The colloidal stability of natural allophane in alkaline aqueous environment containing different oxyanions

The specific interaction of monovalent oxyanions (KH₂PO₄ and KH₂AsO₄) to positively charged natural allophane reduces the colloidal stability of allophane at pH 5. The colloidal stability can be well predicted by the Derjaguin-Landau and Verwey-Overbeek (DLVO) theory with surface charge density. So, far, research at alkaline conditions, where the oxyanions become divalent and the allophane is negatively charged, is lacking. Our hypothesis is that the divalent oxyanions can increase the colloidal stability of allophane at pH 9, mainly due to the strong contribution of their specific interaction to allophane surface. In addition, we confirm the applicability of DLVO theory to the colloidal stability of allophane in the presence of specifically interacting oxyanions in alkaline solutions. We examined the electrophoretic mobility (EPM) and stability ratio (W) of allophane as functions of electrolyte concentration and oxyanion species at pH 9, using electrophoretic and dynamic light scattering methods. Furthermore, the critical coagulation ionic strength (CCIS) and surface charge density can be estimated from the experimental results. The order of CCIS is HPO₄^2- > HAsO₄^2- > SO₄^2- ∼ SeO₄^2- > Cl^-. The results of the CCIS and stability ratio reveal the effect of ionic specificity, which is not explicitly considered by the DLVO theory. Nevertheless, the DLVO theory with surface charge density from EPM including the effect of ionic valence and species could well predict the experimental data. Thus, the CCIS in the presence of any electrolyte can in principle be estimated once the surface charge density of allophane is determined.