Adsorption Equilibria of Cu 2+ , Zn 2+ , and Cd 2+ on EDTA-Functionalized Silica Spheres Diego Q. Melo, † Vicente O. S. Neto, ‡ Juliene T. Oliveira, † Allen L. Barros, † Elis C. C. Gomes, § Giselle S. C. Raulino, ‡ Elisane Longuinotti, † and Ronaldo F. Nascimento* ,† † Department of Analytical Chemistry and Physico-Chemistry, Federal University of Ceara ́ , Rua do Contorno, S/N, Campus do Pici, Bl. 940 CEP: 60451-970, Fortaleza, CE, Brazil ‡ Department of Hydraulic and Environmental Engineering, Federal University of Ceara ́ , Rua do Contorno, S/N Campus do Pici, Bl. 713 CEP: 60451-970, Fortaleza, CE, Brazil § Department of Organic and Inorganic Chemistry, Federal University of Ceara ́ , Rua do Contorno, S/N Campus do Pici, Bl. 940 - CEP: 60451-970, Fortaleza, CE, Brazil ABSTRACT: Ethylenediaminetetraacetic acid (EDTA) functionalized silica spheres were used to remove metal ions from aqueous solutions. The adsorption kinetics of Cu 2+ , Zn 2+ , and Cd 2+ (60 mg·L -1 , pH 5.5) were fitted to the pseudosecond order model. Adsorption equilibria were reached within 20 min, indicating that chemisorption may be the limiting step in the adsorption process. Adsorption isotherms were analyzed with nonlinear models by considering the ERRSQ error function and the determination coefficient R 2 . The data with monoion solutions (10 mg·L -1 to 300 mg·L -1 ) were tested with Langmuir, Freundlich, and Redlich-Peterson isotherm models. The best fit was found with the Langmuir model, and maximum adsorption capacities followed the order: Cu 2+ > Zn 2+ > Cd 2+ . Breakthrough curves were obtained using filled columns. The adsorbed ions were quantitatively recovered on elution with hydrogen chloride (0.10 M). After three adsorption-recovery cycles, the metal ions could still be recovered almost quantitatively, which demonstrates the good performance of the EDTA-functionalized silica spheres. 1. INTRODUCTION The contamination of natural waters by biological and chemical agents has become a matter of vivid public interest. Among the various toxic pollutants that can be found in water, toxic metal ions deserve special attention since they are also bioaccumu- lative. Therefore, their occurrence in the environment may result in risks to fauna, flora, and human health. Consequently, the implementation of removal technologies for the treatment of effluents from various industries (mining, textile, painting, electroplating, pesticide-producing) has become a matter of urgency, since in many cases the effluents are discarded into water bodies with no suitable treatment. 1-3 The processes used for metal ion removal from an aquatic environment include chemical precipitation, membrane filtration, ion-exchange, and adsorption, 4 the latter being the most popular due to the simplicity and the low cost. 5 A wide range of materials have been used in adsorption processes: mineral adsorbents, such as zeolites, 6 silica, 7 and alumina, 8 as well as organic adsorbents, such as activated carbon, 9 sugar cane bagasse, 10 coconut fiber, 11 chitin, and chitosan. 12 The use of activated carbon and silica has been widely investigated for the removal of metal ions from aqueous matrices. Silica is an adsorbent of particular interest due to its high surface area and its physical and chemical stability. Derivatives are also expected to be efficient for the removal of metal ions, which makes them an important material for adsorption studies. In this work, silica spheres functionalized with APTS (3- aminopropyltriethoxysilane) and EDTA (ethylenediaminetetra- acetic acid) (synthesis described elsewhere) 13 was used as an adsorbent for the removal of Cu 2+ , Zn 2+ , and Cd 2+ ions from aqueous solution in both batch and fixed-bed column systems. In this study equilibrium isotherms and kinetics were evaluated by testing Langmuir, Freundlich, and Redlich-Peterson isotherm models, and pseudofirst order, pseudosecond order, and Weber-Morris models. 2. MATERIALS AND METHODS 2.1. Preparation of Solutions. Analytical-grade chemicals and ultrapure water (Millipore Direct Q3 Water Purification System) were used to prepare the solutions. Monoelement and multielements stock solutions of Cu 2+ , Zn 2+ , and Cd 2+ (500 mg·L -1 ) were prepared with CuSO 4 ·5H 2 O, ZnSO 4 ·8H 2 O, and CdSO 4 ·8/3H 2 O (Merck, Sã o Paulo, Brazil), respectively. The acetate buffer was prepared with sodium acetate and glacial acetic acid. NaOH (0.10 mol·L -1 ) and HCl (0.10 mol·L -1 ) solutions were used for pH adjustments. Erlenmeyers (50.0 Received: December 17, 2012 Accepted: February 20, 2013 Published: March 1, 2013 Article pubs.acs.org/jced © 2013 American Chemical Society 798 dx.doi.org/10.1021/je3013364 | J. Chem. Eng. Data 2013, 58, 798-806