Is the maximum adsorption capacity obtained at high VOC pressures (> 1000 Pa) really meaningful in real-world applications for the sorptive removal of VOCs under ambient conditions (< 1 Pa)?

Abstract

The general performance metric for sorbent is its maximum adsorption capacity in which the maximum amount of an adsorbate loadable on an adsorbent is determined from isotherms. Such a metric is useful to ascertain the maximum possible capacity obtained at high adsorbate partial pressures (e.g., > 1,000 Pa). However, this is often misleading to predict the performance of materials in real-world applications where the ambient trace pollutant levels are much lower, e.g., < 0.1 Pa. The use of such concept at such low pressure condition is also very problematic if the isotherm follows Henry's law. Overall, although such a metric is commonly reported, its utility is significantly limited for treating trace level pollutants. This study demonstrates the inadequacy of such definition based on experimental and theoretical basis. It is thus recommended that researchers must provide a complete isotherm analysis (e.g., Langmuir or Henry's law) in the 0-10 Pa pressure region as supplementary datasets in support of any bold claims that a proposed sorbent is very fit for low pollutant pressure ( < 10 Pa) air quality management (AQM) purposes. Also, if the adsorption isotherm volumetric method is used, it is suggested that the isotherms should be constructed to cover a good range of intervals over 0-10 Pa region for AQM purposes (e.g., 0, 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, 3, and 10 Pa). It is futher recommended, if possible, to expand such test over an extended range (e.g., 30, 100, 300, and 1000 Pa) for a facile assessment of performance on a log-log isotherm plot.