Influence of H2SO4 and ferric iron on Cd bioleaching from spent Ni–Cd batteries

► Recovery of Cd from Ni–Cd spent batteries. ► Sulphuric acid and ferric ions as main leaching agents. ► The higher amount of iron ions increases Cd leaching efficiency. ► Cd leaching efficiency with H2SO4 depends on maintaining the low pH values. The paper is concerned with biohydrometallurgical methods of cadmium recovery from spent Ni–Cd batteries. Cd leaching efficiency from electrode material in different media (H2SO4 and Fe2(SO4)3 solutions), at different Fe(III) concentrations and using the bacteria Acidithiobacillus ferrooxidans were investigated. The main aim of this study was to understand which from the bioleaching products (sulphuric acid or ferric sulphate) play a main role in the bioleaching process of Cd recovery. The influence of Fe ions on Cd leachability was confirmed. The best leaching efficiency of Cd (100%) was reached by bioleaching and also by leaching in Fe2(SO4)3 solution. The results of X-ray diffraction confirmed that no cadmium was present in solid residuum obtained after the Cd bioleaching as well as Cd leaching using solely ferric iron. The use of H2SO4 solution resulted in the lowest efficiency of Cd leachability, the presence of hydroxides in electrode materials caused neutralization of the leaching solution and inhibition of Cd leaching.