Primary biodegradation soils

Since lignins are polymers of phenolics and are major plant constituents with resistance to microbial decomposition, they are the primary source of phenolic units for humic acid synthesis (178, 179). Once transformed, these humic acids become further resistant to microbial attack and can become bound to soils (180) form interactions with other high molecular weight phenolic compounds (ex. lignins, fulvic acids) and with clays (181) and influence the biodegradation of other organic substrates in soils (182, 183). 

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). 

If sorption and partitioning mechanisms dominate the fate of PAHs in soils, then the PAHs remaining in SOM should be primarily parent compounds which are sorbed to organic surfaces. Slow rates of desorption become the primary limitation for biodegradation however, the presence of adapted PAH-minerali-zing communities in contaminated soils suggests that PAH desorption occurs at sufficient rates over time to establish and maintain adapted microbial communities [36,264,356]. PAH biodegradation appears to proceed, albeit at much slower rates than predicted or desired [264,278,279]. 

Aerobic degradation of diethyl phthalate by acclimated soil and activated sewage sludge microbes was studied using an acclimated shake flask CO2 evolution test. After 28 d, loss of diethyl phthalate (primary degradation) was >99%, with a lag phase of 2.3 d, and ultimate biodegradation (CO2 evolution) was 95%. The half-life was 2.21 d (Sugatt et al., 1984). 

Biological. 1-Naphthylamine added to three different soils was incubated in the dark at 23 °C under a carbon dioxide-free atmosphere. After 308 d, 16.6 to 30.7% of the 1-naphthylamine added to soil biodegraded to carbon dioxide (Graved et al., 1986). Li and Lee (1999) investigated the reaction of 10 mL of 7 mM 1-naphthylamine with 4 g of a Chalmers soil (pH 6.5, 11.1% sand, 72.8% silt, 16.0% clay). After 120 h, the soil was washed with acetonitrile and the extractant analyzed using GC/MS. The primary transformation product was a dimer tentatively identified as TV-(4-aminonaphthyl)-1-naphthylamine. The investigators hypothesized that the formation of this compound and two other unidentified dimers was catalyzed by minerals present in the soil. 

Microbial degradation in soils is greatest for the aromatic fractions of fuel oils, while the biodegradation of the aliphatic hydrocarbons decreases with increasing carbon chain length. Evaporation is the primary fate process for these aliphatics (Air Force 1989). 

In ambient air, the primary removal mechanism for acrolein is predicted to be reaction with photochemically generated hydroxyl radicals (half-life 15-20 hours). Products of this reaction include carbon monoxide, formaldehyde, and glycolaldehyde. In the presence of nitrogen oxides, peroxynitrate and nitric acid are also formed. Small amounts of acrolein may also be removed from the atmosphere in precipitation. Insufficient data are available to predict the fate of acrolein in indoor air. In water, small amounts of acrolein may be removed by volatilization (half-life 23 hours from a model river 1 m deep), aerobic biodegradation, or reversible hydration to 0-hydroxypropionaldehyde, which subsequently biodegrades. Half-lives less than 1-3 days for small amounts of acrolein in surface water have been observed. When highly concentrated amounts of acrolein are released or spilled into water, this compound may polymerize by oxidation or hydration processes. In soil, acrolein is expected to be subject to the same removal processes as in water. 

The primary mechanisms of degradation of chemicals in soil, water, sediment, air, and biota environments are classified as biotic (biodegradation, phytodegradation, and respiration) or abiotic (hydrolysis, photolysis, and oxidation/reduction), as shown in Figure 6.7. Biodegradation, the transformation of chemicals by microorganisms, has potential to occur in any environmental compartment that... 

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