A Kinetic Model for the Adsorption of Gold from I 2 /I 2 Solutions onto a Porous Polymer Membrane M. Pe ´rez-Tello, 1 F.Rodrı´guez-Felix, 2 M. M. Castillo-Ortega, 2 V. M. Sa ´nchez-Corrales 1 1 Departamento de Ingenierı ´a Quı ´mica y Metalurgia, Universidad de Sonora, Hermosillo, Sonora, Mexico 83000 2 Departmento de Investigacio ´n en Polı ´meros y Materiales Universidad de Sonora, Hermosillo, Sonora, Mexico 83000 Received 24 July 2010; accepted 16 July 2011 DOI 10.1002/app.35272 Published online 20 October 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: A model for the adsorption of gold from I 2 /I  aqueous solutions onto a cellulose acetate (CA)-polyaniline (PANI) porous membrane is presented. The adsorption of gold is represented by an ion-exchange overall reaction in which AuI 2  ions replace the Cl  ions at the active sites of the polyaniline matrix. The model incorporates the external mass transfer of AuI 2  from the bulk solution to the membrane sur- face, followed by the pore diffusion of AuI 2  to reach the active sites in the membrane. The overall ion-exchange reac- tion was assumed to achieve local instantaneous equilibrium. Verification of the kinetic model with the experimental data showed that the effective diffusivity of AuI 2  within the membrane is about 8.3  10 6 cm 2 /s. The potential applica- tions of the present formulation are discussed. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 124: 1695–1706, 2012 Key words: diffusion; adsorption; modeling; membranes INTRODUCTION The synthesis of polyaniline-based (PANI) composite membranes has received great attention in recent years by the research community. This is because PANI presents a number of characteristics which makes it an excellent candidate for electronic, envi- ronmental, and engineering applications. Such prop- erties include good electrical conductivity, chemical stability under environmental conditions, and a suit- able ion-exchange capacity. 1–6 Despite its potential, PANI usually is not used as the sole component in membranes. This is because the mechanical strength of PANI is not sufficient to guarantee a long-term use of the material for most applications in which PANI has been tested. Therefore, other materials such as cellulose acetate (CA) 7–9 and poly(methyl methacrylate) 10–12 (PMMA) are mixed with PANI in various proportions to obtain composite membranes with improved mechanical characteristics. The addi- tion of CA and PMMA to PANI typically produces a porous membrane structure which may be particu- larly useful for potential applications such as adsorption and ion exchange, in which a large sur- face area for the solid-fluid contact is desirable. In a previous work, 13 the present authors reported on the synthesis of CA membranes modified with polyacrylic acid using various plasticizers and coated with PANI. The membranes were characterized in terms of their morphology, pore size, electrical conduc- tivity, strain at break, and glass transition temperature. Preliminary experiments showed that one of the for- mulations of the composite membranes coated with PANI was capable of adsorbing gold from a synthetic gold-iodide (AuI 2  ) aqueous solution at room temper- ature. This indicated that such a formulation may have the potential to treat aqueous streams typically pro- duced during leaching of gold from ore minerals. 14 In a subsequent paper, 15 the authors showed that the adsorption equilibrium of gold on the membrane at room temperature followed a Langmuir-type rela- tionship: q Aeq ¼ k 1 C Aeq 1 þ k 2 C Aeq (1) where C Aeq and q Aeq are the equilibrium concentra- tions of gold in the aqueous solution and the solid phase, respectively, and the k symbols are empiri- cally determined parameters. Ion-chromatography analyses of the bulk solution before and after the solid–liquid contact showed that the concentration of total Cl  ions increased upon the adsorption of gold on the membrane. Early equilibrium stability diagrams 16–18 indicate that under the conditions tested in the experiments, AuI 2  is the predominant species of gold in the aqueous phase. Therefore, the authors suggested an ion-exchange mechanism by which the AuI 2  ions adsorbs on the active sites of the PANI structure while the Cl  ions attached to the PANI structure are simultaneously released to Correspondence to: M. Pe ´rez-Tello (mperezt@iq.uson.mx). Journal of Applied Polymer Science, Vol. 124, 1695–1706 (2012) V C 2011 Wiley Periodicals, Inc.