Formate supplementation can increase nickel recovery by Halothiobacillus halophilus

Acidophilic thiobacilli are traditional biotechnological agents for metal recovery from sulfide ores. Major industrial strains belong to autotrophic bacteria which are used without any organic supplements to stimulate the process. The autotrophic strain Acidithiobacillus ferrooxidans ATCC 21834 is known to use formate as a source of energy under special laboratory conditions. We showed the presence of formate dehydrogenase in the type strain of another autotrophic species Halothiobacillus halophilus representing another genus of thiobacilli. This finding prompted studies of bioleaching stimulation by formate. Canadian sulfide nickel ore was chosen for model investigation as leached substrate and the moderate acidophilic strain H. halophilus DSM 6132 was used as the leaching agent. In bench-scale bioleaching experiments, inoculation of the ore with H. halophilus supplemented with 0.3 % formate increased the recovery of nickel 70-fold as compared with formate-free inoculation (1008.0 vs. 13.8 mg Ni/L per 34 days). Bacteria H. halophilus belong to moderate acidophilic microorganisms; thus, the results were obtained with initial pH 7.4 and final pH 5.4. The mechanism of formate stimulation is under discussion.