Phosphate Adsorption on γ‑Alumina: A Surface Complex Model Based on Surface Characterization and Zeta Potential Measurements

An innovative approach using solid/liquid interface characterization (zetametry measurements, ATR-IR), coupled with ex situ FTIR has been used to build a phosphate complexation model at the alumina surface via the PHREEQC software. According to the phosphorus coverage impregnated on the alumina surface, two mechanisms of adsorption have been identified: a specific surface complex adsorbed on the predominant alumina facet and a surface precipitation process. At low phosphorus coverage (P coverage < 1.2 at/nm2), the adsorption proceeds mainly through a monodentate complex adsorbed on μ1-AlIV-OH sites located on the (110) facet, and a bidentate involving μ1-AlV-OH (110) is also discussed. However, the surface concentration of these particular sites is limited. Thus, above the threshold of 1.2 at/nm2, a surface precipitation occurs with the formation of an amorphous AlPO4 phase, experimentally observed by FTIR and zetametry measurements.