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Arsenic-alkali slag (AAS) is one of the most hazardous solid wastes generated by the antimony smelting process, posing a significant environmental risk. However, the economic method for effective As immobilization in AAS is still lacking. In this study, we proposed an economical and novel method for the removal of As from AAS using siderite (FeCO3). After the sodium carbonate is recovered from the AAS leaching liquor, the process continues. Aqueous As(V) is immobilized as scorodite via continuous dissolution–oxidation of siderite (FeCO3) by air injection. The residual As(V) is immobilized via Fe-As coprecipitation by lime neutralization. The dissolution–oxidation of siderite produces Fe(III), ensuring continuous Fe(III)-As(V) coprecipitation and scorodite crystallization. The results showed that the removal efficiency of As from AAS reached 99.98%. The mixture of scorodite and Fe-As coprecipitation demonstrated good As stability in the Toxicity Characteristic Leaching Procedure (2.6 mg/L) and at pH 4 (1.84 mg/L), pH 6 (2.13 mg/L), and pH 8 (3.20 mg/L) in a 50-d long-term stability test. Combined results from chemical analysis, X-ray diffraction, and infrared spectroscopy revealed that the existence of high ionic strength Na+ and Cl– were the critical factors in the dissolution of siderite and the crystallization of scorodite. This study provides an effective treatment method for the removal and fixation of arsenic in metallurgical AAS. [ABSTRACT FROM AUTHOR]

In order to deposit arsenic residues from copper production in a stable way, the trivalent arsenic must first be xidized to arsenic(V). A well-known but quite expensive method for this is oxidation with hydrogen peroxide. In order to enable the oxidation of arsenic on a large scale in the future, a potentially cheaper method has to be found, which offers the possibility of oxidizing extremely high arsenic concentrations. As a novel alternative, electrochemical oxidation using a boron-doped diamond electrode is investigated. Based on previous work, this paper concentrates on the presence of interfering ions during oxidation. Furthermore, it is shown that the electrochemically xidized arsenic(V) can be precipitated as scorodite. Finally, an economic analysis shows the potential financial benefit of oxidation via BDD electrodes compared to hydrogen peroxide. [ABSTRACT FROM AUTHOR]

In situ treatment of acidic arsenic-containing wastewater from the non-ferrous metal smelting industry has been a great challenge for cleaner production in smelters. Scorodite and iron arsenate have been proved to be good arsenic-fixing minerals; thus, we used lead slag as an iron source to remove arsenic from wastewater by forming iron arsenate and scorodite. As the main contaminant in wastewater, As(III) was oxidized to As(V) by H2O2, which was further mineralized to low-crystalline iron arsenate by Fe(III) and Fe(II) released by lead slag (in situ generated). The calcium ions released from the dissolved lead slag combined with sulfate to form well-crystallized gypsum, which co-precipitated with iron arsenate and provided attachment sites for iron arsenate. In addition, a silicate colloid was generated from dissolved silicate minerals wrapped around the As-bearing precipitate particles, which reduced the arsenic-leaching toxicity. A 99.95% removal efficiency of arsenic with initial concentration of 6500 mg/L was reached when the solid–liquid ratio was 1:10 and after 12 h of reaction at room temperature. Moreover, the leaching toxicity of As-bearing precipitate was 3.36 mg/L (As) and 2.93 mg/L (Pb), lower than the leaching threshold (5 mg/L). This work can promote the joint treatment of slag and wastewater in smelters, which is conducive to the long-term development of resource utilization and clean production. [ABSTRACT FROM AUTHOR]

The incorporation of sulfate ( S O 4 2 − ) into the scorodite (FeAsO4·2H2O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of S O 4 2 − speciation and local structure in scorodite lattice is still lacking. In this study, X‑ray difraction (XRD), Fourier transform infrared spectroscopy (FTIR), sulfur K-edge X‑ray absorption near edge structure (XANES), and extended X‑ray absorption fine structure (EXAFS) spectroscopic analyses in combination with density functional theory (DFT) calculations were used to determine the local coordination environment of S O 4 2 − in the naturally and hydrothermally synthesized scorodite. The S O 4 2 − retention in natural scorodite and the effect of pH value and initial Na+ concentration on the incorporation of S O 4 2 − in synthetic scorodite were investigated. The results showed that trace amounts of S O 4 2 − were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that S O 4 2 − was homogeneously distributed inside the natural and synthetic scorodite particles, and its content in the synthetic scorodite increased slightly with the initial Na+ concentration at pH of 1.2 and 1.8. The FTIR features and XANES results indicated that the coordination number (CN) of FeO6 octahedra around S O 4 2 − in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 Å with an average of ~1.47 Å, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 Å with an average of ~3.33 Å. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) Å with a CNS-Fe = 4 for S O 4 2 − in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that S O 4 2 − in the scorodite lattice may be in the form of a Fe2(SO4)3-like local structure. The present study is significant for understanding the formation mechanism of scorodite in natural environments and hydrometallurgical unit operations for waste sulfuric acid treatment. [ABSTRACT FROM AUTHOR]

With the objective of enhancing the stability of scorodite, its encapsulation with hydroxyapatite (Ca5(PO4)3OH) (HAP) and fluorapatite (Ca5(PO4)3F) (FAP) surface coatings, the two most stable of the calcium phosphates, inert to pH and redox potential variations, are presented in this work. The experimental work includes: (1) determination of the metastable zone for HAP and FAP precipitation, (2) the synthesis of crystalline scorodite under atmospheric conditions using hydrothermal scorodite seed and its characterization, (3) the coating of scorodite with hydroxyapatite and fluorapatite with supersaturation-controlled heterogeneous crystallization, and (4) the long-term stability of the encapsulated scorodite solids. Hydroxyapatite and fluorapatite were prepared with homogeneous precipitation from a metastable solution to which reagents were added at a controlled flow rate. Crystalline scorodite was produced with seeding precipitation and encapsulated with a direct apatite (HAP or FAP) deposition that was controlled by adjusting the pH and reagent addition. The stability tests in oxic and anoxic environments over the pH range of 5–9 showed the release of arsenic from the apatite-coated scorodite to be much lower than from naked scorodite, thereby demonstrating that apatite-based encapsulation of hazardous materials is technically feasible and merits further consideration for development into an arsenic stabilizing technology. [ABSTRACT FROM AUTHOR]

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 situ treatment of acidic arsenic-containing wastewater from the non-ferrous metal smelting industry has been a great challenge for cleaner production in smelters. Scorodite and iron arsenate have been proved to be good arsenic-fixing minerals; thus, we used lead slag as an iron source to remove arsenic from wastewater by forming iron arsenate and scorodite. As the main contaminant in wastewater, As(III) was oxidized to As(V) by H2O2, which was further mineralized to low-crystalline iron arsenate by Fe(III) and Fe(II) released by lead slag (in situ generated). The calcium ions released from the dissolved lead slag combined with sulfate to form well-crystallized gypsum, which co-precipitated with iron arsenate and provided attachment sites for iron arsenate. In addition, a silicate colloid was generated from dissolved silicate minerals wrapped around the As-bearing precipitate particles, which reduced the arsenic-leaching toxicity. A 99.95% removal efficiency of arsenic with initial concentration of 6500 mg/L was reached when the solid–liquid ratio was 1:10 and after 12 h of reaction at room temperature. Moreover, the leaching toxicity of As-bearing precipitate was 3.36 mg/L (As) and 2.93 mg/L (Pb), lower than the leaching threshold (5 mg/L). This work can promote the joint treatment of slag and wastewater in smelters, which is conducive to the long-term development of resource utilization and clean production.

With the objective of enhancing the stability of scorodite, its encapsulation with hydroxyapatite (Ca5(PO4)3OH) (HAP) and fluorapatite (Ca5(PO4)3F) (FAP) surface coatings, the two most stable of the calcium phosphates, inert to pH and redox potential variations, are presented in this work. The experimental work includes: (1) determination of the metastable zone for HAP and FAP precipitation, (2) the synthesis of crystalline scorodite under atmospheric conditions using hydrothermal scorodite seed and its characterization, (3) the coating of scorodite with hydroxyapatite and fluorapatite with supersaturation-controlled heterogeneous crystallization, and (4) the long-term stability of the encapsulated scorodite solids. Hydroxyapatite and fluorapatite were prepared with homogeneous precipitation from a metastable solution to which reagents were added at a controlled flow rate. Crystalline scorodite was produced with seeding precipitation and encapsulated with a direct apatite (HAP or FAP) deposition that was controlled by adjusting the pH and reagent addition. The stability tests in oxic and anoxic environments over the pH range of 5–9 showed the release of arsenic from the apatite-coated scorodite to be much lower than from naked scorodite, thereby demonstrating that apatite-based encapsulation of hazardous materials is technically feasible and merits further consideration for development into an arsenic stabilizing technology.

The synthesis of biogenic scorodite combined with the oxidation of As(III) catalysed by granular activated carbon (GAC) was previously demonstrated. However, the colloidal size of the formed scorodite particles is still a bottleneck, as it would hinder the easy separation of the precipitates in a full-scale application. Here, we studied the effect of GAC concentration on biological scorodite precipitation at thermoacidophilic conditions in batch experiments. Higher arsenic removal efficiency and precipitation of larger and more stable scorodite particles were found only in biotic tests and at low catalyst concentration of 4 g L−1. Furthermore, with 4 g L−1 GAC, the Fe and As predominantly precipitated in solution while with 20 g L−1 GAC the Fe and As predominantly precipitated on the GAC. For experiments with 4 and 20 g L−1 of GAC, the average particle size was 66 and 2.6 μm, respectively. This could be explained by the lower saturation level of the solution at the lower GAC level. This study shows for the first time that the oxidative catalytic capacity of GAC can be used to influence crystallization of scorodite.

In this paper, a scheme is proposed for the treatment of arsenic-containing lead slime by the combination of acid pressure oxidation leaching and forming scorodite. On the basis of thermodynamic calculations, the effects of six factors including acid concentration, oxygen partial pressure (pO2), liquid to solid ratio (L/S), agitating speed, leaching time and temperature for the removal of arsenic were studied in an acid pressure oxidation leaching process, then the optimum leaching conditions were established: L/S of 10 mL/g, leaching time of 2.5 h, pO2 of 2.0 MPa, leaching temperature of 170 °C, acid concentration of 100 g/L and stirring speed of 300 r/min. Under the optimal conditions, the leaching rate of arsenic from lead slime reached 99.10% and the arsenic content of the leaching residue was about 0.80%. After a decontamination procedure, the total arsenic concentration in the acid solution obtained from leaching experiments was 37.18 g/L, and the initial pH was 0.50. Finally, as high as 98.5% of arsenic extracted from the lead slime was stabilized in the form of scorodite (FeAsO4·2H2O) by the precipitation process under the following conditions: initial pH value of 1.0, Fe(II)/As molar ratio of 1.3, pO2 of 2.5 MPa, temperature of 160 °C and precipitation time of 2.0 h.

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 Fe3+ reduction. Results: Carbon-fiber (CF) was shown to function as an electron-mediator to catalyze chemical (abiotic) redox-coupling between As(III) oxidation and Fe3+ reduction. Accordingly, by taking advantage of Fe3+ 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Micro- to nanoscale gold fills the microcracks in arsenopyrite, and is closely associated with Bi minerals. • Gold is polycrystalline and consists of nanocrystallites 20–30 nm in size. • Arsenopyrite is replaced by scorodite. During this sulfide oxidation, second generation of colloidal-like gold was co-precipitated with scorodite. • A possible low-temperature hydrothermal origin of scorodite is indicated. Arsenopyrite and pyrite are important carriers of Au that is present in the form of microscopic and "invisible" gold. Such gold can be expected to be released upon the process of oxidation of these sulfides. In studied gold mineralization at Drenjak, related to the granitoids of the Oligocene-Miocene Kopaonik Ore District, the main arsenopyrite-pyrite-(bismuthinite) sulfide assemblage was formed in the quartz bodies and partly replaced by scorodite (FeAsO 4 ·2H 2 O) and Bi-arsenate. With only up to 5 ppm of gold recorded by LA-ICP-MS, aforementioned sulfide minerals are here practically free from lattice-bound gold. Native gold is present in the mineralization, generally ranging in grain size from 20 µm down to the nanoparticles, as revealed by TEM. It is mainly hosted by arsenopyrite and scorodite. In arsenopyrite, gold is polycrystalline and consists of nanocrystallites 20–30 nm in size. It was deposited after arsenopyrite, healing its cavities and microcracks, often associated with native bismuth and bismuthinite. The abundance of gold and Bi minerals correlates with the presence of scorodite. Two types of gold are present in scorodite and Bi arsenate: i) prevailing relict Au grains retained after arsenopyrite oxidation, ii) colloidal-like gold co-precipitated with the arsenates. Based on our observations, we additionally present and discuss indications of low-temperature hydrothermal origin of scorodite, formerly known only as a weathering product [ABSTRACT FROM AUTHOR]

Scorodite is an important mineral not only for arsenic (As) removal from industrial wastewaters but also in the mobility and final fate of As in waste rocks, contaminated soils and sediments, and mine tailings. Because of the mineral's high As-loading capacity and stability, numerous studies have been done to understand its formation. Unfortunately, most of these studies were limited to elevated temperatures (>70 °C), so the processes involved in scorodite formation under ambient conditions remain unclear. This study provides evidence of the catalytic effects of hematite on the formation of scorodite at 25 °C in a pyrite-rich natural geologic material. Scorodite peaks were detected in the XRD patterns of the leaching residues with and without hematite, but those in the former were stronger and more pronounced than the latter. These results suggest that the formation of scorodite was catalysed by hematite, a generalisation that is further supported by strong characteristic IR absorption bands of scorodite at 819 cm–1 (As–O bending vibration), 785 and 725 cm–1 (As–O stretching vibrations), and 2990 cm−1 (OH-vibration) as well as the distinct XPS binding energies of Fe(III)–As (709.7 eV), As(V)–O (44.8, 44.31 and 43.7 eV), O2− (530.5 eV) and coordinated water (531.3 eV) in scorodite. This phenomenon could be attributed to three possible mechanisms: (1) more rapid precipitation promoted by the "seeding" effect of hematite particles, (2) additional supply of Fe3+ from hematite dissolution under acidic conditions, and (3) enhanced oxidations of Fe2+ to Fe3+ and As(III) to As(V) on the surface of hematite. Image 1 • Scorodite formation was promoted in the presence of hematite. • Hematite particles acted as "seeds" that enhanced scorodite precipitation. • Partial hematite dissolution supplied Fe3+ that enhanced scorodite formation. • Hematite catalysed Fe2+ and As(III) oxidations that promoted scorodite formation. • Hematite could be used to catalyse scorodite formation at room temperature. [ABSTRACT FROM AUTHOR]

For the metallurgical industries, the safe disposal of arsenic-bearing waste is animportant issue. In this study, a two-stage arsenic precipitation process was proposed, which consists of formation of scorodite at lower pH followed by precipitation of arsenical ferrihydrite at higher pH. The experimental results show that arsenic can be removed effectively from a solution with arsenic concentration of 5 g/L as precipitate of crystalline scorodite through oxidizing ferrous ions with air stream at pH of 2at first. Then, the pH value was raised to 3 and 4, and almost all the residual arsenic was removed from the solution due to adsorption onto amorphous ferrihydrite at the molar ratio of Fe/As between 2 and 4. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests demonstrate that the complex precipitates generated under these conditions (Fe/As ratio of 4, final pH between 3 and 4) were very stable and no leached arsenic can be detected using acetic acid buffer solution (pH 4.93 ± 0.05) in tests of 72 h. Experimental results suggest that a high Fe/As ratio was necessary, so that the excess iron promotes the formation a product of good stability, arsenical ferrihydrite (AsFH: AsO 4 3−·Fe(OH)(H 2 O) 1+x) under a relatively high pH in acidic solution. Unlabelled Image • Arsenic was removed almost completely in a two-stage arsenic precipitate process. • The arsenate adsorbed by ferrihydrite can be transformed into scorodite. • The excess iron can form an arsenic deficient ferrihydrite which could restrain the leaching of arsenic. • The formed complex precipitates were stable in the TCLP test. [ABSTRACT FROM AUTHOR]

The stability of scorodite and scorodite with crystalline polyferric sulfate (PFS) added was investigated using the Toxicity Characteristic Leaching Procedure (TCLP) and leaching tests using NaOH solutions. The leaching behaviors of iron and arsenic were analyzed using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results indicated that the addition of PFS reduced the concentration of arsenic dissolved into the leaching solution significantly from 10 to 30 mg/L to below 0.01 mg/L through its adsorption and effect on the equilibrium of scorodite dissolution. The PFS addition facilitated the leaching of iron from the scorodite to precipitate as two-line ferrihydrite that coated the surface of scorodite particles. This potentially slowed down the diffusion of arsenic from the scorodite to the leaching solution. The thermodynamic calculations for the scorodite dissolution process showed that the formation/addition of ferrihydrite was effective in controlling the equilibrium arsenic concentration. With consideration of the ferrihydrite formation in the calculations, the scorodite was shown to be stable between pH 2.0–8.8, which was a wider range than the scorodite stability range of pH 2.0–5.8. • The stability of scorodite under circumneutral and alkaline conditions was improved by the addition of crystalline PFS. • The leaching of arsenic from scorodite was controlled by the addition of PFS via adsorption and changing equilibrium. • The model diagram for the leaching process of scorodite with PFS added was drawn. [ABSTRACT FROM AUTHOR]

Abstract 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 (H 2 O 2) 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 H 2 O 2 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 (S n 2−). 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. Highlights • A complete process of treatment arsenic sulphide residue by transforming into scorodite. • Arsenic was extracted effectively from arsenic sulphide residue without producing arsenic containing hazardous materials. • Experiment results shows potential application value of the process in hydrometallurgy industry. [ABSTRACT FROM AUTHOR]

Arsenic–enriched wastewater (A–EW) is a hypertoxic sewage from the utilization of crude antimony oxides in lead anode slime metallurgy. In traditional methods, the H+ accumulation inhibits the arsenic immobilization during scorodite synthesis. In this study, a novel multivalent iron source comprised of Fe(OH) 3 and FeSO 4 ·7H 2 O was proposed to resolve the adverse effects of pH fluctuation during immobilizing A–EW as scorodite. Various approaches, such as scanning electron microscopy and X–ray photoelectron spectroscopy, were applied to characterize the synthesized scorodite. This work was divided into two parts. In thermodynamics, H n AsO 4 (3−n)- (n = 1, 2, 3) and Fe(OH) n (3−n)+ (n = 0, 1, 2, 3) can feasibly coprecipitate as scorodite according to their △ r G m , T θ ranged from −111.10 kJ mol−1 to −33.53 kJ mol−1. In experimental research, A–EW was immobilized as scorodite by optimizing conditions as initial pH = 2.0, molar ratio of Fe to As = 1.2, molar ratio of Fe(II) to Fe(III) = 4:6, arsenic concentration = 40 g/L, and temperature = 95 °C. The arsenic precipitation ratio is 99.60%, and the micromorphology of synthesized scorodite presents a regular octahedron having size of 5–10 μm. The low leachability of As (0.41 mg/L) in toxicity characteristic leaching procedure (TCLP) confirmed that the prepared scorodite is nonhazardous. The solution pH is stable at 2.0 as the H+ depletion (0.5660 mol) by Fe(OH) 3 dissolution and Fe2+ oxidization balanced with that (0.5657 mol) generated from As(V)–Fe(III) coprecipitation. In general, the A–EW was effectively immobilized by proposed multivalent iron source, and can be potentially applied to safely dispose other industrial effluents, such as high arsenic leachates and arsenic–bearing waste acid from nonferrous metallurgy. Scorodite formation mechanism and protons balance in weak acidic solution using a multivalent iron source. [Display omitted] • H n AsO 4 (3−n)- and Fe(OH) n (3−n)+ can coprecipitate as scorodite from △ r G m , T calculations. • 99.60% of A–EW was immobilized as 5–10 μm octahedral scorodite at a stable pH. • Fe(OH) 3 neutralizes extra H+ from oxidative coprecipitation of Fe(II) and As(V). • Sodium salts were recovered from solution by an evaporative crystallization. [ABSTRACT FROM AUTHOR]

Owing to the chemical stability of scorodite, the synthesis of scorodite for arsenic fixation has gained interest as a treatment method for arsenic-containing wastewater generated at nonferrous metal smelting plants and other facilities. Diverse synthesis processes have been investigated such as hydrothermal, oxidation, and solid iron oxide methods. The solid iron oxide method using hematite as an iron source is particularly promising owing to its many advantages. Though the overall reaction equation for this process does not include Fe2+, the reaction is influenced by Fe2+ in the solution, significantly increasing the reaction rate. In the present study, we examine the mechanism of this reaction electrochemically to elucidate the role of Fe2+. By analyzing the open-circuit potentials and the polarization behaviors of each local reaction and conducting short-circuiting experiments based on the predicted local-cell model, we demonstrate the electrochemical nature of the reaction, which involves the reduction of hematite and the oxidation of Fe2+. In addition, the multistep reaction involved in the formation of scorodite is discussed using potential–pH diagrams based on thermodynamic reference data. [Display omitted] • Acceleration mechanism of Fe2+ on the scorodite synthesis using Fe 2 O 3 was elucidated. • Polarization behavior involved in the scorodite formation was clarified. • Thermodynamic discussion was presented based on E –pH diagrams. [ABSTRACT FROM AUTHOR]

The need to sustainably produce raw materials encourages mining companies to develop and incorporate new economically and environmentally efficient processes. Therefore, there is a need to investigate the behavior and stabilization of hazardous elements present in effluents from metal recovery processes such as arsenic. This study evaluates the incorporation of an effluent solution from a copper smelter that is to be treated in a copper hydrometallurgical plant (heap leaching). The treatment is applied to recover compounds of interest such as copper, acid and water, in addition to confining impurities as stable residues in the leach residues. Here, we assess the capacity of the mineral to retain arsenic. To do this, a mixed solution of effluent and process solution was prepared, with a concentration of 1 g/L of arsenic. The solution was irrigated in leach columns loaded with a heap mineral with varying pH levels (0.8; 1.5 and 2) and solution potentials (510 and 540 mV). The concentrations of arsenic and iron in the solution and in the solid residues were measured to determine the capacity of the mineral to retain arsenic and how it was retained. The pH level plays an important role since, at a higher pH, the presence of arsenic and iron in the solution decreases, therefore increasing in the solid residue. Finally, a retention of 57% of arsenic is reached at pH 2. The characterization of the residues by scanning electron microscopy (SEM) confirms that arsenic is associated with Fe, S and O, forming ferric arsenates, while an X-Ray analysis identifies the arsenic compounds as crystalline scorodite.

Abstract Scorodite was encapsulated by precipitating a crystalline polyferric sulfate (PFS) coating layer on the surface of scorodite particles to restrain the release of arsenic. The encapsulation experiments were carried out at atmospheric pressure, 90 °C, and 1.5 pH by adding 20 g scorodite particles/L to a 0.1-0.4 mol/L ferrous sulfate solution followed by the introduction of pure oxygen to oxidize and precipitate the ferrous ions as PFS. After encapsulation the samples were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning transmission electron microscopy. The results indicated that the coating was crystalline PFS, and the scorodite particles were completely encapsulated by the coating. The stability of the scorodite encapsulated with PFS was evaluated by several toxic leaching tests which use an HAc-NaAc buffer solution of 4.93 pH, a NaOH solution of 9.3 pH, a NaH 2 PO 4 -NaOH buffer solution of 8.6 pH, or Na 2 S solutions of 7.6 and 8.1 pH and a reducing potential (E h) within the −100 to 0 mV range as leaching solutions. The results of the leaching tests indicated that the scorodite encapsulated with PFS was highly effective at restraining the release of arsenic in both weakly acidic and alkaline solutions under both oxic and anoxic conditions. Graphical abstract Unlabelled Image Highlights • Scorodite was encapsulated by PFS, and the coating layer was characterized. • The layer restrained the dissolution of arsenic in scorodite effectively in leaching solutions. • The layer showed to be effective in both weakly acidic and alkaline solutions under both oxic and anoxic conditions • The layer also had efficacy in the existence of phosphate. [ABSTRACT FROM AUTHOR]

A cleaning process, including the steps of alkaline leaching, precipitation of lead and stabilization of arsenic, was proposed for separation of arsenic from lead anode slime generated in lead smelter before valuable metals recycling. The anode slime was first leached by NaOH solution at oxygen pressure, and the optimum conditions were obtained as follows: α (NaOH) = 1.2 (where α represents excess coefficient), temperature 160 °C, liquid-solid ratio 5:1, oxygen partial pressure 1.2 MPa, where the leaching rates of arsenic and lead reached 95.65% and 0.96%. Second, the leaching solution underwent oxidative precipitation for lead separation in the form of PbO 2 using sodium persulfate as oxidizing agent. Under the optimum conditions, the precipitation rate of lead was 94.53%. Then, the sodium arsenate was crystallized from the latter precipitation solution. Finally, the re-dissolving arsenic was solidified by the atmospheric oxidation method to form scorodite crystal. A suitable pH range for scorodite formation was below pH 6.0, where the highest precipitation rate of As was 88.48%. However, for solution pH higher than 6.0, the precipitates were very fine and showed inferior crystallinity as well as being unsafe for long-term stockpiling. It is suggest that this process has potential application for dearsenification from all arsenic-rich anode slime. [ABSTRACT FROM AUTHOR]

Applicability of the bioscorodite method (use of the thermo-acidophilic Fe(II)-oxidizing archaeon Acidianus brierleyi for arsenic (As) oxidation and immobilization at 70 °C) was tested for synthetic copper refinery wastewaters of a wide range of dilute initial As(III) concentrations ([As(III)]ini = 3.3-20 mM) with varying initial [Fe(II)]/[As(III)] molar ratios ([Fe(II)]ini/[As(III)]ini = 0.8-6.0). Crystallization of scorodite (FeAsO4»2H2O) tends to become increasingly challenging at more dilute As(III) solutions. Optimization of conditions such as initial pH, seed feeding and initial [Fe(II)]/[As(III)] molar ratio was found critical in improving final As removal and product stability: Whilst setting the initial pH at 1.2 resulted in an immediate single-stage precipitation of crystalline bioscorodite, the initial pH 1.5 led to a two-stage As precipitation (generation of brown amorphous precursors followed by whitish crystalline bioscorodite particles) with a greater final As removal. The formation process of bioscorodite precipitates differed significantly depending on the type of seed crystals fed (bio- versus chemical- scorodite seeds). Feeding the former was found effective not only in accelerating the reaction, but also in forming more recalcitrant bioscorodite products (0.59 mg/L; Toxicity Characteristic Leaching Procedure (TCLP) test). Under such favorable conditions, 94-99% of As was successfully removed as crystalline bioscorodite at all dilute As(III) concentrations tested by setting [Fe(II)]ini/[As(III)]ini at 1.4-2.0. Providing an excess Fe(II) (closer to [Fe(II)]ini/[As(III)]ini = 2.0) was found beneficial to improve the final As removal (up to 98-99%) especially from more dilute As(III) solutions. [ABSTRACT FROM AUTHOR]

The attention of mining industry has directed towards the processing of complex arsenic-bearing minerals due to a decrease in the traditional base metal reserves. Arsenic present in the minerals is usually mobilized through hydrometallurgical and pyrometallurgical processes. Arsenic has become a worldwide environmental challenge in the metals and mining industry. Hence, arsenic in the process streams must be immobilized properly prior to the discard of waste. The initial step for arsenic fixation is the oxidation of trivalent arsenic in order to improve both arsenic removal and stability of the final arsenical residues. Arsenic immobilization step could be accomplished using hydrometallurgical and pyrometallurgical techniques. Whereas, pentavalent arsenic is commonly precipitated using hydrometallurgical processes consisting of lime neutralization, sulfide precipitation, co-precipitation of arsenic with ferric ions and scorodite precipitation. In the pyrometallurgical methods arsenic and sulfur are captured using a fixing agent such as calcium and ferrous salts to produce a stable residue. This paper aims to provide a comprehensive review on past, current and future arsenic immobilization techniques related to the mining industry with a large focus on the practised processes and new developments. [ABSTRACT FROM AUTHOR]

The release of hazardous gas increases with the development of industry. The research of gas-sensitive materials has attracted attention. Nanoscale iron oxide (α-Fe2O3) is one of the research hotspots of gas-sensitive materials because it is a cheap, non-toxic semiconductor material. In this study, pomegranate-shaped α-Fe2O3 was synthesized using an in situ corrosion method of scorodite. Spherical-shaped α-Fe2O3 nanoparticles were included in the octahedral shells. The forming process of the structure was analyzed by a variety of measurements. The shell was formed first through the deposition of Fe(OH)3, which was produced by hydrolyzing scorodite. Then, the corrosion was continued and Fe(OH)3 precipitation was produced below the shell. The particles aggregated and formed spheres. The pomegranate-shaped α-Fe2O3 was formed when the scorodite was hydrolyzed completely. The gas-sensing properties of α-Fe2O3 were investigated. The results showed that pomegranate-shaped α-Fe2O3 was responsive to a variety of gases, especially xylene. The value of Ra/Rg was 67.29 at 340 °C when the concentration of xylene was 1000 ppm. This indicated the pomegranate-shaped α-Fe2O3 has potential application as a xylene gas sensor.

This study investigated the effect caused by Cu(II) in synthetic As(III)-bearing copper refinery wastewaters, on microbial scorodite (FeAsO 4 ·2H 2 O) formation using the thermo-acidophilic Fe(II)- and As(III)-oxidizing archaeon, Acidianus brierleyi . Microbial Fe(II) oxidation and cell growth became only marginal in the presence of 8–16 mM Cu(II), with its As(III) oxidation ability being severely inhibited. Consequently, scorodite formation was disabled by Cu(II) addition. However, feeding scorodite seed crystals readily alleviated Fe(II)- and As(III)-oxidation ability of Ac. brierleyi at 8 mM Cu(II), forming crystalline scorodite within 24 days in shake flasks. Zeta potential analysis indicated cell attachment to the scorodite seed crystal surface, implying its role in providing the immediate support for microbial colonization and enabling more robust microbial reactions. Most of Cu(II) was neither adsorbed nor co-precipitated and remained in the solution phase during scorodite crystallization, with or without the presence of seed crystals. Addition of seed crystals at 0.015, 0.03, 0.075 and 0.15% resulted in As immobilization of 96, 97, 97 and 98%, respectively, by day 24. This study demonstrated that despite of its inhibitory effect on Ac. brierleyi cells, scorodite can still be crystallized in the presence of Cu(II) by feeding scorodite seeds from synthetic copper refinery As(III)-bearing wastewaters. [ABSTRACT FROM AUTHOR]

Synthesis of scorodite (FeAsO 4 ·2H 2 O) using dynamic action agglomeration and the oxidation effect from ultrasound irradiation was investigated. The effect of different reaction temperatures (90, 70, 50, and 30 °C) on the size and morphology of scorodite particles synthesized under O 2 gas flow and ultrasound irradiation was explored because the generation of fine bubbles depends on the solution temperature. At 90 °C, the size of scorodite particles was non-homogeneous (from fine particles (<1 μm) to large particles (>10 μm)). The oxidation–reduction potential (ORP) and yield at 90 °C showed lower values than those at 70 °C. The scorodite particles, including fine and non-homogeneous particles, were generated by a decrease in the oxidation of Fe(II) to Fe(III) and promotion of dissolution caused by the generation of radicals and jet flow from ultrasound irradiation. Using ultrasound irradiation in the synthesis of scorodite at low temperature (30 °C) resulted in the appearance of scorodite peaks in the X-ray diffraction (XRD) pattern after a reaction time of 3 h. The peaks became more intense with a reaction temperature of 50 °C and crystalline scorodite was obtained. Therefore, ultrasound irradiation can enable the synthesis of scorodite at 30 °C as well as the synthesis of large particles (>10 μm) at higher temperature. Oxide radicals and jet flow generated by ultrasound irradiation contributed significantly to the synthesis and crystal growth of scorodite. [ABSTRACT FROM AUTHOR]

A novel process for preparing scorodite particles with a diameter of approximately 20 µm from Fe(II) and As(V) in aqueous solution has been developed by DOWA Metals and Mining. In the present study, the dissolution characteristics of iron and arsenic from the scorodite particles synthesized by this process have been investigated under different conditions. The results show that the concentration of arsenic dissolved from the particles in aqueous solution is very low, but it has a complicated dependence on the temperature and pH of the solution. Transmission electron microscopy (TEM) with an energy dispersive X-ray spectrometer (EDS) was used to analyze the morphology, structure, and composition of the scorodite particles. The results indicate that the scorodite particles exhibit a nearly octahedral shape with planes composed of almost (111) planes in the orthorhombic structure. The concentration of iron at the surface of the particles is higher than that of iron inside of the particles. This characteristic morphology, along with the minimal surface defects of the scorodite particles, is considered to be responsible for the low dissolution of arsenic from the particles in aqueous solution. Atmospheric temperature and solution conditions were also found to be important for the safe, long-term storage of arsenic using scorodite particles.

Applicability of the bioscorodite method (use of the thermo-acidophilic Fe(II)-oxidizing archaeon Acidianus brierleyi for arsenic (As) oxidation and immobilization at 70 °C) was tested for synthetic copper refinery wastewaters of a wide range of dilute initial As(III) concentrations ([As(III)]ini = 3.3–20 mM) with varying initial [Fe(II)]/[As(III)] molar ratios ([Fe(II)]ini/[As(III)]ini = 0.8–6.0). Crystallization of scorodite (FeAsO4·2H2O) tends to become increasingly challenging at more dilute As(III) solutions. Optimization of conditions such as initial pH, seed feeding and initial [Fe(II)]/[As(III)] molar ratio was found critical in improving final As removal and product stability: Whilst setting the initial pH at 1.2 resulted in an immediate single-stage precipitation of crystalline bioscorodite, the initial pH 1.5 led to a two-stage As precipitation (generation of brown amorphous precursors followed by whitish crystalline bioscorodite particles) with a greater final As removal. The formation process of bioscorodite precipitates differed significantly depending on the type of seed crystals fed (bio- versus chemical- scorodite seeds). Feeding the former was found effective not only in accelerating the reaction, but also in forming more recalcitrant bioscorodite products (0.59 mg/L; Toxicity Characteristic Leaching Procedure (TCLP) test). Under such favorable conditions, 94–99% of As was successfully removed as crystalline bioscorodite at all dilute As(III) concentrations tested by setting [Fe(II)]ini/[As(III)]ini at 1.4–2.0. Providing an excess Fe(II) (closer to [Fe(II)]ini/[As(III)]ini = 2.0) was found beneficial to improve the final As removal (up to 98–99%) especially from more dilute As(III) solutions.

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite. [Display omitted] • The release of arsenic was positively correlated with the reduction of scorodite. • Parasymplesite was the main product of scorodite bio-reduction. • Thiosulfate promoted the bio-reduction of scorodite. • The formation of parasymplesite was affected by thiosulfate. [ABSTRACT FROM AUTHOR]

Hydrometallurgical iron-arsenic (Fe–As) coprecipitation residue (FACR), a massive industrial solid waste, is a potentially cheapest Fe(III) source for the removal and fixation of As in waste sulfuric acid (WSA) as stable scorodite. This study innovatively proposed a method by controlling a low supersaturation degree of ferric arsenate in WSA via stepwise addition and slow dissolution of FACR, ensuring continuous ferric arsenate precipitation and scorodite crystallization. The results showed that As removal efficiency in WSA reached 99.92%. The decrease in the weight of solid waste was up to 37.1%. X-ray diffraction and infrared spectroscopy results indicated that scorodite is the principal crystalline As mineral in the products. The solid products displayed excellent stability with the TCLP concentrations of leached As and divalent metals (CuII, PbII, ZnII, and CdII) far below the US-EPA regulatory limits. Scanning/Transmission electron microscopy and X-ray photoelectron spectroscopy results revealed that such high TCLP stability could be ascribed to the adsorption of As and trace divalent metals on the raw/secondary Fe-(oxy)hydroxide minerals, such as FeO(OH), goethite, and ferrihydrite. This work provides an economical and efficient method for simultaneous treatment of WSA and FACR, which can support the sustainable development of the hydrometallurgical industry. [Display omitted] • First attempt in using FACR as an iron source for WSA treatment. •Removal efficiency of As from waste sulfuric acid reaches up to 99.92%. •Weight loss ratio of solid waste before and after treatment reaches up to 56.8%. •TCLP concentrations of As and valuable metals are below the EPA regulatory limits. •Scorodite and Fe-(oxy)hydroxide occurs in products are responsible for As fixation. [ABSTRACT FROM AUTHOR]

• Simultaneous oxidation and removal of As(III) to form scorodite. • Arsenic removal efficiency reach 97% for high As(III) concentration of 15 g/L. • A critical role of HO•, Fe(III) and Fe(IV) in the oxidation of As(III). • A synthetic transformation mechanism of scorodite via a four-stage process. Due to the lack of affordable and green disposal technologies, high arsenic-containing waste acid from the heavy nonferrous metallurgical smelter causes serious environmental pollution and threatens human health. In this study, a cost-effective and environmentally-friendly solution for simultaneous oxidation and precipitation of arsenic as well as formation of scorodite by adding ferrous sulfate as an iron source is proposed under elevated temperature (170 °C) and pressurized oxygen (1 MPa) conditions. And that the total reaction can be interpreted as 4Fe2+ + 4HAsO 2 + 3O 2 + 10H 2 O → 4FeAsO 4 ·2H 2 O + 8H+. Consequently, the final oxidation and removal efficiency of arsenic reached 98.95% and 97.42%, respectively, resulting in scorodite with an arsenic leaching concentration of 1.4 mg/L in the TCLP leaching test. The results prove that the HO•, Fe(III) and Fe(IV) produced by the oxidation of Fe(II) is of capable oxidizing As(III) to As(V). Most importantly, the synthetic transformation mechanism of scorodite via a four-stage process is revealed, including the oxidation of As(III), formation of a mixture of amorphous hydronium jarosite and iron arsenate as precursors, partial dissolution of the precursor as well as nucleation and growth of scorodite particles. [ABSTRACT FROM AUTHOR]

Scorodite precipitation is widely employed for the fixation of arsenic in hydrometallurgical industries. However, its stability in reducing environment is not yet fully understood. This work investigated the effect of hydroquinone-induced Fe(III) reduction on the stability of scorodite. The results showed that ≤ 3% (11.3 mg L − 1 ) arsenic and < 4% (11 mg L − 1 ) Fe(II) were released into solution after 72–90% of Fe(III) was reduced to Fe(II) at pH 4–8. This indicated that most of the reductive decomposition generated Fe(II) and As(V) were retained in the solid phases as secondary minerals. The XRD, ATR-FTIR, Raman, SEM-EDS and chemical analysis suggested that amorphous FeHAsO 4 ⋅ xH 2 O was very likely the major sink for arsenic immobilization. At pH 9, the scorodite decomposed completely resulting in ~ 19% (71.2 mg L − 1 ) arsenic and ~ 3% (8.2 mg L − 1 ) Fe(II) release and a residual solid of Fe/As ~ 1.2. Both amorphous ferrous arsenates and Fe(II, III) (hydr)oxides were probably responsible for the retention of arsenic in this case. The solubility of scorodite in terms of dissolved arsenic concentration upon iron reduction at pH 7–9 was significantly lower than that in oxic environment under the conditions of this work. [ABSTRACT FROM AUTHOR]

In this work, we determined the full set of thermodynamic properties for the ferric orthoarsenate, FeAsO 4 ·0.75H 2 O. This phase could be expected in metallurgical waste forms from pressure oxidation of arsenopyrite-rich ores and its solubility is therefore of interest for the understanding of arsenic mobility in such waste forms. Using a combination of solubility experiments at low pH (< 2) and relaxation calorimetry, we have determined log K sp of − 26.54 ± 0.35 for the reaction FeAsO 4 · 0.75 H 2 O = Fe 3 + + AsO 4 3 – + 0.75 H 2 O and S o for the ferric orthoarsenate of 154.2 ± 0.5 J mol − 1 K − 1 , respectively. These data give Δ f G o (FeAsO 4 ·0.75H 2 O) = − 993.15 ± 2.57 kJ mol − 1 and Δ f H o (FeAsO 4 ·0.75H 2 O) = − 1140.38 ± 2.59 kJ mol − 1 . The calculated enthalpy was checked by thermodynamic considerations of the system FeAsO 4 -H 2 O and found in a good agreement with the expected value. Heat capacity at temperatures higher than the ambient ones was fit by a Berman-Brown polynomial C p = k 0 + k 1 T –0.5 + k 2 T − 2 + k 3 T − 3 with the coefficients k 0 = 1.774 × 10 2 , k 1 = 6.531 × 10 2 , k 2 = − 1.265 × 10 7 , and k 3 = 1.754 × 10 9 (fit valid between 280 and 465 K). Ferric orthoarsenate is less soluble than scorodite (FeAsO 4 ·2H 2 O) and could be thus an interesting alternative for the disposal of arsenical waste. Its use will be certainly hampered by the high temperature required for its synthesis (> 200 °C). One could, however, attempt to tweak the pressure oxidation technology such that the formation of ferric orthoarsenate would be enhanced, hence producing a waste form with relatively low arsenic solubility. [ABSTRACT FROM AUTHOR]

Detection and sizing of natural colloids involved in the release and transport of toxic metals and metalloids is essential to understand and model their environmental effects. Single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) was applied for the detection of arsenic-bearing particles released from mine wastes. Arsenic-bearing particles were detected in leachates from mine wastes, with a mass-per-particle detection limit of 0.64 ng of arsenic. Conversion of the mass-per-particle information provided by SP-ICP-MS into size information requires knowledge of the nature of the particles; therefore, synchrotron-based X-ray absorption spectroscopy (XAS) was used to identify scorodite (FeAsO·2HO) as the main species in the colloidal particles isolated by ultrafiltration. The size of the scorodite particles detected in the leachates was below 300-350 nm, in good agreement with the values obtained by TEM. The size of the particles detected by SP-ICP-MS was determined as the average edge of scorodite crystals, which show a rhombic dipyramidal form, achieving a size detection limit of 117 nm. The combined use of SP-ICP-MS and XAS allowed detection, identification, and size determination of scorodite particles released from mine wastes, suggesting their potential to transport arsenic. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]