Fluoride-containing water: A global perspective and a pursuit to sustainable water defluoridation management -An overview

High levels of fluoride, though, naturally occurring (which can reach as high as 2,800 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 250–1,000 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 the 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.