A STUDY OF THE BEHAVIOR OF METHYLMERCURY COMPOUNDS IN AQUEOUS-SOLUTIONS, AND OF GAS-LIQUID DISTRIBUTION COEFFICIENTS, USING HEAD SPACE ANALYSIS

The distribution of methylmercury species in aqueous solutions as well as their gas/liquid partitioning, has been studied as a function of pH and halide concentrations using Head Space analysis with Gas Chromatography-Microwave Induced Plasma detection. The methylmercury concentration in the vapour phase, being directly related to its concentration in the liquid phase, gives, as a result of differences in volatility, information on the nature of the methylmercury species in solution. In methylmercury solutions at high pH, MeHgX (X denotes Cl, Br, I) is transformed into MeHgOH: for MeHgCl, this process occurs at lower pH than for MeHgBr and MeHgI, respectively, according to their stability constants. Gas/liquid distribution coefficients of MeHg-compounds have been determined in a temperature-range of 25-80-degrees-C; the volatility of the compounds decreases in the following order: MeHgI > MeHgBr > MeHgCl > MeHgOH. Addition of potassium halides to MeHgCl-solutions results in a salting-out effect; for iodide this effect could not be observed experimentally since it was obscured by side-reactions, In the presence of excess iodide, MeHgI is partially transformed into the ionic MeHgI2-, and in acidic regions, an additional decomposition reaction of methylmercury takes place. Both side-reactions were also observed when an excess bromide was added, but to a lesser extent; in a chloride matrix, none of these side-effects seemed to take place. In methylmercury solutions at very high pH, besides MeHgOH, also MeHg(OH)2- is formed.