Enhanced Carbon Dioxide Adsorption in Sandstone Analogues with Coal Fly Ash: Role of Mineral Composition in Geo-Sequestration

Nanotechnology offers promising pathways to enhance CO2 geo-sequestration by improving trapping mechanisms like adsorption. However, the widespread deployment of commercial nanomaterials is hindered by their high cost and energy-intensive synthesis. This study investigates the viability of coal fly ash, a plentiful and low-cost industrial byproduct, as a sustainable alternative for enhancing CO2 adsorption in sandstone formations. Volumetric adsorption experiments were conducted on Berea sandstone treated with Class C fly ash (0.1–0.75 wt %) at reservoir-relevant conditions of 50–70 °C and pressures up to 10 MPa. The results demonstrate that fly ash treatment significantly enhances CO2 uptake in a concentration-dependent manner. At 50 °C and 10 MPa, adsorption capacity increased from 0.203 mmol/g (untreated) to 0.238 mmol/g (0.1 wt %), 0.340 mmol/g (0.5 wt %), and 0.393 mmol/g (0.75 wt %), representing a 17% to 94% improvement. These enhancements arise from the introduction of additional surface area and reactive mineral sites that promote physisorption and incipient mineral carbonation, with FTIR spectra further confirming incipient mineral carbonation through the emergence of carbonate bands at 1439, 873, and 712 cm–1 following CO2 exposure. The adsorption behavior was best described by the Freundlich and Sips isotherm models (R2 > 0.97), confirming heterogeneous surface interactions. Crucially, even a minimal fly ash concentration (0.1 wt %) yields a geologically significant storage increment, potentially sequestering an additional 0.93 million tonnes of CO2 in a representative reservoir model. This work establishes fly ash as an effective, economical, and environmentally sustainable adsorbent for enhancing CO2 storage security in deep geological formations, transforming a waste product into a valuable resource for carbon management.