Metal complex formation on the surface of amorphous aluminium hydroxide. Part IV. Interaction of oxovanadium(IV) and vanadate(V) with aluminium hydroxide in the presence of succinic, malic and 2-mercaptosuccinic acids

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Ternary complexes formed on the surface of amorphous aluminium hydroxide by adsorption of oxovanadium (IV) from aqueous solutions (pH range 4–8), in the presence of succinic, malic and 2-mercaptosuccinic acids, have been studied by means of electron spin resonance (ESR) spectra. Their structure depends on the nature of the ligands as well as on the pH and the composition of the exchange solution. At low pH values, all the surface-adsorbed ligands yield the same type of complex, by coordinating the metal ion only through a carboxylate group, the tendency increasing in the order succinic⋍malic<2-mercaptosuccinic. In the higher pH range, ternary species are favoured by high ligand-to-metal ratios in solution, but only with malic and 2-mercaptosuccinic acids. In this case, dissociated OH and SH groups of the ligands also participate in metal binding. The extent of metal sorption depends on the coordinating behaviour of the ligand on the surface and in solution. In fact, with increasing ligand concentration in solution, the metal sorption is first favoured, due to the formation of ternary species on the surface, and later inhibited, particularly in the case of 2-mercaptosuccinic acid which is a strong VO (IV) chelator in solution. The interaction of VO 3 − -exchanged hydroxide with 2-mercaptosuccinic acid yields VO (IV)-containing gels through a redox process in which surface-adsorbed and solution species take part.