Carbonate radical in natural waters

Abstract

Sunlight plays an important role in natural cleansing of field waters through direct photolysis and indirect photolysis by different transients. The carbonate radical (·CO3-) is a secondary radical that results from the scavenging of hydroxyl radical (·OH) by carbonate/bicarbonate and is itself strongly electrophilic. 'N',' N'-dimethylaniline (DMA) was found to be a selective probe for measuring the steady state concentration of this radical in a variety of natural waters. The pH, nitrate, total carbonate/bicarbonate, and DOC in selected field waters were quantitatively analyzed. Steady-state carbonate radical concentrations [·CO3-]ss in natural waters were investigated in a photoreactor and in summer sunlight. The measured [·CO 3-]ss strongly depended on light intensity; the range of [·CO3-]ss found in natural waters was 5 * 10-15 to 10-13 'M'. A linear relationship of [·CO3 -]ss to the major components in natural waters was derived. A novel method was used to produce the ·CO3 - with KOONO as solid ·OH source. Using a competition kinetic method, the second-order constants of nine pesticides were measured ranging from 2.0 * 107 'M-1s -1' for fenthion to 2.0 * 105 ' M-1s-1' for methyl parathion. Thioanisole, dibenzothiophene, and fenthion were selected as probes to determine the reaction pathway with ·CO3 -. Using HPLC, GC, GC-MS and LC-MS for structural confirmation, the major photodegradation products were the corresponding sulfoxides, followed by further oxidation to the sulfones. The photoreactivities of these probes in different reaction matrices were investigated. Since fenthion provided an excellent structural template in order to observe the influence of photo-oxidation on hydrolysis rates, the hydrolysis kinetics of fenthion and five metabolites were studied at different pHs and temperatures. Fenthion and its metabolites were relatively stable in neutral media, and their stability decreased as pH increased. The photo-oxidation of fenthion with ·CO3 - results in intermediates that are rapidly degraded through hydrolytic cleavage. Carbonate radical is a selective oxidant existing in natural waters with relatively high steady-state concentration that could contribute to limiting the persistence of electron-rich compounds.