Highlights

• Overexposure to fluoride via drinking water causes several health effects including fluorosis
• Endemic fluorosis is still persisted in several countries even with advancement in research
• Most of fluorosis management techniques suggested in the past have come with their own drawbacks
• Defluoridation techniques based on aluminium materials pose serious health risks to the public
• A method which removes excess F- from drinking water without affecting water quality has a scope

Excerpt:

5.2. Defluoridation techniques

Among the defluoridation methods developed to overcome the problem of excess F^– in drinking water, the Nalgonda technique, the use of activated alumina, and reverse osmosis are very well employed to bring down the F^– concentration within the desirable limit. Although these defluoridation methods can successfully remove excess F- reduce it well below the acceptable limit, these methods are not feasible in the actual fields due to several drawbacks. For example, the Nalgonda technique is based on a precipitation process that requires careful monitoring of residual alkalinity and concentrations of Al³⁺ and SO ions in defluoridated water, which exceed desirable limits (Meenakshi and Maheshwari, 2006). Similarly, using activated alumina for defluoridation, residual aluminium concentration in treated water exceeds its permissible limit (Shreyas et al., 2013). Besides, this technique requires either periodic regeneration or disposal of spent alumina. More concerning issue of using defluoridation methods based on aluminium materials is that presence of any residual aluminium along with F^– in treated water forms fluoroalumino complexes (AlF_x) due to the strong affinity of Al³⁺ for F^–. These Al-F complexes are known to enhance the accumulation of both F^– and Al³⁺, and cause neurotoxic health effects (Wasana et al., 2015). This suggests that adopting a defluoridation method based on Al³⁺ materials may pose additional adverse health effects on the consumers that may worsen compared to the presence of F- in drinking water. To overcome drawbacks associated with alumina and its derivatives, several other materials for F^– removal are proposed in the literature (Bhatnagar et al., 2011), and these materials are based on the adsorption technique. This adsorption process is reported to have higher removal capacities compared to the Nalgonda technique. Further, the adsorption technique is also economically feasible and easy to operate. However, reports published on field studies are limited. However, this technique produces excess sludge, which has to be disposed of or regenerated (Bhatnagar et al., 2011, Shreyas et al., 2013). But disposing of spent adsorbents causes more harm to the environment as it contains dangerously high amounts of fluoride. Thus exploring environmentally safe routes for sludge disposal or using this F^–-bearing sludge for alternate use needs to be considered while evaluating an adsorption technique for defluoridation of drinking water. In addition, this methodology is also pH and temperature-sensitive (Alkurdi et al., 2019, Alhassan et al., 2020, Hegde et al., 2020). The ion-exchange process is another high-performance (95 %) defluoridation technique that uses ion-exchange resin for the removal of F- This technique is not extensively employed since the demerits outweigh merits viz. highly expensive and cannot be implemented in remote areas. The membrane-based techniques: reverse osmosis and nanofiltration, face the same issue. Despite this, these techniques are considered the most efficient among all due to their ease of operation, quality of treated water and high durability. However, because of their prohibitive set-up cost, removal of essential minerals and difficulty in managing brine/retentate, they are not a popular choice (Damtie et al., 2019). Similarly, electrocoagulation and electrodialysis are electrochemical-based techniques that are considered highly desirable. The electrodialysis technique is not only used for fluoride removal but also for other contaminants from aqueous media. A major disadvantage of this technique is that a high amount of electricity is required for its operation (Haldar and Gupta, 2020), which is not easily available in several underdeveloped and developing regions. The electrocoagulation process, similar to the Nalgonda technique, produces aluminium complexes after its operation and problem associated with sludge disposal exists.

From the above-adduced facts, there is a necessity to develop a fluorosis management technique that is technically and economically feasible to implement in the affected areas. Particularly, the fluorosis technique would be implemented that should at least selectively remove excess F^– from drinking water without compromising with other water quality parameters. In this direction, a few of the defluoridation techniques, those based on non-toxic elements such as calcium and magnesium, have found to be potential techniques and shown promising defluoridation capacities (Islam and Patel, 2007, Pemmaraju and Rao, 2011, MacDonald et al., 2011, Mourabet et al., 2012, Khare et al., 2019, Sankannavar and Chaudhari, 2019). However, the safe disposal of resulting F^–-bearing materials is another problem that demands research.

Graphical Abstract