EarthFax Engineering, Inc. and its affiliate EarthFax Development Corp. have demonstrated the degradation, in soil, of polychlorinated dibenzo-p-dioxins ("PCDDs") and polychlorinated dibenzo furans ("PCDFs") under pilot-scale conditions through the use of white-rot fungi ("WRF"). The site where this work was demonstrated was a former wood treatment facility located at a hardwood sawmill in North Carolina. Soil at the site had become contaminated with dioxins and furans, in addition to pentachlorophenol, lindane, and other pesticides.
Previous investigations at the site had identified the general extent and magnitude of contamination from the past wood-treatment activities. Based on these investigations, soil was excavated from the area of greatest contamination for use in the pilot-scale test.
The pilot-scale test consisted of four treatment cells, each with a capacity of approximately two cubic yards (4'Wx4'Lx4'D). Each cell was constructed with plywood walls, set on a concrete pad, and lined with 6-mil polyethylene sheeting.
Dioxin degradation was evaluated in two of the cells. Cell A consisted of contaminated soil mixed with 20% (w/w) fungal inoculum (i.e., sawdust thoroughly colonized with the WRF Pleurotus ostreatus). Cell B consisted of contaminated soil mixed with 40% (w/w) fungal inoculum.
Each cell was loaded with the soil/inoculum mixture, with perforated piping installed on the bottom and top of the mix. The cells were covered with 6-mil polyethylene sheeting and a regenerative air blower was attached to the piping in a manner that allowed air to be injected into the bottom and withdrawn from the top of each cell.
Soil/inoculum samples from Cells A and B were collected at the time of cell construction (i.e., post-inoculation) and 282 days later. The initial samples were analyzed in accordance with EPA Method 8290, with the final samples being analyzed in accordance with Method 8280.
The percent degradation values of the dioxins were approximately 61 to 69% for penta-CDDs, 61 to >87% for hexa-CDDs, 63 to 70% for hepta-CDDs, and 72 to 80% for the octa-CDDs. No tetra-CDDs were detected in the initial or final samples.
Percent degradation of the furans was greater than 80% for tetra-CDFs, 51 to 63% for penta-CDFs, 53 to 73% for hexa-CDFs, 67 to 68% for hepta-CDFs, and 76 to 79% for octa-CDFs. The observed degradation of PCDDs and PCDFs confirms the laboratory results of Takada et al. (1996), who demonstrated the ability of a WRF to substantially degrade highly chlorinated PCDDs and PCDFs in aqueous culture, and extends those results to degradation of these compounds in soil under field conditions.
As a measure of total contaminant degradation, the initial and final PCDD and PCDF values were converted to total equivalent concentrations ("TEQs") of 2,3,7,8-tetra-CDD using the toxicity equivalency factors which were developed by the U.S. Environmental Protection Agency (1989). The resulting TEQ degradation was 69% in Cell A and 63% in Cell B. These results are presented graphically in Figure 1.