Vapor pressure/volatility 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). 

More than 25 different substituted urea herbicides are currently commercially available [30, 173]. The most important are phenylureas and Cycluron, which has the aromatic nucleus replaced by a saturated hydrocarbon moiety. Benzthiazuron and Methabenzthiazuron are more recent selective herbiddes of the class, with the aromatic moiety replaced by a heterocyclic ring system. With the exception of Fenuron, substituted ureas (i.e., Diuron, Fluometuron, Fig. 10, Table 3) exhibit low water solubilities, which decrease with increasing molecular volume of the compound. The majority of the phenylureas have relatively low vapor pressures and are, therefore, not very volatile. These compounds show electron-donor properties and thus they are able to form charge transfer complexes by interaction with suitable electron acceptor molecules. Hydrolysis, acylation, and alkylation reactions are also possible with these compounds. 

Substituted dinitroanilines (Fig. 10, Table 3) are an important series of selective herbicides commercially introduced in agriculture in the 1960s. Trifluralin is the most prominent member of this series. Nitralin and Benfluralin have also received widespread usage, while Profluralin is a relatively recent herbicide of this class. Dinitro anilines show very low water solubilities. Nitralin and Benfluralin have low vapor pressures and are nonvolatile, while Trifluralin is relatively volatile. All these compounds have been shown to be relatively immobile in soil systems. 

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... 

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 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. 

Aniline is released in the presence of denitrifying and methanogenic microbial activity238b. The pKa value suggests that in moist soils, aniline will be protonated, and bound to soil, which inhibits degradation. Volatilization does not take place from dry soils, based on the vapor pressure. Studies have been made on the metabolism of aniline-derived products, such as herbicides and fungicides, in soil. Chloroanilines bind to organics in... 

Substituted Anilines. Some properties of three substituted aniline herbicides are given in Table VIII. The compounds are slightly soluble in water only to 0.05-0.5 ppm. Nitralin and benefin have low vapor pressures and are nonvolatile while trifluralin which has a vapor pressure of 1.14 X 10" mm of Hg at 25°C and is relatively volatile. Applying... 

The mechanism of volatilization, in which herbicide molecules leave the soil or plant surface in the vapor phase and move into atmospheric air, is a significant cause of dissipation. Since the volume of atmospheric air is substantial compared with the number of herbicide molecules, the dilution factor is very large and therefore subsequent quantification is difficult. The volatility of a herbicide depends on its inherent vapor pressure, which of course is determined by its chemical structure. As shown by Guenzi and Beard, quite small changes in chemical structure can result in substantial changes in vapor pressure. The actual quantity of herbicide lost to the atmosphere also depends upon the concentration in the soil, the soil water content, air flow over the soil surface, humidity, temperature, diffusion rates within the soil air and at the surface, adsorption to soil particles, and water solubility. 

The rate of volatilization from a solid surface to which a herbicide is not adsorbed, as can occur on a plant leaf, at constant temperature depends only on the vapor pressure of the herbicide and the rate of movement away from the surface. Diffusion is the major mechanism governing movement... 

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. ... 

The molecular property of vapor pressure is one of the key parameters controlling the volatilization rate of herbicides, and methods of measurement have been reviewed by Spencer and Cliath and Plimmer. A lack of consistency is apparent in numerical values reported in the literature, and results are often quoted at a wide variety of temperatures. Both Plimmer and Taylor and Glotfelty have pointed out the problems in obtaining reliable measurements. Some authors refer to vapor density rather than vapor pressure, and Eq. (7.1) relates the two terms ... 

Movement of herbicides in the vapor phase can be important for transport from soil to plant (Section 9.3.2), and also some compounds transported to leaves from roots via the xylem may be then lost from leaves by volatilization. Worthing and Ashton and Crafts give the vapor pressures for most herbicides. 

Arsenic is a gray crystalline solid that does not melt at atmospheric pressure but simply volatilizes to give a dense, malodorous yellow vapor. Its main use as an element is to harden lead-antimony alloys, for example, those in storage batteries or lead shot. Arsenic compounds are highly toxic hence, many have been used as potent herbicides and insecticides. 

---