Effects of water parameters on the degradation of microcystin-LR under visible light-activated TiO2 photocatalyst

A study was performed to determine the effect of pH, alkalinity, natural organic matter (NOM) and dissolved oxygen in the performance of nitrogen and fluorine doped TiO2 (NF-TiO2) for the degradation of hepatotoxin microcystin-LR (MC-LR) in synthetic and natural water under visible light irradiation. The initial degradation rate of MC-LR was fastest under acidic conditions (3.50 +/- 0.02 x 10(-3) mu M min(-1) at pH 3.0) and decreased to 2.29 +/- 0.07 x 10(-3) and 0.54 +/- 0.02 x 10(-3) mu M min(-1) at pH 5.7 and 7.1, respectively. Attractive forces between the opposite charged MC-LR and NF-TiO2 are likely responsible for the enhancement in the photocatalytic decomposition of MC-LR resulting from increased interfacial adsorption. For carbonate buffered solutions, the photocatalytic activity of NF-TiO2 was reduced when increasing the carbonate concentration up to 150 mg CaCO3 L-1. The scavenging of radical species by the bicarbonate ion at pH 7.1 is discussed. In the presence of NOM, the degradation rates decreased as pH and initial concentration of the NOM increased. The inhibition was higher with fulvic acid than humic acid under alkaline conditions. Oxygenated solution yields higher NF-TiO2 photocatalytic degradation of MC-LR compared to nitrogen sparged solution at pH 5.7. The involvement of specific reactive oxygen species implicated in the photodegradation is proposed. Finally, no significant degradation is observed with various natural waters spiked with MC-LR under visible light (lambda > 420 nm) but high removal was achieved with simulated solar light. This study provides a better understanding of the interactions and photocatalytic processes initiated by NF-TiO2 under visible and solar light. The results indicate solar photocatalytic oxidation is a promising technology for the treatment of water contaminated with cyanotoxins. (C) 2011 Elsevier Ltd. All rights reserved.