Voltammetric and other properties of methylmercury compounds and distribution of methylmercury in environmental samples
The reduction of aqueous methylmercury solutions has been studied using polarography and cyclic voltammetry at the dropping mercury electrode. By scanning a broad range of pH, pCl and methylmercury concentrations, a detailed reduction mechanism has been proposed. Reduction is shown to proceed in two le‾ steps, involving first the formation of a methylmercury radical, which then accepts another electron to form (following C-Hg bond rupture) Hg° and the corresponding hydrocarbon. The intervention of a dimerization reaction between the two e‾ transfer steps, significantly distorts the shapes of the 2 polarographic waves. The effect of Cl‾ on the half wave potential of the first wave has been used to estimate the formation constant of the dichloro complex anion. The linearity of the polarographic diffusion current with methylmercury concentration down to ~10-5 M has been used to study the stability of methylmercury chloride solutions in water and in HCl at room temperature. Decomposition rates at elevated temperatures gave an activation energy of 24 kcals mole-1 deg-1 in 6 M HCl. The volatility of a 10 mM methylmercury chloride solution from water and from 2 M HCl has been determined at 25°C by means of a gas saturation method. Analyses performed on a gas chromatograph with electron-capture detector showed enhanced volatility (~2 fold) from water compared with 2 M HCl. The effect of HCl on the extraction of methylmercury chloride into benzene has been studied. Enhanced extractability in the presence of fish has been discovered, and this is suggested to be an effect of “oils” in the fish. Raman, NMR and UV spectrochemical techniques have been used to search for chloro complexes of methylmercury in high chloride media. A bathochromic shift in the UV on addition of Cl‾ has been taken as evidence for the formation of the dichloro anion. A UV method has been proposed for HgII in the presence of methylmercury, since the latter absorbs very little at the λmax for HgCl4=. Analyses of assorted fish from the Upper Susquehanna River basin revealed that ~83% of the mercury in them was methylmercury. Results ranged from about 0 - 1.2 ppm Hg (as methylmercury) and a high methylmercury accumulation in young predator fish (walleyes, smallmouth bass) seemed to taper off in older fish. Lobster (Homarus Americanus) of the North Atlantic showed a trend in the distribution of methylmercury within the organs. Stronger muscles seem to bind more methylmercury than the weaker ones.
Methyl mercury, Electric properties, Mercury, Toxicology
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