Comparison and optimization of the bio-cyanidation potentials of B. megaterium and P. aeruginosa for extracting gold from an oxidized copper-gold ore in the presence of residual glycine

Considering the environmental issues of cyanide ion, cyanogenic bacteria which are capable of cyanide genesis may be regarded as a potential alternative to conventional cyanidation. The capacities of two cyanogenic bacterial strains namely Pseudomonas aeruginosa and Bacillus megaterium for the cyanide production and gold bioleaching from the pure gold particles and an oxidized gold-bearing ore (6.2 g/ton) were compared. Also, in order to maximize the cyanide bio-production and subsequently the gold recovery, the influences of the initial pH, glycine, FeSO_4, NaH₂PO₄, and MgSO₄ concentrations in the growth media were studied and optimized. In the optimal culture condition, P. aeruginosa and B. megaterium produced 66.35 and 67.05 mg/l cyanide, respectively, and 2.5 g/l glycine remained unconsumed. Therefore, bioleaching experiments were performed under the synergistic effect of the cyanide-glycine mixture through the active and passive approaches that the latter averagely improved the Au dissolution by 8%. The concentration of Au in the leachate of B. megaterium was higher in comparison with the other bacterium, and reached 205 and 32.2 μg/l for the pure gold and the original ore, respectively. Moreover, performing a pretreatment stage to remove metal contents before cyanidation, increased the final Au concentration to 44.4 μg/l. Finally, raising the bioleaching pH to 10 increased the Au concentration to 56.8 μg/l which represents >80% gold recovery. Although the Au extraction efficiencies of both strains were low, the observed improvements due to the ferrous and magnesium addition, copper removal, and pH increase are promising for the future works.