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Title: Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms-Final Technical Report

Abstract

The premise of this project was that if we understood the fundamental chemistry that controls the branching among product formation pathways for the degradation of CCl₄, we could design remediation strategies that minimize the formation of CHCl₃ and thereby provide badly needed alternatives for remediation of the large plumes of CCl₄ that contaminate several DOE sites. To this end, we performed a series of coordinated batch, spectroscopic, and modeling experiments, to study the effect of a variety of factors on the yield of CHCl₃ from CCl₄ during reduction with zero-valent iron (Fe⁰). The factors studied include those with direct implications for field performance (e.g., the concentration of CCl₄ relative to the amount of iron surface area) and others chosen for diagnosis of the reaction mechanism (e.g., incorporation of deuterium into CCl₄ reduction products in the presence of D₂O). The key mechanistic findings of this study are (i) that CCl₃• probably is not an intermediate in the formation of CF, but CCl₃⁻ probably is, (ii) the high reductive capacity of the Fe⁰ core favors the concerted 2e⁻ reduction, and (iii) magnetite on Fe⁰ favors the benign product formation pathway. The latter conclusion is based on the observation that one type ofmore » nano-sized Fe⁰ that is coated with magnetite shell produces low yields of chloroform (0-40%), whereas others produce the higher yields of chloroform (60-100%) that are typical of most methods for reducing CCl₄ (including biodegradation). Since nano-Fe⁰ can, in principle, be introduced into the deep subsurface by injection, our results would suggest that the right type of nano-Fe⁰ introduced in the right way might be highly effective at dechlorinating CCl₄ with minimal formation of CHCl₃ or other undesirable by-products. This conclusion may offer a breakthrough in the search for remediation technologies that are suitable for the deep CCl₄-contamination at DOE sites such as the 200-W area of Hanford.« less

Authors:
; ;
Publication Date:
Research Org.:
Oregon Health & Science University; Pacific Northwest National Laboratory
Sponsoring Org.:
USDOE - Office of Environmental Management (EM)
OSTI Identifier:
900346
Report Number(s):
ER63485-F
TRN: US200717%%70
DOE Contract Number:
FG07-02ER63485
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; BIODEGRADATION; CAPACITY; CARBON TETRACHLORIDE; CHEMISTRY; CHLOROFORM; DESIGN; DEUTERIUM; DIAGNOSIS; IRON; MAGNETITE; PERFORMANCE; PLUMES; REACTION KINETICS; SIMULATION; SURFACE AREA; Carbon Tetrachloride, Chloroform, Groundwater, Remediation, In Situ Chemical Reduction, Dechlorination, Zerovalent Iron Metal, Nanoparticles

Citation Formats

Tratnyek, Paul G, Amonette, James E, and Bylaska, Eric J. Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms-Final Technical Report. United States: N. p., 2007. Web. doi:10.2172/900346.
Tratnyek, Paul G, Amonette, James E, & Bylaska, Eric J. Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms-Final Technical Report. United States. doi:10.2172/900346.
Tratnyek, Paul G, Amonette, James E, and Bylaska, Eric J. Wed . "Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms-Final Technical Report". United States. doi:10.2172/900346. https://www.osti.gov/servlets/purl/900346.
@article{osti_900346,
title = {Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms-Final Technical Report},
author = {Tratnyek, Paul G and Amonette, James E and Bylaska, Eric J},
abstractNote = {The premise of this project was that if we understood the fundamental chemistry that controls the branching among product formation pathways for the degradation of CCl₄, we could design remediation strategies that minimize the formation of CHCl₃ and thereby provide badly needed alternatives for remediation of the large plumes of CCl₄ that contaminate several DOE sites. To this end, we performed a series of coordinated batch, spectroscopic, and modeling experiments, to study the effect of a variety of factors on the yield of CHCl₃ from CCl₄ during reduction with zero-valent iron (Fe⁰). The factors studied include those with direct implications for field performance (e.g., the concentration of CCl₄ relative to the amount of iron surface area) and others chosen for diagnosis of the reaction mechanism (e.g., incorporation of deuterium into CCl₄ reduction products in the presence of D₂O). The key mechanistic findings of this study are (i) that CCl₃• probably is not an intermediate in the formation of CF, but CCl₃⁻ probably is, (ii) the high reductive capacity of the Fe⁰ core favors the concerted 2e⁻ reduction, and (iii) magnetite on Fe⁰ favors the benign product formation pathway. The latter conclusion is based on the observation that one type of nano-sized Fe⁰ that is coated with magnetite shell produces low yields of chloroform (0-40%), whereas others produce the higher yields of chloroform (60-100%) that are typical of most methods for reducing CCl₄ (including biodegradation). Since nano-Fe⁰ can, in principle, be introduced into the deep subsurface by injection, our results would suggest that the right type of nano-Fe⁰ introduced in the right way might be highly effective at dechlorinating CCl₄ with minimal formation of CHCl₃ or other undesirable by-products. This conclusion may offer a breakthrough in the search for remediation technologies that are suitable for the deep CCl₄-contamination at DOE sites such as the 200-W area of Hanford.},
doi = {10.2172/900346},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Mar 07 00:00:00 EST 2007},
month = {Wed Mar 07 00:00:00 EST 2007}
}

Technical Report:

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  • Most approaches that have been proposed for the remediation of groundwater contaminated with carbon tetrachloride (CCl4) produce chloroform (CHCl3) as the major product and methylene chloride (CH2Cl2) as a minor product. Both of these products are nearly as persistent and problematic as the parent compound, but competing reaction pathways produce the more desirable products carbon monoxide (CO) and/or formate (HCOO-). Results scattered throughout the chemical and environmental engineering literature show that the branching between these reaction pathways is highly variable, but the controlling factors have not been identified. If we understood the fundamental chemistry that controls the branching among these,more » and related, product-formation pathways, we could improve the applicability of a host of remediation technologies (both chemical and biological) to the large plumes of CCl4 that contaminate DOE sites across the country. This project will provide the first complete characterization of the mechanisms and kinetics of competing degradation reactions of CCl4 through laboratory experiments in simple model systems closely coordinated with theoretical modeling studies. The results provide strategies for maximizing the yield of desirable products from CCl4 degradation, and the most promising of these will be tested in column model systems using real site waters and matrix materials.« less
  • Quantify the kinetics of all competing product-formation pathways, over a range of conditions relevant to groundwater remediation, using well-mixed batch reactors and analysis primarily by chromatography. At OGI, batch experiments were conducted on Fe(0) systems (both Fisher Electrolytic and Nano-sized iron). The experiments were done with and without buffer. The buffered experiments tried to contrast two buffers: an organic buffer (EPPS, presumably a H atom donor), and the inorganic borate. In the buffered experiments, the pH was varied (7.3 and 8.4). For the pre-exposure treatment, after trying a variety of methods, like shaking and not shaking for varied amounts ofmore » time, it was decided to stick with not shaking and have a pre-exposure of 24 hours. The unbuffered data did not show any marked trend with increasing mass of Felc. However, 3.5 g of Fe showed about 100% conversion to CHCl3, and 1g of Fe showed 50% conversion. At pHs 8.4 and 7.3, there was no trend observed for branching ratios between EPPS and Borate buffer. kCT (disappearance rate constant of carbon tetrachloride) values were found to be different from CT and CF fits. Experiments with nano-iron (unbuffered, buffered with both buffers at pH 8.3), did not show any trend with respect to Fisher Iron, except for the unbuffered experiments, where the CF ''yield'' was less in the nano iron case. Future experiments involve testing for chloride, formate and CO, and performing experiments over a wider range of pH and buffers. Batch experiments were conducted at PNNL to compare the efficiency and product distribution of representative Fe(II) and Fe(0) systems applied to dechlorination of CCl4. These experiments involved (1) a smectite clay with Fe(III) in its structure that had been reduced to Fe(II) by dithionite treatment, (2) the same clay to which Fe(II) was added as an exchangeable cation, (3) electrolytic Fe(0) from Fisher, and (4) a mixture of the reduced clay and Fe(0). Experiments were conducted in headspace vials at pH 7 in either bicarbonate or bis-tris propane buffers. Reactant and product concentrations were determined by headspace analysis using GC/MS. Results from the first run of this experiment showed relatively little dechlorination by the Fe(II) system, and from 50-80% dechlorination by the Fe(0) system after 48 h. Essentially no CHCl3 was seen in the Fe(II) system, whereas as 30% of the original CCl4 was converted to CHCl3 in the Fe(0) system. Very low amounts of CH2Cl2 were seen in all treatments. Plans are underway to repeat this experiment and use a cryo-GC capability that will allow simultaneous determination of CO as well as the chlorinated methanes. This is important because CO represents the end product of the second hypothetical dechlorination pathway that may compete with the CHCl3 pathway and will aid in mass balance calculations. An additional experiment to evaluate the Henry's Law constant for CCl4 in aqueous solutions in the presence and absence of clay showed no significant difference due to the presence of clay, although slightly higher gas-phase concentrations were seen when clay was present.« less
  • Most approaches that have been proposed for the remediation of groundwater contaminated with carbon tetrachloride (CCl{sub 4}) produce chloroform (CHCl{sub 3}) as the major product and methylene chloride (CH{sub 2}Cl{sub 2}) as a minor product. Both of these products are nearly as persistent and problematic as the parent compound, but competing reaction pathways produce the more desirable products carbon monoxide (CO) and/or formate (HCOO{sup -}). Results scattered throughout the chemical and environmental engineering literature show that the branching between these reaction pathways is highly variable, but the controlling factors have not been identified. If we understood the fundamental chemistry thatmore » controls the branching among these, and related, product-formation pathways, we could improve the applicability of a host of remediation technologies (both chemical and biological) to the large plumes of CCl{sub 4} that contaminate DOE sites across the country. This project will provide the first complete characterization of the mechanisms and kinetics of competing degradation reactions of CCl{sub 4} through laboratory experiments in simple model systems closely coordinated with theoretical modeling studies. The results provide strategies for maximizing the yield of desirable products from CCl{sub 4} degradation, and the most promising of these will be tested in column model systems using real site waters and matrix materials.« less
  • Most approaches that have been proposed for the remediation of groundwater contaminated with carbon tetrachloride (CCl{sub 4}) produce chloroform (CHCl{sub 3}) as the major product and methylene chloride (CH{sub 2}Cl{sub 2}) as a minor product. Both of these products are nearly as persistent and problematic as the parent compound, but competing reaction pathways produce the more desirable products carbon monoxide (CO) and/or formate (HCOO{sup -}). Results scattered throughout the chemical and environmental engineering literature show that the branching between these reaction pathways is highly variable, but the controlling factors have not been identified. If we understood the fundamental chemistry thatmore » controls the branching among these, and related, product-formation pathways, we could improve the applicability of a host of remediation technologies (both chemical and biological) to the large plumes of CCl{sub 4} that contaminate DOE sites across the country. This project will provide the first complete characterization of the mechanisms and kinetics of competing degradation reactions of CCl{sub 4} through laboratory experiments in simple model systems closely coordinated with theoretical modeling studies. The results provide strategies for maximizing the yield of desirable products from CCl{sub 4} degradation, and the most promising of these will be tested in column model systems using real site waters and matrix materials.« less
  • Most approaches that have been proposed for the remediation of groundwater contaminated with carbon tetrachloride produce chloroform as the major product and methylene chloride as a minor product. Both of these products are nearly as persistent and problematic as the parent compound, but competing reaction pathways produce the more desirable products carbon monoxide and/or formate. Branching between these reaction pathways is highly variable, but the controlling factors have not been identified. To improve the applicability of reductive remediation technologies to the large plumes of carbon tetrachloride at several DOE sites, we are pursuing the complete characterization of the mechanisms andmore » kinetics of competing degradation reactions of carbon tetrachloride through laboratory experiments closely coordinated with theoretical modeling studies. The results are beginning to suggest strategies for maximizing the yield of desirable products from carbon tetrachloride degradation, which will be tested in column model systems using real site waters and matrix materials.« less