Molecular scale assessment of defluoridation of coal-mining wastewater by calcined Mg/Al layered double hydroxide using F 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 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. 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.