Surface Reactivity of Aluminum Hydroxides Precipitated in the Presence of Low Molecular Weight Organic Acids

The surface reactivities of aluminum hydroxides precipitated in systems at the initial Al concentration of 1.1 × 10 −3 M and the OH/Al molar ratio of 3.0 in the individual presence of 0, 10 −6 , and 10 −4 M , respectively, of citric, L‐malic, DL‐aspartic, and p ‐hydroxybenzoic acids were investigated. The organic acids enhanced the specific surface of the precipitation products of hydrolytic reactions of Al through structural distortion. The development of surface charge and active sites for the retention of phosphate and Ca were thus promoted. The enhancement of the surface reactivities of the precipitation products of Al by the organic acids followed the sequence of the stability constants of the complexes of Al with the organic acids, namely, citric acid > malic acid > aspartic acid > p ‐hydroxybenzoic acid. At the concentration of 10 −4 M organic acids, deviation from the latter sequence occurred in the development of positive surface charge and in the retention of phosphate. Specific adsorption of anions, while favoring the retention of Ca, counteracted the promoting influence of high specific surface on the development of the net surface positive charge and on the retention of phosphate of the precipitation products of Al. Among the organic acids used, with the exception of p ‐hydroxybenzoic acid, a concentration of 10 −4 M , in contrast to 10 −6 M , was sufficiently high to stabilize the high specific surface and phosphate retention capacity of the precipitation products of Al during aging. This information is of significance in understanding the role of low molecular weight organic acids in the formation of short‐range ordered aluminum hydroxides and their subsequent surface reactivities with respect to P and Ca.