*5.5.2 Borne char and activated animal charcoal*

Bone char is the oldest known water defluoridation agent, and it was first used in USA from 1940s to the 1960s. The technique was later introduced into other countries and it is now among the most used methods in the developing countries of the world. The method involves the use of animal charcoal which is packed into columns and water percolated through the charcoal media [120]. Mutheki et al. [121] compared to the field and laboratory performance of bone char filters and other filters based on a combination of bone char and calcium-phosphate pellets. They found average uptake F capacities to be 1.2 ± 0.3 mg/g and 3.0 ± 1.0 mg/g, respectively. A study to explore activated carbon from fish bladder showed a maximum F removal of 1.43 mg/g [122]. In related work, Kawasaki et al. [123] who investigated four types of animal biomass and Singanan [124] who studied certain bone char adsorbents for F removal, reported more or less similar defluoridation potential. However, some authors reported 99% F removal from borehole water containing 11 mg/L F based on a cartridge bone char

affixed onto a domestic faucet as a flow-through defluoridizer [125]. Therefore, some authors contend that F removal bone char is an efficient protocol for defluoridation of brackish water [22]. However, care must be taken about the preparation of the bone char, related to the taste and odor released into the treated water [126]. Besides, large amounts of organic materials are needed for gasification by expensive treatment at 800–1400 K in an inert atmosphere to obtain the adsorbents.

#### *5.5.3 Graphene*

Graphene is an emerging carbon material with sp2 -hybridized single-carbon atom-layer structure [127] that is also a promising adsorbent for water defluoridation [128]. Some workers have studied amine grafted graphene oxide encapsulated chitosan hybrid beads for water defluoridation and found a defluoridation capacity of 4.65 mg/g [129]. However, Li et al. [127], while studying graphene samples obtained from exfoliating graphite materials, reported high F adsorption capacities of 17.65 mg/g. Also, a team investigating the F adsorption by graphene-aluminum-silver-carbon quantum dots reported an adsorption capacity of 12.04 mg/g [130]. Nonetheless, some workers have shown that enhancing graphene oxide using cupric oxide, improved its F uptake capacity to 34 mg/g [131]. The results collaborate with those of a team of researchers, which used aluminum modified graphene oxide (GO) and showed it to have superior F removal efficacies of 38.31 mg/g [132]. This shows the high potential use of graphene in F water remediation.

Besides activated carbons and graphene, the application of carbonaceous minerals to water defluoridation has been reviewed elsewhere [80]. However, Abe et al. [133] reported that the water defluoridation capacities of various carbons follow the order: bone char > coal charcoal > wood charcoal > carbon black > petroleum coke.

#### **5.6 Biosorption**

Biosorption utilizes animal and plant remains in the pulverized form "as is" without prior gasification or charring. A study realized in Tanzania, reported F removal efficiencies, which was 4.1–47.3%, for several biosorbents [134]. In a particular defluoridation study, which was conducted by Yadav et al. [135] using three agricultural-based biomasses as adsorbents tested on groundwater containing 5 mg/L F, the authors reported F removal efficacies of 40–58%. Elsewhere, Gandhi and Sekhar [136], found that the F biosorption capacity of *Strychnos potatorum* seed powder was 0.9945–1.052 mg/g. Further recently, the F removal of palm kernel shellbased adsorbent was evaluated and found to be 2.35 mg/g [137]. It is found, therefore, that, plant biomasses exhibit limited F uptake capacities when compared to other adsorbents unless they are formidably treated.

Also, defluoridation studies utilizing algal biosorbents derived from *Spirogyra IO1* [138], *Ulva fasciata* sp. [139], *polyalthia longifolia* [140], and *Spirogyra* IO2 to adsorb F [141] have been reported. In the defluoridation evaluation of *Spirogyra* IO2, [141], for example, sorption capacity of 1.272 mg/g was reported. Also, the defluoridation capacities of fungal biosorbents have been studied. In evaluating *Fusarium oxysporum* to remove F from water, low defluoridation capacity of just 19% was reported [142]. These similar findings were collaborated with those by Ramanaiah et al. [143] fungal biosorbent of *Pleurotus ostreatus* 1804. However, other researchers have shown that the F biosorption potential of *Saccharomyces* sp. biomass reached 91% of F removal [144]. Similarly, *Aspergillus* and Calcium treated *Aspergillus* biosorbents revealed

F adsorption capacities of 8.09 mg/g [145]. Differences in the capacity of various biomasses to sequestrate F from water are related to, among other factors, differences in active functional groups in the biomasses [134].

Thus, even though Mukkanti and Tembhurkar [146] have recently reported a high F adsorption capacity of 26.31 mg/g for an adsorbent developed from clamshell waste, it is apparent that the usual water defluoridation capacities of untreated biomasses are low when compared to the other adsorbents [90, 118]. Furthermore, the source biota for the prized F biosorbents may be non-existent in the regions where they are needed for easy defluoridation of water. Plus, untreated biomasses degrade easily under chemical and biological attacks.

#### **5.7 Layer double hydroxides**

Layered double hydroxides (LDHs), also called anionic clay and hydrotalcite-like compounds, are a "host-guest" layered materials, which have the general formula [M2+ 1-xM3+x(OH)2] x+(An−x/n).mH2O, where *M*2+ and *M*3+ are metal cations that occupy octahedral positions in hydroxide layers; *x* is the molar ratio M3+/(M2+ + M3+) and *A* denotes interlayer charge-compensating anions [147]. LDHs have attracted a lot of attention as F adsorbents in the recent past. Lu et al. [148] assessed F removal based on NiAl layered double hydroxides (NiAl-LDHs) and reported a low equilibrium F concentration of just 0.2388 mg/L in the treated water. Sadik et al. [149] also reported high F removal rates of 99.2% for calcined LDHs synthesized from seawater (LDHsw). New data have provided the maximum F adsorption capacity of 6.67 mg/g for Fe3O4/Al(OH)3 [150] and 12.63 mg/g for tri-metal Mg/Ce/Mn oxide-modified diatomaceous matrix [151]. However, the mechanism of F adsorption onto an LDH and calcined layered double hydroxide (CLDH) had been earlier evaluated with maximum defluoridation capacities of 1.3 mg/g and 20 mg/g, respectively [152]. The F sorption quantities were somehow similar to the 22.78 mg/g and 20.28 mg/g that have recently been reported for Ce-Ti and Ce-Ti/Fe3O4 hybrid oxides, respectively [153].

However, several studies have reported enhanced defluoridation capacities for LDHs. In a study involving calcined Mg–Al–CO3 LDHs, for example, competitive F adsorption was evaluated and water defluoridation capacity of 1.94 mmol/g was reported [154]. Then, Kang et al. [155] reported water defluoridation capacity of 50.91 mg/g for Mg/Fe l CLDHs. Furthermore, other studies have documented a high F adsorption capacity of 146.6 mg/g for Ca-Al LDHs [156] and 270.3 mg/g for Fe–Mg– La triple-metal hydroxide composite [157]. Consequently, other authors have focused on the optimization of defluoridation conditions for LDHs. Elhalil et al. [158], as such, while evaluating showed that optimum adsorbent dosages of water defluoridation using calcined Mg/Al LDH were in the range of 0.29–0.8 g/L. Also, the suggested F adsorption equilibrium time [149, 156] and solution acidity [149, 155] of LDHs are within 1 h and pH 6–7, respectively.

### **6. Phytoremediation**

This is a technique of defluoridation and removal of other contaminants from the environment, which uses plants to absorb and accumulate excess F from soil and water through their roots into their systems. The plants are then removed at the predetermined time and disposed of safely. Several researchers evaluated technology phytoremediation with varying degrees of success. The tolerance capacity of *Solanum*  *Water Defluoridation Methods Applied in Rural Areas over the World DOI: http://dx.doi.org/10.5772/intechopen.105102*

*tuberosum* to accumulate F, has been tested and found that after 87 days, the F levels in the leaves, root, shoot, and potato tuber of the plants had increased to 3.96, 3.02, 2.8, and 1.56 mg, respectively [159]. Some researchers also tested the uptake of Al and F by four green algae species and found that *Pseudokirchneriella subcapitata* showed the highest aluminum and fluoride absorption under the test conditions [160]. Sirisha et al. [161] studied phytoremediation of Cr and F in industrial wastewater using the aquatic plant *ipomoea aquatica plant*. They found that the F removal rapidly reached 37% in just 10 min and similar results have been found by researchers in related tests [162] indicating that phytoremediation id a promising green technology for use in environmental fluoride remediation [163].
