Food for Thought

• Nov 01, 2000

Bacteria, which are ubiquitous on Earth, help us in many ways. In addition to enabling us to digest our food and produce pharmaceutical products, bacteria can help clean up certain environmental problems.

Chlorinated volatile organic compounds (VOCs) have been released from a variety of sources. When such chemicals enter groundwater, they are difficult and expensive to remove. Luckily, over time, some bacteria naturally helps degrade VOCs and their byproducts. Unfortunately, this natural bioremediation is too slow and incomplete to meet environmental requirements efficiently. For nearly a decade, researchers have sought ways to enhance the bioremediation of VOCs in groundwater.

All soils contain some organic carbon, which can be used by fermentation bacteria to produce the molecular hydrogen needed for the growth of other bacteria. The hydrogen is especially important to reductive dehalogenators (anaerobic bacteria), which can dehalogenate (dechlorinate) VOCs. The dechlorination process mineralizes the VOCs, rendering them inert in groundwater. During the reductive degradation of chlorinated compounds, bacteria facilitate the degradation of the chlorinated compound by replacing a chlorine molecule with hydrogen. In general, this process requires reducing conditions, reflected by a low reduction-oxidation (redox) potential as measured in millivolts (mV) using a silver-silver chloride electrode referenced to a standard hydrogen electrode (Eh).

Although the rapid growth of microorganisms creates reducing conditions of less than +300 millivolts (mV) as Eh, a redox potential less than -300 mV as Eh is required for complete degradation of VOCs. Adding reducing agents to groundwater to enhance the environment for bacteria is the subject of this article.

Field study
Geomatrix Consultants investigated a site where soil and groundwater had been affected by VOCs -- in particular, tetrachloroethene (PCE), a solvent commonly used in dry cleaning and semiconductor manufacturing facilities. After soil around a former underground solvent tank was excavated and replaced, VOCs were no longer detected in the soil. In groundwater, however, concentrations of vinyl chloride (VC) detected in monitoring well A reflected incomplete degradation of PCE close to the area of excavation (Figure 1). The regulatory requirement for site closure was that VC concentrations in groundwater not exceed the California maximum contaminant level (MCL) for VC, which is 0.5 microgram per liter ((g/L). To eliminate the need for continued monitoring, we selected reductive bioremediation to degrade these residual VOCs in place.

Reducing conditions - such as the potential for oxidation reduction (redox) measured in site monitoring wells before the study -- indicated moderately low redox, but not low enough to support complete decholrination of the VC. By stimulating redox reactions in affected groundwater, we sought to enhance dechlorination of VOCs, including complete dechlorination of VC. We developed a sucrose/yeast extract that was injected into the groundwater to feed the fermentation bacteria so they would enhance conditions for the dechlorinators. Sucrose supplies organic matter for fermentation, the byproducts of which are hydrogen and other electron donors. Yeast extract supplies the microbial community with necessary growth factors that cannot be synthesized by the cell (such as vitamins and amino acids).

In our study, two doses of sucrose/yeast extract, ten months apart, were injected into site monitoring well A. If the measured redox potential declined, we would know that the sucrose/yeast extract was improving the conditions necessary for the indigenous bacteria to dechlorinate VC.

Results
Figure 1 shows the changes in Eh after the sucrose/yeast extract was injected into monitoring well A. Before the first injection, Eh measurements ranged from 298 to 348 mV (moderate redox potential). The solution is estimated to have extended to a radius of only about 2.6 and 4.2 feet during the first and second injections, respectively. Yet the decrease in Eh was measured even 100 feet from the point of injection.

In monitoring well A, Eh continued to decrease for four months (through November 1995), but in distant monitoring wells, Eh began to increase after about three months (Figure 2). Seven months after the initial injection (February 1996), the Eh in all wells had reached levels equal to or greater than original levels. A second injection of the sucrose/yeast extract solution in May 1996 again lowered Eh throughout the site. Eh remained low for about three months in the distant wells, but a distinctly low Eh remained in well A for more than a year after the injection.

Figure 1. Eh intensities in monitoring wells before and after injections.

Concentrations of the PCE degradation product (VC), as measured by laboratory analysis, followed a pattern similar to that of Eh -- falling after the first injection, rising and then falling again after the second injection. The results of sampling and testing before and after injections are given in Table 1. In December 1994, before any injections, VC was found in monitoring well A at 3.1 micrograms per liter ((g/L). In November 1995 (four months after the first injection), VC in well A had declined to 0.59 (g/L, but by February 1996 was up to 2.9 (g/L. After the sucrose/yeast extract was injected again (in May 1996), and Eh declined a second time, no VC was detected in August or November 1996 (laboratory detection limit of 0.5 (g/L). Although VC was detected at just 0.5 (g/L in monitoring well A in February 1997, it was not detected in any wells in April 1997.

Table 1. Residual vinyl chloride concentrations and redox potential in monitoring well A before and after bioremediation.

The sucrose/yeast extract served as a reducing agent by feeding the fermentation bacteria, which then supplied hydrogen for reductive dehalogenation. Lowering the redox potential of the groundwater enhanced reductive bioremediation. VC was removed and the cleanup goal attained. The site was closed by the regulatory agency.

This technology has been used successfully to dechlorinate VOCs at other sites where VOCs were found at concentrations hundreds of times higher than at this site (concentrations representative of free product).

Currently, Geomatrix Consultants and the University of California Berkley have a patent pending on this technology.

Figure 2. Eh intensities in monitoring well during bioremediation.

Notes: (g/L = micrograms per liter. mV = millivolts. Eh = the potential relative to a standard hydrogen electrode. The actual measurement was made with a silver/silver chloride electrode (SSCE). The Eh is calculated from the measured potential using the Nernst Equation: Eh = E(SSCE) + 199 mV.

Thomas A. Delfino is a principal with Geomatrix Consultants Inc., Oakland, Calif., and leads Geomatrix's Resource Optimization practice.

James H. Honniball is a project scientist with Geomatrix.

They can be reached via e-mail at TDelfino@geomatrix.com and Jhonniball@geomatrix.com respectively.

This article originally appeared in the 11/01/2000 issue of Environmental Protection.