Titania-assisted photoreduction of Cr(VI) to Cr(III) in aqueous media: Kinetics and mechanisms

The kinetics of photon-assisted reduction of chromium-oxyanion, HCrO(4)(-), at the surface of titania particles dispersed in the aqueous solution was investigated as a function of the initial HCrO(4)(-) concentration and the initial salicylic acid (SA) concentration of the solution, at fixed titania loading (1 g/L), solution pH (2), and dissolved oxygen concentration (0.028 mM). Detailed analysis of the kinetic data was carried out to determine the rate constants of the various electron and hole scavenging processes. Dissolved oxygen competes very strongly with HCrO(4)(-) anions for the photogenerated electrons, suggesting that its presence is detrimental to the reduction of HCrO(4)(-) anions. In the absence of salicylic acid, recombination dominates the water and the lattice oxidation processes, thereby removing the photogenerated electrons and leading to lower HCrO(4)(-) reduction rates. Salicylic acid rapidly scavenges the photogenerated holes, thus preventing the electron-hole recombination and assisting in the transfer of electrons to HCrO(4)(-) anions leading to their increased rate of reduction. Only a third of the incident photons were absorbed by the titania leading to the generation of the electron-hole pairs, indicating inefficient use of energy. The quantum yield for the reduction of HCrO(4)(-) anions to Cr(3+) cations varied from 18 to 39 pct as HCrO(4)(-) concentration increased from 0.4 to 1 mM (in the absence of SA) and varied from 39 to 98 pct as SA concentration increased from 0.0036 to 0.1 mM (at [HCrO(4)(-)] = 1 mM).