Non-singular sorption-desorption behavior is a widely known phenomenon observed for organic compounds, including agrochemicals, in liquid phase experiments with soils, sediments (and other sorbents). The hysteresis is recognized to affect environmental fate of chemicals but its nature often remains obscured. When degradation of sorbing chemicals, loss of the system integrity, slow sorption/desorption kinetics and some artifacts are ruled out, one meaningful understanding proposed in the literature to explain sorption-desorption hysteresis considers formation of metastable states in a sorbent. In fact, formation of the metastable states over the sorption branch and their persistence across desorption reflect the often-intuitive statement that the reason of hysteresis is “a change of a sorbent during sorption”. This sorbent change should have its own thermodynamic cost. This contribution presents a new approach calculating the free energy cost for appearance of sorption-desorption hysteresis caused by formation (or modification) of metastable states in a sorbent. The approach involves integration over the sequence of sorption-desorption isotherms in properly defined coordinates. This approach is illustrated by using experimental data on non-singular sorption-desorption of some compounds including pesticides on soils, sediments, organic matter and clay sorbents. Albeit the approach does not depend on a specific model, it leads to particularly simple relationships when sorption and desorption isotherms follow the Freundlich model. The principal novelty of this methodology is that it makes focus on integral free energy-based measures accounting for specific shapes of sorption and desorption isotherms. Added free energy values quantifying hysteresis as deviation from equilibrium (or initially metastable) states are associated with sorbed concentration ranges and provide a basis for further analysis, which may involve both empirical correlations (e.g., in terms of linear free energy relationships and linking to sorbate molecular descriptors) and theoretical modeling of formation of metastable states in a complexed matrix composed of minerals and natural organic matter.

Sorption-desorption hysteresis linked to formation of metastable states: How much does it cost (in terms of free energy)

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.