Recently, sulfate radical-based advanced oxidation technologies have shown significant implications for environmental remediation to decompose water pollutants. In this study, we evaluated the performance of heterogeneous activation of peroxymonosulfate (PMS) to generate sulfate radicals using cobalt catalyst immobilized on titanium dioxide nanoparticles (Co/TiO2). The Co/TiO2 catalyst was prepared via an incipient wetness impregnation method employing Degussa P-25 TiO2 and Co(NO3)center dot 6H(2)O. The activity of Co/TiO2 system was compared with those of Co(NO3)(2) solution for homogeneous PMS activation and neat Co3O4 for heterogeneous PMS activation. More emphasis was given to the effect of cobalt loading and heat treatment on the physicochemical properties of Co/TiO2 and cobalt leaching. The results showed that heat treatment of Co/ TiO2 at 500 degrees C, where cobalt existed as Co3O4, induced negligible Co leaching and enhanced catalytic activity to decompose 2,4-dichlorophenot. The Co/TiO2 catalyst at Co/Ti molar ratio of 0.1 showed the highest activity via heterogeneous PMS activation. On the other hand, Co/TiO2 catalysts with Co/Ti molar ratio of above 0.2 exhibited rather much lower activity which was initiated predominantly via a homogeneous pathway from leached cobalt, although they contained considerable amounts of Co3O4. The formation of Co-OH complexes at the surface of Co/TiO2 nanoparticles, due to the ability of TiO2 to dissociate H2O for the formation of surface hydroxyl groups, was proposed to facilitate the heterogeneous PMS activation. However, high cobalt loading covering the TiO2 surface diminished the beneficial role of TiO2 due to the reduction in the concentration of surface hydroxyl groups and thus decreased the heterogeneous PMS activation. The activity of Co3O4 in Co/TiO2 catalysts was much higher than that of neat Co3O4 due to the presence of surface hydroxyl groups and uniform distribution of well-defined 10-15 nm nanocrystalline Co3O4 particles at the surface of 30-40 nm TiO2 nanoparticles. (c) 2007 Elsevier B.V. All rights reserved.