Surface complexation reactions of inorganic anions on hydrotalcite-like compounds.

Complexation reactions of environmentally important inorganic anions such as nitrate, chloride, sulfate, arsenate, and phosphate on the surface of hydrotalcite-like compounds (HT) were investigated to understand the role of HT in the immobilization of hazardous anions in an alkaline environment. The effects of surface complexation reactions on the solid state properties of HT were also evaluated to understand their stability. Synthetic HT was used for the adsorption and post-adsorption experiments. The obtained adsorption isotherms showed that the order of selectivity of HT for anions was NO(3)<Cl<SO(4)<<AsO(4)<PO(4). To distinguish the adsorption mechanisms (inner-sphere complexes or outer-sphere complexes) of these anions, zeta potential measurements and infrared absorption spectroscopic analysis were performed. The results indicated that NO(3) and Cl were adsorbed as diffuse ions on the outer surfaces of the HT, while SO(4) formed outer-sphere complexes with a strong electrostatic interaction. Moreover, AsO(4) and PO(4) formed inner-sphere complexes via a ligand substitution reaction on the HT surfaces. And it was suggested that oxyanions with low ionic potential, such as AsO(4) and PO(4), had a tendency to form inner-sphere complexes with the HT surfaces. The formation of inner-sphere complexes shifted the isoelectric point and the surface charge of the HT. Furthermore, the solubility of the HT was reduced by the inner-sphere complexes with PO(4) and AsO(4). It was revealed that the formation of inner-sphere complexes on the HT surfaces contributed to the stabilization of the HT, as well as a decrease in the mobility of these anions.