Date of Award

1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Louis J. Thibodeaux

Abstract

Hydrophobic chemicals have accumulated in sediments for most of the last century as the result of industrial and municipal discharges as well as urban and agricultural runoff to surface waters. Beginning with the Clean Water Act, these pollutant sources have been significantly reduced resulting in the sediments becoming a source of pollutants to the overlying water ecosystem. It is therefore important to determine the rate at which the contaminants associated with the sediment are released to the water. Removal or isolation of sediment-associated contaminants is often desirable. One option for isolation of contaminated sediments from the aquatic ecosystem is capping, the placement of clean (uncontaminated) sediment on top of the contaminated sediment. After cap placement, molecular and Brownian colloidal diffusion will be the dominant release mechanisms for sediment associated contaminants. Adsorbed contaminant molecules will "piggy-back" diffusing colloids. Experimental results demonstrating the mobility of natural colloids in diffusion controlled environments were used to determine effective colloid diffusivities via a mathematical model. In addition, preliminary verification of a simple mathematical model for colloid enhanced contaminant release rate is presented. 22 mg/L of natural dissolved organic carbon increased the flux of phenanthrene and pyrene by approximately 20% and 45% respectively while the model predicted 10% and 35% enhancement. Experimental and mathematical model results demonstrating the efficacy of capping at isolating contaminated sediments are presented and discussed. Three to thirteen millimeter caps of different sediments were used and an approximately 10 fold decrease in the release rate through the cap compared to the release from uncapped sediments after 50 days. Cap effectiveness was shown to be greatest immediately after placement and to decrease with time. The mathematical model predictions of the release dynamics compared favorably with the observed experimental results. The modeling of capped systems provides the basis for sound engineering design of caps as chemical containment barriers. The cap properties found to be most significant in the chemical barrier property of a cap were the thickness and organic carbon content. Both these properties tend to increase the breakthrough time and to decrease the magnitude of the chemical release rate through the cap.

Pages

274

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