Mercury methylation and reduction in marine systems using the high specific activity radiotracer 197Hg

Bratkic, A. (Speaker), Neža Koron (Contributor), Milena Horvat (Contributor), Sergio Ribeiro Guevara (Contributor), Mitja Vahčič (Contributor)

Activity: Talk or presentationTalk or presentation at a conference


Mercury (Hg) methylation and reduction are key processes in Hg cycling
in aquatic systems. The transformation of Hg+2, the most abundant Hg
species in waters and sediments, to the neurotoxic species methylmercury
(MeHg) in these aquatic compartments determines the Hg impact
in biota at different levels of the trophic webs, and also in external consumers.
On the other side, Hg+2 reduction to Hg0 in waters is a depuration
process of the ecosystem by further Hg0 vapour evasion. A well
know technique to evaluate Hg+2 reduction and methylation rates in
laboratory experiments is the addition of a known amount of labelled
Hg+2 to incubation setups, measuring the ratio of labelled Hg in the
species produced respective to the spike. But two methodological aspects
revealed crucial to obtain reliable results: high sensibility of the
tracer to allow spikes at natural levels, and separation techniques that
allow measuring very low concentrations of the Hg species produced
with negligible contamination of the labelled Hg+2 spiked.
The high specific activity 197Hg (T1/2=64.14 h) radiotracer showed
excellent performance in the evaluation of Hg+2 methylation and reduction
processes in marine environments. It was produced by irradiation
of Hg targets enriched to 52 % in 196Hg (natural abundance:
0.15%) in a nuclear research reactor and evaluated in the Hg products
by the X and gamma ray emissions with High Purity Germanium well
and coaxial type detectors. A specific protocol to extract MeHg with
minimum Hg+2 contamination allowed to determine MeHg production
as low as 0.1 % of the spike in marine waters, whereas the Hg+2 reduction
potential was assessed by purging and trapping the radiolabelled
Hg0 in a permanganate solution, allowing detection as low as 0.001%
of the spike. In both cases the addition of labelled Hg was at 1 to 10 ng
L-1 levels. In sediments incubated in anoxic conditions to study bacterial
Hg methylation, the 197Hg radiotracer spiked at concentrations as
low as 0.02 ng g-1 allowed MeHg recoveries with a detection limit of
0.001 % of the spike. Hg+2 reduction was also measured in sediment
experiments. The labelled Hg0 vapour released during incubation was
flushed out by a flow of N2 from the incubation vessel and trapped on
selenium-coated paper. Potentials for labelled Hg+2 reduction ranged
from 0.02 to 0.1% of the spike.
Period13 Sep 2019
Event titleInternational Conference on Mercury as a Global Pollutant 2019
Event typeConference
Conference number14
Degree of RecognitionInternational