Demonstration of the "conditioning effect" in soil organic matter in support of a pore deformation mechanism for sorption hysteresis

Hysteresis, or isotherm nonsingularity, is a confounding issue in sorption research that undermines the commonplace assumption of reversibility in environmental fate and effects models for organic compounds in soil media. Until now, a molecular-level mechanism for true hysteresis when the sorbate is retrievable, structurally intact, has not been forthcoming. We show here that two organic soils exhibit the "conditioning effect", which refers to the enhancement in sorption of a compound following brief exposure of the sorbent to high concentrations of the same or a similar compound. The conditioning effect has been used in support of a pore deformation mechanism for hysteresis in glassy polymers. By this mechanism, the sorbate causes irreversible changes in the structure of internal nanopores (holes) in the organic matrix upon its sorption. Trichloromethane was the test solute for dichloromethane-conditioned Pahokee soil (44.6% organic carbon), and chlorobenzene and 1,2,4-trichlorobenzene were the test solutes for benzene-conditioned Mount Pleasant silt loam (4.5% organic carbon). In each case, the isotherm of the test solute in the conditioned soil was shifted upward of, and was less linear than, the corresponding isotherm in the nonconditioned control. Application of the polymer-based Dual-Mode (partitioning-hole filling) Model shows an expansion of the hole domain as a result of conditioning. The memory of the conditioning effect persists for longer than 96 days at 21 degrees C but is lost upon heating the sample at 100 degrees C. A three-step (sorption-desorption-resorption) experiment demonstrated hysteresis followed by enhanced resorption, implying a mechanistic relationship between hysteresis and the conditioning effect. The results indicate that irreversible pore deformation is a mechanism for hysteresis in natural organic matter materials and suggest that slow matrix relaxation may contribute to the often-observed long-term resistance of some contaminants to desorption.