The mechanism of photocatalysis over titanium dioxide particle was reported (Zhang and Itoh, 2006). The generated electron–hole pairs must be trapped in order to avoid recombination. The hydroxyl ions (OH−) are the likely traps for holes, leading to the formation of hydroxyl radicals which are strong oxidant agents, while the traps for electrons are adsorbed oxygen species, leading to the formation of superoxide species (O2−) which are unstable, reactive and may evolve in several ways.
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Barakat et al. (2004) studied the photocatalytic degradation using UV-irradiated TiO2 suspension for destroying complex cyanide with a con-current removal of copper. Results revealed that free copper (10−2 M) was completely removed in 3 h. The co-existence of Cu(II) and CN− enhanced the removal efficiency of both CN− and copper; the removal (%) increased with increase of Cu:CN− molar ratio reaching a complete removal for both copper and cyanide at a ratio of 10:1 (Fig. 15).