advances in analysis of trace elements in environmental samples by X-ray based techniques (IUPAC Technical Report)

Trace elements analysis is a fundamental challenge in environmental sciences. Scientists measure trace elements in environmental media in order to assess the quality and safety of ecosystems and to quantify the burden of anthropogenic pollution. Among the available analytical techniques, X-ray based methods are particularly powerful, as they can quantify trace elements in situ. Chemical extraction is not required, as is the case for many other analytical techniques. In the last few years, the potential for X-ray techniques to be applied in the environmental sciences has dramatically increased due to developments in laboratory instruments and synchrotron radiation facilities with improved sensitivity and spatial resolution. In this report, we summarize the principles of the X-ray based analytical techniques most frequently employed to study trace elements in environmental samples. We report on the most recent developments in laboratory and synchrotron techniques, as well as advances in instrumentation, with a special attention on X-ray sources, detectors, and optics. Lastly, we inform readers on recent applications of X-ray based analysis to different environmental matrices, such as soil, sediments, waters, wastes, living organisms, geological samples, and atmospheric particulate, and we report examples of sample preparation.

Synchronous microbial vanadium (V) reduction and denitrification in groundwater using hydrogen as the sole electron donor

Groundwater co-contaminated by vanadium (V) (V(V)) and nitrate requires efficient remediation to prevent adverse environmental impacts. However, little is known about simultaneous bio-reductions of V(V) and nitrate supported by gaseous electron donors in aquifers. This study is among the first to examine microbial V(V) reduction and denitrification with hydrogen as the sole electron donor. V(V) removal efficiency of 91.0 ± 3.2% was achieved in test bioreactors within 7 d, with synchronous, complete removal of nitrate. V(V) was reduced to V(IV), which precipitated naturally under near-neutral conditions, and nitrate tended to be converted to nitrogen, both of which processes helped to purify the groundwater. Volatile fatty acids (VFAs) were produced from hydrogen oxidation. High-throughput 16S rRNA gene sequencing and metagenomic analyses revealed the evolutionary behavior of microbial communities and functional genes. The genera Dechloromonas and Hydrogenophaga promoted bio-reductions of V(V) and nitrate directly coupled to hydrogen oxidation. Enriched Geobacter and denitrifiers also indicated synergistic mechanism, with VFAs acting as organic carbon sources for heterotrophically functional bacteria while reducing V(V) and nitrate. These findings are likely to be useful in revealing biogeochemical fates of V(V) and nitrate in aquifer and developing technology for removing them simultaneously from groundwater.

Efficient photocatalytic oxidation of arsenite from contaminated water by Fe2O3-Mn2O3 nanocomposite under UVA radiation and process optimization with experimental design

The efficiency of photocatalytic oxidation process in arsenite (As(III)) removal from contaminated water by a new Fe₂O₃-Mn₂O₃ nanocomposite under UV_A radiation was investigated. The effect of nanocomposite dosage, pH and initial As(III) concentration on the photocatalytic oxidation of As(III) were studied by experimental design. The synthesized nanocomposite had a uniform and spherical morphological structure and contained 49.83% of Fe₂O₃ and 29.36% of Mn₂O₃. Based on the experimental design model, in photocatalytic oxidation process, the effect of pH was higher than other parameters. At nanocomposite concentrations of more than 12 mg L^-1, pH 4 to 6 and oxidation time of 30 min, photocatalytic oxidation efficiency was more than 95% for initial As(III) concentration of less than 500 μg L^-1. By decreasing pH and increasing the nanocomposite concentration, the photocatalytic oxidation efficiency was increased. Furthermore, by increasing the oxidation time from 10 to 240 min, in addition to oxidation of As(III) to arsenate (As(V)), the residual As(V) was adsorbed on the Fe₂O₃-Mn₂O₃ nanocomposite and total As concentration was decreased. Therefore, Fe₂O₃-Mn₂O₃ nanocomposite as a bimetal oxide, at low doses and short time, can enhance and improve the efficiency of the photocatalytic oxidation and adsorption of As(III) from contaminated water resources. Furthermore, the energy and material costs of the UV_A/Fe₂O₃-Mn₂O₃ system for photocatalytic oxidation of 1  mg L^-1 As(III) in the 1 L laboratory scale reactor was 0.0051 €.

Effects of Fe-S-As coupled redox processes on arsenic mobilization in shallow aquifers of Datong Basin, northern China

High arsenic groundwater generally coexists with elevated Fe²⁺ concentrations (mg L^-1 levels) under reducing conditions, but an explanation for the extremely high arsenic (up to ∼2690) concentrations at very low Fe²⁺ (i.e., μg L^-1 levels) in groundwater of Datong Basin remains elusive. Field groundwater investigation and laboratory microcosm experiments were implemented in this study. The field groundwater was characterized by weakly alkaline (pH 7.69 to 8.34) and reducing conditions (Eh -221.7 to -31.9 mV) and arsenic concentration averages at 697 μg L^-1. Acinetobacter (5.9-51.3%), Desulfosporosinus (4.6-30.2%), Brevundimonas (3.9-19%) and Pseudomonas (3.2-14.6%) were identified as the dominant genera in the bacterial communities. Bacterially mediated arsenate reduction, Fe(III) reduction, and sulfate reduction are processes occurring (or having previously occurred) in the groundwater. Results from incubation experiment (27 d) revealed that nitrate, arsenate, and Fe(III)/sulfate reduced sequentially with time under anoxic conditions, while Fe(III) and sulfate reduction processes had no obvious differences, occurring almost simultaneously. Moreover, low Fe²⁺ concentrations were attributed to initially high pH conditions, which relatively retarded Fe(III) reduction. In addition, arsenic behavior in relation to groundwater redox conditions, matrices, and solution chemistry were elaborated. Bacterial arsenate reduction process proceeded before Fe(III) and sulfate reduction in the incubation experiment, and the total arsenic concentration (dominated by arsenite) gradually increased from ∼7 to 115 μg L^-1 as arsenate was reduced. Accordingly, bacterially mediated reductive desorption of arsenate is identified as the main process controlling arsenic mobility, while Fe(III) reduction coupled with sulfate reduction are secondary processes that have also contributed to arsenic enrichment in the study site. Overall, this study provide important insights into the mechanism controlling arsenic mobility under weakly alkaline and reducing conditions, and furnishes that arsenate reduction by bacteria play a major role leading to high accumulation of desorbed arsenite in groundwater.

Influence of calcium and phosphate on pH dependency of arsenite and arsenate adsorption to goethite

In the environment, simultaneous presence of arsenic (As) of different oxidation states is common, which hampers our understanding of As behavior. In the current study, the pH dependency of arsenite (As(III)) and arsenate (As(V)) adsorption to goethite under the influence of calcium (Ca²⁺) (as a major cation) and phosphate (PO₄^3-) (as a major anion) was studied, and the reliability of the CD-MUSIC model prediction was tested. The results show that the presence of the major ions led in general to a weaker and more complicated pH dependency of As adsorption. Calcium promoted As(V) adsorption especially at high pH, which can reverse the direction of the pH dependency. The presence of Ca²⁺ can even decrease As(III) adsorption when As(V) and/or PO₄^3- are present. Phosphate competed strongly with both As(III) and As(V) in their adsorption, especially at intermediate and low pH. In the multi-component system, As(III) adsorbs weaker than As(V) over the environmental relevant pH range, therefore it is often the dominant As species in solution and soluble As(III) concentration generally decreases with increasing pH. In the same pH range, As(V) adsorption shows a complicated pH dependency. Soluble As(V) reaches a minimum around pH 6 at high concentration of major bivalent cations (e.g. Ca²⁺), whereas soluble As(V) will decrease with pH at low bivalent cation concentrations. The experimental results can be reliably predicted and explained with the CD-MUSIC model. The outcome of this study can provide understanding needed in the risk assessment and remediation of As contaminated soils and water.

Iron-based subsurface arsenic removal technologies by aeration: A review of the current state and future prospects

Arsenic contamination in groundwater is a critical issue and one that raises great concern around the world as the cause of many negative health impacts on the human body, including internal and external cancers. There are many ways to remove or immobilize arsenic, including membrane technologies, adsorption, sand filtration, ion exchange, and capacitive deionization. These exhibit many different advantages and disadvantages. Among these methods, in-situ subsurface arsenic immobilization by aeration and the subsequent removal of arsenic from the aqueous phase has shown to be very a promising, convenient technology with high treatment efficiency. In contrast to most of other As remediation technologies, in-situ subsurface immobilization offers the advantage of negligible waste production and hence has the potential of being a sustainable treatment option. This paper reviews the application of subsurface arsenic removal (SAR) technologies as well as current modeling approaches. Unlike subsurface iron removal (SIR), which has proven to be technically feasible in a variety of hydrogeochemical settings for many years, SAR is not yet an established solution since it shows vulnerability to diverse geochemical conditions such as pH, Fe:As ratio, and the presence of co-ions. In some situations, this makes it difficult to comply with the stringent guideline value for drinking water recommended by the WHO (10 μg L^-1). In order to overcome its limitations, more theoretical and experimental studies are needed to show long-term application achievements and help the development of SAR processes into state-of-the-art technology.

Mobilization of arsenic on nano-TiO2 in soil columns with sulfate reducing bacteria

Arsenic (As) remediation in contaminated water using nanoparticles is promising. However, the fate and transport of As associated with nano-adsorbents in natural environment is poorly understood. To investigate the fate of adsorbed As on nano-TiO₂ in changed redox condition from oxic to anoxic, we added the As(V)-TiO₂ suspension in groundwater to an autoclaved soil column which inoculated a sulfate-reducing bacterium, Desulfovibrio vulgaris DP4. The dissolved As(V) in effluent increased to 798 μg/L for the biotic column and to 1510 μg/L for the abiotic control, and dissolved As(III) was observed only in biotic column. The total As (dissolved plus particulate) in the biotic column effluent (high to 2.5 mg/L) was substantially higher than the abiotic control (1.5 mg/L). Therefore SRB restrained the release of dissolved As, and facilitated the transport of particulate As. Micro-XRF analysis suggested that the nano-TiO₂ with As was mainly retained in the influent front and that its transport was negligible. Our pe-pH calculation and XANES analysis demonstrated that generated secondary iron minerals containing magnetite and mackinawite mainly were responsible for dissolved As retention, and then transported with As as particulate As. The results shed light on the mobilization of adsorbed As on a nano-adsorbent in an anoxic environment.

Microscopic insight into precipitation and adsorption of As(V) species by Fe-based materials in aqueous phase

The mechanism of As(V) removal from the drinking water and industrial effluents by iron materials remains unclear at the molecular level. In this work, the association of Fe-based materials with As(V) species was explored using density functional theory and ab initio calculations. Solvent separated ion pair structures of [FeH₂AsO₄]²⁺_aq species may be dominant in an acidic solution of FeAs complex. The association trend of H₂AsO₄^- species by Fe³⁺_aq is found to be quite weak in the aqueous solution, which may be attributed to the strong hydration of Fe³⁺_aq and [FeH₂AsO₄]²⁺ species. However, the association of H₂AsO₄^- species by colloidal clusters is quite strong, due to the weakened hydration of Fe(III) in colloidal structures. The hydrophobicity of Fe-based materials may be one of the key factors for their As(V) removal efficiency in an aqueous phase. When the number of OH^- coordinated with Fe(III) increases, the association trend of As(V) by colloidal ferric hydroxides weakens accordingly. This study provides insights into understanding the coprecipitation and adsorption mechanisms of arsenate removal and revealing the high efficiency of arsenate removal by colloidal ferric hydroxides or iron salts under moderate pH conditions.

Analysis of the functional gene structure and metabolic potential of microbial community in high arsenic groundwater

Microbial functional potential in high arsenic (As) groundwater ecosystems remains largely unknown. In this study, the microbial community functional composition of nineteen groundwater samples was investigated using a functional gene array (GeoChip 5.0). Samples were divided into low and high As groups based on the clustering analysis of geochemical parameters and microbial functional structures. The results showed that As related genes (arsC, arrA), sulfate related genes (dsrA and dsrB), nitrogen cycling related genes (ureC, amoA, and hzo) and methanogen genes (mcrA, hdrB) in groundwater samples were correlated with As, SO₄^2-, NH₄⁺ or CH₄ concentrations, respectively. Canonical correspondence analysis (CCA) results indicated that some geochemical parameters including As, total organic content, SO₄^2-, NH₄⁺, oxidation-reduction potential (ORP) and pH were important factors shaping the functional microbial community structures. Alkaline and reducing conditions with relatively low SO₄^2-, ORP, and high NH₄⁺, as well as SO₄^2- and Fe reduction and ammonification involved in microbially-mediated geochemical processes could be associated with As enrichment in groundwater. This study provides an overall picture of functional microbial communities in high As groundwater aquifers, and also provides insights into the critical role of microorganisms in As biogeochemical cycling.