Enhanced Cd(II) Uptake by the Bassanite Phase Contained in Waste Calcite Produced via the Carbonation of Flue Gas Desulfurization (FGD) Gypsum

Abstract

The uptake of Cd(II) by waste calcite, a by-product of the carbonation of flue gas desulfurization (FGD) gypsum, was investigated in a batch experiment. The uptake of Cd(II) by and the dissolution of Ca(II) from the waste calcite particles were monitored simultaneously as a function of exposure time to 0.89 mM Cd(II) solution. The reagent-grade calcite particles were also examined for a comparative study. The waste calcite contained bassanite, dolomite, and muscovite as the major impurity phases. X-ray diffraction study revealed that the bassanite phase dissolved almost completely during the 1st hour of the exposure to the solution while other phases were intact for two days. The amount of removed Cd(II) was found to be proportionally related to dissolved Ca(II) reflecting the exchange nature of the adsorption. The use of waste calcite instead of reagent-grade calcite enhanced Ca(II) dissolution and thereby Cd(II) uptake significantly. The waste calcite removed about 90% of initial Cd(II) while reagent calcite removed only 6% during the exposure for 2 days to 0.89 mM Cd(II) solution. The enhanced Ca(II) dissolution and Cd(II) uptake by the waste calcite were attributed to the fast-dissolving bassanite phase which provides the substantial quantity of Ca(II) and sulfate ions to the calcite/solution interface.