This technology is a two-stage, in situ photolytic and biological detoxification process for shallow soil contamination. The first step in the process degrades the organic contaminants with ultraviolet (UV) radiation. The photolytic degradation rate is several times faster with artificial UV light than with natural sunlight. The degradation process is enhanced by adding detergent-like chemicals (surfactants) to mobilize the contaminants. Photolysis of the contaminants converts them to more easily degraded compounds. Periodic sampling and analysis determines when photolysis is complete. Biodegradation, the second step, further destroys organic contaminants and detoxifies the soil.
When sunlight is used to treat shallow soil contamination, the soil is first tilled with a power tiller and sprayed with surfactant. The soil is tilled frequently to expose new surfaces and sprayed often. Water may also be added to maintain soil moisture.
When UV lights are used, parabolic reflectors suspended over the soil increase the amount of UV irradiation (see figure below). After photolysis is complete, biodegradation is enhanced by adding microorganisms and nutrients and further tilling the soil.
When these techniques are applied to soils with deep contamination, soil needs to be excavated and treated in a specially constructed shallow treatment basin that meets Resource Conservation and Recovery Act requirements. When soil contamination is shallow, photolysis and housing prevent contaminants from migrating to groundwater.
The only treatment residuals are soil contaminated with surfactants and the end metabolites of the biodegradation processes. The end metabolites depend on the original contaminants. The surfactants are common materials used in agricultural formulations. Therefore, the soils can be left on site.
This photolytic and biological soil detoxification process destroys organics, particularly dioxins such as tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyls (PCB), other polychlorinated aromatics, and polynuclear aromatic hydrocarbons.
This technology was accepted into the SITE Emerging Technology Program in 1989; the evaluation was completed in 1992. The Emerging Technology Report (PB95-159992) is available for purchase from the National Technical Information Services. The Emerging Technology Bulletin (EPA/540/F-94/502) and Emerging Technology Summary (EPA/540/SR-94/531) are available from EPA.
Bench-scale tests conducted on dioxin-contaminated soil showed that the effectiveness of surface irradiation to degrade TCDDs or PCBs is strongly influenced by soil type. Early tests on sandy soils showed greater than 90 percent removals for both TCDDs and PCBs. Using a 450-watt mercury lamp, the irradiation time was more than 20 hours for greater than 90 percent destruction of TCDD and more than 4 hours for greater than 90 percent destruction of PCBs. However, a high humic content decreased the effectiveness of the UV photolysis. Soil contaminated with PCBs in the bench-scale tests had a high clay content. The highest removal rate for these soils was 30 percent, measured over a 16-hour irradiation time.
The bench-scale tests used a medium-pressure mercury UV lamp; sunlight was ineffective. No significant improvement in PCB destruction was achieved using a pulsed UV lamp.
The process was also tested with Fenton's reagent chemistry as an alternate method of degrading PCBs to more easily biodegraded compounds. PCB destruction ranged from nondetectable to 35 percent. Data indicates that no significant change in PCB chlorine level distribution occurred during treatment.
Other studies examined PCB biodegradability in (1) soil treated with a surfactant
and UV radiation, (2) untreated soil, and (3) soil known to have PCB-degrading
organisms. Study results were as follows:
Isolation and enrichment techniques have made it possible to isolate microorganisms capable of biodegrading PCBs in contaminated soil.
EPA PROJECT MANAGER:
Randy Parker
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7271
Fax: 513-569-7571
TECHNOLOGY DEVELOPER CONTACT:
Duane Graves
IT Corporation
312 Directors Drive
Knoxville, TN 37923-4709
423-690-3211
Fax: 423-694-3626