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NIEHS/EPA Superfund Basic Research Program 'Research Brief- ' Num ber 56
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From: "Inhof.Christina", INTERNET:inhof@niehs.nih.gov
To: "'Research Brief List'", INTERNET:sf-brief@bobo.niehs.nih.gov
Date: 10/6/99 5:42 PM
RE: NIEHS/EPA Superfund Basic Research Program 'Research Brief- ' Num ber 56
SBRP "Research Brief" - Number 56
Title: Electrolytic Strategies for Remediation of Chlorinated Solvents in
Groundwater
Chlorinated solvents are among the most widespread contaminants of
groundwater in this country. Several of these compounds, including vinyl
chloride and trichloroethylene, are prominent pollutants at Superfund sites,
where in many cases they have migrated into aquifers and seriously degraded
the groundwater quality of surrounding communities. Most chlorinated
solvents are harmful to humans - for example, some are known or probable
human carcinogens, while others are associated with birth defects and liver
damage - so, it is important to protect communities from exposure to these
compounds. This is being accomplished in part by the extensive efforts on
hazardous waste sites to remediate contaminated aquifers.
The task of cleaning up groundwater has proven to be one of the most
difficult and expensive challenges facing remedial project managers and the
environmental cleanup industry. Conventional methods of groundwater
remediation use "pump-and-treat" strategies, which can be energy intensive
and sometimes produce a contaminant disposal at ground surface. Experience
has also shown that chemical residuals are very difficult to remove using
this approach.
In the last few years, promising alternatives to pump-and-treat systems have
been under development. One such approach is the use of permeable, reactive
barriers that contain zero-valent iron (ZVI). These barriers are actually
incorporated into the aquifer and can be thought of as subsurface reaction
walls. This in situ form of treatment works by stripping the chlorine atoms
from chlorinated hydrocarbons via reductive dehalogenation reactions and
leaves behind innocuous products such as methane and ethane. However, this
process has its own shortcomings. The long-term efficiency of ZVI barriers
may be compromised by the oxidation of iron surfaces, which can lead to less
favorable reaction kinetics and eventual contaminant breakthrough. New
groundwater treatment methods are greatly needed.
Researchers at the University of Arizona are developing novel
electrochemical methods for the in situ remediation of chlorinated
contaminants. These methods offer the means to overcome the potential
shortcomings of ZVI barriers. To begin, electrolytic reactors can be
designed to take advantage of the "redox" properties of halogenated target
compounds. Heavily chlorinated compounds such as perchloroethene and carbon
tetrachloride can be completely dehalogenated at the cathode of an
electrolytic reactor via processes that are analogous to reductions by ZVI.
Easily oxidized compounds such as vinyl chloride can be oxidized at the
anode. In addition, electrode materials can be selected to avoid corrosion,
and process kinetics can be controlled to a degree that is absent in current
ZVI systems.
Significant progress has been made in identifying materials -- including
copper, nickel, zinc and iron -- that efficiently dehalogenate chlorinated
solvents at the cathode terminal in electrolytic systems. Using these
materials, the reaction products, kinetics and mechanisms have been
characterized for the reductive dehalogenation of several common chlorinated
groundwater contaminants.
Anodic oxidation of halogenated solvents has also been investigated. The
oxidation of trichloroethene was examined using a conductive ceramic anode
made of partially reduced titanium oxide. Trichloroethene was rapidly
converted to carbon dioxide and carbon monoxide with the cogeneration of
molecular oxygen. An investigation of the mechanistic details - for
example, the dependence of trichloroethene transformation on hydroxyl
radical formation - is underway.
Moreover, recent studies have shown that gas-phase chlorinated hydrocarbons,
including perchloroethene, trichloroethene and carbon tetrachloride, can be
electrolytically reduced. These findings open the way to reactor design for
treatment of solvent-contaminated gases derived from soil-vapor extraction,
bioventing, gas sparging and related processes. Ongoing studies are directed
toward the selection of electrode materials and other design features that
are relevant to the treatment of gas-phase contaminants.
Transfer of these electrolytic methods to the field for practical
applications will depend on reactor design and scale up. Several bench- and
pilot-scale electrolytic reactors have been constructed and evaluated under
conditions that would likely be encountered in the field.
These recent findings show that the electrolytic decomposition of
chlorinated solvents is promising for in situ treatment of contaminated
groundwater. Both oxidative and reductive transformation strategies are
available, and reaction kinetics, which are very fast, are to some extent
under operational control via selection of electrode potential, material,
specific surface area and manipulation of reactor hydraulics. Perhaps the
most encouraging aspect of the research to date is the treatability of
gas-phase contaminants in electrolytic reactors, which may be useful for the
destruction of gases brought to the surface using solvent extraction
techniques.
As a class, chlorinated solvents represent some of the most toxic,
environmentally persistent compounds in groundwater. In some areas of the
country these compounds have rendered the groundwater unsafe for human
consumption, depriving some communities of a much-needed source of water.
This well-recognized problem has been difficult to confront, so these
studies represent exciting developments towards a potential new technology
that addresses some of the most troublesome aspects of groundwater
remediation.
_______________________________________________________________
For more information please contact:
Robert Arnold, Ph.D.
PO Box 210011
Department of Civil Engineering
University of Arizona
Tucson AZ 85721-0072
Phone: (520) 621-2410
Email: arnold@engr.arizona.edu
Eric Betterton, Ph.D.
Department of Atmospheric Sciences
University of Arizona
P.O. Box 210081
Tucson, AZ 85721-0081
Phone: (520) 621-2050
Email: better@atmo.arizona.edu
To learn more about this research please refer to the following sources:
Chen, G., E. A. Betterton and R. G. Arnold. 1999. Electrolytic oxidation of
trichloroethylene using a ceramic anode. Journal of Applied
Electrochemistry. (Accepted)
Liu, Z., E. A. Betterton and R. G. Arnold. 1999. Electrolytic reduction of
low-molecular-weight chlorinated aliphatic compounds-structural and
thermodynamic effects on process kinetics. Environmental Science and
Technology. (Submitted)
Liu, Z., R. G. Arnold, E. A. Betterton and K. D. Festa. 1999. Electrolytic
reduction of CCl4 - Effects of cathode material and potential on kinetics,
selectivity and product stoichiometry. Environmental Engineering Science
16:1-13.
Betterton, E. A., R. G. Arnold, R. J. Kuhler and G. A. Santo. 1995.
Reductive dehalogenation of bromoform in aqueous solution. Environmental
Health Perspectives 103(5):89-91.
Warren, K. D., R. G. Arnold, T. L. Bishop, L. C. Lindholm and E. A.
Betterton. 1995. Kinetics and mechanism of reductive dehalogenation of
carbon tetrachloride using zero-valence metals. Journal of Hazardous
Materials 41:217-227.
_______________________________________________________________
As always, your feedback is welcomed.
Beth Anderson
Program Analyst
Superfund Basic Research Program
National Institute of Environmental Health Sciences
tainer@niehs.nih.gov
_______________________________________________________________
To check out the Superfund Basic Research Program web site, look us up at
http://www.niehs.nih.gov/sbrp/home.htm
'Research Briefs' are available on our webpage at the following
address: http://www.niehs.nih.gov/sbrp/newweb/resbrief.htm . They can
also be accessed from the SBRP Homepage, by choosing Research Briefs.
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From: "Inhof.Christina" <inhof@niehs.nih.gov>
To: "'Research Brief List'" <sf-brief@bobo.niehs.nih.gov>
Subject: NIEHS/EPA Superfund Basic Research Program 'Research Brief- ' Num
ber 56
Date: Wed, 6 Oct 1999 17:32:39 -0400
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