The influence of silver ions (Ag+) on the bioleaching of cobalt from spent lithium batteries using Acidithiobacillus ferrooxidans (A. ferrooxidans) bacteria was investigated. The best effect was observed at Ag+ concentration of 0.02 g/L, and the amount of leached cobalt was 98.4% in a period of 7 days, whereas in the absence of Ag+, the amount of leached cobalt was only 43.1%. Based on the results of SEM, XRD and EDX investigation, it is deduced that Ag+ first reacted with cobalt to form AgCoO2 as an intermediate. A mechanism that is based on catalytic interactions is proposed to explain the enhanced leaching of Co using A. ferrooxidans. (C) 2013 Elsevier Ltd. All rights reserved.