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Final 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 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 the research project were 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.
Summary/Accomplishments (Outputs/Outcomes):Electron transfer from Fe0 to targeted contaminants is affected by initial composition of the iron, 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 nitrate removal was most rapid at low pH (2-4); however, 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 ferrous iron (Fe2+) solution greatly enhanced nitrate reduction at near-neutral pH and coincided with formation of a black precipitate. X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed that both the black precipitate and black oxide coating on the iron surface were magnetite. In this system, Fe(II) was 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 found that metolachlor and dicamba destruction by Fe0 were enhanced by adding Fe or Al salts and creating pH and redox conditions favoring green rust formation. Batch and column tests indicate that Al, Fe2+, or Fe3+ may enhance removal of nitrate, nitrogenated, and chlorinated organics by a permeable reactive iron barrier. Cementation occurred only in the inlet zone, suggesting that lepidocrocite and goethite, rather than magnetite, are responsible for cementation and clogging. The feasibility of enhancing destruction through addition of cations, however, must be evaluated within a ground water environment before field implementation.
In the presence of magnetite particles, significant Fe2+(aq) was adsorbed at pH of approximately 7.3. The surface-bound Fe2+ (S.B.Fe2+) then could be oxidatively precipitated by nitrate via the reaction: 12 S.B.Fe2+ + NO3- + 13 H2O 
4 Fe3O4↓ + NH4+ + 22 H+. The reaction self-accelerated as the magnetite produced supplied more reactive sites, but with the build-up of acidity, the reaction decelerated and stopped at pH less than 6.8. Introducing a sub-stoichiometric dose of O2 or Fe3+(aqueous) was as effective as seeding magnetite particles, suggesting that lepidocrocite is a precursor for initiating the reaction. Our results extend previous research by relaxing reaction conditions so nitrate can react with ferrous iron at pH less than 8.0. The proposed reaction may be an important link between the nitrogen cycle and the Fe(II)/Fe(III) redox couple in the bio- or geosphere.
The composition of Fe0 surfaces significantly affects reactivity. Samples of unannealed and annealed (heat-treated under H2/N2) Fe0 were analyzed using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Annealed samples contained less iron carbide and more sulfur. The carbide concentration was essentially unchanged by trichloroethylene (TCE) and H2O/TCE exposure, whereas sulfur decreased in proportion to chlorine adsorption following the dechlorination reaction.
A sonication method was developed to strip the corrosion coating off iron grains layer by layer. Transformation of the constituents and morphology of the corrosion coating along its depth and over reaction time can then be investigated by composition analysis, XRD, and SEM. The sonication method recovered more than 90 percent iron oxides produced by the Fe0-dissolved oxygen (DO) redox reaction. Magnetite and lepidocrocite were identified as the corrosion products. Initially, lepidocrocite was the main product in the presence of DO. As the oxide coating thickened, the inner layer transformed to magnetite, which was retained as the only stable corrosion product once DO was depleted. The method confirms phase transformations between γ-FeOOH and Fe3O4 within a stratified coating. The sonication technique allows investigation of complicated processes in Fe0-oxide-contaminant systems. A two-layer semiconductor model was developed.
We also investigated the use of cationic surfactants to accelerate Fe0-mediated destruction of HMX and RDX. Soils contaminated from military operations often contain multiple explosives. Differences in solubility and reactivity make developing a single remediation treatment difficult. When Fe0 was used to treat munitions-contaminated soil, we observed high rates of RDX and TNT destruction (98%) but not HMX. 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 contaminated soil slurry that also contained RDX and TNT. Batch experiments at equal molar concentrations showed Fe0 preferentially reduced RDX over HMX. The cationic surfactants hexadecyltrimethyl ammonium bromide (HDTMA) and didecyldimethyl ammonium bromide (DDDMA) were most effective in increasing HMX concentration in solution, but DDDMA more effectively facilitated HMX transformation by Fe0.
We investigated the potential of selected graphitic/activated carbon materials to improve Fe0 reduction of HMX, RDX, and TNT. The efficacy of cast iron particles (ZVI) and electrolytic (99% pure) iron (eZVI) were compared in combination with graphite and activated carbon materials under aerobic and anaerobic conditions. Activated carbon inhibited aerobic degradation or had no effect. Under aerobic conditions, graphite enhanced degradation of HMX by both ZVI and eZVI but had little effect on RDX degradation. Under anaerobic conditions, HMX was rapidly degraded by all treatments, but degradation was faster when graphite was present. Anaerobic degradation of RDX was similarly enhanced by graphite, but was not as rapid. Our results indicate graphite may be most beneficial when conditions are least favorable for reduction. We are investigating differences in effectiveness based on structure of the carbon materials to gain insight into the degradation reactions and determine how to improve remediation protocols.
In 2002, a field project was initiated to treat contaminated soil from a Nebraska agricultural cooperative. The soil contained an average of 653 mg atrazine, 177 mg metolachlor, and 4,220 mg nitrate-N per kg. The soil was mixed and windrowed using a high speed, tractor-pulled implement. Fifty, 50-pound paper bags of finely-ground iron metal were placed on the windrow, along with 20, 50-pound paper bags of commercial aluminum sulfate (commonly used to acidify soil). The amendments were incorporated to obtain final concentrations of 5 percent (w/w) Fe0 and 2 percent Al2(SO4)3. Within 5 months, no metolachlor was detected (less than 1 mg kg-1, greater than 99% destruction), atrazine concentration decreased to 277 mg/kg, and nitrate-N decreased to 2,742 mg kg-1. After 1 year, atrazine concentration decreased to 127 mg kg-1 (81% reduction) and nitrate-N decreased to 2,410 mg kg-1 (43% reduction). Although remediation of metolachlor contamination was complete after treating with Fe0+Al2(SO4)3, secondary treatment was necessary to further decrease concentrations of atrazine and nitrate-N. Table sugar (sucrose) was added as a supplemental carbon source by incorporating 128, 25-pound paper bags into the soil with the soil-mixing implement. Adding readily available carbon while maintaining anaerobic conditions resulted in more than 95 percent and more than 88 percent decrease in atrazine and nitrate-N concentrations.
Conclusions:- Electron capture detection response, vertical attachment energies, and attachment rate constants from electron scattering data, may be useful in predicting dehalogenation rates of chlorinated contaminants treated with Fe0.
- Cl and Br salts enhance degradation and may be used to restore and maximize Fe0 reactivity. Adding Fe2+, Fe3+, and Al3+ can significantly enhance reduction by Fe0.
- A ferrous iron (Fe2+) solution greatly enhanced nitrate reduction by zerovalent iron at near-neutral pH and coincided with formation of a black precipitate. XRD and SEM confirmed that both the black precipitate and black oxide coating on the iron surface were magnetite.
- Nitrite reduction rate was proportional to dissolved Fe2+ concentration and nitrite reduction out-competed water reduction in terms of contributing to the overall iron corrosion. Our observations help explain the complicated interactions between water reduction and nitrite reduction, the roles of surface-bound Fe2+, and the evolution of the iron corrosion coating.
- In the presence of nitrate, Fe2+ (aqueous) will be adsorbed to the Fe0 or Fe oxides; the surface-complexed Fe(II) may be oxidized and become structural Fe(III), resulting in a steadily increasing ratio of Fe(III)/Fe(II) in the oxides formed. The transformation of amorphous iron oxides into magnetite, a non-passivating oxide, triggers rapid nitrate removal.
- Our results extend previous research by relaxing reaction conditions so nitrate can react with ferrous iron at pH less than 8.0. The reaction could add an important link between the nitrogen cycle and the Fe(II)/Fe(III) redox couple in the bio- or geosphere.
- Magnetite was the dominant corrosion product under anoxic/other conditions. Lepidocrocite and goethite were identified in column inlet zone and cementation occurred, suggesting they cause hydraulic clogging. A two-layer semiconductor model was developed and refined.
- Metolachlor dechlorination was greater under Eh/pH favoring green rust formation.
- The cationic surfactants HDTMA and DDDMA increase HMX solubility and DDDMA most effectively facilitated Fe0-mediated transformation but HMX is slowed when RDX is present.
- Graphite may facilitate electron transfer in otherwise unfavorable conditions.
Journal Articles on this Report: 7 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. Effects of dissolved oxygen on formation of corrosion products and concomitant oxygen and nitrate reduction in zero-valent iron systems with or without aqueous Fe2+. Water Research 2005;39(9):1751-1760. |
R829422E03 (Final) |
not available |
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Huang YH, Zhang TC. Enhancement of nitrate reduction in Fe-0-packed columns by selected cations. Journal Of Environmental Engineering-Asce. 2005;131(4):603-611. |
R829422E03 (Final) |
not available |
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Huang YH, Zhang TC. Modeling of nitrate adsorption and reduction in Fe-0-packed columns through impulse loading tests. Journal Of Environmental Engineering-Asce. 2005;131(8):1194-1202. |
R829422E03 (Final) |
not available |
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Papastavros E, Shea PJ, Langell MA. Oxygen, carbon, and sulfur segregation in annealed and unannealed zerovalent iron substrates. Langmuir 2004;20(26):11509-11516. |
R829422E03 (Final) |
not available |
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Satapanajaru T, Comfort SD, Shea PJ. Enhancing metolachlor destruction rates with aluminum and iron salts during zerovalent iron treatment. Journal Of Environmental Quality. 2003;32(5):1726-1734. |
R829422E03 (Final) |
not available |
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Shea PJ, Machacek TA, Comfort SD. Accelerated remediation of pesticide-contaminated soil with zerovalent iron. Environmental Pollution 2004;132(2):183-188. |
R829422E03 (Final) |
not available |
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Zhang TC, Huang YH. Effects of selected good's pH buffers on nitrate reduction by iron powder. Journal Of Environmental Engineering-Asce. 2005;131(3):461-470. |
R829422E03 (Final) |
not available |
Supplemental Keywords: cleanup, sediments, restoration, engineering, environmental chemistry, contaminated aquifers, contaminated groundwater, contaminated sediment, dehalogenation, ecology assessment models.
, 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 ironProgress and Final Reports:
2003 Progress Report
2004 Progress Report
Original Abstract