Fluoride removal from drinking water

Abstract

Fluoride in drinking water exerts bidirectional effects on teeth and bones within specific concentration ranges. Low-concentration fluoride (1–1.5 mg/L) enhances the anti-caries resistance of dental enamel. However, concentrations between 1.5–4 mg/L cause enamel lesions known as dental fluorosis. Prolonged exposure to higher fluoride levels (4–10 mg/L) may progress from dental fluorosis to more severe skeletal fluorosis. Globally, particularly in parts of the United States, Africa, and Asia, groundwater fluoride concentrations are often elevated, sometimes exceeding 30 mg/L, posing potential threats to public health. This review systematically summarizes research progress in drinking water defluoridation, focusing on two main approaches: membrane separation technology and adsorption technology. The membrane technology section covers methods such as reverse osmosis, nanofiltration, dialysis, and electrodialysis. Adsorption technology, as a traditional defluoridation method, centers on the application of adsorbents including alumina/aluminum-based materials, clays and soils, calcium-based minerals, synthetic compounds, and carbon-based materials. The article also reviews studies on fluoride removal from aqueous solutions using natural zeolites, modified zeolites, and cross-linked polystyrene-based ion exchange resins. In recent years, layered double metal oxides (LDMOx) have shown promising prospects as emerging adsorbents for fluoride removal, and this paper briefly discusses the latest developments in this area.

Introduction
Fluorine primarily exists in nature within minerals such as fluorite, fluorite, cryolite, and fluorapatite. These minerals are poorly soluble in water, so fluoride in groundwater typically originates from suitable geochemical dissolution conditions or industrial discharge of fluoride-containing wastewater.

The mechanism by which fluorides affect teeth and bones lies in their ability to replace hydroxyl groups in hydroxyapatite, forming fluorapatite. Moderate fluoride levels (around 0.7 mg/L) enhance enamel hardness; but excessive fluoride (above 1.5 mg/L) causes tissue overmineralization, leading to brittle teeth and bones, and resulting in dental fluorosis and skeletal fluorosis. The World Health Organization and many countries set the safe limit for fluoride in drinking water at 1.5 mg/L. However, due to the narrow margin between fluoride's benefits and toxicity, its precise safety threshold remains to be determined.

With industrial development, pollution from high-fluoride water bodies has become increasingly severe. Globally, over 260 million people regularly consume freshwater with excessive fluoride levels, with tropical regions facing heightened risks due to higher water consumption. Therefore, developing efficient and economical defluoridation technologies is crucial. This paper aims to systematically review existing defluoridation methods and research trends, providing references for scientific exploration and technological applications in this field.

Chapter Excerpt
Fluoride Removal Methods in Aqueous Solutions
The objective of fluoride removal is to reduce water fluoride concentrations below safety standards. Current technologies primarily fall into two categories: membrane separation and adsorption methods. (Industrial high-fluoride wastewater often undergoes pretreatment using calcium/magnesium/barium salt precipitation to remove fluoride as insoluble fluorides; however, this method is generally unsuitable for advanced drinking water treatment.)

Conclusion

This paper reviews primary fluoride removal technologies for drinking water. Membrane separation techniques (including reverse osmosis, nanofiltration, dialysis, and electrodialysis) and adsorption methods (involving aluminum-based materials, clay minerals, calcium-based compounds, synthetic adsorbents, and carbon materials) represent the two dominant approaches. Zeolites and ion exchange resins also find application in fluoride removal. Notably, novel adsorbents like layered double metal oxides are gaining prominence due to their high selectivity and ease of regeneration. Future fluoride removal technologies must continue balancing high efficiency, low cost, environmental friendliness, and operational simplicity.

Reference:https://www.culligan.com/blog/how-to-remove-fluoride-from-water