Journal of Colloid and Interface Science 270 (2004) 47–55 www.elsevier.com/locate/jcis Cu(II) retention on a humic substance R.A. Alvarez-Puebla, a C. Valenzuela-Calahorro, b and J.J. Garrido a,∗ a Department of Applied Chemistry, Public University of Navarra, Campus Arrosadía, E-31006 Pamplona, Spain b Department of Inorganic Chemistry, Faculty of Pharmacy, University of Granada, E-18071 Granada, Spain Received 11 February 2003; accepted 26 August 2003 Abstract Humic substances (HS) are macromolecular products derived from a physical, chemical, and microbiological process called “humifi- cation.” These substances play an important role in the mobility and bioavailability of nutrients and contaminants in the environment. Adsorption isotherms provide a macroscopic view of the retention phenomena. However, complementary techniques are needed in order to study the retention mechanism. The application of the classical models and some modern ones, based on humic substances chemistry, do not accurately describe these adsorption data. The aim of this paper is to model isotherms and combine adsorption data with spectroscopy and microscopy techniques to study the Cu(II) retention on a HS. The adsorption isotherms shape varies significantly with the solution pH from L-type (pH 2–6) to S-type (pH 8). FTIR shows that, when pH is 2 the retention of Cu(II), as [Cu(H 2 O) 6 ] 2+ , is the preferred retention mechanism. The quantity of Cu(II) retained as [Cu(OH)(H 2 O) 6 ] + rises, as pH increases. At pH 4, Cu(II) begins to precipitate, which is the preferred mechanism at pH 8.02. The presence of HS has a great influence on the precipitation process of Cu(II), giving rise to amorphous precipitates. As it is shown by SEM-XRF, Cu(II) distributes heterogeneously on HS surface and accumulates on the humic phases. The presence of different anions (chloride and nitrate) slightly modifies the HS behavior as cation exchanger. When Cl - ions are present, part of the Cu(II) form [CuCl 4 ] 2- , which is stable in solution due to its negative charge; when the anion present is NO - 3 the formed complex, [CuNO 3 ] + , is retained on the HS.  2003 Elsevier Inc. All rights reserved. Keywords: Adsorption; Humic substance; Isotherms; Cu(II); FTIR; SEM 1. Introduction The common definitions for humic substances (HS) and their fractions are ambiguous and, in some instances, arbi- trary [1]. However, it could be said that they are macro- molecular products derived from a physical, chemical, and microbiological process called “humification” of organic molecules from plants, animals, microorganism tissues, and metabolic products. The HS are the most widespread nat- ural nonliving organic materials in all terrestrial and aquatic environments, up to 80% of soil organic matter and up to 60% of dissolved organic carbon [2]. Based upon their sol- ubility in acids and alkalis, HS can be fractioned into humic acids (HA), fulvic acids (FA) and humin. The HA can be di- vided into brown humic acids (BHA) and gray humic acids (GHA) [3]. Due to their colloidal and polyfunctional charac- * Corresponding author. E-mail address: j.garrido@unavarra.es (J.J. Garrido). ter, these substances play an important role in the mobility and bioavailability of nutrients and contaminants in the en- vironment [4]. One of the main features of HS in the en- vironment is their capacity to interact with metal ions to form soluble complexes, colloidal substances, and/or insolu- ble substances [3]. The interactions between HS and metals have great interest from chemical, biological, and environ- mental perspectives, due to the capacity of HS to complex metal ions. The study of interactions between HS and metal ions requires the characterization of both. The chemical be- havior of transition metals is well known [5]. The behavior of humic substances is scarcely known, despite their phys- ical and chemical similitudes, as a consequence of their fractal nature [6]. Elemental analysis is probably the most frequent tool used in the characterization of HS. This tech- nique provides information about the distribution of major elements and is useful to distinguish different fractions of HS [2]. Carboxyl and phenolic groups are the main acidic groups. The determination of such groups in HS is closely connected with the retention of metal ions. 0021-9797/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.068