Reactivity of Cyanobacteria Metabolites with Ozone: Multicompound Competition Kinetics

Cyanobacterial blooms occur at increasing frequency and intensity, notably in freshwater. This leads to the introduction of complex mixtures of their products, i.e., cyano-metabolites, to drinking water treatment plants. To assess the fate of cyano-metabolite mixtures during ozonation, a novel multicompound ozone (O₃) competition kinetics method was developed. Sixteen competitors with known second-order rate constants for their reaction with O₃ ranging between 1 and 10⁸ M^-1 s^-1 were applied to cover a wide range of the O₃ reactivity. The apparent second-order rate constants (k_app,O3) at pH 7 were simultaneously determined for 31 cyano-metabolites. k_app,O3 for olefin- and phenol-containing cyano-metabolites were consistent with their expected reactivity (0.4-1.7 × 10⁶ M^-1 s^-1) while k_app,O3 for tryptophan- and thioether-containing cyano-metabolites were significantly higher than expected (3.4-7.3 × 10⁷ M^-1 s^-1). Cyano-metabolites containing these moieties are predicted to be well abated during ozonation. For cyano-metabolites containing heterocycles, k_app,O3 varied from <10² to 5.0 × 10³ M^-1 s^-1, giving first insights into the O₃ reactivity of this class of compounds. Due to lower O₃ reactivities, heterocycle- and aliphatic amine-containing cyano-metabolites may be only partially degraded by a direct O₃ reaction near circumneutral pH. Hydroxyl radicals, which are formed during ozonation, may be more important for their abatement. This novel multicompound kinetic method allows a high-throughput screening of ozonation kinetics.