Sorption of methylmercury by clays and mineral oxides

Experiments on the sorption behaviour of methylmercury (CH3Hg+) by clays and mineral oxides showed that adsorption was rapid for all the colloids investigated and that the adsorption kinetics conformed to a first 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 reactive sites, probably owing to hydrophobic interactions between the methyl groups of the methylmercury in solution and those of CH3Hg+ already adsorbed. The determination of the adsorption parameters led the sorbents to be classified according to their sorption capacities. The maximum adsorption capacity was found in expandable clays having high cation exchange capacities. Hence, it could be suggested that one of the mechanisms involved in methylmercury retention by days is related to an interlayer adsorption. This was confirmed by X-ray diffraction analysis. Considering mineral oxides, the sorption capacity measured for goethite (iron oxyhydroxide, alpha-FeOOH) is higher than for hausmanite (Mn3O4). For these sorbents, surface complexation and surface precipitation are probably responsible for the CH3Hg+ sorption.