Project Abstract

Development of a Vitamin B12-Amended Bioremediation Process for the Reductive Dechlorination of Chlorobiphenyls at all Chlorine Positions

Principal Investigators

Sandra Woods
Oregon State University
E-mail: Sandra.Woods@orst.edu

Goal

The goal of this project is to develop a vitamin B12-amended anaerobic process that will result in the complete reductive dechlorination of highly chlorinated biphenyls, especially those heavily substituted at the ortho position. Our objectives are (1) to determine the effect of parent compound structure on biotransformation pathways and kinetics, and (2) to demonstrate and optimize PCB reductive dechlorination in a laboratory-scale anaerobic process.

Rationale

Vitamin B12s is a nucleophile with the capability of reductively dechlorinating chlorobiphenyls at all positions. By coupling vitamin B12 with a biological reductant, reductive dechlorination can be supported. A microorganism was selected for study due to its ability to reduce cobalt, and potentially, the cobalt center of vitamin B12.

Approach

During the first year of the project, we are concentrating on meeting objective 1 by evaluating several reducing systems. Our model compound is 2,3,4,5,6-pentachlorobiphenyl, selected for study because one ring is fully chlorinated. We will expand the study to include other chlorobiphenyls as the work progresses. Our reducing systems include a PCB-dechlorinated microbial consortium, chemically-reduced vitamin B12, biologically-reduced vitamin B12, and a PCB-dechlorinating consortium amended with vitamin B12. We are separately evaluating the reduction of the catalyst and the reductive dechlorination of the chlorobiphenyl.

Status

We have determined a complete reductive dechlorination pathway for 2,3,4,5,6-pentachlorobiphenyl in a system containing titanium citrate-reduced vitamin. 2,3,4,5,6-Pentachlorobiphenyl is reductively dechlorinated primarily at the meta and para position, but its products are dechlorinated at the ortho, meta, and para positions to yield each of the six possible dichlorobiphenyl intermediates.

Once the ability of vitamin B12s to yield ortho, meta, and para dechlorination was identified, we began to examine the ability to microbially-reduce vitamin B12. Fully oxidized B12 at a concentration of 46mM was incubated with a pure culture of Shewanella alga, formerly Geobacter sulfurreducens, under anaerobic conditions at pH 7. Lactate was provided as the electron donor. Over a period of 10 days, vitamin B12 was reduced from the fully oxidized form, B12a, to a reduced form, B12r. The rate at which this organism is capable of reducing vitamin B12 is currently under investigation.

The vitamin B12/Shewanella alga system was evaluated for its ability to reductively dechlorinate a range of halogenated organic compounds, including carbon tetrachloride, dibromodichloromethane, bromotrichloromethane, and tetrachloroethylene. All of the halogenated methanes were transformed rapidly except tetrachloroethylene. Further evaluation of transformation rates at an elevated pH are planned since the literature has shown that dechlorination rates using vitamin B12 are pH dependent. The reduction of 2,3,4,5,6-pentachlorobiphenyl in this system has been assessed at pH 7 and pH 8.4. There does not appear to be any significant dechlorination of the 2,3,4,5,6-pentachlorobiphenyl at pH 7. However, when the pH was raised to 8.4 it appeared as though some transformation occurred. Currently the evaluation of extent of apparent transformation is being reexamined. In addition, the use of other possible catalytic systems are being evaluated.