Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability

A method was developed to measure mercury methylation and demethylation rate constants simultaneously in aquatic samples. Solutions containing stable isotope tracers of Hg-199(2+) and (CH3Hg+)-Hg-202 were spiked into lake sediments at subambient concentrations. The formation of (CH3Hg+)-Hg-199 and the decrease in (CH3Hg+)-Hg-202 were measured simultaneously in time series experiments using gas chromatographic separation and isotope-specific detection by inductively coupled plasma mass spectrometry. Specific rate constants For the two processes were calculated and compared to rate constants obtained by monitoring changes in concentration of the ambient methylmercury in the same sample. The inorganic mercury tracer Hg-199(2+) was methylated at a faster rate compared with the ambient inorganic Hg2+, which indicates that the added tracer Hg2+ is more available for transformation reaction than the ambient Hg2+. The degradation of tracer and ambient methylmercury proceeded at a similar rate, showing no significant differences between added tracer and ambient methylmercury. The calculated half-life for methylmercury in sediments was 1.7 d, suggesting a rapid turnover and low persistence of methylmercury in lake sediments. Different Hg species were investigated regarding their availability for methylation reactions. Compared to Hg(NO3)(2), Hg-fulvate showed reduced availability and freshly precipitated HgS was hardly available.