Excess fluoride (F?) in drinking water leads to detrimental health effects including dental and skeletal fluorosis. More than 260 million people worldwide are affected by excess fluoride (>1.5 mg/L) in their drinking water. In this three-phase study, we have used modified activated carbon and cerium oxide nanomaterials for aqueous fluoride removal. The overarching goal of this research was to develop cost-effective drinking water fluoride removal technologies.
In Phase I, a citric acid (0.3 M) modified granular activated carbon (CAGAC) was effectively used to remove >70% of aqueous fluoride within 60 min. The maximum adsorption capacity of CAGAC was two times (1.65 mg/g) that of unmodified GAC (0.88 mg/g). To address the need for defluoridation technologies adaptable in rural and socio-economically challenged communities, commonly available lime (Citrus aurantiifolia) juice was used in lieu of citric acid in Phase II of this research. The lime modified GAC (LGAC) showed an adsorption capacity of 1.63 mg/g. Both CAGAC and LGAC worked effectively over a wide range of pH (4-8) even though the point-of-zero-charge (PZC) was 4.89 for CAGAC and 3.05 for LGAC indicating that the fluoride removal was not controlled by electrostatic interaction alone, both surface adsorption and intra-particle diffusion also took part.
In Phase III, graphene oxide-ceria (GO-CeO2) nanohybrid was used for fluoride removal as the activated carbon-based systems were slow in kinetics. The nanohybrid exhibited ultra-rapid kinetics for fluoride removal. The equilibrium (85% removal of 10 mg F?/L) was achieved within 1 minute which is the fastest kinetics for fluoride removal reported so far. The maximum fluoride adsorption capacity of GO-CeO2 nanohybrid was 8.61 mg/g at pH 6.5 and that increased to 16.05 mg/g at pH 4. The experimental results and characterization data indicated that both electrostatic interaction and surface complexation participated in the fluoride removal process. The oxygen ions present in CeO2 lattice were replaced by fluoride ions to make a stable CeF3 complex. During fluoride removal, the GO sheets acted as electron mediators and helped to reduce Ce4+ to Ce3+ at the CeO2 NPs-GO interface, and the additional Ce3+ enhanced fluoride removal by the nanohybrid
