1. Introduction

In halogen family, fluorine occurs in the form of gas due to its high reactivity and electronegativity in nature. Fluoride is an anionic form of fluorine and makes different organic and inorganic compounds due to its smaller size and greater tendency to behave as a ligand. A few compounds of fluoride are soluble in water and occurs in water resources ^{[1, 2, 3]}. High fluoride concentrations have been found in ground waters of more than 20 nations. Fluoride is the main inorganic pollutant of natural origin found in more than permissible limit in groundwater of 19 states of India viz. Andhra Pradesh, Assam, Bihar, Chhattisgarh, Delhi, Karnataka, Kerala, Madhya Pradesh, Gujarat, Haryana, Jammu & Kashmir, Jharkhand, Maharashtra, Orissa, Punjab, Rajasthan, Tamil Nadu, Uttar Pradesh, West Bengal ^[4]. However, the groundwater contributes only 0.6% of the total water resources on earth and the preferred source of drinking water both in rural and urban areas. Ground water provides 80 % of the total drinking water need in India. At present, surface water and ground water sources both are being deteriorated due to increasing population, industrialization, and urbanization. Ground water flows through a variety of geological structures in shallow aquifers of the earth. Some researchers have reported that various factors either natural or anthropogenic are also responsible for groundwater deterioration ^{[5, 6]}. The fluoride occurs mainly in the form of fluorspar, cryolite, sellaite and fluorapatite. Fluorspar occurs in sedimentary rocks whereas cryolite in igneous rocks. Higher fluoride concentration in groundwater (up to more than 30 mg/L) has occurred broadly, remarkably in Africa, Asia, and the USA [7-12]^[7].

Typically, the water sources have become hazardous not only for consumption but for other activities like irrigation and industrial requirements. Accordingly, there is a need for attention on the future impact of water resources in planning and developmental issues. The usual amount of fluoride essentially depends on the geological, chemical and physical characters of the concerned aquifer, temperature, the porosity as well as acidity of the soil and surrounding rocks. Therefore, the present article describes the fluoride sources, its effects, and methods existing for fluoride removal from water and wastewater.

2. Chemical Behavior of Fluoride

Naturally, fluorine is found in the form of CaF₂. The fluoride enters the soil through weathering of rocks and precipitation or waste runoff. However, the endemic fluorosis in India is principally caused by hydro-geochemical reasons. It has been established that the lower calcium and higher bicarbonate alkalinity increase the high fluoride content in groundwater ^{[12, 13]}. Elrashidi and Lindsay, 1986 ^[14] explained that the fluoride content of soils varies from less than 20 mg/L to an unevenly high-level concentration in highly affected areas. However, fluoride may present in the soil in the form of fluorapatite, fluorspar etc. ^{[15, 16]}. The fluoride ion is substituted by hydroxyl ion due to redox reactions in soil or rocks and finally increases the concentration of fluoride ions in circulating water. Fluoride amount may also increase in ground water where cation exchange of sodium for calcium occurs ^[17]. The large-scale withdrawal of water for agriculture purpose, low rainfalls, poor recharging and pollution due to industrial effluents are few important reasons to increase the fluoride content in groundwater.

Fluoride ion is strongly electronegative in character. Therefore, fluoride ion is attracted by positively charged Ca⁺⁺ ions in teeth and bones. Islam and Patel, 2011 ^[18] told that the high fluoride concentration exposure in workers of furnace create the problem of bladder cancer. The too much ingestion of fluoride may cause dental ^[19] and skeletal disorders ^[20]. Fluoride displaces the OH^- ions from hydroxyapatite [Ca₅(PO₄)₃OH], which is the main mineral constituent of teeth particularly in the enamel and bones, finally forms the harder and tougher fluorapatite [Ca₅(PO₄)₃F].

As a strong ligand and in various pH ranges of water, fluoride may form soluble complexes with polyvalent cations like Ca⁺², Mg⁺², Fe³⁺, Al³⁺etc.^[21]. Under acidic conditions, fluoride forms metal complexes. Some researchers have reported that fluoride (CaF₂) and fluorapatite [Ca₅(PO₄)₃F] have been considered to be the main minerals responsible for ground water fluoride contamination ^{[22, 23, 24]}. Several researchers have explained that the dissolution of these two minerals may be increased by calcite precipitations which withdraw the calcium ions from the solution as given below in equation (1) and finally decreases the aqueous calcium ion activity ^{[22, 25, 26, 27, 28]}.

(1)

3. Reasons for Natural Fluoride Concentration in Ground Water and Its Health Implications

The occurrence of a higher amount of fluoride in ground water of developing nations is mainly due to the lack inappropriate infrastructure for water treatment. The ground water sources are linked with different types of rocks, volcanic activities, industrial and agricultural activities which are directly responsible for higher fluoride concentration in groundwater. However, the geology of the area plays an important role to increase the amount of fluoride in water resources. The amount of fluoride in ground water mainly depends on geological settings and types of rocks. Crystalline rocks, sedimentary rocks, and metamorphic rocks play a key role to increase the fluoride concentration in groundwater through several geological activities. Besides this, the reaction time of ground water with aquifer minerals is also a key factor to increase the natural fluoride concentration in groundwater ^[29].

Fluoride possesses the interesting qualities related to human health especially in the prevention of dental caries. The calcium and phosphorus metabolism in the human body is also affected by the higher concentration of fluoride in drinking water. Generally, fluoride in lower quantity is an essential component for normal mineralization of bones and formation of dental enamel ^[30]. The excessive intake of fluoride may create the problem of fluorosis. López et al. 2006, ^[31] told that about five million people are affected with fluorosis in Mexico which covers about 6% of the population. In the Indian context, fluorosis was first reported in Nellore district of Andhra Pradesh in 1937 as reported by Shortt in 1937 ^[32]. Susheela, 1985 ^[33] reported that the industrial fluorosis leads to inhalation of fluoride dust or gas or fumes in humans. Central Pollution Control Board (CPCB) under Ministry of Environment & Forest, Govt. of India (1998), has given a permissible limit of industrial emission of fluoride as less than 25 mg/Nm^{3 [34]}. The prolonged ingestion of high fluoride concentrations i.e. >1.5ppm in drinking water causes severe health problems like dental and skeletal fluorosis. BIS 10500 ^[35] and WHO ^[36] have determined the standard maximum value of fluoride in drinking water as 1.5 mg/L.

4. Fluoride Detection Methods for Fresh Water Samples

Several lower level detection methods are available to determine the fluoride in freshwater samples (Table 1).

Table 1. Fluoride Detection methods

Download as

PowerPoint Slide

Larger image(png format)

• Tables index

Veiw figure

View current table in a new window

5. Occurrence and Detection of Fluoride in Fresh Water Sources of Uttarakhand, India: Case Studies

Uttarakhand is a Himalayan state of India with prosperous biodiversity ^[51] and also possess a large number of freshwater resources including rivers, springs, lakes, ponds, glaciers and locally available several traditional water resources ^[52]. The water quality of surface and ground water sources of the state has been monitored ^{[47, 48, 49, 53]}. During the study of 2014, our research group has detected the presence of fluoride in Alaknanda river, Madhuganga river, Gadoligadhera, Kevrugadhera, Gazaldgadhera, Adwani gadhera at six selected sites of Pauri districts of Uttarakhand state with maximum values as 0.43 and 0.23 mg/l during pre-monsoon and post-monsoon seasons respectively ^[49]. In another study, we have described the spatial and temporal variations in surface water quality of Pithoragarh district in Kumaun region of Uttarakhand. Under this study, the average fluoride values were found as 0.38 and 0.31 mg/l in pre-monsoon season and post-monsoon season of 2011 in the monitoring of Ram Ganga river, Thuligad, Raigad, Thalika Gadhera, Kaliyapatal and Ghurghatiya water sources of Pithoragarh ^[47].We have also studied the drinking water quality of 18 selected locations of Dehradun district in Uttarakhand and found the average fluoride values as 0.52 and 0.46 mg/l in pre – and post-monsoon seasons respectively ^[48]. Recently, our research group have assessed the groundwater quality of Bhagwanpur Industrial Area of Haridwar district in the state and found the mean values of fluoride as 0.1225 and 0.3275 mg/l during March and April months of 2015 respectively, which were within the limit of BIS ^[54].

6. Fluoride Determination in Natural Water Samples by SPADNS Method

Fluoride ions possess dual importance in water supplies. Fluoride concentration less than 0.8 mg/l causes dental cavities while higher concentration causes dental fluorosis. Therefore, it has become necessary to maintain its concentration within limits. Among several methods of fluoride determination in water and waste water samples, Colorimetric i.e. SPADNS method is one of the most popular methods in laboratories. Under acidic conditions, fluoride ions react with Zirconium-SPADNS [Sodium 2-(parasulphophenylazo)-1,8-dihydroxy-3,6-naphthalene disulphonate] solution. Its dissociation and formation of colorless complex anion take place. As soon as a number of fluoride ions increases, the color becomes lighter ^{[46, 55]}. The chemical reaction of such method is given below:

(2)

7. Fluoride Removal Methods

Defluoridation is a method of removal of fluoride from drinking water. The defluoridation method may generally be categorized into two categories such as adsorptive methods and additive methods. In adsorptive methods, a bed of better surface activity is preferred and fluoride contaminated water passes through the bed. Thus, preferentially the fluoride ions get adsorbed on the surface bed material due to greater surface activity and complete a decline in fluoride concentration in the out-stream water whereas, in additive methods, suitable reagents are added for the defluoridation purpose. The fluoride ions in water react with the added chemical reagents and form an insoluble complex ^[56]. In other words, the defluoridation of drinking water can be achieved mainly by two options such as the treatment of water at the source and the treatment of water at the household level. The treatment at the water source is the preferred method in most of the developed countries whereas the treatment of the water at household can be preferred in less developed countries due to several advantages over treatment at the community level ^{[57, 58]}. Mostly, the fluoride removal techniques from drinking water work on the principles of precipitation, use of lime softening, alum-lime adsorption methods with a diversity of materials like activated alumina, bone char, synthetic calcium hydroxylapatite and bauxite. Defluoridation of water and wastewater can be achieved mainly by the important techniques as given in Figure 1.

Download as

• PPT
  PowerPoint Slide
• PNG
  Larger image(png format)

Veiw figureFigures index

•  NEW
  View larger figure in new window

Figure 1. Different Defluoridation methods [1,5,53,58,59]

The efficient elimination of water pollutants especially fluoride from fresh water is a major issue globally. The techniques like membranes separation, adsorption, ozonolysis, an advanced oxidation process; photocatalysis etc. are some of the important techniques for the treatment of water. Nowadays, nanoscience has emerged as an indispensable tool for the treatment of waste water. At the nanoscale, the material represents the remarkable variation in their properties because their surface area gets increased with a decrease in particle size. The particles of 1 to 100 nm size are called as nano-particles. Nano-sized materials possess numerous novel and interesting physical, chemical and biological properties like small size, porosity, ease of separation, high reactivity, large surface area, catalytic potential, chemical, thermal, electromagnetic, mechanical, optical and huge number of active sites to interact with pollutants etc. Several types of nano-materials have been prepared and being used for water treatment ^[1]. Usually, the nanoparticles of alumina, maghemite, iron, iron oxide, iron hydroxide, titanium oxide, nickel oxide, silica, stannous oxide, titanium oxide, zinc sulfide, zinc oxide, zirconia, anatase, akageneite, cadmium sulfide, cobalt ferrite, copper oxide etc. are found useful in water treatment.

Adsorption is a surface process in which contaminants are adsorbed on the solid surface. Generally, adsorption occurs by physical forces but, weak chemical bondings also contribute in the adsorption process. The process of adsorption is controlled by some factors like nature of the adsorbent and adsorbate temperature, pH, the concentration of pollutants, contact time, particle size etc. At equilibrium, the correlation between the amounts of contaminant adsorbed and in water is called as adsorption isotherm. There are several important renowned models such as Freundlich, Langmuir, Halsey, Henderson, Smith, intra-particle diffusion, Lagergren and Elovich liquid film diffusion etc. which were used to explain the results of adsorption process ^{[60, 61]}.

After the comparative study, it has been found that the adsorption method as the better and cost effective water treatment method among several water treatment methods like evaporation, aerobic, anaerobic, ion exchange, solvent extraction, electro-dialysis, micro- and ultrafiltration, reverse osmosis, precipitation, distillation, oxidation etc ^[62]. For defluoridation by adsorption, the method covers the adsorption of fluoride ions onto the surface of an active material. In adsorption method, the contaminated water is passed through a defluoridating material which retains the fluoride ions either by physical, chemical or ion exchange procedures. After a definite period, the adsorbent needs the replacement with the newer one. Numerous adsorbent materials have been employed for defuoridation of water as per their specific character of adsorption. The parameters considered for the removal of fluoride from water by adsorption method were adsorbent dose, contact time, thermal pre-treatment of adsorbent, initial fluoride concentration, and pH value. Initially, several adsorbent materials have been used to find out an effective and cheap defluoridating agent.

Nowadays, the most commonly used adsorbents are activated alumina and activated carbon. Mainly defluoridation methods involve the utilization of aluminum compounds due to the similarities of the charges on the fluoride ions with hydroxyl ions and the ease of replacement of the fluoride ions with that of hydroxyl ions to form fluoride-aluminum hydroxide complex. Researchers have used the acidic alumina ^[63], amorphous Al (OH)₃, gibbsite or alumina (Al₂O₃) ^[64] for defluoridation. Usually, the defluoridation ability of activated alumina gets affected due to the hardness of water as well as surface loading also. Farrah and co-workers, 1987 ^[64] studied the interaction of fluoride ions with amorphous aluminum hydroxide, gibbsite and aluminum oxide over a range of pH from 3 to 8 and Fluoride ion concentrations about 1.9–19 mg/L. A research group has established the efficiency of alum-impregnated activated alumina in defluoridation of water and it was 92.6% at pH 6.5 with a contact time of 3 hours when 25 mg/L fluoride was initially present in water ^[65]. Sometimes in the cases of acidic solutions, the adsorption of fluoride was found to be retarded due to the electrostatic repulsion whereas at higher pH, the adsorption of fluoride ion takes place due to electrostatic repulsion of fluoride ion to the negatively charged surface of alumna as well as the competition for active sites by excessive amount of hydroxyl ion ^[66]. Moreover, Langmuir and Freundlich isotherm models have been described for the equilibrium behaviors of the adsorption method. In 1975 Nawlakhe ^[67] and his research team reported Nalgonda procedure, which was named after a village of Andhra Pradesh in India, is based on adsorption of fluoride on flocs of aluminum hydroxide, in which two chemicals, alum, and lime are rapidly mixed with the fluoride-contaminated water followed by succeeding gentle stirring. The developed flocs of aluminum hydroxides take the majority of the dissolved fluoride and are removed later. Afterward, this Nalgonda method was adopted in other nations.

When the fluoride contaminated water passes through a packed column of activated alumina i.e. through the grains of aluminium oxide, contaminants in the water were adsorbed onto the surface of these grains of activated alumina ^{[68, 69, 70]}. Higher defluoridation capacity was reported over a pH range of 4.0–9.0 by using both untreated and thermally treated hydrated alumina ^[71] (Shimelis et al. 2006). The pH plays a valuable role in defluoridation of drinking water. Modification increases the adsorption ability of adsorbent material especially alumina. In this direction, numerous studies have been completed using the manganese oxide in fluoride removal like MnO₂ coated alumina ^{[72, 73, 74, 75]} and MnO₂ coated activated carbon ^{[76, 77]}. Magnesia modified activated alumina ^[78] was also found suitable for the removal of fluoride from water. When the surface of alumina was impregnated with alum, its defluoridation capacity was found 92.6% at pH 6.5 although it decreased further at higher pH conditions ^[66]. A research group has found that the aligned carbon nanotubes which were prepared by the decomposition of xylene using ferrocene as a catalyst, absorb 4.5 mg/g fluoride from 15 mg/L fluoride at pH 7 ^[79]. Li and his group have explained the adsorption of fluoride on alumina supported on carbon nanotubes ^{[80, 81, 82]} and these nanotubes were prepared by the pyrolysis propylene–hydrogen mixture using Nickel as a catalyst. Furthermore, a range of adsorbents have been used for the removal of fluoride from drinking water by the researchers such as activated carbon ^{[83, 84, 85, 86, 87]}, activated clay ^[88] and impregnated alumina [69,89-94], solid industrial wastes such as red mud, spent catalysts and fly ash [95-101]^[95], sorbents[102-104]^[102], alum ^{[105, 106]} and other materials [107-113]^[107] etc.

Ion exchange process removes the fluoride from drinking water up to 90-95% maintaining the taste and color of water. Strongly anion-exchange resin having quaternary ammonium functional groups was employed for the defluoridation purpose ^[5]. Moreover, there are some restrictions with this method like the efficiency of ion-exchange method is reduced due to the presence of other interfering ions and expensive in nature ^[1].

Coagulation and Precipitation method of defluoridation of water have high efficiency and are commercially available methods. Calcium hydroxide and aluminum hydroxide were used in this method ^{[1, 114]} but this method is found as expensive, pH dependent with the presence of other co-ions ^[115].

Membrane filtration technique of defluoridation of water includes the reverse osmosis and nano-filtration methods. Reverse osmosis is a physical process and is capable of removing the contaminants through the semi-permeable membrane after the application of high pressure. Nano-filtration is a comparatively low-pressure technique having slightly bigger pores than membrane process. Although, these are highly selective and efficient methods but have some disadvantages like higher running cost with the production of toxic wastewater ^{[1, 116, 117, 118, 119]}.

The dialysis method separates the solutes through the membrane which possesses the pores as less restrictive in comparison to nano-filtration ^{[42, 43, 120, 121]}. This technique is reported as highly selectively, efficient with higher installation and maintenance cost ^{[1, 115, 118]}.

8. Programmes Adopted for Fluoride Mitigation in India

Definitely, Government may play an important role in fluorosis mitigation. Besides this, multi-institutional linkages will be helpful in implementing the fluorosis mitigation programin the country. Moreover, different projects can be implemented by the government with community participation. The government of India has launched a program named as “National Drinking Water Mission” to provide safe drinking water for the community and also to combat the problem of fluorosis in the country. Fluorosis mitigation project in Dhar District in Madhya Pradesh; Fluorosis mitigation project at Sonbhadra, Uttar Pradesh; Integrated Fluorosis Mitigation, Madhya Pradesh; Fluorosis Mitigation in Nuapada District, Orissa; Sachetana Plus Fluoride Mitigation Project in Karnataka; Project SARITA in Dungarpur of Rajasthan; Mitigation of Fluorosis in Nalgonda District’s Villages etc. are some important programmes which have been conducted by the government.

9. Conclusion

The occurrence, detection and the removal methods of Fluoride in fresh water samples have been studied in the paper. Primarily, the detection of fluoride in water samples either fresh or waste water samples is the most important point which should be considered. Therefore, movable water testing laboratories covering fluoride analysis including cheaper fluoride testing kits are required in rural areas especially in hilly parts which are the places having fresh water resources without any water testing facilities.

The chemical and geographical conditions impact the defluoridation efficiency accordingly. The selected method for defluoridation may vary as per the geological conditions of the area. Therefore, the technology should be adopted using the actual water sample which is to be treated before implementation in the particular area. Fluoride contaminated water creates the health difficulties globally including India. In this paper, we emphasized the various techniques to resolve the problems of higher concentration of fluoride ions in fresh water. Among several defluoridation technologies, adsorption method possesses many advantages over the other methods such as more accessibility, cheaper method, availability of a broad range of adsorptive materials, simple operation etc. Fluoride removal from water samples by activated alumina is accepted treatment technology by the researchers. Therefore, various International Organizations have classified the activated alumina adsorption as one of the best established available technologies for fluoride removal. In the view of above facts, it is proposed that adsorption based techniques are better than other contemporary techniques for fluoride removal from fresh water in urban as well as rural areas. This technique is not only a cost effective method but also don’t need specific instruments and power supply.

Acknowledgements

The authors are thankful to the G.B. Pant Institute of Himalayan Environment and Development (GBPIHED), Kosi-Katarmal, Almora, Uttarakhand (under Ministry of Environment, Forest and Climate Change, Govt. of India) for financial support under Integrated Eco-development Research Programme in the Himalayan Region-NMHS. The author (Bhavtosh Sharma) is specially thankful to Prof. Durgesh Pant, Director USERC for providing the necessary facilities to complete this work.

References

[1]	Habuda-Stanić, M.,  Ergović Ravančić, M. and  Flanagan, A. “A Review on Adsorption of Fluoride from Aqueous Solution”, Materials, 7(9). 6317-6366. 2014.
	In article		View Article
[2]	Fluoride and Fluorides: Environmental Health Criteria 36; World Health Organization (WHO): Geneva, Switzerland, 1984.
	In article
[3]	Fluorides-Environmental Health Criteria 227, World Health Organization (WHO), Geneva, Switzerland, 2002.
	In article
[4]	Ministry of water Resources (MOWR) GOI, New Delhi. 2016. https://wrmin.nic.in/forms/list.aspx?lid=327.
	In article
[5]	Meenakshi and Maheshwari, R.C., “Fluoride in drinking water and its removal”, J. Hazard Materials, B137. 456-463. 2006.
	In article
[6]	Kass, A., Yechieli Gavrieli, Y., Vengosh, A. and Starinsky, A., “The impact of freshwater and wastewater irrigation on the chemistry of shallow groundwater: a case study from the Israeli Coastal aquifer”, J. Hydrol., 1-4. 314-331. 2005.
	In article		View Article
[7]	Azbar, N. and Turkman, A., “Defluoridation in drinking waters”, Water Sci. Technol., 42, 403-407. 2000.
	In article
[8]	Chernet, T., Trafi, Y. and Valles, V., “Mechanism of degradation of the quality of natural water in the lakes region of the Ethiopian rift valley”, Water Res., 35. 2819-2832. 2002.
	In article		View Article
[9]	Agarwal, M., Rai, K., Shrivastav, R. and Dass, S., “Defluoridation of water using amended clay”, J. Cleaner Produc., 11, 439-444. 2003.
	In article		View Article
[10]	Apambire, W. B., Boyle, D. R. and Michel, F. A., “Geochemistry, genesis and health implications of fluoriferous groundwaters in the upper regions of Ghana”, Environ. Geol., 33, 13-24. 1997.
	In article		View Article
[11]	Moturi, W.K.N., Tole, M.P. and Davies, T.C., “The contribution of drinking water towards dental fluorosis: a case study of Njoro division, Nakuru district, Kenya”, Environ. Geochem. and Health, 24, 123-130. 2002.
	In article		View Article
[12]	Hem, J.O., “Study and interpretation of chemical characteristics of natural water”, U. S. Geological Survey Water Supply Paper, p. 1473. 1959.
	In article
[13]	Bulusu, K.R. and Pathak, B.N.. “Discussion on water defluoridation with activated alumina”, J. Environ. Eng. Div., 106(2). 466-469. 1980.
	In article
[14]	Elrashidi, M.A. and Lindsay, W.L., “Solubility of aluminum fluoride, fluorite and fluoriphogopite minerals in soils”, J. Soil Sci. Soc. Am., 50, 594-598. 1986.
	In article		View Article
[15]	Shailaja, K. and Johnson, M.E.C., “Fluorides in groundwater and its impact on health”, J. Environ. Biol., 28, 331-332. 2007.
	In article		PubMed
[16]	Bishnoi, M. and Shalu, A., “Potable groundwater quality in some villages of Haryana, India: focus on fluoride”, J. Environ. Biol., 28. 291-294. 2007.
	In article		PubMed
[17]	Edmunds, W.M. and Smedley, P.L., Groundwater geochemistry and health: an overview, in: Appleton, Fuge, McCall (Eds.), Environmental Geochemistry and Health. Geological Society Special Publication, 113, pp. 91-105. 1996.
	In article		View Article
[18]	Islam, M. and Patel, R.K., “Thermal activation of basic oxygen furnace slag and evaluation of its fluoride removal efficiency”, Chem. Eng. J., 169. 68-77. 2011.
	In article		View Article
[19]	Jamodei, A.V., Sapkal, V.S. and Jamode, V.S., “Defluoridation of water using inexpensive adsorbents”, J. Ind. Inst. Sci., 84. 163-171. 2004.
	In article
[20]	Gonzales, C., Hotokezaka, H., Karadeniz, E.I., Miyazaki, T., Kobayashi, E. and Darendeliler M.A., “Effects of fluoride intake on orthodontically induced root respiration”, Am. J. Ortho Dentofacial Orthop., 139, 196-205. 2004.
	In article		View Article  PubMed
[21]	Nordstrom, D.K. and Jenne, E.A., “Fluorite solubility equilibria in selected geothermal waters”, Geochimica et Cosmochimica Acta, 41, 175-188. 1977.
	In article		View Article
[22]	Handa, B.K., “Geochemistry and genesis of fluoride- containing ground waters in India”, Groundwater, 13. 275-281. 1975.
	In article		View Article
[23]	Edmunds, W.M. and Smedley, P.L., 2005. Fluoride in natural waters. In: Selinus, O. (Ed.), Essentials of Medical Geology. Elsevier Academic Press, London, pp. 301-329.
	In article
[24]	Reddy, D.V., Nagabhushanam, P., Sukhija, B.S., Reddy, A.G.S. and Smedley, P.L., Fluoride dynamics in the granitic aquifer of the Wailapally watershed, Nalgonda district, India”, Chemical Geol., 269. 278-289. 2010.
	In article		View Article
[25]	Deshmukh, A.N., Valadaskar, P.M. and Malpe, D.B., “Fluoride in environment: a review”, Gondwana Geological Magazine, 9, 1-20. 1995.
	In article
[26]	Chae, G.T., Yun, S.T., Kwon, M.J., Kim, S.Y. and Mayer, B., “Batch dissolution of granite and biotite in water: implication for fluorine geochemistry in groundwater”, Geochem. Journal, 40, 95-102. 2006.
	In article		View Article
[27]	Mamatha, P. and Rao, S.M., “Geochemistry of fluoride rich groundwater in Kolar and Tumkur districts of Karnataka”, Environ. Earth Sci., 61, 131-142. 2010.
	In article		View Article
[28]	Banerjee, A., “Groundwater fluoride contamination: A reappraisal”, Geoscience Front.,6(2). 277-284. 2015.
	In article		View Article
[29]	Brunt, R., Vasak, L. and Griffioen, J., Fluoride in Ground water: Probability of occurrence of excessive concentration on global scale, International Groundwater Resources assessment centre, Report nr. SP 2004-2, 2004, Accessed on web on 17. 11. 2016.
	In article
[30]	Bell, M.C. and Ludwig, T.G. The supply of fluoride to man: ingestion from water, in: Fluorides and Human Health, WHO Monograph Series 59, World Health Organization, Geneva. 1970.
	In article
[31]	López, V.A., Reyes, B.J.L., Song, S. and Herrera, U.R., “Temperature effect on the zeta potential and fluoride adsorption at the Al2O3/aqueous solution interface”, J. Colloid Interface Sci., 298(1). 1-5. 2006.
	In article		View Article  PubMed
[32]	Shortt, W.E. Endemic fluorosis in Nellore District, South India. Ind. Med. Gazette, 72-396. 1937.
	In article
[33]	Susheela, A.K., “Epidemiology and Control of Fluorosis in India”, Fluoride, 18(2). 120-21. 1985.
	In article
[34]	Emission Regulations – Part II. (1998). New Delhi: Central Pollution Control Board (India). Pp. 18.
	In article
[35]	Bureau of Indian Standard (BIS), Drinking water specification, BIS-10500. 2012.
	In article
[36]	Guidelines for drinking-water quality. Edn 4, World Health Organization, 2011, https://apps.who.int/iris/bitstream/10665/44584/1/9789241548151_eng.pdf accessed on 15 December, 2016.
	In article
[37]	Ferreira, H.S., Ferreira, S.L.C., Cervera, M.L. and de la Guardia, M., “Development of a non-chromatographic method for the speciation analysis of inorganic antimony in mushroom samples by hydride generation atomic fluorescence spectrometry”, Spectrochim. Acta,Part B, 64. 597–600. 2009.
	In article		View Article
[38]	Bosch, M.E., Sanchez, A.J.R., Rojas, F.S. and Ojeda, C.B. “Arsenic and antimony speciation analysis in the environment using hyphenated techniques to inductively coupled plasma mass spectrometry: a review”, Int. J. Environ Waste Manage., 5. 4-63. 2010.
	In article		View Article
[39]	Torok, P. and Zˇemberyova, M. “Utilization of W/Mg(NO3)2 modifiers for the direct determination of As and Sb in soils, sewage sludge and sediments by solid sampling electrothermal atomic absorption spectrometry”, Spectrochim Acta, Part B 65, 291-296. 2010.
	In article		View Article
[40]	Sharma, B. and Tyagi, S. “Simplification of Metal Ion Analysis in Fresh Water Samples by Atomic Absorption Spectroscopy for Laboratory Students”, J. Lab. Chem. Education, 1(3). 54-58. 2013.
	In article
[41]	Agrahari, S.K., Kumar, S.D. and Srivastava, A.K., “Development of Carbon paste electrode containing benzo-15-crown-5-for trace determination of the uranyl ion by using a voltametric technique”, J. AOAC Int. 92, 241-247. 2009.
	In article		PubMed
[42]	Santos, V.S., Santos, W.J.R., Kubota, L.T., Tarley, C.R.T., “Speciation of Sb(III) and Sb(V) in meglumine antimoniate pharmaceutical formulations by PSA using carbon nanotube electrode”, J. Pharm. Biomed. Anal. 50, 151-157. 2009.
	In article		View Article  PubMed
[43]	Tanguy, V., Waeles, M., Vandenhecke, J. and Riso, R.D., “Determination of ultra-trace Sb(III) in seawater by stripping chronopotentiometry (SCP) with a mercury film electrode in the presence of copper”, Talanta, 81, 614-620. 2010.
	In article		View Article  PubMed
[44]	Sanghavi, B.J. and Srivastava, A.K., “Simultaneous voltammetric determination of acetaminophen, aspirin and caffeine using an in situ surfactant-modified multiwalled carbon nanotube paste electrode”, Electrochim Acta, 55, 8638-8648. 2010.
	In article		View Article
[45]	Shahrokhian, S. and Ghalkhani, M. (). Glassy carbon electrodes modifed with a flm of nanodiamond–graphite/chitosan: Application to the highly sensitive electrochemical determination of Azathioprine . Electrochim Acta, 55, 3621-3627. 2010.
	In article		View Article
[46]	Guide Manual: Water & Waste Analysis, Central Pollution Control Board (CPCB), https://cpcb.nic.in/upload/Latest/Latest_67_guidemanualw&wwanalysis.pdf accessed on 14.12.2016.
	In article
[47]	Singh, P., Dobhal, R., Seth, R., Aswal, R.S., Singh, R., Uniyal D.P. and Sharma B. “Spatial and Temporal Variations in Surface Water Quality of Pithoragarh District, Uttarakhand (India)”, Anal. Chem. Lett., 5(5). 267-290. 2015.
	In article		View Article
[48]	Tyagi, S., Singh, P., Dobhal. R. and Uniyal, D.P., Sharma, B., Singh, R., “Spatial and temporal variations in quality of drinking water sources of Dehradun district in India”, Int. J. Environ. Technol. Manage., 18(5/6). 375-399. 2015.
	In article		View Article
[49]	Tyagi, S., Singh, P., Sharma, B. and Singh, R., “Assessment of Water Quality for Drinking Purpose in District Pauri of Uttarakhand, India”, Appl. Ecol. Environ. Sci., 2(4). 94-99.2014.
	In article
[50]	Neal, M., Neal, C., Wickham, H. and Harman, S., “Determination of bromide, chloride, fluoride, nitrate and sulphate by ion chromatography: comparison of methodologies for rainfall, cloud water and river waters at the Plynlimon catchments of mid-Wales”, Hydrol. Earth Syst. Sci., 11(1). 294-300. 2007.
	In article		View Article
[51]	Sundriyal, M. and Sharma, B., “Status of Biodiversity in Central Himalaya”, Appl. Ecol. Environ. Sci., 4(2). 37-43. 2016.
	In article
[52]	Sharma, B., “Sustainable Drinking Water Resources in Difficult Topography of Hilly State Uttarakhand, India”, Ameri. J. Water Res., 4(1). 16-21. 2016.
	In article
[53]	“Water Resources Management & Treatment Technologies”, edited byBhavtosh Sharma, OP Nautiyal, Durgesh Pant, Published by USERC, Deptt. of Science & Technology, Govt. of Uttarakhand. 2016.
	In article
[54]	Sharma, B., Savera, K.K., Kausik, S., Saini, P., Bhadula, S., Sharma, V. and Singh, P., “Assessment of Ground Water Quality of Bhagwanpur Industrial Area of Haridwar in Uttarakhand, India”, Applied Ecology and Environmental Sciences, 4(4). 96-101. 2016.
	In article
[55]	Engineering Chemistry by Baskar C., Baskar S. and Dhillon R.S., published by Wiley India Pvt. Ltd. 2012.
	In article
[56]	Patil, A.R. and Kulkarni, B.M., “Study of ion exchange regime and activated alumina as defluoridation media”, J. Institution of Engineers (India)”, 03.14-16.1989.
	In article
[57]	Iyenger, L., Small Community Water Supplies: Technology, People and Partnerships. In: Smet J, van Wijk C (ed). Technologies for fluoride removal. Delft, Netherlands. IRC. Pp. 499-514. 2003.
	In article
[58]	Mohapatra, M., Anand, S., Mishra, B.K., Giles, D.E. and Singh, P., “Review of fluoride removal from drinking water”, J. Environ Management. 91. 67-77. 2009.
	In article		View Article  PubMed
[59]	Tomar, V. and Kumar, D.A, “Critical study on efficiency of different materials for fluoride removal from aqueous media”, Chem. Cent. J. 7. 1-15. 2013.
	In article		View Article  PubMed
[60]	Da˛browski, A., “Adsorption from theory to practice”, Advances Colloid Inter. Sci., 93. 135-224. 2001.
	In article		View Article
[61]	Qiu, H., Lv, L., Pan, B., Zhang, Q., Zhang, W., Zhang, Q., “Critical review in adsorption kinetic models”, J. Zhejiang Univ Sci A, 10(5).716-724. 2009.
	In article
[62]	Kumar, S., Gupta, A. and Yadav, J.P., “Fluoride removal by mixtures of activated carbon prepared from Neem (Azadirachta indica) and Kikar (Acacia arabica) leaves”. Ind. J. Chem. Tech, 14. 355-361. 2007.
	In article
[63]	Goswami, A. and Purkait, M.K., “The defluoridation of water by acidic alumina”, Chem Eng Res and Des, 90, 2316-2324. 2012.
	In article		View Article
[64]	Farrah, H., Slavek, J. and Pickering, W.F., “Fluoride interactions with hydrous aluminum oxides and alumina”, Aust. J. Soil Res., 25, 55-69. 1987.
	In article		View Article
[65]	Tripathy, S.S., Bersillon, J.J. and Gopal, K. “Removal of fluoride from drinking water by adsorption onto alum-impregnated activated alumina”, Sep. Purif. Technol., 50. 310-317. 2006.
	In article		View Article
[66]	Ku, Y. and Chiou, H.M., “The adsorption of fluoride ion from aqueous solution by activated alumina”, Water Air Soil Pollut., 133. 349-361. 2002.
	In article		View Article
[67]	Nawlakhe, W.G., Kulkarni, D.N., Pathak, B.N. and Bulusu, K.R., “Defluoridation of water by Nalgonda technique”, Ind. J. Environ. Health, 17. 26-65. 1975.
	In article
[68]	Camacho, L.M., Torres, A., Saha, D. and Deng, S., “Adsorption equilibrium and kinetics of fluoride on sol–gel- derived activated alumina adsorbents”, J. Colloid Interface Sci., 349. 307-313. 2010.
	In article		View Article  PubMed
[69]	Malay, K.D. and Salim, A.J., “Comparative study of batch adsorption of fluoride using commercial and natural adsorbent”, Res. J. Chem. Sci. 1. 68-75. 2011.
	In article
[70]	Tang, Y., Guan, X., Su, T., Gao, N. and Wang, J., “Fluoride adsorption onto activated alumina: modeling the effects of pH and some competing ions”, Colloids Surf, 337, 33-38. 2009.
	In article		View Article
[71]	Shimelis, B., Zewge, F. and Chandravanshi, B.S., “Removal of excess fluoride from water by aluminum hydroxide”, Bull. Chem. Soc. Ethiopia, 20. 17-34. 2006.
	In article
[72]	Maliyekkal, S.M., Sharma, A.K. and Philip, L., “Manganese-oxide-coated alumina: a promising sorbent for defluoridation of water”, Water Res., 40. 3497-3506. 2006.
	In article		View Article  PubMed
[73]	Gadhari, N.S., Sanghavi, B.J., Karna, S.P., Srivastava, A.K., “Potentiometric stripping analysis of bismuth based on carbon paste electrode modified with cryptand [2.2.1] and multiwalled carbon nanotubes”, Electrochim Acta, 56, 627-635. 2010.
	In article		View Article
[74]	Mobin, S.M., Sanghavi, B.J., Srivastava, A.K., Mathur, P. and Lahiri, G.K. “Biomimetic Sensor for Certain Phenols Employing a Copper(II) Complex”, Anal Chem, 82. 5983-5992. 2010.
	In article		View Article  PubMed
[75]	Sivasankar, V., Ramachandramoorthy, T. and Darchenc, A., Manganese dioxide improves the efficiency of earthenware in fluoride removal from drinking water. Desalination, 272. 179-186. 2011.
	In article		View Article
[76]	Shahrokhian, S., Ghalkhani, M., Adeli, M. and Amini, M. K., Multi-walled carbon nanotubes with immobilised cobalt nanoparticle for modifcation of glassy carbon electrode: Application to sensitive voltammetric determination of thioridazine. Biosens Bioelectron, 24. 3235-3241. 2009.
	In article		View Article  PubMed
[77]	Ma, Y., Wang, S.G., Fan, M., Gong, W.X. and Gao, B.Y., “Characteristics and defluoridation performance of granular activated carbon coated with manganese oxides”, J. Hazard. Mater, 168, 1140-1146. 2009.
	In article		View Article  PubMed
[78]	Teng, S.X. and Wang, S.G., “Removal of fluoride by hydrous manganese oxidecoated alumina: Performance and mechanism”, J. Hazard. Mater, 168. 1004-1011. 2009.
	In article		View Article  PubMed
[79]	Li, Y.H., Wang, S.G., Zhang, X.F., Wei, J.Q., Xu, C.L., Luan, Z.K. and Wu, D. H., “Adsorption of fluoride from water by aligned carbon nanotubes”, Mater. Res. Bull., 38. 469-476. 2003a.
	In article		View Article
[80]	Li, Y.H., Wang, S., Zhang, X., Wei, J., Xu, C., Luan, Z., Wu, D. and Wei, B. “Removal of fluoride from water by carbon nanotube supported alumina”, Environ Technol., 24, 391-398. 2003b.
	In article		View Article  PubMed
[81]	Li, Y.H., Wang, S., Cao, A., Zhao, D., Zhang, X., Xu, C., Luan, Z., Ruan, D., Liang, J., Wu, D. and Wie, B. “Adsorption of fluoride from water by amorphous alumina supported on carbon nanotubes”, Chem. Phys. Lett., 322, 1-5. 2001.
	In article		View Article
[82]	Kasprzyk-Hordern, B., Kondakal, V.V.R., Baker, D.R., “Enantiomeric analysis of drugs of abuse in wastewater by chiral liquid chromatography coupled with tandem mass spectrometry” J. Chromatogr. A., 1217, 4575. 2010.
	In article		View Article  PubMed
[83]	Hanumantharao, Y., Kishore, M. and Ravindhranath, K., “Preparation and development of adsorbent carbon from Acacia farnesiana for defluoridation”, Int. J. Plant Anim. Environ. Sci. 1, 209-223. 2011.
	In article
[84]	Karthikeyan, G. and Siva Ilango, S., “Fluoride sorption using Morringa Indica-based activated carbon”, Iran. J. Enviton. Health Sci. Eng. 4. 21-28. 2007.
	In article
[85]	Hernández-Montoya, V., Ramírez-Montoya, L.A., Bonilla-Petriciolet, A., Montes-Morán, M.A., “Optimizing the removal of fluoride from water using new carbons obtained by modification of nut shell with a calcium solution from egg shell”, Biochem. Eng. J., 62, 1-7. 2012.
	In article		View Article
[86]	Sivabalan, R., Rengaraj, S., Arabindoo, B. and Murugesan, V., “Fluoride uptake characteristics of activated carbon from agriculture-waste”, J. Sci. Ind. Res., 61, 1039-1045. 2002.
	In article
[87]	Mariappan, P., Yegnaraman, V. and Vasudevan, T., “Defluoridation of water using low cost activated carbons”, Ind. J. Environ. Protect., 22, 154-160. 2002.
	In article
[88]	Agarwal, M., Rai, K., Shrivastav, R. and Dass, S. “Defluoridation of water using amended clay”, J. Cleaner Produc., 11, 439-444. 2003.
	In article		View Article
[89]	Tripathy, S.S. and Raichur, A.M., “Abatement of fluoride from water using manganese dioxide-coated activated alumina”, J. Hazard. Mater., 153, 1043-1051. 2008.
	In article		PubMed
[90]	Waghmare, S.S. and Arfin, T., “Fluoride Removal from Water by Aluminium Based Adsorption: A Review”, J. Biol. Chem. Chron., 2(1), 1-11. 2015.
	In article
[91]	Khichar, M. and Kumbhat, S., “Defluoridation-A review of water from aluminium and alumina based compound”, Int. J. Chemical Studies, 2(5), 4-11. 2015.
	In article
[92]	Sharma, S.K. and Sharma, M.C., “Application of Biological Adsorbent Materials for Removal of Harmful Inorganic Contaminants from Aqueous Media – An Overview”, Current Trends in Technol Sci, 4. 1. 2015.
	In article
[93]	Das, N., Pattanaik, P. and Das, R., “Defluoridation of drinking water using activated titanium rich bauxite”, J. Colloid Interface Sci., 292. 1-10. 2005.
	In article		View Article  PubMed
[94]	Yang, M., Hashimoto, M.T., Hoshi, N. and Myoga, H., “Fluoride removal in a fixed bed packed with granular calcite”, Water Res., 33, 3395-3402. 1999.
	In article		View Article
[95]	Jagtap, S., Thakre, D., Wanjari, S., Kamble, S., Labhsetwar, N. and Rayalu, S., “New modified chitosan-based adsorbent for defluoridation of water”, J. Colloid Interface Sci. 332. 280-290. 2009.
	In article		View Article  PubMed
[96]	Sujana, M.G., Mishra, A. and Acharya, B.C., “Hydrous ferric oxide doped alginate beads for fluoride removal: Adsorption kinetics and equilibrium studies”, Appl. Surf. Sci. 270. 767-776. 2013.
	In article		View Article
[97]	Davila-Rodriguez, J.L., Escobar-Barrios, V.A. and Rangel-Mendez, J.R., “Removal of fluoride from drinking water by a chitin-based biocomposite in fixed-bed columns”, J. Fluor. Chem. 140, 99-103. 2012.
	In article		View Article
[98]	Swain, S.K., Patnaik, T., Patnaik, P.C., Jha, U., Dey, R.K., “Development of new alginate entrapped Fe(III)–Zr(IV) binary mixed oxide for removal of fluoride from water bodies. Chem. Eng. J., 215-216, 763-771. 2013.
	In article		View Article
[99]	Cengeloglu, Y., Kir, E. and Ersoz, M., “Removal of fluoride from aqueous solution by using red mud”, Sep. Purif. Technol., 28, 81-86. 2002.
	In article		View Article
[100]	Piekos, R. and Paslawaska, S., “Fluoride uptake characteristic of fly ash”, Fluoride, 32. 14-19. 1999.
	In article
[101]	Chaturvedi, A.K., Pathak, K.C. and Singh, V.N., “Fluoride removal from water by adsorption on China clay”, Appl. Clay Sci., 3, 337-346. 1988.
	In article		View Article
[102]	Kamble, S.P., Jagtap, S., Labhsetwar, N.K., Thakare, D., Godfrey, S., Devotta, S., Rayalu, S.S., “Defluoridation of drinking water using chitin, chitosan and lanthanum-modified chitosan”, Chem. Eng., J. 129. 173-180. 2007.
	In article		View Article
[103]	Viswanathan, N. and Meenakshi, S., “Development of chitosan supported zirconium(IV) tungstophosphate composite for fluoride removal”, J. Hazard. Mater. 176. 459-465. 2010.
	In article		View Article  PubMed
[104]	Mohan, S.V., Ramanaiah, S.V., Rajkumar, B. and Sarma, P.N., “Removal of fluoride from aqueous phase by biosorption onto algal biosorbent Spirogyra Sp.-102: sorption mechanism elucidation”, J. Hazard Mater., 141. 465-474. 2007.
	In article		View Article  PubMed
[105]	Miramontes, P., Bautista-Margulis, R.G. and Perez-Hernandeza, A., “Removal of arsenic and fluoride from drinking water with cake alum and a polymeric anionic flocculent”, Fluoride, 36. 122-128. 2003.
	In article
[106]	Sujana, M.G., Soma, G., Vasumathi, N. and Anans, S., “Studies on fluoride adsorption capacities of amorphous Fe/Al mixed hydroxides from aqueous solutions”, J. Fluorine Chem., 130. 749-754. 2009.
	In article		View Article
[107]	Zhang, Yi., Wang, D., Liu, B., Gao, X., Xu, W., Liang, P. and Xu, Y., “Adsorption of Fluoride from Aqueous Solution Using Low-Cost Bentonite/Chitosan Beads”, AmericanJ. Anal Chem, 4. 48-53. 2013.
	In article		View Article
[108]	Khatibikamal, V., Torabian, A., Janpoor, F. and Hoshyaripour, G., “Fluoride removal from industrial wastewater using electrocoagulation and its adsorption kinetics”, J. Hazardous Materials, 179 (1-3). 276-280. 2010.
	In article		View Article  PubMed
[109]	Waghmare, S.S.and Arfin, T., “Fluoride Removal from Water by various techniques: Review” Int. J. Innovative Sci. Engineering Technol., 2 (9). 560-571. 2015.
	In article
[110]	Meenakshi, S. and Viswanathan, N., “Identification of selective ion-exchange resin for fluoride sorption”, J. Colloid Interface Sci., 308. 438-450. 2007.
	In article		View Article  PubMed
[111]	Abe, I., Iwasaki, S., Tokimoto, T., Kawasaki, N., Nakamura, T. and Tanada, S., “Adsorption of fluoride ions onto carbonaceous materials”, J. Colloid Interface Sci., 275. 35-39. 2004.
	In article		View Article  PubMed
[112]	Ramos, R.L., Ovalle-Turrubiartes, J. and Sanchez-Castillo, M.A., “Adsorption of fluoride from aqueous solution on aluminum-impregnated carbon”, Carbon, 37. 609-617. 1999.
	In article		View Article
[113]	Jagtap, S., Thakre, D., Wanjari, S., Kamble, S., Labhsetwar, N. and Rayalu, S., “New modified chitosan-based adsorbent for defluoridation of water”, J. Colloid Interface Sci. 332. 280-290. 2009.
	In article		View Article  PubMed
[114]	Potgeiter, J.H., “An experimental assessment of the efficiency of different defluoridation methods”, Chem, SA 317-318. 1990.
	In article
[115]	Piddennavar, R., “Review on defloridation techniques of water”, Int. J. Eng. Sci. 2, 86-94. 2013.
	In article
[116]	Maheswari, R.C. and Hoelzel, G., “Potential of membrane separation technology for fluoride removal from underground water”, Proceedings of the Water Environment Federation, 17. 620-636. 2002.
	In article
[117]	Bhatnagar, A., Kumar, E. and Sillanpää, M., “Fluoride removal from water by adsorption—A review”, Chem. Eng. J., 171. 811-840. 2011.
	In article		View Article
[118]	Chakrabortty, S., Roy, M. and Pal, P., “Removal of fluoride from contaminated groundwater by cross flow nanofiltration: Transport modeling and economic evaluation”, Desalination, 313, 115-124. 2013.
	In article		View Article
[119]	Seadar, J.D. and Heneley, J.E., “The Separation Process Principles”, second ed., NJ: Wiley, pp. 521-523. 2005.
	In article
[120]	Bosch, M.E., Sanchez, A.J.R., Rojas, F.S. and Ojeda, C.B., “Arsenic and antimony speciation analysis in the environment using hyphenated techniques to inductively coupled plasma mass spectrometry: a review”, Int. J. Environ Waste Manage., 5. 4-63. 2010.
	In article		View Article
[121]	Khani, H., Rofouei, M.K., Arab, P., Gupta, V.K. and Vafaei, Z., “Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: application to potentiometric monitoring of mercury ion (II)”, J. Hazard. Material, 183. 402-409. 2010.
	In article		View Article  PubMed