Heterogeneous photocatalysis of Cr(VI) in the presence of citric acid over TiO2 particles: Relevance of Cr(V)–citrate complexes

TiO2-photocatalytic reduction experiments of Cr(VI) (0.8 mM) under near UV (366 nm) irradiation in the presence of citric acid (0 ≤ [citric acid] (mM) ≤ 40) were performed at pH 2 under air bubbling. Addition of citric acid facilitates Cr(VI) reduction, hindering the electron-shuttle mechanism taking place in pure water. TOC monotonously decreases until all Cr(VI) was reduced. The maximum rate of Cr(VI) reduction was attained for an initial citric acid/Cr(VI) molar ratio, R, equal to 1.25, a further increment in R being detrimental; however, Cr(VI) decay in the presence of citric acid was always faster than in its absence. Cr(VI) reduction takes place through Cr(V) species, readily complexed by citrate and detected by EPR spectroscopy. Quantitative EPR determinations indicate that an important fraction (nearly 15%) of the reduced Cr(VI) is transformed to Cr(V)–Cit, which also undergoes a photocatalytic transformation. The detrimental effect taking place at high conversions for R > 1.25 can be ascribed to secondary steps, i.e., the competition between Cr(VI) and Cr(V) complexes for conduction band electrons or to the competition of Cr(V)–Cit and Cit for holes.