Thermal activation of palygorskite for enhanced fluoride removal under alkaline conditions

Fluoride contamination of groundwater is a global environmental problem. Based on adsorption treatment strategies, magnesium-based materials have shown potential for defluoridation of this water source. In this research, we calcined raw palygorskite (RPAL), a natural magnesium-rich clay, at different temperatures (100-1000 degrees C), to examine the defluoridation performance. The evolution of the morphology, physical-chemical properties, and crystal structure was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Al-27 and Si-29 solid-state nuclear magnetic resonance (NMR) spectroscopy. Palygorskite calcined at 200 degrees C (CP200) had F- uptake of similar to 1.6 mg F-/g within a neutral-to-basic pH region of F-contaminated groundwater (e.g., pH 7-9), which is 5-6 times larger than that for RPAL. Additionally, CP200 and CP400 exhibited comparatively greater adsorption performance for fluoride removal compared with activated alumina (AA) under alkaline conditions (e.g., 1.5-3 times at pH >= 9). The dependence of fluoride uptake by the calcined samples appeared to be related to major changes in the crystal structure of palygorskite, particularly the dehydration of zeolite waters and partially coordinated water within pores. The exposed > MgOH2 groups at tunnel edges may play an important role in fluoride removal, especially at elevated pH. The higher stability under alkaline conditions and the lower sensitivity to pH compared with AA demonstrated the potential of calcined palygorskite as an effective magnesium-based clay for the remediation of fluoride contamination of groundwater.

Automated summary

This research examined the potential of calcined palygorskite for defluoridation of groundwater contaminated with fluoride, showing higher adsorption performance compared to activated alumina under alkaline conditions. The study found that changes in the crystal structure and exposed > MgOH2 groups in the calcined samples may play a crucial role in fluoride removal.