Phosphate Adsorption by Synthetic Amorphous Aluminosilicates

Summary Phosphate adsorption isotherms were determined for four synthetic amorphous aluminosilicate gels with A1: A1 + Si molar ratios of 0.29 to 0.88. The concomitant silicate release and acid consumed to maintain the pH of the suspensions constant were also measured. The adsorption isotherms were analysed applying a two‐term Langmuir equation–assuming two types of sites. The experimental points fitted the predicted adsorption curves only up to a certain amount of phosphate adsorbed. The deviation at high phosphate adsorption values suggested the presence of more than two types of adsorption site. A comparison of phosphate adsorbed with the silicate released and acid consumed to maintain the pH constant indicated that, for a 3 h reaction time at concentrations below about 10 μmol cm −3 , phosphate exchanges mainly with aquo and hydroxo ligands and with adsorbed silicate. At higher concentrations phosphate is adsorbed (i) on sites arising from the disruption of hydroxy aluminium polymers in the gels and (ii) by the displacement of structural silicate.