Phenoxyalkanoate degradation

Chlorinated phenols are used for the impregnation of timber and the production of phenoxyalkano-ate herbicides that are degraded by dioxygenation to chlorophenols. 

Muller TA, SM Byrde, C Werlen, JR van der Meer, H-P Kohler (2004) Genetic analysis of phenoxyalkanoic acid degradation in Sphingomonas herbicidovorans MH. Appl Environ Microbiol 70 6066-6075. 

Phenoxyalkanoates. There has been considerable interest in the persistence of chlorinated phenoxyalkanoates—and particularly of phenoxyacetates and phenoxypropionates, which have been used as herbicides. This has therefore stimulated studies on the degradation of these aryl-alkyl ethers. Considerable effort has been directed to elucidating the subsequent... 

Loos MA (1975) Phenoxyalkanoic acids. In Kemey PC, Kaufman DD (eds) Herbicides chemistry, degradation and mode of action, vol 1. Marcel Dekker New York, pp 1-128 Lovley DR (1993) Dissimilatory metal reduction. Annual Review of Microbiology 47 263-290 Macalady DL, Wolfe NL (1983) New perspectives on the hydrolytic degradation of the organo-phosphorothionate insecticide chloropyrifos. J Agric Food Chem 31 1139-1147 Macalady DL, Wolfe NL (1985) Effects of sediment sorption and abiotic hydrolysis. J Agric Food Chem 33 167-173... 

The results of many studies of the degradation of phenoxyalkano-ates using pure cultures of microorganisms have been reported. 

Loos, M.A. (1975) Phenoxyalkanoic acids. In Herbicides, Chemistry, Degradation and Mode of Action. Vol. I, 2nd Edition, Kearney, P.C., Kaufmann, D.D., Editors, Marcel Dekker, New York. 

Lindane is relatively non-persistent, especially under anaerobic conditions, and although its more highly chlorinated residues may present the same problems as those of polychlorophenols, the less chlorinated residues should follow pathways similar to those established for the microbial degradation of the chlorinated phenoxyalkanoic acid herbicides. Recent evidence (46) indicates that certain microbes can dechlorinate DDT anaerobically, thereby making available intermediates which may undergo further aerobic attack, leading in principle to total degradation. The ultimate fate of the hexachloronorbornene nucleus of cyclodienes is still uncertain and this question continues to attract attention. 

Miiller, M.D. Buser, H.-R., Conversion reactions of various phenoxyalkanoic acid herbicides in soil. 1. Enantiomerization and enantioselective degradation of the chiral 2-phenoxypropionic acid herbicides Environ. Sci. Technol 1997, 31, 1953-1959. 

Effects of Long-Term Phenoxyalkanoic Acid Herbicide Field Applications on the Rate of Microbial Degradation... 

The phenomenon of cross-enhancement, or the ability of soils pretreated with specific herbicides to degrade other structurally related herbicides more rapidly than in untreated control soils, has been known for nearly 40 years. Both enhanced degradation and cross-adaptation have been observed under field conditions for phenoxyalkanoic acid herbicides (3-5). ... 

The objectives of this review are to briefly summarize the various factors influencing the breakdown of the phenoxyalkanoic acid herbicides in soils of North America and Europe, and then to discuss the effects of their repeated field usage on the rate of their microbial degradation. 

Phenoxyalkanoic acids are formulated as esters and amine salts and chemical hydrolytic mechanisms are considered responsible for the rapid conversion of the phenoxyalkanoic esters to the corresponding phenoxy-alkanoic anion in moist soils ( 5). The dimethylamine salt of 2,4-D similarly undergoes chemical conversion in the soil, by dissociation, to the acid anion (5j. However, abiotic processes do not appear to result in any significant degradation of phenoxyalkanoic acids in the soil (5j. 

The effects of concentration on the degradation of 2,4-D in soil have been studied using (14C)labeled herbicide with the release of (14C)carbon dioxide being used as a measure of the rate of breakdown of the 2,4-D. In some of these studies, the loss of 2,4-D was considered to be.biphasic with a slow initial evolution of (14C)carbon dioxide beipg followed by a more rapid release (12-14). In contrast, other studies have shown the evolution of (14C)carbon dioxide from (14C)2,4-D treated systems to be uniform with time (13, 15, 16). A similar phenomenon has been reported for the breakdown of (14C)MCPA in soil (17). In all the above experiments no attempts were made to specifically analyze for (14C)2,4-D or (14CJMCPA actually remaining, and it has been cautioned (2, 5) that evolution of (14C)carbon dioxide is not a true measure of the decomposition rate of (14C)phenoxyalkanoic acids in soils. 

Radioactive studies in which solvent-extractable (14C) was characterized by chromatographic assay have revealed no lag-phase when low concentrations <10 ppm) of (14C)2,4-D and (14C)2,4,5-T were incubated with moist soils (18-21). Degradation of several phenoxyalkanoic acids at soil concentrations of <5 ppm using... 

Since the phenoxyalkanoic acid herbicides are degraded in the soil by biological processes, factors that affect microbial activity will directly affect their breakdown. Soil pH, soil type, soil organic matter, herbicide formulation, and herbicide concentration can all influence the rate of microbial decomposition ( 4, 5). Greater effects are experienced with moisture and temperature, since these factors have a profound influence on microbial activity and thus on herbicide breakdown (4 5). It has been concluded ( 5), that soil temperature above 10°C and moistures above the wilting point are necessary for biological degradation of phenoxyalkanoic acids. 

Field studies with single applications of phenoxyalkanoic acid herbicides have indicated that breakdown is rapid under temperature and moisture conditions that favour microbiological activity (5). Enhanced degradation of these herbicides, under field conditions, was first noted in the late 1940s. The use of plant bioassay procedures, led to the discovery that the persistence of 2,4-D, but not 2,4,5-T, was decreased,by pretreatment of the soil with 2,4-D (26, 27). This enhanced breakdown was later confirmed using (14C)2,4-D and radiochemical analytical techniques (29). The breakdown of the (14C)2,4-D being more rapid in soil from the treated plots, tested 8 months after the last field application, than in soil from plots treated for the first time. 

There is evidence from a relatively few studies to show that repeated treatments of 2,4-D and MCPA result in enhanced degradation rates under field conditions as a result of adaptation of soil microorganisms. Even less is known about the phenomenon of cross-enhancement under field conditions, where previous applications of either 2,4-D and MCPA may result in enhanced breakdown of both phenoxyalkanoic herbicides. 

Their discovery can be traced to the classical investigations of Knoop (1904). By feeding phenoxyalkanoic acids to dogs, he established that the degradation of fatty acids in the animal organism begins with oxidation at the carbon atom in the -position with respect to the carboxy radical, and terminates with decomposition... 

Most organic herbicides used to control aquatic weeds are resistant to chemical degradation. While herbicides such as the phenoxyalkanoic acids are readily converted to salts, and the ester derivatives are easily hydrolyzed to acids, the basic herbicide molecules are stable. Dichlobenil is practically immune to degradation by refluxing in concentrated acid and base. Similarly, diquat and l,l -dimethyl-4,4 -bipyridinium ion (para-... 

Differential metabolism appears to be an important basis of selectivity also in the phenoxyalkanoic acids. 0-Dealkylation leading to detoxification of 2,4-D by side-chain removal, as indicated by release of glyoxylic acid from side-chain-labeled 2,4-D, has been implicated as being important to selectivity in certain plant species. In the case of the phenoxyacetic acids, however, side-chain removal might also be achieved by O-demethylation of the dichloroanisole resulting from an initial decarboxylation. In many plants such as susceptible black currant Ribes nigrum L.), these pathways occur to only a minor extent, but they constitute a major degradative mechanism in red currant Ribes sativum Syme), where they confer tolerance." ... 

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