Adsorption of hydrolyzable metal ions at the oxide—water interface. I. Co(II) adsorption on SiO2 and TiO2 as model systems

Complete isotherms of adsorption density vs. pH and concentration have been determined for Co(II) adsorption on SiO2 (point-of-zero charge pHPZC = 2.0) and TiO2 (pHPZC = 5.6). Isotherms for Fe(III), Cr(III), and Ca(II) on SiO2 have also been determined as a function of pH at fixed added concentration. The adsorption data indicate that the primary hydration sphere of free or hydrolyzed metal ions is not altered in the adsorption process. While a specific adsorption potential of ca. −5 kcal mole−1 exists for Co2+ adsorption on TiO2, for SiO2, it is −2 kcal mole−1, or less. Adsorption does not show a simple plateau monolayer behavior, but rather a steady trend with pH and concentration to saturation at the precipitation condition. The qualitative correlation between adsorption and hydrolysis is shown to break down when examined in detail. A complete description is shown to require many interactions for all hydrolysis products and the free ion.