Manipulating Soil Populations For Bioremediation Of Xenobiotics

Bioremediation manages microorganisms to reduce, eliminate, contain, or transform contaminants present in soils, sediments, water, or air. Various forms of bioremediation have been documented throughout recorded history. The composting of agricultural residues and sewage treatment are based on the use of microorganisms to catalyze chemical transformations. Composting dates back to 6000 BCE, with the modern use of bioremediation beginning over 100 years ago with the design and operation of the first biological sewage treatment plant in Sussex, England, in 1891. Over the past several decades, in situ degradation of biologically foreign chemical compounds (solvents, explosives, polycyclic aromatic hydrocarbons, heavy metals, radionuclides, etc.) has been used as a cost-effective alternative to incineration or burial in landfills (Alexander, 1994). An advantage of bioremediation over other methods is that it transforms contaminants instead of simply moving them from one source to another as in the practice of land filling (Table 17.7). Also it is relatively low cost compared to other methods of removal. In a bioremediation process, microorganisms break down contaminants to obtain chemical energy. It involves the manipulation of microorganisms and their metabolic processes (enzymes) to degrade compounds of concern. Figure 17.3 shows the microbial degradation pathway of a pesticide, DDT (Bumpus and Aust, 1987).

Phanerochaete chrysosporium (and several other species) uses a peroxidase enzyme system that acts in concert with H2O2, produced by the fungus, to degrade many recalcitrant organics, especially those with structures similar to lignin, which naturally degrades in soil systems. Degradable contaminants include DDT, lindane, chlordane, TNT, and PCBs. Bacteria and fungi have been shown to break down practically all hydrocarbon contamination in the natural environment.

Through studying natural processes, researchers have been able to determine what conditions are necessary for degradation and what organisms are active degraders of specific pollutants and manipulate them. Examples of aerobic bacteria managed for their degradative abilities include Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium. These bacteria have been reported to degrade pesticides and hydrocarbons, both alkane and polyaromatic compounds (PAHs). Many of these organisms are capable of using the contaminant as the sole source of C and energy. In the absence of O2, anaerobic bacteria have been used in the remediation of polychlorinated biphenyls (PCBs) and dechlori-nation of trichloroethylene (TCE) and chloroform. Ligninolytic fungi, such as P. chrysosporium, have the ability to degrade an extremely diverse group of persistent PAHs. Methylotrophs use the methane monooxygenase pathway to degrade a wide range of compounds including the chlorinated aliphatics, trichloroethylene,

496 Chapter 17 Management of Organisms and Their Processes in Soils TABLE 17.7 Description of Common Bioremediation Approaches

In situ bioremediation of soil The goal of aerobic in situ bioremediation is to supply oxygen and nutrients to the microorganisms in the soil and does not require excavation or removal of contaminated soils. In situ techniques can vary in the way they supply oxygen to the organisms that degrade the contaminants. Two such methods are bioventing and injection of hydrogen peroxide. Remediation can take years to reach cleanup goals.

Bioventing Bioventing systems deliver air from the atmosphere into the soil above the water table through injection wells where contamination is located. Nutrients, nitrogen, and phosphorus may be added to increase the growth rate of the microorganisms.

Injection of hydrogen peroxide This process delivers oxygen by circulating hydrogen peroxide through contaminated soils to stimulate the activity of indigenous microbial populations.

Ex situ bioremediation of soil Ex situ techniques require excavation and treatment of the contaminated soil. Ex situ techniques include slurry- and solid-phase techniques.

Liquid slurry phase Contaminated soil is combined with water and other additives in a bioreactor and mixed to keep indigenous microorganisms in contact with the contaminants. Solid-phase bioremediation treats soils in aboveground treatment areas equipped with collection systems to prevent contaminants from escaping.

Contaminated soils are excavated and spread on a pad with a system that collects leachates or contaminated liquids that seep out of the contaminated soil. The soil is periodically turned to mix air or provide nutrients.

Contaminated soil is piled to several meters over an air distribution system. Moisture and nutrient levels are incorporated to maximize microbial activity. Biodegradable contaminates are mixed with straw, hay, or other C-rich compounds to facilitate optimum levels of air and water to microbial populations. The three commonly used designs are static pile composting, mechanically agitated in-vessel composting, and windrow composting.

Solid phase

Landfarming

Soil biopiles

Composting and 1,2-dichloroethane. Geobacter sulfurreducens (Fig. 17.4) grows with insoluble Mn(IV) oxides as the electron acceptor.

Several factors influence the success of bioremediation and should be considered on a site-by-site basis. Factors include the existence of a microbial population capable of degrading the contaminant, availability of contaminants in the microbial populations, the type of contaminant, its concentration, and environmental factors such as soil type, temperature, pH, the presence of oxygen, or other electron

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