Sorption–desorption hysteresis of phenanthrene – Effect of nanopores, solute concentration, and salinity

Phenanthrene sorption and desorption from sediment/soil in fresh and saline water were measured, and effects of nanopores, solute concentration, and salinity on sorption–desorption hysteresis were discussed. The extent and kinetics of sorption–desorption hysteresis depend much on the pore distribution of the sorbents, and greater but slower-developed hysteresis occurred on the sorbent with higher specific surface area and more nanopores. In saline water, phenanthrene sorption was enhanced as compared to freshwater, with log KF increasing from 2.84 and 3.08 to 2.96 and 3.33 for the two sorbents, respectively; however, the sorption–desorption hysteresis was weakened, as indicated by the lower hysteresis index in saline water as compared to those in freshwater. In successive desorption, the irreversible sorbed amount of phenanthrene increased with increasing phenanthrene concentration until a maximum (Qmaxirr) was achieved, and the subsequent sorption became reversible. In saline water, Qmaxirr is much lower (10 mg kg−1) as compared to freshwater (36 mg kg−1), and phenanthrene sorption was almost reversible, especially at high concentrations. N2 sorption illustrated that soil organic matter had changed to a more condensed conformation in saline water, as indicted by the reduced surface area (from 9.6 to 7.3 m2 g−1), which is unfavorable for irreversible sorption.