Chemical Treatment of Highly Toxic Acid Mine Drainage at A Gold Mining Site in Southwestern Siberia, Russia

Association of rare earth elements with secondary mineral phases formed during alkalinization of acid mine drainage

Rare Earth Elements (REE) are present in acid mine drainage (AMD) in micromolar concentrations and AMD discharge may lead to an environmental risk. Alkaline Passive Treatment Systems (PTS) are often used to treat AMD and trap toxic trace elements. This study was set up to identify mechanisms by which REE are trapped in or on secondary phases formed in a PTS. Batch alkalinization experiments were performed to simulate PTS by sequentially increasing the pH of AMD collected from the Tharsis mining area inside the Iberian Pyrite Belt and synthetic AMD water samples via CaCO3 addition. The solids that precipitated up to pH ~4 and between pH 4-6 were collected and characterized by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in combination with Scanning Electron Microscope/Energy Dispersive X-ray spectroscopy (SEM/EDX) and synchrotron-based X-ray Absorption Spectroscopy (XAS) and synchrotron-based Micro-X-ray Fluorescence (μ-XRF). Results reveal that REE are mostly scavenged between pH 4-6 in association with Al and Fe phases, whereas a smaller fraction is scavenged at pH ~4 by association with gypsum. Synchrotron-based analysis evidences the incorporation of La3+ into the gypsum structure by substituting Ca2+, indicating a co-precipitation mechanism with gypsum occurring mainly at low pH. Results from parallel adsorption and co-precipitation tests suggest that the REE scavenging between pH 4-6 could be due to a combination of adsorption and co-precipitation on Al(OH)3 and ferrihydrite. This implies that in PTS, REE would be mainly found in Al- (and Fe-) oxyhydroxides occurring in deeper layers of the PTS, i.e., where higher pH-values occur, though a small fraction, especially the light REE, could also be found incorporated into gypsum in the upper layers.

Sulphidogenic Bioprocesses for Acid Mine Water Treatment and Selective Recovery of Arsenic and Metals

Human communities need water and mineral resources, the supply of which requires the implementation of recycling and saving strategies. Both closed and active mining sites could beneficiate of the implementation of nature-based solutions, including bioreactors involving sulphate-reducing prokaryotes (SRP), in order to separate and recover arsenic (As) and metals from aqueous stream while producing clean water. Selective precipitation strategies can be designed based on the selection of microbial communities adapted to the pH conditions, generally acidic, and to available low-cost electron donors. Laboratory batch and continuous experiments must be implemented for each type of mine water in order to determine the optimal flow-sheet in which As could be precipitated as sulphides (orpiment or realgar), inside the bioreactor or offline, through stripping of biologically produced hydrogen sulphides (H2S). The respective concentrations and proportions of As and metals and the initial acid mine drainage pH are key parameters that will influence the feasibility of efficient selective precipitation. SRP-based bioreactors could be combined with complementary treatment steps in optimised mine water management solutions that will minimise the production of As-contaminated end-solid waste.

Acid mine drainage treatment using zero-valent iron nanoparticles in biochemical passive reactors

Acid mine drainage (AMD) is the major effluent generated from metal and coal mines, causing serious ecological risks and degradation of aquatic habitats and surrounding soil quality. Biochemical passive reactors (BPRs) are an option for improving AMD affected water. This study investigates the effect of the size and concentration of zerovalent iron nanoparticles (nZVI) on the efficiency of batch BPRs during AMD remediation. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were also used as complementary techniques for the investigation of the changes in microbial cells and nZVI properties after the AMD remediation. The results from the batch experiment showed that the concentration of nZVI increases the pH and decreases ORP during AMD treatment, thus favouring the removal of metals. The results also suggest that metal sulfide precipitation occurred in all the batch with reactive mixture but was greater in reactors amended with nZVI of larger size. This study revealed that the presence of nZVI in the BPR leads to metal removal as well as the inhibition of sulfate-reducing bacteria (SRB) activity. Microscopy study indicated that the addition of nZVI creates a morphological change on certain microorganisms in which the cellular membrane was fully covered with nZVI, inducing cell lysis process. These results show that nZVI is a promising reactive material for the treatment of AMD in BPR systems.

Implication of biofilms in the sustainability of acid mine drainage and metal dispersion near coal tailings

In this study, the ecology of biofilms collected from sediments and efflorescent crusts (EFC) along an acid mine drainage (AMD) system was determined using 16S bacterial metagenomics. The dissolution of coal tailing and EFC by bacteria hosted in biofilms was investigated. Results revealed the predominance of acidophilic bacteria such as Acidithiobacillus ferrooxidans, Leptospirilum ferrooxidans, Acidithrix, Leptospirilum sp, Acidimicrobiaceae, Sulfobacillus, Acidiphilium, and Acidithiobacillus sp. in the biofilms, some of which have been reported to oxidize sulfide minerals and contribute to AMD formation. The experimental results further highlighted the ability of the bacteria in biofilms to leach out metals such as Co, Fe, and Zn, while decreasing the pH of the solution. The bioleaching of EFC was very fast, and increased diversity of the bacterial inoculum contributed to accelerating the leaching rate. Compared to abiotic leaching, the dissolution of minerals by acidophilic bacteria increased the percentage of free hydrated metal speciated forms over the inorganic complex speciated forms, suggesting the potential of biofilms to enhance the dispersion of metals in aquatic systems.

Editorial for Special Issue “Novel and Emerging Strategies for Sustainable Mine Tailings and Acid Mine Drainage Management”

Climate change is one of the most pressing problems facing humanity this century [...]

Performance Evaluation of Fe-Al Bimetallic Particles for the Removal of Potentially Toxic Elements from Combined Acid Mine Drainage-Effluents from Refractory Gold Ore Processing

Acid mine drainage (AMD) is a serious environmental issue associated with mining due to its acidic pH and potentially toxic elements (PTE) content. This study investigated the performance of the Fe-Al bimetallic particles for the treatment of combined AMD-gold processing effluents. Batch experiments were conducted in order to eliminate potentially toxic elements (including Hg, As, Cu, Pb, Ni, Zn, and Mn) from a simulated waste solution at various bimetal dosages (5, 10, and 20 g/L) and time intervals (0 to 90 min). The findings show that metal ions with greater electrode potentials than Fe and Al have higher affinities for electrons released from the bimetal. Therefore, a high removal (> 95%) was obtained for Hg, As, Cu, and Pb using 20 g/L bimetal in 90 min. Higher uptakes of Hg, As, Cu, and Pb than Ni, Zn, and Mn also suggest that electrochemical reduction and adsorption by Fe-Al (oxy) hydroxides as the primary and secondary removal mechanisms, respectively. The total Al3+ dissolution in the experiments with a higher bimetal content (10 and 20 g/L) were insignificant, while a high release of Fe ions was recorded for various bimetal dosages. Although the secondary Fe pollution can be considered as a drawback of using the Fe-Al bimetal, this issue can be tackled by a simple neutralization and Fe precipitation process. A rapid increase in the solution pH (initial pH 2 to >5 in 90 min) was also observed, which means that bimetallic particles can act as a neutralizing agent in AMD treatment system and promote the precipitation of the dissolved metals. The presence of chloride ions in the system may cause akaganeite formation, which has shown a high removal capacity for PTE. Moreover, nitrate ions may affect the process by competing for the released electrons from the bimetal owing to their higher electrode potential than the metals. Finally, the Fe-Al bimetallic material showed promising results for AMD remediation by electrochemical reduction of PTE content, as well as acid-neutralization/metal precipitation.