Sunlight degradation, microbial

Section 3 Developing Formulations of Microbial Pesticides to Resist Sunlight Degradation... 

Pesticides vary widely in their chemical and physical characteristics and it is their solubility, mobility and rate of degradation which govern their potential to contaminate Controlled Waters. This, however, is not easy to predict under differing environmental conditions. Many modern pesticides are known to break down quickly in sunlight or in soil, but are more likely to persist if they reach groundwater because of reduced microbial activity, absence of light, and lower temperatures in the sub-surface zone. 

Another source of chlordecone release to water may result from the application of mirex containing chlordecone as a contaminant and by the degradation of mirex which was used extensively in several southern states. Carlson et al. (1976) reported that dechlorinated products including chlordecone were formed when mirex bait, or mirex deposited on soil after leaching from the bait, was exposed to sunlight, other forms of weathering, and microbial degradation over a period of 12 years. Chlordecone residues in the soil could find their way to surface waters via runoff. 

In summary, photodecomposition of methoprene is facile and leads to a multiplicity of products. The lower photostability in sunlight of methoprene compared to epifenonane has been mentioned previously. Because of its photochemical lability and its ready microbial degradation (see below), methoprene is microencapsulated for aquatic use as a mosquito larvicide. 

FIGURE 5 Photolytic degradation of sterile aquatic plant leachate (Juncus effusus, 0.2 pm pore size filtrate after 20 weeks of microbial decomposition) to C02 under replicated, aseptic conditions exposed to full sunlight (UV-B + UV-A + PAR), to UV-A + PAR only, and to PAR only (upper). A severe rainstorm occurred during the incubations, which reduced light severely for an hour (middle). The net change in C02 production per amount of light received per interval under these conditions (lower). 

Carbofuran is soluble in water and moderately persistent in soil (half-life 30-120 days). Carbofuran is degraded by chemical, photochemical, and microbial processes. Hydrolysis is more rapid in alkaline conditions. Carbofuran breaks down in sunlight. Carbofuran has a high potential for leaching into groundwater. Carbofuran is mobile in sandy loam, silty clay, and silty loam soils. In surface water, carbofuran is subject to hydrolysis, particularly under alkaline conditions. Hydrolysis of carbofuran (half-lives) in water is 690, 8, and 1 weeks at pH values of 6, 7, and 8, respectively. As in soils, photodegradation and microbial transformation may also contribute to degradation. Carbofuran is not volatile and does not adsorb to sediment or particles. 

The persistence of fenthion in the environment is dependent on several factors, including photolysis, metabolism in plants and insects, and microbial degradation. Estimates of the half-life of fenthion in soil vary from < 1 day in studies cited by the US EPA for aerobic soil metabolism to 3-6 weeks, cited by Extoxnet. Half-lives for aquatic degradation range from 2.9 to 21.1 days for various ocean, river, swamp, or lake aquatic conditions. Sunlight accelerates degradation of fenthion 20-fold in river water and fivefold in seawater. 

Methoprene degrades rapidly in sunlight, both in water and on inert surfaces. It is metabolized rapidly in soil under both aerobic and anaerobic conditions (half-life =10-14 days). The major microbial degradation product is carbon dioxide. Degradation in both freshwater and saltwater is also quite rapid with a half-life of 10-35 days at 20°C. Methoprene is not very soluble in water (< 2 ppm) and as a result is not... 

All environmental studies show the formation of the same key products of PBO degradation. It is clear that PBO degradation is facilitated by sunlight, moisture and soil microbial activity, Under strictly anaerobic conditions, with the total exclusion of oxygen, PBO degradation is negligible. Under these conditions, such as arc found in deep soil and sediment layers, PBO would be only slowly degraded. 

In the ambient atmosphere, NDMA should be rapidly degraded upon exposure to sunlight. The half-life for direct photolysis of NDMA vapor is on the order of 5 to 30 minutes. In surface water exposed to sunlight, NDMA would also be subject to photolysis. On soil surfaces, NDMA would be subject to removal by photolysis and volatilization. The volatilization half-life of NDMA from soil surfaces under field conditions has been found hours. In subsurface soil and in water beyond the penetration NDMA would be susceptible to slow microbial decomposition under both aerobic and anaerobic conditions. In aerobic subsurface soil, the half-life of NDMA has been found to be about 50 to 55 days. Degradation has been found to proceed slightly faster under aerobic conditions than under anaerobic conditions. 

Limited available data suggest that NDMA would be subject to slow photolysis in natural waters exposed to sunlight (Polo and Chow 1976 Callahan et al. 1979). In unlit waters, it appears that NDMA would be rather persistent, eventually degrading as the result of microbial transformation (Kaplan and Kaplan 1985, Kobayashi and Tchan 1978, Tate and Alexander 1975). There is evidence which suggests that formaldehyde and methylamine may form as biodegradation products of NDMA (Kaplan and Kaplan 1985). Insufficient data are available to predict the rate at which NDMA would degrade in water. NDMA is not expected to chemically react under the conditions found in natural waters (Callahan et al. 1979, O.liver et al. 1979). ... 

Hydrolyzed by strong acids and alkalis, and in aqueous solutions in sunlight. Thermally stable up to 200°C. In soil, microbial degradation involves hydrolysis to ethylmercaptan, carbon dioxide and di-isobutylamine. ti/2 (soil) 1.5 to 10 weeks Unstable in highly alkaline media... 

Photodegradable polymers undergo degradation from the action of sunlight. In many cases, polymers are attacked photochemically and broken down to small pieces. Further microbial degradation must then occur for true biodegradation to be achieved [Ashwin, 2011]. 

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