Effects of fluoride and polymeric additives on the dissolution of calcite and the subsequent formation of fluorite

Fluoride is a problematic contaminant of the ground water arising from both natural and man-made origins. In this report, the fluoride uptake by calcite (calcium carbonate) was examined with emphasis on the effects of model organic compounds (polymers) and the possible adhesion of fluorite (calcium fluoride) on calcite. The primary route of the fluoride removal was fluorite formation through dissolution-and-recrystallization. While all model polymers affected the kinetics of fluorite formation, the positively charged poly(ethylene imine) had the most inhibiting influence. The observation of the calcite single crystals using atomic force microscopy revealed that the inhibitory effect was attributed to the retarded diffusion of fluoride to the calcite surface, which was also in accordance with the analysis of bulk experiments with calcite powders. It was also found that the oriented growth of fluorite was induced on the calcite (1 0 4) surface probably by the epitaxy with the (1 2 0) plane of fluorite. The current results could be of critical implications in the process design of fluoride removal by considering organic interferences and minimizing the need of flocculation step.