Demonstration of the “Conditioning Effect” in Soil Organic Matter in Support of a Pore Deformation Mechanism for Sorption Hysteresis

Abstract

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.

Diffusion-Controlled Hysteresis

“…In the case of swelling, for example, the total sorption capacity on the desorption path may be larger than the initial sorption capacity. Another mechanism is the so-called conditioning effect, which manifests itself in the fact that subsequent (ad)sorption measurements on the same material tend to give higher sorption uptake (Lu and Pignatello, 2002;Wang et al, 1998). For the CFCMS sample we have found that after we had performed two adsorption runs described above (Fig.…”

Thermodynamic analysis of water vapour sorption behaviour of juvenile and mature wood of Abies alba Mill.

“…The dual-mode model by Vrentas and Vrentas [79] could perhaps solve the problem, as it considers that the phenomenon of hysteresis occurs because the material is in two different physical states in the adsorption and desorption processes [80,81]. Hill et al [82] assumed that thermodynamic equilibrium was reached locally in each process.…”

The water vapor sorption behavior of natural fibers

“…During desorption, relaxation of the matrix to the state it was in during adsorption is kinetically hindered. 54 The free volume (that is internal cell wall space not occupied by macromolecules; also termed microcapillaries) is greater on desorption compared to adsorption, hence adsorption and desorption are occurring within different physical environments. 55 The effect of matrix crosslinking upon hysteresis was tested using humic soils treated with Al 3þ ions, where it was found that increased hysteresis was linked to a higher crosslinking density.…”