Sorption hysteresis on soils and sediments: Using single‐point desorption isotherms to obtain characteristic free energy values

Sorption–desorption hysteresis (SDH) may control distributions of chemicals across diverse environmental phases, including soils and sediments. Formation of metastable states caused by pore deformation or inelastic swelling of a sorbent and their persistence during desorption have been considered in the literature as one reason for “true” SDH. Such metastable states may lead to the lack of closure of the sorption–desorption loop at non‐zero sorbate concentrations, a phenomenon often noted in soil and environmental literature. SDH has often been characterized using single‐point desorption isotherms (DIs) that combine sorbed states reached during single desorption steps started from different points along a sorption isotherm (SI). This work aimed to demonstrate how single‐point DIs could be used to characterize SDH in the liquid phase in terms of the Gibbs free energy accumulated in some non‐relaxed sorbed states belonging to a DI, as compared with the states of the same composition (sorbed concentration) belonging to an SI. Based on the literature data on SIs and single‐point DIs of pesticides and some hydrocarbons on soils, artificial soils, minerals and sediments, these quantities of extra free energy were determined and their changes in the selected sorbate‐sorbent systems were examined. When the extent of SDH decreases with increasing solute concentration, these additional free energies decrease. They may remain constant or even increase, suggesting, in the latter case, that more work is needed to perturb a sorbent structure at higher sorbed concentrations. Magnitudes of calculated extra free energies may be helpful in examining whether irreversible pore deformation in a sorbent and formation of metastable states are the reasons for SDH. This paper proposes a novel approach for quantifying liquid‐phase SDH in cases in which a thermodynamic justification is sought. It advances the ability to predict the fate of chemicals in typical soil/sediment environments. Highlights Desorption from soils and sediments to solutions may be hysteretic due to formed metastable states Hysteresis is quantified in terms of excessive free energies of metastable states Extra free energies are sorbate‐ and sorbent‐dependent, varying across sorption isotherms Single‐point desorption isotherms allow quantification of the free energy excess in metastable states