Molecular scale assessment of defluoridation of coal-mining wastewater by calcined Mg/Al layered double hydroxide using 19F solid-state NMR, XPS, and HRTEM

Calcination is an effective way to improve the F − adsorption capacity of layered double hydroxide (LDH) materials, however, a molecular scale understanding of the enhanced defluoridation capability of calcined LDHs (CLDH) is lacking. This study investigated the mechanisms of F − adsorption by CLDH using 19 F solid-state NMR, X-ray photoelectron spectroscopy (XPS), and high-resolution TEM . Under calcination process, LDH underwent three periods: surface dehydration below 200 °C, structural dehydroxylation at 200–400 °C, and release of interlayer carbonate groups above 400 °C. Additionally, XPS and XRD characterization showed that CLDH could not recover to the original structural symmetry even after rehydration and reconstitution. The F − affinity was greatly enhanced for the calcined LDH, especially at high pH. At pH 10, the adsorption capacity could reach 22.0 mg F − /g for CLDH (500 °C calcined), about 6 times larger than that of LDH. The XRD analyses revealed that the F-adsorbed CLDH had a poorer crystalline degree as the calcination temperature increased, consistent with the TEM observation of abundant defects and Mg/Al oxides on the CLDH sheets. 19 F solid-state NMR spectra of the CLDH after F − adsorption showed that the formation of surface Al–F is the predominant F − adsorption mode at pH 7, whereas the Mg–F local coordination mode is the pronounced F − adsorption mechanism under alkaline conditions (pH 10). The present study provided a comprehensive understanding of CLDH in F − adsorption and suggested that calcination is a promising treatment for promoting the efficacy of polluted anion scavenging.