Kinetics of Phosphate Adsorption by Calcium Carbonate and Ca‐Kaolinite

Abstract

AbstractThe objective of the present investigation was to elucidate the time‐dependence of phosphate adsorption by calcium carbonate and Ca‐kaolinite and to suggest a mechanism of adsorption. The phosphate adsorption by calcium carbonate and Ca‐kaolinite at low phosphate concentrations in solution could be described by Langmuir adsorption isotherms, indicating that a monolayer of phosphate is formed on the surface. The calculated maximum surface saturation was 25µg/g for calcium carbonate and 187µg/g for Ca‐kaolinite. A second‐order kinetic equation was developed which considers both the change in phosphate concentration in solution and the surface saturation of the adsorbent during the adsorption process. The rate constant of phosphate adsorption was about 30,000 times greater for calcium carbonate than for Ca‐kaolinite. About 80% of the phosphate adsorption by calcium carbonate was completed within 10 sec. The variation of the second‐order rate constant with phosphate concentration was different in the two systems. The rate constant decreased in the calcium carbonate‐phosphate system and increased in the Ca‐kaolinite‐phosphate system with increasing phosphate concentration. It was suggested that phosphate ions are adsorbed by displacing coordinated water molecules and/or coordinated anions.