An EXAFS investigation of the mechanism of competitive sorption between Co(II) and Ni(II) at γ-alumina/solution interface

Co(II) and Ni(II) are two common toxic heavy metals, and may simultaneously exist in contaminated water, soil, and sediment systems in Earth’s surface environment. Under this circumstance, competitive adsorption between the two metals may influence their migration, toxicity, and bioavailability. In this research, the competitive sorption of Co(II) and Ni(II) on γ-Al2O3 was studied using both macroscopic sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results suggest that Ni(II) reduced the amount of Co(II) sorption in a binary-solute system at pH 6.0. This is because both Co(II) and Ni(II) form inner-sphere surface complexes during sorption on γ-Al2O3 and compete for the surface reactive sites. However, Co(II) exhibited a negligible influence on sorption amount of Ni(II) under the same conditions, which suggests Ni(II) has a stronger affinity to alumina surface. At pH 7.5, Co(II) and Ni(II) sorption density were much higher than that at pH 6.0, but there no mutual competitive effect was observed. EXAFS analysis further revealed that formation of layered double-hydrated precipitates was the dominant sorption mechanism for both Co(II) and Ni(II) at pH 7.5. Because this type of sorption does not rely on surface reactive sites, there was no competition between Co(II) and Ni(II). This finding sheds light on risk assessment and remediation of Ni/Co pollution.