Insights into Adsorption Behavior and Mechanism of Br- onto Nickel-Aluminum Layered Double Hydroxides Intercalated with Different Inorganic Anions

Layered double hydroxides (LDHs) have garnered significant attention from researchers in the field of adsorption due to their unique laminated structures and ion exchange properties. LDHs with various anion intercalation showed different adsorption effects on adsorbing ions, but the corresponding adsorption mechanisms are ambiguous. In this study, three types of NiAl-LDHs were synthesized, utilizing NO3-, CO32-, or Cl- as the interlayer anions. Batch tests were conducted to study their adsorption performances for Br-. Among them, the LDH with a NO3- intercalation layer exhibited the highest adsorption capacity for Br-, reaching up to 1.40 mmol g-1. The adsorption kinetics, mechanism, and renewability of these NiAl-LDHs were systematically compared. As a result, the type of Br- adsorption by all three materials was single molecular layer chemisorption. Moreover, the thermodynamic results of adsorption suggested that the adsorption of Br- was a spontaneous exothermic process. X-ray photoelectron spectroscopy, X-ray diffraction, and point of zero charge analysis collectively indicated that the adsorption of Br- by LDHs primarily occurred through interlayer ion exchange and electrostatic interactions. Structural characterizations of the adsorbents revealed that Br- entered the interlayers of the three LDHs, causing varying degrees of reduction in the interlayer spacing. Density functional theory calculations indicated that the interlayer binding energy of LDH with NO3- intercalation was the lowest, thereby making it more susceptible NO3- to be exchanged with Br-. Finally, the stability of the NiAl-LDHs was studied. The NiAl-LDHs retains a high removal efficiency of Br- even after 5 cycles of adsorption and desorption.