The role of carbonate radical in limiting the persistence of sulfur-containing chemicals in sunlit natural waters

Carbonate radical (.CO3−) is a selective oxidant that may be important in limiting the persistence of a number of sulfur-containing compounds in sunlit natural waters. Thioanisole, dibenzothiophene (DBT), and fenthion were selected to investigate the degradation pathway initiated by .CO3−; electron-rich sulfur compounds are particularly reactive towards the .CO3−. Using HPLC, GC, GC–MS and LC–MS for structural confirmation, the major photodegradation products of thioanisole and DBT were the corresponding sulfoxides. The sulfoxide products were further oxidized through reaction with .CO3− to the corresponding sulfone derivatives. Fenthion showed a similar pathway with appearance of fenthion sulfoxide as the major product. The proposed mechanism involves abstraction of an electron on sulfur to form a radical cation, which is then oxidized by dissolved oxygen. Each of the sulfur probes were further investigated in a sunlight simulator under varying matrix conditions. The highest rate constants occurred in the .CO3− matrix, and the lowest occurred in a matrix of dissolved organic carbon (DOC) and bicarbonate. In synthetic and natural field water, thioanisole photodegraded faster than under direct photolysis, with half-lives of 75.1 and 85.8 min, respectively. Fenthion photodegraded more rapidly than thioanisole. DBT photodegraded rapidly in a .CO3− matrix with a half-life of 24.8 min, while the half-life of direct photolysis was 350 min. Photodegradation products of each compound were also investigated. Ultimately, .CO3− was found to contribute toward the photodegradation of sulfur-containing compounds in natural waters.