Paraquat

CAS RN: 4685-14-7

Environmental Fate

TERRESTRIAL FATE: Based on a classification scheme(1), Koc values of 15,473-1,000,000(2-7), indicate that paraquat is expected to be immobile in soil(SRC). These values along with field studies and soil column leaching studies have demonstrated that paraquat is strongly adsorbed to soil constituents and does not leach under environmental conditions(8-10). Volatilization of paraquat from moist soil surfaces is not expected(SRC) given an estimated Henry's Law constant of <3.0X10-14 atm-cu m/mole(SRC), based upon its vapor pressure, <7.5X10-8 mm Hg(11), and water solubility, 6.2X10+5 mg/L(11). Paraquat is not expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(11). The biodegradation of paraquat is very slow because it strongly adsorbs to clay or organic matters in soils making paraquat unavailable to biodegrade(12). When not in the adsorbed-state, paraquat biodegrades(13-15). Some photodegradation may occur on plant leaves or soil surfaces exposed to sunlight(11-12). Field degradation half-lives in soil are reported as 99-4747 days(7,16-17).
AQUATIC FATE: Based on a classification scheme(1), reported Koc values of 15,473-1,000,000(2-7), indicate that paraquat is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(8) based upon an estimated Henry's Law constant of <3.0X10-14 atm-cu m/mole(SRC), derived from its vapor pressure, <7.5X10-8 mm Hg(9), and water solubility, 6.2X10+5 mg/L(9). Paraquat does not undergo hydrolysis based on tests conducted at pH 5, 7 and 9(10). According to a classification scheme(11), BCFs of 0.05-6.9 reported in rainbow trout, green sunfish, bluegill and channel catfish(12-13), suggest bioconcentration in aquatic organisms is low(SRC). Paraquat present in solution in the unabsorbed state may biodegrade easily in water(14). But when paraquat is adsorbed to clay or organic matter in water, biodegradation will be very slow(14-15). Paraquat will be completely removed from most surface waters in 8-12 days due to sorption to suspended solids and sediment in water; after being fixed to sediment and suspended solids, the disappearance of paraquat is slow(7).
FIELD STUDY: Paraquat resides are removed rapidly from water by adsorption on aquatic weeds and by strong adsorption to bottom mud. Decaying weeds also transport paraquat to bottom mud(1).
Paraquat Concn in 2 Trials

Sample 1 day after treatment 4 days after treatement 16 days after treatment 32 days after treatment
Water (mg/L) 0.31 0.12 not detected not reported
Weed (mg/kg) 13.70 25.80 21.0 0.55
Mud (mg/kg) 3.70 not reported not reported not reported
Water (mg/L) 0.37 not detected not detected not detected
Weed (mg/kg) 25.50 40.0 37.8 27.8
Mud (mg/kg) not detected 0.97 0.23 0.32
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), paraquat, which has a vapor pressure of <7.5X10-8 mm Hg at 25 deg C(2), is expected to exist solely in the particulate phase in the ambient atmosphere. Particulate-phase paraquat may be removed from the air by wet and dry deposition(SRC). Paraquat does not contain chromophores that absorb at wavelengths >290 nm(3) and, therefore, is not expected to be susceptible to direct photolysis by sunlight(SRC).
Find more information on this substance at: PubChem, PubMed