Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (2004) 137–143 Attachment of two distinct humic acids onto a silica gel surface Alexandre G.S. Prado ∗ , Barbara S. Miranda, José A. Dias Instituto de Quimica, Universidade de Brasilia, Caixa Postal 4478, 70919-970 Brasilia, Distrito Federal, Brazil Received 7 May 2003; accepted 23 April 2004 Available online 17 June 2004 Abstract A method for the attachment of two different humic acids onto silica gel has been developed. In the first step, the precursor was prepared by reacting the silylant agent 3-aminopropyltrimethoxysilane (APTS) with silica gel. Elemental analysis of the product showed the presence of 1.23 mmol of silylant agent per gram of support. Humic acid extracted from peat soil, HA PS , and commercial humic acid, HA FL , were connected by hydrogen bonds to the amine groups previously anchored onto silica gel, producing new materials named SiHA PS and SiHA FL , with 84.7 ± 0.04 and 101.7 ± 0.04 mg of humic acid per gram of modified silica for SiHA PS and SiHA FL , respectively. Thermogravimetry, infrared, 13 C and 29 Si CP/MAS–NMR spectra have confirmed the success of the attachment and the scanning electron microscopy–energy dispersive spectrometer (SEM–EDS) technique showed a good dispersion of the humic acids onto modified silica surface for both materials. © 2004 Elsevier B.V. All rights reserved. Keywords: Humic acid; Silica; Immobilization 1. Introduction Development of inorganic–organic hybrids with a variable number of simple or complex anchored molecules bonded on inorganic surfaces is finding scientific and technologi- cal applications in several areas [1]. Among a large num- ber of inorganic oxides, silica gel fulfills a significant role due to its ability to anchor organic chains designed to em- brace the desired functional groups while also increasing chemical and thermal stabilities of the ligands [2]. The ac- tive hydrogen atoms of the silanol groups dispersed on a silica gel surface can react with organosilyl groups of the general formula (RO) 3 SiY, with Y being an organic chain containing the required functional groups [3]. This type of reaction creates a versatile surface, producing materials with some organic character bonded to the inorganic framework. This class of materials is superior to organic carrier poly- mers from the point of view of practical advantages such as structural stability, swelling behavior, thermal properties, accessibility of reactive centers, and insolubility in water and organic solvents [4]. These hybrids have been used in a wide range of applications, such as heterogeneous catalysis [5], metal ion and pesticide preconcentration [6,7], ion ex- ∗ Corresponding author. Tel.: +55 61 307 2166; fax: +55 61 273 4149. E-mail address: agsprado@unb.br (A.G.S. Prado). change [8], separation of metal ions [9,10], stationary phases for chromatography [11,12], biotechnology [13], and in con- trolled release pesticides [14,15]. The attachment of humic acids, which are a very important class of naturally occur- ring complex agents [16,17], can lead to materials with in- teresting properties. Humic acids are formed by a mixture of weak-acid polyelectrolites [18,19], which present a com- plex class formed by organic macromolecules with a large variety of molar mass distributions, substructures and func- tionalities [20,21]. Moreover, these natural materials present a high ability to adsorb and to complex metal ions, organic compounds and other possible contaminants [22–24]. Thus, the preparation of hybrids with humic acid properties should lead to new materials with complexation characteristics of humic acids and with the insolubility, thermal and chemical stability characteristics of silica gel. This type of material utilizes the immobilized humic acid for removing contami- nants from water and/or other solvents, while avoiding hu- mic acid loss by solubilization. In addition, this material can be reused without waste [25]. The present investigation reports a methodology for the attachment of two distinct humic acids onto modified silica gel surface and their characterization by thermogravimetric analysis, infrared, scanning electronic microscopy (SEM) coupled with energy dispersive spectrometer (EDS), and 13 C and 29 Si CP/MAS–NMR spectroscopies. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.065