Changes in the adsorption activity of metal oxyhydroxide hydrogels during their ageing in electrolyte solutions

Adsorption properties of Ti(IV), Zr(IV), In(III), Cr(III) and Fe(III) oxyhydroxides aged in sodium chloride and sulphate solutions have been studied. The main research method was determination the rates of heterogeneous hydrolysis reactions for RhCl63− and IrCl62− complexes. The rate constants of these reactions were a measure of adsorption activity of the oxyhydroxides on which surface they occur. For comparison the corresponding data for freshly formed oxyhydroxides are presented. It was found that adsorption activity (the values of rate constants Kobs) of ferro- and indiogels decrease slowly with increasing of duration and temperature of ageing. The sulphate electrolyte promotes the decrease of sorption activity more than chloride electrolyte. In contrast, the samples of titano- and chromogels aged during 2 h at 80°C have much greater adsorption activity than freshly formed ones, and this activation holds for a very long time. The zirconogel keeps in these conditions stable adsorption activity for a very long time. Apparently, after precipitation the chromo-, titano- and zirconogels some metastable non-crystalline or micro-crystalline structures stable to further ageing form rapidly. The oxyhydroxides behaviour is obviously connected with the electronic structure of outer electronic shell of their central ions. The chromium(III) oxyhydroxide with a unsymmetrical, partially filled 3d-sublevel is the most effective sorbent and has the most stable sorption properties.