The removal mechanism of batch electrocoagulation (EC) process for removing Cr ions was investigated. The influence of operation parameters on removal mechanisms was discussed. The utilization rate of in situ electro-generated Fe ions in the sludge was introduced to investigate the removal mechanism and optimize the EC process. The Fe elements' utilization rate resulting from the specific adsorption is much higher than that resulting from precipitation and co-precipitation. The initial pH determines which removal mechanism dominates the Cr removal. At neutral initial pH condition, Cr ions are mainly removed by flocs' surface complexation reaction (specific adsorption). The utilization rate of Fe ions at neutral pH condition reaches its maximum and is higher than that at other pH conditions. The influence of electrode material on EC performance was investigated on the basis of utilization rate of Fe ions. For EC with Fe/Al electrode combination, although direct dissolution of Al ion from Al electrodes will improve the Cr(VI) removal efficiency, the utilization rate of generated metal ions is much lower, when compared with EC with Fe/Fe electrode. The utilization rate of in situ electro-generated metal ions could be considered as a new index to evaluate EC performances. (C) 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.