Visible light photocatalytic reduction of Cr(VI) on TiO2 in situ modified with small molecular weight organic acids

Visible light photoreduction of toxic Cr(VI) over TiO2 was achieved through surface modification with small molecular weight organic acids (SOAs) as sacrificial organics. Because neat anatase TiO2 is active only under UV light irradiation (lambda < 387 nm), no photoreduction of Cr(VI) was observed in TiO2 dispersions being irradiated with visible light (lambda > 420 nm). However, when a small amount of colorless SOAs was added into the TiO2 dispersion, a charge-transfer-complex (CTC) was formed between TiO2 and SOA, which was sensitive to visible light irradiation and induced the photo-oxidation of SOA and photoreduction of Cr(VI). It was observed that about 95% of added Cr(VI) (0.2 mmol L-1) was removed in the visible light-illuminated TiO2 (1.0 g L-1) dispersions at pH 3.0 within 2 h by adding 0.2 mmol L-1 tartaric acid as a SOA. The SOA-induced photoreduction of Cr(VI) proceeded via a CTC-mediated path, being governed by chemical structures of sacrificial SOAs. A higher energy of the highest occupied molecular orbital or lower ionization potential of SOAs is favorable to electron transfer within TiO2-SOA complex, thereby accelerating the photoreduction of Cr(VI). The Cr(VI) removal was further enhanced by increasing SOA concentration and/or decreasing solution pH. (C) 2010 Elsevier B.V. All rights reserved.