Removal of chloride ions from aqueous phase as layered double hydroxides via uptake strategies: A critical review

Chloride ions (Cl⁻) are considered detrimental to water reuse across various sectors, and their selective removal presents significant challenges, making it a critical topic of interest in wastewater treatment. Recently, the precipitation of Cl⁻ as layered double hydroxides (LDHs) using various uptake strategies has garnered substantial attention. This review provides a comprehensive analysis of methods for transforming Cl⁻ into the form of LDH-Cl, encompassing mixed metal oxide (MMO), LDHs/calcined LDHs sorption, and Friedel's salt precipitation (FSP). The review delves into the Cl⁻ removal performance, mechanisms, reaction behaviors, and influencing factors of these methods. Specially, within the context of zero liquid discharge for industrial wastewater, the feasibility, recovery potential, and the pros and cons of these treatment processes are analyzed and compared. Among these methods, chloride binding by MMO is identified as the most promising approach for the effective removal of Cl⁻ from wastewater, given the challenges in high sludge yield encountered with the FSP method and the limited sorption capacity of LDHs/CLDHs. Finally, we propose several key strategies for improving Cl⁻ removal efficiency, which encompass mitigating side reactions, developing more reactive and cost-efficient agents, managing generated chemical sludge for resource utilization, and optimizing overall process design to ensure the long-term stability and reliability of the treatment system. These strategies are essential for the application of selective Cl⁻ removal technologies in engineering practice.