Solid-liquid interfacial reaction of Zn 2+ ions on the surface of amorphous aluminosilicates with various Al/Si ratios

Abstract

The adsorption behavior of Zn 2+ ions onto the surface of amorphous aluminosilicates was studied using both potentiometric and spectroscopic methods (XANES: X-ray Absorption Near-Edge Structure). The aluminosilicates were prepared with different Al/Si ratios in order to compare the reactivities of surface aluminol and silanol groups toward Zn 2+ ions. Potentiometric experiments were performed by maintaining the reacting suspensions at constant pH, ionic strength, and solid concentration, while Zn concentration was increased by stepwise addition. Our results showed that the surface aluminol and silanol groups possess significantly different reactivities toward Zn 2+ ions. The reaction of Zn 2+ ions with aluminol groups occurs through three processes: (i) surface complexation, (ii) dissolution, and (iii) re-sorption. A stoichiometric relationship was confirmed for the surface complexation between the aluminol groups and Zn 2+ ions: two moles of H + ions were released for one mole of Zn 2+ ion adsorption. Following the surface complexation process, measurable amounts of zinc and aluminum ions were found to be mobilized from the surface of the solid to the liquid phase; subsequently, these ions precipitated on the solid surface, and possibly formed a co-precipitate with the hydrotalcite-type structure. On the other hand, a stoichiometric relationship was not obtained for the sorption of Zn 2+ ions on silanol groups, and therefore, it was concluded that Zn 2+ ions are retained on the surface of amorphous aluminosilicates by two different reactions. One reaction involves the surface complexation between Zn 2+ ions and surface aluminol groups, which proceeds rapidly. The other reaction is the slow retention of Zn 2+ ions onto silanol and/or aluminol groups, which could be the surface precipitation of Zn(OH) 2 or the co-precipitation of Zn 2+-Al 3+ hydroxides. It can be suggested that the total sorption behavior of Zn 2+ ions on amorphous aluminosilicates with different Al/Si ratios can be represented as the sum of the individual reactions of Zn 2+ ions toward the aluminol and silanol groups. The potentiometric results were confirmed by XANES data. It was clearly evident that only the aluminol groups were responsible for surface complexation of Zn 2+ ions. An equilibrium constant was calculated for this reaction.