Bimetallic nanoparticles have been used for effective reduction of chlorinated compounds; however, the study of cation effect on degradation is limited. This study examined the effect of three selected cations normally co-present in soil and groundwater contamination sites on the degradation kinetics and removal efficiency of pentachlorophenol (PCP) by Pd/Fe nanoparticles. Degradation of PCP by Pd/Fe nanoparticles was carried out in aqueous solutions containing different cations in sulfate form, Na2SO4, CuSO4, NiSO4, and Fe-2(SO4)(3), respectively. The observed inhibitory effect of Na2SO4 on degradation of PCP was contributed to the existence of SO42- ions. Overcoming the inhibitory effect of SO42- ions, Cu2+, Ni2+, and Fe3+ could facilitate the degradation kinetics and efficiencies of PCP by Pd/Fe nanoparticles. XANES absorption spectra were performed to characterize their valences. The enhancement effect of Cu2+ and Ni2+ ions result from the presence of reduced forms of copper and nickel on Pd/Fe surfaces. The presence of reduced forms of copper and nickel on Pd/Fe nanoparticles were confirmed by ICP-MS analysis. The addition of Fe3+ ions caused a decrease in pH and can reasonably account for the enhancement seen in the PCP degradation process. These observations lead to a better understanding of PCP degradation with Pd/Fe nanoparticles and can facilitate the remediation design and prediction of treatment efficiency of PCP at remediation sites. (C) 2011 Elsevier B.V. All rights reserved.