Biochar Promotes Arsenopyrite Weathering in Simulated Alkaline Soils: Electrochemical Mechanism and Environmental Implications.

Oxidation dissolution of arsenopyrite (FeAsS) is one of the important sources of arsenic contamination in soil and groundwater. Biochar, a commonly used soil amendment and environmental remediation agent, is widespread in ecosystems, where it participates in and influences the redox-active geochemical processes of sulfide minerals associated with arsenic and iron. This study investigated the critical role of biochar on the oxidation process of arsenopyrite in simulated alkaline soil solutions by a combination of electrochemical techniques, immersion tests, and solid characterizations. Polarization curves indicated that the elevated temperature (5-45 °C) and biochar concentration (0-1.2 g·L-1) accelerated arsenopyrite oxidation. This is further confirmed by electrochemical impedance spectroscopy, which showed that biochar substantially reduced the charge transfer resistance in the double layer, resulting in smaller activation energy (Ea = 37.38-29.56 kJ·mol-1) and activation enthalpy (ΔH* = 34.91-27.09 kJ·mol-1). These observations are likely attributed to the abundance of aromatic and quinoid groups in biochar, which could reduce Fe(III) and As(V) as well as adsorb or complex with Fe(III). This hinders the formation of passivation films consisting of iron arsenate and iron (oxyhydr)oxide. Further observation found that the presence of biochar exacerbates acidic drainage and arsenic contamination in areas containing arsenopyrite. This study highlighted the possible negative impact of biochar on soil and water, suggesting that the different physicochemical properties of biochar produced from different feedstock and under different pyrolysis conditions should be taken into account before large-scale applications to prevent potential risks to ecology and agriculture.

Safe, accurate, and precise sulfur isotope analyses of arsenides, sulfarsenides, and arsenic and mercury sulfides by conversion to barium sulfate before EA/IRMS.

The stable isotope ratios of sulfur (δ34S relative to Vienna Cañon Diablo Troilite) in sulfates and sulfides determined by elemental analysis and isotope ratio mass spectrometry (EA/IRMS) have been proven to be a remarkable tool for studies of the (bio)geochemical sulfur cycles in modern and ancient environments. However, the use of EA/IRMS to measure δ34S in arsenides and sulfarsenides may not be straightforward. This difficulty can lead to potential health and environmental hazards in the workplace and analytical problems such as instrument contamination, memory effects, and a non-matrix-matched standardization of δ34S measurements with suitable reference materials. To overcome these practical and analytical challenges, we developed a procedure for sulfur isotope analysis of arsenides, which can also be safely used for EA/IRMS analysis of arsenic sulfides (i.e., realgar, orpiment, arsenopyrite, and arsenian pyrite), and mercury sulfides (cinnabar). The sulfur dioxide produced from off-line EA combustion was trapped in an aqueous barium chloride solution in a leak-free system and precipitated as barium sulfate after quantitative oxidation of hydrogen sulfite by hydrogen peroxide. The derived barium sulfate was analyzed by conventional EA/IRMS, which bracketed the δ34S values of the samples with three international sulfate reference materials. The protocol (BaSO4-EA/IRMS) was validated by analyses of reference materials and laboratory standards of sulfate and sulfides and achieved accuracy and precision comparable with those of direct EA/IRMS. The δ34S values determined by BaSO4-EA/IRMS in sulfides (arsenopyrite, arsenic, and mercury sulfides) samples from different origins were comparable to those obtained by EA/IRMS, and no sulfur isotope fractionations were introduced during sample preparation. We report the first sulfur isotope data of arsenides obtained by BaSO4-EA/IRMS.

Supergene geochemistry of arsenic and activation mechanism of eucalyptus to arsenic source.

Arsenic (As) migration and transformation in the supergene environment and eucalyptus planting have essential effects on ecology or even human health, respectively. However, the combined environmental impact of As migration and transformation and eucalyptus planting has not been studied. Here we report a case of soil As contamination caused by eucalyptus planting and address the fate of As in Longmen county, Guangdong Province, China. We found high As content in weathered arsenopyrite bearing granite or granite-derived soil, where a large area of eucalyptus is planted. The release of organic acids from eucalyptus roots promoted the electrochemical reaction of arsenopyrite to produce AsO33-. In the subsequent supergene migration process, As species change from arsenite to arsenate with the addition of oxygen and the effect of clay minerals, last with As infiltration, precipitation, and enrichment, forming the As contamination in soil. The whole process reveals the activation process of eucalyptus to the As source (arsenopyrite), the migration and transformation process of As in the supergene environment, and the formation mechanism of soil As contamination. This finding provides a new perspective of soil As contamination around arsenopyrite bearing granite of the Nanling area with eucalyptus planting and proposes that the negative effects of Nanling eucalyptus planting may be greater than expected.

Application of Silicate-Based Coating on Pyrite and Arsenopyrite to Inhibit Acid Mine Drainage.

The prevention of acid generation from sulfidic mine wastes is a problem that challenges the global scientific community for decades. A promising strategy is related to the formation of coating layer around sulfides for inhibiting surface oxidation. In the current research, the conditions favoring the formation of an efficient silicate-based coating around pyrite and arsenopyrite were studied, using batch tests. The coating solutions contained silicate-oxyanions, an oxidant (H2O2) and buffered at pH 6. The effect of Si concentration (0.1-50 mM), liquid/solid ratio (5-100 mL/g) and contact time (up to 24 h) was investigated. Pyrite tailings treated with a solution of 1 mM Si/0.1 M H2O2 at L/S:100 mL/g for 24 h resulted in the optimum formation of a coating, which reduced the amount of SO4-2-released by 72%, compared to the sample treated in the absence of Si. However, silicate treatment had a negative effect on arsenopyrite tailings inducing As mobilization.

RNA transcript response by an Acidithiobacillus spp. mixed culture reveals adaptations to growth on arsenopyrite.

Biooxidation of gold-bearing refractory mineral ores such as arsenopyrite (FeAsS) in stirred tanks produces solutions containing highly toxic arsenic concentrations. In this study, ferrous iron and inorganic sulfur-oxidizing Acidithiobacillus strain IBUN Ppt12 most similar to Acidithiobacillus ferrianus and inorganic sulfur compound oxidizing Acidithiobacillus sp. IBUNS3 were grown in co-culture during biooxidation of refractory FeAsS. Total RNA was extracted and sequenced from the planktonic cells to reveal genes with different transcript counts involved in the response to FeAsS containing medium. The co-culture's response to arsenic release during biooxidation included the ars operon genes that were independently regulated according to the arsenopyrite concentration. Additionally, increased mRNA transcript counts were identified for transmembrane ion transport proteins, stress response mechanisms, accumulation of inorganic polyphosphates, urea catabolic processes, and tryptophan biosynthesis. Acidithiobacillus spp. RNA transcripts also included those encoding the Rus and PetI proteins involved in ferrous iron oxidation and gene clusters annotated as encoding inorganic sulfur compound metabolism enzymes. Finally, mRNA counts of genes related to DNA methylation, management of oxidative stress, chemotaxis, and motility during biooxidation were decreased compared to cells growing without mineral. The results provide insights into the adaptation of Acidithiobacillus spp. to growth during biooxidation of arsenic-bearing sulfides.

Changes in biooxidation mechanism and transient biofilm characteristics by As(V) during arsenopyrite colonization with Acidithiobacillus thiooxidans.

Chemical and surface analyses are carried out using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM-EDS), atomic force microscopy (AFM), confocal laser scanning microscopy (CLSM), glow discharge spectroscopy (GDS) and extracellular surface protein quantification to thoroughly investigate the effect of supplementary As(V) during biooxidation of arsenopyrite by Acidithiobacillus thiooxidans. It is revealed that arsenic can enhance bacterial reactions during bioleaching, which can strongly influence its mobility. Biofilms occur as compact-flattened microcolonies, being progressively covered by a significant amount of secondary compounds (S n2- , S0, pyrite-like). Biooxidation mechanism is modified in the presence of supplementary As(V), as indicated by spectroscopic and microscopic studies. GDS confirms significant variations between abiotic control and biooxidized arsenopyrite in terms of surface reactivity and amount of secondary compounds with and without As(V) (i.e. 6 µm depth). CLSM and protein analyses indicate a rapid modification in biofilm from hydrophilic to hydrophobic character (i.e. 1-12 h), in spite of the decrease in extracellular surface proteins in the presence of supplementary As(V) (i.e. stressed biofilms).

Improving arsenopyrite oxidation rate laws: implications for arsenic mobilization during aquifer storage and recovery (ASR).

Aquifer storage and recovery (ASR) and aquifer recharge (AR) provide technical solutions to address water supply deficits and growing future water demands. Unfortunately, the mobilization of naturally present arsenic due to ASR/AR operations has undermined its application on a larger scale. Predicting arsenic mobility in the subsurface during ASR/AR is further complicated by site-specific factors, including the arsenic mobilization mechanisms, groundwater flow conditions, and multi-phase geochemical interactions. In order to ensure safe and sustainable ASR/AR operation, a better understanding of these factors is needed. The current study thus aims to better characterize and model arsenic remobilization at ASR/AR sites by compiling and analyzing available kinetic data on arsenic mobilization from arsenopyrite under different aqueous conditions. More robust and widely applicable rate laws are developed for geochemical conditions relevant to ASR/AR. Sensitivity analysis of these new rate laws gives further insight into the controlling geochemical factors for arsenic mobilization. When improved rate laws are incorporated as the inputs for reactive transport modeling, arsenic mobilization in ASR/AR operations can be predicted with an improved accuracy. The outcomes will be used to guide groundwater monitoring and specify ASR/AR operational parameters, including water pretreatment requirements prior to injection.

A method for preparation and cleaning of uniformly sized arsenopyrite particles.

BACKGROUND: The oxidative dissolution of sulfide minerals, such as arsenopyrite (FeAsS), is of critical importance in many geochemical systems. A comprehensive understanding of their dissolution rates entails careful preparation of the mineral surface. Measurements of dissolution rates of arsenic from arsenopyrite are dependent on the size and degree of oxidation of its particles, among other factors. In this work, a method was developed for preparation and cleaning of arsenopyrite particles with size range of 150-250 µm. Four different cleaning methods were evaluated for effectiveness based on the removal of oxidized species of iron (Fe), arsenic (As) and sulfur (S) from the surface. The percentage oxidation of the surface was determined using X-ray photoelectron spectroscopy (XPS), and surface stoichiometry was measured using scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS). RESULTS: Results indicate that sonicating the arsenopyrite particles and then cleaning them with 12N HCl followed by 50% ethanol, and drying in nitrogen was the most effective method. This method was successful in greatly reducing the oxide species of Fe while completely removing oxides of As and S from the arsenopyrite surface. CONCLUSIONS: Although sonication and acid cleaning have been widely used for mineral preparation, the method described in this study can significantly reduce grain size heterogeneity as well as surface oxidation, which enables greater control in surface and dissolution experiments.

New insights into the controversy of reactive mineral-controlled arsenopyrite dissolution and arsenic release.

Serious arsenic (As) contaminations could commonly result from the oxidative dissolution of As-containing sulfide minerals, such as arsenopyrite (FeAsS). Pyrite (Py) and calcite (Cal) are two typically co-existing reactive minerals and represent different geological scenarios. Previous studies have shown that a high proportion of Py can generate a stronger galvanic effect and acid dissolution, thereby significantly promoting the release of arsenic. However, this conclusion overlooks calcite's antagonistic effect on the release of As in the natural environment. That antagonistic effect could remodel the linear relationship of pyrite on the oxidative dissolution of arsenopyrite, thus altering the environmental risk of As. We examined As release from arsenopyrite along a gradient of Py to Cal molar ratios (Py:Cal). The results showed that the lowest As release from arsenopyrite was surprisingly found in co-existing Py and Cal systems than in the singular Cal system, let alone in the singular Py system. This phenomenon indicated an interesting possibility of Py assistance to Cal inhibition of As release, though Py has always been regarded as a booster, also evidenced in this research, for As release from arsenopyrite. In singular systems of Py and Cal, As continued to be released for 60 days. However, in co-existing Py and Cal systems, As was released non-linearly in three stages over time: initial release (0-1 Day), immobilization (1-15 Days), and subsequent re-release (>15 Days). This is a new short-term natural attenuation stage for As, but over time, this stage gradually collapses. During the re-release stage (> 15 Days), a higher molar ratio of Py:Cal (increasing from 1:9 to 9:1) results in a lower rate constant k (mg·L-1·h-1) of As release (range from 0.0011 to 0.0002), and a higher abundance of secondary minerals formed (up to 26 mg/g goethite and hematite at Py: Cal=9:1). This demonstrates that increasing the Py:Cal molar ratio results in the formation of more secondary minerals which compensate for the higher potential antagonistic mechanisms generated by pyrites, such as acid dissolution and galvanic effect. These results explain the mechanisms of the high-risk characteristics of As both in acidic mine drainage and karst aquifers and discover the lowest risk in pyrite and calcite co-existing regions. Moreover, we emphasize that reactive minerals are important variables that can't be ignored in predicting As pollution in the future.

Novel insights into the kinetics and mechanism of arsenopyrite bio-dissolution enhanced by pyrite.

Arsenopyrite and pyrite often coexist in metal deposits and tailings, thus simultaneous bioleaching of both sulfides has economic (as well as environmental) significance. Important targets in bio-oxidation operations are high solubilization rates and minimized accumulation of Fe(III)/As-bearing secondary products. This study investigated the role of pyrite bioleaching in the enhancement of arsenopyrite dissolution. At a pyrite to arsenopyrite mass ratio of 1:1, 93.6% of As and 93.0% of Fe were solubilized. The results show that pyrite bio-oxidation can promote arsenopyrite dissolution, enhance S0 bio-oxidation, and inhibit the formation of jarosites, tooeleite, and amorphous ferric arsenate. The dry weight of the pyrite & arsenopyrite residue was reduced by 95.1% after bioleaching, compared to the initial load, while only 5% weight loss was observed when pyrite was absent. A biofilm was formed on the arsenopyrite surface in the presence of pyrite, while a dense passivation layer was observed in the absence of pyrite. As(III) (as As2O3) was a dominant As species in the pyrite & arsenopyrite residue. Novel and detailed findings are presented on arsenopyrite bio-dissolution in the presence of pyrite, and the presented approach could contribute to the development of novel cost-effective extractive bioprocesses. ENVIRONMENTAL IMPLICATION: The oxidation of arsenopyrite presents significant environmental hazards, as it can contribute to acid mine drainage generation and arsenic mobilization from sulfidic mine wastes. Bioleaching is a proven cost-effective and environmentally friendly extractive technology, which has been applied for decades in metal recovery from minerals or tailings. In this work, efficient extraction of arsenic from arsenopyrite bioleaching was presented through coupling the process with bio-oxidation of pyrite, resulting in lowered accumulation of hazardous and metastable Fe(III)/As-bearing secondary phases. The results could help improve current biomining operations and/or contribute to the development of novel cost-effective bioprocesses for metal extraction.

Selective depression mechanism of polyaspartic acid and calcium oxide on arsenopyrite after copper ions activation and its effect on flotation separation performance.

The arsenopyrite activated by copper ions have similar flotation properties to chalcopyrite. Polyaspartic acid (PASP) and calcium oxide (CaO) using as combination depressants for the selective separation of copper-activated arsenopyrite and chalcopyrite were carried out by micro-flotation experiments, contact angle measurements, surface adsorption capacity tests, zeta potential measurements, X-ray photoelectron spectroscopy (XPS) analyses, inductively coupled plasma-optical emission spectrometer (ICP-OES) tests and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses, and its depression mechanism was investigated. The results of flotation experiments showed that the recovery of arsenopyrite after addition of the depressants reached only 7.80 %, while the recovery of chalcopyrite reached 94.02 %. The results of contact angles, adsorption capacity tests and zeta potential measurements showed that the PASP-CaO can selectively enhance the hydrophilicity of arsenopyrite surface, but has little effect on the chalcopyrite. XPS analyses and ICP-OES tests further verified that the depressants first eliminated the activation of copper ions and then selectively adsorbed on the surface of arsenopyrite. ToF-SIMS analyses showed that the PASP-CaO would achieve selective depression of arsenopyrite in the form of PASP, PASP-Ca complexes and Ca(OH)+, respectively. Finally, the mechanism diagram of PASP-CaO selectively depressing arsenopyrite was derived. These results will provide an excellent theoretical reference for the flotation separation of copper arsenic sulfide ore.

Activated carbon-mediated arsenopyrite oxidation and arsenic immobilization: ROS formation and its role.

The oxidative dissolution of arsenopyrite (FeAsS) is a significant source of arsenic contamination in nature. Activated biochar (AC), a widely used environmental remediation agent, is prevalent in ecosystems and participated in various geochemical processes of arsenic and iron-containing sulfide minerals. However, the impact of AC-arsenopyrite association on reactive oxidation species (ROS) generation and its contribution to As transformation were rarely explored. Here, ROS formation and the redox conversion of As during the interaction between AC and arsenopyrite were investigated. AC-mediated arsenopyrite oxidation was a two-stage process. At stage I, the heterogeneous electron transfer from arsenopyrite facilitated O2 reduction on AC, enhancing arsenopyrite dissolution and ROS formation. TBA, PBQ and catalase inhibited 86.40 %, 79.39 % and 49.66 % of As(III) oxidation, respectively, indicating indicated that HO˙, (O2•)- and H2O2 were responsible for As(III) oxidation. However, at stage II, the mobility of As was highly restricted, especially increasing AC addition. Besides adsorption, AC retained appreciable As through catalyzing insoluble ferric arsenate formation and growth by promoting Fe(II) and As(III) oxidation and functioning as nuclei. These findings deepen our understanding of the coupling behavior of AC-arsenopyrite and its influence on geochemical cycling of arsenic in mined surroundings, which has important implications for mitigating arsenic pollution.

Mechanism of arsenic release process from arsenopyrite chemical oxidation.

Oxidation of arsenopyrite is one of the main causes of arsenic pollution in the environment. This study, examind changes in the surface properties of arsenopyrite in the presence of oxygen. Furthermore, X-ray photoelectron spectroscopy, in situ Raman analysis, high-resolution transmission electron microscopy, and ab initio molecular dynamics were carried out on arsenopyrite, and the oxidation properties and processes of the arsenopyrite surface, along with the Fe/As/S oxidation processes, were analysed In situ-Raman spectroscopy data clearly showed that in the presence of oxygen, Fe2+ ions were transformed into Fe3+ ions on the surface of arsenopyrite at 207 cm-1, and the content of iron oxides on the surface of arsenopyrite increased significantly over time. The presence of iron promoted the oxidation of As(III), and the oxidation process was found to affect the oxidation of As atoms on the surface of arsenopyrite due to the presence of FeS bonds. The presence of As3+ intensified the oxidation of arsenopyrite surface (139 cm-1).The main reason was the presence of O2·-/HO2· on the surface of arsenopyrite, the change of Fe2+ â†’ FeOH2+ â†’ Fe3+ on the surface of arsenopyrite, and the change of As3+ â†’ As4+ â†’ As5+, which strengthened the overflow of As, leading to reconstruction and changes on the surface of arsenopyrite.

Current Trends in Metal Biomining with a Focus on Genomics Aspects and Attention to Arsenopyrite Leaching-A Review.

The presented review is based on scientific microbiological articles and patents in the field of biomining valuable metals. The main attention is paid to publications of the last two decades, which illustrate some shifts in objects of interest and modern trends both in general and applied microbiology. The review demonstrates that microbial bioleaching continues to develop actively, despite various problems in its industrial application. The previous classic trends in the microbial bioleaching persist and remain unchanged, including (i) the search for and selection of new effective species and strains and (ii) technical optimization of the bioleaching process. Moreover, new trends were formed during the last decades with an emphasis on the phylogeny of leaching microbiota and on genomes of the leaching microorganisms. This area of genomics provides new, interesting information and forms a basis for the subsequent construction of new leaching strains. For example, this review mentions some changed strains with increased resistance to toxic compounds. Additionally, the review considers some problems of bioleaching valuable metals from toxic arsenopyrite.

ROS formation driven by pyrite-mediated arsenopyrite oxidation and its potential role on arsenic transformation.

Pyrite-mediated arsenopyrite oxidation is an important process affecting arsenic (As) mobility. The iron sulfides-induced reactive oxidation species (ROS) can exert significant influence on As transformation. However, the impact of pyrite-arsenopyrite association on ROS production and its contribution to As transformation were rarely estimated. Here, ROS formation and the redox conversion of As during the interaction between pyrite and arsenopyrite as function of O2, pH and pyrite surface oxidation were investigated. Pyrite promoted arsenopyrite oxidation and As(III) oxidation due to heterogeneous electron transfer. The electron transfer from arsenopyrite facilitated O2 reduction on pyrite surface with increasing ROS formation. Hydroxyl radical (HO˙), superoxide (O2•)- and hydrogen peroxide (H2O2) were the main reactive species for As(III) oxidation. Iron (hydr)oxides produced from pyrite surface oxidation provided fast electron transfer channels for efficient O2 reduction as evidenced by electrochemical experiment, further verifying the promoted effect of surface-oxidized pyrite (SOP) on arsenopyrite dissolution. However, total As and As(V) obviously decreased during SOP-mediated arsenopyrite oxidation. Iron (hydr)oxides retained appreciable As through adsorption to limit its mobility, and decreased HO˙ production to inhibit As(III) oxidation via decomposing H2O2. This work furthers our understanding of arsenic transformation in the environment which has important implications for mitigating arsenic pollution.

Arsenopyrite dissolution in circumneutral oxic environments: The effect of pyrophosphate and dissolved Mn(III).

The oxidative dissolution of As from arsenopyrite, one important arsenic mineral in reducing conditions, poses an environmental hazard to natural aquatic systems. The dissolution of arsenopyrite occurs slowly due to the surface precipitates of iron oxides in circumneutral oxic environments. However, the presence of natural ligands and coexisting metals may change the release of Fe species, which would be of critical importance to the dissolution of arsenopyrite. Here, we investigated the oxidative dissolution of arsenopyrite induced by pyrophosphate (PP) and dissolved Mn(III) species as a natural occurring Mn species with strong complexation affinity to PP. With the presence of PP, the formation of Fe(II)-PP complexes and its rapid oxidation to dissolved Fe(III)-PP species resulted in a substantial increase in the generation of hydroxyl radicals (•OH) under ambient dark conditions, contributing to faster dissolution of arsenopyrite and higher percentage of As(V) in the dissolved products. Dissolved Mn(III), though considered as an extra oxidant besides oxygen, unexpectedly acted as a radical scavenger for •OH and inhibited the production of As(V). Moreover, the oxidation of sulfur species differed in the two systems as significant formation of thiosulfate was observed with the presence of PP, which did not occur in the system with dissolved Mn(III). Overall, the effects of dissolved Mn(III) and PP on the dissolution of arsenopyrite and the subsequent transformation of Fe, As and S species have important implications for disentangling the interactions among these metastable elements, and for assessing their transport and environmental impacts in aquatic systems.

Arsenic Release from Various Sizes of Excavated Crushed Rock Mixed with Soil at Various Ratios under Three Mass Water Contents for Cropland.

The purpose of this experiment was to investigate the feasibility of treating arsenopyrite-containing excavated crushed rock (ECR) in cropland by examining the amounts of arsenic released from various sizes of ECR mixed with soils at different ratios under three water levels using a batch incubation experiment. A total of 4 particle sizes of ECR were mixed with soil from 0% to 100% in 25% increments under three mass water contents such as 15%, 27%, and saturation. The results showed that the amount of As released from ECR mixed with soil was in the order of 27% saturation and 15% for 180 days regardless of the ECR:soil ratios, and the increase in the amount of As released before 90 days was slightly greater than that after 90 days. The maximum and minimum contents of released As were observed at 350.3 mg·kg-1 (ECR:Soil = 100:0, ECR size = 0.0-0.053 mm, and Ɵm = 32.2%), indicating that the smaller the ECR particle size resulted in a higher extractable As concentration. The amount of As released was higher than the relevant standard (25 mg·kg-1), except for ECR with a mixing ratio (25:75) and particle size (4.75-10.0 mm). In conclusion, we assumed that the amount of As released from ECR was influenced by the higher surface area of smaller ECR particle sizes and mass water content, which determine the porosity of the soil. However, further studies are needed on the transport and adsorption of released As depending on the physical and hydrological properties of the soil to determine the size and incorporation rate of ECR into the soil in view of the government standard.

(Bio)dissolution of arsenopyrite coupled with multiple proportions of pyrite: Emphasis on the mobilization and existential state of arsenic.

The formation of arsenic-bearing acid mine drainage (AMD) via the oxidation of arsenopyrite refuse ore has attracted significant attention. Pyrite, as main a concomitant mineral, is a crucial factor that affects the (bio)dissolution of arsenopyrite, but there are still some points on the detailed action mechanism under normal environmental conditions that need further study. In this study, the effect mechanism of pyrite with a systematic pyrite content (0, 10, 25, 50, 75, 90, and 100 wt %) on arsenopyrite oxidation and arsenic release in the presence of Acidithiobacillus ferrooxidans was investigated. The X-ray diffraction (XRD), scanning election microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical analyses were also carried out. Results showed that the existence of pyrite and Acidithiobacillus ferrooxidans significantly accelerated the dissolution of arsenopyrite and the oxidation of As (Ⅲ) to As (Ⅴ), resulting from the galvanic effect, an increase in the Fe3+/Fe2+ ratio and the oxidation-reduction potential (Eh) value, and a decrease in pH level. As the detected main intermediate products, element sulphur was considered as the dominating obstructive factor during arsenopyrite oxidation, while the added pyrite could accelerate its oxidation. Moreover, a close relationship between different mineral proportions and the galvanic effect was also observed and discussed. Finally, suggestions on AMD governance and source control are proposed.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite.

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO42- on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO42- in simulated AMD environments, and the effects of SO42- concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO42- could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO42-, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite-As(III) complex along with the formation of reactive oxygen species in the presence of O2. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO42- concentration. In the pH range of 2.0-4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO42-. This study reveals how SO42- promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

Arsenic release from arsenopyrite weathering in acid mine drainage: Kinetics, transformation, and effect of biochar.

Arsenopyrite (FeAsS) oxidative dissolution provides an important source for the occurrence of high arsenic in acid mine drainage (AMD). Biochar is a potent material that can dramatically sequestrate an array of heavy metals in water. However, little is known about the role of biochar on the fate of As from arsenopyrite in AMD. This study investigates the effects of biochar concentrations, AMD acidities, and temperatures on the release of As from arsenopyrite in a simulated AMD over a range of environmentally relevant conditions. Results show that biochar inhibits As release and further acidification without changing the arsenopyrite weathering mechanism. Arsenopyrite is first oxidized to Fe(II), As(III) and S0 and ultimately oxidized to Fe(III), As(V) and SO42-, respectively. Higher concentration, temperature or higher acidity promotes the arsenic release rate. Electrochemical studies showed that biochar inhibited As release and acidification for reduced the charge transfer resistance at the double layer and film resistance at the passivation layer, which was mainly attributed to Fe(III) ions in AMD being adsorbed, oxidized, and As complexed to biochar-Fe-As(V). This study reveals the release mechanism of As from arsenopyrite weathering in AMD and suggests the applicability of biochar in mitigating arsenic pollution and further acidification in sulfide mineral mine drainage.