In this paper, the special sorption properties of mercury(II) on calcium-montmorillonite is shown. Mercuric hydroxide (Hg(OH)2) produced by the hydrolysis of mercuric ions is fairly soluble, solubility is 3.2 × 10−4 mol/dm3. As a result, indifferently of the usual behavior of other hydroxides, it remains in the aqueous solution. The sorption properties
are determined by the fact: there is no precipitation at higher pH values. Montmorillonite contains permanent as well as pH-dependent
charges, so the cation exchange on the permanent changes, in the interlayer space of montmorillonite and the sorption of neutral
mercuric hydroxide molecules can be studied simultaneously. So, two interfacial processes of the same substance, namely the
ion exchange of hexahydrate mercuric(II) ions and the sorption of neutral mercuric hydroxide molecules, can be studied in
the same system.