Environmental Engineering Reference
In-Depth Information
chemo-deluoridation units are also installed in Chichkavtha village of Nagpur district,
and people have been using treated water for >6 months.
17.3.2 Electrocoagulation
In the electrocoagulation method, a coagulant is generated in situ by the electrochemi-
cal reaction (generally oxidation) of a suitable anode (commonly aluminum), which leads
to locculation. The locculated mass is then settled and separated similar to traditional
water treatment techniques. In this way, an aluminum (or iron) anode is placed in the
water stream and electric current is passed that produces Al 3+ ions, which subsequently
hydrolyses to form hydroxide precipitate with luoride. The electrocoagulation principle is
quite similar to the Al coagulation-locculation method. Instead of adding an Al salt, the
Al 3+ ions are generated by electrolytic oxidation at the anode, often offering better control.
The generation of the Al 3+ ion can be represented as Al (s) → Al 3+ (aq) + 3e . The settled
mass is removed by any separation method like iltration. Various combinations, such as
electrodialysis + ion exchange membranes, have been evaluated; however, not many are
being used currently in the ield (Ayoob et al., 2008). One such method developed by CSIR-
NEERI India is further discussed in a later part of this chapter.
17.3.3 Contact Precipitation Method
As reported by Bailey et al. (2006) in a contact precipitation method, luoride is removed
from water through the addition of calcium- and phosphate-based compounds. The pres-
ence of a bone charcoal medium acts as a catalyst for the precipitation of luoride in the
form of CaF 2 and/or luorapatite. Evaluation studies at a community level in Tanzania
have shown results with high eficiency using ield water. The method is reported to show
good reliability with acceptable water quality and low-cost advantages for deluoridation
of water (Fawell et al., 2006).
17.3.4 Magnesia-Based Precipitation
Magnesium oxide (MgO) can be effectively used in a precipitation-iltration type tech-
nique to remove luoride from water. The mechanism for removal of luoride ions is sug-
gested to be based on chemisorption or adsorption. MgO after adding to water forms
magnesium hydroxide (Mg(OH) 2 ), which can combine with luoride to produce an insolu-
ble magnesium luoride (MgF 2 ) precipitate. Addition of MgO, however, leads to an increase
in pH and other compounds are commonly added to the water to maintain the pH and
promote sedimentation. The following luoride-magnesium chemistry is reported (Ayoob
et al., 2008):
MgO + H 2 O → Mg(OH) 2
Mg(OH) 2 + 2F → MgF 2 ↓ + 2OH
The solubility product K sp for MgF 2 is 7.4 × 10 −11 , while the solubility for MgF 2 in pure water
is 2.64 × 10 −4 M.
This process is however, reported to be associated with limitations such as high ini-
tial cost, large MgO dose, alkaline pH of the treated water, as well as separation and
disposal of any precipitate. The pH of the treated water was reported to be as high as
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