Revisiting regeneration performance and mechanism of anion exchanger-supported nano-hydrated zirconium oxides for cyclic water defluoridation

• Regeneration of F-loaded HZO@D201 nanocomposite was systematically investigated. • 1% NaOH-2% NaCl binary solution fully refresh F-loaded HZO@D201 within 10 min. • Regeneration was dominated by ligand exchange between fluoride and hydroxide ions. • Desorption reagent was recycled for cyclic runs with constant desorption efficiency. Nano-hydrous zirconium oxide (HZO) was among the most attractive adsorbents for water defluoridation in terms of environmental friendliness, high capacity and water insolubility. However, the systematic investigation on regeneration of the exhausted nano-HZO, which is crucial to the potential application, is rarely reported. Herein, the nanocomposite HZO@D201 containing nano-HZO for inner-sphere complexation with fluoride, and the strongly basic anion exchanger D201 host for electrostatic attraction, was adopted to investigate the regeneration performance and mechanism. The NaOH-NaCl binary solution was employed as the regenerant. In specific, NaOH was responsible for desorbing fluoride from nano-HZO through surface deprotonation reaction, and NaCl for suppressing electrostatic attraction. By utilizing the response surface methodology, the desorption efficiency of the fluoride-loaded HZO@D201 (denoted as F-HZO@D201) exceeded 95% after treatment with 1% NaOH-2% NaCl for 10 min in the batch assay. As a comparison, the treatment with 5% NaOH-5% NaCl for 24 h was adopted as the empirical regeneration condition previously. The desorption efficiency reached ∼100% with 2-bed volume regenerant in the column test. The adsorption isotherm plots of the desorbed F-HZO@D201 overlapped with the pristine HZO@D201, demonstrating that F-HZO@D201 was fully refreshed. Evolution of crystalline structure of HZO, XPS F1s spectra, FT-IR spectra, and concentration of each reagent was monitored, suggesting ligand exchange between fluoride and hydroxide ion as the main desorption mechanism. After adding CaCl 2 and NaOH to remove fluoride and to restore the alkali concentration, respectively, the used regenerant was recycled in a 5-cyclic regeneration run with constant desorption efficiency (>95%).