90    THE EFFECTS OF CARBONATE ON CALCIUM FLUORIDE
PRECIPITATION
Joseph G. Rabosky, Private Consultant
Pittsburgh, Pennsylvania 15202
James P. Miller, Jr., Professor Emeritus
University of Pittsburgh
Pittsburgh, Pennsylvania 15202
INTRODUCTION
The adverse health impacts of fluorides in our environment are primarily linked to excessive
concentrations in drinking water supplies. However, in order to provide a viable source of potable
water, contamination must be prevented. In the United States the procedure chosen to safeguard
water supplies is effluent control, i.e., contamination flowing into natural waters is regulated by
means of standards which impose maximum permissible concentrations. Regulation of these discharges requires treatment to reduce contaminants to within maximum allowable limits; consequently,
wastewater treatment is widely practiced in the United States. By providing proper wastewater treatment, our drinking water supplies will be protected.
Past research describes several defluoridation techniques in which the sole objectives were to reduce
fluoride concentrations to levels which either approached solubility or exhibited few adverse health
effects. The reporting authors had varying degrees of success with their respective methods
[1,2,3,4,5,6]; however, little understanding was gained as to why some research was more successful
than others when identical removal processes were employed.
The primary objective of this study was to determine the effects of pH and carbonate on defluoridation using calcium salts. An ion that competes with fluoride for calcium, namely carbonate, was
introduced into the calcium fluoride equilibrium system to study its effect on precipitation at various
pH levels. Because calcium carbonate is slightly less soluble than calcium fluoride, carbonate effectively competes for available calcium. When the solubility product of calcium carbonate is exceeded,
calcium is removed from solution in the carbonate form. Establishment of this competitive scheme in
a pH range of 5 to 11 permitted the effects of the carbonate equilibrium on calcium defluoridation to
be demonstrated.
The results of the study revealed several significant observations. The first suggests that pH influences defluoridation efficiency by affecting species form. Essentially, if ions are in the proper species
form to combine with calcium, the resulting competition between these species and fluoride for
available calcium reduces defluoridation efficiency. Secondly, particle size analysis indicated that a
pore diameter of 0.45 microns may be too large to filter calcium fluoride whose particle size ranged
from 0.25-0.75 microns for samples collected during this study. Lastly, with the exception of only
dilute solutions, incorporation of activity coefficients into solubility calculations are necessary to
accurately predict theoretical fluoride concentrations. Wastewaters may not fall into the category of
dilute solutions; therefore, consideration must be given to ionic strength and inclusion of activity
coefficients for calculation of theoretical predictions.
PRELIMINARY DISTILLATION
During the initial stages of the study, the effect of interferences on fluoride determinations was
checked. Samples were collected and divided for comparison purposes. Analytical results indicated
only very slight differences between distilled and undistilled samples; therefore, the study was continued without distillation. All reported results were based on this premise.
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