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 (l < 387 nm), no photoreduction of Cr(VI) was observed in TiO2
dispersions being irradiated with visible light (l > 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 L1) was removed in
the visible light-illuminated TiO2 (1.0 g L1) dispersions at pH 3.0 within 2 h by adding 0.2 mmol L1
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.