3.1. Identification of the best conditions for the metallic doping of
bone chars
Table 1 shows the fluoride adsorption results for the 36 metaldoped
bone char samples obtained from L9 experimental design.
Overall, the fluoride adsorption capacities ranged from 4.7 to
28.6 mg/g and they increased for almost all the modified adsorbents
based on the fact that raw bone char showed a fluoride uptake of
≈7.32 mg/g [7]. Note that fluoride ion shows a high electronegativity
and a small ionic size and it can be classified as a hard bases
having a strong affinity for metallic ions, including trivalent cations
such as Al3+ and Fe3+ [27,28]. Therefore, the increase in the adsorption
properties of metal-modified bone chars must be associated with the presence of these cations in the adsorbent surface. However,
the improvements observed in the adsorption properties of
these new adsorbents highly depend on both the nature of the
metallic species and the doping conditions. Specifically, the incorporation
of aluminum sulfate in the surface showed the greatest
impact for increasing the fluoride adsorption capacity of bone char.
Adsorption properties of bone char may increase up to 600% when
using this salt, and the modified adsorbent has outstanding fluoride
uptakes, i.e., qF− > 23 mg/g. On the other hand, aluminum nitrate,
aluminum chloride and iron chloride can enhance fluoride removal
but in a minor percentage (up to 400, 124 and 88%, respectively).
Herein, it is convenient to remark that the metal doping stage gives
rise to a partial dissolution of the inorganic phase of bone char (due
to the low pH of the solution), and the final yield obtained after
the modification step ranges from 8 to 93%, see Table S1 of supplementary
material, depending on the metallic salt. In particular, the
yield for aluminum chloride salt was higher than those obtained
for other salts