Continuous Bioleaching of Arsenic-Containing Copper-Zinc Concentrate and Shift of Microbial Population under Various Conditions

Shifts in the Microbial Populations of Bioleach Reactors Are Determined by Carbon Sources and Temperature

In the present study, the effect of additional carbon sources (carbon dioxide and molasses) on the bio-oxidation of a pyrite-arsenopyrite concentrate at temperatures of 40-50 °C was studied, and novel data regarding the patterns of the bio-oxidation of gold-bearing sulfide concentrates and the composition of the microbial populations performing these processes were obtained. At 40 °C, additional carbon sources did not affect the bio-oxidation efficiency. At the same time, the application of additional carbon dioxide improved the bio-oxidation performance at temperatures of 45 and 50 °C and made it possible to avoid the inhibition of bio-oxidation due to an increase in the temperature. Therefore, the use of additional carbon dioxide may be proposed to prevent the negative effect of an increase in temperature on the bio-oxidation of sulfide concentrates. 16S rRNA gene profiling revealed archaea of the family Thermoplasmataceae (Acidiplasma, Ferroplasma, Cuniculiplasma, and A-plasma group) and bacteria of the genera Leptospirillum, with Sulfobacillus and Acidithiobacillus among the dominant groups in the community. Temperature influenced the composition of the communities to a greater extent than the additional sources of carbon and the mode of operation of the bioreactor. Elevating the temperature from 40 °C to 50 °C resulted in increases in the shares of Acidiplasma and Sulfobacillus and decreases in the relative abundances of Ferroplasma, Leptospirillum, and Acidithiobacillus, while Cuniculiplasma and A-plasma were more abundant at 45 °C. A metagenomic analysis of the studied population made it possible to characterize novel archaea belonging to an uncultivated, poorly-studied group of Thermoplasmatales which potentially plays an important role in the bio-oxidation process. Based on an analysis of the complete genome, we propose describing the novel species and novel genus as "Candidatus Carboxiplasma ferriphilum" gen. nov., spec. nov.

Electrochemical Characteristics of the Oxidation of Sulfur- and Iron-Containing Compounds by Acidophilic Microorganisms

The electrochemical features of the interactions of sulfur- and iron-containing compounds (ferrous sulfate, elemental sulfur, pyrite tailings, cysteine, sodium thiosulfate) with a model acidophilic consortium, including the genera Leptospirillum, Sulfobacillus, Acidithiobacillus, Ferroplasma, and Acidiplasma, were studied. The method of cyclic voltammetry recorded redox processes at the electrode/solution interface in the presence of the studied sulfur- and iron-containing compounds. In general, the modeling consortium led to the intensification of these processes. The characteristics of the diffuse layer near the electrode/solution interface were studied using electrochemical impedance spectroscopy. The introduction of microorganisms and/or substrates into the supporting electrolyte led to a decrease in the slope and can be interpreted as evidence of their effect on the diffusion part of the double layer. Its contribution decreases in favor of ion transport. All this, in general, does not contradict the assumption of cell adsorption on the electrode surface. Confocal laser scanning microscopy confirmed this assumption and showed cell adhesion to the surface. The data obtained confirm the importance of bioelectrochemical processes of the studied group of microorganisms in biotechnological processes associated with the leaching of metals from sulfide ores.

Two-Stage Oxidative Leaching of Low-Grade Copper-Zinc Sulfide Concentrate

Bioleaching may be effectively used to extract nonferrous metals from sulfide ores and concentrates. At the same time, some minerals are refractory and their bioleaching rate is often comparatively low that does not allow the required metal extraction rate to be achieved. In the present work, we studied the two-stage process, which included stages of biological and chemical leaching, to improve copper extraction from low grade Cu-Zn sulfide concentrate containing chalcopyrite, tennantite, pyrite, and sphalerite. Bioleaching was conducted in the continuous mode in three laboratory scale reactors connected in series. The pulp density was 10% and the residence time was 7 days. The temperature was 40 °C in the 1st reactor and 50 °C in the 2nd and 3rd reactors. Bioleaching allowed the extraction of 29.5 and 78% of Cu and Zn, respectively. The solid bioleach residue obtained was then treated for additional Cu and Zn recovery using high temperature leaching at 90 °C for 25 h. The liquid phase of the bioleaching pulp contained Fe³⁺ ions, which is the strong oxidant, and the leach solution was supplemented with NaCl. In the presence of the maximal NaCl concentration (1 M), Cu and Zn extraction reached 48 and 84%. Thus, two-stage leaching may allow to increase bioleaching efficiency and may be used to improve the bioleaching rate of refractory minerals, such as chalcopyrite.

Options for Increasing the Rate of Bioleaching of Arsenic Containing Copper Concentrate

In the present work the effect of alkaline sulfide leaching (ASL) on the extraction of copper and zinc from low-grade copper concentrate containing chalcopyrite, tennantite, sphalerite, and pyrite during batch and continuous bioleaching experiments was studied. It was demonstrated that ASL and further bioleaching may be a promising approach for treatment of copper–zinc concentrates containing tennantite as this approach allows increasing copper extraction degree in comparison to one-stage bioleaching by 1.6–2.3 times. Thus, ASL was shown to be effective for pretreatment of tennantite containing concentrates to improve bioleaching for copper extraction. At the same time, ASL led to decrease in zinc extraction by 1.4–1.5 times. Therefore, the development of combined hydrometallurgical processes including ASL and bioleaching for effective metal leaching requires further studies to avoid negative effect on zinc extraction.

Extraction of Metals from Polluted Soils by Bioleaching in Relation to Environmental Risk Assessment

Environmental pollution has particular implications for the whole geosystem and increases the global risk to human and ecological health. In this regard, investigations were carried out on soil samples to perform the quality status assessment by determining: pH, texture, structure and metal concentration, as well as carrying out an assessment of anthropogenic activity by determining pollution indices: C_f (contamination factor), C_d (degree of contamination), PLI (pollution load index), E_r (ecological risk index) and PERI (potential ecological risk index). Analyses on soil samples showed high concentrations of metals (Cu: 113-2996 mg kg^-1; Pb: 665-5466 mg kg^-1; Cr: 40-187 mg kg^-1; Ni: 221-1708 mg kg^-1). The metal extraction experiments were carried out by bioleaching using Thiobacillusferrooxidans, microorganisms at different amounts of bioleaching solution (20 mL and 40 mL 9K medium) and a stirring time of up to 12 h. The results on the degree of contamination, pollution loading index PLI (2.03-57.23) and potential ecological risk index PERI (165-2298) indicate that the soils in the studied area have a very high degree of pollution. The decontamination procedure by bioleaching showed a decrease, but at the end of the test (12 h), the followed indices indicate high values, suggesting that bioleaching should continue. The depollution yield after 12 h of treatment is, however, encouraging: Cu 29-76%, Pb: 10-32%, Cr: 39-72% and Ni 44-68%. The use of yield-time correlation equations allows the identification of the optimal exposure time on the bioleaching extraction process to obtain optimal results. The aim of the research is to determine the soil quality, soil environmental risk, extraction of metals from polluted soils by bioleaching and to identify influencing factors in achieving high remediation yields.