Kinetics and mechanism of the abiotic decomposition of dimethyl polysulfides with three, four and five sulfur atoms under dark, oxic conditions

Abstract

Environmental contextDimethyl polysulfides are malodorous compounds formed from decomposing algal matter. The decomposition of dimethylpolysulfides with 3–5 sulfur atoms was studied in aqueous solution under dark, oxygenated conditions and compared with observations of natural systems. The half-lives of dimethyl tri- and tetrasulfides are very long (176–100000 years), while the half-life of dimethyl pentasulfide (<2 years) is similar to the observed time of its removal from natural aquatic systems. AbstractThe presence of malodorous dimethyl polysulfides (DMPSs) has been documented in various aquatic systems. In this work, we studied the kinetics and mechanisms of the chemical decomposition of DMPSs with 3–5 sulfur atoms in aqueous solutions in the presence of oxygen and absence of light. DMPSs are shown to undergo reaction with hydroxyl ions, which results in their decomposition. The orders of the decomposition reactions with respect to dimethyl trisulfide (DMTS), dimethyl tetrasulfide (DM4S) and dimethyl pentasulfide (DM5S) are 2.0±0.3, 1.7±0.3 and 2.0±0.2, respectively. The reaction orders with respect to OH− are 0.59±0.06, 0.56±0.08 and 0.58±0.11, respectively. The activation energies of these reactions are 170kJmol−1K−1, 114kJmol−1K−1 and 75kJmol−1K−1, respectively. The initial products of the decomposition are Me2Sn−1 and Me2Sn+1 and the apparent final products are elemental sulfur and dimethyl disulfide (DMDS). DMDS, which is formed during the decomposition of DMTS, is depleted in 34S (ϵ=−13.2 ‰), while the DM4S is enriched 34S (ϵ=4.7 ‰). A mechanism for the decomposition of DMPSs is proposed based on the results. Under these conditions, half-lives for the decomposition of DMPSs in Lake Kinneret vary from 2 months for DM5S to 100000 years for DMTS. The relatively short time scale of the reported odour episodes indicates that other chemical, photochemical or biological processes are responsible for the decomposition of DMTS and DM4S.