Grantee Research Project Results
Final Report: Characterization and Kinetics of Contaminant Oxidation and Hydrogen Peroxide Decomposition in the Presence of Subsurface Material
EPA Grant Number: R823402Title: Characterization and Kinetics of Contaminant Oxidation and Hydrogen Peroxide Decomposition in the Presence of Subsurface Material
Investigators: Valentine, Richard L.
Institution: University of Iowa
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1995 through September 30, 1998 (Extended to August 26, 1999)
Project Amount: $189,242
RFA: Exploratory Research - Engineering (1995) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Safer Chemicals
Objective:
Hydrogen peroxide is sometimes used to supply oxygen to facilitate aerobic biodegradation in the subsurface environment where it readily decomposes to produce oxygen. More recently, it also has been used to oxidize subsurface contaminants. The chemistry involved in homogeneous aqueous phase decomposition and oxidation reactions is comparatively well understood. Much less is known, however, about the nature and importance of surface catalyzed reactions, which may be more important in the subsurface environment.This research characterizes the role of several different types of subsurface materials and water quality parameters in catalyzing the decomposition of hydrogen peroxide, and resulting oxidation of selected organic contaminants by reaction with intermediates produced as a consequence of hydrogen peroxide decomposition. An important overall objective is to develop mechanistic descriptions and models describing hydrogen peroxide loss and contaminant oxidation based on unifying concepts and elementary reactions. Our studies focused on the role of several iron and copper oxides and the influence of several inorganic and organic ligands, which may complex with the oxide surfaces. Several model contaminants and organic probe compounds were studied, which included phenol, quinoline, benzoic acid, and atrazine. Experiments were conducted in batch, column, and a unique continuous flow recycle reactor.
Summary/Accomplishments (Outputs/Outcomes):
Hydrogen peroxide decomposition in batch and recycle flow reactor studies followed a rate expression first order in hydrogen peroxide and iron and copper oxide concentration. The catalytic activity varied by over an order of magnitude on a mass basis, in the order of ferrihydrite > semi-crystalline oxide > goethite. On a surface area basis, the catalytic activity however, was much more comparable. This indicates that site density and site activity of these iron oxides were similar, even though their morphology greatly differed.The catalytic activity of all three iron oxides decreased with increasing concentrations of carbonate or phosphate. The reduction due to the presence of carbonate is quite large over environmentally significant ranges of 0.5 to 10 mM. It is, therefore, expected that carbonate concentration is an important parameter in determining the fate of hydrogen peroxide in the subsurface environment. Phosphate had a much more pronounced effect for comparable concentrations, presumably because it is so strongly adsorbed. The presence of humic acid had mixed effects. It generally decreased the hydrogen peroxide decomposition rate depending on its concentration.
Probe studies also showed that the rate of hydrogen peroxide decomposition was unaffected by the presence of butanol, an aqueous phase hydroxyl radical scavenger. This is consistent with the existence of rate limiting reactions that do not involve hydroxyl radical.
Several organic compounds were used as probes to study surface catalyzed oxidation. In comparison to the effect on hydrogen peroxide decomposition, the catalytic activity toward quinoline oxidation was highest for goethite, much less for the semi-crystalline material, and negligible in the presence of ferrihydrite. Goethite also exhibited a catalytic activity toward the oxidation of phenol and atrazine in batch and fixed?bed studies. Therefore, one can conclude that organic oxidation is not necessarily directly related to the rate or extent of hydrogen peroxide decomposition.
The presence of phosphate, a strongly adsorbing ligand, greatly reduced the rate of hydrogen peroxide decomposition, but did not influence the ultimate amount of quinoline degraded in the presence of quinoline, only the length of time to achieve that loss (i.e., there was a constant relationship between the amount of hydrogen peroxide decomposed and the amount of quinoline transformed). This ratio also was very small (typically 0.01 mole/mole), showing that the reactions are not very efficient. The influence of natural organic matter was complicated. At high concentrations, no quinoline loss was observed. At lower concentrations tested, enhanced removals were observed. This was attributed to the several possible roles that natural organic matter (NOM) could have (e.g., radical scavenger, reactant).
A model describing hydrogen peroxide decomposition kinetics and simultaneous contaminant loss was developed. Key features include a rate limiting reduction of surface bound Fe(III) by hydrogen peroxide and formation of both superoxide anion and hydroxyl radicals. This rate limiting reaction should be contrasted to the classical "Fenton's" reaction, which is the oxidation of Fe(II) by hydrogen peroxide. Therefore, hydrogen peroxide is both an oxidant and reductant in these systems. This model also includes formation of hydroxyl radicals on the surface as well as in the water phase. Inclusion of scavenging reactions are included that account for a measurable "solids concentration effect," whereby increasing catalytic solids concentrations can actually decrease the efficiency or extent of contaminant oxidation. This model successfully described hydrogen peroxide decomposition and quinoline oxidation catalyzed in the presence of aquifer sand whose surface was coated iron oxides.
A model for the effect of adsorbing species, carbonate and phosphate, on
hydrogen peroxide decomposition in the presence of iron oxides was developed
based upon several assumptions: (1) adsorption is described by a simple Langmuir
isotherm, (2) all sites are equivalent with respect to both adsorption and
catalytic activity, (3) adsorption of either carbonate or phosphate deactivates
the site, and (4) adsorption of carbonate and other species is weak relative to
phosphate. These assumptions are supported by experimental data in which
adsorption isotherms could be directly utilized to predict the effect of
changing carbonate or phosphate concentrations on the rate of hydrogen peroxide
decomposition.
A comparison of the catalytic activity of an alumina supported
copper oxide was made to that of goethite. On a mass basis, the supported copper
oxide was approximately ten times more active than goethite toward catalyzing
hydrogen peroxide decomposition. It is estimated to be over two orders of
magnitude more active when normalized to copper content.
The catalytic activity toward oxidation of phenol and benzoic acid also was studied at catalyst concentrations where hydrogen peroxide decomposition rates were equal. The supported copper oxide was found to be more efficient for phenol oxidation while goethite catalyzed benzoic acid oxidation more efficiently than the copper oxide. The differences are hypothesized due to differences in adsorption of the contaminants. Most notably, benzoic acid was adsorbed to both oxides, while phenol was not adsorbed. This resulted in a reduction in the rate of hydrogen peroxide decomposition, which could be directly related to the amount of benzoic acid oxidized. It is believed that the increased efficiency of benzoic acid loss in the presence of goethite is attributable to enhanced reaction with surface bound benzoic acid.
Lastly, magnetite and hematite catalysis of hydrogen peroxide loss and contaminant oxidation was studied at pH 3-4. This pH was chosen because the activity was relatively small at neutral pH values. Both quinoline and phenol were degraded by hydrogen peroxide in solutions containing the mineral, and in mineral-free solutions prepared by equilibrating the water with the mineral. Furthermore, this was very efficient when considering the slow rate of simultaneous hydrogen peroxide loss in these solutions. Analysis of both solutions indicated the release of small amounts of iron into solution. It is hypothesized that it is the released iron acting as the primary reactant with hydrogen peroxide in solution leading to contaminant degradation as a consequence of "Fenton-like" reactions.
A number of conclusions can be made regarding influencing factors and
mechanisms involved in oxide surface catalyzed hydrogen peroxide decomposition
and contaminant oxidation.
First, the three common iron oxides?ferrihydrite,
goethite, and semi-crystalline iron oxide?are similarly active in catalyzing
hydrogen peroxide decomposition, and the reaction was a true surface reaction at
neutral pH values (i.e., it is not attributable to iron released into solution
at neutral pH). In no studies at neutral pH did the filtered solutions show
catalytic activity. However, differences exist in their influence on contaminant
loss. Only goethite was capable of causing quinoline loss. This is hypothesized
due to differences occurring at the surface level, not in solution. The
comparison of contaminant loss in the presence of copper oxide and goethite
indicates that specific interactions with the surfaces may be important in
governing contaminant loss. In other words, generalizations on contaminant
oxidation may not always be applicable.
Perhaps, the most important
conclusion relevant to the use of hydrogen peroxide in remediation of the
contaminated subsurface environments is that at neutral pH values, surface
catalyzed contaminant oxidation is not very efficient. Generally, much less than
1 percent of the hydrogen peroxide is useful toward contaminant oxidation.
However, at sufficiently low pH (e.g., 3), iron containing minerals can be a
source of soluble iron, which can be an efficient catalyst for contaminant
oxidation occurring in solution. We conclude that surface reactions need not be
hypothesized to explain the degradation of the contaminants if the water is at
low pH where dissolution of many iron-containing minerals readily occurs.
The model results (applicable to neutral pH) are consistent with the inherent assumptions on site reactivity, and with observations on the influence of several common water constituents. Scavenging of reactive radicals are important reactions accounting for the low efficiency of contaminant oxidation at neutral pH. The rate limiting reaction is the reduction of Fe(III) to Fe(II) by the oxidation of hydrogen peroxide to super oxide anion. A number of reactions also lead to hydroxyl radical formation. The presence of both types of intermediates points to the potential importance of both contaminant oxidation and reduction reactions. It is hypothesized that reactions may involve both surface bound contaminants and those in solution.
Both organic and inorganic adsorbing ligands will reduce the rate of hydrogen peroxide decomposition by deactivating catalytic sites. This may translate into slower hydrogen peroxide loss and slower contaminant oxidation, but not necessarily in a less efficient use of hydrogen peroxide. In fact, a reduction in rate of hydrogen peroxide decomposition may actually increase the efficiency of contaminant oxidation.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 3 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Miller C, Valentine RL. Mechanistic studies of surface catalyzed H2O2 decomposition and contaminant degradation in the presence of sand. Water Research August 1999,33(12):2805-2816. |
R823402 (Final) |
not available |
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Valentine RL, Wang HC. Iron oxide surface catalyzed oxidation of quinoline by hydrogen peroxide. Journal of Environmental Engineering 1998;124(1):31-38. |
R823402 (Final) |
not available |
Supplemental Keywords:
remediation, subsurface contamination, chemical oxidation, Fenton's reaction., RFA, Scientific Discipline, Waste, Water, Environmental Chemistry, Bioremediation, Drinking Water, Groundwater remediation, Environmental Engineering, groundwater disinfection, water quality parameters, aquatic restoration, biodegradation, kinetics, chemical transport, kinetic studies, treatment, hydrogen peroxide decomposition, water quality, water treatment, drinking water contaminants, other - risk management, organic contaminantsRelevant Websites:
http://www.cee.engineering.uiowa.eduProgress 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.