Soil, reductive degradations

The fate of the dinitroaniline herbicides in soil is extremely complex and many metabolites have been identified. Golab and Althaus reported 28 metabolites identified in a degradation study of trifluralin in soil. Major degradation products of dinitroaniline herbicides were formed by nitro reduction, A-dealkylation (mono-dealkylated and completely dealkylated) and the ring formation of benzimidazole. 

In laboratory tests, several fungi and cultures of actinomycetes isolated from garden soil readily degraded disulfoton (Bhaskaran et al. 1973). In flooded soil under anaerobic conditions, the reduction of disulfoton sulfoxide to disulfoton was due to biological conversion (Tomizawa 1975). 

Table 8.11 Reduction in compound availability for soil microbial degradation as a result of ageing. Reprinted with permission from Alexander M (2000) Aging, bioavaUability and overestimation of risk from environmental pollutants. Environ Sci Technol 34 4259 265. Copyright 2005 American Chemical Society...

Reductive degradations of soil and coal-derived humic acids with sodium amalgam have been reported to produce a variety of phenolic compounds (12, 13, 14) however, this procedure has been questioned by... 

Several mechanisms are involved in degradation, among them, dealkylation, reduction, oxidation, hydrolysis and combinations of these. Degradation products do not accumulate in the soil, and none of the degradation products are to be found in quantities greater than 4% of the total herbicide initially applied. Nonextract-able, so-called soil-bound degradation products located in the fulvic acid and humic acid fractions of the soil are formed in substantial quantities. 

Information about the biochemical precursors of volatile sulfur compounds has been obtained from artificial culture stiidies, reviewed by Kadota and Ishida (1972), and from incubation studies of natural and amended soils, reviewed by Bremner and Steele (1978). Table 10-5 summarizes our knowledge about the origin of sulfides, which have been observed to emanate from soils as degradation products under aerobic as well as anaerobic conditions. As indicated above, H2S is formed not only by the degradation of organic compounds, but also by anaerobic reduction of sulfate. 

The stmctures of HA and FA are not yet determined. A combination of many techniques is required to determine the structure of HSs. Schulten et al. have employed p)>Tolysis-gas chromatography with electron impact and field ionisation mass spectrometry (Py-GC/MS), in-source pyrolysis-field ionisation mass spectrometry (Py-FIMS), CP/MS NMR, oxidative and reductive degradation, colloid chemical methods, and electron microscopy to develop a carbon network structure for soil HS (Schulten (1994), Schulten and Schnitzer (1993)). The elemental composition of FLA was C3()8H32s09oN5 fot MW of 5540 Da. This indicates of the complexity of such compounds and the extensive techniques required. If carbohydrates or proteinaceous materials are bonded covalently with HA, %C content decreases and %0 content increases. 

Resorcinol (CsH Oa) can be produced by the sodium-amalgam reductive degradation of soil hmnic acid. It became a fixms of research when it was discovered to be one of the strongest precursors of trihalomethane (THM) in water. This is its ability to become carcinogenic and mutagenic m the presence of chlorine in water (25,28,41). 

It has already been mentioned that the degradation of s-triazine herbicides such as atrazinc in soil can be described by two reaction types only, hydrolysis and reductive dealkylation (see Figure 10.3-8). Application oF these two reaction types to a specific s-triazinc compound such as atrazinc provides the reaction network shown in Figure 10,3-12. This can also be vcriFicd by running this example on h ttp //www2,chemie,uni-erlangen.de/semces/eros/,... 

A commercial technology (69), the SABRE process, treats contaminated water and soil ia a two-stage process by adding a readily degradable carbon and an inoculum of anaerobic bacteria able to degrade the contaminant. An initial aerobic fermentation removes oxygen so that the subsequent reduction of the contaminant is not accompanied by oxidative polymerization. 

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