Soil degradation anaerobic sediments

Under anaerobic conditions, mirex was slowly dechlorinated to the 10-monohydro derivative by incubation with sewage sludge bacteria for two months (Andrade and Wheeler 1974 Andrade et al. 1975 Williams 1977). The primary removal mechanism for mirex was anaerobic degradation as demonstrated by the 6-month stability of the compound in nine aerobic soils and lake sediments (Jones and Hodges 1974). 

This is an ex situ anaerobic bioremediation technology for metal-contaminated soils, sludges, and sediments. While metals are the primary pollutant treated, the biological system also degrades and removes organics such as hydrocarbons. 

Taken collectively, the data clearly indicated differential vaporization as the primary mode of toxaphene loss from leaf surfaces and gave no indication that chemical reactivity played even a minor role. If toxaphene had been degraded either on surfaces or during its brief residence time in the air prior to sampling, changes in the chromatographic profile would have been erratic with new peaks observed in the capillary chromatograms such as occur in samples of anaerobic soil and ditch sediment where microbial decomposition is extensive. 

Al ough alachlor is no longer used in the U.S., the three chemical compounds have very similar structural (Figure 1) and chemical properties. Alachlor degradataion data may be useful as a model for this chemical class. Caution must be used in interpolating these data however since the ESA metabolite of metolachlor is formed more slowly and at lower concentrations in soil (18). The objective of this study was to compare atrazine and alachlor sorption, mineralization, and degradation potential, processes that are major contributors to the environmental fate of pesticides, from surface soil to aquifer sediments in laboratoiy studies. In addition, ctegradation of alachlor was compared under aerobic and anaerobic conditions. 

Aerobic degradation of hydrocarbons requires access to electron acceptors, generally oxygen in natnral sitnations, added hydrogen peroxide in terrestrial systems, or nitrate or snlfate nnder anaerobic conditions that prevail at deeper levels of the soil or sediment. 

Molecular hydrogen is an important intermediate in the degradation of organic matter by microorganisms in anoxic habitats such as freshwater and marine sediments, wet land soils, and the gastrointestinal tract of animals. In these particular conditions H2 is produced during fermentation of carbohydrates, lipids, nucleic acids, and proteins by anaerobic bacteria and,... 

The major part of the biosphere is aerobic and consequently priority has been given to the study and assessment of biodegradability under aerobic conditions. Nevertheless, there are environmental compartments that can be permanently (e.g. anaerobic digesters) or temporarily anaerobic (e.g. river sediments and soils) and surfactants do reach these. The majority of surfactants entering the environment is exposed to and degraded under aerobic conditions. This is the predominant mechanism of removal even in cases of absence of wastewater treatment practices (direct discharge) and it is estimated that less than 20% of the total surfactant mass will potentially reach anaerobic environmental compartments [1]. Only in a few cases, however, will the presence of surfactants in these compartments be permanent. The presence of surfactants in anaerobic zones is not exclusively due to the lack of anaerobic degradation. Physico-chemical factors such as adsorption or precipitation play an important role as well as the poor bioavailability of surfactant derivatives (chemical speciation) in these situations. 

Mirex is a very persistent compound in the environment and is highly resistant to both chemical and biological degradation. The primary process for the degradation of mirex is photolysis in water or on soil surfaces photomirex is the major transformation product of photolysis. In soil or sediments, anaerobic biodegradation is also a major removal mechanism whereby mirex is slowly dechlorinated to the 10-monohydro derivative. Aerobic biodegradation on soil is a very slow and minor degradation process. Twelve years after the application of mirex to soil, 50% of the mirex and mirex-related compounds remained on the soil. Between 65--73% of the residues recovered were mirex and 3-6% were chlordecone, a transformation product (Carlson et al. 1976). 

Chlordecone is similar to mirex in structure and is also highly persistent in soils and sediments (halflife expected to be analogous to 10 years duration for mirex) because of its resistance to biodegradation, although some microbial metabolism of chlordecone has been reported (Lai and Saxena 1982 Ordorff and Colwell 1980). No evidence of microbial degradation was detected for chlordecone exposed to hydrosoils from a reservoir (not previously contaminated with chlordecone) and from Bailey Creek (contaminated with chlordecone) under either anaerobic or aerobic conditions for 56 days (Huckins et al. 1982). 

When lindane was incubated in aerobic and anaerobic soil suspensions for 3 wk, 0 and 63.8% was lost, respectively (MacRae et al., 1984). Using settled domestic wastewater inoculum, lindane (5 and 10 mg/L) did not degrade after 28 d of incubation at 25 °C (Tabak et al., 1981). When lindane was incubated in river water samples and sediments for 3 wk, 80% of the applied amount had degraded. Under sterilized conditions, >95% was recovered after 12 wk. Under unsterile and sterile conditions, 20 and 80% of the recovered lindane was bound to sediments (Oloffs et al., 1973 Oloffs and Albright, 1974). 

Biological. Microbial degradation of trichloroethylene via sequential dehalogenation produced cis- and /ra/3s-l,2-dichloroethylene and vinyl chloride (Smith and Dragun, 1984). Anoxic microcosms in sediment and water degraded trichloroethylene to 1,2-dichloroethylene and then to vinyl chloride (Barrio-Lage et al., 1986). Trichloroethylene in soil samples collected from Des Moines, lA anaerobically degraded to 1,2-dichloroethylene. The production of 1,1-dichloroethylene was not observed in this study (Kleopfer et al., 1985). 

Environmental Fate. It is not known if 3,3 -dichlorobenzidine, like benzidine, is oxidized by clay minerals or if cations in water ean have the same oxidizing effect. 3,3 -Dichlorobenzidine does not appear to biodegrade easily, but the few studies in this area did not state the type(s) or concentrations of mieroorganisms used in eaeh study. More systematic studies with other organisms may prove useful. A reeent study (Nyman et al. 1997) provides evidence that in the span of a year up to 80% of 3,3 -dichloro-benzidine can degrade to benzidine in anaerobic mixtures of sediment/water. Further research to identify the pathways and produets of deeomposition of 3,3 -dichlorobenzidine in various soils is needed. The toxieologieal profile for benzidine eontains information on the environmental fate of that compound (ATSDR 1995). 

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