This step was performed on two underground waters (Louza 2
and Gafsa SM4) naturally fluoridated. In Table 4, we present the
results of the defluoridation of the two waters. The values were
obtained at optimum conditions (pH 7.2, contact time 60 min,
adsorbent amount 15 g L1). The sorption of fluoride on the
cuttlefish bone was effective. Indeed, the residual fluoride was
below the WHO standards. Except for fluoride, the other physicochemical
parameters of the treated water were not modified.
Analysis of the precipitate obtained showed that it was mainly
composed of calcium carbonate and a small amount of CaF2. For
CaF2 precipitation, supersaturation of F and Ca2+ was the major
premise. Indeed, with Louza 2 water,
CF = 3.39 mg L1 = 1.79 104 M;
CCa
2+ = 210 mg L1 = 5.25 103 M. The ionic product
Q = CCa (CF)2 = 5.25 103 (1.79 104)2 = 16.9 1011 was
higher than solubility product Ks of CaF2 (Ks = 3.5 1011). Turner
et al. [25] and Nath and Dutta [44] confirmed that besides sorption,
precipitation of CaF2 could also occur.