Adsorption mechanism of borate with different calcined layered double hydroxides in a molar ratio of 3:1

Layered double hydroxides (LDHs) with a molar ratio of 3:1, which contain different divalent metals (Zn-LDH, Mg-LDH, and Ca-LDH), were synthesized. They were calcined at 500°C (Zn-CLDH, Mg-CLDH, and Ca-CLDH) for borate removal. The characterization of different CLDHs before and after adsorption of boric acid was performed by XRD, FTIR, and SEM. For Ca-CLDH (3:1), the main removal mechanism was the formation of ettringite with borate, which is the same as Ca-CLDH (2:1). However, the latter has a much higher adsorption of borate than the former, because for Ca-CLDH (2:1), the amount of ettringite produced during regeneration is more than that of Ca-CLDH (3:1). For Zn-CLDHs (3,1 and 2,1), borate was removed completely by ion exchange and intercalation. However, the adsorption rate of Zn-CLDH (2:1) is higher than that of Zn-CLDH (3:1), even though the final adsorption capacity is similar. In addition, Zn-LDH (2:1) containing polymerized boron formed in a short time, but as the reaction progresses, more Zn-LDH (3:1) containing polymerized boron is gradually formed. For two ratios of Mg-CLDHs, borate was first immobilized on the surface of metal oxide by electrostatic adsorption and then entered the interlayer of Mg-LDHs by intercalation, resulting in a decrease in borate concentration. However, Mg-LDH (3,1) has a faster and better adsorption of borate than Mg-LDH (2,1), and the structure regeneration time of Mg-CLDH (3,1) was shorter than that of Mg-CLDH (2,1). Therefore, when using CLDHs for boron removal, the metal type and ratio of LDHs should be considered.