Biological degradations, dioxin

Dioxins are not decomposed by heat or oxidation in a 700°C incinerator, but pure compounds are largely decomposed at 800°C. Chlorinated dioxins lose chlorine atoms on exposure to sunlight and to some types of gamma radiation, but the basic dioxin structure is largely unaffected. The biological degradation rate of chlorinated dioxins is slow, although measured rates differ widely. 

However several POPs, particularly the OCPs and dioxins, remain at low levels in the Australian environment and several remain persistent at low levels in body fats and fluids of Australians. The levels reflect the past use and persistence of OCPs in the Australian environment, contamination of the food chain and the capacity of the body to metabolise and store in body fats. The dioxins remain due to the ubiquitous nature of their sources with combustion as a major source and their persistence. Future trends are likely to mean very low-level residues in human fats of DDE, cyclodienes, HCB, HCHs and dioxins in the long term. Their rate of decline will probably depend on removing HCB from chlorinated industrial chemicals and OCPs from the environment (e.g. remediation of contaminated soils) by hazardous waste treatment methods (e.g. physical, chemical and biological degradation or fixation) or secure landfill. 

An environmental protocol has been developed to assess the significance of newly discovered hazardous substances that might enter soil, water, and the food chain. Using established laboratory procedures and C-labeled 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), gas chromatography, and mass spectrometry, we determined mobility of TCDD by soil TLC in five soils, rate and amount of plant uptake in oats and soybeans, photodecomposition rate and nature of the products, persistence in two soils at 1,10, and 100 ppm, and metabolism rate in soils. We found that TCDD is immobile in soils, not readily taken up by plants, subject to photodecomposition, persistent in soils, and slowly degraded in soils to polar metabolites. Subsequent studies revealed that the environmental contamination by TCDD is extremely small and not detectable in biological samples. 

Polychlorinated dibenzo-1,4-dioxins are extremely toxic compounds [127] which are difficult to degrade biologically. This especially applies to 2,3,7,8-tetrachlorodibenzo[l,4]dioxin 15 (Seveso dioxin, TCDD) which is teratogenic. TCDD (LD50 = 45 pg kg" in rats) is formed as a byproduct in the commercial synthesis of 2,4,5-trichlorophenol 14. It is an intermediate in the production of the germicide hexachlorophene 16 and the herbicide (2,4,5-trichlorophenoxy)acetic acid 17 ... 

Recent reports have demonstrated that the white rot fungus Phanerochaete chrysosporlum produces a peroxldase-llke lignin degrading enzyme which can attack recalcitrant pesticides such as DDT and pesticide related compounds such as dioxin (2,3,7,8-tetrachlorodlbenzo-p-dloxln) (19). The fungi certainly eidilblt a broad range of degradatlve capabilities which deserve attention In future research efforts on biological elimination of pesticide wastes. 

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