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

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 CdCl2, 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" Cd2+ adsorption. At equal aqueous activities of Cd2+, adsorption was systematically lower for the pre-equilibrated input solutions. The effect of variable solution composition on Cd2+ adsorption was reproduced by a Ca2+-Cd2+ cation exchange model and by a surface complexation model for the calcite-aqueous solution inter-face. For the range of experimental conditions tested, the latter model predicted binding of aqueous Ca2+ and Cd2+ 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 Cd2+ 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 It. The process exhibited first-order kinetics with respect to the concentration of adsorbed Cd2+, with a linear rate constant at 25degreesC of 0.03 h(-1). Assuming that diffusion into the calcite lattice was the mechanism of slow uptake, a Cd2+ solid-state diffusion coefficient of 8.5 X 10(-21) cm(2) s(-1) was calculated. Adsorbed Cd2+ had a pronounced effect on the dissolution kinetics of calcite. At maximum Cd2+ surface coverage (similar to10(-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 Cd2+ cations after their removal. Copyright (C) 2003 Elsevier Science Ltd.