SiO₂‑Amine Materials from Geothermal Silica Waste with Potential Application in Direct CO₂ Air Capture

Direct CO₂ air capture (DAC) has become a key strategy for tackling climate change, as it can help reduce the concentration of this greenhouse gas (GHG) in the atmosphere, which, as of July 2026, has risen to 429.12 ppm. Most DAC techniques are based on adsorption processes, in which sorbents must have high adsorption capacity, good selectivity, easy regeneration, low energy penalty, suitable adsorption-desorption kinetics, excellent chemical stability, and low cost. Amine functionalization of porous materials has been proposed to improve CO₂ adsorption capacity and selectivity, and using waste to obtain economically viable adsorbents has attracted interest. Thus, this research aims to develop an efficient SiO₂-amine adsorbent for direct CO₂ capture from air using geothermal silica waste. The silica waste comes from two Mexican geothermal power plants. Silica waste was characterized by XRF, XRD, and N₂ adsorption-desorption, then functionalized with TEPA, PEHA, and PEI by wet impregnation. The resulting SiO₂-amine materials were characterized by FT-IR and TG. In addition, CO₂ capture tests were carried out at 25, 30, and 35 °C with 400 ppm of CO₂. CO₂ uptake ranged from 0.17 to 0.65 mmol CO₂/g, with the CT-TEPA material achieving the highest value at 30 °C (0.65 mmol CO₂/g). The kinetic analysis showed that the nonlinear PSO model adequately described the temporal evolution of CO₂ capture. Stability evaluation over 10 consecutive adsorption-desorption cycles shows a 3.09% mass loss, mainly due to amine decomposition during desorption. This study systematically evaluates the influence of waste origin, surface treatment, and amine type on adsorbent performance under representative DAC conditions, offering an alternative strategy for developing CO₂ capture materials from locally available waste resources.