Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites

Abstract

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed 19F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six 19F NMR peaks centered at −140, −148, −156, −163, −176, and −183 ppm was 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 −140, −148, −156, and −163 ppm to Al–F (F coordinated to surface Al) and those at −176 and −183 ppm to Mg–F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al–F is the predominant mode of F– sorption at low pH, whereas the formation of Mg–F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F– uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F– removal under alkaline conditions.

Fluorine speciation and origin of Early-Middle Triassic bentonite deposits in Sichuan Basin, South China

“…Farrah et al (1987) confirmed the capacity of Al(OH) 3 to scavenge F − from aqueous phases from pH 4 to 7, and the oxides dissolve and form F-Al complexes under strongly acidic conditions. Ren et al (2021) suggested that F uptake by layered double hydroxides is enhanced under alkaline conditions.…”

Kinetics, isotherm, mechanism, and recyclability of novel nano-sized Ce4+-doped Ni–Al layered double hydroxide for defluoridation of aqueous solutions

“…Fluoride ions may form hydrogen bonds with the hydroxyl groups on the LDH surface. This type of interaction can contribute to the adsorption of uoride and the subsequent changes observed in the ATR-IR spectrum (Ren et al 2021). Furthermore, the Ni-Al-Ce LDH nanomaterial demonstrated good recyclability, maintaining 71% of its uoride adsorption capacity after four consecutive cycles.…”

“…The layered structure of Ce-doped Ni-Al LDH nanoparticles provides a large surface area for uoride adsorption. The mechanism might involve the interaction between F − and the active sites of the LDH surface(Ren et al 2021). LDH surface sites + F → LDHF (Surface adsorption)The incorporation of Ce 4+ ions into the LDH lattice enhances the surface reactivity and increases the binding sites for uoride ions.Reusability of Ce-doped Ni-Al LDH in Fluoride AdsorptionWe used 0.1 M NaOH solution as a desorption agent to regenerate Ce 4+ doped Ni-Al LDH after uoride adsorption from aqueous solution.…”

Controllable synthesis of layered double hydroxide nanosheets to build organic inhibitor-loaded nanocontainers for enhanced corrosion protection of carbon steel

“…Layered double hydroxides (LDHs), also known as hydrotalcitelike compounds, have been widely studied due to their lamellar structure and anion exchange properties, which make them highly versatile for various applications. [1][2][3] Their structure includes the positive charge of hydroxide layers formed by the hydroxide of divalent and trivalent cations and the negative charge of counter anions, which neutralize the total charge of LDHs and permit the hydroxide layers to stack on top of each other by electrostatic forces. 4,5 LDHs are represented by the formula [M 1−x 2+ M x 3+ (OH) 2 ](A n− ) x/n $yH 2 O, where M 2+ is a divalent metal cation (Mg 2+ , Zn 2+ , Ca 2+ , etc.…”