SORPTION OF METHYLMERCURY BY CLAYS AND MINERAL OXIDES

Experiments on the sorption behaviour of methylmercury (CH3Hg+) by cla ys and mineral oxides showed that adsorption was rapid for all the col loids investigated and that the adsorption kinetics conformed to a fir st order model. The doubly inflected isotherms observed in most of the sorption studies seem to indicate the occurence of several adsorption mechanisms. Adsorption continued even after saturation of the reactiv e sites, probably owing to hydrophobic interactions between the methyl groups of the methylmercury in solution and those of CH3Hg+ already a dsorbed. The determination of the adsorption parameters led the sorben ts to be classified according to their sorption capacities. The maximu m adsorption capacity was found in expandable clays having high cation exchange capacities. Hence, it could be suggested that one of the mec hanisms involved in methylmercury retention by days is related to an i nterlayer adsorption. This was confirmed by X-ray diffraction analysis . Considering mineral oxides, the sorption capacity measured for goeth ite (iron oxyhydroxide, alpha-FeOOH) is higher than for hausmanite (Mn 3O4). For these sorbents, surface complexation and surface precipitati on are probably responsible for the CH3Hg+ sorption.