Enhanced Defluoridation Using Novel Millisphere Nanocomposite of La-Doped Li-Al Layered Double Hydroxides Supported by Polymeric Anion Exchanger

Jianguo Cai,Chao Shan,Yanyang Zhang,Yue Qian,Bingcai Pan

doi:10.1038/s41598-018-29497-1

Abstract

A novel nanocomposite bead LaLiAl-LDH@201 was fabricated by doping a small amount of La into nanocrystalline Li/Al layered double hydroxides (LDHs) pre-confined inside polystyrene anion exchanger D201 (LiAl-LDH@201). A systematic characterisation of the resultant LaLiAl-LDH@201 (XRD, SEM-EDS, TEM-EDS, and XPS) evidenced the successful incorporation of La into the Li/Al LDHs, with their interlayer distance expanded to allow more exchangeable sites for fluoride uptake. The resultant LaLiAl-LDH@201 showed high and stable defluoridation performance over a wide range of pH from 4 to 9. The superior uptake capacity and affinity for fluoride of LaLiAl-LDH@201 over LiAl-LDH@201 were driven by both the increased anion exchange capacity of the embedded LDHs and the specific La-F interaction evidenced via XPS and TEM-EDS characterisation. Fixed-bed column test confirmed that the working capacity of LaLiAl-LDH@201 for defluoridation of authentic fluoride-rich groundwater was nearly twice that of LiAl-LDH@201. The fluoride-loaded LaLiAl-LDH@201 could be conveniently regenerated in situ by using NaOH + NaCl binary solution, achieving desorption efficiency above 98%. Moreover, negligible capacity loss, La leaching, or structure alteration was observed after five adsorption-regeneration cycles, indicating the high stability of LaLiAl-LDH@201. Therefore, the novel millisphere nanocomposite LaLiAl-LDH@201 was promising for efficient defluoridation from water and wastewater.

Highlights

Adsorptive removal of fluoride by activated alumina (AA) is one of the most widely used approaches to ensure the safety of drinking water[6,7,8]
The as-synthesised adsorbent LaLiAl-layered double hydroxides (LDHs)@201 is present as spherical beads of 0.6–1.0 mm in diameter, with a large portion of La distributed in the outer sphere (Fig. 1a)
All the above analysis implied that some proportion of the doped La was present in the form of itsoxides, while the rest proportion was infused into the Li/Al-LDH via substitution for Al

Summary

Introduction

Adsorptive removal of fluoride by activated alumina (AA) is one of the most widely used approaches to ensure the safety of drinking water[6,7,8]. In our recent study[16], a nanocomposite LiAl-LDH@201 with enhanced stability (over pH 3.5–12) and improved fluoride removal capacity was developed by impregnating Li/Al LDHs into a millimetric polystyrene anion exchanger. Many studies suggested doping of high-valent metals such as La, Ce and Zr into metal oxides sorbents (such as AA, Fe2O3, etc.) could enhance their F removal capability[18,19,20]. A novel millimetric nanocomposite La doped LiAl-LDH@201 (denoted as LaLiAl-LDH@201) was prepared to enhance the capacity and selectivity of the original sorbent. The resultant material was characterised with comprehensive techniques, and the defluoridation performance of LaLiAl-LDH@201 was evaluated with particular interest in the effects of pH and various competing anions. The performance of defluoridation from two practical groundwater samples (from Shandong and Yunnan Provinces) by LaLiAl-LDH@201 was evaluated via cyclic adsorption-regeneration in fixed-bed mode

Methods

Results

Conclusion