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 (kapp,O3)
at pH 7 were simultaneously determined for 31 cyano-metabolites. kapp,O3 for olefin- and phenol-containing cyano-metabolites
were consistent with their expected reactivity (0.4–1.7 ×
106 M–1 s–1) while kapp,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, kapp,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.