Herbicides volatility

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). 

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. 

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. 

Phenylcarbamates. Phenylcarbamate herbicides represent one of two subgroups of carbamate herbicides, the phenylcarbamates and the thiocarbamates (299). Both groups are prone to volatilization losses the thiocarbamates are particularly susceptible and should be sod-incorporated immediately after apphcation (2). The carbamate herbicides are used, in general, for the selective pre-emergence control of grass and broadleaved weeds (299). Exceptions would include barban, desmedipham, and phenmedipham which are appHed post-emergence. 

The encapsulation of herbicides has received much attention. Encapsulated alachlor is a high volume herbicide product generally sold as a Hquid formulation, although a dry granule version is also available. The capsules, produced by interfacial polymeri2ation (11), are reported to be spherical with a diameter of 2—15 p.m (75). Two thiocarbamate herbicides, EPTC and vemolate [1929-77-7], were encapsulated by interfacial polymeri2ation because they are volatile compounds. When appHed in unencapsulated form, they must be incorporated in the soil within two hours in order to provide effective weed control. When appHed as a microencapsulated formulation, the rate of volatili2ation is lower and soil incorporation can be delayed 24 hours (76). 

The major use of alkan olamines in agricultural products is as a neutralizer for acidic herbicides. They also contribute increased water solubiUty, reduced volatility, and more uniform solutions. Various ethan olamines are reported in formulations to improve potato tuber size (219) and enhance the resistance to salt of some crops (220). 

The efficient recovery of volatile nitrosamines from frankfurters, followed by gc with chemiluminescence detection, has been described (133). Recoveries ranged from 84.3 to 104.8% for samples spiked at the 20 ppb level. Methods for herbicide residues and other contaminants that may also relate to food have been discussed. Inorganic elements in food can be deterrnined by atomic absorption (AA) methods. These methods have been extensively reviewed. Table 8 Hsts methods for the analysis of elements in foods (134). 

Baird is the 20-acre site of a former chemical mixing and batching company. Poor waste disposal practices resulted in the contamination of groundwater, soil, the municipal water supply, and a brook adjacent to the site. Over one hundred contaminants, including chlorinated and nonchlorinated volatile organics, heavy metals, pesticides, herbicides, and dioxins, had been identified in site soil and groundwater. Remediation activities included soil excavation and incineration, and groundwater treatment (the audit focused on the soil excavation and incineration... 

Contractors at Sites B, D, G, I, and J had incomplete sampling practices and as a result were not able to evaluate PPE levels based on monitoring data. Eor example, both contractors SSAHPs at Site I lacked provisions for monitoring site hazards such as metals, pesticides, herbicides, and semi-volatile organic compounds (SVOCs) that could not be evaluated with a PID. Since worker exposures to the range of hazards on site had not been characterized, PPE was not selected based on its performance relative to the nature and level of site hazards. 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

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. 

Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

Hamilton, D. J. (1980) Gas chromatographic measurement of volatility of herbicide esters. J. Chromatogr. 195, 75-83. 

These days, allelopathic plants as catch crops or trap crops found utilization in plant protection of tropical regions against parasitic weeds. They do not eliminate the parasite completely but decrease the seed bank in the soil. Other applications of allelopathy for weed control include the use of plant residues as a natural herbicide agent, e.g., water extracts, pellets, flours, by-products of crop processing, etc. The strategies for use of volatile compounds as soil fumigants are developed. 

Phenoxyalkanoic acid herbicides are not amenable to direct gas chromatographic determination because of the high polarity or low volatility of the compounds and must be converted to their... 

With the exception of picloram and phenols (Fig. 10, Table 3), acidic pesticides are considered nonvolatile from aqueous and soil systems [153]. Some ester formulations of these compounds also behave as herbicides. They do not ionize in solution and are less water-soluble than the acid or salt forms. They are eventually hydrolyzed to acid anions in aqueous and soil systems, but in the ester form are non-ionic and relatively volatile. 

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. 

---