Deep eutectic system based C3N4-Zr composite material for highly efficient removal of fluoride in hydrochloric acid

The fluorine chemical industry's rapid growth demands efficient strategies for treating and utilizing the costly and inefficient byproduct, fluoride-containing hydrochloric acid, posing a significant challenge. In this study, a cost-effective method was developed for preparing an acid-resistant and efficient defluorination material. Carbon nitride (C3N4) was chosen for its exceptional acid resistance and was modified via template-free doping with different zirconium source. Characterization results through XRD, FTIR, BET, XPS, SEM, and TEM confirm the successful synthesis of CN-Zr using zirconium chloride (ZrCl4) with a specific surface area of 87.77 m2/g. Furthermore, the adsorption results indicate outstanding fluoride adsorption performance of CN-Zr in hydrochloric acid, with a maximum adsorption capacity of 145.34 mg/g and minimal zirconium leaching. Moreover, the adsorption mechanism was systematically studied using Zeta potential, FTIR and XPS characterization. The findings indicate that Zr was integrated with C3N4 uniformly by forming a deep eutectic system (DES), utilizing exposed terminal amino groups for effective hydrogen bonding with HF which predominates as the primary fluorine existence in hydrochloric acid. Meanwhile, the enhanced ion exchange, complexation of Zr-F and strong electrostatic attraction all contribute to improving adsorption. Finally, the assessment of recyclability performance and operation cost demonstrate that the material can maintain 80 % of its initial adsorption capacity after five cycles. The cost analysis results indicate that the adsorption cost is only 0.3157 CNY/g, making it considerable competitive in the industry.