Unlocking the full adsorption potential of microporous carbon for formaldehyde in complex air mixtures

Abstract

The widespread application of microporous activated carbon (AC) in air purification systems is challenged by its questionable efficacy against low-boiling, polar compounds, such as formaldehyde (FA), particularly when present in complex, multi-component streams (e.g., aromatic hydrocarbons [BTX], oxygenated [ethanol], and water vapors). Aiming to enhance its practical utility, this study undertakes a systematic investigation into the FA adsorption behavior of a commercial AC bed adsorbent with precise control over the packing particle sizes (i.e., small [AC-S: 0.075-0.212 mm], medium [AC-M: 0.212-0.6 mm], and large [AC-L: 0.6-1.7 mm]). Emphasis is also placed on the effects of relative humidity [0.015 % to 100 %], inlet FA concentrations [50-500 ppm]), and gas phase composition (non-competing single and competing multi-component streams) on the breakthrough (BT) adsorption dynamics of FA onto AC bed adsorbents. The 99 % BT adsorption capacity (mg g(- 1)) for 100 ppm FA under trace humidity conditions (5 ppm H2O) is moderately influenced by AC particle size (AC-M (12.0) > AC-S (9.8) > AC-L (5.8)). BTX severely reduces FA uptake (by 77.5-96.0 %) and induces a roll-up effect. However, humidity and ethanol synergistically enhance FA adsorption by increasing the number of polar surface groups, which in turn suppresses the competition from BTX. The dominance of physisorption for FA is suggested by its adherence to the Freundlich isotherm. Kinetics transitions (e.g., from pseudo-first-order for smaller AC particles to pseudo-second-order for larger ones) is indicative of their differing diffusion mechanisms. Spectroscopic studies reveal that water and ethanol play a dual role: they aid adsorption via hydrogen bonding and paraformaldehyde formation at moderate levels, while blocking sites at saturation. This research offers crucial insights into tuning AC properties, paving the new way for the design of application-ready adsorbents for complex environments.