Grantee Research Project Results
Final Report: Fenton-Like Reductions for the Enhanced Desorption and Degradation of Biorefractory Contaminants
EPA Grant Number: R826163Title: Fenton-Like Reductions for the Enhanced Desorption and Degradation of Biorefractory Contaminants
Investigators: Watts, Richard J. , Teel, Amy L. , Hess, Thomas F.
Institution: Washington State University
EPA Project Officer: Aja, Hayley
Project Period: February 1, 1998 through January 31, 2001
Project Amount: $307,481
RFA: Exploratory Research - Environmental Engineering (1997) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
The overall objective of this research project was to investigate the Fenton's reaction conditions that promote the generation of hydroxyl radicals (OH), superoxide, and hydroperoxide, and to determine which of these species may be responsible for the enhanced treatment of sorbed contaminants. Modified Fenton's reagent is an increasingly popular advanced oxidation process (AOP) based on the traditional Fenton's reaction, in which the decomposition of a solution of dilute hydrogen peroxide is catalyzed by excess iron (II), resulting in near-stoichiometric generation of OH. Hydroxyl radicals, a relatively non-specific oxidant, readily attacks even highly chlorinated compounds. Fenton's reagent usually is modified for in situ chemical oxidation (ISCO) by using higher concentrations of hydrogen peroxide, and by varying the type of catalyst (i.e., iron (III), iron chelates, or iron oxyhydroxide minerals). Such modifications promote the generation of other oxygen transient species, such as superoxide radical anion and hydroperoxide anion. These species may potentially reduce highly chlorinated contaminants that typically are resistant to many biological and chemical treatment processes, and may promote the enhanced treatment of sorbed contaminants, which usually are not transformed by common oxidation and reduction processes.
Summary/Accomplishments (Outputs/Outcomes):
The degradation of carbon tetrachloride by modified Fenton's reagent was investigated using a range of hydrogen peroxide concentrations and a 1 micro moles per liter (µM) iron (III) catalyst. Hydroxyl radicals, the documented reactive species in modified Fenton's reactions, is not reactive with carbon tetrachloride, yet carbon tetrachloride degradation was observed in the Fenton's reactions and was confirmed by chloride generation. Because carbon tetrachloride is not reactive with OH, a reductive mechanism that may involve superoxide radical anion was proposed for carbon tetrachloride degradation in modified Fenton's systems. Carbon tetrachloride degradation was observed only when high carbon tetrachloride concentrations (> 0.25 µM) were treated using aggressive Fenton's conditions (H2O2 ≥ 0.3 M). Although carbon tetrachloride generally is considered unreactive with superoxide, in an aggressive Fenton's reaction with a high concentration of carbon tetrachloride, a sufficient number of molecular collisions may occur to promote significant degradation of carbon tetrachloride.
The degradation of carbon tetrachloride by Fenton's reagent served as the basis for probe compound development for detecting transient oxygen species. Modified Fenton's systems are so reactive that tracking probe degradation products (the common method for verifying a reaction) is impossible because degradation products react so rapidly. Because of the low potential for tracking reaction products, the goal of this segment of the research was to develop probe compounds that are reactive with only one transient species as a basis for tracking OH, superoxide, and hydroperoxide under varying conditions of modified Fenton's reactions. The results of the matrix of reactions of various potential probe compounds, with all three potential reactants, showed that 1-hexanol is only reactive with OH, carbon tetrachloride is only reactive with superoxide, and 1,3,5-trinitrobenzene is only reactive with hydroperoxide anion.
Mechanisms of oxidation and reduction in modified Fenton's reagent were then investigated using the probe compounds developed. The probe compounds were treated with concentrations of hydrogen peroxide ranging from 14.7 µM to 1470 µM (0.05–5 percent) under pH regimes of 3, 5, 7, and 9 using both unchelated iron catalysts and iron complexed with nitrilotriacetic acid (NTA). All probe compounds were destroyed more rapidly at higher pH regimes. At all pH conditions, isopropanol did not affect the reduction reaction, demonstrating that OH are not responsible for carbon tetrachloride and 1,3,5-trinitrobenzene degradation.
The conditions that promote the generation of oxidants and reductants subsequently were investigated using central composite rotatable designs. The probe compounds were treated with modified Fenton's reagent at four different pH regimes (pH 3, 5, 7, and 9) using chelated and soluble iron catalysts. The results showed OH and superoxide were generated only at acidic pH regimes using soluble iron catalyst, but were generated at all pH regimes using chelated iron as the catalyst.
The generation of OH and superoxide was then investigated in Fenton's reactions, catalyzed by three synthetic iron oxide catalysts (goethite, hematite, and ferrihydrite). The OH and superoxide generation was observed only in goethite- and ferrihydrite-catalyzed hydrogen peroxide reactions, and was accompanied by the release of trace soluble iron in these systems. However, only OH generation was observed in hematite-catalyzed reactions.
The enhanced desorption of chloroaliphatic compounds from a silty loam soil by modified Fenton's reagent was studied using a series of probe compounds of varying hydrophobicities. Hexachloroethane, which has negligible reactivity with OH, was transformed more rapidly in modified Fenton's reactions (≥ 0.3 M hydrogen peroxide) than it was lost by gas-purge desorption, supporting the existence of a non-OH mechanism. The addition of excess isopropanol to scavenge OH slowed, but did not stop the desorption and degradation of hexachloroethane. In the presence of the reductant scavenger chloroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible for enhanced contaminant desorption. In the presence of excess isopropanol, toluene, which has negligible reactivity with reductants, was displaced from the soil, but not degraded. This indicates that enhanced desorption of sorbed compounds occurs independently of their degradation.
To further define the mechanism of enhanced contaminant desorption, dodecane was used as a probe compound in gas-purge desorption experiments and in various oxidation and reduction reactions. To determine the species responsible for contaminant desorption, the three predominant transient oxygen species found in Fenton's reactions were each generated in separate systems. Each of these reactions was purged with nitrogen gas to volatilize desorbed dodecane for capture in Supelco-30 gas absorbent (ORBO) tubes. The degradation of sorbed dodecane promoted by the three transient oxygen species was compared to standard gas purge analysis (purging with nitrogen in deionized water). Dodecane desorption from the silica sand measured by gas-purge methodology was negligible for more than 24 hours. Dodecane sorbed to silica sand showed no enhanced desorption in the presence of OH generated by the traditional Fenton's reaction. Similarly, hydroperoxide anion generated by the decomposition of sodium perborate did not displace dodecane from the silica sand. However, the generation of superoxide using a superoxide-driven Fenton-like reaction with the addition of ethanol, a OH scavenger, resulted in > 90 percent desorption of dodecane from the silica sand after 3 hours, as measured by its capture in the gas absorbent tubes. The results show that superoxide is the desorbing agent in modified Fenton's reactions, and that conditions that result in its generation greatly enhance the effectiveness of Fenton's processes for in situ subsurface remediation.
Although considered an oxidation process, modified Fenton's reactions also act through a reductive mechanism to degrade many compounds that are not reactive with OH, such as carbon tetrachloride, which broadens the scope of this process for hazardous waste treatment and remediation.
Fenton's reactions catalyzed by soluble iron only are effective at acidic pH; however, those catalyzed by iron chelates provide effective generation of oxidants and reductants at neutral as well as acidic pH regimes. Goethite-and ferrihydrite-catalyzed Fenton-like systems promote both oxidation and reduction reactions. Hematite-catalyzed decomposition of hydrogen peroxide generates only OH. Depending on the catalyst, varying proportions of the reactions occurs via a range of homogeneous and heterogeneous catalyst.
Enhanced contaminant desorption was promoted by reductants, followed by oxidation and reduction of contaminants in the aqueous phase. Superoxide is the primary desorbing agent in modified Fenton's reactions; reaction conditions that result in its generation greatly enhance the effectiveness of Fenton's processes for in situ subsurface remediation.
Vigorous Fenton-like reactions, in which reductants and OH are generated, may provide a universal treatment matrix in which contaminants are desorbed and then oxidized and reduced in a single system.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 12 publications | 6 publications in selected types | All 6 journal articles |
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Howsawkeng J, Watts RJ, Washington DL, Teel AL, Hess TF, Crawford RL. Evidence for simultaneous abiotic-biotic oxidations in a microbial-Fenton's system. Environmental Science & Technology 2001;35(14):2961-2966. |
R826163 (Final) |
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Teel AL, Warberg CR, Atkinson DA, Watts RJ. Comparison of mineral and soluble iron Fenton's catalysts for the treatment of trichloroethylene. Water Research 2001;35(4):977-984. |
R826163 (1998) R826163 (Final) |
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Teel AL, Watts RJ. Degradation of carbon tetrachloride by modified Fenton's reagent. Journal of Hazardous Materials 2002;94(2):179-189. |
R826163 (Final) |
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Watts RJ, Bottenberg BC, Hess TF, Jensen MD, Teel AL. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton's reactions. Environmental Science & Technology 1999;33(19):3432-3437. |
R826163 (1998) R826163 (Final) |
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Watts RJ, Haller DR, Jones AP, Teel AL. A foundation for the risk-based treatment of gasoline-contaminated soils using modified Fenton's reactions. Journal of Hazardous Materials 2000;76(1):73-89. |
R826163 (Final) |
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Watts RJ, Stanton PC, Howsawkeng J, Teel AL. Mineralization of a sorbed polycyclic aromatic hydrocarbon in two soils using catalyzed hydrogen peroxide. Water Research 2002;36(17):4283-4292. |
R826163 (Final) |
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Supplemental Keywords:
soil, sediments, adsorption, organics, treatment, remediation, cleanup, oxidation, engineering , toxics, waste, water, contaminated sediments, environmental chemistry, environmental engineering, groundwater remediation, HAPS, chloroform, Fenton-like reductions, biorefractory contaminants, contaminated sediment, degradation, enhanced desporption, hydroxyl radicals, kinetic studies, reactivity, sediment treatment, soil sediment, hazardous waste treatment, Fenton's reagent., Scientific Discipline, Toxics, Water, Waste, Chemical Engineering, Contaminated Sediments, Environmental Chemistry, HAPS, Environmental Engineering, Groundwater remediation, degradation, biorefractory contaminants, sediment treatment, soil sediment, contaminated sediment, sorbed contaminants, Chloroform, enhanced desporption, kinetic studies, remediation, hydroxyl radicals, reactivity, Fenton's reagentProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.