Adsorption–desorption characteristics of nitrate, phosphate and sulfate on Mg–Al layered double hydroxide A. Halajnia a, ⁎, S. Oustan a , N. Najaﬁ a , A.R. Khataee b , A. Lakzian c a Department of Soil Science, Faculty of Agriculture, University of Tabriz, 51664-16471, Tabriz, Iran b Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran c Department of Soil Science, Faculty of Agriculture, Ferdowsi University of Mashhad, Iran abstract article info Article history: Received 21 December 2012 Received in revised form 22 April 2013 Accepted 6 May 2013 Available online 25 May 2013 Keywords: Adsorption capacity Adsorption selectivity Adsorption kinetics Hysteresis index FTIR spectroscopy The adsorption–desorption behavior of nitrate on Mg–Al layered double hydroxide (4:1) was compared with that of phosphate and sulfate as two common anions in soil solution. Based on the results, the kinetics of anion adsorption on the LDH followed a pseudo-second order model. The adsorption process was found to be exothermic for nitrate and endothermic for phosphate and sulfate. The adsorption data were best described by the Freundlich model for nitrate and by the Langmuir model for phosphate and sulfate. In this study, the synthesized LDH exhibited higher adsorption rate and adsorption capacity for nitrate compared to sulfate and phosphate. The maximum adsorption capacity values were 1.90, 0.28 and 0.13 mmol g -1 for nitrate, phosphate and sulfate, respectively. However, the Langmuir equation constants related to the adsorp- tion energy were found to be 0.210, 10.731 and 3.021 L mmol -1 , respectively. The distribution coefﬁcient (K d ) was higher for nitrate than phosphate and sulfate at high initial concentrations. Moreover, the values of hysteresis index based on the Freundlich exponent were 97.5, 22.5 and 79.0% for nitrate, phosphate and sulfate, respectively. The FTIR spectroscopy indicated a strong band related to the nitrate intercalated into the interlayer space of the LDH. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Due to the high mobility of nitrate and low anion exchange capac- ity of soils, this anion easily leaches through the soil proﬁle. Nitrate leaching is a serious concern for two reasons, decreasing nitrogen- fertilizer use efﬁciency (NUE) and water resources quality problems. A signiﬁcant amount of nitrogen fertilizer can be lost in the form of nitrate, so that the average nitrogen use efﬁciency is estimated less than 50% (IAEA, 2008). Nitrate contamination of drinking water from agricultural activities is a serious problem in many parts of the world (Prakasa Rao and Puttanna, 2006). Increasing concerns to the environmental impacts associated with nitrate leaching has led to the development of various methods to increase nitrogen fertilizer efﬁciency such as the use of slow release fertilizers, urease enzyme and nitriﬁcation inhibitors and multi-stage application of these fertil- izers. Recently, use of layered double hydroxides (LDHs) as a slow release fertilizer or anionic exchanger has been proposed for increas- ing NUE as well as nitrate buffering capacity of soils (Komarneni et al., 2003; Torres-Dorante et al., 2008, 2009). Layered double hydroxides belong to a group of lamellar non silicate compounds with positively charge. LDHs are commonly represented by the formula of [M 2+ 1 - x M 3+ x (OH) 2 ] x+ [A m- x/m ].nH 2 O, where, M 2+ and M 3+ are divalent and trivalent cations. The value of x is equal to M 3+ /(M 2+ +M 3+ ) and A m- is the charge balancing interlayer anion. In the majority of cases the x values change between 0.10 and 0.33 (Forano et al., 2006). LDHs with positively charged brucite-like sheets and relatively weak interlayer bonding exhibit excellent ability to capture different families of anions such as halides, non-metal oxyanions, anionic metal complexes, organic anions and anionic polymers (Forano et al., 2006; Goh et al., 2008). It is generally believed that LDHs have greater afﬁnities for anions with higher charge density. For common inorganic anions, the LDH selectivity decreases in the following order: CO 3 2- > HPO 4 2- > SO 4 2- > Cl - > NO 3 - (Das et al., 2006; Goh et al., 2008; Lv et al., 2008; Tezuka et al., 2004; You et al., 2001). Therefore, nitrate adsorption efﬁciency of LDHs is strongly reduced in the presence of other anions. For successful application of LDHs to nitrate removal, these compounds should have high capacity and or selectivity for nitrate in the presence of other anions in complex solutions such as soil solution. There are few studies about effective LDHs with a high capacity and selectivity to nitrate adsorption. Génin et al. (2001) illustrated the potential of Fe(II)–Fe(III) green rust for reducing nitrate in soil solution. Tezuka et al. (2004, 2005) found that Ni–Fe-LDH (4:1) had a high selectivity for nitrate com- pared with Mg–Al, Co–Fe and Mg–Fe because of appropriate basal Applied Clay Science 80–81 (2013) 305–312 ⁎ Corresponding author. Tel.: +98 511 878 9213. E-mail addresses: halajnia@yahoo.com (A. Halajnia), oustan@hotmail.com (S. Oustan), n-najaﬁ@tabrizu.ac.ir (N. Najaﬁ), a_khataee@tabrizu.ac.ir (A.R. Khataee), alakzian@yahoo.com (A. Lakzian). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.05.002 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay