Removal mechanism of polymeric borate by calcined layered double hydroxides containing different divalent metals

The removal mechanism of polymeric borate by calcined layered double hydroxides is not clear. In this work, layered double hydroxides containing different divalent metals were synthesized and calcined to produce calcined layered double hydroxides (Zn-, Mg-, and Ca-CLDH). Then, Zn-, Mg-, and Ca-CLDH were applied to remove polymeric borate. Zn-CLDH showed better performance for the removal of borate than Ca-CLDH, and hardly any borate was removed by Mg-CLDH. Based on the characterization results, the detailed removal process of polymeric borate by different calcined layered double hydroxides is discussed. Because there is little H3BO3 that can act as a trigger, and ligand promoted dissolution of the complex H3BO3 and MgO is prevented. Therefore, Mg-CLDH could not transform to the layered structure to immobilize the borate. For the Zn-CLDH, Zn-CLDH transformed into Zn-LDH, and polymeric borate was absorbed into the interlayer of layered double hydroxides, which is the dominant mechanism of borate removal by Zn-CLDH. Reconstruction of the Ca-LDH from the Ca-CLDH was more rapid than the other calcined layered double hydroxides. However, formation of borate-containing ettringite was the main removal mechanism in the first stage. With increasing reaction time, the reaction between CO32- and Ca2+ released from ettringite, and the regeneration of Ca-LDH to form CaCO3 were the main reasons for borate removal in the second stage. (C) 2015 Elsevier B.V. All rights reserved.