DISSOLUTION OF MAGNETITE BY MERCAPTOCARBOXYLIC ACIDS

Mercaptocarboxylic acids dissolve magnetite by creating reactive surface complexes in which the ligand is bound to surface iron(III) centers via both -COO- and -S- groups. These surface complexes dissolve rather rapidly by a mechanism that probably involves both sequential redox decomposition-dissolution and dissolution-redox decomposition. In either case the causative factor of rate enhancement is the weakening of Fe-O-Fe bonds caused by the ligand. Very early in the dissolution process a further catalytic pathway is established by dissolved Fe(II)-L complexes. The dependence of the rate of all these processes on solution variables (pH, ligand nature and concentration) is essentially determined by the changes brought about in the number of surface complexes created by adsorption. The reactivity of these complexes is much less sensitive to the mentioned variables. Hydrogen peroxide addition gives rise to a complex sequence of reactions that involves free radicals and leads to the oxidation of the ligand as the main stoichiometric reaction.