Kinetic study of phosphate reaction with aluminum oxide and kaolinite

Abstract

The reaction of aqueous phosphate (∼3 × 10<sup>-4</sup>M) with alumina and kaolinite shows a rapid (12-24 hr) adsorption step followed by a slow process, obeying a first-order rate law, which extends over the next 60 days and probably involves nucleation and growth of a hexagonal AlPO<sub>4</sub> phase. For kaolinite with 10.7 m<sup>2</sup>/g surface area, solids concentration 7.5 g/l, pH = 4.6, as well as for α-Al<sub>2</sub>O<sub>3</sub> with surface area 10 m<sup>2</sup>/g, solids concentration 2.5 g/l, and pH = 4.3, the first-order rate constant at 50°C is 0.022 day<sup>-1</sup>. Increasing temperature increases the rate, with an activation energy of 2.4 Kcal/mol. Addition of fluoride ion (10<sup>-4</sup>M) decreases the amount of phosphate initially adsorbed but does not affect the first-order rate. The rate constant increases proportionally to the solid surface available, and decreases with increasing pH to essentially zero at pH ≥ 7. Successive addition of phosphate aliquots to the same sample of solid uses up the sites available for the rapid initial step but does not strongly influence the rate of the slow step.