CO adsorption on PEHA-functionalized geothermal silica waste: a kinetic study and quantum chemistry approach

The use of solid waste and industrial by-products as alternatives to traditional raw materials to develop CO 2 adsorbents has recently gained interest. In the present work, silica waste produced in a geothermal plant was impregnated with pentaethylenehexamine (PEHA) to prepare an amine-modified CO 2 adsorbent. The SiO 2 -PEHA adsorbent was characterized, and the effects of temperature and initial CO 2 concentration on the CO 2 adsorption were assessed. Experimental CO 2 adsorption results were fitted to pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (IP diffusion) kinetic models. In addition, using the Gaussian 16 computational chemistry software package with the 6-311+(d,p) basis set coupled with the B3LYP hybrid density functional theory model, an adsorption mechanism between PEHA and CO 2 was proposed. The results showed that the CO 2 adsorption rate increases as the temperature and the initial CO 2 concentration increase. This happens as the resistance to CO 2 diffusion decreases until the mass transfer limitations dominate. The maximum CO 2 adsorption capacity was 1.05 mmol CO 2 per g material at 60 °C with 10 vol% CO 2 . The PSO model showed the best fit for the CO 2 adsorption process on the SiO 2 -PEHA material. This suggests a chemical reaction between CO 2 and PEHA molecules. Quantum chemical techniques allowed corroborating the formation of ammonium carbamate as a product of the exothermic reaction of CO 2 adsorption on the SiO 2 -PEHA material. The use of geothermal silica waste to prepare amine-modified CO 2 adsorbent materials was succesfully tested.