Nanosized biogenic iron oxide was synthesized by dissimilatory iron-reducing bacterium, Shewanella sp. This biogenic iron oxide was evaluated as a catalyst in the heterogeneous catalytic ozonation of para-chlorobenzoic acid (pCBA). XRD and TEM analyses showed that the biogenic iron oxide was magnetite phase (Fe3O4) and was composed of nanosized irregular particles in the range of 10.0 +/- 4.0 nm in diameter. Catalytic ozonation was carried out at acidic pH levels (similar to 2.5) in the presence of the biogenic magnetite. It was clearly shown that the biogenic magnetite enhanced the degradation of pCBA by the production of (OH)-O-center dot resulting from the catalytic decomposition of ozone on the surface of the particles. Functional groups on the surface of the biogenic magnetite played a role of catalytic active sites, and this was confirmed by FT-IR and titration analyses. However, the biogenic magnetite showed a lower catalytic efficiency than the commercial nanosized magnetite, resulted from the formation of 4 times bigger aggregates of the biogenic magnetite than the commercial one in aqueous solutions. The R-ct values representing the ratio of hydroxyl radicals and ozone were found to be divided into two regions during reaction. The R,, values during first period (1 min) were much greater than those during second period, and this was caused by initial rapid decrease of pCBA. (c) 2008 Elsevier B.V. All rights reserved.