Project Abstract

The Effect of Environmental Conditions on Reductive Dechlorination Rates

Principal Investigators

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

Goal

The goal of this project is to develop an understanding of the effect of environmental conditions on the rates of reductive dechlorination reactions.

Rationale

Reductive dechlorination is a common anaerobic biodegradation mechanism for chlorinated aromatic compounds. This research will test the hypothesis that the rate of reductive dechlorination reactions depend upon the apparent redox potential, or EH, of the reaction medium.

Approach

The project has two phases. In the first phase, a reactor system was developed to allow measurement of biodegradation rates under constant conditions of biomass, pH, sulfate, sulfide, and acetate concentrations. In the second phase, batch experiments are being conducted to measure degradation rates under various, controlled environmental conditions. A model anaerobic system fed acetate, pentachlorophenol (PCP), and nutrients has been selected for study.

Status

This project was completed in early 1995. A reactor system was developed to control and/or monitor pH, apparent oxidation-reduction potential, and acetate, biomass, sulfate, and sulfide concentrations. An additional platinum electrode was added to monitor apparent redox measurements and to help identify possible poisoning of the controlling platinum electrode. Both hydrogen peroxide and potassium ferricyanide were used to raise and control the apparent redox potential above the "natural" system potential of -250 to -260 mV, and both titanium citrate and hydrogen were used to lower the apparent redox potential. Pentachlorophenol dechlorination were modeled as first order with respect to PCP concentration. When multiple PCP additions were made, without manipulating redox potential, the rate of dechlorination was observed to increase with the number of PCP additions. This suggests either induction of enzyme(s) or growth of a sub-population responsible for dechlorination. Overall biomass concentration changes relatively little during a typical experiment. The trend of increasing dechlorination rate with multiple PCP additions was shown to persist during "mild" redox potential increases of 50 to 150 mV. However, dramatically decreased dechlorination rates were measured during 250 mV perturbations. PCP dechlorination continued at elevated redox potentials that essentially halted acetate consumption and methane production, lending support to the hypothesis of a separate dechlorinating sub-population.