Reductive Dissolution Of Iron Oxyhydroxides Research Articles

Geochemical behavior and fate of arsenic in middle Gangetic plain, Terai region of India, and its health risk quantification using Monte Carlo simulation and sensitivity analysis

The present study investigates the groundwater solute chemistry, hydrogeochemical behavior of arsenic (As) and the assessment of health risks through ingestion and dermal contact pathways to the adults and children of the Bahraich district, falling under the middle Gangetic plain, Ghaghara river sub-basin, Terai region of India. A Monte Carlo simulation and a sensitivity analysis were also performed to quantify the uncertainties and impact of various input variables in risk calculations, respectively. Concentrations major ions (Ca2+, Mg2+, Na+, K+, SO42−, PO43− NO3−, HCO3−, F− and Cl−), dissolved organic carbon (DOC), and trace metals (As, Fe, and Mn) as well as physical parameters (EC, pH, and Eh) were measured on the collected groundwater and river water samples. The concentration of As in the groundwater samples was found in range between 0.64 μg/L to 104 μg/L and ∼37% of the samples had As>10 μg/L (WHO and BIS acceptable limit of 10 μg/L for drinking water). River water samples also displayed high As concentrations (mean of 14 μg/L) with ∼73% of samples having As>10 μg/L, which could be attributed to infiltration from As-enriched sallow groundwater of the nearby regions. The groundwater of the study area was predominantly of Ca2+-Mg2+-HCO3- type. Bivariate weathering plots showed groundwater was influenced by silicate weathering and carbonate dissolution, along with the ion exchange and reverse ion exchange processes, with a minor contribution from evaporate dissolution. Most of the groundwater samples (n = 57) were anoxic and had low SO42− and NO3−, and high Mn, Fe and DOC concentrations. The observed values and correlations between various measured parameters, including groundwater As concentrations and saturation indices calculations, indicated the reductive dissolution of iron oxyhydroxide as the major process for As mobilization in the study area. The results of total non-carcinogenic risk (HI) estimated by the deterministic and probabilistic techniques were nearly identical for both adults and children, but an overestimation was observed in the case of carcinogenic risk calculated by the deterministic approach. Health risk results also showed that children were more susceptible to non-carcinogenic risk, whereas adults were at a higher risk of cancer in the study area. Sensitivity analysis indicated that the concentration of As in groundwater and exposure duration (ED) were the most effective variables for non-carcinogenic (HI) and carcinogenic (TILCR) risk estimation in both adults and children.

Geochemical, Topographical, and Meteorological Controls on Groundwater Arsenic Contamination in Sharda River Basin of Uttar Pradesh, India

Groundwater arsenic (As) contamination is a health threat for millions of people in the Gangetic plains of India. It is, therefore, critical to understand the mechanism of As enrichment to reduce the As exposure. Geochemical analysis of 30 groundwater samples collected across the banks of River Sharda was performed for the identification of major geochemical processes controlling groundwater geochemistry. Shallow wells (3-10 m) are found to be contaminated with As and is confined to newer alluvium of Holocene age. The average temporal decline of the groundwater level was observed across 10 monitoring stations is 0.067 m/yr. Decreasing rainfall, lesser recharge and huge groundwater extraction for irrigation might have impacted the groundwater to flow faster, while the increase in temperature and weathering regime favoured arsenic mobilisation. The long-term trends of rainfall show a decline of 1.97 mm/yr and the temperature increase is observed to be 0.0049°C/yr. These changes in rainfall and temperature also might have impacted the As mobilisation in groundwater. NO3- was found to be low in samples with low As concentrations, indicating the prevalence of reducing conditions. Whereas high concentrations of Fe were observed for high As samples, indicating their common source of origin. Also, the alkaline nature of aquifer and high concentrations of HCO3- might have contributed to As enrichment. Results from scatter plots and correlation matrix also support this sequential reduction leading to the reductive dissolution of iron oxyhydroxides and thus enriching the concentration of As in the groundwater.

Hydrochemistry and Arsenic Contamination of Shallow Aquifers in Bidyananda and Nazimkhan Unions, Rajarhat Upazilla, Kurigram, Bangladesh

ABSTRACT In this study, the spatial distribution and hydrochemistry of arsenic (As) in shallow alluvial aquifers of Bidyananda and Nazimkhan Union of Rajarhat opazilla, Kurigram district of Bangladesh have been evaluated. Nineteen groundwater samples (depth: 10 to 50 m) were collected from the studied area for physical and chemical analysis. Based on analytical data of chemical analysis, a Piper trilinear diagram have been prepared which revealed that the groundwater of the studied area is Ca-HCO3 type, where Ca is the dominant metal. Physical data and permeability index values indicate good quality of the groundwater for drinking and irrigation. The concentrations of Fe, Mn and As in almost all samples exceed the WHO and Bangladesh national standard indicating significant pollutions. Groundwater As concentrations in the aquifer (10–50 m of depth) ranged from <0.005 to 0.323 mgL-1 and geochemical conditions changed from oxidized to successively more reduced, higher As concentrations with depth. Correlation studies revealed a good correlation between As and Mn (R2=0.20) in groundwater implying that natural reductive dissolution of MnO(OH) and FeO(OH) may be responsible for As release into the groundwater. However, a poor correlation between As and Fe (R2 = 0.0084) in groundwater is probably due to the precipitation of dissolved Fe and Mn as siderite (FeCO3) and rhodochrosite (MnCO3), respectively, under reducing conditions. The poor correlations among dissolved As, Fe and SO42- concentrations (As vs. SO42- R2 = 0.0674; Fe vs. SO42-, R2=0.1542) in groundwater suggest that pyrite/sulfide oxidation is not the dominant process for As release. A high concentration of As, Fe and Mn indicate that the natural reductive dissolution of iron oxyhydroxides is probably the dominant process for As mobilization in the groundwater of the studied area.

GIS mapping-based impact assessment of groundwater contamination by arsenic and other heavy metal contaminants in the Brahmaputra River valley: A water quality assessment study

Groundwater aquifers serve as the primary source of potable water for the vast majority of the population in the Brahmaputra River valley. However, these groundwater reserves form a substantial part of a rapidly weathering alluvial drainage resulting in major contamination by Arsenic (As) and associated heavy metals thereby raising key concern on their safety and sustained usage. The current study entails a comprehensive qualitative analysis of the potable groundwater reserves using a combination of geographical information system (GIS) and geochemical approaches to determine the severity of groundwater contamination by As and other heavy metals in a total of twenty administrative districts of Assam, India. Furthermore, the groundwater solute chemistry and water quality status in the 20 districts of the study area, which encompasses the Brahmaputra River valley, was also assessed to evaluate the sustainability of these potable groundwater reserves. The findings show that the groundwater aquifers present in these districts of the Brahmaputra River valley in Assam are severely contaminated with As with the highest concentration at ∼352.56 ppb being observed in the district of Nagaon. The districts of Tinsukia, Dibrugarh, Sivasagar, Dhemaji, Lakhimpur, Jorhat, Golaghat, Sonitpur, Nagaon, Morigaon, Darrang, Udalguri, Kamrup, Nalbari, and Baksa also showed the presence of elevated levels of dissolved As which on an average exceeded the WHO permissible limit of 10 parts per billion as well as the Bureau of Indian Standards limit of 50 parts per billion. Moreover, notable contamination by other heavy metals including lead, manganese, iron, and zinc exceeding the WHO and BIS prescribed limits were prominent. Also, the groundwater was found to harbour elevated concentrations of ions such as HCO3−, Ca2+ and Mg2+ which impart severe restrictions on to the safety, aesthetics and palatability of these potable water reserves. Furthermore, the dissolved As content was found to exhibit a significant correlation to the elemental contaminants such as iron and manganese suggesting that the reductive dissolution of iron oxyhydroxides may serve as the underlying mechanism for the As enrichment of these groundwater reserves. Additionally, the impact assessment study through the spatial distribution of the groundwater quality index (GWQI) of the areas studied showed that the groundwater reserves in the majority of the administrative districts are unfit for human consumption, without the intervention through requisite water treatment and/or sustainable remediation approaches. These findings indicate the need for serious introspection and reconsiderations on the part of the stakeholders for the sustainable use of these groundwater resources for anthropogenic activities.

Reactive transport model for predicting arsenic transport in groundwater system in Datong Basin

High As concentration in groundwater of Datong Basin has emerged as an issue of great concern in the past decade because of its serious impact on the health of many people. The present study is to our knowledge the first attempt model the transport of arsenic in this system using a 1D reactive transport model (PHREEQC). Based on published field data from Shanyin Field Site (Datong Basin), the importance of the (bio)geochemical processes for arsenic mobility is evaluated quantitatively using geochemical models of growing complexity. Arsenic mobility during the surface water and groundwater interactions can be explained by the reductive dissolution of iron oxyhydroxides, sulfate, nitrate and arsenate, and by surface sorption/de-sorption occurring in response to changes in geochemical conditions. Both the Fe(III) surface complexation model and the 1D reactive transport model were employed to depict the arsenic transport in shallow groundwater system during the surface water-groundwater interactions. The surface complexation model results showed that a total of 0.34 mmol and 0.56 mmol arsenic were sorbed onto Fe(III) oxyhydroxides for Well 1-2S and Well 2-2S, respectively, reflecting the important role of surface sorption process on arsenic transport. The 1D model results revealed that redox processes were also critical. The improved calibration of model simulations involving both of these components indicates that the combination of processes is critical to understanding and characterising the transport of arsenic in groundwater during groundwater surface water interaction, especially close to the surface water body.

THE INFLUENCE OF REDUCTIVE DISSOLUTION OF IRON OXIDES BY S(-II) ON URANIUM MOBILITY

This study investigates possible redox transformations of uranium under Ntransient redox conditions. NSpecific focus lies on the fate of U as reductive dissolution of iron oxyhydroxides by S(-II) is initiated. In batch experiments sulfide was incrementally added to a lepidocrocite suspension containing adsorbed U(VI). The partitioning of uranium was monitored during the progressing transformation of lepidocrocite into FeS. Synchrotron-based X-ray absorption spectroscopy was used to resolve the oxidation state of uranium.Upon addition of sulfide intermediate release of U from the solid to the solution was observed. The mobilization of U was followed by immobilization in later stages. XAS reveals that this immobilization coincides with reduction of NU(VI) to U(IV). Consequently, reduction of U(VI) and precipitation of U(IV) solids, due to a shift from oxic to sulfate reducing conditions is possible. NHowever, kinetic effects might lead to an intermediate mobilization of U that should be considered for the risk assessment of nuclear waste repositories and the remediation of sites, contaminated with radionuclides.

Organic and inorganic colloids impacting total iodine behavior in groundwater from the Datong Basin, China

The geochemical behaviors of colloids in aquifers played an important role in determining the fate of iodine in groundwater system. To decipher the impact of colloids on iodine mobilization in aquifers, three successive pore-sized colloids filtration (0.45μm, 30kDa and 5kDa) were conducted on in-situ during groundwater sampling. The results showed that the distribution ratios (f) of total iodine (IT) and iron in the dissolved solution (i.e., 5kDa ultrafiltered) were from 0.78 to 0.99 and from 0.56 to 0.94, respectively. Natural organic matter (NOM) in the colloidal fractions obtained the f values ranging from 0.14 to 1.00. The decreased Eh values from recharge area to discharge area indicated redox potential of groundwater system changed from oxidizing to reducing along groundwater flowpath, and interestingly, the corresponding iodine fractions in groundwater were decreasing in dissolved solution and increasing in colloidal fractions. Inverse correlation between Fe and DOC and positive correlation between iodine and DOC suggested the occurrence of reductive dissolution of iron oxyhydroxides and degradation of organic iodine in groundwater system. Iodine distribution increased in dissolved solution and decreased in colloids with pH increase. Moreover, as pH increased, f (Fe) and f (DOC) decreased in dissolved solution and increased in colloids. Relatively weak correlation of f (IT) with f (Fe) and strong relationship between f (IT) and f (DOC) in the large grain size fractions suggested the Fe-OM complexes controlled iodine distribution in groundwater, which depends on the presence of Fe bridges. Negligible association of iodine with Fe and NOM in the small grain size fractions might be attributed to the effects of abundant OH− content in groundwater.

On the relation between fluvio-deltaic flood basin geomorphology and the wide-spread occurrence of arsenic pollution in shallow aquifers

Pollution of groundwater with natural (geogenic) arsenic occurs on an enormous, world-wide scale, and causes wide-spread, serious health risks for an estimated more than hundred million people who depend on the use of shallow aquifers for drinking and irrigation water. A literature review of key studies on arsenic concentration levels yields that Holocene fluvial and deltaic flood basins are the hotspots of arsenic pollution, and that the dominant geomorphological setting of the arsenic-polluted areas consists of shallow-depth meandering-river deposits with sand-prone fluvial point-bar deposits surrounded by clay-filled (clay plug) abandoned meander bends (oxbow lakes). Analysis of the lithofacies distribution and related permeability contrasts of the geomorphological elements in two cored wells in a point bar and adjacent clay plug along the Ganges River, in combination with data of arsenic concentrations and organic matter content reveals that the low-permeable clay-plug deposits have a high organic matter content and the adjacent permeable point-bar sands show high but spatially very variable arsenic concentrations. On the basis of the geomorphological juxtaposition, the analysis of fluvial depositional processes and lithofacies characteristics, inherent permeability distribution and the omnipresence of the two geomorphological elements in Holocene flood basins around the world, a generic model is presented for the wide-spread arsenic occurrence. The anoxic deeper part (hypolimnion) of the oxbow lake, and the clay plugs are identified as the loci of reactive organic carbon and microbial respiration in an anoxic environment that triggers the reductive dissolution of iron oxy-hydroxides and the release of arsenic on the scale of entire fluvial floodplains and deltaic basins. The adjacent permeable point-bar sands are identified as the effective trap for the dissolved arsenic, and the internal permeability heterogeneity is the cause for aquifer compartmentalization, with large arsenic concentration differences between neighboring compartments.

Evidence of decoupling between arsenic and phosphate in shallow groundwater of Bangladesh and potential implications

Reductive dissolution of iron oxyhydroxides and reduction of arsenic are often invoked as leading causes of high dissolved As levels in shallow groundwater of Bangladesh. The second of these assumptions is questioned here by comparing the behavior As and phosphate (P), a structural analog for As (V) which also adsorbs strongly to Fe oxyhydroxides but is not subject to reduction. The first line of evidence is provided by a detailed groundwater time-series spanning two years for three wells in the 6–9 m depth range showing removal of As(III) from shallow groundwater during the monsoon without of loss of P. The data indicate a loss of ∼90% of the dissolved As from groundwater in the intermediate well relative to a level of 3 μmol/L As predicted by conservative mixing between groundwater sampled from the shallower and the deeper well. In contrast, P concentrations of ∼30 μmol/L in the intermediate well closely match the prediction from conservative mixing. Reduction therefore appears to inhibit the release of As to groundwater at this site relative to P instead of enhancing it. A re-analysis of existing groundwater As and P data from across the country provides a broader context for this finding and confirms that, without reduction, elevated concentrations of As would probably be even more widespread in shallow aquifers of Bangladesh. Without providing definite proof, X-ray absorption spectroscopy of sediment from the time-series site and elsewhere suggests that the loss of As from groundwater may be coupled to precipitation of As sulfide. Further study is needed to assess the implications of these observations for shallow aquifers that have been subjected to increased withdrawals for irrigation in recent decades.

Temporal variation of groundwater level and arsenic concentration at Jianghan Plain, central China

Elevated arsenic in groundwater affects some 60 thousand people in the Jianghan Plain, yet mitigation efforts are hindered by persistent uncertainty about the proximal source of As and mechanisms for its mobilization. Samples were collected every month from May 2012 to April 2014 in a set of 39 monitoring wells and two rivers in the field monitoring site, Jianghan Plain, to evaluate the temporal variation of groundwater level and chemical composition for As and other constituents. Results showed that groundwater tables fluctuated during the sampling period, with higher water levels during the rainy season and lower water levels during the dry season. Wells at 25m and 50m deep had lower water level than those of the wells at 10m deep. Water levels of the wells located in the south of the field monitoring site were always 1m higher than the wells in the north. Only five wells showed no significant changes in As concentrations during the monitoring period; 23 wells exhibited seasonal variations in groundwater As concentration that were consistent from year to year; the rest of the 11 wells not only showed seasonal variations but also exhibited increasing trends over the monitoring period. Seasonal As concentration changes in groundwater were positively correlated to groundwater level changes, with lower concentration corresponding to lower water level during dry season and vice versa during rainy season. The rise in As concentration during the rainy season could be attributed to enhanced reductive dissolution of iron oxyhydroxides and/or reductive desorption of As(V) as the conditions turn to be more reducing, while during the dry season more As could be scavenged onto fresh iron oxyhydroxides. Meanwhile, temporal variations showed compatible trends between As and Fe, Fe(II), S2−, indicating that As variation would be related to Fe and S cycling in the aquifer systems. Additionally, the availability of labile organic carbon as a driver of microbial reduction may play an important role in controlling the spatial and temporal variation of groundwater As concentration in the Jianghan Plain.

Iodine mobilization in groundwater system at Datong basin, China: Evidence from hydrochemistry and fluorescence characteristics

Characterizing the speciation of iodine in groundwater is essential for understanding its hydrogeochemical behavior in aquifer systems. To quantify the variations in iodine speciation and assess factors controlling the distribution and transformation of iodine, 82 groundwater samples and 1 rain water were collected from the Datong basin, northern China in this study. Factor analysis (FA) and excitation emission matrix with parallel factor analysis (EEM–PARAFAC) were used to clarify the potential relationships among iodine species and other hydrochemical parameters. The iodine concentrations of groundwater range from 6.23 to 1380μgL−1 with 47% of samples exceeding its drinking water level of 150μgL−1 as recommended by the Chinese government. 57% of samples have ratios of iodate to total iodine greater than 60%, while iodide as the major species in 22% of the samples. Significant amounts of organic iodine with concentrations higher than 100μgL−1 were observed in 9 groundwater samples. Redox conditions of groundwater system strongly affect iodine concentration and speciation of inorganic iodine in groundwater, and extremely reducing condition restricts the iodine release from sediments into groundwater. The results of FA show that iodine mobilization in groundwater is related to the nature of dissolved organic matter. EEM-PARAFAC model demonstrates the dominance of terrestrial DOM sources and the presence of microbial activities in groundwater system of the Datong basin. It is proposed that degradation of organic matter and reductive dissolution of iron oxyhydroxides are major hydrogeochemical processes responsible for the mobilization of iodine release and the genesis of organic iodine.

Is organic matter a source or redox driver or both for arsenic release in groundwater?

Arsenic (As) contamination in groundwater is a global public health issue, because groundwater is the main source of drinking water worldwide. Arsenic contamination in groundwater is directly or indirectly associated with organic matter (OM). Therefore, this study investigated the role of OM in mobilization of As in the subterranean aquifers. The reduced concentrations of terminal electron acceptors (dissolved O2, NO3-, and SO42-), enhanced concentrations of dissolved organic carbon (DOC), HCO3- and NH4+ ions, and strong correlations between NH4+ and DOC indicate the reducing geochemical environment of aquifer in Bangladesh, where DOC might have originated from microbial oxidation of sedimentary OM, humic substances, surface derived terrestrial OM and petroleum. Groundwater samples exhibit variations in the concentrations of DOC (0.20–5.09mg/l), fluorescence intensity and As depending on the depth and locations of the sampling site suggesting that the aquifers have different characteristics and different sources of DOC. The DOC concentrations in the upper aquifer (up to 41m) are higher than those in the middle and lower aquifers (>55m) suggesting that the recent inflow of surface derived terrestrial OM with recharge water and petroleum in the shallow parts of the aquifer promoted the peak of high dissolved As, where older water mixes with recent recharge water containing organic carbon. Hydrogeochemical data and sediment geochemistry indicates that As mobilization from the sediment occurs through the microbial-mediated reductive dissolution of iron oxyhydroxide, where OM is the main redox driver facilitating As release into groundwater. Organic matter can strongly influence the solubility and mobility of As mainly through redox reactions, competitive adsorption, desorption and complexation reactions. Furthermore, based on the results of sediment and groundwater geochemistry, and As speciation in plant tissue, it can be concluded that OM is not only a redox driver, but also one of the sources of As in groundwater.

Hydrogeochemistry of high iodine groundwater: a case study at the Datong Basin, northern China

High iodine concentrations in groundwater have seldom been reported and there have been few systematic studies on high iodine groundwater worldwide. To better understand the sources and processes responsible for iodine enrichment in the groundwater of the Datong Basin, the hydrochemical characteristics of groundwater and geochemical features of aquifer sediments were studied. High iodine groundwater mainly occurs in the center of the Datong Basin with iodine concentrations ranging between 3.31 and 1890 μg L(-1). Most samples with iodine concentrations higher than 500 μg L(-1) are from wells with depths between 75 and 120 m. High pH and a reducing environment are favorable for iodine enrichment in the groundwater, with iodide as the dominant species that accounts for 63.2-99.3% of the total iodine. Sediment samples from a borehole specifically drilled for this study contain 0.18-1.46 mg kg(-1) iodine that is moderately correlated with total organic carbon (TOC). The results of sequential extraction experiments show that iodine is mostly bound to iron oxyhydroxides and organic matter in the sediments. The mobilization processes of iodine are proposed to include reductive dissolution of iron oxyhydroxides and transformations among iodide, iodate and organic iodine driven by microbial activities under alkaline and reducing conditions.

Evaluation of Groundwater for Arsenic Contamination Using Hydrogeochemical Properties and Multivariate Statistical Methods in Saudi Arabia

The aim of this research is to evaluate arsenic distribution and associated hydrogeochemical parameters in 27 randomly selected boreholes representing aquifers in the Al‐Kharj geothermal fields of Saudi Arabia. Arsenic was detected at all sites, with 92.5% of boreholes yielding concentrations above the WHO permissible limit of 10 μg/L. The maximum concentration recorded was 122 μg/L (SD = 29 μg/L skewness = 1.87). The groundwater types were mainly Ca+2‐Mg+2‐‐Cl− and Na+‐Cl−‐, accounting for 67% of the total composition. Principal component analysis (PCA) showed that the main source of arsenic release was geothermal in nature and was linked to processes similar to those involved in the release of boron. The PCA yielded five components, which accounted for 44.1%, 17.0%, 10.1%, 08.4%, and 06.5% of the total variance. The first component had positive loadings for arsenic and boron along with other hydrogeochemical parameters, indicating the primary sources of As mobilization are derived from regional geothermal systems and weathering of minerals. The remaining principal components indicated reductive dissolution of iron oxyhydroxides as a possible mechanism. Spatial evaluation of the PCA results indicated that this secondary mechanism of arsenic mobilization may be active and correlates positively with total organic carbon. The aquifers were found to be contaminated to a high degree with organic carbon ranging from 0.57 mg/L to 21.42 mg/L and showed high concentrations of ranging from 8.05 mg/L to 248.2 mg/L.

Occurrence and geochemical behavior of arsenic in a coastal aquifer–aquitard system of the Pearl River Delta, China

Elevated concentrations of arsenic, up to 161μg/L, have been identified in groundwater samples from the confined basal aquifer underlying the aquitard of the Pearl River Delta (PRD). Both aquatic arsenic in pore water and solid arsenic in the sediments in the basal aquifer and aquitard were identified. Arsenic speciation of groundwater in the basal aquifer was elucidated on a pH-Eh diagram. In the PRD, arsenic is enriched in groundwater having both low and high salinity, and arsenic enriched groundwater is devoid of dissolved oxygen, has negative Eh values, is slightly alkaline, and has abnormally high concentrations of ammonium and dissolved organic carbon, but low concentrations of nitrate and nitrite. Results of geochemical and hydrochemical analyses and sequential extraction analysis suggest that reductive dissolution of iron oxyhydroxide could be one of the important processes that mobilized solid arsenic. We speculate that mineralization of sedimentary organic matter could also contribute to aquatic arsenic. Scanning electron microscope analysis confirms that abundant authigenic pyrite is present in the sediments. Sulphate derived from paleo-seawater served as the important sulfur source for authigenic pyrite formation. Co-precipitation of arsenic with authigenic pyrite significantly controlled concentrations of aquatic arsenic in the coastal aquifer–aquitard system.

Kinetics of Fe(II)-Catalyzed Transformation of 6-line Ferrihydrite under Anaerobic Flow Conditions

The readsorption of ferrous ions produced by the abiotic and microbially mediated reductive dissolution of iron oxy-hydroxides drives a series of transformations of the host minerals. To further understand the mechanisms by which these transformations occur and their kinetics within a microporous flow environment, flow-through experiments were conducted in which capillary tubes packed with ferrihydrite-coated glass spheres were injected with inorganic Fe(II) solutions under circumneutral pH conditions at 25 degrees C. Synchrotron X-ray diffraction was used to identify the secondary phase(s) formed and to provide data for quantitative kinetic analysis. At concentrations at and above 1.8 mM Fe(II) in the injection solution, magnetite was the only secondary phase formed (no intermediates were detected), with complete transformation following a nonlinear rate law requiring 28 and 150 h of reaction at 18 and 1.8 mM Fe(II), respectively. However, when the injection solution consisted of 0.36 mM Fe(II), goethite was the predominant reaction product and formed much more slowly according to a linear rate law, while only minor magnetite was formed. When the rates are normalized based on the time to react half of the ferrihydrite on a reduced time plot, it is apparent that the 1.8 mM and 18 mM input Fe(II) experiments can be described by the same reaction mechanism, while the 0.36 input Fe(II) experiment is distinct. The analysis of the transformation kinetics suggests that the transformations involved an electron transfer reaction between the aqueous as well as sorbed Fe(II) and ferrihydrite acting as a semiconductor, rather than a simple dissolution and recrystallization mechanism. A transformation mechanism involving sorbed inner sphere Fe(II) alone is not supported, since the essentially equal coverage of sorption sites in the 18 mM and 1.8 mM Fe(II) injections cannot explain the difference in the transformation rates observed.

Geochemical controls of elevated arsenic concentrations in groundwater, Ester Dome, Fairbanks district, Alaska

Ester Dome, an upland area near Fairbanks, Alaska, was chosen for a detailed hydrogeochemical study because of the previously reported elevated arsenic in groundwater, and the presence of a large set of wells amenable to detailed sampling. Ester Dome lies within the Fairbanks mining district, where gold-bearing quartz veins, typically containing 2–3 vol.% sulfide minerals (arsenopyrite, stibnite, and pyrite), have been mined both underground and in open cuts. Gold-bearing veins on Ester Dome occur in shear zones and the sulfide minerals in these veins have been crushed to fine-grained material by syn- or post-mineralization movement. Groundwater at Ester Dome is circumneutral, Ca–HCO 3 to Ca–SO 4 type, and ranges from dilute (specific conductance of 48 µS/cm) to more concentrated (specific conductance as high as 2070 µS/cm). In general, solute concentrations increase down hydrologic gradient. Redox species indicate that the groundwaters range from oxic to sub-oxic (low dissolved oxygen, Fe(III) reduction, no SO 4 reduction). Waters with the highest Fe concentrations, as high as 10.7 mg/L, are the most anoxic. Dissolved As concentrations range from < 1 to 1160 µg/L, with a median value of 146 µg/L. Arsenic concentrations are not correlated with specific conductance or Fe concentrations, suggesting that neither groundwater residence time, nor reductive dissolution of iron oxyhydroxides, control the arsenic chemistry. Furthermore, As concentrations do not covary with other constituents that form anions and oxyanions in solution (e.g., HCO 3, Mo, F, or U) such that desorption of arsenic from clays or oxides also does not control arsenic mobility. Oxidation of arsenopyrite and dissolution of scorodite, in the near-surface environment appears to be the primary control of dissolved As in this upland area. More specifically, the elevated As concentrations are spatially associated with sulfidized shear zones and localities of gold-bearing quartz veins. Consistent with this interpretation, elevated dissolved Sb concentrations (as high as 59 µg/L), also correlated with occurrences of hypogene sulfide minerals, were measured in samples with high dissolved As concentrations.

Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers

Non-conservative behavior of dissolved inorganic phosphate (DIP) in estuaries is generally ascribed to desorption from iron and aluminum (hydr)oxides with increasing salinity. Here, we assess this hypothesis by simulating the reversible adsorption of phosphate onto a model oxide (goethite) along physico-chemical gradients representative of surface and subsurface estuaries. The simulations are carried out using a surface complexation model (SCM), which represents the main aqueous speciation and adsorption reactions of DIP, plus the ionic strength-dependent coulombic interactions in solution and at the mineral–solution interface. According to the model calculations, variations in pH and salinity alone are unlikely to explain the often reported production of DIP in surface estuaries. In particular, increased aqueous complexation of phosphate by Mg 2+ and Ca 2+ ions with increasing salinity is offset by the formation of ternary Mg-phosphate surface complexes and the drop in electrical potential at the mineral–water interface. However, when taking into account the downstream decrease in the abundance of sorption sites, the model correctly simulates the observed release of DIP in the Scheldt estuary. The sharp increase in pH accompanying the admixing of seawater to fresh groundwater should also cause desorption of phosphate from iron oxyhydroxides during seawater intrusion in coastal aquifers. As for surface estuaries, the model calculations indicate that significant DIP release additionally requires a reduction in the phosphate sorption site density. In anoxic aquifers, this can result from the supply of seawater sulfate and the subsequent reductive dissolution of iron oxyhydroxides coupled to microbial sulfate reduction.

Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide

Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate- and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the reactivity of Hg, including the propensity of inorganic Hg to be methylated, are poorly understood. Under abiotic conditions, using a laboratory flow reactor, bisulfide (HS −) was added at 40 to 250 μM h −1 to 5 g L −1 goethite (α-FeOOH) suspensions to which Hg(II) was adsorbed (30–100 nmol m −2) at pH 7.5. Dissolved Hg initially decreased from 10 3 or 10 4 nM (depending on initial conditions) to 10 −1 nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (β-HgS (s)) formed, based on identification using Hg L III-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. The apparent coordination of oxygens surrounding Hg(II), measured with EXAFS spectroscopy, increased during one flow experiment, suggesting desorption of monodentate-bound Hg(II) while bidentate-bound Hg(II) persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 nM or 30 nM), suggesting that byproducts of bisulfide oxidation and Fe(III) reduction, primarily polysulfide and potentially Fe(II), enhanced the dissolution of β-HgS (s) and/or desorption of Hg(II). Rapid accumulation of Fe(II) in the solid phase (up to 40 μmol g −1) coincided with faster elevation of dissolved Hg concentrations. Fe(II) served as a proxy for elemental sulfur [S(0)], as S(0) was the dominant bisulfide oxidation product coupled to Fe(III) reduction, based on sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. In one experiment, dissolved Hg concentrations tracked those of all sulfide species [S(-II)]. These results suggest that S(-II) reacted with S(0) to form polysulfide, which then caused the dissolution of β-HgS (s). A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy, although there was no clear evidence that either surface-bound or mineralized Fe(II) strongly affected Hg speciation. Examination of interrelated processes involving S(-II) and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg reactivity models.

Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide

Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide