Preparation, Characterization, and Regeneration of Aluminum (Hydr)Oxide–Amended Molecular Sieves for Fluoride Removal from Drinking Water

Molecular sieves and zeolites showed increased fluoride adsorption capacities when amended with aluminum (hydr)oxide (AlOOH). When normalized by the AlOOH content, the adsorption capacities of most amended molecular sieves were higher than the maximum theoretical value expected for monolayer surface coverage, suggesting fluoride removal from processes beyond adsorption, such as precipitation. Although the mass-normalized adsorption capacities of most amended materials were less than that of an equivalent mass of pure AlOOH, several molecular sieves with pores of 1 to several nanometers showed mass-normalized adsorption capacities similar to pure AlOOH, possibly due to their larger pores, which may have facilitated fluoride adsorption after aluminum (hydr)oxide precipitation. Energy-dispersive X-ray spectroscopy detected elevated fluorine in a representative AlOOH-amended molecular sieve after repeated fluoride adsorption, also consistent with fluoride uptake by processes beyond only monolayer coverage. Regeneration of the adsorbents with low-concentration sodium hydroxide solution led to partial recovery of the fluoride adsorption capacity, which decreased over the course of sequential adsorption batch experiments, possibly due to loss of aluminum.