Herbicides dissipation

Several processes may play a role in the environmental dissipation of -triazine herbicides. Dissipation processes can include microbial or chemical degradation in soil metabolism or conjugation in plants photodegradation in air, water, and on soil and plant surfaces and volatilization and transport mechanisms. This chapter will address photolytic degradation and abiotic hydrolysis of the currently used triazine herbicides, the triazinone herbicides (metribuzin and metamitron), and the triazinedione herbicide hexazinone. 

Nearly all thin film photolysis studies involving triazine herbicides have utilized model surfaces such as filter paper (Jordan et al., 1964 Morita et al., 1988), aluminum (Jordan et al., 1965), glass (Pape and Zabik, 1972 Chen et al., 1984 Hubbs and Lavy, 1990), and silica gel (Lotz et al., 1983). A shortcoming of the use of model surfaces is that herbicide dissipation due to volatility losses is often not accounted for (Hubbs and Lavy, 1990). Konstantinou et al. (2001) studied the sunlight photolysis of atrazine, propazine, and prometryn on soil (sandy clay loam, clay loam, and... 

Michael, J.L. and D.G. Neary (1993). Herbicide dissipation studies in southern forest ecosystems. Environ. Toxicol. Chem., 12 405 410. 

Otto, S., Riello, L., During, R.-A., Hummel, H.E. (1997) Herbicide dissipation and dynamics modelling in three different tillage... 

Maximizing Herbicide Utility. Most herbicides dissipate rather rapidly after application. That is, they volatilize, are decomposed by light or microorganisms, and are leached into soil, etc. In any case, they become unavailable to perform their function. Ultimately, this dissipation becomes desirable in that... 

Castro M, Silva-Ferreira AC, Manaia CM, Nunes O. (2005). A case study of mohnate application in a Portuguese rice field Herbicide dissipation and proposal of a clean-up methodology. Chemosphere 59 1059-1065. 

The reaction was also tested in soils and sand for chloroacetanilide herbicides. Enhanced herbicide dissipation occurred in sand or sandy soils, but became insignificant in clayey soils (29). The reduced enhancement in clayey soils may be attributed to the strong adsorption of herbicides to soil, which may have limited the interaction between the herbicide (adsorbed) and thiosulfate anion (in solution). In sand or sandy soils, the transformation of propachlor was the most rapid, which was followed by acetochlor and alachlor. Metolachlor transformation was only marginally enhanced. 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Dinitroaniline herbicides, in general, are very lipophilic, hence they are insoluble in water. They are stable under acidic or alkaline conditions. Dinitroaniline herbicides are potentially dissipated in the environment via photodegration and volatilization. 

Wu, T.L. 1980. Dissipation of the herbicides atrazine and alachlor in a Maryland cornfield. Jour. Environ. Qual. 9 459-465. 

In crop protection as well, understanding plant metabolism is of paramount importance to increase selectivity and to address resistance of chemical compounds. Moreover, dissipation of a compound in the aquatic ecosystem is very similar to the excretion phenomena of the bodies. An extensive amount of evidence has been accumulated to support the involvement of CYPs in the metabolism and detoxification of herbicides, fungicides and insecticides. The understanding of their biotransformations at the molecular level may be extremely helpful for herbicide- or insecticide-synergistic development. 

The environmental impact of sulfonylurea herbicides is also of significance. Based on hydrolytic studies, Dinelli et al. [168] predicted the dissipation rate of four herbicides following aquifer contamination. The chemical... 

G. Dinelb, A. Vicari, A. Bonetti, P. Catizone, Hydrolytic Dissipation of Four Sulfonylurea Herbicides , J. Agric. Food Chem. 1997, 45, 1940-1945. 

Soil. The major soil metabolite is 2,6-dichlorobenzamide which degrades to 2,6-dichloro-benzoic acid. The estimated half-lives ranged from 1 to 12 months (Hartley and Kidd, 1987). Under field conditions, dichlobenil persists from 2 to 12 months (Ashton and Monaco, 1991). The disappearance of dichlobenil from a hydrosol and pond water was primarily due to volatilization and biodegradation. The times required for 50 and 90% dissipation of the herbicide from a hydrosol were approximately 20 and 50 d, respectively (Rice et al., 1974). Dichlobenil has a high vapor pressure and volatilization should be an important process. Williams and Eagle (1979) found... 

Surface Water. The time required for 50 and 90% dissipation of the herbicide from New York pond water was approximately 21 and 60 d, respectively (Rice et al., 1974). 

Zimdahl and Clark (1982) reported half-lives of 15-38 and 33-100 d for the herbicide in clay loam soil, and sandy loam soil, respectively. They also reported that soil moisture increased the dissipation rate. At 20 °C, the dissipation rates of metolachlor in the clay loam and sandy loam soils at 20, 50, and 80% soil moisture contents were 0.028, 0.053, 0.062, and 0.016, 0.028, and 0.037/day, respectively. The half-lives of metolachlor in soil maintained at temperatures of 30 and 40 °C were approximately 3.85 and 2.75 wk, respectively (Bravermann et al., 1986). The reported half-lives of metolachlor in soil is approximately 6 d (Worthing and Hance, 1991) and 3-4 wk (Bowman, 1988). 

Fang, C.H. Studies on the degradation and dissipation of herbicide alachlor on soil thin layers, J. Chin. Agric. Chem. Soc., 17 47-53, 1979. 

Nash, R.G. Dissipation from soil, in Environmental Chemistry of Herbicides. Volume TR. Grover, Ed. (Boca Raton, FL CRC Press, Inc. 1988), pp. 131-169. 

Newton, M., Roberts, R, Allen, A., Kelpas, B., White, D.,and Boyd, P. Deposition and dissipation ofthree herbicides in foliage, litter, and soil of brushfields of southwest Oregon. J. Agric. Food Chem., 38 574-583,1990. 

Muir DCG (1991) Dissipation and transformation in water and sediment. In Grover R (ed). Environmental Chemistry of Herbicides, Volume 2. CRC Press, Boca Raton, pp 3-88... 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

Rouchard, J., O. Neus, R. Bulche, K. Cools, H. Ealen, and T. Dekkers (2000). Soil dissipation of diuron, chlorotoluron, simazine, pro-pyzamide and diflufenican herbicides after repeated applications in fruit tree orchards. Arch. Environ. Contam. Toxicol., 39 60-65. 

Triazine herbicides absorb sunlight weakly at wavelengths >290 nanometers (nm), thus, dissipation of the triazine herbicides in the atmosphere and in surface waters via photodegradation occurs mainly by indirect photolysis or photosensitized reactions. 

Current information on the photochemical dissipation of the triazine herbicides in the atmosphere is very limited. No studies concerning the vapor-phase photolysis of these herbicides have been reported, and only two studies have investigated the phototransformation of triazine herbicides when associated with atmospheric aerosols. Photodegradation of atrazine and terbuthylazine was observed in these studies, but the significance of photodegradation in the dissipation of atmospheric concentrations of these herbicides has yet to be established. 

The increased rates of photodegradation of the triazine herbicides observed in the presence of naturally occurring sensitizers indicate that photodegradation plays a significant role in the dissipation of these herbicides in natural waters. With most of the sensitizers studied thus far, cyanuric acid was the stable end product, rather than complete mineralization of the triazine herbicide. 

Although hydrolysis of the triazine herbicides is temperature and pH dependent, these herbicides are considered to be hydrolytically stable under the pH and temperature conditions encountered in natural waters. However, the relatively slow hydrolysis rates in natural waters may be enhanced somewhat by the presence of dissolved organic carbon (DOC) (in the form of fulvic acids and a variety of low-molecular-weight carboxylic acids and phenols) that has been shown to catalyze the hydrolysis of several triazine herbicides. Although microbial degradation is probably the most important mechanism of dissipation of the triazine herbicides in soils, abiotic hydrolysis of these herbicides also occurs. Hydrolysis in soils is affected by the pH, organic matter (humic acid) content, and the type and content of clay in the soil. 

Hubbs, C.W. and T.L. Lavy (1990). Dissipation of norflurazon and other persistent herbicides in soil. Weed Sci., 38 81-88. 

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