Review
Fluoride removal from water by adsorption—A review

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Abstract

Fluoride contamination in drinking water due to natural and anthropogenic activities has been recognized as one of the major problems worldwide imposing a serious threat to human health. Among several treatment technologies applied for fluoride removal, adsorption process has been explored widely and offers satisfactory results especially with mineral-based and/or surface modified adsorbents. In this review, an extensive list of various adsorbents from literature has been compiled and their adsorption capacities under various conditions (pH, initial fluoride concentration, temperature, contact time, adsorbent surface charge, etc.) for fluoride removal as available in the literature are presented along with highlighting and discussing the key advancement on the preparation of novel adsorbents tested so far for fluoride removal. It is evident from the literature survey that various adsorbents have shown good potential for the removal of fluoride. However, still there is a need to find out the practical utility of such developed adsorbents on a commercial scale, leading to the improvement of pollution control.

Highlights

► Fluoride removal from water using ‘adsorption’ process has been reviewed. ► A list of ∼ 100 adsorbents has been presented along with their adsorption capacities. ► The influence of various parameters on fluoride removal has also been presented. ► Recently developed novel adsorbents for defluoridation have been discussed.

Introduction

Fluoride (F) contamination in groundwater has been recognized as one of the serious problems worldwide [1]. Fluoride is classified as one of the contaminants of water for human consumption by the World Health Organization (WHO), in addition to arsenic and nitrate, which cause large-scale health problems [2]. Elevated fluoride concentrations in the groundwater occur in various parts of the world [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Fluoride is widely distributed in the geological environment [13] and generally released into the groundwater by slow dissolution of fluorine-containing rocks [14]. Various minerals, e.g., fluorite, biotites, topaz, and their corresponding host rocks such as granite, basalt, syenite, and shale, contain fluoride that can be released into the groundwater [15], [16], [17]. Thus, groundwater is a major source of human intake of fluoride. Besides the natural geological sources for fluoride enrichment in groundwater, various industries are also contributing to fluoride pollution to a great extent [18]. The industries which discharge wastewater containing high fluoride concentrations include glass and ceramic production, semiconductor manufacturing, electroplating, coal fired power stations, beryllium extraction plants, brick and iron works, and aluminium smelters [19]. The effluents of these industries have higher fluoride concentrations than natural waters, ranging from ten to thousands of mg/L [20]. It is estimated that more than 200 million people worldwide rely on drinking water with fluoride concentrations that exceed the WHO guideline of 1.5 mg/L [21]. Depending on the concentration and the duration of continuous uptake, the impact of fluoride in drinking water can be beneficial or detrimental to mankind. Fluoride in drinking water has a narrow beneficial concentration range in relation to human health. Small amounts in ingested water are usually considered to have a beneficial effect on the rate of occurrence of dental caries, particularly among children [22]. On the contrary, excess intake of fluoride leads to various diseases such as osteoporosis, arthritis, brittle bones, cancer, infertility, brain damage, Alzheimer syndrome, and thyroid disorder [23], [24]. Fluorosis is a common symptom of high fluoride ingestion manifested by mottling of teeth in mild cases and embrittlement of bones and neurological damage in severe cases [25]. There are some reports indicating that fluoride may interfere with DNA synthesis [26]. The excess concentrations of fluoride can also interfere with carbohydrates, lipids, proteins, vitamins and mineral metabolism [27]. Fluoride toxicity may occur by several ways. While ingested, fluoride initially acts locally on the intestinal mucosa, it can later form hydrofluoric acid in the stomach, which leads to gastro-intestinal irritation or corrosive effects [27]. Following ingestion, the gastro-intestinal tract is the earliest and most commonly affected organ system. Fluoride can also interfere with a number of enzymes disrupting oxidative phosphorylation, glycolysis, coagulation, and neurotransmission [27]. It is well recognized that individuals with kidney disease have a heightened susceptibility to the cumulative toxic effects of fluoride [28]. In addition, fluoride has been shown to poison kidney function at high doses over short-term exposures in both animals and humans [28]. It has also been concluded by several research groups that fluoride has the ability to interfere with the functions of the brain and pineal gland [28]. Pineal gland is a major site of fluoride accumulation within the body, with higher concentrations of fluoride than either teeth or bone [28].Fluoride exposure has also been linked to bladder cancer—particularly among workers exposed to excess fluoride in the workplace [28]. Thyroid activity is also known to be influenced by fluoride. Keeping the view of toxic effects of fluoride on human health, there is an urgent need to find out an effective and robust technology for the removal of excess fluoride from drinking water.

Section snippets

Technologies for fluoride removal from water

The traditional method of removing fluoride from drinking water is liming and the accompanying precipitation of fluorite [29]. The precipitation and coagulation processes with iron(III) [30], activated alumina [31], alum sludge [32] and calcium [33] have been widely investigated. In addition, ion exchange [34], [35], [36], [37], [38], reverse osmosis [39], [40] and electrodialysis [41] have also been studied for the removal of excess amounts of fluoride from drinking water. However, the

Activated alumina

Activated alumina has been an adsorbent of interest for years among the researchers for fluoride removal. Farrah et al. [47] studied the fluoride ion interaction with amorphous Al(OH)3, gibbsite or alumina (Al2O3) over a wide pH range (3–8) and F concentrations (0.1–1.0 mM). It was found that at pH < 6 and total F:Al ratios >2.5, most of the amorphous AI(OH)3 gel dissolved through the formation of AlF complexes, with the distribution of fluoro ions being determined by the equilibrium F value.

Conclusions and future perspectives

This review has attempted to cover a wide range of adsorbents which have been used so far for the removal of fluoride from the water and wastewater. Based on the literature reviewed, the following concluding remarks can be made:

  • Removal of fluoride by activated alumina is an established treatment technology. The WHO and USEPA classify activated alumina adsorption as one of the best demonstrated available technology (BDAT) for fluoride removal [30]. Although activated alumina is a robust

Acknowledgments

We wish to thank all the anonymous reviewers whose comments/suggestions have significantly improved the quality of this manuscript. Amit Bhatnagar acknowledges his post-doctoral scholarship (FCT-DFRH-SFRH/BPD/62889/2009) supported by the Portuguese Foundation for Science and Technology (FCT).

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