NIEHS/EPA Superfund Basic Research Program 'Research Brief' - Num ber 53

[Joseph R Parrish Jr <JoeParrish@compuserve.com>]

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From:	"Inhof.Christina", INTERNET:inhof@niehs.nih.gov
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DATE:	08/25/99 4:04 PM

RE:	NIEHS/EPA Superfund Basic Research Program 'Research Brief' - Num ber 53

 
SBRP "Research Brief" - Number 53

Title: Supercritical Fluid Research is Leading to a Potential New Process
for Remediation of PCB-Contaminated Soils and Sediments

Many Superfund sites are extensively contaminated with polychlorinated
biphenyls (PCBs), a group of organochlorine compounds that not only degrades
very slowly in the environment, but also possesses many toxic properties.
Because PCBs have the potential to adversely affect the health of wildlife
and humans for a long time when left in the environment, methods are needed
to treat the large quantities of these compounds remaining on hazardous
waste sites. 

Cleaning up PCB-contaminated soils and sediments once meant either
landfilling or incinerating the contaminated materials.  Experience with
these techniques shows they have major drawbacks.  For instance, some
hazardous waste landfills have been reported to leak their contents into the
environment. Although incineration is an effective method for destroying
PCBs, there has been public opposition to this technology because of concern
about the potential release of toxic combustion by-products into the
atmosphere. The limitations of these conventional technologies have sparked
a demand for new strategies to clean up the PCBs on Superfund sites. 

A group of researchers at Syracuse University - a partner in the University
at Albany-SUNY Superfund Basic Research Program - is working on a potential
new solution to the disposal of PCB-contaminated soils and sediments.  These
researchers are developing a two-stage remediation process that uses
supercritical fluids to extract and destroy the PCBs in contaminated
materials.  

Supercritical fluids are substances that have been heated and pressurized
above their critical point, which is the highest temperature and pressure at
which the gaseous form of a pure substance can be compressed into a liquid.
At temperatures and pressures beyond the critical point, a substance is
technically neither a gas or a liquid, but is considered a "fluid" which
possesses key properties of both gases and liquids.  For example, a
supercritical fluid expands to fill in a contained space like a gas, but it
has a density closer to that of a liquid.  This unique combination of
properties makes many supercritical fluids excellent solvents capable of
dissolving a wide range of chemicals. 

The group at Syracuse University has extensively studied the process of
"supercritical fluid extraction" for its ability to remove PCBs from soil
and sediment. To create the supercritical fluid for these studies, high
pressures (80-350 atm) and moderate temperatures (40-60* C) were used.  The
extraction procedure consisted of pumping the supercritical fluid into an
enclosed extraction vessel containing the PCB-contaminated sample. As the
supercritical fluid flowed through the sample, PCBs were extracted, flowing
along with the supercritical fluid to a separation vessel, where the
pressure was lowered to allow for collection of the PCBs. 

Recent studies show that supercritical carbon dioxide augmented with
methanol (a co-solvent that enhances the extraction power of supercritical
carbon dioxide) is very effective at extracting PCBs from both spiked and
real-world samples.  One study with PCB-contaminated sediment from the Saint
Lawrence River showed that 99.8% of the PCBs (concentration of 2200 parts
per million) could be removed within 45-60 minutes using a supercritical
carbon dioxide/methanol mixture.  

The researchers also investigated a process known as "supercritical water
oxidation" to determine how well it could decompose PCBs. This process works
by subjecting the PCBs to an aqueous stream that is heated and pressurized
above the critical point of water (374*C and 217.7 atm).  The addition of
oxygen initiates a series of oxidative reactions that can break down organic
compounds into harmless products such as water and carbon dioxide.  

Oxidation reaction experiments were conducted in a laboratory-scale unit
with individual PCB congeners and a commercial PCB mixture, Aroclor 1248.
Complete oxidation was achieved with the PCB congeners, 2-monochlorobiphenyl
and 3,3',4,4'-tetrachlorobiphenyl, while 99.99% of the Aroclor 1248 in a
simulated soil extract solution (5245 parts per million PCB) was oxidized.
These experiments were carried out at 450-550*C, 250 atm, and 6-54 sec
residence time.  

The results of the above experiments suggest supercritical fluid extraction
and supercritical water oxidation are viable methods for the extraction and
destruction of PCBs from soils and sediments. Because of this success at the
laboratory scale, future work is planned to design and construct a mobile
bench-scale unit for on-site testing of the technology.

When fully developed, this two-stage supercritical fluid technology promises
to be an environmentally sound, economically competitive method for
remediating sites with high levels of PCBs. Carbon dioxide and water are
non-toxic, environmentally benign materials, which in a supercritical state
are very effective at remediating contaminated soils and sediments.
Moreover, the most recent economic analysis of this technology indicates it
costs between $198 to $318 per cubic meter of soil, which is competitive
with other technologies. These features make supercritical fluids especially
attractive as an alternative to landfilling and incineration for
environmental clean up activities.  By providing a safe and cost-effective
means of cleaning up highly contaminated Superfund sites, this technology
will greatly aid in reducing health risks to surrounding communities.  
_____________________________________________________________________

For more information please contact:    

Lawrence Tavlarides, Ph.D.
Syracuse University
L.C. Smith College of Engineering and Computer Science
Department of Chemical Engineering & Materials Science
Syracuse, NY  13244
Phone:  (315) 443-1883
Email:  LLTAVLAR@ecs.syr.edu

To learn more about this research please refer to the following sources: 

Anitescu G., Z.H. Zhang, L.L. Tavlarides.  1999.  A kinetic study of
methanol oxidation in supercritical water. Ind. Eng. Chem. Res. 38: (6)
2231-2237.  

Anitescu G. and L.L. Tavlarides.  1999.  Solubility of individual
polychlorinated biphenyl (PCB) congeners in supercritical fluids: CO2,
CO2/MeOH and CO2/n-C4H10. J. Supercrit. Fluids 14: (3) 197-211.  

Anitescu, G. and L.L. Tavlarides. 1997. Solubilities of solids in
supercritical fluids. Part 1: New quasistatic experimental method for
polycyclic aromatic hydrocarbons in pure fluids. J. Supercrit. Fluids
10:175-189. 

Anitescu, G. and L.L. Tavlarides. 1997. Solubilities of solids in
supercritical fluids. Part 2: Polycyclic aromatic hydrocarbons in CO2-CO
solvent mixtures. J. Supercrit. Fluids 11:37-51. 

Chen, P., W. Zhou and L.L. Tavlarides. 1997. Remediation of polychlorinated
biphenyl contaminated soils/sediments by supercritical fluid extraction.
Environ. Prog. 163:227-236.

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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 53
Date: Wed, 25 Aug 1999 15:56:09 -0400