**12. Chelating ion exchangers with the thiol, thiourea and isothiourea functional groups**

In the group of ion exchangers with the thiol functional groups (Chelite S, Duolite ES-465, Imac GT 73) Imac TMR resin is very important. It is the macroporous ion exchanger with the PS-DVB matrix, which besides the thiol ones, also possesses the sulphone groups. Imac TMR is used for selective sorption of Hg(II) ions from the process solutions as well as Ag(I), Au(III), Pt(IV) and Pd(II). The sorption process of Hg(II) with the saline solution proceeds according to the reaction:

$$\begin{array}{rcl} \mathsf{R} \longrightarrow \mathsf{S} \mathsf{H} + \mathsf{H} \mathsf{g}^{\mathsf{2}+} \longrightarrow (\mathsf{R} \longleftarrow \mathsf{S})\_{2} \mathsf{H} \mathsf{g}^{\mathsf{2}+} + \mathsf{2H}^{+} \\\\ \mathsf{R} \longrightarrow \mathsf{S} \mathsf{H} + \mathsf{H} \mathsf{g} \mathsf{C} \longmapsto \mathsf{R} \longrightarrow \mathsf{S} \mathsf{H} \mathsf{g} \mathsf{C} \mathsf{l} + \mathsf{H}^{+} \end{array}$$

Also ion exchangers with the isothiourea functional groups (Ionac SR 3, Lewatit TP 214, Purolite S 920, Srafion NMRR) exhibit high affinity for Hg(II) ions. They are also selective for noble metal ions. Depending on the pH, the isothiourea groups occur in the following forms:

For the first form, coordination bond formation is possible, whereas for the second one the sorption process proceeds in accordance with the anion exchange mechanism:
