Journal of Hazardous Materials 140 (2007) 104–109 Chromium(VI) adsorption from aqueous solution onto Moroccan Al-pillared and cationic surfactant stevensite Abdelaziz Benhammou a,∗ , Abdelrani Yaacoubi a , Lahbib Nibou b , Boumediane Tanouti a a Facult´ e des Sciences Semlalia, Avenue My Abdellah, BP-2390, Marrakech, Morocco b Ecole Sup´ erieure de Technologie, Route Dar Si Aissa, BP-89, Safi, Morocco Received 8 April 2006; received in revised form 3 June 2006; accepted 19 June 2006 Available online 27 June 2006 Abstract Batch adsorption of the chromium(VI) onto Moroccan stevensite pillared by Keggin aluminium hydroxypolycation (Al-stevensite) and cationic surfactant cetyltrimethylammoniumbromide (CTA-stevensite) was investigated. The results showed that the CTA-stevensite has a higher affinity than that of Al-stevensite for chromium(VI) adsorption. The adsorption capacities for natural stevensite, Al-stevensite and CTA-stevensite calculated according to the Dubinin–Kaganer–Radushkevich isotherm (DKR) are 13.7, 75.4 and 195.6 mmol kg -1 , respectively. The study of the pH effect showed that the optimal range corresponding to the Cr(VI) maximum adsorption on Al-stevensite is pH 3.5–6 and that on CTA-stevensite is pH 2–6. The adsorption rates evaluated according to the pseudo-second-order model are 7.2, 207.2 and 178.5 mmol kg -1 min -1 for the natural stevensite, Al-stevensite and CTA-stevensite, respectively. The low values of the adsorption energy calculated by (DKR) suggest that anion exchange is the main mechanism that governs the chromate adsorption. © 2006 Elsevier B.V. All rights reserved. Keywords: Moroccan stevensite; Pillared; Adsorption mechanism; Isotherm; Kinetic 1. Introduction Chromium is present in the environment predominantly in one of two valence states: trivalent chromium Cr(III), which occurs naturally and is an essential nutrient, and hexavalent chromium Cr(VI) which is highly toxic and is classified as a human carcinogen [1,2]. In aqueous solution, Cr(VI) exists in the form: chromate CrO 4 2- , dichromate Cr 2 O 7 2- and hydrogen chromate HCrO 4 - . Most studies seem to favour the dominant species dependence on the pH and the concentration of the solu- tion: CrO 4 2- is predominant in basic solutions, H 2 CrO 4 - is predominant at pH < 1 while HCrO 4 - and Cr 2 O 7 2- are pre- dominant at pH 2–6. Chromium is used in various industrial applications, including the production of stainless steels and refractory products like brick and mortar. It is also used in man- ufacturing pigments, the treatment of metal surfaces and in the tanning of leather [3,4]. Various methods were used to remove chromium from waste waters such as membrane filtration [5], ∗ Corresponding author. Tel.: +212 24 62 91 24; fax: +212 24 62 91 24. E-mail address: Benhammou ab@yahoo.fr (A. Benhammou). anion exchange resin [6] and adsorption onto different adsor- bents [7]. The adsorption study of chromium(VI) from aqueous solu- tions by natural or modified clay minerals (kaolinite, bentonite, illite, etc.) was the object of several researches [8–10]. These studies reveal a weak adsorption of chromate anions by nat- ural clay minerals due to the negative charge. Two methods are generally used to improve the affinity of clays for chromate removal: (i) the reduction of the chromium(VI) to chromium(III) by an organic or an inorganic reducing agent and the adsorption of chromium(III) [11–13] and (ii) surface modification by the adsorption of cationic surfactant or metallic hydroxypolycation [14,15]. Recent studies [16,17] showed that the clay minerals rich in Fe(II) can induce a considerable elimination of the chromium(VI) in solution by the reduction/adsorption mech- anism. The effectiveness of retention increases with Fe(II) released in solution. Krishna et al. [18] showed that the adsorption of chromium(VI) onto montmorillonite is car- ried out via the anions exchange between the chromium(VI) and the anions bromide complexed on the cationic surfac- tant CTA-Br. Several studies have shown that clays modified 0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.06.077