Zero-Valent Metal Treatment of Halogenated Vapor-Phase Contaminants in SVE Offgas |
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| EPA Agreement Number | R828771-0-01 |
| Investigators | A. L. Roberts and D.H. Fairbrother (JHU) |
| Type of Research | Basic Research (Laboratory) |
| Project Period | 10/1/01 - 9/30/02 |
| Objectives of Research | |
| This research is exploring the use of zero-valent metals and bimetallic reductants for treating vapor phase organohalides found in offgas from soil vapor extraction systems. Zero-valent metal technology has emerged as a highly promising approach for in situ treatment of groundwater contaminants, but its applicability to vapor phase contaminants has been virtually unexplored. This project is significantly extending the scope of investigations currently underway (though funding obtained from other sources) that pertain to use of zero-valent metals for treatment of anoxic landfill gases. Specifically, in this project we intend to test whether similar treatment systems could prove useful for treating organohalides in gas streams that contain O2. Although it might be anticipated that the presence of O2 could lead to more rapid passivation of reactive surfaces due to the formation of a thick coating of iron oxides and hence decreased reactivity, there is also ample reason to believe that the presence of O2 will not significantly impair treatment system performance, provided that the reactive surface is judiciously selected. If successful, an approach based on zero valent metals or bimetallic reductants could be used to treat offgases obtained from soil vapor extraction (SVE) systems. Results will form the basis of process models that can be used to design aboveground treatment systems for SVE offgases that contain organohalides. | |
| Project Summary/Accomplishments | |
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Preliminary work focused on screening the reactivity of various
iron-based, bimetallic reductants.
The rates of reaction of cis-1,2-dichloroethylene
(cis-DCE), a “model”
organohalide, with a variety of bimetallic reductants were compared in
batch studies. Initial
experiments compared the rates of Ni/Fe, Co/Fe, and Cu/Fe reductants to
that of Fe alone. These studies revealed significant differences between
the various reductants. While
Co/Fe and Cu/Fe exhibited slightly accelerated rates of reaction as
compared to Fe alone, the reaction with Ni/Fe were significantly faster.
Given our initial successes with the Ni/Fe reductants, we have
focused solely on this system in subsequent experiments.
Additional batch reactors were used to investigate the effect of
nickel loading on the observed rates of reaction.
We have developed a methodology for the controlled production of
the bimetallic reductants with variable loadings of the catalytic metal
using electroless deposition. Auger
electron spectroscopy was used to determine the average nickel loading
on the iron particles. Correlation
of the reaction rate data from the batch reactor studies with the
surface composition information indicates that the nickel loading has a
significant impact on the rate of reduction of cis-DCE.
At low Ni loadings, the rate of cis-DCE
removal increases quickly with nickel concentration, but at higher Ni
loadings the rate increases more slowly.
At maximum, the Ni/Fe system exhibited rates of cis-DCE
reduction that were an order of magnitude greater than those of
Fe. Current efforts are focused on testing the applicability of the Fe and Ni/Fe bimetallic reductants to the treatment of gas phase contaminants in column reactors. Plexiglass columns, fitted with a series of gas sampling ports along their lengths, were packed with either iron or nickel-plated iron, filled with water, and allowed to drain gravimetrically. Cis-DCE is introduced to the gas phase by bubbling a 50:50 mix of carbon dioxide and nitrogen through a saturated solution of the organohalide. The concentration of the organohalide can be accurately manipulated by varying the ratio of the cis-DCE-containing gas stream with a second water-saturated gas stream. The gas stream was introduced to the columns several weeks ago, and samples are periodically taken for gas chromatographic analysis. Preliminary results indicate that the gas phase concentration of cis-DCE is decreasing along the length of the columns and product species such as ethene and ethane are growing in, suggesting that the contaminant is partitioning into the aqueous phase and undergoing reductive dehalogenation at the iron or bimetallic surface. Additionally, it is clear that the rate of formation of products in the Ni/Fe system is approximately an order of magnitude faster than the rate observed in the column with Fe alone. |
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| Publications/Presentations | none |
| Future Activities | |
| The column reactors will remain continuously operational for an extended period of time in order to allow us to study the longevity of these reductants. Additional sampling ports on the columns will allow us to remove iron or bimetallic grains for analysis. If the efficiency of the columns appears to change with time, surface analysis of the reductant grains using Auger electron spectroscopy and/or X-ray photoelectron spectroscopy will allow us to correlate changes in the surface composition of the reductants with changes in the reaction kinetics. Additional columns will be added in the future to allow us to explore the effect of variables such as flow rate, specific water retention, nickel loading, concentration of cis-DCE, and the presence of other atmospheric constituents in the gas stream. In particular, the effect of O2 on the efficiency of this treatment system will be explored | |
| Supplemental Keywords | waste reduction, pollution prevention, environmental engineering |