Thermodynamics, kinetics, and activation energy studies of the sorption of chromium(III) and chromium(VI) to a Mn3O4 nanomaterial

In this study, a manganese oxide, Mn3O4 was used to remove chromium(III) and chromium(VI) from aqueous solutions. The Mn3O4 nanomaterial was synthesized through a precipitation method, and was characterized using XRD, which confirmed the material had a crystal structure similar to hausmannite. In addition, using Scherrer's equation it was determined that the nanomaterial had an average grain size of 19.5 +/- 1.10 nm. A study of the effects of pH on the binding of chromium(III) and chromium(VI) showed that the optimum binding pH was 4 and 3, respectively. Batch isotherm studies were performed to determine the binding capacity of chromium(III), which was determined to be 18.7, 41.7, and 54.4 mg/g respectively for 4, 21, and 45 degrees C. Chromium(VI) on the other hand had lower binding capacities of 2.5, 4.3, and 5.8 mg/g for 4 degrees C, 21 degrees C, 45 degrees C, respectively. Thermodynamic studies performed indicated the sorption process was for the most part controlled by physisorption. The Delta G for the sorption of chromium(III) and chromium(VI) ranged from -0.9 to -13 kJ/mol, indicating a spontaneous reaction was occurring. The enthalpy indicated a endothermic reaction was occurring during the binding and show Delta H values of 70.6 and 19.1 kJ mol for chromium(III) and chromium(VI), respectively. In addition, AS for the reaction had positive values of 267 and 73 J/mol for chromium(III) and chromium(VI) which indicate a spontaneous reaction. In addition, the sorption process was found to follow pseudo second order kinetic and the activation energy studies indicated the binding process occurred through chemisorption. (C) 2014 Elsevier B.V. All rights reserved.