Journal of Colloid and Interface Science 313 (2007) 574–578 www.elsevier.com/locate/jcis Chromium removal from water using LTA zeolites: Effect of pH Elena I. Basaldella a,∗ , Patricia G. Vázquez a , Fabio Iucolano b , Domenico Caputo b a Centro de Investigación y Desarrollo en Ciencias Aplicadas (CINDECA) (CONICET-CIC), Universidad Nacional de La Plata, calle 47 N ◦ 257, 1900-La Plata, Argentina b Dipartimento di Ingegneria dei Materiali e della Produzione (DIMP), Settore Chimica Applicata, Facoltà di Ingegneria, P. le Tecchio 80, 80125 Napoli, Italy Received 16 January 2007; accepted 29 April 2007 Available online 3 May 2007 Abstract The effect of pH changes on the ability of the synthetic zeolite NaA to remove Cr 3+ from water by ion exchange was investigated. The exchange rate was improved by working near neutrality. Despite of the occurrence of simultaneous adsorption, precipitation or cation exchange phenomena, spectroscopic analyses of samples taken at different contact times suggested the presence of an unique chromium environment in the solid phase. The increase in pH observed during the ion exchange favored polymerization–precipitation of chromium species present in solution, which, in turn, improved the metal removal capacity of zeolite NaA above the values expected for a pure cationic exchange reaction.  2007 Elsevier Inc. All rights reserved. Keywords: Chromium removal; Zeolite A; Ion exchange; Heavy metal retention 1. Introduction Chromium removal from industrial wastewaters is a process of great significance on account of the widespread use of this hazardous metal in many industrial and manufacturing processes and of its serious environmental impact. Leather tan- ning, steelworks, chromium electroplating are, among others, typical processes responsible for polluting natural waterways. Therefore, removal of this metal from wastewaters is a very rel- evant process. Knowing that Cr(VI) is the most toxic cationic form [1–3], understanding of the behavior of chromium cations as well as of the relationships between Cr(III) and Cr(VI) in water media is critical. On the other hand, chromium-based catalysts are also usu- ally employed in various chemical processes, including the selective oxidation of hydrocarbons. Recently, several studies were reported on synthesis and characterization of chromium- catalysts supported by mesoporous molecular sieves and zeo- lites. In them, cation exchange properties of the supports were found to play a fundamental role [4–6]. * Corresponding author. Fax: +54 221 421 0711. E-mail address: eib@quimica.unlp.edu.ar (E.I. Basaldella). As others heavy metal cations, chromium is removed from aqueous media by alkaline precipitation, though ion exchange- based processes using zeolites have been proposed as an al- ternative method [7–10]. Studies carried out by Pansini et al. [7,8] allowed to foresee possible applications of natural zeo- lites, such as phillipsite and/or chabazite containing tuffs for chromium removal from wastewater. In fact zeolites possess excellent properties to retain di- and trivalent ions. The main divalent ions that displace Na + in zeolite A are, to date, Mg 2+ , Ni 2+ , Fe 2+ , and Cu 2+ [11]. To remove chromium from water using zeolite materials, the pH of the liquid media is generally neutral or acidic. Low pH values are found in solutions that re- sult from electroplating plants or when a reduction of Cr(VI) to Cr(III) is carried out, stabilizing this cation to pH 3.5–4. Ad- ditionally, it was reported that during this exchange reaction, a partial loss of zeolite structure has been observed [7]. In this research, the objectives were to assess the influence of the initial pH of chromium solutions on the exchange capacity of NaA zeolite and the modifications that this process under- goes when the zeolite is previously exchanged with Ca(II). The nature of chromium species present in solid and solution phases after the exchange reaction were analyzed by several spectro- metric techniques. 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.04.066