Electrochemical remediation of radionuclide- and trichloroethene-contaminated groundwater using zero-valent iron/iron oxides and mineralogical characterization of iron hydroxides formed from zero-valent iron

Author

Yul Roh

Date of Award

12-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

John E. Foss

Committee Members

John Ammons, Michael Essington, Chris Cox

Abstract

Recently, metal-based degradation of chlorinated organic compounds and immobilization of radionuclides and heavy metals for groundwater remediation have become popular topics in environmental science and engineering research. Although the zero-valent iron (Fe°)-based reactive barrier has been introduced as a cost-effective remediation technology for groundwaters contaminated by chlorinated organic compounds and reducible metals, the current approach of the technology has been criticized for its low process efficiency (reaction rate) and the effective life of the medium (longevity).

The objectives of this study are to (1) develop and evaluate electro-enhanced technology using Fe° for mobile radionuclides [e.g., TCO₄^- and UO₂(CO)₃^2-] and trichloroethene (TCE) in groundwater, (2) develop and evaluate an iron-oxide-based electroremediation for TCE-contaminated groundwater, (3) perform mineralogical characterization of the precipitates formed during contaminated groundwater treatment, and (4) understand the mechanisms of the chemical processes involved in Fe° oxidation and the effect of cations and anions on the initial precipitation and subsequent phase transformation of the precipitates.

Application of direct current on the Fe° medium column results in a significant enhancement of the dechlorination rate of TCE and the longevity of Fe° as a reductive medium for radionuclides. Several factors affect the removal of radionuclides and TCE, including (1) electrode configuration, (2) applied voltage and current, (3) distance between cathode and anode, (4) material used as electrodes, and (5) influent concentration iv and water chemistry. Reduction of radionuclides and coprecipitation with eroded iron as an iron hydroxide mineral appears to be the preferred removal process. The dechlorination mechanism appears to be reductive using the electrons supplied by the iron oxidation and external power supply. Because the electro-enhanced process is a remediation technology based on chemical reduction and precipitation of radionuclides and degradation of chlorinated organic compounds through external current supply rather than the addition of chemicals, this Fe° barrier technology has the potential to simplify the decontamination process for treating groundwater contaminated -with radionuclides, heavy metals, and chlorinated organic compounds.

Direct current applied to palladized multiphasic iron oxide minerals not only increases the efficiency and effective life of the medium by controlling iron oxidation but also increases the dechlorination rate of solvents by providing an external supply of electrons. The palladized multiphasic iron oxide medium has some advantages over the palladized Fe° medium. Because the palladized multiphasic oxide is less susceptible to passivation by iron hydroxide coating than palladized Fe°, no significant reduction in flow rate or generation of ferrous iron (Fe²⁺) in the effluent are anticipated during TCEcontaminated groundwater treatment.

The mineralogical characterization of the precipitates in reactive Fe° columns shows that the principal corrosion products are magnetite, intermediate products (green rusts), and hydrated forms of ferric oxides, such as akaganeite (β-FeOOH), goethite (α- FeOOH), or lepidocrocite (γ-FeOOH), depending on the degree of oxidation and the chemical environment. Other minerals, including calcite, aragonite, siderite, mackinawite, V and native metal sulfur, are also precipitated on the surface of reactive medium, depending on the groundwater chemistry. Factors affecting the mineralogy of the precipitates include (a) contaminant type and chemical composition of groundwater and (b) occurrence of sulfate-reducing bacteria. These amorphous and crystalline minerals formed on the reactive medium and reduced the effectiveness and porosity of the Fe° medium.

A pyroaurite-type mineral was synthesized as a host iron mineral for the radionuclides through anodic corrosion of iron from Fe° foam. The results showed that reduced uranium and technetium were incorporated in the trioctahedral layer of the pyroaurite structure by substitution of ferrous/ferric iron. The excess positive charges generated by the substitution were neutralized by the carbonate complex in the interlayer of the mineral. The electrochemical synthesis of a crystalline pyroaurite-type mineral could be an effective and economical approach to remediating groundwaters contaminated by uranium, technetium, and other reducible heavy metals. The formation of such a wellcrystalline waste product is fortuitous because its solubility is expected to be much lower than that of an amorphous iron oxide phase, thereby immobilizing the entrapped contaminants more effectively.

Recommended Citation

Roh, Yul, "Electrochemical remediation of radionuclide- and trichloroethene-contaminated groundwater using zero-valent iron/iron oxides and mineralogical characterization of iron hydroxides formed from zero-valent iron. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/7494