Kinetics of methanethiol oxidation by oxygen under aqueous conditions

Abstract

Methanethiol is one of the most abundant volatile organic sulfur compounds in natural aquatic systems and one of the main products of dimethylsulfoniopropionate decomposition. This study focuses on the kinetic parameters of the reaction of methanethiol and its deprotonated form, methanethiolate, with dissolved oxygen in aqueous solutions at various reactant concentrations, pH, and temperatures. The reaction proceeds through two distinct pathways: a slow reaction between protonated methanethiol and oxygen under acidic to neutral conditions, and a fast reaction between methanethiolate and oxygen under basic conditions. At the environmentally relevant pH and concentrations, the reaction order with respect to methanethiol is 2.2 ± 0.4 for the protonated form and 1.6 ± 0.2 for the deprotonated form, while in both cases it is 1.0 ± 0.3 with respect to oxygen. Dimethyl disulfide was the only product detected in both reaction pathways. The ratio between the consumption rates of oxygen and methanethiolate was approximately 1:4, while the ratio of oxygen to methanethiol consumption rates was close to 1:2. This implies that dimethyl disulfide is not the only product of methanethiol oxidation. The half-life of methanethiol in oxic water column at 25 °C and typical marine methanethiol concentrations of 0.02–2 nM was estimated to be 80 to 1200 years. Rates of chemical oxidation of MT in the surface waters are lower than the rates of its photooxidation and degassing. Thus, the contribution of the chemical oxidation of MT in marine systems to its budget is negligible but should be considered for the MT-rich aphotic hydrothermal and limnic waters. In systems with methanethiol concentrations exceeding 1 mM, such as bioreactors, chemical oxidation rather than microbial decomposition is likely the primary mechanism for methanethiol removal, even at low oxygen levels. Under fully oxic conditions, the rate of chemical oxidation of methanethiol is expected to surpass the rate of microbial degradation observed under anoxic conditions typically utilized in these reactors.