ReviewReview of fluoride removal from drinking water
Introduction
The fluoride occurs mainly as sellaite (MgF2), fluorspar (CaF2), cryolite (Na3AlF6) and fluorapatite [3Ca3(PO4)2 Ca(F,Cl2)]. As fluorspar it is found in sedimentary rocks and as cryolite in igneous rocks. These fluoride minerals are nearly insoluble in water. Hence fluorides will be present in groundwater only when conditions favour their dissolution or high fluoride containing effluents are discharged to the water bodies from industries.
Fluoride in drinking water has a profound effect on teeth and bones. Fluoride displaces hydroxide ions from hydroxyapatite, Ca5(PO4)3OH, the principal mineral constituent of teeth (in particular the enamel) and bones, to form the harder and tougher fluoroapatite, Ca5(PO4)3F. Up to a small level this strengthens the enamel. However, fluoroapatite is an order of magnitude less soluble than hydroxyapatite, and at high fluoride concentration the conversion of a large amount of the hydroxyapatite into fluoroapatite makes the teeth and (after prolonged exposure) the bones denser, harder and more brittle. In the teeth this causes mottling and embrittlement, a condition known as dental fluorosis. With prolonged exposure (Dissanayake, 1991) at higher fluoride concentrations dental fluorosis progresses to skeletal fluorosis (Table 1). Fluoride is thus considered beneficial in drinking water at levels of about 0.7 mg/L but harmful once it exceeds 1.5 mg/L which is the World Health Organisation limit being followed in most of the nations (WHO, 1985, Smet, 1990) and is also the Australian recommended limit (NHMRC, 2004). The difference between desirable doses and toxic doses of fluoride is ill-defined, and fluoride may therefore be considered as an essential mineral with a narrow margin of safety (WHO, 1984).
With the increase in industrial activities water bodies with excess levels of fluoride are becoming a matter of great concern. High fluoride concentrations in groundwater, up to more than 30 mg/L, occur widely, notably in the United States of America, Africa and Asia (Czarnowski et al., 1996, Azbar and Turkman, 2000, Wang et al., 2002, Agarwal et al., 2003, Moges et al., 1996, Gaciri and Davies, 1992, Chernet et al., 2002, Mjengera and Mkongo, 2002, Moturi et al., 2002, Apambire et al., 1997). Long back it was estimated (WHO, 1984) that more than 260 million people worldwide consume drinking water with a fluoride content of >1.0 mg/L. The majority of these people live in tropical countries where the problem is exacerbated by the need to drink more water because of the heat. It is thus absolutely essential to bring down the fluoride levels to acceptable limits for which tremendous research and development efforts are being put all over the world. The present paper reviews the techniques available and ongoing efforts for fluoride removal from drinking water.
Section snippets
Methods of defluoridation from aqueous solutions
The objective in fluoride removal is to treat the contaminated water so as to bring down fluoride concentration to acceptable limits. The defluoridation techniques can be broadly classified into two categories, namely membrane and adsorption techniques. (High concentrations of fluoride in industrial effluents are usually brought down to ∼ 30 mg/L following precipitation method making use of calcium/magnesium/barium hydroxide slurry to reject fluoride as CaF2, MgF2 or BaF. This method is not
Conclusion
A brief review on fluoride removal for drinking water has been presented. The fluoride removal methods have been broadly divided in two sections dealing with membrane and adsorption techniques. Reverse osmosis, nanofiltration, dialysis and electro-dialysis have been discussed under membrane techniques. Adsorption which is a conventional technique deals with adsorbents such as: alumina/aluminium based materials, clays and soils, calcium based minerals, synthetic compounds and carbon based
Acknowledgements
The authors are thankful to Prof. B.K. Mishra, Director, Institute of Minerals and Materials Technology for his kind permission to publish this paper. The authors are thankful to Department of Science and Technology, India and DEST, Australia for financial support under INDO-AUS Strategic Research Fund Scheme.
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