Evidence of underestimation in PAH sorption/desorption due to system nonequilibrium and interaction with soil constituents.

The hypothesis evaluated was that sorption/desorption experiments conducted under nonequilibrium conditions would sorb/desorb smaller amounts of polycyclic aromatic hydrocarbons (PAHs) in comparison to those conducted under an equilibrium condition. The magnitude of phenanthrene's (PHEs) sorption coefficient was approximately the same for both equilibrium and nonequilibrium conditions. However, when the initial PHE concentrations were varied, sorption nonequilibrium resulted in a 1.4-4.8% decrease in the actual PHE sorbed than those obtained under equilibrium conditions. Similarly, thermodynamic nonequilibrium led to a smaller pyrene (PYR) desorption. For example, when PYR was initially spiked at 10 mg/L, a total of 3.1 mg PYR/kg was desorbed under desorption nonequilibrium, whereas 3.6 mg/kg when equilibrium had been achieved. Mechanistic analysis of sorptive phenomena with respect to soil organic matter (SOM), clay minerals, and dissolved organic matter (DOM) demonstrated that both expandable clays and aliphatic SOM served as main sorbents for PAH sorption/desorption; that DOM was not a primary desorptive factor; and the interaction with clay minerals were stronger than that with SOM.