Photocatalytic reduction of Cr(VI) over titania suspensions

• Cr(VI) photoreduction under UVA light and titania was investigated.
• Highly acidic conditions and increased catalyst dosages favour reduction rates.
• Reduction kinetics shift from 1st to lower order as Cr(VI) concentration increases.
• Three titania samples were tested, pure anatase with high BET area gave best results.
• Phthalates and lower alcohols scavenge holes, thus enhancing Cr(VI) reduction.

The photocatalytic reduction of hexavalent chromium over titania suspensions was investigated using a 9 W UV-A lamp as the irradiation source. Experiments were carried out at acidic conditions, with varying chromium and photocatalyst concentration. Commercially available titania powders were tested, predominantly Evonik P-25 (70:30 anatase:rutile), as well as Kronos vlp7000 and Millenium PC-100 (100% pure anatase). Four different organic substrates (phthalate, methanol, ethanol, propanol) were employed as sacrificial electron donor at varying initial concentrations.Reduction is strongly favoured at acidic conditions with complete conversion of 10 mg L−1 Cr(VI) being achieved after 60 min at pH = 2.5 in the presence of 2 g L−1 TiO2 P-25 and 50 mg L−1 methanol. The role of electron donors is equally important as the respective extent of reduction is only 30% in the absence of a hole scavenger. At lower Cr(VI) concentrations, the reduction appears to obey first order kinetics (as evidenced measuring initial rates and computing kinetic constants), but it shifts to lower reaction orders at higher concentrations. All four organics can promote Cr(VI) reduction with phthalate being slightly more effective than the alcohols. Kronos vlp7000, whose surface area is about 3–5 times greater than the other two, results in the higher reduction rates.

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