Ultrathin Mg-Al layered double hydroxide prepared by ionothermal synthesis in a deep eutectic solvent for highly effective boron removal

Abstract

Ultra-thin layered double hydroxides (LDHs) with small particle sizes were synthesized using a deep eutectic solvent (DES). Products were characterized and applied towards boron removal. The results showed that the LDHs were successfully prepared in a DES (I-LDH) with a narrow diameter distribution (10–40nm) and a monolayer of around 0.7nm. Due to the reduced size, the interaction between the carbonate and metal layers was weak; therefore, I-LDH was found to have a higher sorption capacity to boron than that of LDH prepared through the urea method. The calcinated products (I-CLDH) also exhibited excellent sorption efficiency. The sorption mechanisms of I-LDH, I-CLDH, and U-CLDH were studied. Ion exchange was found to be the predominant mechanism of boron removal by I-LDH. However, for samples after calcination, two stages were present; the first stage included surface complexation and electrostatic attraction, and the second stage included immobilizing boric acid into Mg(OH)2 and attracting borate as an interlayer anionic species into the newly formed LDHs. In addition, pH was found to have a slight influence on sorption performance, although co-existing anions, especially carbonates, played a role in hindering sorption. Base on the Langmuir sorption isotherm, the sorption density of I-CLDH showed a good capacity for removing borate due to the ultrathin structure and high surface area. In addition, the influence of temperature, shaking speed, and dosage on borate sorption by I-CLDH has been studied. I-CLDH was also tested for borate removal from actual wastewater.