Organic Chemicals in the Environment Effect of Starch Encapsulation and Temperature on Volatilization of Atrazine and Alachlor

Volatilization of agricultural chemicals is one process whereby chemicals may enter into parts of the environment where they were not intended. Starch encapsulation of pesticides has been proposed as a way of modifying pesticide behavior in the soil environment. This study was conducted to assess how starch encapsulation and temperature affect volatilization of atrazine [6.chloro-N-ethyI-N’-(l-methylethyl)-l,3,5-triazine-2,4-diamine] and alachlor [2-chloro-N-(2,6diethylphenyl)-N-(methoxymethyl)acetamide]. Volatilization measured using agroecosystem chambers as model systems. Herbicides were applied at rates of 1.7 kg ha-t for atrazine and 2.8 kg ha -I for alachlor, as either a commercial formulation or a starch encapsulated formulation, to the surface of moist soils maintained at temperatures of 15, 25 and 35 °C. Air was drawn through the chambers (2.5 s min-t) and herbicide i n t he v apor phase was t rapped i n polyurethane foam plugs. Volatilization of both herbicides increased as temperature increased. Volatilization of atrazine was less when applied as starch.encapsulated formulation than the commercial formulation. After 35 d cumulative volatilization of atrazine ranged from < 1% of that applied as starch-encapsulated formulation at 15 °C, to 14% of that applied as the commercial formulation at 35 °C. Cumulative volatilization of alachlor was greater when applied as starchencapsulated formulation than as the commercial formulation. After 35 d, cumulative volatilization of alachlor ranged from > 2% of that applied as either formulation at 15 °C to 32% of that applied as starch encapsulated formulation at 35 °C. Differences in volatilization behavior between these herbicides are likely to be due to differences in chemical properties of these herbicides. ENVIRONMENTAL CONTAMINATION by agricultural chemicals is of increasing concern and the level of research directed at improving our understanding of pesticide behavior has increased (Burkart et al., 1990). Volatilization of pesticides is one process whereby chemicals enter parts of the environment where they were not intended. Volatilization losses decrease efficacy of the chemical and potentially expose humans and sensitive crops to the chemical. A method that may modify pesticide behavior and is receiving increased attention involves encapsulating the chemical in a starch matrix (Wing et al., 1987). Starch encapsulation is a controlled-release technology (Schreiber et al., 1987). Pesticides encapsulated the starch granule must be released from the starch matrix before they are effective. Release is largely a diffusion process. When starch granules are applied to soil they imbibe H20 and swell. The encapsulated chemical B.J. Weinhold and T.J. Gish, USDA-ARS, Hydrology Lab., Natural Resources Institute, Beltsville, MD 20705; and A.M. Sadeghi, USDA-ARS, Pesticide Degradation Lab., Natural Resources Institute, Beltsville, MD 20705. Received 30 Jan. 1992. *Corresponding author. Published in J. Environ. Quai. 22:162-166 (1993). dissolves in the imbibed H20 and diffuses out of the starch matrix. A number of factors influence the rate at which the release process proceeds: starch granule characteristics (i.e., size of granules, type of starch) (Wing et al., 1987, 1988), characteristics of the encapsulated chemical (i.e., solubility), and environmental factors (i.e., H20 availability, temperature, and microbial activity). Under similar environmental conditions, a more soluble chemical (e.g., alachlor) will be released more quickly than a less soluble chemical (e.g., atrazine) (Wienhold and Gish, 1992). For a given chemical, as n20 availability and temperature increase, rate of release will also increase. In addition, since starch can be enzymatically broken down by soil microbes, microbial activity increases rate of release (Wienhold and Gish, 1992). Environmental factors, pesticide properties, and management practices are some of the variable known to influence volatilization of pesticides. Volatilization of a soil-applied herbicide is largely regulated by adsorptiondesorption of the chemical from soil particles and organic matter into the solution phase, convection and diffusion to the solution-atmosphere interface, and volatilization into the atmosphere (Spencer et al., 1973). Environmental factors influencing this process include soil temperature and soil H20 content (Spencer et al., 1973). Volatilization rates tend to increase as temperature increases and volatilization rates from moist soil tend to be higher than from dry soil (Spencer and Cliath, 1974; Glotfelty et al., 1984). Chemical characteristics of a pesticide affecting volatilization are related to one another by Henry’s law coefficient (the ratio of a pesticides vapor density to H20 solubility). Hemy’s law coefficient has been a useful chemical benchmark in evaluating the potential volatility of pesticides (Jury et al., 1983). Management practices that increase surface litter (i.e., no-till or conservation tillage) tend to increase volatilization. Surface litter intercepts a portion of sprayed pesticide and prevents adsorption by the soil (Glotfelty, 1987). Practices that increase the distance pesticides must diffuse (i.e., soil incorporation) before entering the atmosphere tend to decrease volatilization (Taylor et al., 1976, 1977). Herbicides applied as commercial formulation are adsorbed by the soil quickly (Koskinen and Harper, 1990). Herbicides applied as the starch encapsulated formulation must first diffuse from the starch granules or be released as the granules degrade (Schreiber and White, 1980) before they can be adsorbed by the soil. It has been hypothesized that controlled release of a herbicide into the soil environment should reduce volatilization (Schreiber et al., 1987). Turner et al. (1978) reported that chloropropham (1-methylethyl 3-chlorophenylcari Trade names or company names are included for the benefit of the reader and imply no endorsement or preferential treatment of the product listed by the USDA.