In the present study, exposure of Cr(III)-tar to full light of medium pressure mercury lamps and a xenon lamp was conducted in batch reaction systems at 25 degrees C and different pH values to predict the potential for Cr(III) oxidation. The results indicated that the more intense irradiation and higher pH facilitated Cr(III)-tar oxidation. It appears that a ligand-to-metal charge-transfer occurs for Cr(III)-tar after irradiation, leading to the generation of Cr(II) and tar(center dot) or (OH)-O-center dot. The accompanying photochemical decomposition of tar(center dot)/or tar, together with O-2, further caused the formation of (OH)-O-center dot through multiple pathways, which ultimately converted Cr(II) to Cr(VI) step by step. H2O2, a direct source of (OH)-O-center dot under irradiation, significantly enhanced photooxidation of Cr(III)-tar. but not obviously of aqueous Cr(III) or Cr(III)-tar in dark, implying that Cr(II) acts as a precursor of Cr(III) oxidization to Cr(VI). (C) 2011 Elsevier B.V. All rights reserved.