Environmental Engineering Reference
In-Depth Information
n-magnesia was crystalline with high phase purity, and the particle size varied in the
range of 3-7 nm. With the help of various spectroscopic, microscopic, and macroscopic
studies, the mechanism of luoride uptake by n-magnesia was explained as luoride
removal thorough isomorphic substitution of hydroxyl groups by luoride in a brucite lat-
tice. This reaction was observed since both the F
−
and OH
−
ions are isoelectronic in nature
and of similar size and with comparable ionic radii. The Langmuir maximum sorption
capacity for luoride removal was reported as high as 267.82 mg/g. Fluoride adsorption
by n-magnesia was less sensitive to pH variations, and only a slight decrease in luoride
adsorption was observed at higher pH, which was due to the competition from OH
−
ions.
Fluoride uptake was most affected in the presence of phosphate followed by bicarbonate
and nitrate. The luoride uptake was systematically studied for n-alumina using aqueous
solutions (Kumar et al., 2011). The maximum sorption capacity of n-alumina for luoride
was reported as 14.0 mg/g at 25°C, with maximum luoride removal at pH 6.15.
17.7.8.3 Regeneration of Adsorbents
Techno-economic viability of any adsorbent signiicantly depends on its regeneration
and reuse potential for many cycles of operation. Various regeneration media have been
reported in the past few decades for the effective regeneration in luoride adsorption stud-
ies, which obviously depend on the nature of the adsorbent used. Some of the reported
regeneration methods are listed in Table 17.1.
17.7.8.4 Disposal of Adsorbent
Disposal of exhausted adsorbent is one of the important parameters that determine its
overall environmental impact, and this has been a challenge in many developing coun-
tries. In any adsorption process, the exhausted material has to be either used or disposed
of properly after a number of cycles of the adsorption/desorption process. Therefore, it is
usually necessary to remove luoride ions for safe disposal, which otherwise may ind its
way to water bodies and contaminate them. This is important for both regenerant as well
as exhausted adsorbent. Safe disposal of exhausted adsorbent has been reported by a few
authors. Iyenger (2005) has reported different methods for the treatment of spent media
generated during the regeneration of AA and further used in the manufacturing of bricks.
These methods have been discussed as follows:
• Addition of CaCl
2
to spent alkali regenerant to precipitate luoride and then mix-
ing the supernatant with an acid regenerant
• Simple mixing of spent alkali/acid regenerants
• Mixing alkali/acid regenerants and using certain additives like alum or lime to
remove luoride and to improve settling properties of the sludge
About 85% luoride could be removed, using option 3, from a mixed alkali-acid regener-
ant. In this way, disposal of spent regenerants can be carried out by mixing spent alkali
and acid regenerants, adjusting pH by addition of lime and settling the sludge for 24 h.
The supernatant solution, with low luoride and near-neutral pH, could then be drained
off. However, the drained water could have high TDS, hardness, and sulfate. Sludge could
be collected periodically and used for brick making at the village level itself. In any case,
regeneration requires use of certain chemicals and minimizing its requirement would be
important from a cost and environmental point of view.
