Project Abstract

Composting of hPAH Contaminated Soil

Principal Investigators

James H. Johnson, Jr., Devasia V. Karimpanal, and Michael M. Gaines
Howard University
E-mail: jj@scs.howard.edu

Introduction

Recalcitrant organic compounds are such a major threat to the environment that developing environmentally friendly and economically viable technology to cleanup these contaminants is a top priority on today's research agenda. Though existing technologies such as incineration are proven to destroy waste to a great extent, disadvantages of high cost, environmental concerns, and public empathy often are barriers to their utilization. Thus while developing alternative technologies for contaminant cleanup, attention must be given to develop technologies that are low cost and environmentally compatible.

Engineers and scientists at Howard University are reevaluating an age-old process operating in nature to keep the biosphere clean and habitable. This process is called composting. The Howard researchers, under the leadership of James H. Johnson Jr., are trying to engineer the process of composting to enhance the microbial degradation of targeted contaminants in a short period of time. The success of this technology is dependent upon a suitable environment and sufficient time for microbes to degrade even the most recalcitrant compounds. Recent interest in exploring land farming for contaminated soil clean up is also exploiting the inherent capacity of the microbes to breakdown the contaminants into environmentally benign products.

The Research

Composting is a biological degradative process by which complex organic compounds are converted to residuals and carbon dioxide and water. Why then are some organic compounds left out while others are preferentially degraded? A basic biological principle is in play here i. e. spend less to gain much. So microbes go after energy sources (food) that could be broken down by expending the least energy. Each organic compound has its own characteristics and behaves in a unique way. If we can change the attribute of the bacteria or the attribute of the organic compound, any type of compound could be degraded. By changing the attribute of the bacteria, we engineer the bacteria which preferentially go after the target compound that is to be degraded. By changing the attribute of the compound, we make the compound behave like those easily degraded by the microbes. A combination of the above two strategies can lead to successful bioremediation of contaminated soil.

The model compound selected for the research is benzo(a)pyre (BaP), a recalcitrant and carcinogenic polycyclic aromatic hydrocarbon (PAH). Bacteria exposed to contaminants are shown to induce enzymes that degrade those specific compounds. Experiments conducted in liquid media has shown that these bacteria are capable of degrading BaP. To make use of this bacterial quality, bacteria are isolated from PAH contaminated sites and used as inocula in the compost. Initial experiments w ere to evaluate the behavior of BaP. Water solubility is a key for bacteria to use any substance as a food source. Lack of bioavailability of BaP was a major limitation that prevented the transport of BaP to bacterial cell. Thus experiments were modified to overcome this limitation. One approach was to use biological amendments such as composted sludge to enhance bacterial growth. Soil binding of BaP also prevented mass transfer. This work has demonstrated the ability of nonionic synthetic surfactants to mobilize BaP from soil.

The Results

Understanding barriers and limitations of BaP degradation and overcoming these barriers were the focal points of Howard's research in developing composting technology for PAH contaminated soil bioremediation. These studies showed that mass transfer limitations and kinetic limitations prevent degradation of BaP. When necessary steps were taken to overcome mass transfer limitation, BaP degradation was achieved. Composting of soil without amendments did not result in the degradation of BaP. On the other hand, BaP degradation was evident in sludge amended soil. Sludge is a rich source of food for bacteria. Many bacteria in nature are known to produce biological surfactants that in turn enhances the solubility and transport of PAHs. Again, some of the organic compounds also may have acted as cosubstrates thus aiding in the degradation of BaP. Though Tween 80 , a nonionic surfactant at 6000 ppm was used with compost amendment, there was no significant difference in the degradation of BaP in sludge amended and sludge and surfactant amended soil. But in liquid experiments, surfactants at 5000 ppm were found to enhance BaP degradation. Higher concentrations of surfactants may enhance the degradation in soil systems.

Future Research

The research at Howard generated an understanding of the major limiting factors of BaP degradation and devised a workable strategy to overcome the limitations. Both by manipulating bacterial growth conditions and enhancing the bioavailability through surfactant addition, one of the most recalcitrant and problem PAHs is successfully degraded under different laboratory conditions. The next step is to try similar strategies on field contaminated soil and make modifications, if needed, to pilot test the technology before field demonstration.