Aluminum Hydrolysis Reactions and Products in Mildly Acidic Aqueous Systems

Early stages of the hydrolysis and hydroxide polymerization reactions of aluminum have been studied using a constant slow aluminum perchlorate flux into a solution held at constant pH by addition of dilute NaOH from a pH stat, observing rates of base addition and periodically determining concentrations of the various species of dissolved aluminum. Total final Al concentrations generally were between 2 x 10 -4 and 5 x 10 -4 molal and holding pH's ranged from 4.75 to 5.20. Polymeric ionic species began to form when supersaturation with respect to microcrystalline gibbsite reached an affinity of reaction ( A ) value near 1.0 kcal at pH 4.75, and the A value remained nearly constant during the remainder of the titration. Similar behavior, but with somewhat higher A values, was observed at higher pH's with a maximum A near 1.6 kcal at pH 5.20. Conditional first order rate constants were determined for the formation of polymeric ionic units with OH/Al molar ratios from 2.0 to 2.4. Average log k" values (sec -1 ) ranged from -4.59 at pH 4.75 to -3.25 at pH 5.20 for experiments done at 25°C. Rate constants about 0.6 log units less negative were measured at 35°C, and about 0.9 log units more negative at 10°C. Species whose OH/Al ratio was greater than 2.4 were formed at the higher pH's, and developed into microcrystalline gibbsite. Polymeric aluminum hydroxide ions and colloidal microcrystalline gibbsite may occur in river and lake waters, especially those affected by low-pH precipitation, but positive identification of such material is difficult.