Variations in chloride ion sorption within Layered double hydroxides engineered with different cation types

Layered double hydroxides (LDHs) are new concrete additives that improve Cl^- sorption capacity by varying their chemical composition. In this work, first-principles calculations were used to design the chemical compositions of LDHs with various cations. Accordingly, four cation types of LDHs (CaAl, MgAl, MgFe and ZnAl) were selected to be synthesized, their Cl^- sorption kinetics were further measured. The objective was to establish a high-throughput screening approach to identifying the most promising cations for enhancing the sorption capacity of LDHs towards Cl^-. Results revealed that the absolute value of Cl^- binding energy increased as the interplanar spacing of different LDHs decreased from first-principles calculations. Particularly, CaAl exhibited the highest adsorption capacity for Cl^- (3.25 mmol/g) from experiments, validating the simulation results that it has the highest absolute value of Cl^- binding energy. MgFe-Cl-LDH has the smallest absolute value of Cl^- binding energy with the lowest adsorption capacity for Cl^- as well as the crystallinity. Moreover, multiple factors influencing the Cl^- sorption ability of LDHs, such as chemical composition, crystallinity, microstructures and their synergistic effects, are discussed thoroughly based on the experimental results. This study established a connection between the simulation and experimental approaches in understanding the mechanism of Cl^- adsorption.