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

Sorption-desorption hysteresis (SDH) is often observed in liquid phase (solution) sorption experiments with various chemicals on complex natural materials, including soils and sediments. Sorption-desorption interactions with soils and sediments are of significant fundamental and applied interest since they control the transport and fate of chemicals in environmental systems. SDH expressed as a difference between sorption and desorption isotherms determined in solutions may demonstrate time-independent behavior. This work aims to propose a concept that could mechanistically explain and allow 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 proposed concept introduces the gates present in a sorbent matrix and 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. This predictive potential may be improved by accounting for the assumptions introduced while developing the models.