Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered mackinawite

Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The K_d (L g⁻¹) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility.     

Arsenite adsorption on galena (PbS) and sphalerite (ZnS)

Arsenite, As(III), sorption on galena (PbS) and sphalerite (ZnS) was investigated as a function of solution composition and characterized using X-ray absorption spectroscopy (XAS). Adsorption conformed to a Langmuir isotherm except at the highest surface loadings, and it was not strongly affected by changes in ionic strength. Arsenite sorbed appreciably only at pH > ∼5 for PbS and pH ∼4.5 for ZnS, behavior distinct from its adsorption on other substrates. Arsenite adsorption on PbS and ZnS resulted in the conversion from As-O to As-S coordination. Arsenite does not adsorb through ligand-exchange of surface hydroxyl or sulfhydryl groups. Rather, it forms a polynuclear arsenic sulfide complex on ZnS and PbS consistent with the As₃S₃(SH)₃ trimer postulated by Helz et al. (1995) for sulfidic solutions. This complex was unstable in the presence of oxidizing agents and synchrotron light—it quickly converted to As(V), which was largely retained by the surface. These data illustrate the complexity of As(III) adsorption to even simple sulfide minerals.     

Biogeochemical redox cycling of arsenic in mine-impacted lake sediments and co-existing pore waters near Giant Mine,Yellowknife Bay,Canada

Lacustrine sediments, submerged tailings, and their pore waters have been collected at several sites in Yellowknife Bay, Great Slave Lake, Canada, in order to investigate the biogeochemical controls on the remobilization of As from mining-impacted materials under different depositional conditions. Radiometric dating confirms that a mid-core enrichment of Pb, Zn, Cu and Sb corresponds to the opening of a large Au mine 60 a ago. This was evident even in a relatively remote site. Arsenic was enriched at mid-core, coincident with mining activity, but clearly exhibited post-depositional mobility, migrating upwards towards the sediment water interface (SWI) as well as down-core. Deep-water (15 m) Yellowknife Bay sediments that contain buried mine waste are suboxic, relatively organic-rich and abundant in microbes with As in pore waters and sediments reaching 585 μg/L and 1310 mg/kg, respectively. Late summer pore waters show equal proportions of As(III) and As(V) (16–415 μg/L) whereas late winter pore waters are dominated by As(III) (284–947 μg/L). This can be explained by As(III) desorption mechanisms associated with the conversion of FeS to FeS₂ and the reduction of As(V) to As(III) through the oxidation of dissolved sulfide, both microbially-mediated processes. Processes affecting As cycling involve the attenuating efficiency of the oxic zone at the SWI, sediment redox heterogeneity and the reductive dissolution of Fe(hydr)oxides by labile organic matter, temporarily and spatially variable.     

Changes in the compound classes of dissolved organic matter along an estuarine transect: A study using FTIR and C NMR

In this work, we use Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (¹³C NMR) data to quantify the changes of major chemical compound classes (carboxylic acid, amide, ester, aliphatic, aromatic and carbohydrate) in high molecular weight (HMW, >1 kDa) dissolved organic matter (DOM) isolated along a transect through the Elizabeth River/Chesapeake Bay system to the coastal Atlantic Ocean off Virginia, USA. Results show that carboxylic acids and aromatic compounds are lost along the transect, while HMW DOC becomes enriched in carbohydrate moieties that could have a mid-transect source, perhaps the intensive red tide bloom (Choclodinium polykrikoides) which occurred during our sampling period. Taking the second derivative of the FTIR spectra resolved three pools of de-protonated carboxylic acids at our Dismal Swamp site (used to represent terrestrial organic matter in this area): one carboxylic acid pool, complexed with iron, seems to be lost between the Dismal Swamp and river sites; the second appears biogeochemically active throughout the riverine transect, disappearing in the coastal ocean sample; the third seems refractory, with the potential to be transported to and to accumulate within the open ocean. Five-member ring esters (γ-lactones) were the major ester form in the Dismal Swamp; aliphatic and acetate esters were the dominant esters in the estuary/marine DOM. No amide groups were detectable in Dismal Swamp DOM; secondary amides were present at the estuarine/marine sites. Coupling FTIR with ¹³C NMR provides new insights into the biogeochemical roles of carboxylic acid, amide and ester compounds in aquatic ecosystems.     

Arsenic in mine waters: an international study

 Hydrochemical data are presented for arsenic (As) in the mine waters of 34 gold and base-metal mining localities in seven countries of south-east Asia, Africa and Latin America, encompassing contrasting climatic settings and at least eight discrete styles of primary mineralization. Peak dissolved As concentrations at these sites range from 0.005–72 mg/l, with the United States Environmental Protection Agency (US-EPA) potable water threshold of 50 μg/l exceeded in 25 cases. Arsenate (As⁵⁺) constitutes the dominant species at over 80% of sites. Very high dissolved As concentrations (>1 mg/l) show no systematic between-site relationship with mine water pH/Eh regime. Important determinants of mine water As fluxes include iron hydrochemistry, the presence of cyanic ore processing effluents, site geology, the paragenetic sequence that follows sulphide oxidation, climate and mine management. Human toxicological impacts of As contamination have been recognized at only one case-study site, with a further four considered to warrant more detailed risk assessment. Received: 2 November 1999 · Accepted: 21 March 2000     

Vertical distribution of As(III) and As(V) in a coastal sandy aquifer: factors controlling the concentration and speciation of arsenic in the Stuarts Point groundwater system,northern New South Wales,Australia

Arsenic species were measured in a bundled-piezometer installed in the Holocene barrier of the Stuarts Point coastal sands aquifer, northern New South Wales, Australia. Vertical distribution shows two peaks of elevated As concentration. At a depth of 10–11 m, concentrations of As^Tot, As(V) and As(III) are in the range of 52–85, 38–67 and 14–18 μg/l respectively and the ratio of As(V)/As(III) is well above 1 at 3.7–2.7. The second peak, at a depth of 25 m, shows the highest concentrations of As^Tot, As(V) and As(III) with values reaching 337, 125 and 212 μg/l, respectively. The As(V)/As(III) ratio is below 1 at 0.6–0.7. High As^Tot and As(V) concentrations at shallower depths are associated with acidic conditions and very low concentrations of all ions. Desorption of As from Al-hydroxides and As-enriched Fe-oxyhydroxides are plausible mechanisms releasing As into the groundwater system. The elevated concentration of As^Tot and As(III) at 25 m is potentially related to the leaching of the clay surfaces. Elevated HCO₃^- and alkaline pH conditions at this depth cause desorption of As which is later present as As(III) species in the reducing environment. The high concentrations of HCO₃^- further reduce the possible extent of As sorption on Fe and Mn oxyhydroxides. The identification of As in a groundwater system associated with the coastal barrier sand-dune environment raises serious questions of the suitability of human consumption of untreated groundwater, drawn from these aquifer types. Further investigation both in Australia and globally are needed to classified the extent of this hydrogeochemical occurrence near coastal communities that rely on groundwater.     

Environmental geochemistry of high arsenic groundwater at western Hetao plain, Inner Mongolia

Environmental geochemistry of high arsenic groundwater at Hetao plain was studied on the basis of geochemical survey of the groundwater and a core sediment. Arsenic concentration in groundwater samples varies from 76 to 1093 μg/L. The high arsenic groundwater mostly appears to be weakly alkaline. The concentrations of NO₃⁻ and SO₄²⁻ are relatively low, while the concentrations of DOC, NH₄⁺, dissolved Fe and sulfide are relatively great. Analysis of arsenic speciation in 21 samples shows that arsenic is present in the solution predominantly as As(III), while particulate arsenic constitutes about 10% of the total arsenic. Methane is detected in five samples with the greatest content being 5107 μg/L. The shallow aquifer in Hangjinhouqi of western Hetao plain is of strongly reducing condition. The arsenic content in 23 core sediment samples varies from 7.7 to 34.6 mg/kg, with great value in clay and mild clay layer. The obvious positive relationship in content between Fe₂O₃, Mn, Sb, B, V and As indicates that the distribution of arsenic in the sediments may be related to Fe and Mn oxides, and the mobilization of Sb, B and V may be affected by similar geochemical processes as that of As.     

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H₂O ± NaCl ± HCl ± H₂S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As^(III) and As^(V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As^(III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As^(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P_sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H₃AsO₃) between an aqueous solution (pure H₂O) and its saturated vapor (K = mAs_vapor /mAs_liquid) were found to be independent of As^(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)₃(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H₂S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As^(III)-H₂O (±NaCl) solutions up to 500°C demonstrate that the As(OH)₃ complex dominates As speciation both in dense H₂O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As^(III) measured in the H₂O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic^(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As₂O₃ solid is consistent with the formation, in the vapor phase, of both As₄O₆ and As(OH)₃. Arsenic^(V) (arsenic acid, H₃AsO₄) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)₃ in the vapor phase.The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)₃(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)₃(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H₂O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.     

Arsenic distribution, speciation and solubility in shallow groundwater of Owens Dry Lake, California

Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L⁻¹ and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and δD suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite [As(III)] was the dominant form of inorganic arsenic at Eh values less than about −170 mV while arsenate [As(V)] was predominant at higher Eh values. Organic arsenic was negligible (<0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg⁻¹ and a maximum concentration range (20 to 40 mg kg⁻¹) was reached as solution concentration increased up to 80 mg L⁻¹, indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As₂S₃) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake.     

Arsenite sorption on troilite (FeS) and pyrite (FeS2)

Arsenic is a toxic metalloid whose mobility and availability are largely controlled by sorption on sulfide minerals in anoxic environments. Accordingly, we investigated reactions of As(III) with iron sulfide (FeS) and pyrite (FeS₂) as a function of total arsenic concentration, suspension density, sulfide concentration, pH, and ionic strength. Arsenite partitioned strongly on both FeS and FeS₂ under a range of conditions and conformed to a Langmuir isotherm at low surface coverages; a calculated site density of near 2.6 and 3.7 sites/nm² for FeS and FeS₂, respectively, was obtained. Arsenite sorbed most strongly at elevated pH (>5 to 6). Although solution data suggested the formation of surface precipitates only at elevated solution concentrations, surface precipitates were identified using X-ray absorption spectroscopy (XAS) at all coverages. Sorbed As was coordinated to both sulfur [d(As-S) = 2.35 Å] and iron [d(As-Fe) = 2.40 Å], characteristic of As coordination in arsenopyrite (FeAsS). The absorption edge of sorbed As was also shifted relative to arsenite and orpiment (As₂S₃), revealing As(III) reduction and a complete change in As local structure. Arsenic reduction was accompanied by oxidation of both surface S and Fe(II); the FeAsS-like surface precipitate was also susceptible to oxidation, possibly influencing the stability of As sorbed to sulfide minerals in the environment. Sulfide additions inhibit sorption despite the formation of a sulfide phase, suggesting that precipitation of arsenic sulfide is not occurring. Surface precipitation of As on FeS and FeS₂ supports the observed correlation of arsenic and pyrite and other iron sulfides in anoxic sediments.     

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5 m) and piezometer wells (depth 12.2–79.2 m) demonstrate that the groundwater is mostly the Ca-HCO₃ type and anoxic in nature (mean Eh_SHE = 34 mV). Arsenic concentrations ranged from <10–538 μg/L, with high concentrations only present in the shallow to medium depth (30–50 m) of the aquifer along with high Fe (0.07–9.8 mg/L) and relatively low Mn (0.15–3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO₃, SO₄ and NO₂ along with higher values of DOC and HCO₃, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.     

Arsenic behaviour in gold-ore mill tailings,Massif Central,France: hydrogeochemical study and investigation of in situ redox signatures

Historical Au-ore exploitation at the Chéni mine in the Massif Central, France, generated 525,000 tonnes of finely ground mill tailings deposited in a heap that has spread with time into three settling basins. The tailings, which are rich in quartz (80%), mica and clay minerals (10% of illite, smectite, kaolinite and chlorite), feldspars (5%) but poor in carbonates (<1%), also contain sulphides (around 5%, mainly pyrite and arsenopyrite). Arsenic content of the tailings is around 6 g kg. This paper describes the geochemistry of drainage waters, with special attention paid to in situ values of the three major redox couples, namely Fe(II)/Fe(III), As(III)/As(V) and S(IV)/S(VI). The water samples range from acidic and oxidized (pH 2.9, E_h +700 mV) to moderate pH and weakly reducing (pH 7.6, E_h 15 mV). The waters are rich in SO₄ and Ca and have variable As (0.05–95 mg L⁻¹) and Fe concentrations (0.07–141 mg L⁻¹). Reduced As(III) species predominate over As(V) species (As(III)/As(V) up to 21), whereas oxidized forms of Fe and S are favoured (Fe(II)/Fe(III) up to 0.5, and S(IV)/S(VI) up to 1).Thermodynamic calculations were performed with the PHREEQC and EQ3NR codes based on a revised As database to evaluate saturation indices (SI) of the waters in relation to the main minerals and define which redox couples control the redox state of the system. The important role of carbonates, though only present in small amounts, explains the acid buffering generated by the oxidation of sulphides for waters in the pH 7–7.5 range. Measured E_h appears to fall between the calculated E_h of the Fe(II)/Fe(III) couple and that of the As(III)/As(V) couple, illustrating redox disequilibrium.     

Oxyanion Concentrations in Eastern Sierra Nevada Rivers – 3. Boron, Molybdenum, Vanadium, and Tungsten

Water samples were collected from 10 locations along the Truckee River system, 14 locations along the Walker River system, and 12 locations along the Carson River, and analyzed for B, Mo, V, W, Na, Cl, and pH. Boron concentrations ranged from approximately 2 mol/kg in the upper reaches of the Truckee River to almost 1,200 mol/kg in Pyramid Lake. Molybdenum, V, and W had concentrations in the nanomolal range; Mo varied from a low of about 12 nmol/kg to a high of 3,200 nmol/kg (Walker Lake); V ranged from 9 nmol/kg to approximately 470 nmol/kg; and W varied from a low value around 0.8 nmol/kg (West Walker River) to 1,030 nmol/kg. The high concentrations of these oxyanion-forming trace elements in the rivers reflects (1) the relative stability of these oxyanions (e.g., MoO₄ ^2-, HVO₄ ^2-, WO₄ ^2-, B(OH)₃, and/or B(OH)₄ ^-) in the alkaline, well oxygenated river and lake waters, (2) contributions of hydrothermal waters (especially for B), and (3) weathering of rocks/regolith with high concentrations of these elements. In the case of Mo, V, and W, each exhibited relatively conservative behavior in the upper, oxygenated reaches of all three rivers. During the study period the region experienced a prolonged drought such that the lower reaches of each river were typified by no flow or stagnant waters and probably low oxygen and/or anoxic conditions (although not measured). Reductive processes occurring in the low flow to stagnant reaches of each river could have led to removal of Mo, V, and W from solution as coprecipitates with Fe monosulfides, or via sorption to Fe oxides/oxyhydroxides and/or organic matter. Boron, however, exhibited essentially no or minor removal from these rivers, and instead was added to each river via B-rich hydrothermal waters (e.g., Steamboat Creek from Steamboat Hot Springs), or by B-rich groundwaters via base-flow during the extensive drought.     

Arsenic speciation in Mono Lake, California: Response to seasonal stratification and anoxia

Mono Lake is a closed-basin, alkaline, hypersaline lake located at the western edge of the Great Basin in eastern California. We studied the distribution of arsenic (As) species in the water column of Mono Lake between February and November, 2002. This period captured the seasonal progression from winter mixing, through summer thermal stratification, to autumn overturn. Arsenic speciation was determined by ion chromatography-inductively coupled-plasma-mass spectrometry of samples preserved in the field by flash-freezing in liquid nitrogen. We found that arsenic speciation was dominated (>90%) by arsenate when oxygen was detectable. Once levels fell below 6 μmol/L O₂, arsenic speciation shifted to dominance by reduced species. Arsenate and arsenite co-occurred in a transition zone immediately below the base of the oxycline and low but significant concentrations of arsenate were occasionally detected in sulfidic hypolimnion samples. Thio-arsenic species were the dominant form of As found in sulfidic waters. Maxima of thio-arsenic species with stoichiometries consistent with mono-, di- and trithio-arsenic occurred in succession as sulfide concentration increased. A compound with a stoichiometry consistent with trithio-arsenic was the dominant As species (∼50% of total As) in high sulfide (2 mmol/L) bottom water. Lower concentrations of total As in bottom water relative to surface water suggest precipitation of As/S mineral phases in response to sulfide accumulation during prolonged anoxia.     

Importance of CDOM Distribution and Photoreactivity in a Shallow Texas Estuary

This study examined freshwater discharge of dissolved organic matter (DOM) to the shallow Lavaca–Matagorda (LM) Bay estuarine system along the central Texas coast and investigated whether chromophoric DOM (CDOM) photochemical reactions have the potential to stimulate microbial activity within LM estuarine waters. Dissolved organic carbon (DOC) concentrations ranged from 3 to 10 mg C l⁻¹ and CDOM levels (reported as a ₃₀₅) ranged from 8 to 77 m⁻¹ during April and July, 2007, when the LM system was experiencing very high freshwater inputs. DOC and CDOM levels were well-correlated with salinities > 3, but exhibited considerable variability at salinities < 3. CDOM photobleaching rates (i.e., decrease in a ₃₀₅ resulting from exposure to solar radiation) for estuarine samples ranged from 0.014 to 0.021 h⁻¹, corresponding to photobleaching half-lives of 33–50 h. Our data indicate when Matagorda Bay waters photobleach; dissolved organic carbon utilization is enhanced perhaps due to enhanced microbial respiration of biologically labile photoproducts (BLPs). Net ecosystem metabolism calculations indicate that most of the LM system was net heterotrophic during our study. We estimate that BLP formation could support up to 20% of the daily microbial respiratory C demand in LM surface waters and combined with direct photochemical oxygen consumption could have an important influence on O₂ cycles in the LM system.     

Increasing Iron Concentrations in UK Upland Waters

Iron distributions in rainfall, streams, soils and groundwaters are described for the Upper River Severn catchment of mid-Wales. Iron is mainly supplied from within-catchment sources with highest concentrations occurring under reducing conditions. Iron concentrations have doubled over the past 20 years (~5.0 μg yr⁻¹ for the forest and ~3.7 μg yr⁻¹ for the moorland). For the forested sites, the gradients are particularly high post-1993. UK rivers/lakes monitored by the UK Acid Waters Monitoring Network show similar increases. Generally, Fe correlates with dissolved organic carbon (DOC). The greatest rates of Fe increase coincide with those for DOC. Thermodynamic modelling using WHAM/Model VI indicates that Fe(III) is mainly in microparticulate form (probably oxyhydroxides) apart from under reducing conditions. It is proposed that Fe increases for surface waters are associated with increased microparticulate Fe(III) due to stabilisation against aggregation by binding of DOM to its surface. The results relate to acidification declines and deforestation leading to land disturbance and wetter conditions within the soil. There will be greater acidification reversal following tree harvesting due to lowering of atmospheric SO_x scavenging and this may have resulted in the greater increase in Fe in the later years of the study.     

Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations

Recent discoveries demonstrate that the chemistry of arsenic in sulfidic waters is much more complex that previously believed. One implication is that all earlier thermodynamic data on stabilities of As thioanions require revision. Previously used experimental approaches for determining As thioanion stabilities may be inadequate to deal with the full range of complexity. Here we use computational as well as empirical information to construct a provisional model for equilibrium As thioanion distributions in sulfidic waters. Whereas previous authors have argued for either As(III) or As(V) thioanions, the new model predicts that both are important and can occur simultaneously under commonly encountered pH and ΣS^−II conditions. At the order of magnitude level, the model reasonably predicts the solubility of As₂S₃ in sulfidic solutions, provides tentative peak assignments for published Raman spectroscopic data and plausibly accounts for how sulfide modifies the bacterial toxicity of As. The model yields a thermodynamic justification for how sulfide, which is usually regarded as a reducing agent, can counter-intuitively drive oxidation of As(III) to As(V), as has been observed both in the laboratory and in the field. Despite its uncertain accuracy, the model serves as a useful source of new, testable hypotheses about As geochemistry and highlights crucial experimental data needs.     

Arsenic in surface- and up to 90°C ground waters in a basalt area,N-Iceland: processes controlling its mobility

The concentrations of As in surface- and up to 90 °C ground waters in a tholeiite flood basalt area in N-Iceland lie in the range <0.03–10 μg/kg. With few exceptions surface waters contain <0.5 μg/kg As whereas ground waters generally contain >0.5 μg/kg As. The As content of ground waters increases on the whole with rising temperature. Arsenic is highly mobile in the basalt-water environment of the study area. An insignificant fraction of the As dissolved from the rock is taken up into secondary minerals. Arsenic is less mobile than B but considerably more mobile than Na which has the highest mobility among the major aqueous components. A significant fraction of the As in the basalt occurs in an easily soluble form. The As hosted in the primary minerals is expected to be concentrated in the titano-magnetite. This mineral is stable in contact with both surface- and ground waters and does not, therefore, supply As to the water, explaining the difference in mobility between As and B. Aqueous As concentrations are a reflection of water/rock ratios, i.e. how much rock a given quantity of water has dissolved. This ratio increases with increasing temperature and increasing residence time of the water in contact with the rock. The distribution of As species has been calculated on the assumption of equilibrium at the redox potential retrieved from measurement of aqueous Fe(II) and Fe(III) concentrations. These calculations indicate that pentavalent As is stable in surface waters and in ground waters with an in situ pH of <10 and would occur mostly as H₂AsO₄⁻ and HAsO₄⁻². In higher pH ground waters the concentrations of the arsenite species H₂AsO₃⁻ is significant at equilibrium, up to 65% of the total dissolved As.     

Simultaneous oxidation of arsenic and antimony at low and circumneutral pH,with and without microbial catalysis

Arsenic and Sb are common mine-water pollutants and their toxicity and fate are strongly influenced by redox processes. In this study, simultaneous Fe(II), As(III) and Sb(III) oxidation experiments were conducted to obtain rates under laboratory conditions similar to those found in the field for mine waters of both low and circumneutral pH. Additional experiments were performed under abiotic sterile conditions to determine the biotic and abiotic contributions to the oxidation processes. The results showed that under abiotic conditions in aerated Fe(III)–H₂SO₄ solutions, Sb(III) oxidizes slightly faster than As(III). The oxidation rates of both elements were accelerated by increasing As(III), Sb(III), Fe(III), and Cl⁻ concentrations in the presence of light. For unfiltered circumneutral water from the Giant Mine (Yellowknife, NWT, Canada), As(III) oxidized at 15–78 μmol/L/h whereas Sb(III) oxidized at 0.03–0.05 μmol/L/h during microbial exponential growth. In contrast, As(III) and Sb(III) oxidation rates of 0.01–0.03 and 0.01–0.02 μmol/L/h, respectively, were obtained in experiments performed with acid unfiltered mine waters from the Iberian Pyritic Belt (SW Spain). These results suggest that the Fe(III) formed from microbial oxidation abiotically oxidized As(III) and Sb(III). After sterile filtration of both mine water samples, neither As(III), Sb(III), nor Fe(II) oxidation was observed. Hence, under the experimental conditions, bacteria were catalyzing As and Sb oxidation in the Giant Mine waters and Fe oxidation in the acid waters of the Iberian Pyrite Belt.     

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.