Kinetic Modeling of TiO₂-Catalyzed Photodegradation of Trace Levels of Microcystin-LR

A kinetic model has been developed to investigate the relative importance of major pathways for the photocatalytic degradation of trace levels of the cyanobacterial toxin microcystin-LR (MLR) in solutions containing a complex suite of dissolved organic matter and to test the sensitivity of MLR degradation to rate constants of the key processes. The kinetic model incorporates adsorption of the trace contaminant, other organics and oxygen on the particle surface, surface reactions between adsorbed radical and nonradical species, desorption of surface radical species, solution phase radical reactions, and radical termination pathways. Under conditions where the contaminant adsorbs strongly to semiconductor surface sites, rapid degradation is observed, and a primary degradation step appears to involve reaction between surface-located long-lived organic radicals (formed from hydroxyl radical scavenging by the bulk organic) and adsorbed trace contaminant. MLR degradation is relatively insensitive to changes in light intensity under these strongly adsorbing conditions but highly dependent under weakly adsorbing conditions and when solution phase degradation is important. While not verified independently, desorption of surface bound superoxide appears to lead to the production of organic peroxyl radicals through reaction of superoxide with the bulk organic. These solution phase organic peroxyl radicals are highly reactive and appear to be the primary source of trace contaminant degradation under conditions where the trace contaminant shows no observable adsorption and surface degradation is negligible. Under alkaline conditions, adsorption of carbonate onto the particle surface results in scavenging of surface hydroxyl radicals to form surface carbonate radicals that rapidly quench surface bound superoxide. This prevents organic peroxyl production, the primary agent of solution-phase trace contaminant degradation.