Comparison of bio-dissolution of spent Ni–Cd batteries by sewage sludge using ferrous ions and elemental sulfur as substrate

Bioleaching of spent Ni–Cd batteries using acidified sewage sludge was carried out in a continuous flow two-step leaching system including an acidifying reactor and a leaching reactor. Two systems operated about 30 d to achieve almost complete dissolution of heavy metals Ni, Cd and Co in four Ni–Cd batteries. Ferrous sulphate and elemental sulfur were used as two different substrates to culture indigenous thiobacilli in sewage sludge. pH and ORP of the acidifying reactor was stabilized around 2.3 and 334 mV for the iron-oxidizing system and 1.2 and 390 mV for the sulfur-oxidizing system. It was opposite to the acidifying reactor, the pH/ORP in the leaching reactor of the iron-oxidizing system was relatively lower/higher than that of the sulphur-oxidizing system in the first 17 d. The metal dissolution, in the first 12–16 d, was faster in the iron-oxidizing system than in the sulphur-oxidizing system due to the lower pH. In the iron-oxidizing system, the maximum solubilization of cadmium (2500 mg l−1) and cobalt (260 mg l−1) can be reached at day 6–8 and the most of metal nickel was leached in the first 16 d. But in the sulphur-oxidizing system there was a lag period of 4–8 d to reach the maximum solubilization of cadmium and cobalt. The maximum dissolution of nickel hydroxide (1400 mg l−1) and metallic nickel (2300 mg l−1) occurred at about day 12 and day 20, respectively.