Research paper Adsorption of chromate from aqueous solutions by HDTMA-modified clinoptilolite, glauconite and montmorillonite Tomasz Bajda ⁎, Zenon Kłapyta Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland abstract article info Article history: Received 19 July 2012 Received in revised form 1 October 2013 Accepted 5 October 2013 Available online 21 October 2013 Keywords: Cr(VI) Adsorption Chromate Organo-silicates Alkylammonium surfactants Adsorption of chromate on natural clinoptilolite (Cp), glauconite (Gl), and montmorillonite (Mt) treated with hexadecyl trimethylammonium (HDTMA) bromide at amounts equivalent to 1.0 and 2.0 of their cation- exchange capacities (CEC) was investigated by batch study. In the case of the Cp and Gl, adsorption of HDTMA took place on the zeolite and clay mineral surfaces only and thus relates to the external CEC (ECEC). The amount of chromate removed from the solution by the organo-silicates continuously decreased with increasing pH in the range 1.3–10. The highest values were obtained at pH between 1.3 and 6, and decreased rapidly above pH 6. At an initial chromate concentration of 6.25 mmol/L, its amounts bound to the Cp, Gl and Mt modified using 1.0 CEC loadings of the surfactant were 47, 102, and 168 mmol/kg, respectively. When amount of HDTMA used was equivalent to 2.0 CEC of the silicate, these values increased to 182, 240, and 285 mmol/kg, respectively. The results obtained suggest that the surfactant molecules, bound beyond the CEC to the external surfaces of the Cp and Gl, show a greater chromate adsorption ability in comparison with such molecules located in the interlayer spaces of the Mt. It is also evident that an excess of the surfactant, not adsorbed on the silicate surfaces, participated in the removal of chromate from the solution by formation of a precipitate of alkylammonium chromate. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Chromium most often exists in the environment as Cr(III) and Cr(VI), and the latter is harmful for people and animals. Reduction of Cr(VI) to Cr(III) occurring inside living cells and disturbing biochemical reactions of DNA synthesis may result in various mutations or activate cancerous processes, manifesting themselves as malignant tumors (ATSDR, 2000). Chromium is a common pollutant introduced into natural waters due to the discharge of a variety of industrial waste- waters. Moreover, chromium-based catalysts were also usually employed in various chemical processes, including selective oxida- tion of hydrocarbons. The conventional treatment of chromate-rich effluent is to reduce Cr(VI) to Cr(III) and precipitate Cr(III) as chromium hydroxide or chromium iron hydroxide at high pH, followed by the disposal of the resulting dewatered sludge. The major drawback of the method is the high cost of chemicals used for the reduction of chromate (David et al., 1998). Extensive research was performed to find materials for chromate adsorption and immobilization. In numerous experiments, pure mineral phases, e.g. oxides or hydroxides of iron, aluminium, manganese, and activated carbons; in addition, natural or secondary polymineral mix- tures like soils, claystones, coals, peats were used. Other suitable adsorbents of chromate were organo-silicates. Clay minerals and zeolites modified with quaternary alkylammonium cat- ions were frequently studied because of their potential application as environmental remediation materials (e.g. Bowman, 2003; Boyd et al., 1988; Chmielewska et al., 2003; Stockmeyer, 1991). Such adsorbents to remove chromate from liquid phases were prepared using smectite (Krishna et al., 2001; Li and Bowman, 1998; Majdan et al., 2005), kaolinite (Li and Bowman, 2001), illite (Li et al., 2002), palygorskite and sepiolite (Li et al., 2003). HDTMA surfactant was commonly applied for sur- face modification. Zeolites show a high cation-exchange capacity (CEC) reaching values from 100 to 200 meq/100 g (natural) to around 450 meq/100 g (synthetic). In contrast, adsorption of anions on the surfaces of these silicates is very limited. The anion exchange capacity (AEC) of zeolites can, however, be considerably improved by chemical modification of their surface properties using selected or- ganic compounds. Such organo-zeolites (mainly organo-clinoptilolites) obtained using HDTMA-Br were frequently studied as adsorbents to remove chromates from aqueous solutions (Bajda and Kłapyta, 2006; Bowman, 2003; Haggerty and Bowman, 1994; Mozgawa et al., 2011). The aim of the study reported here was to compare the chromate adsorption capacities of clinoptilolite, glauconite, and montmorillonite from selected Polish deposits modified with HDTMA-Br. The silicate modification procedure described in most papers included washing of the reaction products to remove excess organic salt. In our experiments, unwashed organo-silicates were used to test the role of unadsorbed surfactant in the chromate removal process. Applied Clay Science 86 (2013) 169–173 ⁎ Corresponding author. Tel.: +48 126175232; fax: +48 126334330. E-mail address: bajda@agh.edu.pl (T. Bajda). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.10.005 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay