Layered double hydroxides (LDHs) are potential low-cost filter materials for F removal for drinking water, but the molecular scale defluoridation mechanisms were lacking. In this research, we employed 19F solid-state NMR spectroscopy to identify F sorption products on MgAl LDH and to reveal its relationship with LDH structure. A set of six 19F NMR peaks centered at -139, -147, -155, -164, -176 and -183 ppm were resolved. Combining quantum chemical calculations based on density function theory (DFT) and 19F{27Al} Transfer of Populations in Double Resonance (TRAPDOR) analysis, we could assign the peaks at -139, -147 and -164 ppm to &gt;Al-F, -155 ppm to interlayer F, and -176 and -183 ppm to &gt;Mg-F. Interestingly, we found that the interlayer F is not predominant and insensitive to pH, which accounts for ~20-30% of total sorbed F across a wide pH range. In contrast, the formation of &gt;Al-F predominates F sorption at low pH whereas the formation of &gt;Mg-F favors at high pH (or higher Mg/Al ratio). This supported the fact that 4:1 MgAl LDH performed better than 2:1 LDH at pH ≥ 9. Overall, we explicitly revealed the different roles of &gt;Mg-OH and &gt;Al-OH sites of LDHs on F removal, which explained why classic activated alumina has limitations in defluoridation at high pH. Findings from this research may also provide new insights into material screen for potential filters for F removal at alkaline conditions.

Enhanced F uptake by layered double hydroxides at alkaline conditions: Solid-state NMR evidence of the role of surface >Mg-OH sites

ACS Fall 2021

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