Chemical Engineering Journal 147 (2009) 173–179 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Cadmium(II) removal from aqueous solution using microporous titanosilicate ETS-4 Telmo R. Ferreira a , Cláudia B. Lopes b , Patrícia F. Lito a , Marta Otero b , Zhi Lin a , João Rocha a , Eduarda Pereira b , Carlos M. Silva a,∗ , Armando Duarte b a CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal b CESAM, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal article info Article history: Received 11 March 2008 Received in revised form 9 June 2008 Accepted 28 June 2008 Keywords: ETS-4 Cadmium(II) Ion exchange Nernst–Planck Batch experiments abstract The ability of microporous titanosilicate ETS-4 to uptake Cd 2+ from aqueous solutions has been investi- gated, assessing its potential in water remediation. In order to study the equilibrium and the kinetics of the process, batch stirred tank experiments have been carried out by contacting a fixed volume of solution with known masses of ETS-4. The evolution of the cadmium concentration with time has been moni- tored by inductively coupled plasma mass spectrometry. Concerning equilibrium, Freundlich isotherm performs accurately in the range of experimental conditions studied. The solid loadings measured in this essay surmount significantly the values found in literature for different ion exchangers in the same range of temperatures. A Nernst–Planck based model combining internal and external diffusion resistances has been used to describe the ion exchange process, where the convective mass transfer coefficient and the self-diffusivities of the counter ions are the unique parameters. The Nernst–Planck based model accom- plishes good representations (average absolute deviation of 6.74%), even in the transition from the steep descent to the horizontal branch of the bulk concentration versus time curve. The results obtained using the commonly adopted pseudo first- and second-order models found in literature are worse (average absolute deviation of 215.7% and 12.11%, respectively), although more parameters are involved. © 2008 Elsevier B.V. All rights reserved. 1. Introduction It is widely known that heavy metals present in superficial waters are extremely dangerous to environmental and human health because they are not biodegradable and must be removed to prevent their accumulation. Industrial discharges, mainly from mining, metal finishing, welding, alloys manufacturing plants, pulp industries and petroleum refining, are responsible for heavy metal contamination [1]. Cadmium is one of the most toxic non-essential heavy metals present in the environment, even at low concentra- tions. Therefore, removal of trace levels of cadmium is required. Several methods are available to remove toxic metals from aque- ous waste streams, such as chemical precipitation, electrolysis (membrane separation), and ion exchange. The latter is proba- bly one of the most attractive processes and, consequently, the commonly used in industry, because of its simple and efficient application. However, the cost and the regeneration of the adsor- ∗ Corresponding author at: Department of Chemistry, University of Aveiro, Cam- pus Universitário de Santiago, 3810-193 Aveiro, Portugal. Tel.: +351 234 401549; fax: +351 234 370084. E-mail address: carlos.manuel@ua.pt (C.M. Silva). bent are limiting factors [2], hence it is of interest to research new materials to replace expensive activated carbons and resins. Natu- ral and synthetic zeolites are gaining considerable interest because of their high selective and ion-exchange capacity [3–7]. Generally, microporous titanosilicates are three-dimensional crystalline solids with a well-defined structure containing tita- nium, silicon and oxygen atoms [8]. These materials have a regular crystalline framework formed by a three-dimensional combina- tion of tetrahedral and octahedral building blocks connected with each other by shared oxygen atoms. Each TiO 6 octahedron in the titanosilicate global structure carries a -2 charge, which can be neutralized by extra-framework cations (e.g., Na + and K + ). These compensation species, as well as water molecules or other adsorbed molecules, are located in the channels of the structure and can be replaced by others. Titanosilicates exhibit remarkable physical and chemical properties, such as selective sorption, ion exchange and catalytic activity [8]. Of special importance for environmental uses is their ability to uptake and retain heavy metal species from aqueous media. ETS-4 (Engelhard Titanium Silicates No. 4) contains octahedral and square-pyramidal titanium units, in addition to the tetrahedral silicate units. The 12-ring channels are separated by the 8 ring windows. ETS-4 has been suggested as good ion exchanger [8]. 1385-8947/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2008.06.032