CONDITIONING EFFECT ON ADSORPTION OF ORGANIC COMPOUNDS TO SOIL ORGANIC MATTER

Sorption hysteresis (isotherm nonsingularity) is commonly observed in the sorption of organic contaminants in geosolids. In glassy polymers it has been attributed to irreversible internal pore (hole) deformation by the sorbate. Another manifestation of irreversibility in such polymers is known as the "conditioning effect". This refers to the observed enhancement in sorption of a compound following brief exposure of the sorbent to high concentrations of the same or a similar compound.

In this study, the conditioning effect in soil organic matter (SOM) was investigated for trichloromethane (TCM), monochlorobenzene (MCB) and 1,2,4-trichlorobenzene (TCB), selectively sorbed on two SOM-rich soils. Conditioning was performed by treating the soils with a nearly-saturated aqueous solution of a conditioning agent for 6 - 7 days followed by N2 sparging. The chosen conditioning agents are compounds different from the sorbate but chemically analogous: dichloromethane for TCM; benzene for MCB and TCB. A subsample of the same soil (referred to as "non-conditioned") was handled in exactly the same manner except for absence of the conditioning agent.

Adsorption isotherms were constructed for both conditioned and non-conditioned soils over concentration ranges of 3-4 powers of ten. All isotherms were nonlinear. The conditioning effect was significant for all three sorbate-soil systems: the isotherms for conditioned soils clearly shifted upward and nonlinearity increased compared to the non-conditioned soils. The ratio of the apparent distribution coefficient Kd for conditioned and non-conditioned soils was maximum (1.4-1.7) at very low solute concentration and tended to approach unity as solute concentration approached water solubility.

The "conditioning effect" in SOM according to the polymer model is caused by the irreversible enlargement of existing holes or creation of new holes by sorbing molecules, resulting in sorption that is greater in magnitude and more nonlinear on the conditioned sample. The irreversibility is a consequence of the matrix rigidity. The observed conditioning effect provides support for the glassy-polymer hypothesis of SOM.