Investigating CO2 removal by Ca- and Mg-based sorbents with application to indoor air treatment

Indoor carbon dioxide (CO2) levels serve as an indicator of ventilation sufficiency in relation to metabolic effluents. Recent evidence suggests that elevated CO2 exposure (with or without other bioeffluents) may cause adverse cognitive effects. In shelter-in-place (SIP) facilities, indoor CO2 levels may become particularly elevated. This study evaluates four low-cost alkaline earth metal oxides and hydroxides as CO2 sorbents for potential use in indoor air cleaning applications. Sorbents studied were MgO, Mg(OH)2, Ca(OH)2 and commercially available soda lime. Uncarbonated sorbents characterized with nitrogen adsorption porosimetry showed BET surface areas in the 5.6-27 m2/g range. Microstructural analyses, including X-ray diffraction, thermogravimetric analysis and scanning electron microscopy confirmed the carbonation mechanisms and extent of sorption under environmental conditions typical of indoor spaces. Ca-based sorbents demonstrated higher extent of carbonation than Mg-based sorbents. Laboratory parameterizations, including rate constants (k) and carbonation yields (y), were applied in material balance models to assess the CO2 removal potential of Ca-based sorbents in three types of indoor environments. Soda lime (k = [2.2-3.6] × 10−3 m3 mol CO2−1 h−1, y = 0.49-0.51) showed potential for effective use in SIP facilities. For example, CO2 exposure in a modeled SIP facility could be reduced by 80% for an 8-h sheltering interval and to levels below 5000 ppm for an 8-h period with a practically sized air cleaner. Predicted effectiveness was more modest for bedrooms and classrooms.