Ferric iron in sediments as a novel CO2 mineral trap: CO2–SO2 reaction with hematite

Thermodynamic simulations of reactions among SO₂-bearing CO₂-dominated gas, water and mineral phases predict that Fe^III in sediments should be converted almost entirely to dissolved Fe^II and siderite (FeCO₃), and that SO₂ should simultaneously be oxidized to dissolved sulfate. The reactions are however, subject to kinetic constraints which may result in deviation from equilibrium and the precipitation of other metastable mineral phases. To test the prediction, a laboratory experiment was carried out in a well stirred hydrothermal reactor at 150 °C and 300 bar with hematite, 1.0 m NaCl, 0.5 m NaOH, SO₂ in quantity sufficient to reduce much of the iron, and excess CO₂. The experiment produced stable siderite and metastable pyrite and elemental S. Changes in total dissolved Fe are consistent with nucleation of pyrite at ∼17 h, and nucleation of siderite at ∼600 h. Dissolution features present on elemental S at the conclusion of the experiment suggest nucleation early in the experiment. The experiment did not reach equilibrium after ∼1400 h, as indicated by coexistence of hematite with metastable pyrite and elemental sulfur. However, the results confirm that Fe^III can be used to trap CO₂ in siderite if partly oxidized S, as SO₂, is present to reduce the Fe with CO₂ in the gas phase.     

Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.     

Modeling transport of arsenic through modified granular natural siderite filters for arsenic removal

Groundwater arsenic(As)contamination is a hot issue,which is severe health concern worldwide.Recently,many Fe-based adsorbents have been used for As removal from solutions.Modified granular natural siderite(MGNS),a special hybrid Fe(II)/Fe(III)system,had higher adsorption capacity for As(III)than As(V),but the feasibility of its application in treating high-As groundwater is still unclear.In combination with transport modeling,laboratory column studies and field pilot tests were performed to reveal both mechanisms and factors controlling As removal by MGNS-filled filters.Results show that weakly acid pH and discontinuous treatment enhanced As(Ⅲ)removal,with a throughput of 8700 bed volumes(BV)of 1.0 mg/L As(Ⅲ)water at breakthrough of 10 μg/L As at pH 6.Influent HCO_3~-inhibited As removal by the filters.Iron mineral species,SEM and XRD patterns of As-loading MGNS show that the important process contributing to high As(Ⅲ)removal was the mineral transformation from siderite to goethite in the filter.The homogeneous surface diffusion modeling(HSDM)shows that competition between As(III)and HCO_3~-with adsorption sites on MGNS was negligible.The inhibition of HCO_3~-on As(Ⅲ)removal was connected to inhibition of siderite dissolution and mineral transformation.Arsenic loadings were lower in field pilot tests than those in the laboratory experiments,showing that high concentrations of coexisting anions(especially HCO_3~-and SiO_4~(4-)),high pH,low EBCT,and low groundwater temperature decreased As removal.It was suggested that acidification and aeration of highAs groundwater and discontinuous treatment would improve the MGNS filter performance of As removal from real high-As groundwater.     

A kinetic study of the oxidation of arsenopyrite in acidic solutions: implications for the environment

Arsenopyrite is an important component of many ore deposits and dissolves in the O₂-rich, acidic surface waters that are commonly found in the vicinity of active mines, releasing As, Fe and S to the environment. However, despite the potentially serious effect of this pollution on the human and animal population, the rate at which such oxidation occurs is poorly known. Kinetic experiments were therefore conducted in a mixed flow reactor to investigate the oxidation of arsenopyrite in Fe₂(SO₄)₃ solutions (pH=l.8) having a concentration of l×l0⁻² to 1 ×l0⁻⁵ mol kg⁻¹ at temperatures of 45, 35, 25 and 15 °C. The results of these experiments show that the rate of oxidation of arsenopyrite increases with increasing concentration of dissolved Fe₂(SO₄)₃ and temperature. They also show that As released during the oxidation of arsenopyrite has the form As(III), and that the rate of conversion of As(III) to As(V) is relatively low, although it tends to increase with increasing concentration of dissolved Fe₂(SO₄)₃ and temperature. In the presence of Cl⁻, oxidation of arsenopyrite is accelerated, as is the conversion of As(III) to As(V). These findings indicate that exploitation of arsenopyrite-bearing ores will cause contamination of groundwaters by As at levels sufficient to have a major negative effect on the health of humans and animals.     

A geochemical model for removal of iron(II)(aq) from mine water discharges

A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O₂(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O₂(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10⁻⁶ and 10⁻⁵ mol l⁻¹ is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe²⁺ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines.     

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced ⁵⁶Fe/⁵⁴Fe ratios for Fe(II)_aq that are 2-3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)_aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)_aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO₃) and ca. +0.9‰ for Ca-substituted siderite (Ca_0.15Fe_0.85CO₃) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have ⁵⁶Fe/⁵⁴Fe ratios that are up to 1‰ lower than Fe(II)_aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)_aq.The relative order of δ⁵⁶Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)_aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ⁵⁶Fe >0‰, the calculated δ⁵⁶Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ⁵⁶Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ⁵⁶Fe values. Based on this experimental study, formation of low-δ⁵⁶Fe Fe(II)_aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.     

Lead coprecipitation with iron oxyhydroxide nano-particles

Pb²⁺ and Fe³⁺ coprecipitation was studied with sorption edge measurements, desorption experiments, sorbent aging, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Companion adsorption experiments were also conducted for comparison. The macroscopic chemical and near atomic scale HRTEM data supplemented our molecule scale analysis with EXAFS (Kelly et al., 2008). Coprecipitation of Pb²⁺ with ferric oxyhydroxides occurred at ∼pH 4 and is more efficient than adsorption in removing Pb²⁺ from aqueous solutions at similar sorbate/sorbent ratios and pH. X-ray Diffraction (XRD) shows peaks of lepidocrocite and two additional broad peaks similar to fine particles of 2-line ferrihydrite (2LFh). HRTEM of the Pb-Fe coprecipitates shows a mixture of 2-6 nm diameter spheres and 8-20 by 200-300 nm needles, both uniformly distributed with Pb²⁺. Geochemical modeling shows that surface complexation models fit the experimental data of low Pb:Fe ratios when a high site density is used. Desorption experiments show that more Pb²⁺ was released from loaded sorbents collected from adsorption experiments than from Pb to Fe coprecipitates at dilute EDTA concentrations. Desorbed Pb²⁺ versus dissolved Fe³⁺ data show a linear relationship for coprecipitation (CPT) desorption experiments but a parabolic relationship for adsorption (ADS) experiments.Based on these results, we hypothesize that Pb²⁺ was first adsorbed onto the nanometer-sized, metastable, iron oxyhydroxide polymers of 2LFh with domain size of 2-3 nm. As these nano-particles assembled into larger particles, some Pb²⁺ was trapped in the iron oxyhydroxide structure and re-arranged to form solid solutions. Therefore, the CPT contact method produced more efficient removal of Pb²⁺ than the adsorption contact method, and Pb²⁺ bound in CPT solids represent a more stable sequestration of Pb²⁺ in the environment than Pb²⁺ adsorbed on iron oxyhydroxide surfaces.     

Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part I: Evidence from sediment profiles

This study reexamines the notion that extensive As mobilization in anoxic groundwater of Bangladesh is intimately linked to the dissolution of Fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material. Detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater As concentrations were known to span a wide range. The sediment properties that were measured include (1) the proportion of Fe(II) in the Fe fraction leached in hot 1.2 N HCl, (2) diffuse spectral reflectance, and (3) magnetic susceptibility.In parallel with local concentrations of dissolved As ranging from <5 to 600 μg/L, Fe(II)/Fe ratios in shallow (gray) Holocene sands tended to gradually increase with depth from values of 0.3 to 0.5 to up to 0.9. In deeper (orange) aquifers of presumed Pleistocene age that were separated from shallow sands by a clay layer and contained <5 μg/L dissolved As, leachable Fe(II)/Fe ratios averaged ∼0.2. There was no consistent relation between sediment Fe(II)/Fe and dissolved Fe concentrations in groundwater in nearby wells. The reflectance measurements indicate a systematic linear relation (R² of 0.66; n = 151) between the first derivative transform of the reflectance at 520 nm and Fe(II)/Fe. The magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 (x 10⁻⁹ m³/kg SI) and was linearly related (R² of 0.75; n = 29) to the concentrations of minerals that could be magnetically separated (0.03 to 0.79% dry weight). No systematic depth trends in magnetic susceptibility were observed within the shallow sands, although the susceptibility of deeper low-As aquifers was low (up to ∼200 × 10⁻⁹ m³/kg SI).This set of observations, complemented by incubation results described in a companion paper by van Geen et al. (this volume), suggests that the release of As is linked to the transformation of predominantly Fe (III) oxyhydroxide coatings on sand particles to Fe(II) or mixed Fe(II/III) solid phases with a flatter reflectance spectrum such as siderite, vivianite, or magnetite, without necessarily resulting in the release of Fe to groundwater. The very low As/Fe ratio of magnetically separated minerals compared to the As/Fe of bulk acid leachate (2 vs. 40 10⁻⁶, respectively) suggests that such a transformation could be accompanied by a significant redistribution of As to a mobilizable phase on the surface of aquifer particles.     

Solubility and dissimilatory reduction kinetics of iron(III) oxyhydroxides: A linear free energy relationship

Rates of reduction of Fe(III) oxyhydroxides by the bacterium Shewanella putrefaciens were measured as a function of the bacterial density and the Fe(III) substrate concentration. The results show that an earlier reported positive correlation between the solubility products (^*K_so) and the maximum cell-specific reduction rates (v_max) of predominantly poorly crystalline Fe(III) oxyhydroxides also applies to insoluble and crystalline Fe(III) oxyhydroxides. The mineral solubilities were measured by a dialysis bag technique under acidic conditions (pH 1 up to 2.5) at 25 °C. Initial iron reduction rates by S. putrefaciens were determined in the presence of excess lactate as electron donor. In all cases, the microbial reduction rate exhibited saturation behavior with respect to the Fe(III) oxyhydroxide concentration. On a double logarithmic scale, the maximum rates v_max and the solubility products defined a single linear free energy relationship (LFER) for all the Fe(III) oxyhydroxides considered. The solubility provided a better predictor of v_max than the specific surface area of the mineral phase. A rate limitation by the electron transfer between an iron reductase and a Fe(III) center, or by the subsequent desorption of Fe²⁺ from the iron oxide mineral surface, are both consistent with the observed LFER.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.     

Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), Fe^III-rich (27-138 μM) surface water with low molar Cl:SO₄ ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the Fe^III species). Subsurface sediments contained abundant pore-water HCO₃ (6-20 mM) and were reducing (Eh < −100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water Fe^II concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO₄-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water Fe^II, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S^−II concentrations. Such low concentrations of S^−II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g⁻¹ as Fe) compared to disordered mackinawite (up to 590 μmol g⁻¹ as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.     

Magnetite nanoplates decorated on anodized aluminum oxide nanofibers as a novel adsorbent for efficient removal of As(III)

In this study, arsenic as an environmental top-ranked hazardous substance was efficiently removed by a novel adsorbent fabricated by magnetite Fe₃O₄ nanoplates decorated on anodized aluminum oxide (AAO) nanofibers. AAO nanofibers were prepared by anodic polarization method, and then Fe₃O₄ nanoplates were grown on AAO-based substrate by hydrothermal method to fabricate AAO/Fe₃O₄ nanosorbent. Morphology of the fabricated adsorbents was characterized by field emission scanning electron microscopy (FE-SEM), and their crystallinity was studied by X-ray diffraction (XRD). Arsenic (III) removal potential of the proposed adsorbent from contaminated water samples was investigated by the determination of As(III) amounts in the samples by inductively coupled plasma optical emission spectroscopy before and after adsorption process at sub-μg L^?1 levels. The results showed that without pre- and post-treatments such as pH adjustment, As(III) was removed effectively from contaminated water samples by using the proposed adsorbent. AAO/Fe₃O₄ sorbent showed excellent ability to remove 0.1 mg L^?1 As(III) from water samples up to 96 % uptake. Freundlich adsorption isotherm model was used to interpret the As(III) adsorption on proposed sorbent. The Freundlich isotherm parameters n and k _F were obtained to be 2.2 and 10.2, respectively, representing the high affinity of proposed adsorbent for arsenic removal.     

Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)

The vertical distribution of reduced sulfur species (RSS including H₂S/HS⁻, S⁰, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H₂S_MBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSS_V) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H₂S/HS⁻, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeS_am phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species.     

Experimentelle Sideritbildung aus Calcit + FeCl2

The stability relations of calcite and siderite in the system Ca²⁺-Fe²⁺-CO ₃ ^2– Cl ₂ ^2– -H₂O have been determined between 150 and 400° C in the pressure range from 250 to 2000 bars.It was found that the composition of the fluid phase coexisting with calcite and siderite is very poor in Fe²⁺ and correspondingly very rich in Ca²⁺ (see Tab. 1 and Fig. 3). The mole proportion Ca²⁺/(Ca²⁺+Fe²⁺) exceeds the value 0.98 at temperatures below 250° C when fluid pressure is 1000 bars. The stability field of calcite narrows with decreasing temperature, increasing pressure, and decreasing concentration of dissolved salts (CaCl₂₊+FeCl₂). In our experiments siderite becomes unstable at about 400° C (see p. 158).The experimental data indicate that siderite will be formed by reaction of calcite with FeCl₂-bearing solutions that have very low concentrations of Fe²⁺ as well as high Ca²⁺/(Ca²⁺+Fe²⁺)-ratios. Solutions coexisting with calcite must be very poor in Fe²⁺, otherwise siderite is formed.

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Herrn Prof. Dr. H. G. F. Wikkleb danke ich sehr für sein förderndes Interesse an dieser Arbeit und für eine kritische Durchsicht des Manuskriptes. Der Deutschen Forschungsgemeinschaft gebiihrt Dank für die Arbeitsmöglichkeit mit den Apparaturen, die Herrn Prof. Dr. Winkler zür Verfugung gestellt wurden.     

Iron(III) accumulations in inland saline waterways,Hunter Valley,Australia: Mineralogy,micromorphology and pore-water geochemistry

Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L⁻¹. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (Fe^Ox) relative to the Na-dithionite extractable fraction (Fe^Di), with generally high Fe^Ox:Fe^Di ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe₅HO₃·4H₂O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.     

Groundwater chemistry and redox processes: Depth dependent arsenic release mechanism

Patchy occurrences of elevated As are often encountered in groundwater from the shallow aquifers (<50 m) of the Bengal Delta Plain (BDP). A clear understanding of various biogeochemical processes, responsible for As mobilization, is very important to explain this patchy occurrence and thus to mitigate the problem. The present study deals with the periodical monitoring of groundwater quality of five nested piezometeric wells between December 2008 and July 2009 to investigate the temporal changes in groundwater chemistry vis-a-vis the prevalent redox processes in the aquifer. Geochemical modeling has been carried out to identify key phases present in groundwater. A correlation study among different aqueous redox parameters has also been performed to evaluate prevailing redox processes in the aquifer. The long term monitoring of hydrochemical parameters in the multilevel wells together with hydrogeochemical equilibrium modeling has shown more subtle differences in the geochemical environment of the aquifer, which control the occurrence of high dissolved As in BDP groundwater. The groundwater is generally of Ca-HCO₃ type. The dissolved As concentration in groundwater exceeded both WHO and National drinking water standard (Bureau of Indian Standards; BIS, 10 μg L⁻¹) throughout the sampling period. The speciation of As and Fe indicate persistent reducing conditions within the aquifer [As(III): 87-97% of As_T and Fe(II): 76-96% of Fe_T]. The concentration of major aqueous solutes is relatively high in the shallow aquifer (wells A and B) and gradually decreases with increasing depth in most cases. The calculation of SI indicates that groundwater in the shallow aquifer is also relatively more saturated with carbonate minerals. This suggests that carbonate mineral dissolution is possibly influencing the groundwater chemistry and thereby controlling the mobilization of As in the monitored shallow aquifer. Hydrogeochemical investigation further suggests that Fe and/or Mn oxyhydroxide reduction is the principal process of As release in groundwater from deeper screened piezometric wells. The positive correlations of U and V with As, Fe and Mn indicate redox processes responsible for mobilization of As in the deeper screened piezometric wells are possibly microbially mediated. Thus, the study advocates that mobilization of As is depth dependent and concentrations of As in groundwater depends on single/combined release mechanisms.     

Formation of syngenetic and early diagenetic iron minerals in the late Archean Mt. McRae Shale, Hamersley Basin, Australia: New insights on the patterns, controls and paleoenvironmental implications of authigenic mineral formation

A section through the late Archean Mt. McRae Shale comprising, in ascending order, a lower shale interval (LSI), a banded iron formation (BIF), an upper shale (USI) and a carbonate (C1) has been analyzed for total Fe and Al contents and authigenic Fe present as carbonate, oxide, sulfide and silicate phases. The authigenic mineralogy is controlled by the episodic addition of Fe from hydrothermal activity and removal of Fe by sulfide, relative to rates of clastic sedimentation. The LSI and BIF have mean Fe_T/Al values of 2 and 5, respectively, that record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower Fe_T/Al values (0.6-1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (>0.8) indicate the existence of a water column that became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The USI also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin. The carbonate C1 represents a basinal chemistry where sulfide has been removed and Fe_T/Al values are ∼1 indicating that hydrothermal activity again produced dissolved Fe-rich bottom waters. Detailed iron speciation of the Mt. McRae Shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late Archean Hamersley Basin, just prior to the Paleoproterozoic rise in atmospheric oxygenation, and our refined methods have relevance to all Fe-rich deposits.     

Photoreduction of Fe(III) in the Acidic Mine Pit Lake of San Telmo (Iberian Pyrite Belt): Field and Experimental Work

Light-induced reduction of dissolved and particulate Fe(III) has been observed to occur in the surface waters of the acidic mine pit lake of San Telmo (143,600 m², pH 2.8, Fe_total = 2.72 mM). This photochemical production of Fe(II) is directly related to the intensity of solar radiation and competes with biologically catalyzed reactions (i.e., bacterial re-oxidation of Fe(II)) and physical processes (including ionic diffusion, advection, and convection, which tend to homogenize the epilimnetic concentration of Fe(II) at every moment). Therefore, diel cycles of Fe(II) concentration are observed at the lake surface, with minimum values of 10–20 μM Fe(II) (0.35–0.70% Fe_total) at the sunrise and sunset, and maximum values of 90 μM Fe(II) (3.2% Fe_total) at midday in August 2005. Field and experimental work conducted in San Telmo and other pit lakes of the Iberian Pyrite Belt (IPB) (pH 2.3–3.1, Fe_total = 0.34–17 mM) indicate that the kinetics of the photoreductive reaction is zero-order and is independent of the Fe(III) concentration, but highly dependent on the intensity of solar radiation and temperature. Experimental work conducted with natural Fe(III) minerals (schwertmannite, goethite, and lepidocrocite) suggests that dissolved organic matter is an important factor contributing to the photochemical production of Fe(II). The wavelengths involved in the photoreduction of Fe(III) include not only the spectrum of UV-A radiation (315–400 nm), but also part of the photosynthetically active radiation (PAR, 400–700 nm). This finding is of prime importance for the understanding of the photoreduction processes in the pit lakes of the IPB, because the photo-reactive depth is not limited to the penetration depth of UV-A radiation (upper 1–10 cm of the water column depending on the TDS content), but it is approximately equal to the penetration depth of PAR (e.g., first 4–6 m of the water column in San Telmo on July 2007); thus, increasing the importance of photochemical processes in the hydro(bio)geochemistry of pit lakes.     

Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy

The knowledge of mineralogy and molecular structure of As is needed to better understand the stability of As in wastes resulting from processing of gold ores. In this study, optical microscopy, scanning electron microscopy, electron microprobe, X-ray diffraction and X-ray absorption fine structure (XAFS) spectroscopy (including both XANES and EXAFS regimes) were employed to determine the mineralogical composition and local coordination environment of As in gold ores and process tailings from bench-scale tests designed to mimic a common plant practice. Arsenic-bearing minerals identified in the ores and tailings include iron (III) oxyhydroxides, scorodite (FeAsO₄·2H₂O), ferric arsenates, arseniosiderite (Ca₂Fe₃(AsO₄)₃O₂·3H₂O), Ca-Fe arsenates, pharmacosiderite (KFe₄(AsO₄)₃(OH)₄·6-7H₂O), jarosite (K₂Fe₆(SO₄)₄(OH)₁₂) and arsenopyrite (FeAsS). Iron (III) oxyhydroxides contain variable levels of As from trace to about 22 wt% and Ca up to approximately 9 wt%.Finely ground ore and tailings samples were examined by bulk XAFS and selected mineral grains were analyzed by microfocused XAFS (micro-EXAFS) spectroscopy to reconcile the ambiguities of multiple As sources in the complex bulk EXAFS spectra. XANES spectra indicated that As occurs as As⁵⁺in all the samples. Micro-EXAFS spectra of individual iron (III) oxyhydroxide grains with varying As concentrations point to inner-sphere bidentate-binuclear arsenate complexes as the predominant form of As. There are indications for the presence of a second Fe shell corresponding to bidentate-mononuclear arrangement. Iron (III) oxyhydroxides with high As concentrations corresponding to maximum adsorption densities probably occur as nanoparticles. The discovery of Ca atoms around As in iron (III) oxyhydroxides at interatomic distances of 4.14-4.17 Å and the coordination numbers suggest the formation of arseniosiderite-like nanoclusters by coprecipitation rather than simple adsorption of Ca onto iron (III) oxyhydroxides. Correlation of Ca with As in iron (III) oxyhydroxides as determined by electron microprobe analysis supports the coprecipitate origin for the presence of Ca in iron (III) oxyhydroxides.The samples containing higher abundances of ferric arsenates released higher As concentrations during the cyanidation tests. The presence of highly soluble ferric arsenates and Ca-Fe arsenates, and relatively unstable iron (III) oxyhydroxides with Fe/As molar ratios of less than 4 in the ore and process tailings suggests that not only the tailings in the impoundment will continue to release As, but also there is the potential for mobilization of As from the natural sources such as the unmined ore.     

Mechanisms controlling acid neutralization and metal mobility within a Ni-rich tailings impoundment

Low-quality pore waters containing high concentrations of dissolved H⁺, SO₄, and metals have been generated in the East Tailings Management Area at Lynn Lake, Manitoba, as a result of sulfide-mineral oxidation. To assess the abundance, distribution, and solid-phase associations of S, Fe, and trace metals, the tailings pore water was analyzed, and investigations of the geochemical and mineralogical characteristics of the tailings solids were completed. The results were used to delineate the mechanisms that control acid neutralization, metal release, and metal attenuation. Migration of the low-pH conditions through the vadose zone is limited by acid-neutralization reactions, resulting in the development of distinct pore-water pH zones at depth; the neutralization reactions involve carbonate (pH ⩾ 5.7), Al-hydroxide (pH ⩾ 4.0), and aluminosilicate solids. As the zone of low-pH pore water expands, the pH will then be primarily controlled by less soluble solids, such as Fe(III) oxyhydroxides (pH < 3.5) and the relatively more recalcitrant aluminosilicates (pH ⩾ 1.3). Precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxides and hydroxysulfates control the concentrations of dissolved Fe(III). Concentrations of dissolved SO₄ are principally controlled by the formation of gypsum and jarosite. Geochemical extractions indicate that the solid-phase concentrations of Ni, Co, and Zn are associated predominantly with reducible and acid-soluble fractions. The concentrations of dissolved trace metals are therefore primarily controlled by adsorption/complexation and (or) co-precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxide and hydroxysulfate minerals. Concentrations of dissolved metals with relatively low mobility, such as Cu, are also controlled by the precipitation of discrete minerals. Because the major proportion of metals is sequestered through adsorption and (or) co-precipitation, the metals are susceptible to remobilization if low-pH or reducing conditions develop within the tailings.