Interaction between Al³⁺ and Poly(acrylic acid) (PAA) in Aqueous Solution—Formation Modes for the Al–PAA Complexes

The interaction between Al³⁺ and poly(acrylic acid) (PAA) which is a polyelectrolyte was investigated under various conditions. It was revealed that PAA has a strong binding ability with Al³⁺ not only in liquid phase but also in solid phase. Even when no hydrolysis of Al³⁺ occur at pH 3, the Al³⁺ was deposited together with PAA under various COOH/Al ratios and formed an insoluble Al–PAA complex. It was found that the hydrolysis of Al³⁺ was strongly retarded due to the complexation with PAA even under neutral and alkaline conditions. An excess amount of PAA in mixed solution retarded the precipitation of Al³⁺ at pH 4.5–8. The soluble Al–PAA complex suppressed the formation of the Al₁₃ polycation in solution. Moreover, an insoluble Al–PAA complex was also formed under COOH/Al ratio of 1. The molar ratio of COOH/Al in precipitates was dependent on the pH of the reaction solution. The type of chemical bond involved in the Al–PAA complex was affected by the Al³⁺ hydrolysis and dissociation of H⁺ from the carboxyl groups in PAA. At pH 3, one Al³⁺ combined with three carboxyl groups by substitution reaction with H⁺. Above pH 3, one Al(OH)²⁺ or one Al(OH)⁺₂ combined ionically with two or one COO⁻ groups, respectively. The results of this study can be potentially applied to formulate a method to remove Al³⁺ from various wastewater environments in a wide pH range.