CO₂ adsorption on chemically modified carbons from rice husk: kinetic, equilibrium and thermodynamic study

Currently, a wide variety of CO₂ capture and storage technologies have been studied around the world to reduce this environmental problem. In this concern and with the purpose of contributing to the reduction of high atmospheric concentrations of CO₂, the present investigation was carried out, where rice husk was used to optimally prepare a chemically activated carbon with KOH to efficiently adsorb CO₂. Initially, the rice husk was subjected to different carbonization temperatures, followed by chemical activation under various KOH impregnation ratios, activation temperatures, and activation times. The developed coals were characterized using different analytical techniques: N₂ physisorption measurements, scanning electron microscopy (Sem), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). To understand the CO₂ adsorption behavior in the activated carbon prepared with the best textural and morphological characteristics, the non-linear kinetic models of pseudo-first order, pseudo-second order, Elovich and intra particle diffusion were evaluated. Nonlinear isotherms of Langmuir, Freundlich and Temkin, including the thermodynamic parameters of the solid-gas system. In addition, the carbon activated chemically with KOH was chemically reactivated with urea and magnesium nitrate, the materials obtained were characterized and their CO₂ adsorption capacity was studied under environmental conditions of pressure and temperature. The results show that the optimal conditions found to prepare the chemically activated carbon were first carrying out the carbonization of the natural rice husk at 400 ° C for 1 h, followed by a chemical activation with KOH using a residue mass impregnation ratio carbonaceous / activating agent 1: 3 at 600 °C for 1 h. On the other hand, the results of the characterization indicate that a carbonaceous adsorbent with a porous structure was obtained, with a high surface area of 1384.4 m2/g and a total pore volume of 1.0854 cm³/g, and that it showed a CO₂ adsorption capacity of 2.5382 mmol/g at atmospheric conditions for 30 minutes of saturation. On the other hand, under the same conditions CO₂ adsorption experiments were carried out using the un activated carbonaceous residue, obtaining a maximum adsorption capacity of 1.1557 mmol/g; therefore, the chemically activated material exceeded by 119% the adsorption of CO₂ with respect to the carbonaceous residue. On the other hand, the kinetic study of CO₂ adsorption on chemically activated carbon with KOH showed that the adsorption mechanism was described by the non-linear pseudo first order model; and that the experimental data were fitted to the non-linear Freundlich isotherm model, revealing the heterogeneity of the adsorbent surface. Furthermore, the thermodynamic parameters determined showed that the CO₂ adsorption process in the adsorbent was exothermic, spontaneous and feasible by nature. On the other hand, when the resulting KOH chemically activated carbon was reactivated with urea and magnesium nitrate, it showed a CO₂ adsorption capacity of 2.0642 mmol/g for reactivation with urea and 1.4052 mmol/g for reactivation with nitrate magnesium, respectively. Finally, the results of this study show that the chemical activation with KOH of a carbon based material allows to optimally and economically prepare a valuable activated carbon to be potentially used in large-scale CO₂ capture technologies in order to mitigate climate change. (author)