Mechanism by which ferric iron promotes the bioleaching of arsenopyrite by the moderate thermophile Sulfobacillus thermosulfidooxidans

Study on the influence of active oxygen on the natural oxidation of arsenopyrite under different temperature conditions

Arsenic (As), a toxic element, contaminates farmlands, rivers, and groundwater, posing severe environmental and health risks. Notably, As-containing materials in tailings are affected by temperature variations during long-term storage, and this considerably impact the oxidation and migration of elements in arsenopyrite.This study focused on arsenopyrite and investigated the process of its oxidative dissolution and release of arsenic under different temperature conditions by using in-situ XRD, in-situ XPS and electron paramagnetic resonance spectroscopy(EPR), The role of oxygen free radicals in the oxidation of arsenopyrite was elucidated. It has been established that under high-temperature conditions As, iron (Fe), and sulfur (S) are primarily present As(V)/As(IV), Fe(III), and SO42-, respectively. The O2⋅- generated during the oxidation of As(III) by O2, OH⋅ produced by the Fe(II)/FeOH2+ reaction, and H2O2 formed via their interaction play a crucial role in the photochemical oxidation of arsenopyrite. These findings provide a theoretical basis for the formation of ferric arsenate precipitation, contributing in the adsorption and immobilisation of oxidatively released arsenic.

Appropriate sulfur fertilization in contaminated soil enhanced the cadmium uptake by hyperaccumulator Sedum alfredii Hance

The biogeochemical processes of sulfur and heavy metals in the environment are closely related to each other. We investigated the influence of sulfur addition on hyperaccumulator Sedum alfredii Hance growth, cadmium (Cd) accumulation, soil Cd bioavailability, soil bacterial communities and plant transcriptome responses. The results showed that an appropriate rate of sulfur addition (1.0 or 2.5 g/kg) enhanced the growth of Sedum alfredii Hance plants as well as their accumulation of Cd. A high rate of sulfur addition (5.0 or 10.0 g/kg) causes toxicity to Sedum alfredii Hance plants. The application of an appropriate amount of sulfur to the soil increased the abundance of sulfur-oxidizing bacteria such as Sulfuriferula and Thiobacillus; acid-fast bacillus such as Alicyclobacillus; and cadmium-tolerant bacteria such as Bacillus and Rhodanobacter. This led to a decrease in pH and an increase in bioavailable Cd in the soil. RNA sequencing revealed that the addition of sulfur to soils led to the up regulation of most of the differentially expressed genes (DEGs) involved in "photosynthesis" and "photosynthesis, light reaction" in Sedum alfredii Hance leaves. Moreover, the "plant hormone signal transduction" pathway was significantly enriched with sulfur addition. Sulfur assimilation in Sedum alfredii Hance plants may promote photosynthesis and hormone synthesis, leading to Cd tolerance in these plants. Our study revealed that sulfur fertilization enhanced the efficiency of Cd phytoremediation in Sedum alfredii Hance plants.

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.

(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.

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.

A review on the bioleaching of toxic metal(loid)s from contaminated soil: Insight into the mechanism of action and the role of influencing factors

The anthropogenic activities in agriculture, industrialization, mining, and metallurgy combined with the natural weathering of rocks, have led to severe contamination of soils by toxic metal(loid)s. In an attempt to remediate these polluted sites, a plethora of conventional approaches such as Solidification/Stabilization (S/S), soil washing, electrokinetic remediation, and chemical oxidation/reduction have been used for the immobilization and removal of toxic metal(loid)s in the soil. However, these conventional methods are associated with certain limitations. These limitations include high operational costs, high energy demands, post-waste disposal difficulties, and secondary pollution. Bioleaching has proven to be a promising alternative to these conventional approaches in removing toxic metal(loid)s from contaminated soil as it is cost-effective, environmentally friendly, and esthetically pleasing. The bioleaching process is influenced by factors including pH, temperature, oxygen, and carbon dioxide supply, as well as nutrients in the medium. It is crucial to monitor these parameters before and throughout the reaction since a change in any, for instance, pH during the reaction, can alter the microbial activity and, therefore, the rate of metal leaching. However, research on these influencing factors and recent innovations has brought significant progress in bioleaching over the years. This critical review, therefore, presents the current approaches to bioleaching and the mechanisms involved in removing toxic metal(loid)s from contaminated soil. We further examined and discussed the fundamental principles of various influencing factors that necessitate optimization in the bioleaching process. Additionally, the future perspectives on adding omics for bioleaching as an emerging technology are discussed.

A pathway of the generation of acid mine drainage and release of arsenic in the bioleaching of orpiment

Arsenic in acid mine drainage (AMD) is commonly associated with the bioleaching of arsenic sulfide minerals. Orpiment is iron free and one of the most common arsenic sulfide minerals, but no studies are involved with the relationship between the iron free bioleaching of orpiment and the generation of arsenic-containing AMD. In this study, the iron free bioleaching experiments with Acidithiobacillus thiooxidans (T.t) or Acidithiobacillus caldus (A.c) were carried out. In the experiments with T.t, the pH value decreased with time, and the leached arsenic increased significantly. Meanwhile, the density of planktonic bacteria increased gradually, suggesting that T.t survived in the orpiment pulp. However, in the experiments with initial pH of 1, pH changed little and arsenic was nearly not leached, implying that the bioleaching of orpiment can be inhibited when the initial pH was too low. The XRD patterns and the TFESEM-EDS analyses showed that no elemental sulfur was detected on the orpiment surface. It was supposed that the sulfur was converted to sulfuric acid in the bioleaching process. The CFESEM images showed that no corrosion pits were formed though a few cells adhered to the orpiment surface, and the TEM images showed that no extracellular polymeric substances (EPS) were excreted by the attached cells on the orpiment particles. In the experiments with A.c, similar results were obtained. It is inferred that the bioleaching of orpiment under iron deficient conditions in mining areas generates arsenic-containing AMD, but can be inhibited when the initial pH is too low.

Correlation Between Fe/S/As Speciation Transformation and Depth Distribution of Acidithiobacillus ferrooxidans and Acidiphilium acidophilum in Simulated Acidic Water Column

It is well known that speciation transformations of As(III) vs. As(V) in acid mine drainage (AMD) are mainly driven by microbially mediated redox reactions of Fe and S. However, these processes are rarely investigated. In this study, columns containing mine water were inoculated with two typical acidophilic Fe/S-oxidizing/reducing bacteria [the chemoautotrophic Acidithiobacillus (At.) ferrooxidans and the heterotrophic Acidiphilium (Aph.) acidophilum], and three typical energy substrates (Fe²⁺, S⁰, and glucose) and two concentrations of As(III) (2.0 and 4.5 mM) were added. The correlation between Fe/S/As speciation transformation and bacterial depth distribution at three different depths, i.e., 15, 55, and 105 cm from the top of the columns, was comparatively investigated. The results show that the cell growth at the top and in the middle of the columns was much more significantly inhibited by the additions of As(III) than at the bottom, where the cell growth was promoted even on days 24–44. At. ferrooxidans dominated over Aph. acidophilum in most samples collected from the three depths, but the elevated proportions of Aph. acidophilum were observed in the top and bottom column samples when 4.5 mM As(III) was added. Fe²⁺ bio-oxidation and Fe³⁺ reduction coupled to As(III) oxidation occurred for all three column depths. At the column top surfaces, jarosites were formed, and the addition of As(III) could lead to the formation of the amorphous FeAsO₄⋅2H₂O. Furthermore, the higher As(III) concentration could inhibit Fe²⁺ bio-oxidation and the formation of FeAsO₄⋅2H₂O and jarosites. S oxidation coupled to Fe³⁺ reduction occurred at the bottom of the columns, with the formations of FeAsO₄⋅2H₂O precipitate and S intermediates. The formed FeAsO₄⋅2H₂O and jarosites at the top and bottom of the columns could adsorb to and coprecipitate with As(III) and As(V), resulting in the transfer of As from solution to solid phases, thus further affecting As speciation transformation. The distribution difference of Fe/S energy substrates could apparently affect Fe/S/As speciation transformation and bacterial depth distribution between the top and bottom of the water columns. These findings are valuable for elucidating As fate and toxicity mediated by microbially driven Fe/S redox in AMD environments.

Synergistic oxidation of dissolved As(III) and arsenopyrite in the presence of oxygen: Formation and function of reactive oxygen species

As an important source of arsenic (As) pollution in mine drainage, arsenopyrite undergoes redox and adsorption reactions with dissolved As, which further affects the fate of As in natural waters. This study investigated the interactions between dissolved As(III) and arsenopyrite and the factors influencing the geochemical behavior of As, including initial As(III) concentration, dissolved oxygen and pH. The hydrogen peroxide (H₂O₂) and hydroxyl radical (OH^•) generated from the interaction between Fe(II) on arsenopyrite surface and oxygen were found to facilitate the rapid oxidation of As(III), and the production of As(V) in the reaction system increased with increasing initial As(III) concentration. An increase of pH from 3.0 to 7.0 led to a gradual decrease in the oxidation rate of As(III). At pH 3.0, the presence of As(III) accelerated the oxidation rate of arsenopyrite; while at pH 5.0 and 7.0, As(III) inhibited the oxidative dissolution of arsenopyrite. This work reveals the potential environmental process of the interaction between dissolved As(III) and arsenopyrite, and provides important implications for the prevention and control of As(III) pollution in mine drainage.

Geochemical investigations of noble metal-bearing ores: Synchrotron-based micro-analyses and microcosm bioleaching studies

Auriferous sulphide ores often incorporate micro-fine (or invisible) gold and silver particles in a manner making their extraction difficult. Nobel metals are lost in the tailings due to the refractory nature of these ores. Bioleaching is an environment-friendly alternative to the commonly used and toxic cyanidation protocols for gold extraction from refractory ores. In this paper, we investigate gold and silver bioleaching from porphyry and epithermal mineralisation systems, using iron-oxidizing bacteria Acidithiobacillus ferrooxidans. The invisible Au, sequestered in refractory ores, was characterised in situ by synchrotron micro X-Ray Fluorescence (SR-μ-XRF) and X-ray Absorption Spectroscopy (XAS), offering information on Au unaltered speciation at the atomistic level within the ore matrices and at a micro-scale spatial resolution. The SR-μ-XRF and XAS results showed that 10-20 μm sized elemental Au(0) nuggets are sequestered in pyrite, chalcopyrite, arsenopyrite matrices and at the interface of a mixture of pyrite and chalcopyrite. Moreover, the preliminary bioleaching experiments of the two types of ores, showed that Acidithiobacillus ferrooxidans can catalyse the dissolution of natural heterogeneous Fe-rich geo-matrices, sequestering Au and Ag and releasing particulate phases or partially solubilising them within 60 days. These results provide an understanding of noble metal sequestration and speciation within natural ores and a demonstration of the application of synchrotron-based micro-analysis in characterizing economic trace metals in major mineral structures. This work is a contribution to the ongoing efforts towards finding feasible and greener solutions of noble metal extraction protocols.

Arsenopyrite Bio-Oxidization Behavior in Bioleaching Process: Evidence From Laser Microscopy, SEM-EDS, and XPS

In arsenopyrite bioleaching, the interfacial reaction between mineral and cells is one of the most important factors. The energy of the interface is influenced by the mineralogical and microbiological characteristics. In this paper, the interfacial energy was calculated, and the surface of arsenopyrite during the bioleaching process was characterized by 3D laser microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, in order to assess the dissolution and oxidation behavior of arsenopyrite during bioleaching. The results showed that the contact angles of arsenopyrite were 22 ± 2° when covered with biofilms, but the reaction surface of arsenopyrite turned 103 ± 2°. However, the angle was 45–50° when covered by passive layer, which was half as that of arsenopyrite surface. The interfacial energy of arsenopyrite without biofilms increased from 45 to 62 mJ/m², while it decreased to 5 ± 1 mJ/m² when covered by biofilms during the leaching process. The surface was separated into fresh surface, oxidized surface, and (corrosion) pits. The interfacial energy was influenced by the fresh and oxidized surfaces. Surface roughness increased from 0.03 ± 0.01 to 5.89 ± 1.97 μm, and dissolution volume increased from 6.31 ± 0.47 × 10⁴ to 2.72 ± 0.49 × 10⁶ μm³. The dissolution kinetics of arsenopyrite followed the model of K_t = lnX, and the dissolution mechanisms were mixed controlled: surface reaction control and diffusion through sulfur layer. On the surface of arsenopyrite crystal, the oxidation steps of each element can be described as: for Fe, Fe(II)–(AsS)→Fe(III)–(AsS)→Fe(III)–OH or Fe(III)–SO; for S, As–S(-1) or Fe–S(-1)→polysulfide S→intermediate S–O→sulfate; and for As, As^–1–S→As⁰→As⁺¹–O→As⁺³–O→As⁺⁵–O.