Competitive adsorption between orthophosphate and other phosphates on aluminum hydroxide

The competitive adsorption between potassium dihydrogen phosphate (ortho-P) and myo-inositol hexaphosphate (IP6) or tripolyphosphate (tripoly-P) and the adsorption isotherms of different phosphates on amorphous aluminum hydroxide were investigated in this study. The adsorption isotherms of ortho-P, IP6, and tripoly-P fitted well with the Freundlich isotherm model, and IP6 had greater affinity for the aluminum hydroxide surface than ortho-P and tripoly-P. The addition sequence had a pronounced influence on the adsorption of the phosphates on aluminum hydroxide over the pH range examined in this study. Adding ortho-P after IP6 had little effect on the quantity of IP6 adsorbed on the aluminum hydroxide at pH 4 to 10; however, the adsorption of IP6 decreased by about half when ortho-P was added simultaneously with or before IP6 addition. The adsorption of tripoly-P was significantly reduced due to the presence of ortho-P over the entire pH range, and the maximum reduction in tripoly-P adsorption occurred when ortho-P was introduced before tripoly-P. On the other hand, ortho-P adsorption was mostly unaffected by the introduction of IP6 or tripoly-P when the pH was more than 7, but it exhibited competitive inhibition at more acidic pH levels. Under a specific total phosphorus concentration with various ortho-P/IP6 or ortho-P/tripoly-P ratios, the amount of total phosphorus adsorbed on the aluminum hydroxide depended on the fraction as well as the intrinsic binding affinity of each phosphate species for the surface sites of the adsorbent. The surface charge of the aluminum hydroxide depends more on the other two phosphates than on ortho-P when they coexist.