Oxyhydroxysulfate Mineral Research Articles

Geochemistry and Mineralogy of Precipitates from Passive Treatment of Acid Mine Drainage: Implications for Future Management Strategies

Traditional mining activities in Zaruma-Portovelo (SE Ecuador) have led to high concentrations of pollutants in the Puyango River due to acid mine drainage (AMD) from abandoned waste. Dispersed alkaline substrate (DAS) passive treatment systems have shown efficacy in neutralizing acidity and retaining metals and sulfates in acidic waters, achieving near a 100% retention for Fe, Al, and Cu, over 70% for trace elements, and 25% for SO42−. However, significant solid residues are generated, requiring proper geochemical and mineralogical understanding for management. This study investigates the fractionation of elements in AMD precipitates. Results indicate that Fe3+ and Al3+ predominantly precipitate as low-crystallinity oxyhydroxysulfate minerals such as schwertmannite [Fe3+16(OHSO4)12–13O16·10–12H2O] and jarosite [KFe3+3(SO4)2(OH)6], which retain elements like As, Cr, Cu, Pb, and Zn through adsorption and co-precipitation processes. Sulfate removal occurs via salts like coquimbite [AlFe3(SO4)6(H2O)12·6H2O] and gypsum [CaSO4·2H2O]. Divalent metals are primarily removed through carbonate and bicarbonate phases, with minerals such as azurite [Cu(OH)2·2CuCO3], malachite [Cu2(CO3)(OH)2], rhodochrosite [MnCO3], and calcite [CaCO3]. Despite the effectiveness of DAS, leachates from the precipitates exceed regulatory thresholds for aquatic life protection, classifying them as hazardous and posing environmental risks. However, these residues offer opportunities for the recovery of valuable metals.

Antimony-bearing schwertmannite transformation to goethite: A driver of antimony mobilization in acid mine drainage

Antimony(V) mobility in acid mine drainage (AMD) is often controlled by sorption and coprecipitation with schwertmannite – a poorly-ordered Fe(III) oxyhydroxysulfate mineral. However, due to its metastable nature, schwertmannite transforms over time to more thermodynamically stable Fe(III) phases, such as goethite. This study examines how transformation of Sb(V)-bearing schwertmannite to goethite impacts Sb(V) mobility, while also assessing the role that Sb(V) may play in stabilizing schwertmannite against such transformation. To address these aims, Sb(V)-free, Sb(V)-sorbed and Sb(V)-coprecipitated schwertmannite were allowed to undergo partial transformation to goethite under acid sulfate conditions. Iron K-edge EXAFS spectroscopy revealed that sorbed and coprecipitated Sb(V) partly stabilized schwertmannite against transformation. The onset of schwertmannite transformation to goethite was found to drive clear mobilization of Sb(V) into solution, regardless of the Sb(V) loading or whether Sb(V) was initially sorbed or coprecipitated with the precursor schwertmannite. This initial phase of Sb(V) mobilization was followed by subsequent solid-phase recapture of the released Sb(V), with Sb K-edge EXAFS spectroscopy revealing that this process involved Sb(V) incorporation into the newly-formed goethite. Our findings show that, although schwertmannite transformation to goethite is partially inhibited by co-existing Sb(V), the initial stage of this transformation process drives significant Sb(V) mobilization in AMD systems.

Phase transformation of schwertmannite changes microbial iron and sulfate-reducing processes in flooded paddy soil and decreases arsenic accumulation in rice (Oryza sativa L.)

Rice (Oryza sativa L.) is known to accumulate inorganic arsenic (iAs) and dimethylarsenate (DMA) in its grains, which threatens both human health and rice yield. Although schwertmannite, a metastable Fe (Ⅲ)-oxyhydroxysulfate mineral with extremely high adsorption capacity for iAs, has been proposed to remediate paddy soil to decrease As accumulation in rice, it remains unclear whether the phase transformation of schwertmannite would occur in flooded paddy soil and how its phase transformation changes the soil microbial processes that impact the accumulation of iAs and DMA in grains. Here, we found that amending As-contaminated paddy soil with 0.5%–1% (w/w) schwertmannite decreased the accumulation of iAs and DMA in grains by 37.41%–43.29% and 50.60%–73.89%, respectively, even though schwertmannite has transformed to goethite and secondary FeS was formed in both rhizosphere and bulk soils. The phase transformation of schwertmannite released a considerable amount of SO42− into porewater, thereby increasing the abundances of both sulfate-reducing bacteria and the dsrB gene but decreasing the abundance of iron-reducing bacteria. This result suggested that schwertmannite phase transformation has promoted sulfate-reducing process and weakened iron-reducing process in flooded soil. Such promoted sulfate-reducing process and weakened iron-reducing process in paddy soil can decrease the reductive dissolution of As-bearing (oxyhydr)oxides, increase the formation of secondary FeS mineral for decreasing porewater As concentration, and strengthen the role of Fe plaque as a barrier for As absorption by rice. Additionally, the application of schwertmannite has decreased the abundance of arsM gene and weakened As methylation process in soil. Therefore, the effective decrease of iAs and DMA accumulation in rice grains by schwertmannite can not only be ascribed to the adsorption capacity of schwertmannite for As and the adsorption or incorporation of As by transformation products, but also contributed by the promoted sulfate-reducing process and the weakened iron-reducing process in flooded paddy soil.

A Detailed Sedimentological Analysis of Colour Lake, Axel Heiberg Island, Nunavut

This research project examines sediments in Colour Lake, a non-glacial lake on Axel Heiberg Island in the Canadian High Arctic. Analyzing the mineralogical composition, structural morphology and deposition patterns of the accumulated lake sediments provides information about the depositional environment and can serve as proxies for past environmental conditions. Work focused primarily on a 53 cm long sediment core collected in 2019. The objectives were: (1) to document distinct sediment units and interpret their depositional history by high resolution image analysis and microscopic observations in thin sections; (2) to identify schwertmannite, an iron oxyhydroxy-sulfate mineral within the sediments using scanning electron microscopy and x-ray fluorescence (XRF); (3) to determine this mineral’s relationship with siliciclastic deposits; and (4) to establish sediment chronology by 210Pb and 137Cs dating. Data was organized into pollen charts, incorporating images, scans, sediment stratigraphy, and grain size distributions to better understand sediment layers and their positions within the core. Thirteen lithozones were identified based on observed variations in the core, differing in terms of mud and clay lens distribution, the presence of schwertmannite, lamination patterns (graded or non-graded detrital laminations), the occurrence of biological components, and detrital mineral grain sizes. A recurring lithozone featuring non-graded, amorphous clay with schwertmannite, typically overlying either graded or reverse-graded mud, was observed throughout the core. While the mechanisms responsible for sediment deposition have not been fully explored, they are suspected to be related to hyperpycnal flows and slumping events. An Intervaled Sediment Extruder (ISE) was developed for precise sediment extrusion from a core collected in July for 210Pb and 137Cs dating, which will constrain sedimentation rates and correlate depositional events with past environmental conditions. Future work will focus on linking sediment structures to climatic events, identifying annually laminated layers (varves), and completing dating analyses to better understand past climate variations.

Antimony sorption to schwertmannite in acid sulfate environments

Schwertmannite is a poorly-crystalline Fe(III) oxyhydroxysulfate mineral that may control Sb(V) mobility in acid sulfate environments, including acid mine drainage and acid sulfate soils. However, the mechanisms that govern uptake of aqueous Sb(V) by schwertmannite in such environments are poorly understood. To address this issue, we examined Sb(V) sorption to schwertmannite across a range of environmentally-relevant Sb(V) loadings at pH 3 in sulfate-rich solutions. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Sb(V) sorption (at all loadings) involved edge and double-corner sharing linkages between SbVO6 and FeIIIO6 octahedra. The coordination numbers for these linkages indicate that sorption occurred by Sb(V) incorporation into the schwertmannite structure via heterovalent Sb(V)-for-Fe(III) substitution. As such, Sb(V) sorption to schwertmannite was not limited by the abundance of surface complexation sites and was strongly resistant to desorption when exposed to 0.1 M PO43-. Sorption of Sb(V) also conferred increased stability to schwertmannite, based on changes in the schwertmannite dissolution rate during extraction with an acidic ammonium oxalate solution. This study provides new insights into Sb(V) sorption to schwertmannite in acid sulfate environments, and highlights the role that schwertmannite can play in immobilizing Sb(V) within its crystal structure.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments.

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between SbVO6 and FeIII(O,OH)6 octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

Alkaline modification on schwertmannite promoted the simultaneous immobilization of arsenite and cadmium

Arsenic (As) and cadmium (Cd) co-contamination is of most concern in the environment. Schwertmannite (Sch), a poorly crystalline Fe(III) oxyhydroxy-sulfate mineral, is extremely efficient for As immobilization owing to its unique structure, but ineffective against cationic Cd (Cd(II)) due to the acidic pH after Sch addition. In this study, an alkaline modification method was proposed to improve the treatment efficiency of schwertmannite for As(III)-Cd(II) co-contamination. Results showed that alkali soaking had no obvious effect on mineral phase, and schwertmannite was the main mineral component. Alkali-modified Sch (Sch-OH) consisted of a large number of nanoparticles superimposed on each other, with specific surface area drastically increasing from 69.7 m2/g (Sch) to 261.9 m2/g. The sulfate content in Sch-OH structure decreased by 71.5%, which contributed to the reduction of maximum adsorption capacity for As(III) (from 116.09 to 81.06 mg/g at pH 6.5). The co-existing As(III) enhanced Cd(II) adsorption onto Sch-OH by forming ternary Fe-As-Cd complexes, corresponding to an increase in adsorbed amount from 8.8 to 11.3 mg/g (in 20 mg/L Cd(II) solution). Sch-OH exhibited a higher mineral stability than Sch under flooded condition at 40°C, and its retaining ability for As(III) was stronger. In a Cd-As polluted soil, the content of KH2PO4-extractable As decreased from 6.3 to 4.8 mg/kg after 10 days-incubation with 5% Sch-OH, and the immobilization efficiency of Cd reached 49.1%. It is necessary to explore more modification methods of schwertmannite to broaden its application in metal(loid)s remediation.

An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, CrVIO42- can substitute for SO42- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4−8) and the degree of Cr(VI) substitution (0.16–13 mol% CrVIO42--for-SO42- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.

Schwertmannite modified with ethanol: A simple and feasible method for improving As(III) adsorption capacity

As a poorly crystalline Fe(III)–oxyhydroxy sulfate mineral, schwertmannite (Sch) was effectively utilized for the arsenic contamination purification. In this study, in order to improve the adsorption capacity and realize batch production, ethanol modified Sch (EtOH–Sch) has been successfully chemo–synthesized through H2O2 rapid oxidation. As the volume ratio of EtOH to water was 20%, the As(III) removal efficiency by EtOH–Sch was the highest. Through the characterization, EtOH-Sch showed the clear and discrete sea urchin-like spheres with larger specific surface area and optimized structure. EtOH–Sch exhibited excellent adsorption capacity for As(III) in a wide pH range of 6–12. As(III) removal by EtOH–Sch followed the Langmuir model and pseudo-second-order kinetics, with the maximum As(III) adsorption capacity of as high as 194.93 mg/g. In addition, the enhanced mechanism for As(III) adsorption by EtOH–Sch was proposed, including increased active sites and optimized specific structure. The excellent adsorption capacity as well as relatively good reusability of EtOH-Sch indicated its promising prospects in the application of arsenic remediation.

Schwertmannite: A review of its occurrence, formation, structure, stability and interactions with oxyanions

In this article, we provide a critical review on the occurrence, formation, structure and stability of schwertmannite – an enigmatic Fe(III) oxyhydroxy-sulfate mineral that is of widespread interest in the soil science, environmental geochemistry , hydrometallurgy and wastewater fields. We also review interactions between schwertmannite and selected, environmentally relevant oxyanions (arsenate, phosphate, antimonate, selenate , chromate , and molybdate). Schwertmannite is one of the most common minerals to form via direct precipitation of iron in acid sulfate waters at pH 2 – 4. As such, it has been found to occur in systems that are affected by acid mine drainage , acid sulfate soils , natural acid rock drainage, and hydrometallurgical settings involving oxidation of iron-sulfide minerals. Despite its official recognition as a new mineral almost 30 years ago, schwertmannite’s poor crystallinity , fine particle size, variable composition and metastability have constrained our ability to conclusively resolve its crystal structure, possibly polyphasic nature, and solubility/stability. Nevertheless, since the early 1990’s, there has been a steadily-growing body of literature showing that, in acid-sulfate systems, schwertmannite strongly impacts iron and sulfur cycling, acidity dynamics, and electron flow. In addition, interactions with schwertmannite also strongly influence the mobility and fate of co-associated oxyanionic species. In parallel, many oxyanions have themselves been found to also exert a powerful control on schwertmannite’s geochemical behavior and mineralogical stability. Schwertmannite-oxyanion interactions occur via adsorption to the schwertmannite surface, anionic exchange with sulfate in the schwertmannite tunnel structure, and co-precipitation by substitution for either structural sulfate or Fe(III) during schwertmannite formation. This array of distinct sorption mechanisms results in oxyanions being able to both stabilize and conversely destabilize schwertmannite. This distinction appears to reflect an interplay of kinetic and thermodynamic constraints, which depend heavily on the specific oxyanion, its concentration (i.e. level of loading on/in schwertmannite), and other background geochemical conditions (e.g. pH and presence of other ions). Resolving these complex interactions and their importance in controlling schwertmannite occurrence and stability represents a challenge for future research.

Arsenic(V) removal behavior of schwertmannite synthesized by KMnO4 rapid oxidation with high adsorption capacity and Fe utilization

Adsorption is a simple and efficient way for arsenic contamination purification in water, with a pressing challenge to find a cheap and efficient adsorbent. As a poorly crystalline Fe(III)-oxyhydroxy sulfate mineral, schwertmannite can be As(V) adsorbent because of its tunnel structure and low cost. However, the schwertmannite synthesized commonly by H2O2 rapid oxidation suffers from the low Fe utilization and limited As(V) adsorption capacity. In this research, the schwertmannite is synthesized by KMnO4. The results show that the Fe utilization can be improved from 40% to 56%, with the As(V) adsorption capacities double times better than those synthesized by H2O2 at pH 7 and 2. The As(V) adsorption mechanisms at different pHs and the reason for the improvement of As(V) adsorption capacity are thoroughly investigated. The FTIR and EDS images confirm that As(V) adsorption exchange with SO42− is the dominant mechanism at pH 7 and 2. At pH 11, the As(V) is mainly removed by surface complexation because the surface SO42− is exchanged by OH−. The intraparticle diffusion model fitting and XPS results further reveal that the tunnel structure built by Fe–SO4 in the KMnO4 oxidized schwertmannite is more stable, possibly resulting in the better As(V) adsorption performance.

Effects of adsorbed phosphate on jarosite reduction by a sulfate reducing bacterium and associated mineralogical transformation

Jarosite is one of the iron oxyhydroxysulfate minerals that are commonly found in acid mine drainage (AMD) systems. In natural environments, phosphate and sulfate reducing bacteria (SRB) may be coupled to jarosite reduction and transformation. In this research, the effect of phosphate on jarosite reduction by SRB and the associated secondary mineral formation was studied using batch experiments. The results indicated that Fe3+ is mainly reduced by biogenic S2− in this experiment. The effect of PO43− on jarosite reduction by SRB involved not only a physico-chemical factor but also a microbial factor. Phosphate is an essential nutrient, which can support the activity of SRB. In the low PO43− treatment, the production of total Fe2+ was found to be slightly larger than that in the zero PO43− treatment. Sorption of PO43− effectively elevated jarosite stability via the formation of inner sphere complexes, which, therefore, inhibited the reductive dissolution of jarosite. At the end of the experiment, the amounts of total Fe2+ accumulation were determined to be 4.54 ± 0.17a mM, 4.66 ± 0.22a mM, 3.91 ± 0.04b mM and 2.51 ± 0.10c mM (p < 0.05) in the zero, low, medium and high PO43− treatments, respectively, following the order of low PO43− treatment > zero PO43− treatment > medium PO43− treatment > high PO43− treatment. PO43− loading modified the transformation pathways for the jarosite mineral, as well. In the zero PO43− treatment, the jarosite diffraction lines disappeared, and mackinawite dominated at the end of the experiment. Compared to PO43--free conditions, vivianite was found to become increasingly important at higher PO43− loading conditions. These findings indicate that PO43− loading can influence the broader biogeochemical functioning of AMD systems by impacting the reactivity and mineralization of jarosite mineral.

Fabricating Fe3O4-schwertmannite as a Z-scheme photocatalyst with excellent photocatalysis-Fenton reaction and recyclability

Here we reported an effective method to solve the rate-limiting steps, such as the reduction of Fe3+ to Fe2+ and an invalid decomposition of H2O2 in a conventional Fenton-like reaction. A magnetic heterogeneous photocatalyst, Fe3O4-schwertmannite (Fe3O4-sch) was successfully developed by adding Fe3O4 in the formation process of schwertmannite. Fe3O4-sch shows excellent electrons transfer ability and high utilization efficiency of H2O2 (98.5%). The catalytic activity of Fe3O4-sch was studied through the degradation of phenol in the heterogeneous photo-Fenton process. Phenol degradation at a wide pH (3 - 9) was up to 98% within 6 min under visible light illumination with the Fe3O4-sch as heterogeneous Fenton catalyst, which was higher than that using pure schwertmannite or Fe3O4. The excellent photocatalytic performance of Fe3O4-sch is ascribed to the effective recycling between Fe3+ and Fe2+ by the photo-generated electron, and also profit from the formation of the “Z-Scheme” system. According to the relevant data, photocatalytic mechanism of Fe3O4-sch for degrading phenol was proposed. This study not only provides an efficient way of enhancing heterogeneous Fenton reaction, but also gives potential application for iron oxyhydroxysulfate mineral.

Threshold for labile phosphate in a sandy acid sulfate soil

In acid soils, plant availability of fertiliser P is affected by phosphate binding to Fe or Al oxide minerals. However, little is known about the P pools in acid sulfate soils after phosphate fertilisation, or the capacity of the oxyhydroxy sulfate mineral jarosite (KFe3(OH)6(SO4)2) found in these soils, to bind or release added phosphate. Sulfuric acid sulfate soil (initial pH 3.5, adjusted to 5.5) was amended with phosphate at 0, 96, 385, 578 and 770 mg P kg−1 and incubated under submerged conditions. Soil phosphate pools (labile phosphate, moderately labile phosphate, non-labile phosphate and residual phosphate) and amorphous Fe/Al were measured after two and four weeks. Phosphate sorption and its release were determined in incubated soils and jarosite, separately. In soil, maximum phosphate sorption was about 350 mg phosphate kg−1. Labile phosphate was higher than the control (without phosphate addition) at phosphate rates above 350 mg P kg−1 and was more than 50% of added phosphate. The length of submergence had little effect on phosphate pools. Below the threshold of 350 mg phosphate kg−1, absorbed phosphate was strongly bound to the soil whereas 50% of sorbed phosphate was released above the threshold. The high phosphate binding capacity of jarosite (maximal 360 mg phosphate kg−1) confirmed the importance of Fe oxyhydroxy sulfate minerals in P binding in acid soils. However, at maximal phosphate binding, up to 20% of bound phosphate could be released, suggesting that phosphate bound to jarosite may become plant available. The results of this study will help inform fertiliser requirements for crop growth or remediation of acid sulfate soils using plants.

Producing OH, SO4− and O2− in heterogeneous Fenton reaction induced by Fe3O4-modified schwertmannite

Abstract Heterogeneous Fenton has become a promising economical and environmentally-friendly method for water treatment, however, the production of abundant radical resources (e.g., O2−, OH) are oftentimes limited by finite Fe2+ species to activate H2O2. Therefore, development of a new iron-bearing solid modified with diverse functional groups for generating a large of radicals in heterogeneous Fenton is significance. Here, we constructed a Fe3O4-modified schwertmannite (Fe3O4/Sch) catalyst to activate H2O2, SO42− and O2 to produce three kinds of radicals in heterogeneous Fenton reaction. Specifically, Fe3O4/Sch catalyst was synthesized to improve its surface properties such as the contents of surface Fe2+ species as H2O2 or O2 activators to produce OH or O2−, the SO42− of Sch surface as SO4 − precursor, and electron transfer between Fe3+ and Fe2+. Degradation rate of ciprofloxacin (CIP) for 6 min was up to 97% in Fe3O4/Sch-H2O2 system, which was higher than that in Sch-H2O2 or Fe3O4-H2O2. After four successive cyclic experiments, degradation rate of CIP was still achieved 97% within 10 min in Fe3O4/Sch-H2O2 system. The formation pathways of the relevant active radicals and the degradation pathways of CIP were also proposed by theoretically calculated and LC-MS data. This finding has not only developed an efficient heterogeneous Fenton catalyst, but also given a new idea of SO4 − generation by modified iron oxyhydroxysulfate mineral for application of heterogeneous Fenton technology.

Fate of oxalic-acid-intervened arsenic during Fe(II)-induced transformation of As(V)-bearing jarosite

Jarosite is a metastable Fe(III)-oxyhydroxysulfate mineral that can act as an excellent scavenger for arsenic (As) in acid sulfate soils (ASSs) and in areas polluted by acid mine drainage (AMD). The Fe(II)-induced transformation of jarosite can influence the As mobility in reducing soil and sediment systems. Although organic acids are prevalent in these environments, their influence on the behavior of As during the Fe(II)-induced transformation of jarosite is yet to be fully understood. In this study, we investigated the effects of oxalic acid on the partitioning of As into dissolved, adsorbed, poorly crystalline, and residual phases during the Fe(II)-induced transformation of As(V)-bearing jarosite at pH 5.5 and 1 mM Fe(II) concentration. The results demonstrated that jarosite frequently transformed to lepidocrocite in treatments without oxalic acid or with low oxalic acid (0.1 mM), and As was typically redistributed in the surface-bound exchangeable and residual phases. While a high concentration of oxalic acid (1 mM) retarded the transformation of jarosite and produced goethite as the primary end product, it also changed the Fe(II)-induced transformation pathway and drove most As into the residual phase (approximately 92%). The results indicated that oxalic acid exerts a significant influence on the partitioning and speciation of As during the above-mentioned transformation. X-ray photo electron spectroscopy analysis of the reaction products also revealed that As(V) may be still the dominant redox species. Overall, this study provides critical information for understanding the fate of As during the transformation of secondary minerals under complex influencing factors, thereby assisting in more accurately predicting the geochemical cycling of As in natural systems.

Elucidation of desferrioxamine B on the liberation of chromium from schwertmannite

Schwertmannite is a ferric oxyhydroxy-sulfate mineral, which is commonly found in acid sulfate systems. In this study, the liberation of chromium from Cr(VI)-substituted schwertmannite in the presence of the siderophore desferrioxamine B was studied. In addition, mineral transformation was examined using X-ray diffraction, scanning electron microscopy, and X-ray absorption spectroscopy. Based on solid phase analyses, the transformation of schwertmannite incubated at pH 6.5 in the presence of 100 mg/L desferrioxamine B proceeded through a ligand-reductive induced pathway that released dissolved sulfate, Fe(III), and Cr(III), and generated goethite. Results of XAFS showed that CrO42− substituted for SO42− in the schwertmannite framework. Kinetic analysis using the Kabai and shrinking core models showed that Cr suppressed the transformation of schwertmannite into goethite and the liberation of Cr was controlled by diffusion through the secondary mineral's layer of dissolution. The information provided in this study will be useful in predicting the effects of the interaction between dissolved organic matter and minerals on the mobilization of Cr in acid mine drainage systems.

Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Acid sulfate systems commonly contain the metastable ferric oxyhydroxysulfate mineral schwertmannite, as well as phosphate (PO43−) - a nutrient that causes eutrophication when present in excess. However, acid sulfate systems often experience reducing conditions that destabilize schwertmannite. Under such conditions, the long-term fate of both schwertmannite and PO43− may be influenced by interactions during microbially-mediated Fe(III) and SO42− reduction. This study investigates the influence of PO43− on Fe(III) and SO42− reduction and the subsequent mineralogical transformation(s) in schwertmannite-rich systems exposed to reducing conditions. To accomplish this, varied PO43− loadings were established in microbially-inoculated schwertmannite suspensions that were incubated under anoxic conditions for 82 days. Increased PO43− attenuated the onset of microbial Fe(III) reduction. This delayed consequent pH increases, which in turn had cascading effects on the initiation of SO42− reduction and subsequent mineral species formed. Under zero PO43− loading, goethite (αFeOOH) formed first, followed by mackinawite (FeS) and siderite (FeCO3). In contrast, in higher PO43− treatments, vivianite (Fe3(PO4)2) and/or sulfate green rust (FeII4FeIII2(OH)12SO4) became increasingly important over time at the expense of goethite and mackinawite compared to PO43−-free conditions. The findings imply that PO43− loading alters the rates and onset of microbial Fe(III)- and SO42−- reduction and the subsequent formation of secondary Fe-bearing phases. In addition, schwertmannite reduction and the associated mineralogical evolution under anoxic conditions appears to sequester large quantities of PO43− in the form of green rusts and vivianite.

Schwertmannite transformation via direct or indirect electron transfer by a sulfate reducing enrichment culture

Understanding the mechanism of the microbial transformation of Fe(III)-oxyhydroxysulfate minerals is of considerable interest, because this transformation plays an important role in controlling the behaviour of toxic metals from acid mine drainage (AMD). In this study, we examined a sulfate reducing enrichment culture from AMD-contaminated sediments and predicted the possible pathway of electron transfer when incubated with schwertmannite, a common Fe(III)-oxyhydroxysulfate occurring in the AMD environment. Experiments were designed to distinguish the mechanisms by which bacteria facilitate direct (i.e., bacteria allowed to adhere to the mineral) or indirect (i.e., bacteria separated from the mineral by dialysis bag) electron transfer to reduce the mineral. The effects of adding anthraquinone-2,6-disulfonate (AQDS) as an exogenous electron shuttle were also investigated. Vivianite was detected as the main product of schwertmannite transformation. Reduction of sulfate and iron were more pronounced in direct treatments, while more non-reductive dissolution were observed in indirect treatments. The addition of AQDS lead to the production of more dissolved Fe2+ over 20 d than in the absence of AQDS. Microbial community composition differed in direct and indirect treatments, while the addition of AQDS did not significantly affect the community structure in each treatment. After incubation for 20 d, the growth of Desulfovibrio exceeded that of the originally dominant Citrobacter in direct treatments, while an unknown genus most closely related to Citrobacter within Enterobacteriaceae was predominant in indirect treatments. This monodominant community in indirect treatments was assumed not to transfer electron directly to schwertmannite but to rely on shuttling mechanism. PICRUSt results implied that bacteria in indirect treatment have potential to produce shuttling compounds or complexing agents. The absence of dsr genes and the putative fermentative process suggested that the Enterobacteriaceae might indirectly facilitate the dissolution and transformation of schwertmannite.

Phosphate-Imposed Constraints on Schwertmannite Stability under Reducing Conditions.

Schwertmannite is a ferric oxyhydroxysulfate mineral, which is common in acid sulfate systems. Such systems contain varying concentrations of phosphate (PO43-)-an essential nutrient whose availability may be coupled to schwertmannite formation and fate. This study examines the effect of phosphate on schwertmannite stability under reducing conditions. Phosphate was added at 0, 80, 400, and 800 μmoles g-1 (i.e., zero, low, medium, and high loading) to schwertmannite suspensions which were inoculated with wetland sediment and suspended in N2-purged artificial groundwater. pH remained between 2.7 and 4.3 over the 41 day experiment duration. Fe(II) accumulated in solution due to dissimilatory Fe(III)-reduction, which was most pronounced at intermediate PO43- loadings (i.e., in the low PO43- treatment). Partial transformation of schwertmannite to goethite occurred in the zero and low PO43- treatments, with negligible transformation in higher PO43- treatments. Overall, the results suggest that intermediate PO43- loadings provide conditions which facilitate optimal reductive dissolution of schwertmannite. At zero PO43- loading, reductive dissolution appears to be constrained by the rapid transformation of schwertmannite to goethite, which thereby decreases the bioavailability of solid-phase Fe(III). Conversely, at high loadings, PO43- appears to stabilize the schwertmannite surface against dissolution; probably via the formation of strong surface complexes.