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Environmental fate & pathways

Phototransformation in air

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A number of studies have examined the photodegradation of methanethiol. In reaction with OH radicals,the rate constant for reaction of methanethiol with hydroxyl radicals at room temperature was 2.56 x 10-11cm3/(molecule•sec) with 50% degradation after 46 minutes. The atmospheric lifetime was calculated to range from 66 minutes to 35 hours in this study (Lee and Tang, 1983). An estimated half life of 3.8 hours (also based on reaction with OH radicals) and overall hydroxyl rate constant of 3.3 x 10-11cm3/(molecule•sec) was derived from the EPISuite model (U.S. EPA, 2000). In a study measuring photooxidation of methanethiol with alkyl nitrites as the source of OH radicals, the main products of the reaction were SO2, CH3SO3H and H2SO4, with the final yields of 29, 40 and ≥ 2%, respectively. Degradation was 100% after 4 minutes (Hatakeyama and Akimoto, 1983). In a reaction with NOx, the half life of methanethiol was 2 hours with 85% degraded after 5 hours (Sickleset al.,1978).

Sulfur dioxide is oxidized rapidly by both homogeneous and heterogeneous reactions and is removed from the atmosphere by precipitation and by dry deposition on surfaces, mainly as sulfuric acid (ATSDR, 1998). The acidic photo-oxidation products of methanethiol could contribute to the acidification of the aquatic environment.

Upon reaction with OH radicals, the half life of sodium methanethiolate was estimated to be 10 days using the EPISuite model (U.S. EPA, 2000). The estimated overall hydroxyl rate constant was 1.1 x 10-12cm3/(molecule•sec).