Concentration- and time-dependent sorption and desorption behavior of phenanthrene to geosorbents with varying organic matter composition

Batch equilibration and the decant-refill methods were used to measure sorption and desorption of phenanthrene to four geosorbents with varying organic matter composition at different contact times to better understand the mechanisms underlying sorption and desorption processes. The sorbents were characterized by solid-state 13C cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy. Sorption and desorption isotherms were constructed and fitted to the Freundlich model and the site energy distribution model was used to measure the relative change of distributed sorption energies. Concentration- and time-dependence were observed for both sorption and desorption processes. The high energy sites become filled with increasing sorbate concentration, resulting in the non-linearity of sorption isotherms and the increase of the desorption percentages. The shape of the sorption isotherms changed with the contact time, which was characterized by the variation of Freundlich model parameters (KF and n). More favorable sorption sites could be accessed when the contact time was extended, leading to the decrease of the desorption percentages. As indicated by the value of n, partitioning appeared to contribute more to the sorption of phenanthrene for the soils with higher oxygen-substituted carbon content and less condensed soil organic matter (SOM). Sorption in adsorption domains is energetically more favorable and kinetically less accessible as compared to partitioning domains, and both the composition and conformation of SOM play important roles in the sorption and desorption processes by forming specific interactions with phenanthrene molecules and governing the accessibility of sorption domains.