Fig. 5 shows the effect of pH on the removal of fluoride by
sorption on the cuttlefish bone. As the pH of the fluoride solution
increased from 4.5 to 10.5, fluoride removal was more or less
constant around 85–82% for a pH between 4.5 and 7.5. But, it
drastically dropped to 35% at pH 10.5. The mechanism of fluoride
removal can be better understood by examining the cuttlefish bone
surface composition. Cuttlefish bone, when dissolved in water, will
produce the following species, HCO3
, CO3
2, Ca2+, CaHCO3
+ and
CaOH+. When the solution is more basic than the point at zero
charge (pH > pHzpc), negative species will predominate and the
surface will be negatively charged. Similarly, when the solution is
more acidic than the point at zero charge (pH < pHzpc), positive
species will predominate and the surface will be positively
charged. A number of authors [37–41] showed that Ca2+ and
CO3
2 were the potential determining ions in a pure calcite
solution. Decreasing pH effectively provides more Ca2+ in a
Fig. 5 shows the effect of pH on the removal of fluoride bysorption on the cuttlefish bone. As the pH of the fluoride solutionincreased from 4.5 to 10.5, fluoride removal was more or lessconstant around 85–82% for a pH between 4.5 and 7.5. But, itdrastically dropped to 35% at pH 10.5. The mechanism of fluorideremoval can be better understood by examining the cuttlefish bonesurface composition. Cuttlefish bone, when dissolved in water, willproduce the following species, HCO3, CO32, Ca2+, CaHCO3+ andCaOH+. When the solution is more basic than the point at zerocharge (pH > pHzpc), negative species will predominate and thesurface will be negatively charged. Similarly, when the solution ismore acidic than the point at zero charge (pH < pHzpc), positivespecies will predominate and the surface will be positivelycharged. A number of authors [37–41] showed that Ca2+ andCO32 were the potential determining ions in a pure calcitesolution. Decreasing pH effectively provides more Ca2+ in a
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