Mercury methylation/demethylation and volatilization pathways in estuarine sediment slurries using species-specific enriched stable isotopes

Species-specific enriched stable isotopes have been used to study mercury transformations (methylation, demethylation and volatilization) in estuarine sediments under different environmental conditions (both biotic and abiotic and oxic and anoxic). These experiments have demonstrated the potential for the isotopically enriched species in combination with highly sensitive measurement methods (ICP MS) to facilitate the study of mercury speciation and reactivity. Sediments (sterilized and nonsterilized) were spiked with both isotopically enriched inorganic (¹⁹⁹Hg) and monomethylmercury (MM²⁰¹Hg) at environmental levels to avoid perturbing the system and incubated under both aerobic and anaerobic conditions. The formation of MM¹⁹⁹Hg and the degradation of MM²⁰¹Hg were measured simultaneously during time series experiments by capillary gas chromatography-inductively coupled mass spectrometry. Specific methylation and demethylation rate constants (K_m and K_d) were calculated. Results clearly showed that methylmercury levels in sediments are controlled by competing and simultaneous methylation and demethylation reactions. Operating conditions, likely to drastically modify the reactivity of the media, were of primary importance to assess the relative significance of each mechanism. In estuarine sediments, mercury methylation was enhanced under anaerobic nonsterile conditions, whereas other environmental conditions were either less favorable for monomethylmercury production or more effective for its degradation. The production of total gaseous mercury was found to be minimal, although it could be demonstrated that it was related to the fate and transformation of methylmercury.