Materials Discovery and Characterization for Carbon Dioxide Capture in Soil

Soil respiration processes constitute an enormous fraction of the global carbon budget, releasing CO2 at rates nearly eight times greater than fossil fuel combustion. Accordingly, soil-based CO2 mitigation strategies may present a potent lever to fight climate change. Existing soil-based technologies, such as enhanced weathering and soil carbon amendments, are energy intensive and largely focus on agricultural applications. Uncultivated areas, especially forested regions, thus provide another mitigation opportunity not addressed by current technologies. We propose that highly enriched CO2 concentrations (45-50 times above atmospheric levels) in the shallow soil zone may enable direct capture of CO2 in the gas phase by solid sorbent materials. CO2 adsorption on porous solids is thought to be influenced by a variety of factors, including surface area, pore size and structure, and surface functionality. How these factors collectively influence CO2 adsorption on porous solids under the conditions relevant to soil, however, remains poorly understood. Herein we provide CO2 adsorption isotherms and water co-adsorption data for seven candidate materials (activated carbon, mesoporous carbon, biochar, zeolite 13X, montmorillonite, CMK-3, and CFK-13) and find that superior CO2 adsorption is achieved by high surface area, high micropore volume, and surface-functionalized solids. Further, though surface area and porosity are essential determinants of sorption capacity, specific surface chemistry and pore structure may exert stronger effects on dry CO2 uptake. Consistent with previous work, we find that increased relative humidity tends to reduce CO2 sorption capacity, though the effect appears to vary greatly among different sorbent types. Beyond CO2 mitigation, sorbent amendments to soil may offer co-benefits including CH4 emissions reductions and improved water retention. We expect that these results will enhance understanding of gas sorption on porous solids under soil conditions and inform the development of novel soil-based CO2 mitigation approaches.