UV and visible light activated TiO₂ photocatalysis of 6-hydroxymethyl uracil, a model compound for the potent cyanotoxin cylindrospermopsin

TiO₂ photocatalyses of 6-hydroxymethyl uracil (6-HOMU) a model compound for the potent cyanotoxin, cylindrospermopsin (CYN), were carried out employing visible and UV irradiation using different non-metal doped TiO ₂ materials, nitrogen and fluorine-TiO₂ (NF-TiO ₂), phosphorus and fluorine-TiO₂ (PF-TiO₂) and sulfur-TiO₂ (S-TiO₂). The model compound was readily degraded under UV TiO₂ photocatalysis with pseudo-first-order rate constants (k) of 2.1, 1.0, and 0.44 h^-1 for NF-TiO₂, PF-TiO₂ and S-TiO₂, respectively. Under visible light activated (VLA), NF-TiO₂ was the most active photocatalyst, PF-TiO₂ was marginally active and S-TiO₂ inactive. VLA NF-TiO₂ was effective and increased the k with increasing pH from 3 to 9. The presence of humic acid (HA), Fe³⁺ and Cu²⁺ can enhance the degradation. However, at 20 ppm HA significant inhibition was observed, likely due to shadowing of the catalyst, quenching of ROS or blocking active sites of TiO₂. We probed the roles of different reactive oxygen species (ROS) using specific scavengers and the results indicate that O₂^- plays an important role in VLA TiO₂ photocatalysis. Our results demonstrate that NF-TiO₂ photocatalysis is effective under UV and visible irradiation and over a range of water qualities. VLA NF-TiO₂ photocatalysis is an attractive alternative technology for the CYN contaminated water treatment.