An efficient advanced oxidation process, which is based on the zero-valent iron (ZVI)-mediated decomposition of peroxydisulfate (PDS) that results in the generation of very strong oxidizing species (sulfate radicals) in aqueous solution, is reported. Experiments were conducted in the dark using ZVI loading ranging from 0.05 to 0.40 g L-1 in the presence of PDS. The results showed that ZVI significantly improved the degradation efficiencies of 4-chlorophenol at ambient temperature. The optimum loading of ZVI was 0.20 g L-1 and 88% removal of 4-chlorophenol was observed in 1 h of reaction time. The addition of methanol and tert-butyl alcohol as hydroxyl radical and sulfate radical scavengers proved the presence of sulfate radicals in the ZVI-PDS reaction system. The degradation of 4-chlorophenol was accompanied by the formation of hydroquinone, 1,4-benzoquinone, and small molecule compounds such as oxalic acid and succinic acid. Therefore, hydroquinone pathway was regarded as the main step in the oxidation of 4-chlorophenol. Chloride ion release and formation of oxidation intermediates were evidence of 4-chlorophenol degradation involving sulfate radicals. (C) 2009 Elsevier BM. All rights reserved.