*5.5.1 Plant biomass-derived carbonaceous F adsorbents*

Many activated carbons have been studied for water defluoridation. Hanumantharao et al. [107], for example, evaluated *Acacia farnesiana* carbon and reported a low defluoridation capacity of 0.268 mg/g. Similar limited water defluoridation capacities (< 1.5 mg/g) have also been reported for carbons of: *Neem* [108], *Tamarindus indica* fruit shells [109]*,* family fruit [110]*,* zirconium-impregnated coconut shell [111], rice straw [112], zirconium impregnated cashew nutshell [113], pine cone [114], and zirconium impregnated coconut fiber [115]. In contrast, studies by Mondal et al. [116] using sugarcane charcoal revealed F uptake capacities of 7.33 mg/g. Similarly, investigations using *Pithacelobium dulce*, *Ipomoea batatas,* and *Peltophorum ferrugineum* carbons showed defluoridation capacities of 78.96, 76.62, and 74.48, respectively [117] but much higher defluoridation capacities of 142.86 mg/g and 230.61 meq/g have been reported for *Delonix regia* pod carbon [118], certain carbon nanostructures [119] and for activated coffee husks carbons [55], respectively. In general water defluoridation using activated carbons has been shown to be pH-dependent and most carbon adsorbents have the highest F removal at acidic pH < 3 values [80]. Plus, the adsorbent particle appears to play a leading role in controlling the adsorption efficiency – high sorption occurs for the lowest size.
