Retention and induced aggregation of Co(II) on a humic substance: sorption isotherms, infrared absorption, and molecular modeling Ramon A. Alvarez-Puebla a, * , Julian J. Garrido b , Cristobal Valenzuela-Calahorro c , Paul. J. G. Goulet a a Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, Canada N9B 3P4 b Department of Applied Chemistry, Universidad Publica de Navarra, Campus Arrosadı ´a, E-31006 Pamplona, Spain c Department of Inorganic Chemistry, Faculty of Pharmacy, Universidad de Granada, E-18071 Granada, Spain Received 21 September 2004; accepted for publication 9 November 2004 Available online 8 December 2004 Abstract The solubility of humic substances (HS) increases with pH. However, when in the presence of appropriate concen- trations of transition metals in solution, soluble HS flocculate as a consequence of the aggregation processes promoted by the metallic ions. This is of great environmental interest as HS can retain large amounts of metallic ions at low pH. They can also form soluble complexes with these metallic ions as pH increases, even at pH values greater than the pre- cipitation pH of the cation. The aim of this work is to model isotherms and combine macroscopic data with Fourier Transform infrared absorption (FTIR), and molecular modeling techniques, in order to study the interaction of Co(II) with HS. The variation of the shape of the isotherms from L- to S-type, with pH, indicates a change in the retention processes of Co(II) on HS. Since the solubility of HS, and the shift of the precipitation edge, both increase with pH, the S-shaped isotherms are probably due to a mixed mechanism. This mechanism involves the complexation of metallic ions by soluble HS, the aggregation of these soluble HS macroions (due to the decrease in their charge arising from the complexation of the Co(II) ions), and finally, the surface precipitation of cobalt hydroxide. The experimental data (n s vs C e ) agree well with a model that considers the global sorption process to be the sum of single sorption processes. Speciation diagrams and FTIR results show that, from pH 2 to 6, [Co(H 2 O) 6 ] 2+ is the preferred adsorbed species for the retention of Co(II), while [Co(H 2 O) 5 (OH)] + adsorption increases with n 0 and pH. At pH 8, a surface precipitation pro- cess of an amorphous Co hydroxide begins. The pH-variable sorption isotherms present a hyperbolic shape reflecting 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.11.027 * Corresponding author. Tel.: +1 519 253 3000; fax: +1 519 973 7098. E-mail address: ralvarez@uwindsor.ca (R.A. Alvarez-Puebla). Surface Science 575 (2005) 136–146 www.elsevier.com/locate/susc