EFFECTS OF ORGANIC ACIDS ON PHOSPHATE ADSORPTION BY SYNTHETIC Al OXIDES

Effects of concentration and pH value of several organic acids on phosphate secondary and competitive adsorption by hydroxides, which were synthesized with AlCl3 and NaOH in the presence of various anion ligands, were studied by equilibrium adsorption method. The results showed: (1) the higher the concentration of organic acids, the more reduction in phosphate adsorption amount was. The reduction degree of P adsorption in the presence of citrate was more significant than that in the presence of oxalate; (2) the amount of P secondary adsorption was the least at pH 2 for organic acid solution, whereas the most at pH 3. From pH 4 to 9, the amount of P secondary adsorption became less. These suggested that phosphate adsorption by different Al hydroxides, secondary adsorption after organic acids adsorbed and competitive adsorption between phosphate and organic ligands were all influenced with kind, concentration and pH value of organic acid solutions. The effect mechanism might include dissolution and ligand effect. At very low pH, dissolution of Al hydroxides was dominant, however, at pH more than 3, ligand competition became the most important.