BACKGROUNDThis study attempts to elucidate the complex effect of Ca2+, an important divalent cation widely present in aqueous media, on perchlorate removal by autotrophic perchlorate reducing bacteria (PRB) associated with a zero-valent iron (ZVI) system. RESULTSBatch flask test results showed that, in the presence of larger amounts of Ca2+, perchlorate removal initiated earlier and completed faster. According to the increase of differential chloride concentration, the respective contribution of biodegradation to perchlorate removal was evaluated and found to be improved with increasing amount of Ca2+. It appears that Ca2+ can delay pH increase by combining with OH- and can consequently extend biodegradation time. Despite the major contribution of biodegradation, when ZVI and Ca2+ co-existed, physiochemical perchlorate removal occurred earlier by both adsorption via surface charge neutralization and co-precipitation via formation of larger particles. The positive effect of Ca2+ was further evidenced in subsequent continuous column tests. As empty bed residence time (EBRT) was decreased, more reliable and effective perchlorate removal performance was observed with additional Ca2+ concentration of 240 mg L-1. CONCLUSIONPerchlorate removal by a PRB-ZVI system can be significantly improved with increasing amounts of Ca2+, which can be reasonably understood by the enhanced biodegradation and physiochemical removal process. (c) 2014 Society of Chemical Industry