Determination of methyl mercury in a pilot-scale activated sludge wastewater treatment plant

The methylation of mercury has been investigated and documented mainly in sediments, fish
and microorganisms, while limited number of relevant studies is available for wastewater. The
procedure of mercury methylation can occur via biological pathway (by microorganisms) and
via chemical or photochemical reactions.
Methylation of mercury occurs mainly under anaerobic conditions, but some studies have
shown its existence also under aerobic conditions. The resulting concentration of methyl
mercury, which is a highly toxic compound, depends on the specific rates of
methylation/demethylation of mercury. The factors affecting these procedures are the
availability of inorganic mercury, pH, organic matter concentration, microbial activity, redox
potential and temperature. Bacteria which can methylate mercury are often present in
wastewater, and, therefore, the formation of methyl mercury during wastewater treatment is
possible.
The objective of the present investigation was the determination of methyl mercury in a pilotscale
activated sludge wastewater treatment plant supplied with synthetic wastewater
enriched with mercury. For this purpose, a Liquid-Liquid Extraction / Simultaneous
Derivatization - GC/MS method was developed and applied for the analysis of samples from
the aeration tank, from the treatment plant effluent and from the sludge.
Methyl mercury was not detected in the samples (detection limit 0.07 μg l-1), leading to the
conclusion that mercury is not methylated under the particular experimental conditions of the
pilot-scale water treatment plant.