delta-FeOOH nanoparticles were prepared by the precipitation of Fe(OH)(2) followed by rapid oxidation with H2O2. Samples with different surface areas and particle sizes were obtained by controlling the final pH of the reaction medium. The samples were characterized by X-ray powder diffractometry, Fe-57 Mossbauer spectroscopy, saturation magnetization measurements, total Fe chemical analyses and BET surface area measurements. The catalytic activities of these samples for H2O2 decomposition were strongly influenced by the delta-FeOOH surface area. EPR was used to monitor catalytic H2O2 decomposition in the presence of methanol, indicating that the mechanism of decomposition involves radicals in accord with the Haber-Weiss mechanism. Methylene blue and indigo carmine were used to simulate the degradation of contaminants. Monitoring these experiments with ESI-MS revealed that delta-FeOOH can activate H2O2 to produce reactive radicals, which can further promote the oxidation of the dyes. The dye degradation rates depended on the amount of Fe2+ generated in situ on the delta-FeOOH surface. (C) 2012 Elsevier B.V. All rights reserved.