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Arsenic is a highly toxic and carcinogenic element, which poses a huge threat to millions of people. The toxicity of arsenic links to its solubility. Therefore, the immobilization of arsenic is of great significance. Among a series of arsenic immobilization materials (include arsenate, arsenite and arsenide), scorodite is the most suitable one due to its excellent stability. In this study, we propose a novel method to synthesize scorodite using ferrihydrite as an in situ iron donator. The process occurs via the dissolution of ferrihydrite followed by the coprecipitation of iron and arsenic ions on the surface of ferrihydrite. A series of experiments were carried out to investigate the optimal reaction conditions and applicable scope of initial arsenic concentration for this process. The results indicated that 99.9% of arsenic was removed from aqueous solution and immobilized as stable scorodite at reaction time of 6 h, pH 1.5, Fe/As molar ratio of 1.1 and reaction temperature of 90°C. This process is applicable to the solution with initial arsenic concentration ranging from 1 to 10 g/L, which shows great potential for practical applications.

In this study, the mechanisms by which ferric iron promotes bioleaching of arsenopyrite by the moderately thermoacidophilic strain Sulfobacillus thermosulfidooxidans YN-22 was evaluated for the first time by integrating Fe L-edge and As/S K-edge X-ray absorption near edge structure (XANES) spectroscopy with Fourier transform infrared spectroscopy (FT-IR), synchrotron radiation X-ray diffraction (SR-XRD), Raman spectroscopy, and scanning electron microscopy (SEM), and the leaching parameters were also determined. The results showed that the addition of Fe3+ (6 g/L) significantly promoted bioleaching of arsenopyrite by enhancing the cell growth and the (bio)degradation of elemental sulfur (S0) and orpiment (As2S3) intermediates on the mineral surface. In addition, an increased formation of loose particulate structures containing the Fe(III)-containing products jarosite (mainly ammoniojarosite), scorodite (ferric arsenate) and schwertmannite was observed.

A novel method has been proposed to prepare an arsenate (As(V))-containing solution for scorodite formation using an arsenic (As) sulphide residue obtained from a Pb refinery. Preparation of the As(V) solution from the As sulphide residue included As oxidation leaching by alternating addition of a highly concentrated sodium hydroxide (c-NaOH 600 g/L) solution and hydrogen peroxide (H2O2) at 70 °C. This method gave an As leaching efficiency of up to 99% (total % recovery of As up to 90%), while at the same time, other metals could be recovered effectively (e.g., 95% Pb, 97% Cd, 90% Zn, and 96% Cu). Further oxidation and removal of the remaining arsenite (As(III)) in the leached filtrate by H2O2 indicated that the oxidation efficiency of oxidizing As(III) to As(V) was up to 99%. The As(V) solution produced by this process was suitable for As(V) fixation as scorodite. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the starting materials and output solids. The results showed that the major As phase in the As-containing compound of the initial As sulphide residue was As sulphide. Highly pure elemental sulphur (total % recovery of S up to 30%) was generated during the S(-II) elimination process, and the scorodite from the As(V) fixation process was highly crystalline. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray absorption near edge structure spectroscopy (XANES) were employed to characterize the intermediate filtrate of the process. The results showed that the intermediate filtrate was a multiphase mixture containing As(III), arsenate, sulphate, thio-arsenate, thio-sulphate and polysulphides (Sn2−). The produced scorodite showed good As stability in the TCLP and at pH 3 (0.47 mg/L), pH 5 (0.26 mg/L), and pH 7 (3.41 mg/L) in a 10-day short-term stability test.

Bauxitic Fe-Ni-laterite ore from the Patitira mine in Lokris area (Greece) contains assemblages of unusual As-bearing goethite-type phases in samples with significant organic matter. The material was initially characterized by bulk ICP-MS and SEM-EDS. Furthermore, the distribution and speciation of As were studied by Synchrotron Radiation (SR) spectroscopic techniques, for first time in the literature. The SR μ-XRF elemental maps and the As K-edge μ-XAFS spectra revealed that As is exclusively correlated to Fe, occurring as As5+ in the form of arsenate anions (AsO43−). However, the arsenate anions, being considered as sorbed species on goethite-type phases, exhibit -at molecular scale- a disordered structural environment, resembling locally to the configuration of such anions in natural hydrated Ni arsenates (annabergite) rather than to natural hydrated ferric arsenates (scorodite). The metalloid element in study, possibly derived from As-mineralizations or volcanic rocks, has been transferred in the environment of the laterite re-deposition, where arsenates could interact with the aforementioned Fe-phases.

The critical environmental situation in the region of southwestern Siberia (Komsomolsk settlement, Kemerovo region) is the result of the intentional displacement of mine tailings with high sulfide concentrations. During storage, ponds of acidic water with incredibly high arsenic (up to 4 g/L) and metals formed on the tailings. The application of chemical methods to treat these extremely toxic waters is implemented: milk of lime Ca(OH)2, sodium sulfide Na2S, and sodium hydroxide NaOH. Field experiments were carried out by sequential adding pre-weighed reagents to the solutions with control of the physicochemical parameters and element concentrations for each solution/reagent ratio. In the experiment with Ca(OH)2, the pH increased to neutral values most slowly, which is contrary to the results from the experiment with NaOH. When neutralizing solutions with NaOH, arsenic-containing phases are formed most actively, arsenate chalcophyllite Cu18Al2(AsO4)4(SO4)3(OH)24&middot, 36H2O, a hydrated iron arsenate scorodite, kaatialaite FeAs3O9&middot, 8H2O and Mg(H2AsO4)2. A common specificity of the neutralization processes is the rapid precipitation of Fe hydroxides and gypsum, then the reverse release of pollutants under alkaline conditions. The chemistry of the processes is described using thermodynamic modeling. The main species of arsenic in the solutions are iron-arsenate complexes, at the end of the experiments with Ca(OH)2, Na2S, and NaOH, the main species of arsenic is CaAsO4&minus, the most toxic acid H3AsO3 and AsO43&minus, respectively. It is recommended that full-scale experiments should use NaOH in the first stages and then Ca(OH)2 for the subsequent neutralization.

Arsenic sulfide slag (ASS) is typically by-produced from arsenic-containing wastewater treatment. In this work, a novel hydrothermal treatment method with the assistance of Fe(NO3)3 (HT-Fe(NO3)3) was developed to detoxify ASS by transforming arsenic into scorodite and extracting sulfur in one step. After hydrothermal treatment, As(III) in ASS was oxidized and immobilized into the stable scorodite with a high As immobilization efficiency (~99%), and the toxicity leachability of arsenic-containing solid waste significantly reduced from 634.2 to 2.5 mg/L, well below the discharge standard of solid waste. Further study reveals that the nucleation and growth process was fit well by Avrami-Erofeev model and followed Ostwald step rule, which involved the As2S3 dissolution, formation of amorphous ferric arsenate and then crystallization within the amorphous precursor. In this process, sulfur originated from As2S3 played an important role by serving as the heterogeneous nuclei to decrease the barrier for the formation of amorphous ferric arsenate, and facilitated the transformation of as-formed scorodite from nano-sheet aggregates to the bulk and dense spherical polymorph, which further increased the stability of the arsenic contained solid product. This study will shed light on the development of new technologies for treatment of industrial solid waste and recycle of useful resources.

Arsenic (As), is the 20th most abundant natural element in the earth’s crust. Furthermore, an important source of drinking water contamination and undesirable element overproduced during the extraction of valuable metals from low-grade ores. To date, ferric arsenate, scorodite is the most appropriate carrier for long-term arsenic fixation in metallurgical processes. Previously, the biological scorodite crystallization (bioscorodite) from diluted As(V) streams demonstrated being cost-efficient and sustainable solution for the treatment of diluted As(V) solutions. Since As(III) is predominant in acid effluents and scorodite needs the As to be in the As(V) an oxidation previous oxidation step is required. In this work entitled Expanding the bioscorodite process for As(III) wastewater remediation, we explored at laboratory scale, the possibilities of arsenic immobilization from the treatment acid As(III)-bearing solutions through the biological oxidation and precipitation in a single unit process aiming to broaden the range of application of the bioscorodite concept. In this thesis, the proof of principle, reactor selection and operational conditions of the bioscorodite crystallization were studied. The main findings of this research demonstrated the feasibility of the simultaneous As(III) oxidation catalyzed by granular activated carbon (GAC) and biological Fe(II) oxidation and precipitation in a singles system leading as the main product the formation of highly stable scorodite crystals with low arsenic leachability. Furthermore, the predominance of thermoacidophilic archaea and identification of EPS like-structures indicated that scorodite formation is mediated by the microbial surface or by the exopolymeric organic components. The use of alternative Fe(II) sources such as pyrite and abundant mineral in hydrometallurgical processes was successfully tested indicating the potential of this cheaper source towards reducing the cost of the process. Besides the use of GAC as a cheaper catalyst and the use of cheaper alternative iron sources, other benefits of the proposed biological process include 1) the lower Fe/As molar ratio of the compact precipitate (close to 1), translating into a significant lower Fe consumption and a lower mass of the solid waste residue. 2) Zero cost related to the use of external seeds of stepwise neutralization since thermoacidophilic microorganisms mediate arsenic precipitation and, 3) the reusability of the GAC in the process. The work presented in this thesis demonstrated that the biological scorodite precipitation takes place simultaneously to the catalyzed As(III) by activated carbon oxidation, thereby providing a novel alternative for arsenic immobilization and safe disposal of As from acidic wastewater streams (i.e., metallurgical effluents).

The role of secondary minerals in controlling the migration of arsenic and selected metals (Cu, Pb, Zn) has been investigated in voluminous mining waste dumps at Kaňk (northern part of the Kutna Hora ore district, Czech Republic). These wastes initially contained a large amount of Fe sulfides and arsenopyrite and have been exposed to atmosphere for approximately 500 years. The long-term weathering under acidic conditions caused dissolution of almost all sulfides and significant alteration of primary silicate minerals, producing deeply weathered As-, Fe-, and S-rich waste material (As: 13 g kg−1; Fe: 74 g kg−1; S: 44 g kg−1) with large proportion of clayey matrix filling the space between rock fragments. Detailed mineralogical investigation (powder X-ray diffraction, electron microprobe, Raman spectroscopy) and sequential extraction revealed that arsenic in these wastes is principally stored in reactive but not highly soluble minerals such as poorly-crystalline Fe (oxyhydr)oxides, bukovskýite, and X-ray amorphous hydrated ferric arsenate (HFA). Lesser fraction of As is contained in schwertmannite and less reactive scorodite, jarosite, and well-crystalline Fe (oxyhydr)oxides (especially goethite). Fe (oxyhydr)oxides also appear to be the main reservoirs of Cu and Zn; Pb is principally stored in jarosite. Acidic pore waters (pH ≈ 2.8) collected after rainfall events contain much Al (113 mg L−1), As (2.5 mg L−1), Ca (500 mg L−1), Cu (24 mg L−1), Fe (58 mg L−1), Si (44 mg L−1), SO42− (2170 mg L−1) and Zn (16 mg L−1). Geochemical modeling revealed that most of these elements (Al, Ca, Cu, SO42−, Zn) are controlled by ephemeral Al-Ca-Cu-Zn sulfates (e.g. gypsum, aluminite), which form at the surface of the waste dumps as a result of evaporation of pore solutions during dry seasons. Despite the fact that bukovskýite and other interesting Fe(III) (sulfo)arsenate minerals store a major part of As in the mining wastes, geochemical modeling supported the notion that mobility of As and Fe is controlled by the unspectacular poorly-crystalline As-bearing Fe (oxyhydr)oxides. The data show that the Fe(III) (oxyhydr)oxides, (sulfo)arsenates and hydroxysulfates in the 500-year-old mining waste dump retain arsenic efficiently but not completely. Monitoring wells installed at the site before our research recorded up to 1440 mg L−1 As in shallow groundwater. They argue, however, that this aquifer is disconnected from the larger groundwater bodies at the site and hence does not represent arsenic accumulation of environmental concern.

This study revealed the importance of SO_4^ 2− ions during biogenic scorodite crystallization via a two-stage As-removal process, using a combination of liquid and solid analyses (chemical digestion, FT-IR, SEM, TG-DTA, particle distribution). The first-stage As-removal was induced by microbial oxidation of Fe^2+ and As(III), precipitating SO_4^ 2− -bearing amorphous precursors composed of basic ferric sulfate (MFe_x(SO_4)_y(OH)_z) and ferric arsenate (FeAsO_4·(2+n)H_2O). This was followed by an induction period (a period of constant concentration), where dissolution-recrystallization of unstable amorphous precursors proceeded: Re-dissolved metal ions became locally concentrated on the surface of precursors, which gave the driving force for the second-stage As-removal as secondary layers of crystalline biogenic scorodite (Fe(AsO_4)_0.94(SO4)_0.08·1.69H_2O) out of even more dilute and seeded solution. This phase transformation process was also accompanied by continuous dehydration. This two-stage As-removal via SO_4^ 2- -mediated phase transformation was shown to be key to promote biogenic scorodite formation with greater final As-removal from dilute As(III)-bearing solutions.

Thermodynamic data for the arsenates of various metals are necessary to calculate their solubilities and to evaluate their potential as arsenic storage media. If some of the less common arsenate minerals have been shown to be less soluble than the currently used options for arsenic disposal (especially scorodite and arsenical iron oxides), they should be further investigated as promising storage media. Furthermore, the health risk associated with arsenic minerals is a function of their solubility and bioavailability, not merely their presence. For all these purposes, solubilities of such minerals need to be known. In this work, a complete set of thermodynamic data has been determined for mansfieldite, AlAsO4·2H2O; angelellite, Fe4(AsO4)2O3; and kamarizaite, Fe3(AsO4)2(OH)3·3H2O, using a combination of high-temperature oxide-melt calorimetry, relaxation calorimetry, solubility measurements, and estimates where possible and appropriate. Several choices for the reference compounds for As for the high-temperature oxide-melt calorimetry were assessed. Scorodite was selected as the best one. The calculated Gibbs free energy of formation (all data in kJ·mol–1) is –1733.4 ± 3.5 for mansfieldite, –2319.2 ± 7.9 for angelellite and –3056.8 ± 8.5 for kamarizaite. The solubility products for the dissolution reactions are –21.4 ± 0.5 for mansfieldite, –43.4 ± 1.5 for angelellite and –50.8 ± 1.6 for kamarizaite. Available, but limited, chemical data for the natural scorodite–mansfieldite solid-solution series hint at a miscibility gap; hence the non-ideal nature of the series. However, no mixing parameters were derived because more data are needed. The solubility of mansfieldite is several orders of magnitude higher than that of scorodite. The solubility of kamarizaite, on the other hand, is comparable to that of scorodite, and kamarizaite even has a small stability field in a pH-pε diagram. It is predicted to form under mildly acidic conditions in acid drainage systems that are not subject to rapid neutralization and sudden strong supersaturation. The solubility of angelellite is high, and the mineral is obviously restricted to unusual environments, such as fumaroles. Its crystallization may be enhancedviaits epitaxial relationship with the much more common hematite. The use of the scorodite–mansfieldite solid solution for arsenic disposal, whether the solid solution is ideal or not, is not practical. The difference in solubility products of the two end-members (scorodite and mansfieldite) is so large that almost any system will drive the precipitation of essentially pure scorodite, leaving the aluminium in the aqueous phase.

Scorodite (FeAsO4·2H2O) is the most popular phase for arsenic (As) immobilization while the reductive dissolution of Fe(III) to Fe(II) will promote As release. In the present study, an equilibrium between Fe(III) and Fe(II) was achieved in scorodite preparation system by introducing certain alcohol (methanol, ethanol, isopropanol or tert-butanol), and thus a new mixed-valent iron arsenate black crystal formulated as Fe(II)5.2Fe(III)8.8(HAsO4)4(AsO4)8·H2O was prepared. In comparison with scorodite, the black crystal has higher As content (36.4%, mass fraction) and lower crystal water content (0.73%, mass fraction). Additionally, the leaching concentration of As can be lower than the threshold value (5 mg/L) regulated by identification standards for hazardous wastes of China (GB 5080.3–2007). Therefore, this new mixed-valent iron arsenate crystal could be classified as a non-hazardous and promising As-bearing phase in environmental applications.

Fe(III)–As(V) precipitates were synthesised from the Fe(II)-As(V)-SO42--H2O system at a temperature of 90°C and a constant pH maintained at 1.5 ± 0.05. The precipitates obtained were characterised by X-ray diffraction (XRD), chemical composition analysis, scanning electron microscope (SEM) and Raman spectrum, thermogravimetric analyzer (TGA) and electron probe micro-analyzer (EPMA). The precipitates were in irregular aggregation of about 1–4 μm in size. The precipitates consisted of scorodite, ferric arsenate and an amorphous ferric hydroxide sulphate formulated as Fe(OH)x(SO4)y. The precipitates were stable in modified Toxicity Characteristic Leaching Procedure (TCLP) tests at pH 4.93 for 60 h. Arsenic concentrations in the leaching solutions of 0.27 mg L−1 and 0.59 mg L−1 were obtained for the precipitates prepared initial Fe(II)/As(V) molar ratios of 4.0 and 5.0, respectively. Significantly more iron than arsenic was dissolved with up to 280 mg L−1 of iron reporting to solution. Long-term stabi...

The coprecipitation and subsequent aging of Fe(III)-As(III)-As(V)-SO42− play an important role in controlling As behavior in acidic systems, such as acid mine drainage and hydrometallurgical acid waste. In this study, we investigated the redistribution and transformation of As in the Fe(III)-As(III)-As(V)-SO42− system (As(III)/As(V) ≈ 1) at different Fe/As molar ratios (i.e., 0.25, 0.5, and 1) and pH (1.2 and 1.8) at 60 °C. The results showed that As(III) and SO42− can be incorporated into the amorphous ferric arsenate and scorodite host phases by forming a Fe(AsO4)x(AsO3)y(SO4)z solid solution. As(III) contents in the freshly coprecipitated solids increased with pH and initial As(III) concentrations. During aging process, As(III) contents in the solid products with Fe/As molar ratios of 0.5 and 1 increased with aging time at pH 1.8. In contrast, As(III) was gradually expelled from aging products with aging time at pH 1.2, regardless of Fe/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy characterization results showed that an As(III)-SO42−-doped scorodite was formed at Fe/As molar ratio ≤0.5 during the aging process. It was also found that As(III) had an inhibitory effect on the transformation of poorly crystalline ferric arsenate to scorodite. The present study may have important implications for understanding the geochemical cycle of As, Fe, and SO42− in acidic solutions and give further understanding on the mechanisms involved in As removal and fixation in hydrometallurgical unit operations.

Commercial copper (Cu) is obtained by a hydro-pyrometallurgical process, where the Cu anodes obtained in the furnaces (Cu > 99.5%) are enriched up to 99.99% in “cathodes” by electrorefining at an electrolysis plant. During this process, some impurities accumulate in the electrolyte, mainly arsenic (As), which decrease the quality of the Cu cathode. For this reason, the electrolyte is sent to an electrolyte cleaning plant (ECP) for its purification. Electrolyte sludge (ES) is produced in the last stage of purification and is recirculated back to the furnace due to the high Cu content. This recirculation involves a severe problem of As accumulation in the industrial process. The objective of this work was to develop a procedure to fully dissolve the ES, removing the As and recovering its Cu content. The ES dissolution process was optimised (dissolution efficiency > 99%) in H2SO4 (1.4 M)/HNO3 (1.8 M) medium using a 1:20 g mL−1 solid-to-liquid ratio. As was removed from the ES solution by its precipitation as iron (III) arsenate, with high efficiency (more than 70%). After As removal, the Cu can be precipitated as copper sulphate, which is used in several applications.

Red mud (RM) as waste of industrial aluminum production is piling up in huge ponds. RM could be a cost-effective adsorbent for heavy metals, but adsorption is vulnerable to pH changes, metal ions speciation and the occurrence of iron bearing minerals. In this study, the precipitation and elemental speciation transformation relevant to arsenic fate in responding to the addition of RM during arsenopyrite bio-oxidation by Sulfobacillus thermosulfidooxidans was investigated. The results show that the addition of RM significantly changed the arsenic precipitation and the solution chemistry and thus affected the arsenopyrite bio-oxidation and arsenic fate. An addition of a small amount (≤ 4 g/L) of RM substantially promoted arsenopyrite bio-oxidation with formation of SiO2 @ (As, Fe, Al, Si) spherical nanoparticles that can enhance the stability of the immobilized arsenic. The SiO2-based spherical nanoparticles precipitate was mainly composed of jarosites, amorphous ferric arsenate and crystalline scorodite, and its formation were controlled by Fe3+ concentration and solution pH. An addition of increased amount of RM (≥ 6 g/L) resulted in a significant increase of the solution pH and a decrease in the Fe2+ bio-oxidation activity, and spherical nanoparticles were not formed. Consequently, the dissolution of arsenopyrite was inhibited and the release of arsenic was blocked. This study suggests the applicability of RM in mitigation of arsenic pollution from bio-oxidation of As-bearing sulfide minerals.

AREVA Resources Canada operates the McClean Lake Operation (MLO), a uranium mine and processing facility located in northern Saskatchewan. Uranium-containing ores processed at the MLO contain high concentrations of arsenic that is oxidized to soluble arsenite and arsenate species when leaching and recovering uranium. To reduce the environmental impact of AREVA's mining operations, AREVA has developed a tailings preparation process designed to trap arsenic in a mineral form before the tailings are permanently deposited in the JEB Tailings Management Facility (TMF). Scorodite (FeAsO 4 ·2H 2 O) is predicted to be the primary arsenic species produced from the tailings preparation process. However, scorodite has never been observed in aged tailings samples. Confirming the presence of scorodite in the tailings is important in verifying that the tailings preparation process at the MLO can isolate high concentrations of arsenic from the environment in the form of stable minerals. Herein, X-ray Absorption Near-Edge Structure (XANES) spectroscopy was used to investigate arsenic speciation in a series of samples collected from the JEB TMF in 2013. Arsenic K-edge XANES analysis confirmed that most (86 wt%) of the arsenic content in the tailings samples consisted of iron-containing arsenates. Of these, crystalline scorodite was the most abundant arsenate species followed by poorly crystalline arsenates in the form of poorly crystalline ferric arsenate (FeAsO 4 ·xH 2 O) and arsenate adsorbed on ferrihydrite (AsO 4 –FeOOH). Arsenite adsorbed on ferrihydrite (AsO 3 –FeOOH), and gersdorffite (NiAsS) were also identified as minor arsenic species in the tailings samples. The abundance and distribution of scorodite in the TMF suggests that it is the major arsenic species produced in the tailings preparation process.

Iowaite with designed Mg/Fe ratios of 2.5, 3.0, 3.5, 4.0 and 5.0 was synthesized for treating both arsenate- and arsenite-spiked waters in this study. The experimental results show that there is a positive correlation between the molar ratio of Fe to Mg of synthetic iowaite and its arsenic uptake capacity. The dearsenication mechanisms were investigated based on the XRD, SEM and EDX analyses of solid samples before and after reaction with arsenic-bearing solutions. When the initial solution concentration of arsenic is not higher than 7500 μg/L, anion exchange between arsenate or arsenite in solution and chloride in the interlayer regions of iowaite is the primary mechanism for arsenic removal. However, at a very high initial arsenic concentration in solution (750 mg/L), the dissolution of iowaite and subsequent formation of new minerals, such as scorodite and karibibite, are responsible for the substantial removal of arsenic from water. Iowaite is superior to other types of layered double metal hydroxides (LDHs) such as hydrotalcite and hydrocalumite in terms of water dearsenication, and therefore promising for treatment of arsenic-rich waters, either naturally occurring waters or industrial wastewaters.

An analytical protocol was developed to identify arsenic (As) in soil samples and in the ≤10 µm fraction of surface dust samples (fine surface dust-FSD). Single-particle identification of trace As constituents was undertaken by combining scanning electron microscopy with automated image analyses and high-resolution, transmission electron microscopy. Two forms of As association with iron and aluminum nanoscale phases were identified. In the predominant one, As was identified in oriented aggregates formed by crystalline nanoparticles of Fe-(hydr)oxides. In the FSD samples, As was additionally detected in an assembly of hematite and goethite nanocrystals forming larger particles of few hundreds of nanometers, often entangled with phyllosilicates. These mixed phases carried various elements, such as P, Ba, Pb, among others. Even rare As-bearing phases (e.g., l to 9 particles out of approx. 30,000 particles analyzed), such as arsenopyrite and ferric arsenate, and possibly scorodite were identified in some samples. The developed analytical protocol brings a novel and practical contribution to As speciation in environmental samples.

Objective To enable removal of highly toxic As(III) from acidic waters by inducing indirect microbial As(III) oxidation by Fe-oxidizing bacteria via carbon-assisted redox-coupling between As(III) oxidation and Fe reduction. / Results Carbon-fiber (CF) was shown to function as an electron-mediator to catalyze chemical (abiotic) redox-coupling between As(III) oxidation and Fe reduction. Accordingly, by taking advantage of Fe regeneration by Fe-oxidizing bacteria, it was possible to promote oxidative removal of As(III) as ferric arsenate at moderate temperature. This reaction can be of use under the situation where a high-temperature treatment is not immediately available. Arsenic once concentrated as ferric arsenate on carbon-fibers can be collected to undergo phase-transformation to crystalline scorodite as the next re-solubilization/re-crystallization step at a higher temperature (70°C). / Conclusions While extremely acidophilic Fe-oxidizing bacteria are widely found in nature, the As-oxidizing counterparts, especially those grown on moderately-thermophilic and mesophilic temperatures, are hardly known. In this regard, the finding of this study could make a possible introduction of the semi-passive, low-temperature As-treatment using readily available Fe-oxidizing bacteria.

Secondary arsenic minerals (SAM) formed recently in abandoned adits of the former Au-As mine at Zloty Stok (SW Poland) constitute two assemblages. The first consists of two types of scorodite, pitticite, kaňkite, hornesite, picropharmacolite and minor amounts of jarosite and gypsum. Formation of the Fe arsenates took place under acidic conditions (pH ~3–4) as a result of lollingite, arsenopyrite and pyrite oxidation. Hornesite and picropharmacolite crystallized as products of interactions between acidic arsenic-rich pore solutions with Mg-Ca carbonates from rocks that surround the ore mineralisation. The interaction of carbonates with acid pore solutions caused a rapid increase in pH that reached neutral or weakly alkaline values. The chemical compositions of hornesite and picropharmacolite correspond well to their ideal compositions: (Mg 3.17 Ca 0,07 ) Σ3.24 (AsO 4 ) 1.90 × 8H 2 O and Ca 4.31 Mg 0.92 (HAsO 4 ) 1.91 [(AsO 4 )1.99(SO 4 )0.01] Σ2.00 × 11H 2 O, respectively. The second assemblage of SAM comprises exclusively the Mg-enriched erythrite [(Co 1.66 Mg 1.03 Ni 0.28 Ca 0.05 Zn 0.02 ) Σ3.03 (AsO 4 )1.99 × 8H 2 O)] – annabergite [(Ni 1.48 Mg 0.94 Co 0.66 Ca 0.12 Fe 0.01 Zn 0.01 ) Σ3.20 (AsO 4 )1.92 × 8H 2 O] series. These minerals crystallized from slightly acidic (pH ~5–6) to neutral media. Dissolution of SAM and other secondary phases (e.g., schwertmannite) causes the release of arsenate and sulphate ions into mine waters. These ions can be reduced under anaerobic conditions by different strains of bacteria. The product of this process is orpiment

Nonferrous smelting wastewaters containing high levels of arsenic and sulfuric acid threaten human health and ecological environments due to insufficient disposal technologies and/or possible leakages during transportation to long-term storage facilities. Chemical precipitation is one important route for arsenic removal and immobilization via the formation of arsenic-stabilized minerals. In this study, we propose a new strategy for the treatment of wastewaters containing high levels of arsenic using pyrite as an in situ S and Fe source to prepare environmentally friendly scorodite and As2S3 precipitates. Dissolved pyrite release (S-II) and Fe (II) ions in highly acidic wastewater, both of which drive arsenic precipitation. Sulfide ions readily react with As(III) to generate the stable As2S3 while Fe(II)/As(III) were oxidized to Fe(III)/As(V) by H2O2. The reaction between Fe(III) and As(V) took place on the pyrite surface as a nucleation site to form amorphous ferric arsenate that ultimately transformed into crystalline scorodite. Sulfide ions were oxidized to H2SO4 that further promoted pyrite dissolution. Up to 99.4% of arsenic was successfully removed from copper smelting wastewaters with an initial arsenic concentration of 11200 mg/L. The concentration of leached As from the As-bearing precipitate was 1.78 mg/L under optimized conditions, which were: a 1.4 FeS2/As molar ratio, an initial solution pH of 2, an H2O2/As molar ratio of 1%, a reaction temperature of 90 °C, and a reaction time of 12 h. In summary, a prospective process successfully shows arsenic removal and immobilization from smelting wastewater, and further demonstrates the potential for the efficient and affordable disposal of wastewater.

Multi-metal extraction of flue dust from copper smelting is being pursued to meet the increasingly appealing environmental requirements of our modern life. In this paper, the thermodynamic analysis of acid leaching solution was carried out. Ferric arsenate deposits were found in the range of pH = 0∼11.63, in the form of FeAsO4 and FeAsO4∙2H2O. Cu2AsO4OH, CuHAsO4, and Cu5H2(AsO4)4 forms of Cu2AsO4 were produced in the range of pH = 0.98∼2.33. Zn3(AsO4)2, Zn2AsO4OH, ZnHAsO4, and Zn5H2(AsO4)4 forms of zinc arsenate were produced in the range of pH = 1.40∼4.85. Theoretically, when pH = 0.98∼2.33, copper arsenate salt was produced; however, when the actual experimental results were at pH ≥ 1.2, a small amount of copper co-settling occurred. Theoretical results were verified by single factor experiments. Strictly controlling the pH value of the reaction system can effectively separate arsenic (As) from copper (Cu) and zinc (Zn). Through single factor experimentation, optimal process parameters of iron salt and calcium salt combined with As removal were as follows: reaction time, 5 h; reaction temperature, 85 °C; pH value, 1.2; Fe-As molar ratio, 1.5; no H2O2 added. Under these conditions, the precipitation rate of As was 89.12%, while Cu and Zn did not precipitate. Through XRD analysis, As element was removed from acid leaching solution in the form of scorodite (FeAsO4∙2H2O). This has laid a theoretical foundation for the effective removal of As in acid leaching solution and the reduction of As content.

Acid mine drainage (AMD) formation and toxic arsenic (As) contamination are serious environmental problems encountered worldwide. In this study, we investigated the crucial roles played by common secondary mineral phases formed during the natural weathering of pyrite-bearing wastes—soluble salts (melanterite, FeSO4·7H2O) and metal oxides (hematite, Fe2O3)—on AMD formation and As mobility under acidic conditions (pH 1.5–4) prevalent in historic tailings storage facilities, pyrite-bearing rock dumps and AMD-contaminated soils and sediments. Our results using a pyrite-rich natural geologic material containing arsenopyrite (FeAsS) showed that melanterite and hematite both directly—by supplying H+ and/or oxidants (Fe3+)—and indirectly—via changes in the nature of oxidation layer formed on pyrite—influenced pyrite oxidation dynamics. Based on SEM-EDS, DRIFT spectroscopy and XPS results, the oxidation layer on pyrite was mainly composed of ferric arsenate and K-Jarosite when melanterite was abundant with/without hematite but changed to Fe-oxyhydroxide/oxide and scorodite when melanterite was low and hematite was present. This study also observed the formation of a mechanically ‘strong’ coating on pyrite that suppressed the mineral’s oxidation. Finally, As mobility under acidic conditions was limited by its precipitation as ferric arsenate, scorodite, or a Fe/Al arsenate phase, including its strong adsorption to Fe-oxyhydroxides/oxides.

Mining and mineral processing operations may generate a wide range of As-rich compounds including scorodite (FeAsO4·2H2O), a common secondary arsenate in near-surface environments and some gold mine tailings. Scorodite has a low solubility in a limited pH range, and it is relatively stable under near-surface conditions, but its behavior under the influence of bacteria is not well understood. Considering that reducing conditions are likely to develop in mine tailings with depth and under prolonged disposal conditions, the present study was undertaken to determine the influence of bacteria on the reductive dissolution of scorodite. Because the systematic studies on the microbial reduction of scorodite are lacking, we used two well characterized dissimilatory iron and arsenic reducing bacteria, i.e., Shewanella sp. ANA-3 and Shewanella putrefaciens CN32 in a chemically defined media, at circumneutral pH, containing various phosphate concentrations (i.e., 15, 40 and 400 μM). The initial rates of reduction and the plateau concentrations of reduced species formed in the aqueous phase were greater in the presence of Shewanella sp. ANA-3 than in the presence of Shewanella putrefaciens CN32. The initial rate of reduction was found to increase with increasing phosphate concentration, however, plateau concentrations of the dissolved reduced species, Fe(II) and As(III), formed in the aqueous phase were highest at the lowest phosphate concentration. The solid products of the post-reduction samples were characterized by synchrotron X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (XRD), scanning, transmission and high resolution electron microscopy (SEM/TEM/HRTEM) and energy dispersive spectrometry (EDS). The results indicate that the post reduction solids contained scorodite, a biogenic ferrous arsenite and parasymplesite (Fe3(AsO4)2·8H2O). Phosphate concentrations had an effect on the concentrations of released Fe(II) and As (III) during the microbial reduction and the formation of secondary phases. The influence of bacteria on the reductive dissolution of scorodite must be taken into consideration during mine waste management and disposal operations because of the potential of arsenic releases to terrestrial and aquatic environments as toxic and soluble As(III) species.

Experiments were carried out to test the amenabilities of mineral deposits that contained cobalt deported in arseno-sulfide (cobaltite) and arsenide (skutterudite) minerals, to oxidative bioleaching at mesophilic temperatures and low pH. An ore sample from the Iron Mask deposit (Canada) and a mineral concentrate from a working mine (Bou Azzer, Morocco) were thoroughly characterised, both prior to and following bio-processing. A “top down” approach, using microbial consortia including (initially) 13 species of mineral-degrading acidophiles was used to bioleach the ore and concentrate in shake flasks and bioreactors. Cobalt was successfully liberated from both materials tested (up to 93% from the ore, and 49% from the concentrate), though the chemistries of the leach liquors were very different, with redox potentials being >200 mV lower, and concentrations of soluble arsenic about 7-fold greater, with the concentrate. Addition of pyrite to the arsenide concentrate was found to promote the biomineralisation of scorodite (ferric arsenate), which was detected by both XRD and SEM-EDX, but was not found in bioleached residues of the arseno-sulfide ore. A model was proposed wherein pyrite had three critical roles in facilitating the genesis of scorodite: (i) providing the catalytic surface to promote the oxidation of As (III) to As (V); (ii) acting as a putative “seed” for scorodite crystallisation; (iii) being a secondary source of iron, since the molar ratios of iron:arsenic in the concentrate itself (0.19:1) was well below that required for effective removal of soluble arsenic as scorodite (1:1). This work provided proof of concept that cobalt arseno-sulfide and arsenide ores and concentrates are amenable to bio-processing, and also that it is possible to induce concurrent solubilisation of arsenic from primary minerals and immobilisation in a secondary mineral, scorodite.

Primary production in Mono Lake, a hypersaline soda lake rich in dissolved inorganic arsenic, is dominated by Picocystis strain ML. We set out to determine if this photoautotrophic picoplankter could metabolize inorganic arsenic and in doing so form unusual arsenolipids (e.g., arsenic bound to 2-O-methyl ribosides) as reported in other saline ecosystems and by halophilic algae. We cultivated Picocystis strain ML on a seawater-based medium with either low (37 &micro, M) or high (1000 &micro, M) phosphate in the presence of arsenite (400 &micro, M), arsenate (800 &micro, M), or without arsenic additions (ca 0.025 &micro, M). Cultivars formed a variety of organoarsenic compounds, including a phytyl 2-O-methyl arsenosugar, depending upon the cultivation conditions and arsenic exposure. When the cells were grown at low P, the organoarsenicals they produced when exposed to both arsenite and arsenate were primarily arsenolipids (~88%) with only a modest content of water-soluble organoarsenic compounds (e.g., arsenosugars). When grown at high P, sequestration shifted to primarily water-soluble, simple methylated arsenicals such as dimethylarsinate, arsenolipids still constituted ~32% of organoarsenic incorporated into cells exposed to arsenate but &lt, 1% when exposed to arsenite. Curiously, Picocystis strain ML grown at low P and exposed to arsenate sequestered huge amounts of arsenic into the cells accounting for 13.3% of the dry biomass, cells grown at low P and arsenite exposure sequestered much lower amounts, equivalent to 0.35% of dry biomass. Extraction of a resistant phase with trifluoroacetate recovered most of the sequestered arsenic in the form of arsenate. Uptake of arsenate into low P-cultivated cells was confirmed by X-ray fluorescence, while XANES/EXAFS spectra indicated the sequestered arsenic was retained as an inorganic iron precipitate, similar to scorodite, rather than as an As-containing macromolecule. Samples from Mono Lake demonstrated the presence of a wide variety of organoarsenic compounds, including arsenosugar phospholipids, most prevalent in zooplankton (Artemia) and phytoplankton samples, with much lower amounts detected in the bottom sediments. These observations suggest a trophic transfer of organoarsenicals from the phytoplankton (Picocystis) to the zooplankton (Artemia) community, with efficient bacterial mineralization of any lysis-released organoarsenicals back to inorganic oxyanions before they sink to the sediments.

Iron-arsenate (FeIII-AsV) coprecipitate generated in acid mine drainage and hydrometallurgical practices is one of the potential sources of As contamination in the environment. However, the As behavior as well as the transformation process of the FeIII-AsV coprecipitate is still not fully understood under acidic conditions. This work systematically studied the effect of reaction media (sulfate vs. nitrate), Fe/As molar ratios, and acidic pH values on the transformation rates, routes, and products of the FeIII-AsV coprecipitates at different temperatures during up to 4.5 years of aging. X-ray diffraction (XRD), transmission electron spectroscopy (TEM), Raman spectroscopy, and extended X-ray absorption fine structure spectroscopy (EXAFS) results showed that the FeIII-AsV coprecipitates with Fe/As = 2 transformed to scorodite at pH 4 in both sulfate and nitrate media with poorly crystalline ferric arsenate as precursor. In comparison, for the Fe/As = 4 samples, parascorodite and hematite were formed in sulfate and nitrate media, respectively. Arsenic was released into the liquid phase during the aging process for the samples with Fe/As = 2 and 4. The aqueous As concentrations ([As]aq) in sulfate media were larger than those in nitrate media for both the Fe/As = 2 and 4 samples. In addition, the transformation products were goethite and Na-jarosite at Fe/As = 8 and 16, pH 2.5 with the decrease of [As]aq. The results also indicated that 6-line ferrihydrite was a common intermediate for many Fe-As transformation processes.

Zýkaite, nominally Fe 4 (AsO 4 ) 3 (SO 4 )(OH)⋅15H 2 O, is a common ferric arsenate in damp, moist environments, especially in underground spaces where the humidity does not decline during the year and the temperature fluctuations are minimal. The natural sample used in this study had chemical composition Fe 1.23 (AsO 4 ) 0.93 (PO 4 ) 0.07 (SO 4 ) 0.31 (OH) 0.07 ⋅5.89H 2 O and typical poor crystallinity. The solubility product, derived from solubility experiments, is log K sp = −25.77 for the reaction Fe 1.23 (AsO 4 ) 0.93 (PO 4 ) 0.07 (SO 4 ) 0.31 (OH) 0.07 ⋅5.89H 2 O + 0.07 H + = 1.23Fe 3+ + 0.93AsO 4 3− + 0.07PO 4 3− + 0.31SO 4 2− + 5.96H 2 O. Enthalpy of formation, determined by acid-solution calorimetry, is −2952.2 ± 2.9 kJ mol −1 . Gibbs free energy of formation, calculated from the log K sp value, is −2485.1 kJ mol −1 . The entropies of zýkaite, estimated for a solid and calculated from its enthalpy and Gibbs free energy of formation, are in a fair agreement and document the accuracy of the thermodynamic data. Variable-temperature powder X-ray diffraction showed that the structure of zýkaite gradually collapses and turns amorphous at ∼90 °C. No crystalline phase formed upon heating to 200 °C. Zýkaite is one of the least stable phases in the system Fe 2 O 3 –As 2 O 5 –SO 3 –H 2 O, restricted to environments with low pH and very high activities of As(V) and S(VI). Zýkaite and kaňkite (FeAsO 4 ⋅3.5H 2 O) are often seen on the gently sloping rock faces or on the rock walls, whereas stalactites of hydrous ferric arsenate forms below them on the overhangs. Thermodynamic modeling with our data show that the transformation of zýkaite to other ferric arsenates and sulfoarsenates (scorodite, parascorodite, kaňkite, bukovskýite, hydrous ferric arsenates) is thermodynamically driven. Comparing and combining our thermodynamic data, modeling, variable-temperature X-ray diffraction, and field observations, it seems that zýkaite transforms readily to other phases in the system Fe 2 O 3 –As 2 O 5 –SO 3 –H 2 O via dissolution and precipitation; a solid-state transformation is unlikely.

The need for improved understanding of the phase relationship in the system Fe–AsO4–SO4 involving ferric arsenate, scorodite, and ferrihydrite motivated a series of synthesis experiments at 40 °C and 70 °C, and pH 0.5 to 1.5, from solutions with variable compositions. The precipitates were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM with selected-area electron diffraction, SAED), and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques. Amorphous ferric arsenate is the initial metastable phase forming from solutions with As/S molar values of as low as 0.1, as a precursor to scorodite. The onset of scorodite formation is exponentially delayed with increased pH and at lower temperatures. High-resolution TEM images reveal that amorphous ferric arsenate occurs as randomly oriented stubby nanoparticles showing no changes in size and crystallinity with increased synthesis. Simultaneous fitting of the As K edge and Fe K -edge EXAFS spectra of a series of ferric arsenate precipitates resulted in local structural parameters that are similar to those reported earlier, supporting a local structure made of short polymers containing chains of corner-linked octahedra instead of the framework model or a scorodite-like local structure. The EXAFS modeling results can also be conceptualized as short chains of alternating octahedra and tetrahedra without the Fe–Fe linkages of the original chain model, suggesting that there is no unique EXAFS solution. Experimental results indicate that As can be fixed as scorodite through precipitation under ambient conditions from arsenic-rich effluents in the pH range 0.5–4.5. Improved understanding of the kinetics of phase transformations and their dependence on pH and temperature would facilitate the development of new stabilization technologies involving scorodite or ferrihydrite (or both) by improved controls on the behavior of arsenic in mine wastes. Stabilization of As through controlled precipitation of ferrihydrite during incongruent dissolution of scorodite at pH greater than about 2, which would maximize the uptake of As and optimize the kinetics of FeO6 polymerization without the presence of ferric arsenate, would be the most desirable outcome.

Arseniosiderite and yukonite are among the important arsenate minerals occurring as secondary alteration products in relation to the oxidation of arsenopyrite and arsenian pyrite and as discrete grains in some gold ores, mine tailings, and contaminated soils. Characteristics of these Ca-Fe arsenate species are not well known and our understanding of the conditions promoting their formation and dissolution is limited. Long- and short-range structural characteristics and thermodynamic properties of the Ca-Fe arsenates forming in the Ca-Fe(III)-As(V)-NO3 system were determined to better predict the mineralogical transformations taking place in neutralized sludge and tailings environments, and their influence on arsenic mobilization. Yukonite and arseniosiderite readily form from solutions with highly variable compositions at a wide pH range from slightly acidic to alkaline conditions. Calcium concentrations corresponding to molar Ca/(Ca+Fe+As) ratios as low as 0.1 appear to be adequate for their formation. Our experimental results confirm observations in natural settings and mine tailings where scorodite is progressively replaced by yukonite and arseniosiderite. The initial amorphous precipitates made of small oligomeric units of edge-sharing FeO6 octahedra with bridging arsenate evolve to yukonite through the establishment of corner linkages between the FeO6 chains. Yukonite represents a nanocrystalline precursor and Ca-deficient variety of arseniosiderite. Formation of arseniosiderite is kinetically controlled with faster development of crystallinity at neutral to slightly acidic pH and slower kinetics under alkaline conditions. Calorimetric measurements provided an enthalpy of formation value of −1950.3 ± 3.1 kJ/mol and standard entropy of 237.4 ± 4.4 J/(mol·K) for arseniosiderite [with composition Ca0.663Fe1.093(AsO4)(OH)1.605·0.827H2O], the corresponding Gibbs free energy of formation is −1733 ± 3.4 kJ/mol. A rough estimate of the thermodynamic properties of yukonite is also provided. Arseniosiderite is a stable arsenate between pH 3.5 and 7.5 in solutions saturated with respect to soluble Ca minerals such as calcite, gypsum, anorthite, or Ca-montmorillonite. Arsenic release from mine wastes and contaminated soils can be effectively controlled by arseniosiderite and the conditions promoting its formation such as lime-treatment leading to gypsum saturation in ferric arsenate solutions would prove to be desirable for stabilizing arsenic in the form of arseniosiderite in mine wastes.

A refractory gold concentrate with 19% arsenic was treated by a mixed moderately thermophiles in an airlift bioreactor through an adaptation protocol. The moderately thermophiles could respond well to 20% (w/v) pulp density with less than 10% loss of productivity, and resist arsenic up to 15 g/L. There were a lot of jarosite, arsenolite and sulfur, but not scorodite and ferric arsenate in the bioleached residue. Jarosite passivation and lower sulfur-oxidizing activity of the cells due to the toxicity of the high concentrations of soluble arsenic and iron ions at low pH value should mainly response for the incomplete extraction at high pulp density. The initial bacterial community did not change in nature except for new found P aeruginosa ANSC, but sulfur-oxidizing microorganisms have been dominant microorganisms after a long time of adaptation. Pseudomonas aeruginosa originating from the gold concentrate should be closely relative to the metabolism of the organic matters contained in the refractory gold concentrate.

The metalloids arsenic and antimony are mobilised through the decomposition of arsenopyrite, pyrite, and stibnite at the historic Big River Mine. Their content in the solid substrates reach levels up to ≈25 wt% As and 3.6 wt% Sb, but their concentrations in the mine water are only elevated locally, up to 0.85 mg/L dissolved As and 0.007 mg/L dissolved Sb. Water draining the waste rock are characterised by neutral pH, but at the mine’s former processing site, the oxidation of sulphide concentrates has produced acidic conditions, both in the water (pH 3.8) and solid mining residues (paste pH 2–4). Dissolved metalloid concentrations are being effectively attenuated on site through the formation of secondary As phases in the mining residues, including scorodite, iron sulphoarsenate, and amorphous iron arsenate. Attenuation is also occurring by co-precipitation and adsorption onto iron oxides or hydroxides, and adsorption onto clay mineral surfaces. These processes are complemented by dilution of the dissolved As and Sb by the Big River. Therefore, despite the extremely high levels of As and Sb in the mining residues, metalloid mobilisation is limited, resulting in relatively low concentrations in water leaving the site.

BACKGROUND Ferric arsenate solids in the form of scorodite (FeAsO4·2H2O) particles are suitable carriers for immobilization of arsenic-rich wastes. The stability of scorodite is, however, highly pH dependent (typically at 4≤ pH ≤7) and satisfactory only under oxic disposal conditions. In this work a new stabilization technology based on the concept of encapsulation with aluminum hydroxyl gels is investigated to enhance the stability of arsenical solids over a wider range of disposal conditions. RESULTS The encapsulation system investigated involves blending and short-term ageing of synthetic scorodite particles with amorphous aluminum hydroxyl gels derived from partial hydrolysis of aluminum chloride or aluminum sulfate salts. Of the two gel types, the Al(SO4)1.5-derived gel proved to be the most effective, even at the very low Al(III)/As(V) molar ratio of 0.1, apparently due to in situ development of protective aluminum hydroxide matrix and not as a simple adsorption sink for soluble arsenate species. CONCLUSION Arsenic release from the scorodite–aluminum hydroxyl gel composites was found to be drastically reduced with respect to gel-free scorodite, making this system a very interesting candidate for further development as an effective hazardous material encapsulating material. © 2014 Society of Chemical Industry

A new analytical protocol was developed to provide quantitative, single-particle identification of arsenic in heterogeneous nanoscale mineral phases in soil samples, with a view to establishing its potential risk to human health. Microscopic techniques enabled quantitative, single-particle identification of As-bearing phases in twenty soil samples collected in a gold mining district with arsenic concentrations in range of 8 to 6354 mg kg−1. Arsenic is primarily observed in association with iron (hydr) oxides in fine intergrowth with phyllosilicates. Only small quantities of arsenopyrite and ferric arsenate (likely scorodite) particles, common in the local gold mineralization, were identified (e.g., 7 and 9 out, respectively, of app. 74,000 particles analyzed). Within the high-arsenic subgroup, the arsenic concentrations in the particle size fraction below 250μm ranges from 211 to 4304 mg kg−1. The bioaccessible arsenic in the same size fraction is within 0.86–22 mg kg−1 (0.3–5.0%). Arsenic is trapped in oriented aggregates of crystalline iron (hydr)oxides nanoparticles, and this mechanism accounts for the low As bioaccessibility. The calculated As exposure from soil ingestion is less than 10% of the arsenic Benchmark Dose Lower Limit - BMDL0.5. Therefore, the health risk associated with the ingestion of this geogenic material is considered to be low.

Arsenic is an abundant element associated with a wide range of minerals and a major contaminant in metallurgical wastewater. For the immobilization of arsenic, iron arsenate in the very stable mineral scorodite (FeAsO4 2H2O) is the preferred route. Microorganisms of the natural iron cycle living at pH below 2 and high temperatures can conduct the oxidation of ferrous iron with oxygen, which is not feasible chemically at these extreme conditions. Remarkably, at similar acidic conditions and high temperature these microorganisms can also carry out the oxidation of arsenite (As(III)) to arsenate (As(V)). Using these intrinsic features of the microorganisms, we have investigated the role of a thermoacidophilic mixed culture in the oxidation of As(III) and precipitation of (As(V) in the form of scorodite from a synthetic wastewater containing 6.7mM of As(III) and 0.5%Wt pyrite as main iron Fe(II) source. The results indicate that As(III) was completely oxidized from the synthetic wastewater in the presence of pyrite and scorodite was formed only in presence of the mixed culture at a Fe/As:1.3. This is a combination of biological oxidation and biocrystallisation accomplished to the presence of pyrite not only as the main energy source for the microorganisms, but as catalyst in the As(III) oxidation reaction.

The geochemical mobility of arsenic and some metals in the fine fraction of 20 alluvial sediment samples collected along the thalweg of the San Antonio arroyo was assessed in the abandoned gold mining district of the semi-arid southeastern Baja California peninsula. Acid-digested element concentrations were determined by treating the fine fraction of the sediments with concentrated HNO3 via microwave heating. A separate leaching of the diluted acid-soluble As and some metals from the samples was carried out using 1 M HCl. The most reactive As phase was extracted from the subsamples using an ascorbic solution of Na citrate buffered at pH 8. The high abundance of sulphosalts (tennantite Cu12As4S13 and proustite Ag3AsS3) in the sediments from the head of the arroyo changes downstream to the predominance of arsenic trioxide (arsenolite As2O3) and arsenate (scorodite FeAsO4·2H2O). Acid-digested As concentrations were always found to be high (230–270 mg kg−1) from the San Antonio village until the end of the arroyo. The concentration of reactive As and its relative contribution increased from low values of 2.3 mg kg−1 and 1 %, respectively, at the upstream of the arroyo, to 191–220 mg kg−1 and 75–89 % in the middle and lower parts, and is probably a result of As release from the mineral sulfide fraction of the tailings. Metallic contaminants exhibited different behavior, with a peak of acid-digested and diluted acid-leached concentrations observed in the central part of the arroyo and an almost permanent geochemical mobility.

Jarosite is one of the principal precipitates from process waters of the pressure-oxidation autoclave system at the Macraes gold mine, and this jarosite is discharged with the process waters to a tailings impoundment for long-term storage. Dissolved sulfate and arsenate concentrations in the autoclave, inferred from mineralogy, both exceed 0.1 mol/L (≈10,000 mg/L). Coarse-grained (≈1 mm) jarosite crystals, precipitated as mineral scales, provide insight into the mineralogical nature of the jarosite. The jarosite crystals are zoned, with some zones having up to 12 wt% Al, as part of the jarosite-alunite solid solution series. Substitution of up to 1.7 wt% Na for K also occurs. Both the monovalent and trivalent sites in the jarosites have apparent deficiencies (≈10 atm.%) because of protonation of the hydroxyl groups. There is

Cesarferreiraite, Fe2+Fe23+(AsO4)2(OH)2·8H2O, is a new laueite-group mineral (IMA 2012-099) of triclinic symmetry, from Eduardo pegmatite mine, Conselheiro Pena municipality, Minas Gerais, Brazil. Intimately associated minerals are pharmacosiderite, scorodite, and earlier arsenopyrite, and probably cesarferreiraite replaces the latter. It occurs as fibrous-to-tabular aggregates up to 2 mm. Single crystals, up to 10 μm long with a thickness of about 1–2 μm, are elongated along [001] and flattened on (100). The fibers have almost rectangular cross-section apparently bound by the {100} and {010} pinacoid forms. Color and streak are pale to greenish yellow. Luster is vitreous; individual crystals are transparent and masses are translucent. Cleavage is distinct, presumably on {010} and {100}. Calculated density is 2.934 g/cm3. The mineral is biaxial (+), n (min) = 1.747(3), n (max) = 1.754(3) (589 nm). IR spectrum of cesarferreiraite is unique and can be used for the identification of the mineral. Chemical composition ( n = 4, WDS, calculated for the condition Fe2+:Fe3+ = 1:2, H2O for the ideal structural formula, wt%) is: FeO 11.50, Fe2O3 25.56, CaO 15.41, As2O5 33.51, H2O 26.01, total 100.12. The empirical formula (based on 18 O apfu) is Fe2+0.98Fe3+1.96[(AsO4)1.79(PO4)0.31](OH)1.52·8.08H2O. The strongest eight X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 9.85(95)(010), 6.35(100)(001), 3.671(29)(121), 3.158(32)(130), 2.960(39)(022), 2.884(35)(131), 2.680(29)(211), and 2.540(23)(210). Unit-cell parameters refined from powder data indexed by analogy with related laueite-group minerals (space group: P 1) are: a = 5.383(2), b = 10.363(3), c = 6.878(2) A, α = 96.42(4), β = 109.19(3), γ = 102.30(2)°, V = 347.1(2) A3, and Z = 1. Gladstone-Dale compatibility is −0.020 (excellent). Cesarferreiraite is the arsenate analog of ferrolaueite.

A bioaccessibility test was carried out in four tailings collected at a former mining area in Delita, Cuba. A previous risk assessment study identified arsenic (As) as the main critical contaminant in this area and showed that the tailings had high As concentrations (up to 3.5%). This study aimed at: (i) evaluating As bioaccessibility in four tailings (R1, R2, R3 and R4) from a gold mining area to obtain a better health risk estimate; and, (ii) identifying the mineral phases responsible for most of the bioaccessible As using XRD, SEM-EDS, and XAS. The results showed that bioaccessible As in the tailings ranged from 0.65 to 40.5%. The main factors influencing As bioaccessibility were a high occurrence of amorphous iron arsenate; occurrence, even at low content, of iron oxyhydroxides and stability of mineral phases in the environment of the gastrointestinal tract. Although arsenopyrite, arsenates and goethite were confirmed by mineralogical methods such as optical microscopy, XRD, and SEM-EDS, XAS showed that scorodite–oxidation state As(+V)–was dominant in most of the tailings. This confirms that the low bioaccessibility of As in most of the tailings is due to the slow kinetics of As release from scorodite.

Scorodite is an important arsenic carrier for the immobilization of arsenic generated in the metallurgical industries. The transformation of the amorphous ferric arsenate into scorodite was investigated in a mixed sulfate media (pH 1-2) and elevated temperature (80-95 °C). This transformation process has been traced via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thus, it was found that lower pH and higher temperature was favored to generate more stable scorodite. The scorodite with TCLP leachability of 3.58 mg/L As was produced at 95 °C and initial pH 1.2 for 12 h.

Short-range ordered ferric arsenate (FeAsO4 · xH2O) is a secondary As precipitate frequently encountered in acid mine waste environments. Two distinct structural models have recently been proposed for this phase. The first model is based on the structure of scorodite (FeAsO4 · 2H2O) where isolated FeO6 octahedra share corners with four adjacent arsenate (AsO4) tetrahedra in a three-dimensional framework (framework model). The second model consists of single chains of corner-sharing FeO6 octahedra being bridged by AsO4 bound in a monodentate binuclear (2)C complex (chain model). In order to rigorously test the accuracy of both structural models, we synthesized ferric arsenates and analyzed their local (

Samples from drilling of the Bi9r Tawilah gold prospect in Saudi Arabia reveal the occurrence of a Ca–Fe arsenate phase, which is similar in appearance and chemistry to yukonite. Upon weathering of a granodiorite host, oxidation of arsenopyrite (0–25 m deep) leads to the formation of a very peculiar brown, amorphous to very poorly crystalline aggregate with cellular-like texture. This mixture consists of Ca–Fe arsenate and arsenic-rich ferric oxyhydroxide resulting from the oxidation of arsenopyrite. It is intergrown with colloform ferric oxyhydroxide, the latter resulting from the oxidation of pre-existing pyrite. The EMPA analyses indicate that the Ca-rich domain contains the maximum As 2 O5 content (up to 22.3 wt%) whereas the colloform ferric oxyhydroxide contains the highest amount of Fe 2 O 3 among the sample studied (60.8–63.1 wt%) associated to higher H 2 O content (31.4–33.2 wt%) than in the case of common goethite and lepidocrocite. As far as typical yukonite, scorodite or arsenosiderite are absent in the studied weathered granodiorite, it is believed that oxidation took place at elevated pH (>7) and temperature up to ∼75 °C. The source of Ca 2+ can be derived from alteration of plagioclase in the granodiorite but its possible derivation from strongly corroded marble bands cannot be discarded. It is evident that availability of Ca 2+ and high pH buffered by the dissolution of calcite in the marble, in addition to the prevailing temperature upon weathering, played important roles in the formation of these pseudomorphs at Bi9r Tawilah.

Nearly half a century after mine closure, release of As from the Ylojarvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As2O5) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kankite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylojarvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondarymore » Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit« less

In CSTRs, ferrous iron was biologically oxidized followed by crystallization of scorodite (FeAsO 4 ·2H 2 O) at pH 1.2 and 72 °C. The CSTRs were fed with 2.8 g L −1 arsenate and 2.4 g L −1 ferrous and operated at an HRT of 40 h, without seed addition or crystal recirculation. Both oxidation and crystallization were stable for periods up to 200 days. The arsenic removal efficiency was higher than 99% at feed Fe/As molar ratios between 1 and 2, resulting in effluents with 29 ± 18 mg As L −1 . Arsenic removal decreased to 40% at feed Fe/As molar ratios between 2 and 5. Microorganisms were not affected by arsenic concentrations up to 2.8 g As 5+ L −1 . The bioscorodite solid yield was 3.2 g/g arsenic removed. Bioscorodite crystals precipitated as aggregates, causing scaling on the glass wall of the reactor. The observed morphology through SE microscopy of the precipitates appeared amorphous but XRD analysis confirmed that these were crystalline scorodite. Arsenic leaching of bioscorodite was 0.4 mg L −1 after 100 days under TCLP conditions, but when jarosite had been co-precipitated leaching was higher at 0.8 g L −1 . The robustness of the continuous process, the high removal efficiency and the very low arsenic leaching rates from bioscorodite sludge make the process very suitable for arsenic removal and disposal.

A comprehensive understanding of the risk associated with metal-rich soils and other materials includes identification of the solid phases hosting the metals. Synchrotron microanalysis provides a powerful diagnostic tool to characterize metal-bearing particles in mine tailings, soils, lake sediments, windblown dust, and household dust. A near simultaneous combination of X-ray fluorescence, diffraction, and absorption experiments using a microfocused beam can provide information on elemental concentrations, crystal structure, and oxidation state of individual particles. This approach can distinguish multiple metal-hosting minerals and industrial compounds in a single sample. Our objective is to provide examples of the application of this technique to a range of materials representing potential risk to human or ecosystem health. These examples include arsenic-contaminated materials and metal-rich household dust. We have identified grains of scorodite and other arsenate minerals in mine tailings and...