Effects of ${\mathsf{pCO}}_{\mathsf{2}}$ on the Removal of Fluoride from Wastewater by Calcite

Free-drift batch reactor experiments using calcite (limestone, ${\mathrm{CaCO}}_3$) were used to study fluoride removal through precipitation as fluorite (${\mathrm{CaF}}_2$) from solutions with concentrations reflective of an industrially contaminated site. The influence of ${\mathrm{CO}}_2$ partial pressure (${\mathrm{pCO}}_2$), stirring rate, and fluoride concentration were investigated in this paper. Equilibrium modeling shows that in wastewaters with high fluoride concentrations ($\sim \mathrm{2,000}\mathrm{mg}/\mathrm{L}$), the flux of ${\mathrm{CO}}_{2(\mathrm{g})}$ to ${\mathrm{CO}}_{2(\mathrm{a}\mathrm{q})}$ could not keep up with the consumption of ${\mathrm{CO}}_{2(\mathrm{a}\mathrm{q})}$, resulting in an initial disequilibrium with experimental pH reaching equilibrium quickly, while fluoride removal lagged. Increasing stirring rate significantly decreased the extent of disequilibrium and the time at which the ${\mathrm{CaCO}}_3$-fluoride-${\mathrm{CO}}_2$ system attained equilibrium due to the increased rate of transport of dissolved ${\mathrm{CO}}_2$ to the ${\mathrm{CaCO}}_3$ surface, and simultaneously the rate of transport of the dissolved ${\mathrm{CaCO}}_3$ to the bulk solution. Optimal fluoride removal occurs at ${\mathrm{pCO}}_2{\sim 10}^{-0.52}$ [30% (mol% ${\mathrm{CO}}_2$)] with 96% of the initial $\mathrm{2,000}\mathrm{mg}/\mathrm{L}$ fluoride load removed in less than 80 min with a stirring rate of 300 revolutions per minute. Increasing ${\mathrm{pCO}}_2$ to $\sim 100$ (100% ${\mathrm{CO}}_2$) resulted in very little gain, less than 2%, in fluoride removal, or in the time required to reach equilibrium and therefore significant remediation cost savings can be obtained by using ${\mathrm{pCO}}_2$ 30% when compared to 100%.