Interaction of indigo carmine dye with silica modified with humic acids at solid/liquid interface

Two distinct humic acids, one extracted from Brazilian peat soil, HA(PS), and another one obtained from commercial source, HA(FL) were attachment onto silica gel modified with aminopropyltrimethoxysilane, producing two material named SiHA(PS) and SiHA(FL), respectively. The ability of these materials in removing indigo carmine dye from aqueous solution was followed through series of adsorption isotherms adjusted to modified Langmuir equation. The maximum number of moles adsorbed gave 6.82 +/- 0.12 x 10(-4) and 2.15 +/- 0.17 x 10(-4) mol g(-1) for SiHA(PS) and SiHA(FL), respectively. Same interactions were calorimetrically followed and the thermodynamic data showed endothermic enthalpic values: 12.31 +/- 0.55 and 24.69 +/- 1.05 kJ mol(-1) for SiHA(PS) and SiHA(FL) surfaces, respectively. Gibbs free energies for two adsorption processes of indigo carmine dye presented, negative values, reflecting dye/surface interactions must be accompanied by an increased in entropy values, which are 65 +/- 3 and 98 +/- 5 J mol(-1) K-1 for SiHA(PS) and SiHA(FL) materials, respectively. The adsorption processes for both materials were spontaneous in nature although they presented an endothermic enthalpy for the interaction, resulting in an entropically favored process. (C) 2003 Elsevier B.V. All rights reserved.