Adsorption of phosphate at the aluminum (hydr)oxides–water interface: Role of the surface acid–base properties

In this paper, the adsorption of phosphate on two crystallized aluminum oxides, pseudo-boehmite (pseudo-γ-AlOOH) and α-Al2O3, was studied. The adsorption experiments were carried out in 0.01 M KNO3 solutions at pH 4 and 6, respectively. Adsorption densities were calculated to evaluate the phosphate adsorption affinity for the two aluminum oxides. To better understand the effect of surface properties on the adsorption behavior of phosphate, the surface structure and properties were investigated with X-ray diffraction, N2 adsorption–desorption, electron microscope observation and surface acid–base titrations. The results from adsorption isotherms showed that at both pH 4 and 6, pseudo-γ-AlOOH exhibited greater adsorption capacity than α-Al2O3. For both pseudo-γ-AlOOH and α-Al2O3, the adsorbed amount of phosphate at pH 4 was higher than that at pH 6. Adsorption density calculation indicated that α-Al2O3 had greater affinity for phosphate than pseudo-γ-AlOOH. Information gained from examinations of surface properties revealed that the specific surface area played an important role on adsorption capacity, while the acidity of surface aluminol groups evaluated from acid–base titration curves contributed more to adsorption affinity and consequently the adsorption density.