Thioglycolic acid grafted onto silica gel and its properties in relation to extracting cations from ethanolic solution determined by calorimetric technique

Abstract

Thioglycolic acid was immobilized onto silica gel surface using 3-aminopropyltrimethoxysilane as precursor silylating agent to yield silica. The amount of thioglycolic acid immobilized was 1.03 mmol per gram of silica. This new surface displayed a chelating moiety containing nitrogen, sulfur, and oxygen basic centers which are potentially capable of extracting cations from ethanolic solution, such as MCl 3 (M=Fe, Cr, and Mo). This process of extraction was carried out by the batch method when similar chemisorption isotherms were observed for all cations. The data were adjusted to a modified Langmuir equation. The sequence of the maximum retention capacity was Cr(III) > Mo(III) > Fe(III). The same adsorption was determined by calorimetric titration and the enthalpic values of −35.75±0.02, 32.90±0.15, and −84.08±0.12 kJ mol −1 for chromium, molybdenum, and iron, respectively, were obtained. From the calculated Gibbs free energy −23.4±0.2, −27.2±0.2, and −32.7±0.3 kJ mol −1, the variations in entropy obtained were 42±1, 201±1, and 172±1 J K −1 mol −1 for the same sequence. All thermodynamic values are in agreement with the spontaneity of the proposed cation–basic center interactions for these chelating processes.