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TopNomenclature
Co initial concentration of adsorbate in solution (mg/L)
Ce equilibrium concentration of the adsorbate in the solution (mg/l)
Ct concentration of the adsorbate in the solution at any time t (mg/L)
Qe amount of adsorbate removed from aqueous solution at equilibrium (mg/g)
Qt amount of adsorbate sorbed on the adsorbent surface at any time t (mg/g)
m mass of adsorbent (g)
ppm parts per million
R2 linear regression coefficient
v volume (mL)
Top1. Introduction
Fluoride is one of the very few chemicals that have been shown to cause significant health problems in people through drinking-water. Fluoride has beneficial effects on teeth at low concentrations in drinking-water (0.5 to 1.0 mg/l), but excessive exposure to fluoride in drinking-water (above 1.5 mg/l), or in combination with exposure to fluoride from other sources, can give rise to a number of adverse effects. These range from mild dental fluorosis to crippling skeletal fluorosis as the level and period of exposure increases (Fawell, Bailey, Chilton, Dahi, Fewtrell, & Magara, 2006; Hichour, Persin, Sandeaux, & Gavach, 2000; Mehmet et al., 2007).There are four probable mechanisms of human exposure to the fluoride ions. This can occur through air, dental products, food and beverages other than water and drinking water (Daia, Rena, & Maa, 2004; Ponikvar, Stibilj, & Emva, 2007; Cao, Zhao, Li, Deng, & Yi, 2006). Drinking-water is typically the largest single contributor to daily fluoride intake (Maliyekkal, Shukla, Philip, & Nambi, 2008). Fluoride poisoning can be prevented or minimized by using alternate water sources, improving the nutritional status of population at risk or removing excess fluoride (defluoridation). Defluoridation of drinking water is the only practicable option to overcome the problem of excessive fluoride in drinking water, where alternate source is not available (Maheshwari, 2006).
Fluoride removal from drinking water can be achieved by chemical precipitation (Pinon-Miramontes, Bautista-Margulis, & Perez-Hernandez, 2003), adsorption (Lounici et al., 1997; Ghorai & Pant, 2005), membrane process (Arora, Maheshwari, Jain, & Gupta, 2004; Wisniewski, Rozanska, & Winnicki, 2005) and ion exchange (Meenakshi & Viswanathan, 2007). Among these processes, membrane and ion exchange processes are not very common due to their high installation and maintenance cost. Chemical precipitation by addition of alum and lime mixture into fluoride-contaminated water (Nalgonda technique) is a common method for defluoridation after its inception in India. However, the associated problems like generation of acid/alkali water, residual aluminium and soluble aluminium fluoride complexes, generation of sludge, and relatively higher residual fluoride concentrations are of major concern (Onyango, Kojima, Aoyi, Bernado, & Matsuda, 2004; Maliyekkal, Sharma, & Philip, 2006). Adsorption onto solid surface is a simple, versatile and appropriate process for treating drinking water system, especially for small communities. Adsorption technique is considered as economical and can remove ions over a wide pH range and to a lower residual concentration than precipitation (Maliyekkal et al., 2008).