Performance and Mechanism for Fluoride Removal in Groundwater with Calcium Modified Biochar from Peanut Shell

Abstract

Fluoride in groundwater poses a great risk to humans. Biochar is an effective and environmental-friendly adsorbent for fluoride removal. The objectives of this study were to develop a calcium modified biochar derived from peanut shell and to study its mechanism in the adsorptive removal of fluoride. For these purposes, biochar was prepared using three different techniques. No. 1 biochar was prepared by direct carbonization, No. 2 biochar was modifiied with 30% calcium chloride solution before carbonization, and No. 3 biochar was modified with 30% calcium chloride following carbonization. The No. 2 biochar clearly showed the highest percentage fluoride removal (92.1%) and the fluoride removal efficiency improved by 30%–60% compared with other techniques. The adsorption isotherms and kinetics of the biochar modified with calcium were best described by the Langmuir and pseudo-second-order model, respectively. Based on the calcium content from the energy spectrum, calcium was well loaded onto the biochar. Calcium detached experiments indicated the loaded calcium was the main method for fluoride removal of No. 2 biochar, the adsorption mechanism was clearly demonstrated through the changes of morphology and group during adsorption. Fourier transform infrared spectroscopic (FTIR) analyses indicated the highest fluoride removal efficiency of No. 2 biochar was due to cleavage and structural change in many functional groups. But only C–H was involved in No. 3 biochar fluoride removal process. The good performance of No. 2 biochar for de-fluoridation was due to the calcium stably loaded onto the biochar and many of the changed functional groups there. Biochar modified with calcium before carbonization is an efficient, low-cost, safe technique for de-fluoridation.