Photocatalytic Degradation of the Blue Green Algal Toxin Microcystin-LR in a Natural Organic-Aqueous Matrix

Abstract

The worldwide appearance of toxic cyanobacterial blooms in drinking water supplies has raised concerns about systemic effects on human health. Conventional water treatment methods are poor at removing low concentrations of cyanotoxins, and specialized treatment is usually necessary for treatment of contaminated water. In this study, the applicability of heterogeneous photocatalytic degradation of low concentrations of the cyantoxin microcystin-LR in a natural organic-aqueous matrix is examined using titanium dioxide as the photocatalyst. The initial rate of toxin degradation is strongly pH dependent in a manner mirrored by the pH dependence of toxin adsorption to TiO2. Rapid degradation of toxin occurs in the acidic pH range in the presence of light and TiO2 with a maximum initial rate of degradation occurring at pH 3.5, while at higher pH, a distinct lag is observed prior to commencement of toxin degradation. It is proposed that in the pH range where microcystin-LR adsorbs to TiO2, it is degraded principally by long-lived organic radicals generated through oxidation of adsorbed cyanobacterial exudate. At higher pH, where microcystin-LR adsorption to TiO2 is insignificant, it is proposed that these organic radicals diffuse into solution and (after a lag) initiate oxidation of the toxin in dissolved phase.