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2003 Progress Report: Kinetic and Mechanistic Framework for Remediation Using Zerovalent Iron (SEERII)
EPA Grant Number: R829422E03Title: Kinetic and Mechanistic Framework for Remediation Using Zerovalent Iron (SEERII)
Investigators: Zhang, Tian C. , Shea, Patrick J.
Institution: University of Nebraska at Lincoln
EPA Project Officer: Winner, Darrell
Project Period: August 5, 2002 through August 4, 2004 (Extended to August 4, 2005)
Project Period Covered by this Report: August 5, 2002 through August 4, 2003
Project Amount: $215,061
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (2001)
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Description:
Objective:The objectives of this research project are to: (1) elucidate the kinetics and mechanisms of zerovalent iron (Fe0) treatment processes; (2) develop new approaches to enhance Fe0 performance; and (3) implement a successful cleanup of a contaminated field site.
Progress Summary:Develop Kinetics and Mechanisms of Fe0 Treatment Processes
Electron transfer from Fe0 to targeted contaminants is affected by the initial composition of the iron, the oxides formed during corrosion, and surrounding electrolytes. Under anoxic conditions, Fe0 reduces nitrate to ammonium and magnetite (Fe3O4) is produced at near neutral pH. Batch tests indicated that nitrate removal was most rapid at low pH (2-4); however, the formation of a black oxide film at pH 5 to 8 temporarily halted or slowed the reaction unless the system was augmented with Fe(II), Cu(II), or Al. Bathing the corroding Fe0 in a ferrous iron (Fe2+) solution greatly enhanced nitrate reduction at near-neutral pH and coincided with the formation of a black precipitate. X-ray diffraction and scanning electron microscopy confirmed that both the black precipitate and the black oxide coating on the iron surface were magnetite. In this system, Fe(II) was determined to be a partial contributor to nitrate removal, but nitrate reduction was not observed in the absence of Fe0. Nitrate removal also was enhanced by augmenting the Fe0 treatment system with ferric iron (Fe3+), but no enhancement was observed when Ca2+, Mg2+, or Zn2+ was used.
We determined the effectiveness of Fe0 to dechlorinate metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide) and dicamba (3,6-dichloro-2-methoxy benzoic acid) in the presence of aluminum and iron salts. In treating aqueous solutions of metolachlor (300-400 mg/L) with Fe0, destruction kinetics were greatly enhanced when Al, Fe(II), and Fe(III) salts were added with the following order of destruction kinetics observed (0.5 percent w/v): Al2(SO4)3 > AlCl3 > Fe2(SO4)3 > FeCl3. Central to the mechanism responsible for enhanced metolachlor loss may be the role these salts play in facilitating Fe(II) release. In the process of tracking Al and Fe(II) in the Fe0 + Al2(SO4)3 treatment of metolachlor, we observed that Al was readily sorbed by the corroding iron with a corresponding release of Fe(II). Metolachlor dechlorination also was greater under Eh and pH conditions favoring the formation of green rusts, mixed Fe(II)-Fe(III) hydroxides with interlayer anions that impart a greenish-blue color.
Mixing an aqueous solution of dicamba with 1.5 percent Fe0 (w/v) resulted in 80 percent loss of dicamba within 12 hours. Solvent extraction revealed that dicamba removal by Fe0 was primarily through adsorption; however, when the Fe0 was augmented with Al or Fe(III) salts, dicamba was dechlorinated to an unidentified degradation product. In contrast to dicamba, Fe0 treatment of 3,6-dichlorosalicylic acid (DCSA), the common biological degradation product of dicamba resulted in removal with some dechlorination observed. When DCSA was treated with Fe0 plus Al or Fe(III) salts, destruction was 100 percent. Extracts of this treatment produced the same high-performance liquid chromatography peak observed after treating dicamba with Fe0 + Al or Fe salts.
Molecular modeling suggests that differences in removal and dechlorination rates between dicamba and DCSA may be related to the type of coordination complex formed on the iron surface. Experiments with 14C-labeled dicamba confirmed that Fe-adsorbed dicamba residues are available for subsequent biological mineralization (11 percent mineralized after 125 days). These results indicate that Fe0 could potentially be used to treat dicamba- and DCSA-contaminated water.
The manufacturing process used to produce the Fe0 also can profoundly affect destruction rates. Metolachlor destruction rates with Al or Fe salt-amended Fe0 were greater with annealed iron (indirectly heated under a reducing atmosphere) than unannealed iron. Moreover, the optimum pH for metolachlor dechlorination in water and soil differed between iron sources (pH 3 for unannealed, pH 5 for annealed).
Our results indicate that metolachlor and dicamba destruction by Fe0 may be enhanced by adding Fe or Al salts and by creating pH and redox conditions favoring the formation of green rusts. Batch and column tests indicate that adding Al, Fe2+, or Fe3+ may enhance removal of nitrate, nitrogenated, and chlorinated organics by a Fe0 permeable reactive barrier. Because of the limitations of laboratory conditions (e.g., using pure water to prepare feed solutions and purging with nitrogen gas to deoxygenate), the feasibility of enhancing destruction through addition of catalytic cations must be fully evaluated within a groundwater environment before implementation in the field. In particular, cementation and hydraulic conductivity could be quite different in situ from the column tests conducted in this research. Hydraulic retention time should be determined along with the kinetics of contaminant destruction under the application environment.
Develop New Approaches To Enhance Fe0 Performance
We investigated the use of cationic surfactants to accelerate Fe0-mediated destruction of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Soils contaminated from military operations often contain mixtures of RDX, HMX, and TNT (2,4,6-trinitrotoluene) rather than a single explosive. Differences among explosives in solubility and reactivity make developing a single remediation treatment difficult. When Fe0 was used to treat a munitions-contaminated soil, we observed high rates of destruction for RDX and TNT (98 percent), but not for HMX. Our objective was to determine if HMX destruction by Fe0 could be enhanced by increasing HMX solubility by physical (temperature) or chemical (surfactants) means. To determine electron acceptor preference, we treated RDX and HMX with Fe0 in homogeneous solutions and binary mixtures. Increasing the temperature of the aqueous solution (20 to 55°C) increased HMX solubility (2 to 22 mg L-1) but did not increase destruction by Fe0 in a contaminated soil slurry that also contained RDX and TNT. Batch experiments using equal molar concentrations of RDX and HMX showed that Fe0 preferentially reduced RDX over HMX. By testing various surfactants, we found that the cationic surfactants hexadecyltrimethyl ammonium bromide (HDTMA), didecyldimethyl ammonium bromide, and didodecyldimethyl ammonium bromide were most effective in increasing HMX concentrations in solution. Didecyl and HDTMA also were highly effective in facilitating transformation of HMX by Fe0. HMX destruction kinetics by Fe0 were greater in 5 percent (w/v) HDTMA (0.49 h-1) and 3 percent (v/v) didecyl (0.35 h-1) matrices than in H2O alone (0.09 h-1). These results indicate that cationic surfactants can increase HMX solubility and facilitate Fe0-mediated transformation kinetics, but HMX destruction rates will be slowed when RDX is present.
The water solubilities of HMX, RDX, and TNT increase with increasing temperature, but HMX is the least soluble of the three high explosives at any temperature. HMX can be chemically reduced by Fe0, but kinetics will be slowed when RDX is present. This indicates that when multiple explosives are present in a contaminated soil, most of the RDX may need to be transformed before effective HMX reduction by Fe0. Because cationic surfactants increase solution concentrations of both RDX and HMX, it is likely that removal of RDX is critical to removing HMX from the contaminated soil when Fe0 and surfactants are used. Our results indicate that the cationic surfactants HDTMA and didecyl can be used to greatly increase HMX and RDX solution concentrations and also increase the kinetics of destruction by Fe0.
Implement a Successful Cleanup of a Contaminated Field Site
In 2002, a field project was initiated to treat approximately 50,000 cubic yards of contaminated soil from a Nebraska agricultural cooperative. The soil contained average concentrations of 653 mg atrazine, 177 mg metolachlor, and 4,220 mg nitrate-N per kg. The soil was thoroughly mixed and windrowed using a high-speed, tractor-pulled mixing implement. Fifty, 50-lb paper bags of finely ground iron metal were placed on top of the windrow, along with 20, 50-lb paper bags of commercial aluminum sulfate (commonly used to acidify soil). The amendments were incorporated using the mixing implement to obtain final concentrations of 5 percent (w/w) Fe0 and 2 percent Al2(SO4)3.
Within 5 months of treatment, no metolachlor was detected (less than 1 mg kg-1, greater than 99 percent destruction), atrazine concentration had decreased to 277 mg/kg, and nitrate-N decreased to 2,742 mg kg-1. After approximately 1 year, atrazine concentration was further decreased to 127 mg kg-1 (81 percent reduction) and nitrate-N decreased to 2,410 mg kg-1 (43 percent reduction). Although remediation of metolachlor contamination was complete after treatment with Fe0 + Al2(SO4)3, secondary treatment was determined to be necessary to further reduce concentrations of atrazine and nitrate-N. Table sugar was added as a supplemental carbon source by incorporating 128, 25-lb paper bags into the soil with the high-speed soil mixing implement. Results of this treatment will be discussed in our annual report for Year 2.
Future Activities:Year 2 of the project will focus on additional work concerning the mechanisms of reaction with Fe0, the role of green rust, magnetite and Fe(III) (hydr)oxide surfaces formed during corrosion, and reactions with Fe(II) associated with these surfaces (see Objective 1). We will report on further studies with cationic surfactant to enhance the kinetics of Fe0-promoted remediation of HMX and RDX in water and soil (see Objective 2). The results of secondary on-site treatment of contaminated soil will be discussed (see Objective 3).
We originally proposed that adsorption of Fe2+ (or other cations) onto the magnetite (or green rust), creates an electrical field that can facilitate electron migration from the Fe0 core to the surface. Additional experiments will be conducted in which anions (e.g., phosphate or chromate ions) are introduced into the batch reactor solution at various pH levels. A decrease in reduction rate coinciding with the adsorption of these anions will indirectly support our hypothesis. If the hypothesis is correct, cations that adsorb onto the magnetite and create a positive-charged interface should enhance the reduction of nitrate (or chlorinated/nitrogenated organics). Selected di- and trivalent cations will be tested at various pH levels to determine the effect(s) of adsorption and electrical field enhancement. More tests will be conducted to study the effects of other selected cations on nitrate removal, with an emphasis on substitution of the selected cation for Fe2+ or Fe3+ in iron oxides coating the surface of the iron grains. Mechanisms associated with enhanced nitrate removal will be evaluated. Where feasible, reactions will be modeled.
Several presentations and publications are anticipated during the second year. Dr. Y.H. Huang (who received his Ph.D. in 2002) will be working with Dr. Zhang as a Postdoctoral Research Associate on research related to the project. Dr. Zhang received funding from the Midwest Technology Assistance Center for Small Public Water Systems, "Development of sulfur-limestone autotrophic denitrification processes for treatment of nitrate-contaminated groundwater in small communities." In this project, zerovalent iron will be used to remove sulfate produced by autotrophic denitrification.
Journal Articles on this Report: 4 Displayed | Download in RIS Format
| Other project views: | All 34 publications | 14 publications in selected types | All 14 journal articles |
| Type | Citation | ||
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Huang YH, Zhang TC, Shea PJ, Comfort SD. Effects of oxide coating and selected cations on nitrate reduction by iron metal. Journal of Environmental Quality 2003;32(4):1306-1315. |
R829422E03 (2003) |
not available |
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Park J, Comfort SD, Shea PJ, Machacek TA. Role(s) of didecyl surfactant in reduction of HMX-contaminated soil by Fe0. Agronomy Abstracts. American Society of Agronomy 2002;94:S11-Park 142935. |
R829422E03 (2003) |
not available |
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Satapanajaru T, Comfort SD, Shea PJ. Aluminum and iron salt-catalyzed destruction of metolachlor by zerovalent iron. Journal of Environmental Quality, 2003; 32(5): 1726-1734. |
R829422E03 (2003) |
not available |
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Satapanajaru T, Shea PJ, Comfort SD, Roh Y. Green rust and iron oxide formation influences metolachlor dechlorination during zerovalent iron treatment. Environmental Science and Technology 2003;37(22):5219-5227. |
R829422E03 (2003) R829422E03 (2004) |
not available |
cleanup, sediments, engineering, environmental chemistry, restoration, remediation, performance, contamination, kinetic, treatment process, zero valent iron, Fe. , POLLUTANTS/TOXICS, Water, Geographic Area, Scientific Discipline, Waste, Remediation, Water Pollutants, Contaminated Sediments, Groundwater remediation, Ecology and Ecosystems, State, water quality, dehalogenation, verticle attachment, fate and transport, predictive understanding, reductive treatment, ecology assessment models, kinetic studies, contaminated aquifers, contaminated sediment, remediation technologies, contaminated soil, groundwater contamination, hazardous waste, contaminated groundwater, sediment treatment, environmental engineering, Nebraska (NE), nitrate, zero valent iron
Progress and Final Reports:
Original Abstract
2004 Progress Report
Final Report