Difference in cadmium chemisorption on calcite and vaterite porous particles.

Cadmium is adsorbed on calcium carbonate via chemisorption. All calcium carbonate polymorphs generate otavite (cadmium carbonate), indicating that the crystallographic differences in calcium carbonate should affect the chemisorption equilibrium and kinetics. This study investigates the influences of the polymorph and specific surface area on cadmium adsorption. Here, we synthesise two polymorphs of porous calcium carbonate: calcite and vaterite with a wide range of specific surface areas. Then the equilibrium of cadmium adsorption is evaluated using adsorption isotherm models. Based on the Langmuir model with linear regression analysis, the maximum adsorptions of porous calcite and vaterite particles are 287.8 mg/g and 883.5 mg/g, respectively. The kinetics of cadmium chemisorption show clear differences between polymorphs. The calculated rate constant of the porous calcite particles using a pseudo-second-order reaction and Elovich models are two orders larger than that of porous vaterite particles. Although the adsorbed amount is superior for porous vaterite particles, porous calcite particles exhibit a faster reaction and relatively high adsorbed capacity for cadmium ions.