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Nowadays, due to the strategic status of nickel in the global market, utilizing its disregarded resources like low-grade nickel containing pyrrhotite is of significant importance. A comprehensive set of experiments and analyses were performed to determine the bioleaching capability and mechanism for nickel extraction from hexagonal and monoclinic pyrrhotite. Over 95% Ni extraction was achieved from the hexagonal pyrrhotite sample. Ni extraction from the monoclinic sample reached its maximum value of 67% and 90% at 3% pulp density, with mixed mesophilic and moderately thermophilic cultures, respectively. Characterization analyses indicated that jarosite and elemental sulfur formation in mixed mesophilic bioleaching reduced the samples' bio-oxidation rate and metal dissolution. The kinetics study revealed that the controlling step in thermophilic bioleaching is the chemical reaction; however, the mixed control model was best fitted on mesophilic data. Electrochemistry studies confirmed bioleaching results and indicated that monoclinic pyrrhotite's oxidation rate under the operating conditions is faster than hexagonal pyrrhotite, and the temperature positively correlates with the oxidation rate. Toxicity assessment analysis showed that the final residues of both bioleached samples could be considered environmentally safe., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

This work evaluated a green route for developing an eco-friendly flowsheet to regenerate base and precious metals from waste printed circuits boards (WPCBs). Copper (as nanoparticles with an average diameter of 50 nm) and other base metals were extracted via oxidative acid leaching with high efficiency. Thiocyanate was employed for the first time as a green and economical reagent for the extraction of gold from pretreated WPCB. The effect of various parameters, including reagent dosage and temperature, was evaluated on the gold leaching rate, and 100% gold dissolution was achieved at the optimal condition. It was found that ferric iron concentration as the gold leaching oxidant has a notable effect on gold extraction. Also, at temperatures above room temperature, the recovery rate increases in a short period and then decreases continuously. The activation energy of the optimum gold thiocyanate leaching was found to be 42.84 kJ/mol, indicating chemical reaction to be the rate-controlling step. Gold extraction from the thiocyanate medium was carried out by employing activated carbon, where 100% gold adsorption was achieved in 2 h. Toxicity assessment of final residue revealed that it could be categorized as an environmentally safe waste with negligible risk., (Copyright © 2022 Elsevier Ltd. All rights reserved.)