Sorption of H 3 BO 3 /B(OH) 4  on calcined LDHs including different divalent metals Xinhong Qiu a,⇑ , Keiko Sasaki a , Kwadwo Osseo-Asare b , Tsuyoshi Hirajima a , Keiko Ideta c , Jin Miyawaki c a Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan b Department of Materials Science and Engineering and Department of Energy and Mineral Engineering, Penn State University, University Park, PA 16802, USA c Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8180, Japan graphical abstract article info Article history: Received 5 November 2014 Accepted 27 December 2014 Available online 13 January 2015 Keywords: Boron LDH Anion exchange Complex Intercalation Mechanism abstract LDHs with different divalent metals (Zn-LDH, Mg-LDH and Ca-LDH) have been synthesized and produced calcined LDHs (Zn-CLDH, Mg-CLDH and Ca-CLDH) for borate removal. Based on XRD, SEM, BET, 27 Al NMR, CO 2 -TPD, and 11 B NMR, detailed characterization of different CLDHs before and after reaction with the boron species was systematically performed. The surface area, basicity and the particle charge of the dif- ferent CLDHs, which are related to the hydration and regeneration, were markably influenced by the nat- ure of the divalent metals. Transformation of crystal phases and the types of boron species adsorbed by the different CLDHs varied as time changed. The regeneration of Ca-CLDH required the shortest time. However, Ca-LDH decomposed to release Ca 2+ ions, forming ettringite with borate. Zn-CLDH also rapidly transformed into Zn-LDH. During this reconstruction, B(OH) 4  was intercalated into the interlayer of Zn- LDHs, which is the predominant mechanism of borate removal by Zn-CLDH. Increase in the initial pH caused a competition between borate and OH  so that the removal efficiency of borate by Zn-CLDH decreased. For Mg-CLDH, surface complexation and electrostatic attraction were included in the first stage, immobilizing boric acid into Mg(OH) 2 and attracting borate as interlayer anionic species into the new forming Mg-LDHs in the second stage. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction Boron is widely distributed in natural waters in the form of boric acid (H 3 BO 3 ) or borate ion (B(OH) 4  ) [1]. Due to its special properties, boron (in various compounds) is employed in many important industries and it is one of the elements that is involved in the food chain from both natural sources and human activities [2,3]. Excess concentration in drinking water results in a threat to human health. Therefore, WHO proposed a guideline for boron in drinking water of 2.4 mg/L [4]. Sorption processes are potentially useful and advantageous for boron removal [5,6]. Therefore, various adsorbents have been stud- ied, especially cost-effective sorbents such as MgO [7], dolomite [8] and layered double hydroxides [9,10]. Of these researches, the effi- ciency of using LDHs has been proven [11] and several publications found that calcined LDHs showed higher sorption capacities than the corresponding un-calcined materials [5,12]. Previous investiga- tions into the reaction mechanism of H 3 BO 3 /B(OH) 4  interaction http://dx.doi.org/10.1016/j.jcis.2014.12.093 0021-9797/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: qxinhong@gmail.com (X. Qiu). Journal of Colloid and Interface Science 445 (2015) 183–194 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis