Mercury methylation and reduction potentials in marine water: An improved methodology using 197Hg radiotracer

A highly sensitive laboratory methodology for simultaneous determination of methylation and reduction of spiked inorganic mercury (Hg2+) in marine water labelled with high specific activity radiotracer (197Hg prepared from enriched 196Hg stable isotope) was developed. A conventional extraction protocol for methylmercury (CH3Hg+) was modified in order to significantly reduce the partitioning of interfering labelled Hg2+ into the final extract, thus allowing the detection of as little as 0.1% of the Hg2+ spike transformed to labelled CH3Hg+. The efficiency of the modified CH3Hg+ extraction procedure was assessed by radiolabelled CH3Hg+ spikes corresponding to concentrations of methylmercury between 0.05 and 4 ng L−1. The recoveries were 73.0±6.0% and 77.5±3.9% for marine and MilliQ water, respectively. The reduction potential was assessed by purging and trapping the radiolabelled elemental Hg in a permanganate solution. The method allows detection of the reduction of as little as 0.001% of labelled Hg2+ spiked to natural waters. To our knowledge, the optimised methodology is among the most sensitive available to study the Hg methylation and reduction potential, therefore allowing experiments to be done at spikes close to natural levels (1–10 ng L−1). - Highlights: ► Inorganic mercury methylation and reduction in marine water were studied. ► High specific activity 197Hg was used to label Hg2+ spikes at natural levels. ► Methylmercury extraction had 73% efficiency for 0.05–4 ng L−1 levels. ► High sensibility to assess methylation potentials, below 0.1% of the spike. ► High sensibility also for reduction potentials, as low as 0.001% of the spike.