On the acid–base stability of Keggin Al13 and Al30 polymers in polyaluminum coagulants

The acid–base stabilities of Al13 and Al30 in polyaluminum coagulants during aging and after dosing into water were studied systematically using batch and flow-through acid–base titration experiments. The acid decomposition rates of both Al13 and Al30 increase rapidly with the decrease in solution pH. The acid decompositions of Al13 and Al30 with respect to H+ concentration are composed of two parallel first-order and second-order reactions, and the reaction orders are 1.169 and 1.005, respectively. The acid decomposition rates of Al13 and Al30 increase slightly when the temperature increases from 20 to ca. 35 °C, but decrease when the temperature increases further. Al30 is more stable than Al13 in acidic solution, and the stability difference increases as the pH decreases. Al30 is more possible to become the dominant species in polyaluminum coagulants than Al13. The acid catalyzed decomposition and followed by recrystallization to form bayerite is one of the main processes that are responsible for the decrease of Al13 and Al30 in polyaluminum coagulants during storage. The deprotonation and polymerization of Al13 and Al30 depend on solution pH. The hydrolysis products are positively charged, and consist mainly of repeated Al13 and Al30 units rather than amorphous Al(OH)3 precipitates. Al30 is less stable than Al13 upon alkaline hydrolysis. Al13 is stable at pH < 5.9, while Al30 lose one proton at the pH 4.6–5.75. Al13 and Al30 lose respective 5 and 10 protons and form [Al13]n and [Al30]n clusters within the pH region of 5.9–6.25 and 5.75–6.65, respectively. This indicates that Al30 is easier to aggregate than Al13 at the acidic side, but [Al13]n is much easier to convert to Alsol–gel than [Al30]n. Al30 possesses better characteristics than Al13 when used as coagulant because the hydrolysis products of Al30 possess higher charges than that of Al13, and [Al30]n clusters exist within a wider pH range.