Sorption of phosphate by hydrolytic reaction products of aluminium

THE practical importance of hydrated aluminium oxides in the terrestrial and aquatic environments, is well known primarily through the significant role they have in the retention of nutrients in agriculture 1 and in the control of pollutants such as phosphate in eutrophication processes 2 . Anion adsorption on hydrous aluminium oxides has also attracted the interest of engineers mainly because in tertiary waste-water treatment, the removal of phosphate with alum salts seems to be through adsorption on the hydrated oxide floc particles 3,4 . Consequently the adsorption of anions on hydrated aluminium oxides has been extensively investigated. These studies have shown that organic anions such as citrate compete with phosphate for adsorption sites on the surfaces of the hydrated aluminium oxides thereby decreasing their capacity to retain phosphate 5–8 . Our studies 9 show that citric acid even at very low concentrations hampers the formation of crystalline aluminium hydroxides. We report here that due to the hindrance of the crystallisation processes, the .retention of phosphate by the hydrolytic reaction products of aluminium is enhanced and not reduced when they are precipitated in the presence of low concentration of citric acid which represents an organic acid commonly occurring in nature.