Hydrolysis of aluminium on weakly acidic organic exchangers: Implications for phosphate adsorption

In contrast to its behaviour on soil clays and in aqueous solution, aluminium is more extensively hydrolysed between pH3 and 4.5 on organic exchangers because the weakly acidic carboxyl groups appear to act as sinks for the protons released by hydrolysis and polymerization of the Al(H₂O) ₆ ³⁺ ions. In consequence, the basicity (measured by the OH/Al mole ratio) of Al adsorbed on samples of acid peat, humic acid and Amberlite IRC-50 resin was closely correlated with the apparent pK_a of the acid groups. The amount of phosphate adsorbed depended not only on the amount of Al adsorbed but also on the basicity of the adsorbed Al. Phosphate and OH^– ions competed for Al on the exchange sites with the result that the P/Al mole ratio on the exchanger was inversely related to the basicity of the adsorbed Al. Once adsorbed, the Al was quite stable and P adsorption on the Al-exchanger was unaffected by a rise in pH from 4.5 to 6.4.