Methodological considerations regarding the use of inorganic 197Hg(II) radiotracer to assess mercury methylation potential rates in lake sediment

Methodological considerations on the determination of benthic methyl-mercury (CH₃Hg) production potentials were investigated on lake sediment, using ¹⁹⁷Hg radiotracer. Three methods to arrest bacterial activity were compared: flash freezing, thermal sterilization, and γ-irradiation. Flash freezing showed similar CH₃Hg recoveries as thermal sterilization, which was both 50% higher than the recoveries obtained with γ-ray irradiation. No additional radiolabel was recovered in kill-control samples after an additional 24 or 65 h of incubation, suggesting that all treatments were effective at arresting Hg(II)-methylating bacterial activity, and that the initial recoveries are likely due to non-methylated ¹⁹⁷Hg(II) carry-over in the organic extraction and/or [¹⁹⁷Hg]CH₃Hg produced via abiotic reactions. Two CH₃Hg extraction methods from sediment were compared: (a) direct extraction into toluene after sediment leaching with CuSO₄ and HCl and (b) the same extraction with an additional back-extraction step to thiosulphate. Similar information was obtained with both methods, but the low efficiency observed and the extra work associated with the back-extraction procedure represent significant disadvantages, even tough the direct extraction involves higher Hg(II) carry over.