Scorodite Particles Research Articles

Oxidation of As(III) by pressurized oxygen and the simultaneous precipitation of As(V) as scorodite in acidic sulfate solutions

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+ + 4HAsO2 + 3O2 + 10H2O → 4FeAsO4·2H2O + 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.

Arsenic Trioxide Leaching and Scorodite Crystallization in Methanesulfonic Acid

ABSTRACT Arsenic, one of the most toxic elements, is a byproduct generated from metallurgical processes of arsenic-containing ores. Arsenic can be formed into an arsenic trioxide as a byproduct during the decopperization in electro-refining process when processing high arsenic-bearing copper minerals. However, arsenic trioxide has a very high arsenic mobility, so it requires to be re-dissolved and immobilized into scorodite (FeAsO4▪2H2O). In this study, methanesulfonic acid (MSA) was tested for both arsenic trioxide leaching and scorodite precipitation. Arsenic trioxide leaching tests showed the arsenic extraction of 80% at 21°C in MSA with H2O2 as an oxidant within 6 hours. Meanwhile, arsenic precipitation tests were conducted to achieve above 95% arsenic removal within 6 hours by the formation of scorodite at an initial pH of 0.5 and temperature of 80°C. In addition, coarser scorodite particles were formed in higher temperature and lower solution pH. The optimum condition for scorodite crystallization in MSA medium can be suggested to be at 80°C with the initial pH of 0.5.

Spectroscopic study on the local structure of sulfate (SO42−) incorporated in scorodite (FeAsO4·2H2O) lattice: Implications for understanding the Fe(III)-As(V)-SO42−-bearing minerals formation

AbstractThe incorporation of sulfate (SO42−) into the scorodite (FeAsO4·2H2O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of SO42− speciation and local structure in scorodite lattice is still lacking. In this study, X-ray diffraction (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 SO42− in the naturally and hydrothermally synthesized scorodite. The SO42− retention in natural scorodite and the effect of pH value and initial Na+ concentration on the incorporation of SO42− in synthetic scorodite were investigated. The results showed that trace amounts of SO42− were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that SO42− 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 SO42− 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 SO42− in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that SO42− 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.

Particle size control of biogenic scorodite during the GAC-catalysed As(III) oxidation for efficient arsenic removal in acid wastewaters

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.

Synthesis of porous γ-Fe2O3 via alkaline treatment of size controlled scorodite particles synthesized using ultrasound irradiation and its evaluation as a cathode for lithium-ion battery

Herein, we synthesized scorodite particles using ultrasound irradiation in a strong sulfuric acidic solution (pH 0.5) and investigated the scorodite particles’ size and morphology. The highest crystallinity of scorodite (96%) was synthesized using 200 kHz ultrasonic treatment, and maximum yield of scorodite was obtained in three conditions: 200, 28 kHz, and stirrer. The oxidant generated through 200 kHz ultrasound irradiation enhanced Fe(II) oxidation in the solution resulting in crystal nuclei formation. The crystal nuclei were transformed to scorodite by oxidation with a supply of solute [Fe(III)] from the solution and ostwald ripening. Clustered scorodite (4 μm in size) was obtained via synthesis using 200 kHz ultrasound (pH 0.5). Arsenic could be separated from the clustered scorodite via alkaline treatment; scorodite was changed to porous γ-Fe2O3 comprising nanosized particles. Porous γ-Fe2O3 was evaluated as a cathode in a lithium-ion battery, which showed a 149 mAh g−1 discharge capacity with a 0.5 C rate.

Synthesis, in-situ coating and characterization of scorodite with high leaching stability

Abstract To improve stability of scorodite, a method of simultaneous synthesis and in-situ coating of scorodite was proposed. Scorodite particles with polyhedral and raspberry-like morphologies were synthesized in an Fe(II)−As(V)−H2O system at 90 °C and pH 1.5 by blowing oxygen gas into the system. When the initial Fe/As molar ratio exceeded 1:1, a coating of sulfate-containing iron (hydr)oxides formed on the surfaces of scorodite particles during synthesis. To evaluate the leaching stability of synthesized scorodite samples, toxicity characteristic leaching procedure (TCLP) tests were conducted at pH 4.93 for 60 h, and long-term leaching tests were conducted for 30−40 d within a pH range of 5.40−10.88. The leaching results indicated that the release of arsenic from scorodite was noticeably postponed by the coating, and the average arsenic concentrations in the leaching solutions were as low as 0.12 mg/L in the TCLP tests and lower than 0.5 mg/L in the long-term leaching tests.

Improvement of scorodite stability by addition of crystalline polyferric sulfate

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.

Immobilization of arsenic as scorodite by a thermoacidophilic mixed culture via As(III)-catalyzed oxidation with activated carbon.

In this study we describe the immobilization of arsenic as scorodite (FeAsO4.2H2O) by a thermophilic iron-oxidizing mixed culture from an acidic sulfate medium containing 500 mg L-1 of Fe(II), 500 mg L-1 As(III) and granular activated carbon (GAC) as the main arsenite oxidant. This study shows that crystalline scorodite can only be precipitated in the presence of the ferrous iron-oxidizing mixed culture (pH 1.3 and 70 °C). The efficiency of arsenite oxidation was over 99% with a maximum specific oxidation rate of 280 mgAs(III) gGAC-1 day-1. Ferrous iron and arsenite were also oxidized in the absence of the mixed culture, however, no scorodite precipitated under these conditions; consequently, scorodite precipitation was biologically induced. The precipitated scorodite particles had a size between 0.5 and 10 μm with an average of 5 μm, resulting in low settling rates. Ion activity product calculations and observations by Scanning Electron Microscopy (SEM) indicated that microbial cells served as surface for heterogeneous nucleation. The potential of the thermophilic mixed culture for the scorodite formation explored in this study provides the basis of a new approach for the treatment of As(III) polluted streams.

In-situ oxidative arsenic precipitation as scorodite during carbon catalyzed enargite leaching process

This study explores the in-situ precipitation of scorodite (FeAsO4.2H2O) during the atmospheric enargite leaching in acid chloride media. The temperature, oxygen sparging rate and the different fractions of the catalyst and ferric ions were the effective parameters on the scorodite precipitation process. Crystalline scorodite was precipitated out of the leach solution in the absence of seed at temperatures above 80 °C where the ratio of both AF5 to concentrate and activated carbon (AC) to concentrate were 1:1. The minimal dosage of ferric ions to form the scorodite residue was 5 g/L. Seed addition facilitated the scorodite precipitation process where the primary scorodite particles precipitated after 6 h of enargite leaching. Via this process it was shown that the residual concertation of arsenic in the pregnant leach solution can be reduced as low as 50 mg/L. A series of the toxicity characteristic leaching procedure (TCLP) tests on the generated scorodite exhibited a low arsenic release ranging from 0.2 to 0.7 mg/L, showing an acceptable stability of the scorodite in the residue.

Encapsulation of scorodite using crystalline polyferric sulfate precipitated from the Fe(II)-SO42−-O2-H2O system

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 NaH2PO4-NaOH buffer solution of 8.6 pH, or Na2S solutions of 7.6 and 8.1 pH and a reducing potential (Eh) 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.

Utilization of carbon dioxide to synthesize large scorodite particles under ultrasound irradiation

This study aims to synthesize large scorodite particles at 70 °C for 3 h via the oxidation of a precursor containing Fe(II) under flowing O2 gas by adding CO2 during ultrasound agglomeration process. First, the size of the precursor was increased with flowing O2 gas and stirring until the particle size was large enough to allow aggregation by 200 kHz ultrasound (1.1–2.9 µm). Next, the reaction solution was ultrasonically irradiated for 10 min under the flow of a mixed gas (O2 + CO2). Finally, the agglomerated precursor was oxidized under flowing O2 gas and stirring without ultrasound irradiation. The addition of a mixed gas containing 80 vol % O2 and 20 vol % CO2 and a short period (10 min) of ultrasound irradiation mitigated cavitation-related dispersion while preserving oxidation by O2 and oxidant radicals. Thus, the average size of synthesized particles was >10 µm from the laser diffraction particle size analysis.

Effect of reaction temperature on the size and morphology of scorodite synthesized using ultrasound irradiation

Synthesis of scorodite (FeAsO4·2H2O) 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 O2 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 (<1μm) to large particles (>10μm)). 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 3h. 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 (>10μm) at higher temperature. Oxide radicals and jet flow generated by ultrasound irradiation contributed significantly to the synthesis and crystal growth of scorodite.

Effect of ultrasound intensity on the size and morphology of synthesized scorodite particles

The effect of different ultrasound intensities (11.6, 8.4, 5.3, 2.1, and 0W) on the size and morphology of synthesized scorodite particles was investigated under O2 gas flow at 70°C. The higher ultrasound intensity promoted the oxidation of Fe(II) in the reaction solution. Therefore, higher values of oxidation–reduction potential (ORP) in the solution and the yield of the precipitate were obtained for higher ultrasound intensities. Large scorodite particles (>10μm) were successfully synthesized using ultrasound irradiation. The particle morphology was related to the ultrasound intensity, with the particles synthesized with low intensity showing rough surfaces and those with high intensity showing faceted surface. The higher ultrasound intensity generated a larger amount of fine oxygen bubbles during ultrasound irradiation under O2 gas flow. This increased the area of the interface between the gas phase (oxygen bubbles) and the liquid phase [Fe(II) solution] and promoted the oxidation. Additionally, ultrasound enhanced the transfer of the solutes [Fe(III), As(V)] from the solution to the nuclei with the jet flow by collapsing of the bubbles and the physical stirring by ultrasound irradiation. These characteristics of ultrasound irradiation contributed to the crystal growth of scorodite.

Combining single-particle inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy to evaluate the release of colloidal arsenic from environmental samples.

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 0.64ng 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 (FeAsO4·2H2O) as the main species in the colloidal particles isolated by ultrafiltration. The size of the scorodite particles detected in the leachates was below 300-350nm, 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 117nm. 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. Graphical abstract Analytical approach for the detection and size characterization of As-bearing particles by SP-ICP-MS and XAS in environmental samples.

Synthesis of large scorodite particles using short period time sonication to enhance agglomeration of precursor

The effect of various sonication frequencies (28 kHz, 200 kHz, and 1.7 MHz) on the size of synthesized scorodite particles was investigated. First, the growth of the precursor with oxidation was observed in a synthesis process using stirring. The precursor size changed with the elapsed time and was <1 µm at 0 min, approximately 1 µm at 10 min, and >3 µm at 20 min. The scorodite particles were then synthesized following different procedures such that a different size of the precursor was irradiated by each ultrasound frequency for 10 min during the oxidation process with stirring for a total duration of 180 min. For each frequency, the size of the synthesized particles varied depending on the size of the precursor. Large scorodite particles can be synthesized (70 °C) by irradiating for only 30 min at various frequencies after the size of the precursor particles reaches the optimal range.

Stabilization of iron arsenate solids by encapsulation with aluminum hydroxyl gels

AbstractBACKGROUNDFerric 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.RESULTSThe 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.CONCLUSIONArsenic 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

Size and morphology of scorodite particles synthesized using ultrasound irradiation

The synthesis of large scorodite particles (>10 µm) at low temperature (50–70 °C) with a short reaction time (3 h) using 200 kHz ultrasound irradiation was investigated. Large scorodite particles (>10 µm) were obtained using ultrasound irradiation at 70 °C. The growth of scorodite particles was confirmed by scanning electron microscopy (SEM). The precursor agglomerated at the early stage of the synthesis using ultrasound irradiation. Ultrasound irradiation was applied to agglomerate the precursor. Large scorodite particles were synthesized without fine particles using ultrasound irradiation for a short time at the early stage of the synthesis followed by oxidation using stirring and oxygen flow without ultrasound irradiation.

Dissolution Characteristics and Morphology of Large-sized Scorodite Particles Synthesized from Fe(II) and As(V) in Aqueous Solution

AbstractA 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.

Environmental leaching characteristics of scorodite synthesized with Fe(II) ions

The leaching characteristics of scorodite particles synthesized from Fe(II) under different conditions, such as reaction time and temperature, were investigated. The solution conditions, such as pH and temperature, were changed in leaching experiments. The results showed that the leaching characteristics depend on the synthesis temperature of scorodite particles, particle size, type of solvent and so on. Since the amount of As dissolved from particles with the high Fe/As molar ratio is relatively low, the Fe/As molar ratio of the particles is an important factor to control the dissolution of As from scorodite particles. The results of leaching tests with repeated renewal solution showed that pH of solution is the most important factor in the As dissolution. It was found that the addition of goethite to the scorodite is also effective to reduce the amount of As in solution.

Scorodite encapsulation by controlled deposition of aluminum phosphate coatings

A new stabilization process for scorodite (FeAsO 4·2H 2O) solids based on the concept of encapsulation by controlled deposition of mineral coatings immune to pH or redox potential variations is described. The stability of the encapsulated scorodite with aluminum phosphates under simulated anoxic and oxic environments is demonstrated. Encapsulation experiments were carried out at 95 °C using 50 g/L scorodite in acidic sulphate solution containing 0.16 mol/L of P(V) with Al(III) to P(V) molar ratio of 1 and precipitation pH of 1.7. The encapsulated particles were characterised by XRD, SEM, TOF-SIMS and TOF-LIMS. The coating was crystalline AlPO 4·1.5H 2O ranging in thickness from 2.5 to 3.5 μm. Encapsulation of scorodite particles with hydrated aluminum phosphate appears to be effective in controlling/suppressing the release of arsenic under both oxic and anoxic conditions by more than one order of magnitude.