A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development

A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (+/-1%), and As was completely recovered from HFO-doped gels (+/-4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.     

Investigating arsenic speciation and mobilization in sediments with DGT and DET: a mesocosm evaluation of oxic-anoxic transitions

Mobilization of arsenic from freshwater and estuarine sediments during the transition from oxic to anoxic conditions was investigated using recently developed diffusive sampling techniques. Arsenic speciation and Fe(II) concentrations were measured at high resolution (1-3 mm) with in situ diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques. Water column anoxia induced Fe(II) and As(III) fluxes from the sediment. A correlation between water column Fe(II) and As(III) concentrations was observed in both freshwater (r(s) = 0.896, p < 0.001) and estuarine (r(s) = 0.557, p < 0.001) mesocosms. Porewater sampling by DGT and DET techniques confirmed that arsenic mobilization was associated with the reductive dissolution of Fe(III) (hydr)oxides in the suboxic zone of the sediment; a relationship that was visible because of the ability to measure the coincident profiles of these species using combined DGT and DET samplers. The selective measurement of As(III) and total inorganic arsenic by separate DGT samplers indicated that As(III) was the primary species mobilized from the solid phase to the porewater. This measurement approach effectively ruled out substantial As(V) mobilization from the freshwater and estuarine sediments in this experiment. This study demonstrates the capabilities of the DGT and DET techniques for investigating arsenic speciation and mobilization over a range of sediment conditions.     

Field, experimental, and modeling study of arsenic partitioning across a redox transition in a Bangladesh aquifer

To understand redox-dependent arsenic partitioning, we performed batch sorption and desorption experiments using aquifer sands subjected to chemical and mineralogical characterization. Sands collected from the redox transition zone between reducing groundwater and oxic river water at the Meghna riverbank with HCl extractable Fe(III)/Fe ratio ranging from 0.32 to 0.74 are representative of the redox conditions of aquifers common in nature. One brown suboxic sediment displayed a partitioning coefficient (K(d)) of 7-8 L kg(-1) at equilibrium with 100 μg L(-1) As(III), while two gray reducing sediments showed K(d) of 1-2 L kg(-1). Lactate amendment to aquifer sands containing 91 mg kg(-1) P-extractable As resulted in the reduction of As and Fe with sediment Fe(III)/Fe decreasing from 0.54 to 0.44, and mobilized an equivalent of 64 mg kg(-1) As over a month. Desorption of As from nonlactate-amended sediment was negligible with little change in sediment Fe(III)/Fe. This release of As is consistent with microbial reduction of Fe(III) oxyhydroxides and the resulting decrease in the number of surface sites on Fe(III) oxyhydroxides. Arsenic partitioning (K(d)) in iron-rich, sulfur-poor aquifers with circumneutral pH is redox-dependent and can be estimated by HCl leachable sediment Fe(III)/Fe ratio with typical Fe concentrations.     

Iron(II)-catalyzed oxidation of arsenic(III) in a sediment column

Arsenic contamination in aquatic systems is a worldwide concern. Understanding the redox cycling of arsenic in sediments is critical in evaluating the fate of arsenic in aquatic environments and in developing sediment quality guidelines. The direct oxidation of inorganic trivalent arsenic, As(III), by dissolved molecular oxygen has been studied and found to be quite slow. A chemical pathway for As(III) oxidation has been proposed recently in which a radical species, Fe(IV), produced during the oxidation of divalent iron, Fe(II), facilitates the oxidation of As(III). Rapid oxidation of As(III) was observed (on a time scale of hours) in batch systems at pH 7 and 7.5, but the extent of As(III) oxidation was limited. The Fe(II)-catalyzed oxidation of As(III) is examined in a sediment column using both computational and experimental studies. A reactive-transport model is constructed that incorporates the complex kinetics of radical species generation and Fe(II) and As(III) oxidation that have been developed previously. The model is applied to experimental column data. Results indicate that the proposed chemical pathway can explain As(III) oxidation in sediments and that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.     

Deposition and fate of arsenic in iron- and arsenic-enriched reservoir sediments

Deposition of arsenic to the sediments of Haiwee Reservoir (Olancha, CA) has dramatically increased since March 1996 as a result of an interim strategy for arsenic management in the Los Angeles Aqueduct (LAA) water supply. Ferric chloride and cationic polymer are introduced into the Aqueduct at the Cottonwood treatment plant, 27 km north of the Haiwee Reservoir. This treatment decreases the average arsenic concentration from 25 microg/L above Cottonwood to 8.3 microg/L below Haiwee. Iron- and arsenic-rich flocculated solids are removed by deposition to the reservoir sediments. Analysis of sediments shows a pronounced signature of this deposition with elevated sediment concentrations of iron, arsenic, and manganese relative to a control site. Sediment concentrations of these elements remain elevated throughout the core length sampled (ca. 4% iron and 600 and 200 microg/g of manganese and arsenic, respectively, on a dry weight basis). A pore water profile revealed a strong redox gradient in the sediment. Manganese in the pore waters increased below 5 cm; iron and arsenic increased below 10 cm and were strongly correlated, consistent with reductive dissolution of iron oxyhydroxides and concurrent release of associated arsenic to solution. X-ray absorption near-edge spectroscopy revealed inorganic As(V) present only in the uppermost sediment (0-2.5 cm) in addition to inorganic As(III). In the deeper sediments (to 44 cm), only oxygen-coordinated As(III) was detected. Analysis of the extended X-ray absorption fine structure spectrum indicates that the As(III) at depth remains associated with iron oxyhydroxide. We hypothesize that this phase persists in the recently deposited sediment despite reducing conditions due to slow dissolution kinetics.     

Arsenic behavior in paddy fields during the cycle of flooded and non-flooded periods

The behavior of As in paddy fields is of great interest considering high As contents of groundwater in several Asian countries where rice is the main staple. We determined the concentrations of Fe, Mn, and As in soil, soil water, and groundwater samples collected at different depths down to 2 m in an experimental paddy field in Japan during the cycle of flooded and non-flooded periods. In addition, we measured the oxidation states of Fe, Mn, and As in situ in soil samples using X-ray absorption near-edge structure (XANES) and conducted sequential extraction of the soil samples. The results show that Fe (hydr)oxide hosts As in soil. Arsenic in irrigation waters is incorporated in Fe (hydr)oxide in soil during the non-flooded period, and the As is quickly released from soil to water during the flooded period because of reductive dissolution of the Fe (hydr)oxide phase and reduction of As from As(V) to As(III). The enhancement of As dissolution by the reduction of As is supported by high As/Fe ratios of soil water during the flooded period and our laboratory experiments where As(III) concentrations and As(III)/As(V) ratios in submerged soil were monitored. Our work, primarily based on data from an actual paddy field, suggests that rice plants are enriched in As because the rice grows in flooded paddy fields when mobile As(III) is released to soil water.     

Mercury mobilization in estuarine sediment porewaters: a diffusive gel time-series study

To assess the lability of porewater and sediment solid-phase mercury (Hg), mercapto-substituted siloxane gels were deployed within the sediments of the Penobscot estuary in Maine. Gel deployments occurred in time series and at discrete sediment depths. A sediment distribution coefficient (K(D)) was estimated by modeling the resultant gel Hg uptake. For deployments > 1 day, depth-averaged gel Hg uptake was significantly greater at depth (Zone B 6-20 cm) than in the vicinity of the sediment-water interface (Zone A 0-5 cm), with uptake ultimately reaching 16.7 +/- 4.9 ng Hg g(-1) gel versus 35.5 +/- 3.8 ng Hg g(-1) gel for Zone A versus Zone B, respectively. For Zone A, a simple diffusive model adequately describes gel mass flux, suggesting that Hg repartitioning from the solid phase does not generate a net Hg source term within the time frame of gel deployment. For Zone B, model-determined values of K(D) (K(D) = 25-75) were considerably smaller than literature values typically based on total sediment Hg concentration. The magnitude of the modeled K(D) suggests that it is a small fraction of total sediment-sequestered Hg that is likely sensitive, via interaction with porewater ligands, to the presence of an external sink. These observations of low general Hg reactivity suggest that the net porewater Hg pool may be properly defined as a function of porewater ligand production. Such a definition highlights the importance of microbially mediated diagenesis in controlling Hg cycling within estuarine sediments.     

Depositional influences on porewater arsenic in sediments of a mining-contaminated freshwater lake

Arsenic-containing minerals mobilized during mining activities and deposited to Lake Coeur d'Alene (CDA), Idaho sediments represent a potential source of soluble As to the overlying water. Our objective was to delineate the processes controlling porewater As concentrations within Lake CDA sediments. Sediment and porewater As concentrations were determined, and solid-phase As associations were probed using X-ray absorption near-edge structure (XANES) spectroscopy. Although maximum As in the sediment porewaters varied from 8.4 to 16.2 microM, As sorption on iron oxyhydroxides at the oxic sediment-water interface prevented flux to overlying water. Floods deposit sediment containing variable amounts of arsenopyrite (FeAsS), with majorfloods depositing large amounts of sediment that bury and preserve reduced minerals. Periods of lower deposition increase sediment residence times in the oxic zone, promoting oxidation of reduced minerals, SO4(2-) efflux, and formation of oxide precipitates. Depositional events bury oxides containing sorbed As, transitioning them into anoxic environments where they undergo dissolution, releasing As to the porewater. High Fe:S ratios limit the formation of arsenic sulfides in the anoxic zone. As a result of As sequestration at the sediment-water interface and its release upon burial, decreased concentrations of porewater As will not occur unless As-bearing erosional inputs are eliminated.     

Post-depositional behavior of Cu in a metal-mining polishing pond (East Lake, Canada)

The post-depositional behavior of Cu in a gold-mining polishing pond (East Lake, Canada) was assessed after mine closure by examination of porewater chemistry and mineralogy. The near-surface (upper 1.5 cm) sediments are enriched in Cu, with values ranging from 0.4 to 2 wt %. Mineralogical examination revealed that the bulk of the Cu inventory is present as authigenic copper sulfides. Optical microscopy, energy-dispersion spectra, and X-ray data indicate that the main Cu sulfide is covellite (CuS). The formation of authigenic Cu-S phases is supported by the porewater data, which demonstrate that the sediments are serving as a sink for dissolved Cu below sub-bottom depths of 1-2 cm. The zone of Cu removal is consistent with the occurrence of detectable sulfide and the consumption of sulfate. The sediments can be viewed as a passive bioreactorthat permanently removes Cu as insoluble copper sulfides. This process is not unlike that which occurs in other forms of bioremediation, such as wetlands and permeable reactive barriers. Above the zone of Cu removal, dissolved Cu maxima in the interfacial porewaters range from 150 to 450 microg L(-1) and reflect the dissolution of a Cu-bearing phase in the surface sediments. The reactive phase is thought to be a component of treatment sludges delivered to the lake as part of cyanide treatment. Flux calculations indicate that the efflux of dissolved Cu from the sediments to the water column (14-51 microg cm(-2) yr(-1)) can account for the elevated levels of dissolved Cu in lake waters (approximately 50 microg L(-1)). Implications for lake recovery are discussed.     

Cu and Zn concentration gradients created by dilution of pH neutral metal-spiked marine sediment: a comparison of sediment geochemistry with direct methods of metal addition

The geochemistry of artificially metal contaminated sediments prepared using three methods of metal-spiking was compared in this study. Marine sediments with a gradient of Cu and Zn concentrations were prepared by direct-spiking without and with pH-adjustment to pH 7, and also by serial dilution of direct-spiked sediment (4000 microg g(-1), pH 7 adjusted) with uncontaminated sediment. Porewater concentrations of Cu, Zn, Fe, and Mn in direct-spiked sediments without pH adjustment were orders of magnitude higher than the equivalent sediments adjusted to pH 7 or those prepared by the serial dilution method. Despite pH-adjustment, porewater Cu and Zn concentrations of direct-spiked sediment remained higher than concentrations observed within metal-contaminated natural sediment. The serial dilution of metal-spiked, pH-adjusted sediment substantially decreased Cu and Zn partitioning to the dissolved phase, and minimized the variation of potential competitive ions (H+, Fe2+, Mn2+) over the entire gradient of spiked Cu and Zn concentrations. Metal concentration gradients created using serial dilution of Cu- and Zn-spiked, pH-adjusted sediments produced porewater Cu or Zn, Fe, and Mn concentrations that resemble sediment-porewater partitioning (Cu, Zn, Fe, Mn) typical of metal-contaminated natural sediments. This method is recommended for whole-sediment toxicology studies.     

Iron and arsenic cycling in intertidal surface sediments during wetland remediation

The accumulation and behavior of arsenic at the redox interface of Fe-rich sediments is strongly influenced by Fe(III) precipitate mineralogy, As speciation, and pH. In this study, we examined the behavior of Fe and As during aeration of natural groundwater from the intertidal fringe of a wetland being remediated by tidal inundation. The groundwater was initially rich in Fe(2+) (32 mmol L(-1)) and As (1.81 μmol L(-1)) with a circum-neutral pH (6.05). We explore changes in the solid/solution partitioning, speciation and mineralogy of Fe and As during long-term continuous groundwater aeration using a combination of chemical extractions, SEM, XRD, and synchrotron XAS. Initial rapid Fe(2+) oxidation led to the formation of As(III)-bearing ferrihydrite and sorption of >95% of the As(aq) within the first 4 h of aeration. Ferrihydrite transformed to schwertmannite within 23 days, although sorbed/coprecipitated As(III) remained unoxidized during this period. Schwertmannite subsequently transformed to jarosite at low pH (2-3), accompanied by oxidation of remaining Fe(2+). This coincided with a repartitioning of some sorbed As back into the aqueous phase as well as oxidation of sorbed/coprecipitated As(III) to As(V). Fe(III) precipitates formed via groundwater aeration were highly prone to reductive dissolution, thereby posing a high risk of mobilizing sorbed/coprecipitated As during any future upward migration of redox boundaries. Longer-term investigations are warranted to examine the potential pathways and magnitude of arsenic mobilization into surface waters in tidally reflooded wetlands.     

Biogeochemical mechanisms of selenium exchange between water and sediments in two contrasting lentic environments

The biogeochemical mechanisms of Se exchange between water and sediments in two contrasting lentic environments were assessed through examination of Se speciation in the water column, porewater, and sediment. High-resolution (7 mm) vertical profiles of <0.45 μm Se species across the sediment-water interface demonstrate that the behavior of dissolved Se(VI), Se(IV), and organo-Se are closely linked to redox conditions as revealed by porewater profiles of redox-sensitive species (dissolved O2, NO3-, Fe, Mn, SO4(2-), and ΣH2S). At both sites Se(VI) is removed from solution in suboxic near-surface porewaters demonstrating that the sediments are serving as diffusive sinks for Se. X-ray absorption near edge spectroscopy (XANES) of sediments suggests that elemental Se and organo-Se represent the dominant sedimentary sinks for dissolved Se. Dissolved Se(IV) and organo-Se are released to porewaters in the near-surface sediments resulting in the diffusive transport of these species into the water column, where between-site differences in the depths of release can be linked to differences in redox zonation. The presence or absence of emergent vegetation is proposed to present a dominant control on sedimentary redox conditions as well as on the recycling and persistence of reduced Se species in bottom waters.     

Seasonal and interannual mobility of arsenic in a lake impacted by metal mining

Detailed examination of the water column, sediments, and interstitial waters was conducted in Balmer Lake, Ontario, Canada, in 1993-1994 and 1999 in order to assess the seasonal and interannual controls governing the behavior of As. High-resolution profiles of dissolved (<0.45 microm) Fe, Mn, SO4(2-), and sigmaH2S across the sediment-water interface indicate the presence of reducing conditions in close proximity to the benthic boundary during ice-free periods, which are characterized by fully oxygenated bottom waters. Dissolved As is remobilized as As(III) in suboxic sediment horizons via the redox-controlled dissolution of Fe (and perhaps Mn) oxide phases. During 1993-1994, As fluxes to the water column were relatively low (2-15 microg cm(-2) year(-1)) and contributed between 2 and 18% of the water column inventory. Dissolved As in the lake waters was derived primarily from external mining-related loadings during this period. Between 1993 and 1999, external loadings of As to Balmer Lake decreased while [As]aq within the lake increased, suggesting an increase in the proportion of sediment-derived As. Indeed, benthic dissolved As fluxes in 1999 ranged from 179 to 380 microg cm(-2) year(-1), representing approximately 33-60% of the water column burden. The relatively recent importance of sedimentary arsenic sources is suggested to reflect changes to sediment redox conditions associated with a postulated increase in lake primary productivity. Ironically, the increased contribution of dissolved arsenic to the water column appears to have resulted from an otherwise improvement in water quality. Reduced loadings of Cu, Zn, and Ni to the lake since 1994 appear to have allowed increased phytoplankton production that has stimulated arsenic release.     

Influence of arsenate adsorption to ferrihydrite, goethite, and boehmite on the kinetics of arsenate reduction by Shewanella putrefaciens strain CN-32

The kinetics of As(V) reduction by Shewanella putrefaciens strain CN-32 was investigated in suspensions of 0.2, 2, or 20 g L(-1) ferrihydrite, goethite, or boehmite at low As (10 μM) and lactate (25 μM) concentrations. Experimental data were compared with model predictions based on independently determined sorption isotherms and rates of As(V) desorption, As(III) adsorption, and microbial reduction of dissolved As(V), respectively. The low lactate concentration was chosen to prevent significant Fe(III) reduction, but still allowing complete As(V) reduction. Reduction of dissolved As(V) followed first-order kinetics with a 3 h half-life of As(V). Addition of mineral sorbents resulted in pronounced decreases in reduction rates (32-1540 h As(V) half-life). The magnitude of this effect increased with increasing sorbent concentration and sorption capacity (goethite < boehmite < ferrihydrite). The model consistently underestimated the concentrations of dissolved As(V) and the rates of microbial As(V) reduction after addition of S. putrefaciens (～5 × 10(9) cells mL(-1)), suggesting that attachment of S. putrefaciens cells to oxide mineral surfaces promoted As(V) desorption and thereby facilitated As(V) reduction. The interplay between As(V) sorption to mineral surfaces and bacterially induced desorption may thus be critical in controlling the kinetics of As reduction and release in reducing soils and sediments.     

Arsenite retention mechanisms within estuarine sediments of Pescadero, CA

Arsenic, a toxic metalloid, is commonly associated with sulfide minerals in anoxic sediments. Here we characterize arsenic(III) retention on sediments from a sulfidic estuarine marsh using a series of sorption experiments, and probe the structure of retained arsenite with X-ray absorption spectroscopy. Although the extent of sorption varied with sampling locations, several adsorption characteristics were apparent. A fraction of arsenite adsorbed over the entire pH range examined, although it was most extensive at pH greater than 7, and conformed to a Langmuir isotherm. Iron sulfide phases were responsible for As partitioning in these sediments. Initially, an FeAsS-like precipitate formed with a structure similar to those reported for As(III) sorbed on iron sulfides, a complex that is highly reactive. Following reaction for 21 d, much of the FeAsS-like precipitate was converted to As2S3. A drop in the redox potential accompanied this conversion, suggesting that the evolution of sulfide and other reduced species stabilizes bound arsenic. Processes discerned in this study reveal the importance of sulfide minerals in As sequestration within anoxic environments.     

Chronology of atmospheric deposition of arsenic inferred from reconstructed sedimentary records

Vertical distributions of As, Fe, and organic C were determined in dated (210Pb and 137Cs) sediment cores obtained from two adjacent basins (one perennially oxic and the other seasonally anoxic) of an oligotrophic headwater lake where atmospheric deposition is the only input of anthropogenic As. Despite similar sources in the two basins, the As profiles in the sediments differed markedly. Differences include the following: (i) As concentrations increased sharply upward close to the sediment surface in the perennially oxic basin due to scavenging of upward diffusing As by Fe oxyhydroxides, whereas they decreased in the seasonally anoxic basin where As scavenging by Fe oxyhydroxides did not occur, and (ii) the magnitude and position of major subsurface As maxima differed between the two basins. We applied a one-dimensional transport-reaction equation to porewater As concentration profiles obtained at the two sites to estimate sedimentary As concentrations at the time of deposition as well as subsequent addition or removal of As at various sediment depths. By multiplying As concentrations at the time of deposition by sediment mass accumulation rates, we were able to estimate variations in As fluxes at the sediment surface over the last two centuries. These fluxes were then transformed into atmospheric As deposition fluxes by applying a correction for basin-specific processes using the ratio of expected from atmospheric deposition to measured unsupported 210Pb inventories at the sampling sites. The resulting chronological profiles of atmospheric fluxes of As deposition are similar in both basins, and are consistent with both the history of specific markers for coal combustion and direct historical measurements of As in dry and wet atmospheric deposition in rural areas of North America. We conclude that the history of As inputs can be reconstructed from As sedimentary records using appropriate corrections for diagenesis and basin-specific processes.     

Spatial heterogeneity and kinetic regulation of arsenic dynamics in mangrove sediments: the sundarbans, bangladesh

The biogeochemistry of arsenic (As) in sediments is regulated by multiple factors such as particle size, dissolved organic matter (DOM), iron mobilization, and sediment binding characteristics, among others. Understanding the heterogeneity of factors affecting As deposition and the kinetics of mobilization, both horizontally and vertically, across sediment depositional environments was investigated in Sundarban mangrove ecosystems, Bengal Delta, Bangladesh. Sediment cores were collected from 3 different Sundarbans locations and As concentration down the profiles were found to be more associated with elevated Fe and Mn than with organic matter (OM). At one site chosen for field monitoring, sediment cores, pore and surface water, and in situ diffusive gradients in thin films (DGT) measurements (which were used to model As sediment pore-water concentrations and resupply from the solid phase) were sampled from four different subhabitats. Coarse-textured riverbank sediment porewaters were high in As, but with a limited resupply of As from the solid phase compared to fine-textured and high organic matter content forest floor sediments, where porewater As was low, but with much higher As resupply. Depositional environment (overbank verses forest floor) and biological activity (input of OM from forest biomass) considerably affected As dynamics over very short spatial distances in the mosaic of microhabitats that constitute a mangrove ecosystem.     

Removal of arsenic from contaminated soils by microbial reduction of arsenate and quinone

We investigated bioremediation of As-contaminated soils by reductive dissolution of As using a dissimilatory As(V)-reducing bacterium (DARB), Bacillus selenatarsenatis SF-1. We also examined the effect of anthraquinone-2,6-disulfonate (AQDS), an extracellular electron-shuttling quinone, on the As extraction. When B. selenatarsenatis was incubated with As(V)-laden Al precipitates, no acceleration of As dissolution was observed in the presence of AQDS, even though the microbial reduction of AQDS occurred actively. In contrast, AQDS addition significantly enhanced the reductive dissolution of As and Fe in analogous experiments with As(V)-laden Fe(III) precipitates, whereas As dissolution did not occur in the absence of the As(V) reducer. These results indicate the dissolution of As was accelerated by indirect reduction of solid-phase Fe(III) following microbial AQDS reduction, although As(V) reduction is vital for As extraction. B. selenatarsenatis was able to extract As from two types of industrially contaminated soils through reduction of solid-phase As(V) and Fe(III). The copresence of AQDS with B. selenatarsenatis improved the removal efficiency of As from the contaminated soils, concomitantly releasing Fe(II), suggesting that simultaneous use of DARB and electron-shuttling compounds can be an effective strategy for remediation of As-contaminated soils.     

Kinetics of reductive dissolution of hematite by bioreduced anthraquinone-2,6-disulfonate

The reductive dissolution of hematite (alpha-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone-2,6-disulfonate (AQDS), which is often used as a model electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow rate, pH, and Fe(II) and phosphate concentrations were varied to investigate the redox kinetics in a flow-through reactor. The hematite reduction rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and ADDS sorption on hematite surfaces were independently investigated for interpreting hematite reduction kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to alpha-Al2O3, a redox stable analog of alpha-Fe2O3. Decreasing Fe(ll) and increasing AH2DS sorption by controlling flow rate, influent pH, and Fe(II) and phosphate concentrations increased the rates of reductive dissolution. The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium. This study demonstrated the importance of the geochemical variables for the reductive dissolution kinetics of iron oxides.