Adsorption equilibrium of activated carbon amid fluctuating benzene concentration in indoor environments

Adsorption is commonly employed to remove volatile organic compounds (VOCs) from indoor air. However, the reversibility of this process has been rarely explored in prior research. Meanwhile, some previous research indicated partial reversibility in adsorption and desorption processes, which has not been described in traditional adsorption equilibrium models. In this study, we measured benzene adsorption and desorption isotherms on 6 activated carbons within two concentration ranges: 0-3 ppm and 0-6 ppm. The results showed that the adsorption process was partially reversible and the desorption isotherm was hysteretic for all the samples. Different carbons exhibited distinct isotherm patterns, yet the desorption isotherms in the experimental concentration range can be calculated from the adsorption isotherms using the same empirical equation. In addition, we measured the breakthrough curves using the oscillating benzene concentration (6 ppm to background) on the test columns. Reversible adsorption was observed at low degrees of saturation of the test column. Especially for microporous activated carbon, desorption could be sustained for several hours with purging even at the breakthrough rate of about 10%. An adsorption equilibrium model incorporating partially reversible adsorption and hysteretic desorption isotherm was developed based on the experimental data, which allows a preliminary interpretation of the measured breakthrough curves at fluctuating inlet concentrations.