Isotopic Characterization ( 2 H, 13 C, 37 Cl, 81 Br) of Abiotic Degradation of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies

Axel Horst , Magali Bonifacie (1) , Gérard Bardoux (1) , Hans Richnow (2)

1 IPG Paris - Institut de Physique du Globe de Paris
2 UFZ - Helmholtz Centre for Environmental Research, Department of Isotope Biogeochemistry

Methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) significantly contribute to stratospheric ozone depletion. The atmospheric budgets of both compounds are unbalanced with known degradation processes outweighing known emissions. Stable isotope analysis may be capable to identify and quantify emissions and to achieve a balanced budget. Degradation processes do, however, cause isotope fractionation in methyl halides after emission and hence knowledge about these processes is a crucial prerequisite for any isotopic mass balance approach. In the current study, triple-element isotope analysis (2 H, 13 C, 37 Cl/ 81 Br) was applied to investigate the two main abiotic degradation processes of methyl halides (CH 3 X) in fresh and seawater: hydrolysis and halide exchange. For CH 3 Br, nucleophilic attack by both H 2 O and Cl − caused significant primary carbon and bromine isotope effects accompanied by a secondary inverse hydrogen isotope effect. For CH 3 Cl only nucleophilic substitution by H 2 O was observed at significant rates causing large primary carbon and chlorine isotope effects and a secondary inverse hydrogen isotope effect. Observed dual-element isotope ratios differed slightly from literature values for microbial degradation in water and hugely from radical reactions in the troposphere. This bodes well for successfully distinguishing and quantifying degradation processes in atmospheric methyl halides using triple-element isotope analysis. ■ INTRODUCTION Methyl chloride (CH 3 Cl, chloromethane) and methyl bromide (CH 3 Br, bromomethane) together contribute about 30% to halogen induced ozone loss even though atmospheric concentrations are very low: 540 pptv and 7 pptv, respectively. 1 CH 3 Cl and CH 3 Br are emitted by both anthropogenic and natural sources such as fumigation for quarantine and preshipment treatment (for CH 3 Br), 2 marine macroalgae, 3 salt marshes, 4 soils, 5 biomass burning, 6 and tropical plants. 7 Main degradation processes for both of these compounds are reaction with OH and Cl radicals in the troposphere, 8 degradation in oceans 9 and soils. 10 The atmospheric budgets of both compounds are unbalanced with known degradation processes exceeding the best estimates of known emissions by approximately 20% for CH 3 Cl and 30% for CH 3 Br. 1,11 A better understanding of emission and degradation processes will be necessary in order to better quantify emission and degradation of CH 3 X and to improve budget estimates. Previous studies suggested that degradation in oceans is primarily driven by the abiotic processes hydrolysis and halide exchange as well as microbial degradation. 9,12,13 To a minor extent, hydrolysis may also contribute to degradation of CH 3 Br in soils. 14 Hydrolysis and halide exchange of CH 3 X (CH 3 Cl and CH 3 Br) are both nucleophilic substitution reactions (S N 2) following second order reaction kinetics. The attacking nucleophiles are either water (H 2 O), hydroxide ions (OH −), or halide ions such as Cl − and Br − (Y −): 15−17

Domaines: Environnement et Société

Axel Horst, Magali Bonifacie, Gérard Bardoux, Hans Richnow. Isotopic Characterization ( 2 H, 13 C, 37 Cl, 81 Br) of Abiotic Degradation of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies. Environmental Science and Technology, 2019, ⟨10.1021/acs.est.9b02165⟩. ⟨hal-02392224⟩