Project Abstract

Overcoming the Barriers to In Situ Bioremediation of PCBs

Principal Investigators

John P. Tharakan and Ramesh C. Chawla
Howard University
E-mail: jpt@scs.howard.edu

Introduction

Polychlorinated biphenyls (PCBs) are a family of aromatic chlorinated organic compounds, that were used extensively in industry up to the time their production was banned in 1978. The widespread use of these compounds in industrial applications ranging from dielectric fluids to plasticizers has, over the years, resulted in a tremendous release of PCBs into the environment, where it is estimated that PCBs are now present at over 25% of Superfund sites, and they have also been found in groundwater, air and marine sediments. Clean-up of PCB-contaminated media is quite expensive by existing technologies, which include incineration and solvent extraction. In addition, both these technologies result in the generation of undesirable byproducts or further waste streams that in turn need to be disposed of. Bioremediation of PCB contaminated media is attracting increasing attention because it is relatively inexpensive, it permits the development of in situ treatment schemes and it usually does not result in the generation of additional hazardous waste. Unfortunately, bio-degradation of PCBs is not straightforward. Most microorganisms cannot directly metabolize PCB congeners, and the biodegradability of the specific congener decreases as the degree of chlorination increases. It has been shown that under anaerobic conditions, higher chlorinated PCBs can be reductively dechlorinated, resulting in the generation of lower chlorinated congeners. Under aerobic conditions, and in the presence of biphenyl, many organisms have been identified that can biodegrade the lower chlorinated congeners. However, the use of biphenyl as an in situ cosubstrate is expensive and environmentally problematic. Thus, there was a need to investigate the ability of alternative cosubstrates to support the cometabolic biodegradation of PCBs. Engineers and scientists at Howard University have been investigating the use of various alternative and environmentally acceptable cosubstrates, as well as the management of the cosubstrate concentration, to maximize PCB degradation in the laboratory. This work has also been extended to soil-slurry systems to investigate the feasibility of using this technology to remediate excavated soils instead of incinerating them. Under the leadership of Howard chemical engineering faculty, John P. Tharakan and Ramesh C. Chawla, graduate students (Simon Kizito, '96; Brigette Philpot, '97; Benjamin Reis, '97; and currently, Ebenezar Sada, '98) and postdoctoral researcher (Dr. Raycharn Liou) have been attempting to overcome the barriers to the biological remediation of PCBs. The success of this approach depends on the appropriate choice of cosubstrate and the adequate cycling between anaerobic and aerobic conditions.

The Research

Cometabolic biodegradation is a process by which an organism, that obtains no carbon or energy from a target compound, transforms that compound while growing on a second (co-) substrate. The transformation of the target compound is fortuitous but requires growth of the microbe and the presence of the appropriate enzymes. Thus, not all organisms will cometabolize PCBs. In the studies at Howard, researchers employed micro-organisms (Comomonas testosteroni and Rhodococcus erythropolis) that had been isolated from sites contaminated with PCBs, and which had already demonstrated the ability to biotransform lower chlorinated PCBs in the presence of biphenyl. Investigators examined the ability of alternative cosubstrates, including naphthalene, g-terpenine and terpinen-4-ol, to support biodegradation of PCBs. Naphthalene was chosen because it has structural similarities to biphenyl but it is a lot cheaper. The terpenic compounds were chosen because they are naturally occurring plant compounds and would conceivably be present, or easily transportable to the contaminated sites. These compounds were studied in aerobic aqueous and soil-slurry systems, and in anaerobic soil-column and soil-slurry reactors. In the research protocol, experiments were conducted in the batch, fed-batch or continuously perfused soil-column mode. Samples of cultures were taken at discrete time-intervals and analyzed for cell growth, PCB congener concentration and distribution and cosubstrate level.

The Results

The understanding of cosubstrate mediated biotransformation of PCBs has increased as a result of this research. Our results showed that naphthalene could support the biodegradation of PCBs, but it did not do so to the same extent and at the same rate as biphenyl. The two cosubstrates together were also effective, but not as effective as biphenyl by itself. Further research on the use of biphenyl was warranted, especially to try and minimize the amounts required. Through a carefully controlled series of experiments with step-wise increases in biphenyl concentration, our results demonstrated that an optimum biphenyl concentration existed for maximized cell growth and PCB congener degradation, and this was between 300 and 350 ppm. The research on the use of terpenes as alternative cosubstrates was very encouraging. Preliminary results showed that terpenes supported the growth of C. testosteroni to greater extents than biphenyl and increased the biodegradation of the higher chlorinated congeners. Our initial results in aqueous batch reactors were supported by subsequent triplicate experiments, both in the aqueous phase and also in soil-slurry systems. g-Terpinene and terpinen-4-ol were able to support cell growth and PCB biodegradation, with the analytical results showing that higher chlorinated congeners were reduced by only 40% while lower chlorinated congeners were reduced by as much as 60 to 75%, and some of the mono and di-chlorobiphenyls were transformed to undetectable levels. The terpene work was extended to soil-slurry systems, where similar biodegradation patterns were observed. It appeared that the addition of a 10 to 15% soil slurry did not inhibit cell growth or PCB biodegradation. The percentage of PCB lost by biotic means, however, was reduced; this result was anticipated since some of the PCB congeners would be irreversibly adsorbed to the soil. Our work demonstrated that the cometabolic biotrans-formation of PCBs was possible, even for penta- and hexa- chlorinated congeners. Our work further showed that when soil spiked with Aroclor 1242 was treated anaerobically, there was a significant reduction in the higher chlorinated congeners and a concomitant increase in the lower chlorinated congeners. More recent work has focused on mixed cultures of C. testosteroni and R. erythropolis combined with indigenous soil microbes from contaminated soil. The soil has been obtained from Wurtsmith Air Force base, and already contains several contaminants. Preliminary results using the mixed cultures are forthcoming as the last graduate student on the Wrap-Up phase of this project completes his research.

Future Research

Investigation of the cosubstrate mediated biotransformation of various PCB congeners has provided a great deal of insight into the management of cosubstrates for the effective bioremediation of PCB contaminated media. A number of questions still remain unanswered, including questions on the timing of cosubstrate addition, the effect of cell density on biodegradation and the influence of various co-contaminants, including polycyclic aromatic hydrocarbons (PAH), non-chlorinated organics and aliphatic and long chain hydrocarbons.