Utilization of Waste Adsorbent Generated after Ca/Al-LDH Adsorption of High-Concentration Phosphate: Fluorine Removal

Abstract

Ca–Al-layered double hydroxide (Ca/Al-LDH) is an excellent sorbent for wastewater decontamination and is widely used in the treatment of this phosphate and fluoride-containing wastewater. In this study, we propose a strategy using the wastes generated from treating high-concentration phosphate wastewater process to treat fluoride wastewater to maximize the value of LDH. The effects of environmental parameters such as reaction time, initial fluoride and phosphate concentrations, adsorbent concentration, and pH were investigated. The experimental results showed that when the initial F- concentration was 10 mg L−1, the removal rates of F- were over 90%. To further understand the adsorption mechanism of F- on solid waste, we employed characterization methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR), which were also able to characterize the adsorption products before and after the reaction. It was found that hydroxyapatite was formed after the removal of phosphate, while fluorapatite was formed at the F-substituted hydroxyl position after the removal of fluorine. The adsorption process of fluorine removal is a spontaneous endothermic adsorption process. This study shows that Ca/Al-LDH can be a promising material with a high development value, and the new strategy of sequentially removing phosphorus and fluorine using this material can maximize the value of this adsorbent.