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Free Humic Acid and Anchored onto Silica Gel Surface: A Thermodynamic approach

Alexandre G. S. Prado1, Elaine A. Faria2, Jonas Pertusatti1

1 - QuiCSI Team, Institute of Chemistry, University of Brasília, C.P.4478, 70904-970 Brasilia D.C., Brazil

2 - Federal Institute of Technologic Education of Rio Verde, C.P. 66, 75901-970 Rio Verde G.O., Brazil.

Abstract
Humic acid (HA) obtained from Brazilian peat soil was attached onto silica gel previously modified with 3-aminotrimethoxysilane to produce a material named SiHA. Thermogravimetric data showed that the anchored HA has a significant increase in its thermal stability. Whereas natural HA presents a thermal stability up to 240 oC, the anchored compound presents a thermal stability near 800 oC. The kinetics parameters like activation energy, enthalpy, entropy, Gibbs, pre-exponential factor were also calculated from Coats-Redfern equation. Kinetic parameters show the degradation of humic acid, presenting an excellent stability when immobilized on silica gel due to the fact that activation energy for SiHA decomposition is much higher than free HA.

Keywords: humic acid, kinetic, silica.
Introduction
Humic acid is a mixture of weak-acid polyelectrolytes formed by organic macromolecules exhibiting a large range of molar mass distribution, substructures and functionalities (NIFANTE'VA et al., 2000; ROSA et al., 2005; PERTUSATTI et al., 2007). This natural material is derived from the degradation of animals and plants (NIFANTE'VA et al., 1998). It is one of the main components of organic matter in soils and in natural aquatic environments as dissolved organic carbon (ROSA et al., 2005; PERTUSATTI et al., 2007; NIFANTE'VA et al., 1998; DZHERAYAN et al., 2008). Humic acid has a high ability to bond to organic and inorganic pollutants, such as: dyes, pesticides and heavy metals (PRADO et al., 2003; PRADO; AIROLDI, 2003; KLAVINS et al., 2006; LU et al., 1997). However, this substance is soluble in water,