Surface complexation modeling for predicting solid phase arsenic concentrations in the sediments of the Mississippi River Valley alluvial aquifer, Arkansas, USA

The potential health impact of As in drinking water supply systems in the Mississippi River Valley alluvial aquifer in the state of Arkansas, USA is significant. In this context it is important to understand the occurrence, distribution and mobilization of As in the Mississippi River Valley alluvial aquifer. Application of surface complexation models (SCMs) to predict the sorption behavior of As and hydrous Fe oxides (HFO) in the laboratory has increased in the last decade. However, the application of SCMs to predict the sorption of As in natural sediments has not often been reported, and such applications are greatly constrained by the lack of site-specific model parameters. Attempts have been made to use SCMs considering a component additivity (CA) approach which accounts for relative abundances of pure phases in natural sediments, followed by the addition of SCM parameters individually for each phase. Although few reliable and internally consistent sorption databases related to HFO exist, the use of SCMs using laboratory-derived sorption databases to predict the mobility of As in natural sediments has increased. This study is an attempt to evaluate the ability of the SCMs using the geochemical code PHREEQC to predict solid phase As in the sediments of the Mississippi River Valley alluvial aquifer in Arkansas. The SCM option of the double-layer model (DLM) was simulated using ferrihydrite and goethite as sorbents quantified from chemical extractions, calculated surface-site densities, published surface properties, and published laboratory-derived sorption constants for the sorbents. The model results are satisfactory for shallow wells (10.6 m below ground surface), where the redox condition is relatively oxic or mildly suboxic. However, for the deep alluvial aquifer (21-36.6 m below ground surface) where the redox condition is suboxic to anoxic, the model results are unsatisfactory.     

Effect of phosphate, silicate, and Ca on Fe(III)-precipitates formed in aerated Fe(II)- and As(III)-containing water studied by X-ray absorption spectroscopy

We studied the local coordination and structure of Fe(III)-precipitates formed in aerated Fe(II)- and As(III)-containing water (buffered to pH 7 by 8 mM bicarbonate) using synchrotron-based X-ray absorption spectroscopy (XAS) at the K-edges of Fe, P, Ca, and As. Dissolved phosphate, silicate, and Ca at different ratios relative to each other and to Fe affect the forming Fe(III)-phases in a complex manner. The high affinity of phosphate for Fe(III) results in the predominant precipitation of Fe(III)-phosphate as long as dissolved phosphate is present, with Fe(III) polymerization limited to small oligomers. In Ca-containing solution, Ca uptake by Fe(III)-Ca-phosphate involves the linkage and coagulation of negatively charged Fe(III)-phosphate oligomers via Ca-O-P bonds. In the absence of phosphate, dissolved silicate at Si/Fe ratios above ∼0.5 results in the formation of hydrous ferric oxide (HFO) with mainly edge-sharing Fe-Fe linkage. At lower Si/Fe ratios of ∼0.5-0.1, mainly 2-line ferrihydrite (2L-Fh) with both edge- and corner-sharing Fe-Fe linkage forms. Only in the absence of phosphate at low Si/Fe ratio, lepidocrocite (Lp) forms. In solutions containing sufficient Fe(II), aeration results in the sequential precipitation of Fe(III)-(Ca-)phosphate, HFO or 2L-Fh (depending on solution Si/Fe), and finally Lp. The amount and oxidation state of As co-precipitated with Fe(III) are controlled by the co-oxidation of As(III) with Fe(II), which increases with initial Fe/As ratio, and the competitive uptake of phosphate, As(V) and less strongly sorbing silicate and As(III). This study demonstrates that the diversity and sequence of short-range-ordered Fe(III)-precipitates forming by Fe(II) oxidation in near-neutral natural waters depend on water chemistry. Because differences in the colloidal stability and biogeochemical reactivity of these phases will affect the fate of associated major and trace elements, the different Fe(III)-precipitates and their specific biogeochemical properties must be taken into account when addressing nutrient and contaminant dynamics at redox boundaries in natural and engineered systems.     

Preservation of water samples for arsenic(III/V) determinations: an evaluation of the literature and new analytical results

Published literature on preservation procedures for stabilizing aqueous inorganic As(III/V) redox species contains discrepancies. This study critically evaluates published reports on As redox preservation and explains discrepancies in the literature. Synthetic laboratory preservation experiments and time stability experiments were conducted for natural water samples from several field sites. Any field collection procedure that filters out microorganisms, adds a reagent that prevents dissolved Fe and Mn oxidation and precipitation, and isolates the sample from solar radiation will preserve the As(III/V) ratio. Reagents that prevent Fe and Mn oxidation and precipitation include HCl, H₂SO₄, and EDTA, although extremely high concentrations of EDTA are necessary for some water samples high in Fe. Photo-catalyzed Fe(III) reduction causes As(III) oxidation; however, storing the sample in the dark prevents photochemical reactions. Furthermore, the presence of Fe(II) or SO₄ inhibits the oxidation of As(III) by Fe(III) because of complexation reactions and competing reactions with free radicals. Consequently, fast abiotic As(III) oxidation reactions observed in the laboratory are not observed in natural water samples for one or more of the following reasons: (1) the As redox species have already stabilized, (2) most natural waters contain very low dissolved Fe(III) concentrations, (3) the As(III) oxidation caused by Fe(III) photoreduction is inhibited by Fe(II) or SO₄.     

Seasonal variations of Zn, Cu, As and Mo in arsenic-rich stream at the Mokrsko gold deposit, Czech Republic

Monthly sampling of slightly alkaline arsenic-rich stream in the Mokrsko gold deposit revealed seasonal variations in dissolved Zn, Cu, As and Mo. Concentrations of trace metal cations (Zn, Cu) increased as much as 330 and 178%, respectively, from minimum mean values at autumn to maximum mean values at spring. In contrast, concentrations of trace element oxyanions (As, Mo) revealed opposite seasonal pattern with increase to 189% (As) and 123% (Mo) during summer–autumn, indicating that in-stream biogeochemical process(es) played the main role in controlling the seasonal variations of these trace elements. The trace elements were mainly scavenged by low crystalline Mn oxyhydroxide and Fe oxyhydroxide (ferrihydrite). Results are consistent with sorption and coprecipitation processes controlling seasonal variations of dissolved Zn and Cu, while As and Mo dynamics appear linked to Mn redox reactions. The sorption processes and Mn redox processes are attributed to the changes of pH and oxic/anoxic conditions on the surface of oxyhydroxides, respectively, which are themselves controlled by the balance between photosynthesis and respiration. Under the geochemical conditions of the stream, inferred Mn redox reactions can only be explained by microbial activity.     

Subsurface variations in arsenic mineralogy and geochemistry following long-term weathering of gold mine tailings

Variations in arsenic (As) mineralogy and geochemical controls on its mobility were evaluated in subsurface tailings at the historical Montague and Goldenville mine sites in Nova Scotia, Canada. Tailings at these sites contain some of the highest As concentrations in Nova Scotia and are located in close proximity to local communities. Pore water in the subsurface tailings is characterized by circumneutral to alkaline pH (6.2 to 8.7) and mildly reducing to oxidizing redox conditions (+130 mV to +347 mV). Bulk chemistry, scanning electron microscopy, and synchrotron micro-X-ray diffraction analyses showed As mineral hosts differ with depth. The deepest tailings (max. 2 m) are in direct contact with partially decomposed vegetation, which supports reducing conditions and the precipitation of authigenic As and Fe sulfides. Under reducing conditions, dissolved As concentrations are also controlled by desorption of As from dissolution of Fe and Mn oxides and the sorption or co-precipitation of As with carbonates. These geochemical controls differ from those influencing dissolved As concentrations under oxidizing conditions. In the near surface, As mobility is controlled by oxidative dissolution of primary arsenopyrite, precipitation of secondary Fe arsenates, Fe oxyhydroxides and Mn oxides, secondary Ca-Fe arsenates, and sorption onto Fe oxyhydroxides and gangue minerals. Some of these mineral species are stable under different conditions yet occur in close association, indicating the importance of microenvironments. The results of this study show that the weathering characteristics of these tailings vary with depth, leading to the formation of new As hosts that are distinct from those observed in the near surface. Identification of these As hosts provides an understanding of current controls on As mobility and has implications for future reprocessing and/or remediation efforts.     

Interpreting nanoscale size-effects in aggregated Fe-oxide suspensions: Reaction of Fe(II) with Goethite

The Fe(II)/Fe(III) redox couple plays an important role in both the subsurface fate and transport of groundwater pollutants and the global cycling of carbon and nitrogen in iron-limited marine environments. Iron oxide particles involved in these redox processes exhibit broad size distributions, and the recent demonstrations of dramatic nanoscale size-effects with various metal oxides has compelled us, as well as many others, to consider whether the rate and extent of Fe(II)/Fe(III) cycling depends upon oxide particle size in natural systems. Here, we investigated the reaction of Fe(II) with three different goethite particle sizes in pH 7.5 suspensions. Acicular goethite rods with primary particle dimensions ranging from 7 by 80 nm to 25 by 670 nm were studied. Similar behavior with respect to Fe(II) sorption, electron transfer and nitrobenzene reduction was observed on a mass-normalized basis despite almost a threefold difference in goethite specific surface areas. Scanning electron microscopy (SEM) images, dynamic light scattering (DLS) and sedimentation measurements all indicated that, at pH 7.5, significant aggregation occurred with all three sizes of goethite particles. SEM images further revealed that nanoscale particles formed dense aggregates on the order of several microns in diameter. The clear formation of particle aggregates in solution raises questions regarding the use of primary particle surface area as a basis for assessing nanoscale size-effects in iron oxide suspensions at circum-neutral pH values. In our case, normalizing the Fe(II) sorption densities and rate constants for nitrobenzene reduction by BET surface area implies that goethite nanoparticles are less reactive than larger particles. We suspect, however, that aggregation is responsible for this observed size-dependence, and argue that BET values should not be used to assess differences in surface site density or intrinsic surface reactivity in aggregated particle suspensions. In order to realistically assess nanoscale size-effects in environmentally relevant systems that are likely to aggregate, new methods are needed to quantify the amount of surface area accessible for sorption and reaction in wet nanoparticle suspensions, rather than assuming that this value is equivalent to the surface area determined from the characterization of dry nanoparticles.     

Deciphering As and Cu cycling in sediment pore waters in a large marina (Port Camargue,southern France) using a multi-tracer (Fe,Mn, U,Mo) approach

The sediments of the Port Camargue marina (South of France) are highly polluted by Cu and As (Briant et al., 2013). The dynamics of these pollutants in pore waters was investigated using redox tracers (sulfides, Fe, Mn, U, Mo) to better constrain the redox conditions.In summer, pore water profiles showed a steep redox gradient in the top 24 cm with the reduction of Fe and Mn oxy-hydroxides at the sediment water interface (SWI) and of sulfate immediately below. Below a depth of 24 cm, the Fe, Mn, Mo and U profiles in pore waters reflected Fe and Mn reducing conditions and, unlike in the overlying levels, sulfidic conditions were not observed. This unusual redox zonation was attributed to the occurrence of two distinct sediment layers: an upper layer comprising muddy organic-rich sediments underlain by a layer of relatively sandy and organic-poor sediments. The sandy sediments were in place before the building of the marina, whereas the muddy layer was deposited later. In the muddy layer, large quantities of Fe and Mo were removed in summer linked to the formation of insoluble sulfide phases. Mn, which can adsorb on Fe-sulfides or precipitate with carbonates, was also removed from pore waters. Uranium was removed probably through reduction and adsorption onto particles. In winter, in the absence of detectable pore water sulfides, removal of Mo was moderate compared to summer.Cu was released into solution at the sediment water interface but was efficiently trapped by the muddy layer, probably by precipitation with sulfides. Due to efficient trapping, today the Cu sediment profile reflects the increase in its use as a biocide in antifouling paints over the last 40 years.In the sandy layer, Fe, Mn, Mo and As were released into solution and diffused toward the top of the profile. They precipitated at the boundary between the muddy and sandy layers. This precipitation accounts for the high (75 μg g⁻¹) As concentrations measured in the sediments at a depth of 24 cm.     

Oxidation of As(III) by MnO2 in the absence and presence of Fe(II) under acidic conditions

Oxidation of As(III) by natural manganese (hydr)oxides is an important geochemical reaction mediating the transformation of highly concentrated As(III) in the acidic environment such as acid mine drainage (AMD) and industrial As-contaminated wastewater, however, little is known regarding the presence of dissolved Fe(II) on the oxidation process. In this study, oxidation of As(III) in the absence and presence of Fe(II) by MnO₂ under acidic conditions was investigated. Kinetic results showed that the presence of Fe(II) significantly inhibited the removal of As(III) (including oxidation and sorption) by MnO₂ in As(III)-Fe(II) simultaneous oxidation system even at the molar ratio of Fe(II):As(III) = 1/64:1, and the inhibitory effects increased with the increasing ratios of Fe(II):As(III). Such an inhibition could be attributed to the formation of Fe(III) compounds covering the surface of MnO₂ and thus preventing the oxidizing sites available to As(III). On the other hand, the produced Fe(III) compounds adsorbed more As(III) and the oxidized As(V) on the MnO₂ surface with an increasing ratio of Fe(II):As(III) as demonstrated in kinetic and XPS results. TEM and EDX results confirmed the formation of Fe compounds around MnO₂ particles or separated in solution in Fe(II) individual oxidation system, Fe(II) pre-treated and simultaneous oxidation processes, and schwertmannite was detected in Fe(II) individual and Fe pre-treated oxidation processes, while a new kind of mineral, probably amorphous FeOHAs or FeAsO₄ particles were detected in Fe(II)-As(III) simultaneous oxidation process. This suggests that the mechanisms are different in Fe pre-treated and simultaneous oxidation processes. In the Fe pre-treated and MnO₂-mediated oxidation pathway, As(III) diffused through a schwertmannite coating formed around MnO₂ particles to be oxidized. The newly formed As(V) was adsorbed onto the schwertmannite coating until its sorption capacity was exceeded. Arsenic(V) then diffused out of the coating and was released into the bulk solution. The diffusion into the schwertmannite coating and the oxidation of As(III) and sorption of both As(V) and As(III) onto the coating contributed to the removal of total As from the solution phase. In the simultaneous oxidation pathway, the competitive oxidation of Fe(II) and As(III) on MnO₂ occurred first, followed by the formation of FeOHAs or FeAsO₄ around MnO₂ particles, and these poorly crystalline particles of FeOHAs and FeAsO₄ remained suspended in the bulk solution to adsorb As(III) and As(V). The present study reveals that the formation of Fe(III) compounds on mineral surfaces play an important role in the sorption and oxidation of As(III) by MnO₂ under acidic conditions in natural environments, and the mechanisms involved in the oxidation of As(III) depend upon how Fe(II) is introduced into the As(III)-MnO₂ system.     

Investigations on different positions of electrodes and their effects on the distribution of Cr at the water sediment interface

In most mining areas, significant concentrations of metals such as Pb, Cu, Cr, Zn, Cd, Fe, Mn, Co, Ni, Hg, Ar, halogenated organic compounds and radionuclides are found. Of those, Cr is one of the well-known heavy metals that forms toxic species. It is necessary to study the mobilization and accumulation of Cr at the sediment water interface in an electric field at varying different positions and conditions of the electrode arrangement. The tests were carried out with a natural sediment containing heavy metals from the river Weisse Elster (Germany). The electrokinetic experiments have been performed in columns filled with sediment using electrodes made of conductive polymers (polyethylene with carbon black) at a maximum current density of 0.5 mA/cm². The experimental results suggest that the mobilization and accumulation of Cr highly depends on chemical factors for e.g., pH value, redox potential, respectively redox status and the content of Fe, A1 and organic matter in the soil or sediment. The sorption of Cr (III, VI) is very high in the pH range > 4.5. As expected, a high mobilization of Cr (III, VI) was seen in the case of the experiments with the anode at the sediment, because the pH value was lower than mentioned above. On the opposite, the best conditions for the Cr (III, VI) immobilization is high pH values (cathode at the sediment).     

Contrasting sorption behaviours affecting groundwater arsenic concentration in Kandal Province,Cambodia

Natural arsenic(As)contamination of groundwater which provides drinking water and/or irrigation supplies remains a major public health issue,particularly in South and Southeast Asia.A number of studies have evaluated various aspects of the biogeochemical controls on As mobilization in aquifers typical to this region,however many are predicated on the assumption that key biogeochemical processes may be deduced by sampled water chemistry.The validity of this assumption has not been clearly established even though the role of sorption/desorption of As and other heavy metals onto Fe/Mn(hydr)oxides is an important control in As mobilization.Here,selective chemical extractions of sand-rich and clay-rich sediments from an As-affected aquifer in Kandal Province,Cambodia,were undertaken to explore the potential role of partial re-equilibrium through sorption/desorption reactions of As and related solutes(Fe,Mn and P)between groundwater and the associated solid aquifer matrix.In general,groundwater As is strongly affected by both pH and Eh throughout the study area.However,contrasting sorption behaviour is observed in two distinct sand-dominated(T-Sand)and clay dominated(T-Clay)transects,and plausibly attributed to differing dominant lithologies,biogeochemical and/or hydrogeological conditions.Sorption/desorption processes appear to be re-setting groundwater As concentrations in both transects,but to varying extents and in different ways.In T-Sand,which is typically highly reducing,correlations suggest that dissolved As may be sequestered by sorption/re-adsorption to Fe-bearing mineral phases and/or sedimentary organic matter;in T-Clay Eh is a major control on As mobilization although binding/occlusion of Fe-bearing minerals to sedimentary organic matter may also occur.Multiple linear regression analysis was conducted with groups categorised by transect and by Eh,and the output correlations support the contrasting sorption behaviours encountered in this study area.Irrespective of transect,however,the key biogeochemical processes which initially control As mobilization in such aquifers,may be "masked" by the re-setting of As concentrations through in-aquifer sorption/desorption processes.     

Geochemistry of arsenic during low-temperature water–rock interaction

High arsenic water has been a global focus of both scientists and water supply managers because of its serious adverse impact on human health and wide distribution in the world. Processes of redox, sorption, precipitation, and dissolution release arsenic in both natural systems and in environments intensely modified by human activities. In natural systems, groundwater arsenic is controlled by lithologic geochemistry, sedimentation conditions, hydrogeologic setting and groundwater chemistry. However, in the intensely human-affected systems (such as mining and tilling areas), arsenic mobilization is dependent on the composition of the primary materials, treatment methods, storage design, and local climate. Well-designed experimental systems aid in characterizing sorption, precipitation, and redox processes associated with arsenic dynamics during water-rock interaction. Continued investigations of field sites will further refine understanding of the processes favoring arsenic mobility in the range of natural and man-made systems. The combination of field and experimental studies will lead to better understanding of arsenic cycling in all systems and sustainable management of water resources in arsenic-affected areas.     

Stabilization of Fe/Cu nanoparticles by starch and efficiency of arsenic adsorption from aqueous solutions

Due to the severity of arsenic contamination of soil and water resources around the world, finding new adsorbents for arsenic removal from the water is of high importance. The present study investigates the possible use and effectiveness of starch-stabilized Fe/Cu nanoparticles for adsorption of arsenic from aqueous solutions. First, Fe/Cu nanoparticles at various starch concentrations of 0, 0.02, 0.04 and 0.06 wt% were synthesized and characterized by X-ray diffraction, transmission electron microscopy and zeta potential/particle size analyzer. Then 0.04 wt% stabilized Fe/Cu nanoparticles were tested for the sorption of As(III) and As(V) from synthetic arsenic-contaminated water. To have an understanding about the arsenic adsorption mechanism of nanoparticles, X-ray photoelectron spectroscopy (XPS) was performed before and after adsorption. The results showed that starch provides nanoparticles with a neutral surface and stabilization of nanoparticles is possible with 0.04 wt% or higher concentrations of starch. For 0.04 wt% starch-stabilized Fe/Cu nanoparticles, the adsorption isotherms fit well within the Langmuir equation, with maximum sorption capacities of 90.1 mg/g for As(III) and 126.58 mg/g for As(V) at a pH of 7.0 from the aqueous arsenic solutions. Examining the XPS spectra of nanoparticles before and after adsorption showed that arsenic adsorption by this nanoparticle can be due to the formation of inner-sphere arsenic complexes on the particle surface, and the surface oxygen-containing functional groups involved in adsorption. The high sorption capacity suggests the potential for applying starch-stabilized Fe/Cu nanoparticles to the contaminated waters for removal of arsenic.     

Arsenic enrichment in groundwater of West Bengal,India: geochemical evidence for mobilization of As under reducing conditions

The mechanism of As release and source(s) of As has been investigated in a small part of a watershed in the Murshidabad district of West Bengal. Analyses include major ion and trace element concentrations, as well as O, H and S isotope ratios of groundwater, surface water and a thermal spring. The results indicate that all water samples belong to the Ca–HCO₃ type, except for the thermal spring which is of the Na–HCO₃ type. Shallow and deeper groundwaters have distinct hydrochemical features. High As contents were registered only in the deeper groundwater horizon. Factor analysis and the distribution pattern of major and trace elements indicate that As is present in the aquifer as a scavenged phase by Fe(III) and to a lesser extent by Mn(IV) phases. The release of As into the groundwater occurs gradually in successive stages, corresponding to the actual redox state in the aquifer. The main stage of As release is related to the bacterial reduction of Fe(III) to Fe(II) (i.e. to the simultaneous dissolution of Fe oxyhydroxides). Low redox conditions in highly polluted areas are indicated by low SO₄ concentration and high δ³⁴S values. During bacterial SO₄ reduction, residual SO₄ in groundwater is depleted in the lighter S isotope (³²S). However, the cause of the gradual decrease of the redox state in the groundwater is still not well understood.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Quantifying arsenate partitioning in aquatic systems: Narrowing the laboratory-real world gap with kinetic sediment extractions and the Diffuse Layer Model

Many studies have proposed that silicic acid and phosphate (P^V) can displace arsenic sorbed to iron oxides leading to elevated As concentrations in aquatic systems. While surface complexation models are adept at quantifying sorption to synthetic oxides in laboratory systems their application to complex natural systems remains challenging. In this study we provide a systematic approach to developing a robust use of models for understanding As^V distribution in natural systems in which hydrated iron oxides are the main adsorptive phase. The Waikato River provides a useful laboratory for this work because it contains high H₄SiO₄, As^V and P^V loadings due to geothermal and agricultural inputs. A 15 min oxalate extraction and a 48 h ethylenediaminetetraacetic acid (EDTA) extraction of river sediment contained the same ratios of As:Fe, P:Fe and Si:Fe. Both of these extracts target the poorly ordered iron oxide phases (typically ferrihydrite) and by following the release of elements over time in the EDTA extraction it was possible to demonstrate that the extracted As, P, and Si were associated with the ferrihydrite. This demonstrates for the first time that a single oxalate extraction can quantify ferrihydrite sorbed H₄SiO₄, As and P^V and provides a basis to quantify the role of these ligands in inhibiting As^V sorption to sediments. The measured concentrations of ferrihydrite sorbed As^V, P^V and H₄SiO₄ for the Waikato River suspended sediment allow for the informed selection of appropriate model parameters for applying the Diffuse Layer Model to the system. In this way it was possible to quantify the effect of the individual components in the river water on As^V sorption. This study provides an explanation for the observation that the proportion of sorbed As in the Waikato River is generally significantly lower than that observed in rivers closer to the world average concentrations. More generally the study provides a method to quantify the role of individual water chemistry components on As^V distribution in natural systems.     

Geochemical patterns of arsenic-enriched ground water in fractured,crystalline bedrock,Northport, Maine,USA

High mean As concentrations of up to 26.6 μmol/L (1990 μg/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. Sulfides in the bedrock are the primary source of the As in the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with As concentrations in the ground water barely above the detection limit of 2.0 μmol/L, shows that there are complicating factors. Chemical analyses of water from 35 bedrock wells throughout a small watershed reveal spatial clustering of wells with high As concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium-bicarbonate-dominated water with low As concentrations (CaHCO₃ type), sodium-bicarbonate-dominated water with moderately high As concentrations (NaHCO₃ type), and calcium-bicarbonate-dominated water with very high As concentrations (High-As type). It is proposed that differences in recharge area and ground-water evolution, and possible bedrock composition difference are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption of As is an insignificant control on As concentration. Correlations of As concentrations with Fe and redox parameters indicates that reductive dissolution of Fe(III) oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO₃ and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO₄ concentrations, probably due to precipitation of Fe(III) oxyhydroxides and adsorption of As under oxidizing conditions.     

Removal of trace metals by coprecipitation with Fe,Al and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District,Tennessee

This study examined the sorption of trace metals to precipitates formed by neutralization of 3 natural waters contaminated with acid mine drainage (AMD) in the former Ducktown Mining District, Tennessee. The 3 water samples were strongly acidic (pH 2.2 to 3.4) but had distinctively different chemical signatures based on the mole fractions of dissolved Fe, Al and Mn. One sample was Fe-rich (Fe=87.5%, Al=11.3%, and Mn=1.3%), another was Al-rich (Al=79.4%, Mn=18.0%, and Fe=2.5%), and the other was Mn-rich (Mn=51.4%, Al=25.7%, and Fe=22.9%). In addition, these waters had high concentrations of trace metals including Zn (37,700 to 17,400 μg/l), Cu (13,000 to 270 μg/l), Co (1,500 to 520 μg/l), Ni (360 to 75 μg/l), Pb (30 to 8 μg/l), and Cd (30 to 6 μg/l). Neutralization of the AMD-contaminated waters in the laboratory caused the formation of either schwertmannite at pH<4 or ferrihydrite at pH>4. Both phases were identified by XRD analyses of precipitates from the most Fe-rich water. At higher pH values (∼5) Al-rich precipitates were formed. Manganese compounds were precipitated at pH∼8. The removal of trace metals depended on the precipitation of these compounds, which acted as sorbents. Accordingly, the pH for 50% sorption (pH₅₀) ranged from 5.6 to 7.5 for Zn, 4.6 to 6.1 for Cu, 5.4 to 7.7 for Ni, 5.9 to 7.9 for Co, 3.1 to 4.3 for Pb, and 5.5 to 7.7 for Cd. The pH dependence of sorption arose not only because of changes in the sorption coefficients of the trace metals but also because the formation and composition of the sorbent was controlled by the pH, the chemical composition of the water, and the solubilities of the oxyhydroxide-sulfate complexes of Fe, Al, and Mn.     

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe²⁺, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe²⁺ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe²⁺, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe²⁺ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg⁻¹). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H₂ species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.     

Synchronous release of labile phosphorus,labile ferrum,and labile arsenic in water–sediment interface using ZrO–Chelex measured in the Aibi Lake of Northwest China

In the work presented here, a Zr-oxide diffusive gradients in thin films (DGT) was used to monitor the release flux of phosphorus (P), ferrum (Fe), and arsenic (As) in the water–sediment interface of Aibi Lake—a typical shallow lake located in the arid regions of Northwest China. Results showed that: (1) In the water–sediments interface of Aibi Lake, the ranges (average values) of labile As, labile P, and labile Fe levels in DGTs are 3.846–101.840 (43.934) µg L^?1, 0.006–0.232 (0.070) mg L^?1, and 0.202–52.984 (15.832) mg L^?1, respectively. Among 0–20 cm of the vertical profile there was a stable distribution of three elements, while below the interface as 0 cm–(??80) cm there were relatively large changes of these. (2) Fitting analysis showed that there were significant correlations between labile Fe and labile P, and labile As in four DGTs, which showed that in the water–sediments of Aibi Lake, Fe, P, and As are released simultaneously. (3) Combined with former research, we found that the redox of Fe³⁺ to Fe²⁺ may cause the release of P and As to the sediments and water body from the former Fe–P and Fe–As; the proportion of P/Fe of four DGTs was all relatively lower than 1, suggesting that the redox of Fe³⁺ caused the P to be released. (4) This research showed that the concentrations of P, Fe, and As of the water–sediments interface of the lake was obviously lower than that of the water body and sediments of Aibi Lake as well as others of central and eastern China. ZrO-DGT can accurately reflect the distribution of P, Fe, and As of Aibi Lake. These findings can provide initial verification for the use of ZrO-DGT technology in the research of elements at the water–sediment interface in lakes of Xinjiang Province in Northwest China.     

水库三维Fe、Mn迁移模型——阿哈水库实例研究

通过描述Fe、Mn在水库中的氧化还原变化、吸附、解吸、沉降及再悬浮,考虑水流运动对Fe、Mn迁移变化的影响,研制了水库三维水动力模式及三维Fe、Mn迁移变化模式,从而建立了完整的水库三维Fe、Mn迁移分布模拟模型。该模型在阿哈水库实际运用效果良好。模拟结果表明,水库水流运动整体流速较小,Fe、Mn在水库中平行迁移运动缓慢。进入水库的Fe、Mn负荷大量沉入库底,在水体缺氧条件下,底质中Fe、Mn重新释放,形成明显的Fe、Mn分布垂直梯度,造成水库下层Fe、Mn大大超过国家生活饮用水质量标准。