Impact/Purpose:

Nearly all natural processes are either cyclic or episodic, yet laboratory efforts to simulate fate and transport are almost always steady state column or batch experiments. If the mechanisms of contaminant fate are fundamentally linear (not necessarily first order) and reversible, then there are well established mathematical procedures to model the impact of cyclic or episodic (e.g., a single or stepped spike, etc., events) variations. Unfortunately, many natural processes of importance to contaminant fate are non‑reversible as they occur in lakes, streams, and estuaries. It will be shown that these non‑reversible processes are expected to have previously unexpected and profound impacts on the fate of common contaminants in natural systems of particular importance to the Gulf Coast HSRC regions.

Commonly recognized processes that are known to change over scales of space and time of interest to modeling include: temperature, salinity, pH, redox potential, and oxygen tension. The overall objective of this research will be to develop model procedures and equations which can be used to predict the effects of changes in natural environmental variables on slow, or resistant, desorption and long‑term clean‑up.

Description:

The studies provide the scientific community with a greater understanding of the physiochemical processes of sediment-contaminant interaction. A primary consideration in sediment clean-up is when to stop, or how clean is acceptable. Present mathematical models assume that as long as a clean-up operation continues that the contaminant concentration will be steadily reduced toward zero. Yet, there have been numerous reports that sediment decontamination is biphasic with a fast initial removal followed by a very slow, or no, release, when it is known that contaminants still remain in the sediment. This project has demonstrated that, with some sediments, a fraction of the pollutant is adsorbed irreversibly and is not removable by ordinary means. This means that it will not desorb, as would be predicted by any previous models. This has the potential of reestablishing a limit of clean-up that will protect the environment and public health and at the same time cut remediation costs considerably. Since a major part of the clean-up costs in contaminant sediments is related to removing this last residual of contaminant, the cost savings by reestablishing an effective "clean" level could be several percentages, at least, of a typical clean-up operation. The semi-empirical model will be used in numerous fate prediction and remediation models, whether physical, chemical or biological.

The desorption of contaminants from soils/sediments is one of the most important processes controlling contaminant transport and environmental risks. None of the currently adopted desorption models can accurately quantify desorption at relatively low concentrations; these models often overestimate the desorption and thus the risks of hydrophobic organic chemicals, such as benzene and chlorinated solvents. In reality, desorption is generally found to be biphasic, with two soil-phase compartments. A new duel-equilibrium desorption (DED) model has been developed to account for the biphasic desorption. This model has been tested using a wide range of laboratory and field data and has been used to explain key observations related to underground storage tank (UST) plumes. The DED model relates the amount of a chemical sorbed to the aqueous concentration, with simple parameters including octanol-water partition coefficient, solubility and fractional organic carbon; thus, it is the only biphasic model, to date, that is based on readily available parameters. The DED model can be easily incorporated into standard risk and transport models. According to this model, many regulatory standards of soils and sediments could be increased without increasing the risks.

Two Ph.D. students and one MS student graduated from this program:

• Wei Chen, Ph.D., 1999, Professor/Director Tianjin Key Laboratory of Remediation and Pollution Control for Urban Ecological Environment, Nankai University, China.
• Laine Vignona, Ph.D., 2000, Assistant professor, University of Wisconsin- River Fall.
• Ellen Moore, M.S., 2000, Consultant, Blasland Bouck & Lee, Inc.

URLs/Downloads:

Final Progress Report

Record Details:

Record Type:PROJECT( ABSTRACT )

Start Date:09/01/1998

Completion Date:08/31/2001

Record ID: 169367

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Related Organizations:

Role :OWNER

Organization Name :RICE UNIVERSITY

Organization Name :GCHSRC

Mailing Address :6100 S Main

Citation :Houston

State :TX

Zip Code :77005

Project Information:

Approach :

In recent years, numerous researchers have observed that contaminant desorption processes are often not reversible. This has been observed for both inorganic and organic compounds. In most cases the desorption process has been found to be much slower than the adsorption reaction. Explanations and models advanced to explain these observations have included intra‑particle diffusion limitations, bulk linear driving‑force limitations, simply slow kinetics, experimental artifacts, unaccounted‑for chemical reactions, etc. The general consensus is that at least practical adsorption and desorption of hydrophobic organic contaminants can not be described as linear reversible processes. Desorption from this resistant fraction often accounts for greater than half of the clean‑up effort with little, or no, improvement in environmental quality.

Cost :$46,352.00

Research Component :Hazardous Waste/Remediation

Approach :

In recent years, numerous researchers have observed that contaminant desorption processes are often not reversible. This has been observed for both inorganic and organic compounds. In most cases the desorption process has been found to be much slower than the adsorption reaction. Explanations and models advanced to explain these observations have included intra‑particle diffusion limitations, bulk linear driving‑force limitations, simply slow kinetics, experimental artifacts, unaccounted‑for chemical reactions, etc. The general consensus is that at least practical adsorption and desorption of hydrophobic organic contaminants can not be described as linear reversible processes. Desorption from this resistant fraction often accounts for greater than half of the clean‑up effort with little, or no, improvement in environmental quality.

Cost :$46,352.00

Research Component :Targeted Research

Project IDs:

ID Code :R828598C700

Project type :Center