Interactions of Dilute, Hydrolyzed Aluminum Solutions with Clays, Peat, and Resin

Calcium-saturated montmorillonite, kaolinite, peat, and a sulfonic cation exchange resin were equilibrated with hydrolyzed Al solutions containing 4.0 × 10⁻⁴ or 2.0 × 10⁻³M AlCl₃ and having basicities (OH/Al ratios) of 1.5 or 2.4 to determine the effects of aging, solution concentration, OH/Al ratio, and exchanger surface properties on the fate of added Al. Partially neutralized, dilute Al solutions were used to approximate Al concentrations which may occur in the soil environment. Equilibrating solutions and 0.1M BaCl₂ extracts were analyzed for amounts and basicity of Al, and for the relative amounts of mononuclear and polynuclear Al species. The basicity of adsorhed and nonexchangeable Al was calculated and used to characterize the Al removed from solution and the interactions of Al with exchangers. Decreasing the Al concentration resulted in a lower percentage of detectable Al remaining in solution and a lower percentage of polynuclear species present at a given basicity or length of time. This was attributed to the instability of polynuclear Al species at low Al concentrations which precipitated to form Al(OH)₃. On an equivalent exchange capacity basis, this kaolinite sample displayed the strongest and most rapid interaction with added hydrolyzed Al, as demonstrated by low solution Al, low exchangeable Al, and high amounts of fixed Al. This could result from the abundant edge charge present in this exchanger. Peat and resin reacted much slower than montmorillonite and kaolinite, although peat eventually retained more Al against exchange than montmorillonite or resin. The adsorbed hydrolyzed Al was released by all exchangers at short aging times, but the amounts exchanged decreased and became less basic as aging increased. The use of hydrolyzed Al in amounts less than the exchange capacity of base saturated exchangers produced increases in the pH of the equilibration solution relative to the solution without exchangers. Broad, low temperature DSC endotherms (180-300°C) indicating the presence of precipitated Al were detected in hydrolyzed Altreated samples of montmorillonite, peat, and resin, but not in kaolinite.