One of the major challenges faced by mankind today is to provide safe drinking water to a vast population around the world. It is learnt that fluoride concentrations in groundwater in the world ranges from 0.01 to 48 mg/L. According to the guidelines of World Health Organization (WHO) issued in 2011, the guideline value is 1.5 mg/l for drinking water quality. People in more than 35 nations across the globe face issues of excess fluoride in drinking water, the intensity and severity of which varies with the environmental settings in terms of their geographical and economical status.
Fluorides are released into the environment naturally through weathering and dissolution of rock minerals, in emissions of volcanoes, and in marine aerosols. The two most populated countries of the world, China and India, stand at the top in the list of worst hit nations in groundwater contamination with fluoride. In both these countries, the major source of fluoride pollution is the natural weathering process. Drinking water containing high fluoride content for longer time can result in mottling of teeth, softening of bones and ossification of tendons and ligaments. (Ayoob et al, 2008).
Desirable concentration of fluoride to be maintained in drinking water is 1 mg/L and permissible limit in the absence of alternate sources is 1.5 mg/L as per Indian drinking quality standards (IS 10500-1991).
The burden of disease associated with fluoride, therefore, arises from the adverse effects of both excess and insufficient fluoride. The International Programme of Chemical Safety (ICPS, 1984) note that, “The quantity of fluoride needed for mineralization processes is small, and because of the ubiquitous distribution of fluoride, true deficiency is unlikely to occur in human beings”.
One of the major challenges faced by mankind today is to provide clean water to a vast majority of the population around the world. Emamjomeh et al. (2009) enunciated that fluoride concentrations in groundwater in the world range from 0.01 up to 48mg/L.
Excessive fluoride concentrations have been reported in groundwater of more than 20 developed and developing countries including India where 19 states are facing acute fluorosis problems. (State of Environment Report India, 2009).
Technologies for Defluoridation of Water – An Overview
There are several defluoridation processes tested or employed globally, such as adsorption (Azbar and Turkman, 2000), chemical precipitation ( Azbar and Turkman, 2000,), and electrochemical method (Shen et al., 2003). Currently, some of the popular processes for drinking water defluoridation are the adsorption using activated alumina (Ghorai and Pant, 2005, Tripathy et al., 2006, Chauhan et al., 2007), bone char (Mjengera and Mkongo, 2003, Hernandez-Montoya, 2007) activated carbon (Daifullah et al., 2007, Kumar et al., 2007), other adsorbents (Biswas et al., 2007) and the coagulation using aluminum salts (Pinon – Miramontes et al., 2003).
Other major processes for defluoridation include electro- dialysis (Tahaikt et al., 2006), reverse osmosis (Arora et al., 2004) and nanofiltration (Hu and Dickson, 2006). According to Zuoa et al., (2008), the membrane processes are known to be effective means for defluoridation but this not only removes the beneficial contents present in water during defluoridation, but also increases the operational cost.
Currently, there is a growing interest in Electrocoagulation (EC) process or electrolytic defluoridation. The technique is used to treat restaurant wastewater (Chen et al., 2000), textile wastewater (Bayramoglu et al., 2004), and fluoride-containing wastewater effectively. Electrocoagulation process (EC) using sacrificial aluminum electrodes has been demonstrated to be an effective process since it does not require a substantial investment, presents similar advantages as chemical coagulation and reduces disadvantages, (Hu et al., 2005) and less waste slurry production (Essadki et al., 2009).
Defluoridation efficiency by electrocoagulation process depends on applied current intensity, initial fluoride concentration, initial pH, raw water quality and flow rate. The main reactions involved in the EC are as following:
It is generally accepted that the EC process involves three successive stages: (a) formation of coagulants by electrolytic oxidation of the ‘sacrificial electrode’; (b) destabilization of the contaminants, particulate suspension, and breaking of emulsions; (c) aggregation of the destabilized phases to form flocs.