Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Road, Nanjing, 210023, China; Research Center for Environmental Nanotechnology (ReCENT), Nanjing University, 163 Xianlin Road, Nanjing, 210023, China; Frontiers Science Center for Critical Earth Material Cycling (FSC-CEMaC), Nanjing University, Nanjing, 210023, China.
Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Road, Nanjing, 210023, China; Frontiers Science Center for Critical Earth Material Cycling (FSC-CEMaC), Nanjing University, Nanjing, 210023, China.
Chemosphere. 2022 Sep;303(Pt 2):135072. doi: 10.1016/j.chemosphere.2022.135072. Epub 2022 May 23.
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.
煅烧是提高层状双氢氧化物（LDH）材料 F 吸附能力的有效方法，但对于煅烧 LDH（CLDH）增强除氟能力的分子尺度机制仍缺乏了解。本研究采用 F 固体核磁共振、X 射线光电子能谱（XPS）和高分辨率透射电镜研究了 CLDH 吸附 F 的机理。在煅烧过程中，LDH 经历了三个阶段：低于 200°C 的表面脱水、200-400°C 的结构去羟化以及高于 400°C 的层间碳酸盐基团释放。此外，XPS 和 XRD 表征表明，即使在再水合和再构成后，CLDH 也无法恢复到原始结构的对称性。煅烧后的 LDH 对 F 的亲和力大大增强，尤其是在高 pH 值下。在 pH 值为 10 时，煅烧 500°C 的 CLDH 的吸附容量可达到 22.0mg F/g，约为 LDH 的 6 倍。XRD 分析表明，随着煅烧温度的升高，吸附 F 的 CLDH 的结晶度变差，与 TEM 观察到的 CLDH 片上丰富的缺陷和 Mg/Al 氧化物一致。吸附 F 后的 CLDH 的 F 固体核磁共振光谱表明，在 pH 值为 7 时，表面 Al-F 的形成是 F 吸附的主要模式，而在碱性条件下（pH 值为 10），Mg-F 局部配位模式是 F 吸附的主要机制。本研究全面了解了 CLDH 在 F 吸附中的作用，并表明煅烧是一种有前途的处理方法，可以提高污染阴离子清除的效果。
