Pentachlorophenol transformation in soil: a toxicological assessment

Abstract

Bioremediation is recognised as an economically viable method to treat contaminated soil and the use of microbial inoculants, in particular, white-rot fungi, has been proposed to enhance the remediation process. During bioremediation, a variety of pollutant transformation products will be created which may have toxic synergistic interactions and may not at all be detected by chemical analysis. This work examined the potential for formation of toxic breakdown products during pentachlorophenol (PCP) transformation in soil after inoculation with Phanerochaete chrysosporium; a frequently used fungal inoculant. To monitor toxicity during bioremediation, changes in soil dehydrogenase activity and effects of soil methanol extracts on the growth of a common soil bacterium Bacillus megaterium in liquid culture were determined. After 6 weeks of remediation, soil PCP levels had dropped from an initial 250 mg kg −1 to 2 mg kg −1, and inoculation with P. chrysoporium did not improve PCP remediation over uninoculated PCP-contaminated soil. Soil dehydrogenase activity remained very low in all soils containing PCP and did not recover throughout the experiment (6 weeks) despite the decrease in PCP levels. Soil methanol extracts varied in their toxicity towards growth of B. megaterium and were most toxic after 6 weeks incubation when extracts obtained from PCP-contaminated soil inoculated with P. chrysosporium completely inhibited B. megaterium. A longer incubation time would probably result in removal of toxic products and as soil methanol extracts were used in growth inhibition studies, the bioavailability of these toxic compounds remains in question. However, this work indicates that toxic transformation products may be formed during bioremediation and that ecotoxicological assays are useful to complement chemical analysis during bioremediation of contaminated soil.