Superfund Research Program
The Impact of Chlorine on Hexavalent Chromium Emissions
Superfund sites seldom contain a single toxic chemical, but instead are often contaminated with complex mixtures of organic and inorganic compounds. The interactions and ultimate toxicity of these mixtures are often unknown and their presence complicates efforts to assess the risks of exposure at Superfund sites and to identify the best remediation strategy. In addition to identifying the most efficient overall clean-up strategy, site managers must ensure that remediation of one contaminant does not result in the transformation of another contaminant into a physical and chemical form that is difficult to control or inherently more toxic.
Thermal remediation is an effective remediation technology to clean soils contaminated with volatile organic compounds such as TCE, PCBs and PAHs. Dr. Ian Kennedy at the University of California, Davis is leading a team of researchers in studies to determine the risk to human populations as a result of the release of chlorinated by-products such as heavy metals or dioxins during thermal remediation at Superfund sites. Specifically, they are investigating the impact of the presence of chlorine on the chemical and physical state of chromium (Cr) emissions. Chromium has been detected at over 300 Superfund sites as the result of discharges from chrome plating, leather tanning, and textile operations and airborne emissions of metal-bearing solids chemical manufacturing facilities, the iron and steel industries, and combustion of natural gas, oil, and coal.
The toxicity of chromium depends strongly on several factors:
- Valence state - In the environment, Cr exists primarily in the trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. Cr(III) is an essential nutrient that helps maintain the normal metabolism of glucose, protein and fat, but compounds of Cr(VI) have been found to be mutagenic and carcinogenic. As a result, it is important to be able to predict the valence state of chromium emissions and to design systems to minimize the formation of the hexavalent form. This requires knowledge of the kinetics of chromium oxidation in combustion systems.
- Particle size - The importance of Cr to human health issues increases dramatically as particle size decreases. Metal aerosols emitted from incineration processes are composed of micron and submicron particles that can remain airborne for long periods of time and can penetrate deep into the lung. It is important to design thermal processes to minimize the generation of Cr aerosols emissions. This requires a fundamental understanding of the physical and chemical transformation of Cr and its oxides in flames.
- Chemical state - Metal chlorides are generally much more volatile than the oxides. The presence of metal chlorides increases the possibility for the formation of vapor products rather than aerosols or condensed phase materials, which has significant implications for the selection of emission control equipment.
Dr. Kennedy conducted tests using a laminar (non-turbulent) flame to evaluate the impact of chlorine on the formation of Cr(VI) and the partitioning between solid and vapor phases. The researchers measured total Cr added to the test system and collected emissions from the flame using filters to collect particulate matter and solvents to collect vapor phase species. They measured Cr(VI) emissions using a spectrophotometric method and total Cr on the filter via X-ray fluorescence. The results of the study showed:
- There was a significant increase in Cr(VI) in the post-flame gases with increasing chlorine in the fuel.
- The fraction of Cr(VI) emitted as vapor increased with increasing chlorine in the fuel.
- The fraction of Cr(VI) emitted in the solid phase initially increased with increasing chlorine in the fuel, then leveled off or even decreased with further increases in the chlorine concentration in the fuel.
Dr. Kennedy believes that the formation of an oxychloride (CrO2Cl2) in the cooler post-combustion gases contributes strongly to the increase in Cr(VI) emissions. If true, modification of mixing rates of combustion products with surrounding air could serve as a control strategy. This research represents a significant contribution to our efforts to address important technical issues that limit the implementation of thermal technologies in Superfund site remediation.
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To learn more about this research, please refer to the following sources:
- Guo B, Kennedy IM. 2001. The impact of chlorine on chromium speciation in a laminar diffusion flame. Combustion and Flame 126:1557-1568.
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