Transport of Strontium Through a Ca-bentonite (Almería, Spain) and Comparison with MX-80 Na-bentonite: Experimental and Modelling César Valderrama & Javier Giménez & Joan de Pablo & María Martínez Received: 13 July 2010 / Accepted: 5 October 2010 / Published online: 24 November 2010 # Springer Science+Business Media B.V. 2010 Abstract In this work, the sorption of strontium on a Ca-bentonite (CGA) from Almería (Spain) in column experiments was studied, and the results obtained were compared with the sorption onto the Na- bentonite (MX-80). The code CTXFIT (two site non-equilibrium sorption model) was used in order to fit the experimental data and to determine sorption and transport parameters. The effect of inlet Sr(II) initial concentration and the ionic strength were evaluated. The results obtained showed that the sorption capacities as well as the transport and sorption parameters of both bentonites were affected by the initial metal concentration. In experiments with higher inlet concentrations, columns were saturated faster, leading to shorter breakthrough and exhaustion times. On the other hand, a decrease of sorption and transport parameters was observed at higher ionic strengths, which would confirm ion exchange as the main mechanism of Sr(II) sorption onto both bentonites. The sorption parameters (sorption capacity and retardation factor) obtained indicated that the Ca-bentonite from Almería (Spain) presented better sorption performance than the Na-bentonite, which was related to the physical properties of the Ca-bentonite. Keywords Ca-bentonite . Na-bentonite . Strontium . MX-80 . Breakthrough curve . Two-site non- equilibrium sorption model 1 Introduction Bentonite is an altered volcanic ash deposit that consists essentially of smectite mineral of the mont- morillonite group and has been extensively used as a commercial adsorbent for different metals, due to its high sorption capacity, especially for cations. In addition to the good sorption capacity, bentonite has a very low permeability, which provides a geological barrier, and both a good swelling capacity and a high plasticity, that allow the sealing of rock fractures (Madsen and Müller-Vonmoos 1989). Because of such properties, bentonite has been selected as a potential buffer or backfill material in the design of a high-level nuclear waste (HLNW) repository in a number of disposal programmes (Boult et al. 1998; Bucher and Müller-Vonmoos 1989; Neall et al. 1995; Wieland et al. 1994). In this sense, the behaviour of different commercial bentonites such as Na-bentonite MX-80 (Wyoming, USA) and Ca- Water Air Soil Pollut (2011) 218:471–478 DOI 10.1007/s11270-010-0660-1 C. Valderrama (*) : J. Giménez : J. de Pablo : M. Martínez Departament d’Enginyeria Química, Universitat Politècnica de Catalunya, Av. Diagonal 647, Edifici H Planta 4ª, Barcelona 08028, Spain e-mail: cesar.alberto.valderrama@upc.edu J. de Pablo CTM Centre Tecnològic, Avda. Bases de Manresa 1, 08240 Manresa, Spain