Coupled diffusion and non-linear sorption as controlling factors in the uptake and release of an organic chemical in natural solids.

摘要

Sediment, soil and aquifer contamination resulting from the release of anthropogenic chemicals into the environment is a major environmental and public health concern throughout the industrialized world. At many of these contaminated sites non-polar organic contaminants have been in contact with saturated geosolids for long periods of time. The overall goal of this work is to evaluate, quantify, and model the effect of long-term "aging" (exposure time) of contaminants with some selected (and perhaps exemplary) geosorbent materials, in regards to release when flushed (desorption).;For many systems, a combination of pore diffusion and nonlinear sorption can lead to slower rates of observed uptake than release (hysteretic release). When sorption equilibrium is not attained by the time desorption is begun, observed desorption rates can be unusually slow and appear as "hysteretic", despite being controlled by a single set of fully reversible equilibrium and rate relations. In this work, I test the hypothesis that pore diffusion and nonlinear sorption are controlling processes for phenanthrene transport in porous medium bounded by both an impermeable layer fine-grained material, as relevant to a field site at Dover AFB, DE and also in a sand aquifer setting composed of calcareous rock conglomerates, as relevant to a field site at Borden, Ontario.;The laboratory experiments were conducted for various aging periods using the natural clay and silt geosolids obtained from the Dover site and also using the sand obtained from the Borden site. The fine-grained material was loaded into laboratory columns and penetrated with an Ottawa sand macropore, and the Borden sand was loaded into laboratory columns. These columns were subsequently subjected to breakthrough, continued breakthrough (aging) and elution. The "macropore column" approach permits study of diffusion in a well-characterized physical configuration and under known boundary conditions. The sand column permits study of the effects of diffusion inside the individual sand grains.;To evaluate the extent of system "predictability" during transport, breakthrough of phenanthrene was modeled on the basis of batch-derived sorption parameters and with hydrodynamic properties of the column as determined using dirac injections of tritiated water. To evaluate the applicability of the conceptual model and parameters during desorption, column elution was subsequently conducted under varied conditions of "aging" and modeled using model simulations that fully accounted for the non-equilibrium nature of sorptive uptake. Columns were loaded with steady influent phenanthrene concentrations for various periods of time up to over 2 years prior to the onset of elution. Subsequent elution and sampling was continued until the concentrations in the effluent were below detection using liquid scintillation methods. Finally, deconstruction and extraction of the experimental columns was used to determine the mass of the contaminant that was remaining after elution, as an additional point of comparison with modeled values.;Overall, elution profiles for phenanthrene were shown to be predicable using a model which combined the processes of pore diffusion and nonlinear sorption, provided that the model was also used to simulate initial (non-equilibrium) column loading, with full understanding at the actual time periods and conditions of aging. These predictions were able to capture slow rates of desorption which might otherwise have appeared as "hysteretic" if equilibrium conditions were not understood prior to desorption.