Determination of Monomethylcadmium in the Environment by Differential Pulse Anodic Stripping Voltammetry

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Abstract
A differential pulse anodic stripping voltammetric (DPASV) method was used to differentiate between the cadmium species Cd2+ and MeCd+ (Me = methyl) in aquatic systems. These two species show peaks in the DPASV voltammogram which differ by 112 mV. In model experiments, it was demonstrated that monomethylcadmium is not stable at pH 2, but under higher pH conditions, normally found in fresh and ocean water samples, the identity of MeCd+ was verified by different investigations, including cyclic voltammetry, selective extraction of a complex of diethyldithiocarbamate with MeCd+ into n-hexane, and photochemical dissociation of MeCd+ by UV irradiation. It was also shown that humic acids do not influence the voltammetric determination of monomethylcadmium. For the first time, it was possible to analyze MeCd+ in environmental samples. During different expeditions with the German research vessel Polarstern, monomethylcadmium could be determined above the detection limit of 470 pg L-1 in nearly all surface water samples of the South Atlantic with spot concentrations of up to about 700 pg L-1, whereas in the North Atlantic only 15−30% of the total samples showed MeCd+ concentrations above this limit. The existence of MeCd+ in the remote area of the South Atlantic, as well as positive correlations with the local bioactivity in the ocean, indicates biomethylation as the most probable formation process for this methylated cadmium species. This assumption is supported by the simultaneous occurrence of other methylated heavy metal compounds, such as Me3Pb+. Up to 48% of the total cadmium was found to be monomethylcadmium in some Arctic meltwater ponds.