Volatilization, soil-applied herbicides

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

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. 

Other Potential Problems in Universal Pre-screens. Some soil active herbicides are highly volatile and there are problems in keeping them in the media for use in herbicide studies. This problem was overcome by using a non-volatile fnon-commerclal) thiocarbamate in cell culture studies to screen for heroicide resistant strains (IS). This solution to the problem of volatility does not apply to universal pre-screens. 

Although losses by volatilization are potentially possible for virtually any surface-applied herbicide given certain environmental conditions, herbicides which have particularly high vapor pressures and have been shown to be volatilized from soils include EPTC, triallate, and trifluralin. ... 

Volatilization of NDPA, NDEA, NDMA and Ji-nitrosopendime-thalin were examined in a model system. The nitrosamines were either mixed into predetermined depths of the soil or applied to the soil surface (the conditions were chosen to represent those that would be encountered by nitrosamines coapplied with dini-troaniline herbicides). Volatilization of nitrosopendimethalin was extremely slow regardless of application. The volatile nitrosamines, NDPA, NDEA, and NDMA, in contrast, volatilized so rapidly after application to the surface of moist soil that we predicted that a substantial proportion of the nitrosamine thus applied would enter the atmosphere within a few hours. Incorporation of the nitrosamine in the top 7.5 cm of soil (as might be the case when the herbicide was applied and incorporated in a single operation) decreased total volatilization by at least an order of magnitude. 

Chemical contaminants for which full-scale treatment data exist include primarily volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs). These SVOCs include polychlorinated biphenyls (PCBs), pentachlorophenol (PCP), pesticides, and herbicides. Extremely volatile metals, such as mercury and lead, can be removed by higher temperature thermal desorption systems. The technology has been applied to refinery wastes, coal tar wastes, wood-treating wastes, creosote-contaminated soils, hydrocarbon-contaminated soils, mixed (radioactive and hazardous) wastes, synthetic mbber processing wastes, and paint wastes. 

Pre-plant incorporated herbicides are applied before the crop is sown and are incorporated into the soil. Hence, they are also applied before weeds emerge. The reason for incorporation is usually because the herbicides are volatile and would be lost if they were not incorporated, or light unstable and they would be degraded if they remained on the soil surface. Volatility is a useful characteristic as it allows the redistribution of the compound throughout the soil following incorporation. 

Typically, only 0.01-10% of the mass of pesticide compounds applied to fields is detected in streams [91]. The remaining 90-99% of pesticides adsorb to soil, percolate into groundwater, or volatilize [79]. The major degradates of the most heavily used herbicides found in surface water have not been studied widely. Many chemical properties of pesticides affect the amounts transported to streams. In general, acetanilide herbicides are more soluble in water, and thus more mobile than are the triazines [92], The solubilities of sulfonated degradates of acetanilides (ethane sulfonic acid, or ESA), can be 10 times the solubility of the parent compound [93]. The greater mobilities of the degradates of the acetanilides (amide family) can explain these com-... 

Miellet [80] and Lopez-Avila et al. [81] have reviewed the applications of Soxhlet extraction to the determination of pesticides in soil. This technique has been applied extensively to the extraction of polycyclic aromatic hydrocarbons, volatile organic compounds, pesticides, herbicides and polychlorodibenzo-p-dioxins in soils. Details of the extraction procedures and the analytical finish employed are reviewed in Table 1.1. 

The length of time pesticides persist in the forest floor and soil bears strongly on the probability they will be lost by volatilization (28-31). The phenoxy herbicides are commonly applied to forests as the low-volatile esters. These esters are readily hydrolyzed to their respective acids in soil or on the forest floor. For example, Smith (32) reported that no traces of 2,4,5-T and 2,4-D esters were observed in any of four moist soils after 48 and 72 hours, respectively, and most of them were hydrolyzed in less than 24 hours. The vapor pressures of the acids are much lower than the esters and this hydrolysis, along with subsequent degradation of the acids, results in a very low potential for volatilization of these materials from soil. 

Their volatilization from litter on the forest floor will also be appreciable. With the possible exception of carbaryl, their volatilization after being washed into the soil will be relatively low or insignificant because of their low volatility, low Henry s constants, Kh> and/or their high rates of degradation in the soil environment. The rapid disappearance of the phenoxy herbicides (2, 31) and the insecticide, fenitrothion (28) from vegetation and the forest floor is supporting evidence that volatilization is an important pathway for loss of applied pesticides from the forest canopy and litter on the forest floor. 

What happens to herbicides after they are applied A proportion will be taken up by plants and either stored or metabolized (biochemically transformed to other substances, as we have seen). The metabolites, as well as the remaining parent and other breakdown products, eventually will reach water and soil (6 ), from which they may volatilize into the atmosphere or move on suspended dust or silt [sometimes for great distance (30)] eventually to decompose or be returned to earth in an ever-diminishing cycle. 

In order to minimize waste as well as to direct selectivity, a number of approaches toward dissipation-control are being examined. For example, both volatilization and photodecomposition often can be regulated to a desired degree by incorporation of a non-volatile resin additive into the pesticide formulation (35). The technique appears promising for insecticides, and there is no reason to believe it should not work for herbicides also. Another approach is inhibition of microbial break-down for example, N-methylcarbamate inhibitors of hydrolytic enzymes, such as PCMC ( -chlorophenyl N-methylcarbamate), applied together with a herbicide such as chloropropham [isopropyl N-(3-chlorophenyl)carbamate] which is inactivated by soil microbes, more than doubled the effectiveness (36,37). 

The last ten years have seen important developments in this respect in the three most important families of pesticides fungicides of the triazole group applied at about 100 g/ha, insecticides of the synthetic pyrethroid type at 20 g/ha and herbicides of the sulfonylurea type at 30 and even as little as 5 g/ha exert an effect which could be achieved with the pesticides of 15-20 years earlier only at rates of a few kilograms per hectare. These modern highly efficient preparations form only a small part of the selection of pesticides available today, but a rapid increase in their share of the total is to be expected as a result of purposeful research work. Another approach toward diminishing environmental contamination by chemicals is the development of new active substances which are less volatile, are degraded more rapidly or are more readily adsorbed by soil particles. 

Trifluralin is a preemergence herbicide which must be incorporated into the soil because of its volatility (vapour pressure at 29.5°C is 1.99 10 mm Hg) and its sensitivity to ultraviolet radiation. At application rates of O.S-1.0 kg active ingredient/ha it can be used selectively against annual grass weeds and several broad-leaved weed species in cotton, potatoes, sunflower, soybean, tomato, many vegetables and orchards. Applied in a double dose in two consecutive years, it kills the rhizomes of Johnson grass (Worther, 1974). 

While laboratory studies indicate that low molecular weight nitrosamines including NDPA can volatilize rather rapidly after application to the surface of warm soil, incorporation into the soil of the nitrosamine co-applied with a dinitroaniline herbicide, decreases both the rate and extent of volatilization (49,59). However, in either case volatilization obser d occurred within 3 or 4 days after application. No uptake of C into the stems, leaves and beans was found xj en soybeans were grown in soil treated with 100 ppb of NDPA- C, or N-nitrosopendimetha-lin- C [N-(1-ethyl propyl)-N-nitroso-3,4-dimethy1-2,6-dinitro-benzenamine (60). It should be noted that N-nitrosopendimethalin (a contaminant of the pesticide pendimethalin) (61) was relatively stable in soil and significant quantities could be recovered unchanged after several months. 

Several encapsulated herbicide formulations exist. Encapsulated alachlor herbicide has been sold as a liquid or dry granule formulation. The capsules, produced by interfacial polymerization, are spherical with a diameter of 2-15 /um. Two thiocarbamate herbicides, EPTC and vemolate [1929-77-7], were encapsulated by interfacial polymerization because they are volatile compounds. When applied in imencapsulated form, they must be incorporated in the soil within 2 h in order to provide effective weed control. When applied as a microencapsulated formulation, the rate of volatilization is lower and soil incorporation can be... 

The determination of the herbicide glyphosate at low residue levels is difficult, due to its ionic character, low volatility, low mass, and the lack of chemical groups that might facilitate its detection. Derivatization with FMOC has been used to facilitate its chromatographic retention, allowing the determination of glyphosate and its main metabolite AMPA in water and soil [17,18]. This approach could be applied to food samples as well. 

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