Preparation, characterization of surfactants modified clay minerals and nitrate adsorption Yunfei Xi a,b, ⁎ ,1 , Megharaj Mallavarapu a,b , Ravendra Naidu a,b a Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia, Mawson Lakes, SA 5095, Australia b Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE), University of South Australia, Mawson Lakes, SA 5095, Australia abstract article info Article history: Received 17 August 2009 Received in revised form 20 November 2009 Accepted 23 November 2009 Available online 29 November 2009 Keywords: Anion Adsorption Organoclay Organic surfactants modified clay minerals are usually used as adsorbents for hydrophobic organic contaminants remediation; this work however has shown organoclays can also work as adsorbents for hydrophilic anionic contaminant immobilization. Organoclays were prepared based on halloysite, kaolinite and bentonite and used for nitrate adsorption, which are significant for providing mechanism for the adsorption of anionic contaminants from waste water. XRD was used to characterize unmodified and surfactants modified clay minerals. Thermogravimetric analysis (TG) was used to determine the thermal stability and actual loading of surfactant molecules. Ion chromatography (IC) was used to determine changes of nitrate concentration before and after adsorption by these organoclays. These organoclays showed different removal capacities for anionic ions from water and adsorption mechanism was investigated. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Clay minerals have been widely used in a range of applications because of their high cation exchange capacity, swelling capacity, high specific surface area, and consequential strong adsorption capacity (Bailey et al., 1999; Sen Gupta and Bhattacharyya, 2006; Du and Hayashi, 2006). Bentonite is made up primarily of montmorillonite. Other clay minerals that are included in this study are kaolinite and halloysite. Applications of clay minerals are extended to anionic contaminants particularly inorganic oxyanions, for example, nitrate, chromate and arsenate which are toxic to human and wildlife even at very low concentration. However, these anions can be repelled by the negative charges on clay minerals' surface, so natural unmodified clay minerals are ineffective adsorbents for these contaminants. For this reason, modification using an additive has shown enhanced anions retention capacity. In addition, the optimum modification method should impart high selectivity towards anions. There are some studies that have indicated clay minerals when modified by organic surfactants will become suitable materials for anions retention (Li and Bowman, 1998; Krishna et al., 2000; Li and Bowman, 2001; Kaufhold et al., 2007). Thus, the surface modification of clay minerals has received great interest, for example, ion exchange of the inorganic cations with organic cations usually with quaternary ammonium compounds can change the surface properties (Mortland et al., 1986). The intercalation of cationic surfactants not only changes the surface properties from hydrophilic to hydrophobic but also greatly increases the anions adsorption capacity especially when surfactant loading exceeds the CEC of clay. The resulting adsorption of surfactant molecules via hydrophobic bonding (Xu and Boyd, 1995; Lee and Kim, 2002) and the positive charge of ammonium will attract anions. Such surface property modification is of importance for extended organoclay applications. In particular, at the same time, the hydrophobic nature of the organoclay implies that the material can be used as a filter material for water purification (Beall, 2003). Compared to pure organic surfactants, these organoclays also exhibit a remarkable improvement in properties which includes increased strength and heat resistance, decreased gas permeability and flammability etc. (Dultz and Bors, 2000). All of these applications and improvements mentioned above strongly depend on the structure and properties of the organoclays. So understanding the structure and properties of organo- clays is essential for their applications. Organoclays have been extensively investigated for hydrophobic contaminants immobilization, however, the objective of this study is to investigate the hydrophilic anionic contaminants adsorption on organoclays, and in particular this study investigated the properties of these clays modified with different hexadecyltrimethylammonium bromide dosages and their performance on nitrate adsorption. X-ray diffraction technique was used to investigate the phases and basal spacings of these materials. Thermogravimetric analysis (TG) was used to probe the microenvironment and packing arrangement of organic Applied Clay Science 48 (2010) 92–96 ⁎ Corresponding author. Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia, Mawson Lakes, SA 5095, Australia. Tel: +61 8 83026232. E-mail address: yunfei.xi@unisa.edu.au (Y. Xi). 1 Postal address: X-building, University of South Australia, Mawson Lakes, SA 5095, Australia. 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.11.047 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay