The utilization of fungal biodegradation involves the controlled usage of these specially cultivated fungi to treat contaminants.
White Rot Fungus
White rot fungus has been reported to degrade a wide variety of organopollutants because of its lignin-degrading or wood-rotting enzymes. Two different treatment configurations have been tested for white rot fungus, in situ and bioreactor. An aerobic system using moisturized air on wood chips is used in a reactor for biodegradation. A reactor was used in the bench-scale trial of the process. In the pilot-scale project, an adjustable shredder was used for making chips for the open system. The open system is similar to composting, with wood chips on a liner or hard contained surface that is covered. Temperature is not controlled in this type of system. The optimum temperature for biodegradation with lignin-degrading fungus ranges from 30 to 38° C (86 to 100° F). The heat of the biodegradation reaction will help to maintain the temperature of the process near the optimum.
Although white rot fungus degradation of TNT has been reported in laboratory-scale settings using pure cultures, several factors increase the difficulty of using this technology for full-scale remediation. These factors include competition from native bacterial populations, toxicity inhibition, chemical sorption, and the inability to meet risk-based cleanup levels. White rot works best in nitrogen-limited environments.
In bench-scale studies of mixed fungal and bacterial systems, most of the reported degradation of TNT is attributable to native bacterial populations. High TNT or PCP concentrations in soil also can inhibit growth of white rot fungus. A study suggested that one particular species of white rot fungus was incapable of growing in soils contaminated with 20 ppm or more of TNT. In addition, some reports indicate that TNT losses reported in white rot fungus studies can be attributed to adsorption onto the fungus and soil amendments, such as corn cobs and straw, rather than actual destruction of TNT. Another study tested a variety of white rot fungus for PCP sensitivity. Eighteen species tested for PCP sensitivity were inhibited by 10 mg per liter of PCP when grown on agar plates. Within 2 weeks, 17 of the 18 species grew in the inhibition zones. In liquid-phase toxicity experiments, all 18 species were killed by 5 mg per liter of PCP.
Typical Fungal Biodegradation Process
White rot fungus has the ability to degrade and mineralize a number of organopollutants including the predominant conventional explosives TNT, RDX, and HMX. In addition, white rot fungus has the potential to degrade and mineralize other recalcitrant materials, such as DDT, PAH, PCB, and PCP2-4.
The following factors may limit the applicability and effectiveness of the process:
High TNT concentrations in the soil, sediment, or sludge.
The degradation of contaminants not being sufficient to meet cleanup levels.
Competition from native bacterial populations, toxicity inhibition, and chemical sorption.
Specific data required to evaluate the white rot fungus process include:
Explosives concentration of the contaminated soil, sludge, or sediment.
Final explosive levels required after treatment.
Other contaminants present.
Characterization of soil properties.
This technology has been known for approximately 20 years with very few, if any, commercial applications. A pilot-scale treatability study was conducted using white rot fungus at a former ordnance open burn/open detonation area at Site D, Naval Submarine Base, Bangor, Washington. Initial TNT concentrations of 1,844 ppm were degraded to 1,267 ppm in 30 days and to 1,087 ppm in 120 days. The overall degradation was 41%, and final TNT soil levels were well above the proposed cleanup level of 30 ppm. Additional studies to evaluate the effectiveness of white rot fungus on explosives-contaminated soil are being conducted.
White rot fungus is not native to soil, and some forms of bacteria may become predominant over the growth of fungi. In addition, little is known of the ability of the white rot fungus to compete with other forms of fungi. Many of the preliminary laboratory studies cited use sterile conditions, which allow the white rot fungus to grow without the same limitations encountered in field sites.
Experiments indicate that white rot fungus is viable under specific environmental conditions. Duplicating these conditions in actual site testing may optimize the ability of white rot fungus to remediate hazardous compounds. The timeframe and cost effectiveness of duplicating these conditions have never been taken into account. Several factors are widely believed to optimize the viability and potential of white rot fungus. First, secretion of enzymes is included in nutrient-deficient conditions. The optimum concentration of nitrogen is around 2 to 4 mM. Second, atmospheric concentrations of oxygen result in ligninolytic action but not to the same degree as 100% oxygen. The rate of mineralization is two- to three-fold greater under 100% oxygen. A concentration of oxygen below 5% results in no enzymatic action. Third, pH has been determined to be optimal around 4.5. Fourth, the optimal moisture content is between 40 and 45%.
The costs are estimated at $98 per cubic meter ($75 per cubic yard).