Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Uptake of chloride ion from aqueous solution by calcined layered double hydroxides: Equilibrium and kinetic studies Liang Lv a,b , Jing He a , Min Wei a , D.G. Evans a , Xue Duan a,Ã a Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing University of Chemical Technology, Box 98, 15 Bei San Huan Dong Lu, Chao Yang District, Beijing 100029, China b The West Branch of Zhejiang University of Technology, Zhejiang 324006, China article info Article history: Received 1 September 2005 Received in revised form 23 November 2005 Accepted 29 November 2005 Available online 19 January 2006 Keywords: Chloride Kinetics Layered double hydroxides Hydrotalcite Memory effect Isotherm ABSTRACT Layered double hydroxides (LDH) calcined within a certain temperature range (denoted as CLDH) have been shown to recover their original layered structure in the presence of appropriate anions. In the light of this so-called ‘‘memory effect’’, uptake of chloride ion from aqueous solution by calcined MgAl–CO 3 LDH was investigated in batch mode. The equilibrium isotherm showed that the uptake of chloride ion by CLDH was consistent with the Langmuir and Freundlich equations and that the Langmuir model gave a better fit to the experimental data than the Freundlich model. The maximum uptake capacity of CLDH for chloride ion was 149.5 mg/g, close to the stoichiometric uptake (168 mg/g). The influence of varying pH of solution, initial chloride concentration, adsorbent quantity, and temperature on the kinetics of chloride removal has also been explored. Four kinetic models were used to fit the experimental data, and it was found that the pseudo-second-order kinetics model could be used to describe the uptake process satisfactorily. The calculated value of E a was found to be 56.8 kJ/mol, which suggests that the process of uptake of chloride ion is controlled by the rate of reaction of chloride ion with the CLDH rather than diffusion. A mechanism for removal of chloride ion has been confirmed by X-ray diffraction, FT-IR spectroscopy and TG-MS measurements. & 2005 Elsevier Ltd. All rights reserved. 1. Introduction Layered double hydroxides (LDHs), a class of anionic clays with high anion exchange capacities, are effective adsorbents for removal of a variety of anionic pollutants. The chemical composition of LDHs can be described (Cavani et al., 1991; You et al., 2002) by the formula [M 2+ 1x M 3+ x (OH) 2 ] x+ (A n ) x/n  mH 2 O where M 2+ and M 3+ are metal cations, for example Mg 2+ and Al 3+ , that occupy octahedral sites in the hydroxide layers, A n is an exchangeable anion, and x is the ratio M 3+ /(M 2+ +M 3+ ). Carbonates are the interlayer anions in the naturally occur- ring mineral hydrotalcite, which is a member of this class of materials. The decomposition of MgAl–CO 3 LDH when heated at around 500 1C leads to mixed metal oxides, which are characterized by high specific surface areas and homoge- neous dispersion of metal cations. The mixed metal oxides can take up anions from aqueous solution, with concomitant reconstruction of the original layered structure, as expressed by the following two equations: Mg 1x Al x ðOHÞ 2 ðCO 3 Þ x=2  mH 2 O ! 500  C Mg 1x Al x O 1þx=2 þ x=2CO 2 þðm þ 1ÞH 2 O; ð1Þ Mg 1x Al x O 1þx=2 þðx=nÞA n þðm þ 1 þðx=2Þþ yÞH 2 O ! Mg 1x Al x ðOHÞ 2 ðA n Þ x=n  mH 2 O þ xOH  . ð2Þ ARTICLE IN PRESS 0043-1354/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.11.043 Ã Corresponding author. Tel.: +86 10 64425395; fax: +86 10 64425385. E-mail address: duanx@mail.buct.edu.cn (X. Duan). WATER RESEARCH 40 (2006) 735– 743