The effectiveness of bone char in the defluoridation of water in relation to its crystallinity, carbon content and dissolution pattern

Abstract

The procedure for charring bone has been found to influence the nature of its mineral phase, and also affects the defluoridation capacity of the char. The aim of this study was to examine the efficiency of defluoridation by char produced from various parts of bones and charred for various lengths of time at various temperatures, relating the preparation of the char to its X-ray diffraction pattern, its content of pyrophosphate and its capacity for defluoridation. Bone was charred for 1 and 4 h at 400 °C and 30 min, 4 h and 48 h at 550 °C. Batches of the chars were suspended in amounts of 0.25 g in 100 ml of distilled water containing 0.53 mmol/l fluoride for up to 6 days under gentle agitation. At intervals, the pH and concentrations of fluoride, calcium and phosphate in the water were determined and the degree of saturation with respect to the calcium phosphate salts calculated. The charring procedure reduced the organic content of the bone from the 44-26% in intact bone to 3.4% in bone charred at 400 °C and to almost zero when charred at 550 °C for 48 h. When charred at 400 °C for up to 4 h the X-ray diffraction pattern of the bone showed a poorly crystallized apatite similar to that of untreated bone. Heating for 48 h or more at 550 °C led to considerably sharper apatite reflections, indicative of a well-crystallized salt. The chars produced at 400 °C reduced the fluoride concentration from 10 to 1–2 parts/10 6 over 6 days, whilst the better crystallized chars reduced the fluoride concentration to only 7–8 parts/10 6, irrespective of whether the char originated from compact or spongy bone. Bone samples charred for shorter periods at lower temperatures were poorer apatites, but better fluoride absorbants, an effect unrelated to the carbon residue or pyrophosphate content.