Journal of Dental Research, Vol 71, 1310-1315, Copyright © 1992 by International & American Associations for Dental Research Online Journals
Fluoride sorption characteristics of different grades of bone charcoal, based on batch tests
D. L. Mwaniki
Kenya Medical Research Institute, Medical Research Centre, Nairobi.
Although bone charcoal (char) has been recommended for use in the
de-fluoridation of drinking water in developing countries, parameters
relating to fluoride (F) sorption characteristics by grade or type of bone
char are unclear. Based on batch tests, the rate and capacity of F uptake
were highest with black grade, followed by grey grade and lowest with white
grade. Twenty-four-hour contact-time F sorption isotherms indicated
saturation at capacities of 11.4 mg F per g of black grade, 2.4 mg F per g
of grey grade, and less than 0.3 mg F per g of white grade bone char.
Additional investigations showed that F-removal efficiency of black grade
char decreased steadily with increasing F concentration. Its initial
sorption kinetics were dependent on particle size, and the highest sorption
rate was observed with particle sizes below 0.42 mm. For maximum sorption
rate, the critical mixing time was between five and 15 min. The presence of
chloride anions and elevated temperatures increased the rate of F uptake by
black bone-char. The pH of distilled water following contact with bone char
rose to 11.5 for white bone-char, 8.3 for grey bone-char, and 7.8 for black
bone-char. The effects of processing temperature on the availability of
intra- and extra-apatitic F-binding sites, levels of oxides of calcium and
magnesium, and the carbon content were viewed as possible causes of
observed variations. These findings suggest that the black grade of bone
char was the most efficacious for use in partial de-fluoridation of
drinking water.
