2,4-D

Note: According to WHO (1984) major impurities in technical grade 2,4-D include (2,6-dichlorophenoxy)acetic acid (0.5-1.5%), 2-chlorophenoxyacetic acid (0.1-0.5%), 4-chlorophenoxyacetic acid (0.2-0.8%), bis(2,4-dichlorophenoxy)acetic acid (0.1-2.0%), phenoxyacetic acid (trace0.2%), 2,4-dichlorophenol (0.6-1.0%), 2,6-dichlorophenol (0.001-0.048%), 2,4,6-trichlorophenol (0.001-0.14%), 2-chlorophenol (0.004-0.04%), 4-chlorophenol (0.0004-0.005%), and water (0.10.8%). CASRN: 94-75-7; DOT: 2765; DOT label: Poison; molecular formula: C8H6Cl2O3; FW: 221.04; RTECS: AG6825000; Merck Index: 12, 2865 Physical state, color, and odor: Odorless, white to pale yellow, powder or prismatic crystals. Impure formulations containing 2,4D as the main component may have a phenolic odor. Melting point (°C): 139.6 (Plato and Glasgow, 1969) 138.2-138.8 (Crosby and Tutass, 1966) Boiling point (°C): 160 at 0.4 mmHg (Weast, 1986) Density (g/cm3): 1.416 at 25 °C (quoted, Verschueren, 1983) 1.5600 at 137-141 °C (Acros Organics, 2002) Diffusivity in water (x 10-5 cm2/sec): 0.57 at 20 °C using method of Hayduk and Laudie (1974) Dissociation constant, pKa: 2.73 (Nelson and Faust, 1969)

2.87 (Cessna and Grover, 1978) 2.90 at 25 °C (Jafvert et al., 1990) Flash point: Noncombustible solid (NIOSH, 1997) Entropy of fusion (cal/mol⋅K): 22.0 (Plato and Glasgow, 1969) Heat of fusion (kcal/mol): 9.100 (DSC, Plato and Glasgow, 1969) Henry’s law constant (x 10-5 atm⋅m3/mol): 6.72 and 0.84 were reported at pH values of 1 and 7, respectively (wetted-wall column, Rice et al., 1997a) Bioconcentration factor, log BCF: 0.845 (Chlorella fusca, Geyer et al., 1981) 0.78 (Chlorella fusca, Freitag et al., 1982; Geyer et al., 1984) 1.30 (activated sludge, Freitag et al., 1985) 0.00 (fish, microcosm) (Garten and Trabalka, 1983) Soil organic carbon/water partition coefficient, log Koc: 1.68 (Commerce soil), 1.88 (Tracy soil), 1.76 (Catlin soil) (McCall et al., 1981) 2.05 (German sand), 3.07 (48% sand, 43% silt, 9% clay), 2.14 (78% sand, 12% silt, 10%), 1.76

(22% sand, 55% silt, 23% clay) (Haberhauer et al., 2000) 1.30 (includes salts, Kenaga and Goring, 1980) 1.70-2.73 (average = 2.18 for 10 Danish soils, Løkke, 1984) 2.05 (Spodosol, pH 3.9), 2.11 (Speyer soil, pH 5.8), 2.16 (Alfisol, pH 7.5) (Rippen et al., 1982) 1.77 (Lubbeek II sand loam), 2.33 (Lubbeek II sand), 1.83 (Lubbeek I silt loam), 1.82 (Lubbeek

III silt loam), 2.22 (Stookrooie II loamy sand), 1.80 (Fleron silty clay loam), 1.76 (Bullingen silt loam), 2.79 (Spa silty clay loam), 2.74 (Bernard-Fagne silt loam), 2.48 (Stavelot silt loam), 2.29 (Meerdael silt loam), 2.52 (Soignes silt loam), 1.77 (Heverlee II sandy loam), 1.73 (Nodebais silt loam), 2.91 (Zolder sand - A1 horizon), 2.71 (Zolder sand - A2 horizon) (Moreale and Van Bladel, 1980)

1.652 (European soil, Gawlik et al., 2000) Octanol/water partition coefficient, log Kow: 2.65 (Hansch and Leo, 1985) 2.50 (Riederer, 1990) 2.59 (Freese et al., 1979) 2.15 (aqueous phase contained 0.1 or 0.5 M HCl, Jafvert et al., 1990) Solubility in organics: At 25 °C (g/L): carbon tetrachloride (1), ethyl ether (270), acetone (850), and ethyl alcohol (1,300) (quoted, Bailey and White, 1965) Solubility in water: 890 ppm at 25 °C (Chiou et al., 1977) 900 mg/L at 25 °C (Davidson et al., 1980)

725 ppm at 25 °C (quoted, Bailey and White, 1965) 2.36 mM at 25 °C (shake flask-spectrophometry, Leopold et al., 1960) 2,940 µmol/L at 25 °C (LaFleur, 1979) 620 mg/L at 20 °C (Fühner and Geiger, 1977) Vapor pressure (x 10-3 mmHg): 4.7 at 20 °C (Riederer, 1990) Environmental fate: Biological. 2,4-D degraded in anaerobic sewage sludge to 4-chlorophenol (Mikesell and Boyd, 1985). In moist nonsterile soils, degradation of 2,4-D occurs via cleavage of the carbon-oxygen bond at the 2-position on the aromatic ring (Foster and McKercher, 1973). In filtered sewage water, 2,4-D underwent complete mineralization but degradation was much slower in oligotrophic water, especially when 2,4-D was present in high concentrations (Rubin et al., 1982). In a primary digester sludge under methanogenic conditions, 2,4-D did not display any anaerobic biodegradation after 60 d (Battersby and Wilson, 1989). Faulkner and Woodstock (1964) reported the soil microorganism Aspergillus niger degraded 2,4-D to 2,4-dichloro-5-hydroxyphenoxyacetic acid. Soil. In moist soils, 2,4-D degraded to 2,4-dichlorophenol and 2,4-dichloroanisole as intermediates followed by complete mineralization to carbon dioxide (Wilson and Cheng, 1978; Smith, 1985; Stott, 1983). 2,4-Dichlorophenol was reported as a hydrolysis metabolite (Somasundaram et al., 1989, 1991; Somasundaram and Coats, 1991). In a soil pretreated with its hydrolysis metabolite, 80% of the applied [14C]2,4-D mineralized to 14CO2 within 4 d. In soils not treated with the hydrolysis product (2,4-dichlorophenol), only 6% of the applied [14C]2,4-D degraded to 14CO2 after 4 d (Somasundaram et al., 1989). Steenson and Walker (1957) reported that the soil microorganisms Flavobacterium peregrinum and Achromobacter both degraded 2,4-D yielding 2,4-dichlorophenol and 4-chlorocatechol as metabolites. The microorganisms Gloeosporium olivarium, Gloeosporium kaki, and Schisophyllum communs also degraded 2,4-D in soil forming 2-(2,4-dichlorophenoxy)ethanol as the major metabolite (Nakajima et al., 1973). Microbial degradation of 2,4-D was more rapid under aerobic conditions (half-life = 1.8-3.1 d) than under anaerobic conditions (half-life = 69-135 d) (Liu et al., 1981). In a 5-d experiment, [14C]2,4-D applied to soil water suspensions under aerobic and anaerobic conditions gave 14CO2 yields of 0.5 and 0.7%, respectively (Scheunert). The reported degradation half-lives for 2,4-D in soil ranged from 4 d in a laboratory experiment (McCall et al., 1981a) to 15 d (Jury et al., 1987). Degradation half-lives were determined in six soils: Catlin (1.5 d), Cecil (3.0 d), Commerce (5 d), Fargo (8.5 d), Keith (3.9 d), Walla Walla (2.5 d) (McCall et al., 1981a). Residual activity in soil is limited to approximately 6 wk (Hartley and Kidd, 1987). After one application of 2,4-D to soil, the half-life was reported to be approximately 80 d but can be as low as 2 wk after repeated applications (Cullimore, 1971). The half-lives for 2,4-D in soil incubated in the laboratory under aerobic conditions ranged from 4 to 34 d with an average of 16 d (Altom and Stritzke, 1973; Foster and McKercher, 1973; Yoshida and Castro, 1975). In field soils, the disappearance half-lives were lower and ranged from approximately 1 to 15 d with an average of 5 d (Radosevich and Winterlin, 1977; Wilson and Cheng, 1976; Stewart and Gaul, 1977). Under aerobic conditions, the mineralization half-lives of 2,4-D in soil ranged from 11 to 25 d (Ou et al., 1978; Wilson and Cheng, 1978). The half-lives of 2,4-D in a sandy loam, clay loam, and an organic amended soil under nonsterile conditions were 722 to 2,936, 488 to 3,609, and 120 to 1,325 d, respectively (Schoen and Winterlin, 1987). Disappearance half-lives of 2,4-D were determined in two soils following a 28-d incubation period. In a Cecil loamy sand (Typic Hapludult), half-lives ranged from 3.9 to 9.4 and 6.8 to 115 d at 25 and 35 °C, respectively. In a Wenster sandy clay loam (Typic Haplaquoll), half-lives ranged from 7.0 to 254 and 6.7 to 176 d at

increase of soil moisture content (Ou, 1984). In sandy loam and muck soils, 2,4-D degraded under first-order kinetics. Half-lives were 36 d for aerobic sandy loam, 3.4 d for aerobic muck, and 9.3 d for anaerobic muck (Cheah et al., 1998). Groundwater. According to the U.S. EPA (1986), 2,4-D has a high potential to leach to groundwater. Plant. Reported metabolic products in bean and soybean plants include 4-O-β-glucosides of 4hydroxy-2,5-dichlorophenoxyacetic acid, 4-hydroxy-2,3-dichlorophenoxyacetic acid, N-(2,4dichlorophenoxyacetyl)-L-aspartic acid, and N-(2,4-dichlorophenocyacetyl)-L-glutamic acid. Metabolites identified in cereals and strawberries include 1-O-(2,4-dichlorophenoxyacetyl)-β-Dglucose and 2,4-dichlorophenol, respectively (quoted, Verschueren, 1983). In alfalfa, the side chain in the 2,4-D molecule was lengthened by two and four methylene groups resulting in the formation of (2,4-dichlorophenoxy)butyric acid and (2,4-dichlorophen-oxy)hexanoic acid, respectively. In several resistant grasses, however, the side chain increased by one methylene group forming (2,4-dichlorophenoxy)propionic acid (Hagin and Linscott, 1970). 2,4-D was metabolized by soybean cultures forming 2,4-dichlorophenoxyacetyl derivatives of alanine, leucine, phenylalanine, tryptophan, valine, aspartic and glutamic acids (Feung et al., 1971, 1972, 1973). On bean plants, 2,4-D degraded via β-oxidation and ring hydroxylation to form 2,4dichloro-4-hydroxyphenoxyacetic acid, 2,3-dichloro-4-hydroxyphenoxyacetic acid (Hamilton et al., 1971), and 2-chloro-4-hydroxyphenoxyacetic acid. 2,5-Dichloro-4-hydroxyphenoxyacetic acid was the predominant product identified in several weed species and 2-chloro-4-hydroxyphenoxyacetic acid was present in low quantities in wild buckwheat, yellow foxtail, and wild oats (Fleeker and Steen, 1971). Esterification of 2,4-D with plant constituents via conjugation formed the β-D-glucose ester of 2,4-D (Thomas et al., 1964). Photolytic. Photolysis of 2,4-D in distilled water using mercury arc lamps (λ = 254 nm) or by natural sunlight yielded 2,4-dichlorophenol, 4-chlorocatechol, 2-hydroxy-4-chlorophenoxyacetic acid, 1,2,4-benzenetriol, and polymeric humic acids. The half-life for this reaction is 50 min (Crosby and Tutass, 1966). A half-life of 2 to 4 d was reported for 2,4-D in water irradiated at 356 nm (Baur and Bovey, 1974). Bell (1956) reported that the composition of photodegradation products formed were dependent upon the initial 2,4-D concentration and pH of the solutions. 2,4-D undergoes reductive dechlorination when various polar solvents (methanol, butanol, isobutyl alcohol, tert-butyl alcohol, octanol, ethylene glycol) are irradiated at wavelengths between 254 to 420 nm. Photoproducts formed included 2,4-dichlorophenol, 2,4-dichloroanisole, 4-chlorophenol, 2-and 4chlorophenoxy-acetic acid (Que Hee and Sutherland, 1981). Irradiation of a 2,4-D sodium salt solution by a 660-W mercury discharge lamp produced 2,4dichlorophenol within 20 min. Continued irradiation resulted in continued decomposition. The irradiation times required for 50% decomposition of the 2,4-D sodium salt at pH values of 4.0, 7.0, and 9.0 are 71, 50, and 23 min, respectively (Aly and Faust, 1964). Surface Water. In filtered lake water at 29 °C, 90% of 2,4-D (1 mg/L) mineralized to carbon dioxide. The half-life was <5 d. At low concentrations (0.2 mg/L), no mineralization was observed (Wang et al., 1984). Subba-Rao et al. (1982) reported that 2,4-D in very low concentrations mineralized in one of three lakes tested. Mineralization did not occur when concentrations were at the picogram level. A degradation rate constant of 0.058/d at 29 °C was reported. At original concentrations of 100 mg/L and 100 µg/L in autoclaved water, the amount of 2,4-D remaining after 56 d of incubation were 81.6 and 79.6%, respectively (Wang et al., 1994). In clear and muddy waters, hydrolysis half-lives were 18->50 and 10-25 d, respectively, (Nesbitt and Watson, 1980). Chemical/Physical. In a helium pressurized reactor containing ammonium nitrate and polyphosphoric acid at temperatures of 121 and 232 °C, 2,4-D was oxidized to carbon dioxide, water,

acids were reported as ozonation products of 2,4-D in water at pH 8 (Struif et al., 1978). Reacts with alkalies, metals, and amines forming water soluble salts (Hartley and Kidd, 1987). In water, 2,4-D reacted with OH radicals at a first-order rate constant of 1.6 x 109/M·sec (Mabury and Crosby, 1996a). When 2,4-D was heated at 900 °C, carbon monoxide, carbon dioxide, chlorine, HCl, and oxygen were produced (Kennedy et al., 1972, 1972a). In liquid ammonia containing metallic sodium or lithium, 2,4-D degraded completely (Kennedy et al., 1972a). Total mineralization of 2,4-D was observed when a solution containing the herbicide and Fenton’s reagent (ferrous ions and hydrogen peroxide) was subjected to UV light (λ = 300-400 nm). One intermediate compound identified was oxalic acid (Sun and Pignatello, 1993). Pignatello (1992) investigated the reaction of 2,4-D (0.1 mM) with Fenton’s reagent in an airsaturated acidic solution at 21 °C. When the concentrations of Fe2+ and hydrogen peroxide concentrations were both >1 mM, the reaction time for complete transformation was <1 min. The transformation of 2,4-D to the major product, 2,4-dichlorophenol, decreased with increasing the initial pH. In the presence of Fe3+ and excess hydrogen perioxide, the mineralization of 2,4-D to carbon dioxide and chloride ions is nearly complete. The rate of reaction is sensitive to pH, the optimum pH at approximately 2.75. The reaction with Fe3+ and hydrogen perioxide is accelerated in the presence of UV light (λ >300 nm). In water, 2,4-D reacts with OH radicals at a rate of 8.4 x 1012/M·h at 25 °C (Armbrust, 2000). Gomez et al. (1988) studied the vapor-phase pyrolysis and combustion of 2,4-D in the temperature range 200-1,000 °C. 2,4-D began to decompose at 200-250 °C. In the presence of air, 2,4-D completely degraded when the temperature exceeded 900 °C. HCl and chlorine were identified as products of thermal degradation. The solubilities of the calcium and magnesium salts of 2,4-D acid at 25 °C are 9.05 and 25.1 mM, respectively (Aly and Faust, 1964). Ward and Getzen (1970) investigated the adsorption of aromatic acids on activated carbon under acidic, neutral, and alkaline conditions. The amount of 2,4-D (10-4 M) adsorbed by carbon at pH values of 3.0, 7.0, and 11.0 were 60.1, 18.8, and 14.3%, respectively. Exposure limits (mg/m3): NIOSH REL: TWA 10, IDLH 100; OSHA PEL: TWA 10; ACGIH TLV: TWA 10 (adopted). Toxicity: EC50 (48-h) for Daphnia pulex 3.20 mg/L (Sanders and Cope, 1986). EC50 (24-h) for Daphnia magna 249 mg/L, Daphnia pulex 324 mg/L (Lilius et al., 1995). EC50 (5-min) for Photobacterium phosphoreum 100.7 mg/L (Somasundaram et al., 1990). LC50 (96-h) for carp 5.1-20 mg/L (Spehar et al., 1982), rainbow trout 110 mg/L, fathead minnow 133 mg/L, bluegill sunfish 180 mg/L (Mayer and Ellersieck, 1986), Japanese medaka (Oryzias latipes) 2,780 mg/L (Holcombe et al., 1995), cutthroat trout 64 mg/L, lake trout 45 mg/L (Johnson and Finley, 1980). LC50 (48-h) for bluegill sunfish 900 ppb, for rainbow trout 1.1 ppm (Edwards, 1977). LC50 (24-h) for cutthroat trout 32 to 185 mg/L, lake trout 44.5 to 127.5 mg/L (Mayer and Ellersieck, 1986). Acute oral LD50 for chickens 541 mg/kg, dogs 100 mg/kg, guinea pigs 469 mg/kg, hamsters 500 mg/kg, mice 368 mg/kg, rats 370 mg/kg (quoted, RTECS, 1985), chicks 420 mg/kg (Morgulis et al., 1998). Drinking water standard (final): MCLG: 70 µg/L; MCL: 70 µg/L. In addition, a DWEL of 400 µg/L was recommended (U.S. EPA, 2000). Uses: Systemic herbicide; weed killer and defoliant.