Herbicide degradation

It is a general observation that herbicide degradation occurs more readily ia cultivated than fallow sod, suggesting that rhizosphere organisms are effective herbicide degraders. Whether this can be effectively exploited ia a phytoremediatioa strategy remains to be seea. 

Mortensen SKL, CS Jacobsen (2004) Influence of frozen storage on herbicide degradation capacity in surface and subsurface sandy soils. Environ Sci Technol 38 6625-6632. 

The current methodology to determine residues of alachlor, acetochlor, propachlor, and butachlor in crops and animal products was developed over the last two decades by researchers at the Monsanto Company. These herbicides degrade rapidly in plants and animals to numerous metabolites that can be hydrolyzed to common aniline moieties. Little to no parent herbicide is found as intact residue in crops and animal products therefore, the residue methodology focuses on the determination of the common moieties that are derived from the parent herbicides and their metabolites. Initially, gas chromatography (GC) with flame ionization detection, nitrogen-phosphorus... 

J.A. Shoemaker, Analytical method development for alachlor ESA and other acetanilide herbicide degradation products, Presented at the 49th ASMS Conference, Chicago, IL, May 27-31, 2001. 

Measurement of Chloroacetanilide and Chloroacetamide Herbicide Degradates in Drinking Water by SPE and... 

Soil. Smith (1974) studied the degradation of C-ring- and C-carboxyl-labeled dicamba in moist prairie soils at 25 °C. After 4 wk, >50% of the herbicide degraded to the principal products... 

Lopez A, Mascolo G, Tiravanti G, Passino R (1998) Formation of herbicide degradation byproducts during groundwater disinfection an LC-MS investigation. J Anal Chem 53 856-860... 

All herbicides degrade in soil, but at variable rates (Dawson et al, 1968 Rouchard et al, 2000). The rates of breakdown or deactivation of herbicides are related to a number of soil and environmental factors (Upchurch and Mason, 1962 Upchurch et al, 1966). Surface-applied herbicides volatilize at varying rates, dependent on their vapor pressure (Kearney et al, 1964). Some surface-applied herbicides also break down from ultraviolet light. 

The quantum yield (9) is a measure of the efficiency of the photochemical excitation process, which may result in herbicide degradation and indicates the number of herbicide molecules degraded per photon absorbed. A value of 0 indicates that no chemical reaction occurred, while a value of 1 indicates that all molecules excited due to photon absorption were converted to products. Chain reactions, which can lead to quantum values greater than unity, are unlikely at the very low concentrations found in the aquatic environment. 

Kolpin, D.W., S.J. Kalkhoff, D.A. Goolsby, D.A. Sneck-Fahrer, and E.M. Thurman (1997a). Occurrence of selected herbicides and herbicide degradation products in Iowa s groundwater, 1995. Groundwater, 35 679-688. 

Xiong F, Graham NJD. Rate constants for herbicide degradation by ozone. Ozone Sci Eng 1992 14 283-301. 

Another common laboratory reaction of amines is diazotization to provide unstable and highly reactive diazonium salts. Plimmer et al (14) have isolated an aromatic triazene (XV) from soil containing 3,4-dichloroaniline (XIV) and presented evidence that it is formed by "natural diazotization of the aniline followed by coupling with a second amine molecule (Fig. 5). If this is true— that the natural nitrite commonly found in soil and water can bring about diazotization—a new dimension must be added to both the natural mechanisms of herbicide degradation and the generation of new series of potentially dangerous transformation products. 

Hegg, E. L., Whiting, A. K., Saari, R. E., McCracken, J., Hausinger, R. P, and Que, L. J. (1999). Herbicide-degrading ci-keto acid-dependent enzyme TfdA Metal coordination environment and mechanistic insights. Biochemistry 38, 16714-16726. 

There is evidence from a relatively few field studies to indicate that repeated treatments of 2,4-D and MCPA result in enhanced herbicide degradation rates as a result of adaptation by soil microorganisms. There is also some evidence that cross-enhancement can occur under field conditions, whereby previous applications of either 2,4-D or MCPA, result in enhanced breakdown of both herbicides. 

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