Photocatalytic degradation of microcystin-LR: Conceptual model and pilot scale studies

The applicability of heterogeneous photocatalytic degradation of low concen trations of the cyanobacterial toxin, 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 mir rored 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 concentrations and pH, a distinct lag is observed prior to commencement of toxin degradation. A proposed conceptual model for toxin degradation is dev eloped and the veracity of the proposed model is tested by determining spec ies concentrations for assumed kinetic constants. Very similar dependencies and trends to those observed in the laboratory studies were obtained sugge sting that adsorption and sensitization effects are critical. Tn particular , under conditions where the contaminant adsorbs strongly to semiconductor surface sites, the primary degradation step appears to involve reaction bet ween surface-located long-lived organic radicals and adsorbed trace contami nant. Significant presence of super oxide at low pH appears to enhance trac e contaminant degradation via solution phase formation of highly oxidizing organic peroxy radicals from "bulk" background organic. Under conditions wh ere the trace contaminant shows no observable adsorption, surface degradati on adequately predicts removal due to surface concentration effects. Initia l pilot scale studies using a solar fixed-bed photocatalytic reactor have b een successful in achieving 96% removal of a 100 L microcystin-1R spiked dr inking water solution. The model successfully predicts the much slower degr adation achieved where a trace contaminant is a very small fraction of a na tural organic-aqueous matrix and number of sites is significantly reduced.