Aqueous cadmium uptake by calcite: a stirred flow-through reactor study

Abstract

Uptake of cadmium ions from solution by a natural Mg-containing calcite was investigated in stirred flow-through reactor experiments. Input NaCl solutions were pre-equilibrated with calcite (pH 8.0) or not (pH 6.0), prior to being spiked with CdCl 2. For water residence times in the reactor less than 0.5 h, irreversible uptake of Cd by diffusion into the bulk crystal had a minor effect on the measured cadmium breakthrough curves, hence allowing us to quantify “fast” Cd 2+ adsorption. At equal aqueous activities of Cd 2+, adsorption was systematically lower for the pre-equilibrated input solutions. The effect of variable solution composition on Cd 2+ adsorption was reproduced by a Ca 2+-Cd 2+ cation exchange model and by a surface complexation model for the calcite-aqueous solution interface. For the range of experimental conditions tested, the latter model predicted binding of aqueous Ca 2+ and Cd 2+ to the same population of carbonate surface sites. Under these circumstances, both adsorption models were equivalent. Desorption released 80 to 100% of sorbed cadmium, confirming that fast uptake of Cd 2+ was mainly due to binding at surface sites. Slow, irreversible cadmium uptake by the solid phase was measured in flow-through reactor experiments with water residence times exceeding 0.7 h. The process exhibited first-order kinetics with respect to the concentration of adsorbed Cd 2+, with a linear rate constant at 25°C of 0.03 h −1. Assuming that diffusion into the calcite lattice was the mechanism of slow uptake, a Cd 2+ solid-state diffusion coefficient of 8.5×10 −21 cm 2 s −1 was calculated. Adsorbed Cd 2+ had a pronounced effect on the dissolution kinetics of calcite. At maximum Cd 2+ surface coverage (∼10 −5 mol m −2), the calcite dissolution rate was 75% slower than measured under initially cadmium-free conditions. Upon desorption of cadmium, the dissolution rate increased again but remained below its initial value. Thus, the calcite surface structure and reactivity retained a memory of the adsorbed Cd 2+ cations after their removal.