Secondary adsorption of phosphate on aluminum oxides surfaces as influenced by several organic acids

Synthetic aluminum (Al) oxides (with or without different organic acids and inorganic anions) were used to study the effect of organic acid on phosphate secondary adsorption. The results showed that the maximum of phosphate adsorption (Xm) for the six Al oxides were 0.189-0.838 mmol/g in the absence of other organic acid. The AI(OH)x sample, which was synthesized without additional ligands, had the greatest Xm among these Al oxides, whereas, the AI(OH)x-CA sample, which was synthesized in the presence of citrate, had the least Xm. For most samples, the higher the addition concentrations of organic acids were, the greater the magnitude of phosphate secondary adsorption reduction was. However, while the addition concentrations of oxalate or citrate were more than 2 mmol/L, the grade of phosphate adsorption reduction was decreased. If oxalate or tartrate solution with pH 2 was introduced to AI(OH)x before phosphate addition, the amount of secondary phosphate adsorption was the least among pH 2-9, but if the solution with pH 3 was introduced, that was the greatest. The amount of secondary phosphate adsorption did not change or went down slowly from pH 4 to pH 9. Based on the results above, the possible effect mechanism of organic acid to phosphate adsorption was discussed. The dissolution, complex and charge effect could be one of the most important mechanisms at different conditions. These results had also supplied the evidence that sites on Al oxide surface had affinity to both organic acid and phosphate.