Assessing Thermodynamic Equilibrium and Corrective approach for VOCs Adsorption on Microporous Activated Carbon

Volatile organic compounds (VOCs) are ubiquitous air pollutants arising from industrial, vehicular, and household sources, which present significant risks to both environmental quality and human health. Advances in air purification increasingly rely on high-performing adsorbents for VOC removal, with material innovations in microporous and mesoporous frameworks driving the field toward more efficient and selective remediation methods. However, accurately characterizing adsorbent affinity for VOCs at trace levels remains challenging due to extremely slow adsorption kinetics in highly confining ultramicroporous solids. In this context, accurately determining the Henry constant K H for VOC adsorption in microporous solids at low pressures is limited by difficulties in reaching thermodynamic equilibrium in standard experimental methods. Here, a thermodynamically grounded three-step methodology is introduced for reliable estimation of the Henry constant in such diffusion-limited systems. In practice, this strategy is illustrated by considering cyclohexane adsorption on commercial activated carbon. The approach involves (1) measuring adsorption isotherms at elevated temperatures (which ensure that thermodynamic equilibrium is reached), (2) extrapolating to the target temperature via Polanyi’s adsorption potential theory, and (3) finally deriving K H from the recalculated low-pressure region. Compared to conventional single-temperature volumetric measurements — where equilibrium is hardly attained at low pressure — this method allows reconstructing the adsorption isotherm. This strategy, which is validated by showing that the adsorption isotherms match those obtained from independent breakthrough experiments, reveals systematic underestimation of adsorption at low pressure with standard protocols (up to a 15-fold difference in K H ). This study demonstrates how corrections based on Polanyi’s adsorption potential theory allow overcoming equilibrium limitations, outperforming both measurements and kinetic-based protocols alone. In contrast to previous assumptions that improved measurement criteria or longer equilibration times suffice, our findings emphasize that thermodynamic equilibrium can only be ensured by relying on high-temperature master curves for systematic recalculation. By providing accurate K H values, which are critical for materials screening and process upscaling, this framework enables more robust engineering of sorbent-based air purification technologies. This methodological advance has broad implications for experimental practice in nanostructured adsorbent evaluation and supports safer, more efficient approaches to air pollution control.