Fluoride sorption characteristics of different grades of bone charcoal, based on batch tests

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.