Kinetics of the precipitation reaction between aluminium and contaminant orthophosphate ions

The removal of phosphorous from wastewater in metal-orthophosphate systems typically occurs by simultaneous adsorption on poorly soluble metal hydroxides and by precipitation reactions between metal ions and orthophosphates in solution. To understand the individual contribution of these mechanisms to the removal of phosphorus, the main aim of this study was to determine the kinetics of consumption of contaminant orthophosphates by the precipitation reaction with aluminium ions in a solution free of insoluble aluminium hydroxide. To define the amount of aluminium and phosphorous compounds to be dissolved in water to have this desired reacting condition at a given pH, the solubilities of KH₂PO₄(s), Al(OH)₃(s) and AlPO₄(s) were examined at 25 °C in the pH range ∼2.6 to 7.9. pH-solubility diagrams for these ionic solids were made by solving a system of nonlinear algebraic equations involving dissolution, dissociation and hydrolysis reactions at equilibrium. The kinetics of the reaction between aluminium and orthophosphate ions at a reacting condition free of solids except for the product AlPO₄(s) was investigated in a well-stirred batch reactor at pH ∼3.1 and 3.5 at 25 °C. A detailed kinetic model involving ten species, seven reversible reactions of hydrolysis of soluble aluminium and orthophosphate species and one reversible precipitation reaction between aluminium and phosphate ions revealed a rate constant for the latter reaction of 5.968 × 10¹⁰ L mol^-1 s^-1 (p = 0.191). XRD, TGA/DTGA and EDX analyses of the filtered and dried reacting mixture confirmed that the only solid product of the precipitation reaction was hydrated AlPO₄(s).