Role of Cr(III) deposition during the photocatalytic transformation of hexavalent chromium and citric acid over commercial TiO2 samples

N. V. MONTESINOS; C. SALOU; J.M. MEICHTRY; C. COLBEAU-JUSTIN; M.I. LITTER

Photochemical and Photobiological Sciences

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2016 vol. 15 p. 228 - 234

1474-905X

Removal of Cr(VI) and citric acid (Cit) by heterogeneous photocatalytic Cr(VI) transformation under UVlight over two commercial TiO2 samples (1 g L−1), Evonik P25 and Hombikat UV100, was studied at pH 2and Cr(VI) concentrations between 0.2 and 3 mM, with a fixed [Cit]0/[Cr(VI)]0 molar ratio (MR) of 2.5. In bothcases, up to complete Cr(VI) removal, the temporal profiles of Cr(VI) and Cit were well adjusted to a pseudofirstorder rate law with the same rate constant, evidencing that Cr(VI) removal controls the kinetics of thesystem. Once Cr(VI) is fully removed, Cit degradation continues with a Langmuir?Hinshelwood behaviour. Inall cases, the rate constants decreased with increasing [Cr(VI)]0, and time resolved microwave conductivity(TRMC) measurements revealed that this was due to an increasing retention of Cr(III) on the surface of thephotocatalysts, which reduces the lifetime of the electrons. Both kinetic experiments and TRMC measurementsconfirm that UV100 is not only more efficient than P25 for Cr(VI) and Cit removal, but it is also lessinfluenced by the poisoning of the surface, consistent with its larger specific area. The use of Cit as thesacrificial agent improves the rate and efficiency of the photocatalytic Cr(VI) removal, and also the stability ofthe photocatalyst by preventing Cr(III) deposition, due to the formation of soluble Cr(III)-complexes, envisagedas a general result of the presence of oligocarboxylic acids in the photocatalytic Cr(VI) system.