Competitive adsorption of three different phosphate species on aluminum hydroxide

The competitive adsorption among potassium dihydrogen phosphate (KH ₂PO ₄), sodium hexametaphosphate ((NaPO ₃) ₆) and D-glucose-6-phosphate monosodium salt (C ₆H ₁₂NaO ₉P) was investigated in this study. These phosphates were chosen as the model phosphate compounds to represent reactive phosphate (RP), condensed phosphate (CP) and organic phosphate (OP), respectively. It is revealed that different phosphates have different affinities for the surface sites of Aluminum hydroxide. "Orientation effects" play an important role in determining the affinity of phosphates for the surface sites of aluminum hydroxide. The presence of one phosphate can reduce the adsorption of other phosphates considerably due to the competition for the surface sites of the adsorbent. At a given total phosphorus concentration, the amount of phosphorus adsorbed depends on the concentration fractions as well as the intrinsic binding affinities of each phosphate species for aluminum hydroxide surface sites. Competitive adsorption only becomes vigorous when phosphates are present in high concentrations resulting in intermediate to high adsorption densities. The competition between reactive phosphate and organic phosphate is more extensive than that between reactive phosphate and condensed phosphate, which may attribute to the similarity of P functional group in reactive phosphate and organic phosphate molecules.