Time-independent desorption hysteresis in liquid phase sorption experiments: the concept and the models based on gate-sorption site coupling

Sorption–desorption hysteresis (SDH), expressed as a difference between sorption and desorption isotherms, may demonstrate time-independent behavior in liquid phase experiments with various chemicals on complex natural materials, including soils and sediments. This work proposes a concept that mechanistically explains and allows predictions of time-independent SDH in three different scenarios: (1) sorbed molecules are entrapped and physically blocked from their exchange with the environment; (2) sorbed molecules are irreversibly bound to sorbent matrix such that the sorption sites capable of irreversible binding are not fully occupied in the presence of non-zero concentrations of solutes; (3) SDH is associated with forming of a non-relaxed sorbent state where the free exchange of sorbate molecules with the environment occurs. The novelty of the proposed concept is that it introduces the gates present in a sorbent matrix which are capable of concentration-dependent cooperative opening/closure, thus acting as a switch: sorbate interactions with sorption sites are allowed at increased solute concentrations but not the opposite. Coupling the gates distribution with the distribution of sorption sites allows addressing each scenario of interest and explaining time-independent SDH. The models developed within the concept can represent and even predict desorption data using a minimal number of adjustable parameters. Graphical abstract