Conditioning-Annealing Studies of Natural Organic Matter Solids Linking Irreversible Sorption to Irreversible Structural Expansion

摘要

Sorption of organic compounds to natural organic matter (NOM) solids is often hysteretic, showing increased sorbate affinity along the desorption branch of the isotherm. Hysteresis has important implications for pollutant fate and risk. A proposed mechanism for such manifestation of irreversibility in macromolecular NOM is the pore deformation mechanism. It postulates a sorbate-induced increase in the un-relaxed free volume of NOM during the sorption step. The term 'unrelaxed' represents the excess free volume resulting from structural metastability. In structurally rigid (i.e., glassy) solids, the backbone does not relax reversibly upon chemical desorption. Consequently, the free volumes of such sorbents - and hence their affinities for sorbate molecules - are greater during desorption than sorption. Consistent with this mechanism is the "conditioning effect", the observation of enhanced affinity of sorbate in a second sorption cycle following a prior sorption-desorption cycle of the same or a related compound. We test the pore deformation mechanism by conducting conditioning-annealing experiments on a humic acid (HA), Pahokee Peat (PP) and two synthetic polymers, glassy polyvinylchloride (PVC) and the structurally flexible (i.e., rubbery) polyethylene (PE). Chlorobenzene (CB) was used as the conditioning agent and polychlorinated benzenes as test compounds.