Date of Award

12-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Sciences

Committee Member

Kevin T Finneran, Committee Chair

Committee Member

Cindy Lee

Committee Member

David Freedman

Abstract

Chlorinated ethenes are known or suspected carcinogens. They are also among the most frequently detected organic groundwater contaminants because of wide use and poor disposal practices. Within anaerobic aquifer systems, Fe(III) reduction is a prevalent terminal electron accepting process and has often been reported as a competitive electron acceptor with respect to trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC) reduction. Often, practitioners add excessive amounts of electron donor to compensate for this competition in an effort to avoid incomplete dechlorination or "cis-DCE and VC stall." Contaminated aquifer material incubations were setup to assess dechlorination rates and extent under Fe(III)-reducing conditions, and to evaluate whether excessive electron donor additions increase either the rate or extent of complete dechlorination of TCE or VC using acetate and vegetable oil-based electron donors.

Sediment batch experiments were constructed to specifically assess how Fe(III) speciation and the presence of electron shuttles influenced VC dechlorination. Four treatments were investigated: (1) VC +10 mM ferrihydrite; (2) VC + 10 mM Fe(III)-NTA; (3) VC + 10 mM ferrihydrite + 500 µM AQDS; and (4) VC + 5 mM AQDS. Sediment batch experiments were also set up to specifically evaluate how electron donor concentration affected TCE and VC reduction using acetate and vegetable oils (Newman Zone® Standard without sodium lactate, CAP 18 ME, EOS 598B42) as sole electron donors. Three electron donor strategies were investigated for each series. The first donor strategy was operationally defined as "stoichiometric," in which the electron donor concentration was balanced "electron to electron" with either the TCE or VC present and going to ethene. The second and third approaches were an "excessive" electron donor approach in which 5x and 10x the necessary stoichiometric amount of electron donor was added.

The results of the Fe(III) speciation and electron shuttle batch study suggest Fe(III) is not inhibitory to complete dechlorination. In all incubations amended with Fe(III), VC was completely reduced to ethene, but the rate of VC reduction was faster in incubations that were not amended with Fe(III), which suggest that the presence of Fe(III) may slow the rate of reductive dechlorination.

The results of the high versus low electron donor batch studies demonstrate that TCE and VC were completely degraded when the vegetable oils were amended at stoichiometric and excessive concentrations. Methane accumulations were similar regardless of the treatment applied. TCE and VC reduction were partially degraded in the presence of acetate, regardless of the concentration. Fe(III) reduction did not inhibit ethene production. The data suggest that the addition of substrate may not be necessary if the supply of indigenous electron donor is sufficient and the appropriate microorganisms are present. If electron donor should be added, amendments can start low and be increased.

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