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 F-19 solid-state NMR, X-ray photoelectron spectroscopy (XPS), and high-resolution TEM. Under calcination process, LDH underwent three periods: surface dehydration below 200 degrees C, structural dehydroxylation at 200-400 degrees C, and release of interlayer carbonate groups above 400 degrees 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 degrees 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-19 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.

Automated summary

This study investigated the mechanisms of F- adsorption by calcined LDHs (CLDH) using various analytical techniques. The results showed that calcination greatly enhanced the F- affinity of LDH, especially at high pH. The study also revealed the irreversible changes in LDH structure caused by calcination and the decrease in crystallinity with increasing calcination temperature.