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 rat e constants simultaneously in aquatic samples. Solutions containing stable isotope tracers of Hg-199(2+) and (CH3Hg+)-Hg-202 were spiked into lake sed iments at subambient concentrations. The formation of (CH3Hg+)-Hg-199 and t he decrease in (CH3Hg+)-Hg-202 were measured simultaneously in time series experiments using gas chromatographic separation and isotope-specific detec tion by inductively coupled plasma mass spectrometry. Specific rate constan ts For the two processes were calculated and compared to rate constants obt ained by monitoring changes in concentration of the ambient methylmercury i n the same sample. The inorganic mercury tracer Hg-199(2+) was methylated a t a faster rate compared with the ambient inorganic Hg2+, which indicates t hat the added tracer Hg2+ is more available for transformation reaction tha n the ambient Hg2+. The degradation of tracer and ambient methylmercury pro ceeded at a similar rate, showing no significant differences between added tracer and ambient methylmercury. The calculated half-life for methylmercur y in sediments was 1.7 d, suggesting a rapid turnover and low persistence o f methylmercury in lake sediments. Different Hg species were investigated r egarding their availability for methylation reactions. Compared to Hg(NO3)( 2), Hg-fulvate showed reduced availability and freshly precipitated HgS was hardly available.