The recovery of Zn and Mn from spent alkaline and Zn-C batteries with a biohydrometallurgycal process was studied in a pilot plant that consists of an air-lift bioreactor with a sulphur packed bed where Acidithiobacillus thiooxidans produces an acid-reducing medium; a leaching reactor where the acid-reducing medium is mixed with the battery powder, and a recovery reactor where metals are recovered from the leaching liquor by electrolysis. Results show that with acid medium (350 mMENT#091;H+ENT#093;) produced in 12 days by Acidithiobacillus thiooxidans in the bioreactor, an extraction of 100% of Zn and 67% of the Mn present in the battery powder was reached. The presence of polythionates in the medium produced in the bioreactor allows the dissolution of the manganese. The solid remaining after bioleaching is a manganese oxide. The electrolysis of the leaching liquor produced a cathodic deposit of metallic Zn and an anodic deposit of a manganese oxide in one step at room temperature

The microbial reduction of chromium(VI) to chromium(III) has been one of the most widely studied forms of metal bioremediation. Recently, we have found that Thiobacillus ferrooxidans and Thiobacillus thiooxidans, growing on elemental sulphur, can indirectly promote chromium(VI) reduction by producing reducing agents such as sulphite and thiosulphate, which abiotically reduce chromium(VI). Those species of Thiobacillus are acidophilic bacteria which grow optimally at pH values lower than 4. However, most of those reducing agents are stabilised at higher pH values. Thus, the present paper reports on the ability to reduce chromium(VI) using another specie of Thiobacilli, Thiobacillus thioparus, which is able to grow at pH close to 7.0. T. thioparus cultures were carried out in a fermentation vessel containing medium and sulphur as the sole energy source and maintained at 30ºC and 400 rpm. The pH was adjusted to 6.0, 7.0 or 8.0 and maintained with the automatic addition of KOH. Our results show high chromium (VI) reduction values (close to 100% at the end of bacterial growth) at the three pH values. The results of these experiments are very promising for development of a microbiological process to be used in the detoxification of chromium(VI)-polluted effluents.

In the absence of iron, dissolution of zinc sulfide was enhanced by the action of Acidithiobacillus caldus at 40ºC. The bioleaching mechanism was similar to that observed for mesophilic species of Acidithiobacillus at 30ºC, although the final metal recovery was lower. When iron was added to the cultures, the solubilization of zinc and copper from the sulfides was higher than that in sterile controls. The activity of the cells was through two indirect mechanisms (acid and oxidant mechanisms, for zinc and copper sulfides respectively). A. caldus did not enhance the dissolution of nickel sulfide neither in the absence nor in the presence of iron.