Superfund Research Program
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.
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To learn more about this research, please refer to the following sources:
- Anitescu G, Zhang Z, Tavlarides LL. 1999. A kinetic study of methanol oxidation in supercritical water. Ind Eng Chem Res 38(6):2231-2237. PMID:16190247
- Anitescu G, Tavlarides LL. 1997. Solubilities of solids in supercritical fluids. Part 1: New quasistatic experimental method for polycyclic aromatic hydrocarbons in pure fluids. Journal of Supercritical Fluids 10:175-189.
- Anitescu G, Tavlarides LL. 1997. Solubilities of solids in supercritical fluids. Part 2: Polycyclic aromatic hydrocarbons in Co2-Co solvent mixtures. Journal of Supercritical Fluids 11:37-51.
- Chen P, Zhou W, Tavlarides LL. 1997. Remediation of polychlorinated biphenyl contaminated soils/sediments by supercritical fluid extraction. Environmental Progress 163:227-236.
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