Evidence of inter-species swing adsorption between aromatic hydrocarbons

In this research, the competitive adsorption characteristics between aromatic hydrocarbons were investigated. It is well-known that an industrial effluent may contain a mixture of pollutants. The composition of effluents is usually highly variable in nature to depend upon the feedstock. Hence, one of the pollutants that is present in larger amounts may have the potential to dominate the sorption processes. Although many studies have investigated the competitive adsorption of volatile organic compounds (VOCs) onto activated carbon (AC) in detail, little is known about how the overall process is influenced when a fresh incoming VOC molecule encounters a sorbent bed pre-loaded with other VOCs. Consequently, the objective of the present study was to investigate the stability of pre-adsorbed VOC molecules in the presence of other potentially competitive VOCs in the influent stream. In this regard, the sorbent bed of AC was first preloaded with benzene (50 ppm (0.16 mg L^-1)) and subsequently challenged by either high purity nitrogen or a stream of xylene (at 10, 50, or 100 ppm (0.043, 0.22, or 0.43 mg L^-1)). The desorption rate of preloaded benzene and uptake rate of challenger xylene were assessed simultaneously. The maximum desorption rates of benzene (R_b) against two challenge scenarios (e.g., 100 ppm (0.43 mg L^-1) xylene and pure N₂) were very different from each other, i.e., 663 vs. 257 g kg^-1 h^-1, and their final benzene recoveries were 84% and 42%, respectively. The initially high desorption rate for the former quickly decreased with decreasing benzene residual capacity (C, mg g^-1). Interestingly, the adsorption capacity of xylene increased considerably after the preloading of benzene (relative to no preloading). As such, 10% breakthrough volumes (BTV10) of 100, 50, and 10 ppm (0.43, 0.22, and 0.043 mg L^-1) xylene challenge scenarios increased significantly from 100 to 186, 43.4 to 694, and 600 to 1000 L atm g^-1, respectively. The prevalent mechanisms were analyzed using density functional theory (DFT)-based modelling approaches. The results demonstrated effective replacement of pre-adsorbed molecules with weaker affinity (e.g., benzene) when challenged by molecules with stronger affinity (e.g., xylene) toward the sorbent; this was accompanied by noticeable synergistic enhancement in the adsorption capacity of the latter.