Decomposition of dimethyl polysulfides under solar irradiation in oxic aqueous solutions

Abstract

Environmental contextThe quality of drinking water can be greatly compromised by the presence of dimethyl polysulfides. We studied the rate and mechanism of decomposition of dimethyl polysulfides in aqueous solution under solar irradiation, and found that they decompose photochemically in seconds to minutes, i.e. much faster than under dark conditions. These results suggest that photochemical pathways of dimethyl polysulfide decomposition may prevail in euphotic zones of natural aquatic systems. AbstractThe presence of malodorous dimethyl polysulfides (DMPSs) has been documented in marine and limnic systems as well as in tap water distribution systems. These compounds compromise the quality of drinking water. Under oxic conditions and in the absence of radiation, DMPSs with n ≥ 3 sulfur atoms disproportionate into DMPSs with n+1 and n−1 sulfur atoms, and, finally, to dimethyl disulfide (DMDS) and S8. DMDS, in turn, decomposes to methyl mercaptan (MT) and methanesulfinic acid. Under these conditions, the half-lives of DMPSs vary from months for dimethyl pentasulfide (DM5S) to hundreds of thousands of years for DMDS. In this work, we studied the kinetics and mechanisms of the decomposition reactions of DMPSs with 2–5 sulfur atoms in aqueous solutions in the presence of oxygen and under exposure to solar radiation. The quantum yields of decomposition of DMPSs with 2, 3, 4 and 5 sulfur atoms do not depend on either the concentration of DMPSs or pH, and are 40±10, 2.0±0.2, 35±10 and 10±4 respectively. The quantum yields, which are higher than unity, suggest that under exposure to solar radiation the photochemical decomposition of DMPSs proceeds by a radical chain reaction mechanism. Half-lives of DMPSs in oxic aquatic solutions exposed to solar radiation under a very clear atmosphere and a solar elevation angle of 90° were calculated from the quantum yields and were found to be as low as 43±13s for DMDS, 40±4s for dimethyl trisulfide (DMTS), 2.1±0.6s for dimethyl tetrasulfide (DM4S) and 4.2±1.7s for DM5S.