Environmental Engineering Reference
In-Depth Information
were developed for deluoridation of water, AA is the most widely used absorbent owing
to its high luoride selectivity, relatively low cost, and ease of operation and maintenance
[85,86]. Fluoride adsorption by AA is recommended as a best available technology by the
US Environmental Protection Agency [87]. Extensive reports on the use of AA and its
effectiveness are available in open literature [68,77]. The mechanism of F
−
adsorption by
AA is similar to that of a weak base ion-exchange resin. The major drawbacks of commer-
cially available AA are its poor kinetics of adsorption and high sensitivity to pH. Fluoride
adsorption capacity decreases with the increase in pH and alkalinity. The optimum pH for
luoride removal by AA is in the range of 5-6. At acidic condition (pH <6), the formation of
soluble alumino-luro complexes have been reported [88]. At pH >7, silicate and hydroxyl
ions compete for the adsorption sites, reducing the F
−
adsorption capacity of AA. A recent
attempt by Lee et al. [89] shows that mesoporous aluminas (MA-1 and MA-2) prepared
through a surfactant-assisted route is superior to commercial AA in removing luoride.
A similar study reported that the maximum luoride uptake capacity by nanoalumina is
14.0 mg/g at 25°C, and the maximum luoride removal occurred at pH 6.15 [90]. An excel-
lent review on mesoporous alumina and their characteristics has also been reported [91].
Studies also show that AlOOH, a common starting precursor for alumina, is also a good
adsorbent for this application. The preparation of AlOOH is relatively simple and eco-
friendly, which may make the material attractive and easily acceptable by users. Wang et
al. [92] studied the luoride adsorption capacity of nanoscale AlOOH. The material showed
a maximum adsorption capacity of 3.5 mg F
−
/g of AlOOH, which is comparable to that of
bulk AA. The adsorption of luoride onto nano-AlOOH was strongly pH dependent and
the maximum uptake was observed at pH of 6.8.
Aluminum oxide is highly soluble in water under both acidic (pH <4) and alkaline (pH
>9) conditions. At pH 4, the prominent species in water is
Al H(
2
+
ions and at alkaline
pH, the dominant species is [Al(OH)(H
2
O)
5
]
2+
. This ion can produce dimeric or polymeric
complexes by further deprotonation and loss of water molecules [91]. This instability of
the aluminum oxide may limit its use in the ield especially when repeated reuse of the
adsorbent is expected. Alumina and other aluminum-based oxides loaded with luoride
are typically regenerated by contacting with NaOH (pH >9) followed by acid wash (pH <4).
The regeneration process would dissolve the materials and results in loss of the adsorbent
and thus reduce the economic viability. It is also important to assure the stability of the
material in water after alkali and acid wash.
19.4.1.3 Transition Metal-Based Oxides
Iron oxides exist in many forms in nature and are perhaps one of the widely studied engi-
neered nanoscale systems for environmental remediation. Various kinds of iron NPs with
diverse properties, including low toxicity, chemical inertness, biocompatibility, and super-
paramagnetism, have been synthesized [93-97]. Some of the unique properties of iron oxide
NPs against their bulk counterparts, which make them better material for environmental
remediation, are illustrated in Figure 19.4. The environmental friendliness, low cost, and
the ease of synthesis are other factors that support iron NPs as an attractive choice for a
mass application such as puriication of water. Mohapatra et al. [98] have demonstrated the
use of nanoscale goethite prepared through a new synthetic route using hydrazine sulfate
as an additive. The material was investigated for its ability to remove luoride from water.
The maximum luoride uptake was observed (59 mg F
−
/g [of goethite]) between the pH 6
and 8, proving the applicability of the adsorbent for treatment of natural water systems.
The same group has synthesized mixed nanoscale iron oxides comprising goethite (77%),
