- Fluoride level in natural waters can be exceedingly high reaching 2800 ppm F–.
- Contaminated water and diseases caused by fluoride are prevalent worldwide.
- Recent fluoride removal studies are mostly on the bench-scale phase.
- Investigated techniques usually omit high F– concentration wastewater (>100 ppm).
- Life cycle assessment and cost evaluation for defluoridation are deemed vital.
High levels of fluoride, though, naturally occurring (which can reach as high as 2800 mg F–/L) in the environment can be toxic to various living organisms. Moreover, it can be transported by water and by its confluences and exacerbated by anthropogenic activities making it an environmental and public health concern. World Health Organization has set the standard for drinking water at 1.5 mg F–/L while the average national effluent standard is 15 mg F–/L. Hence, different defluoridation techniques of aqueous solutions were developed in the past years. This study provides an overview of the popular methods in defluoridation (i.e. adsorption, ion-exchangers, precipitation, membrane, electrocoagulation, and electro-dialysis). The mechanisms, critical operational conditions, and research progress are presented. The results further reveal that adsorption, regarded as the primary technique for defluoridation, still needs further development and mostly on its bench-scale and is only proven effective at low initial concentrations. In this study, sorption techniques are also estimated to be 10 to 20 times more expensive in operational costs relative to the other treatments. Furthermore, the majority of the examined literature demonstrated defluoridation at limited initial concentration <100 mg F–/L. In contrast, industrial effluents may reach 250–1000 mg F–/L (up to 10,000 mg F–/L at extreme cases). Inadequate removal of fluoride in water by single treatment also compels researchers to explore hybrid treatments. In addition, due to lack of wastewater treatment facilities requiring high capital cost, bioremediation, a commonly overlooked alternative, is presented for temporarily alleviating fluoride levels. Finally, challenges such as limited literature for disposal of secondary pollution and cost evaluation along with other potential research perspectives are further discussed.
*Original abstract online at https://www.sciencedirect.com/science/article/abs/pii/S0959652620342815