Paraquat
PESTICIDE NAME: Paraquat
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Trade name(s): Paraquat
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Manufacturer(s): Comlets Chemical Industrial Co., Ltd.
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Taiwan, Republic of China
I. Basic information
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A. Molecular structure: C12H14N2Cl2
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B. Chemical name: 1,1-Dimethyl-4,4'-bipyridiniondichloride
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N,N-dimethyl-4,4'-bipyridylium dichloride
C. Derivatives: no information available
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D. Molecular weight: 257 g/mole
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E. Solubility in water: very soluble
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F. Common physical appearance: colorless crystalline solid
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G. Oral LD50(rat): 150 mg/kg
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H. Pesticide classification: bipyridylium herbicide/growth
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regulator
I. Restricted use list (N.Y.): yes
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EPA priority pesticide list: no
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J. Crop use: grass sod, legume grass sod, chickweed, annual
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grasses, broadleaf weeds, apple, peach, pear, nectarine, apricot,
cherry, plum, prune, grape, raspberry, blackberry, blueberry, annual
broadleaf weeds on potatoes.
II. Text
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Paraquat is a soluble, nonselective bipyridylium herbicide which
acts as a cationic pesticide thus undergoing cation exchange in
soils. It is fast acting, non-volatile and is considered to leave no
residuals. Paraquat has been widely treated in the literature and is
used on a variety of crops in New York. Paraquat is considered to be
immobile, strongly adsorbed and is difficult to desorb.
III. Soils information
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A. Degradation and transformation
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Paraquat is rapidly inactivated (not degraded) on contact with
soil. There is little loss from chemical degradation except when pH>9
and with time there can be some photochemical and microbial
degradation(6). Paraquat decomposition is somewhat dependent on soil
type. One study found significant paraquat loss from a highly organic
soil between 48 and 96hrs after application of the pesticide. No
decomposition occurred after 96 hr and degradation was not evident in
other soils. Since any paraquat on the available sites of the surface
of organic colloids would be subject to degradation, the initial
application of the chemical would result in some paraquat being
degraded; eventually all of the herbicide is found in the clay
lattice(2). Biodegradation of high concentrations of paraquat was
reported to be slight in short-term lab experiments(8). In humid
tropical regions, highly weathered soils with kaolinite showed a
decreased capacity to deactivate paraquat as opposed to soils of high
montmorillonite content(8).
The table below presents data concerning paraquat residue on a
medium loam field soil(6)
3-6mo. 18-24mo.
soil rate(lb/A) 0-1in. 0-4in. 0-1in. 0-4in.
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med.loam 1 2.6ppm 1.1ppm 1.7ppm 0.52ppm
5 7.8 3.4 4.1 2.0
2 6.1 1.8 3.4 1.4
100 356 95 205 64
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B. Adsorption and transport
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There is substantial information in the scientific literature
concerning paraquat adsorption and transport in soils. As paraquat is
a cationic pesticide it follows the pathways involved in cation
exchange. It can react with more than one negative site on soil humic
colloids and is readily adsorbed on clay minerals whereas on organic
matter, it is loosely bound and can be leached by a saturated salt
solution(9). There is evidence of a transfer of paraquat from organic
matter sites to clay mineral sites thus restricting
leachability(7,13). Paraquat is more strongly adsorbed by
montmorillonite (an expanding clay) than by kaolinite (a non-expanding
clay)(6). Paraquat adsorption in mineral soils is quick and total(7).
Desorption of paraquat is small and does not follow simple ion
exchange but is possibly due to 1)ion trapping due to reduction in
swelling of matrix, 2)rupture of weak cross-links within adsorbents
where a large amount of ammonium is present, or 3)increase of
selectivity of soils for paraquat over hydrogen ion if H-bonds and Van
der Waals are present(5). Displacement of paraquat is difficult and it
has been found to compete with Ca2+ and Mg2+ for adsorption sites.
Paraquat displaces Na+, Ca2+ and Mg2+ when these cations are on
external or open sites. As the CEC increases, it is more difficult for
paraquat to compete with divalent inorganic cations for adsorption
sites(14).
Paraquat adsorption can be described by the linear Langmuir
isotherm(8,10). In the adsorption isotherm, the area with no paraquat
in solution is referred to as the Strong Adsorption Capacity (SAC).
Addition of hydrogen peroxide will reduce the SAC in heavy soils and
this SAC can be used to estimate the capacity of the soil to deactivate
the herbicide(12).
The tables below present data concerning paraquat adsorption and
desorption in soils. The reference is given at the end of each title.
Paraquat adsorption on mineral soils and peat(7)
Amt. Vol. Init. Final Para. Parition
adsorbent soln. conc. conc. ads. on coefficient
(g) (ml) (ppm) (ppm) peat(ppm) (P ads/P soln.)
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1 100 10 0 1000 infinity
1 100 20 0.05 1995 39900
1 100 40 0.7 3930 5614
1 100 50 0.9 4910 5455
5 100 250 2.89 4942 1710
15 100 750 6.90 4954 718
45 100 2250 17.45 4961 284
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Paraquat adsorption on soils of differing clay and organic matter
contents(8)
Soil type %clay %org.C b(ug/g) k b=ads.max; k calc. from
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Nkpologu 26 1.23 1489 0.052 first straight line
Iwo 15 2.08 5263 0.158 portion of Langmuir
Iwo 54 0.45 5333 0.188
Ikom 42 4.7 7467 0.134
Apomu 6 1.23 2105 0.158
Alagba 13 1.56 3922 0.425
Onne 18 1.04 1233 0.116
Onne 36 0.16 2182 0.046
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Paraquat adsorption and desorption on selected soils (clay and organic
matter content given in table above)(8)
Soil type Ads(ug/g) Des(ug/g) %Des (all extracted with
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Iwo 2079 1010 49 0.01 M CaCl2)
_
Iwo 5013 1341 27
Ikom 5701 2040 36
Apomu 2606 970 37
Alagba 1833,4510 875,2361 48,52
Onne 936,1456 565,508 60,35
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Adsorption coefficients for paraquat on differing sediment
fractions(10)
Sed. Sed. Midpt. ppm meq/100g Av.coeff
frac. conc. Kp Kp range Xc CEC of varia
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Sand 200 300-500 400 380 0.47 0.33
Sand 100 300-750 530 390 0.47 0.39
C.silt 20 20000-30000 25000 5900 4.7 0.15
M.silt 10 30000-40000 35000 7500 6.17 0.09
F.silt 5 150000-350000 250000 10000 7.35 0.08
Clay 1 500000-1000000 750000 17000 17.39 0.21
Clay 0.1 500000-1000000 750000 21000 17.39 0.17
(C=coarse,M=medium,F=fine)
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Paraquat adsorbed on clays with differing ammonium concentrations as a
competing cation(12)
Paraquat adsorbed (meq)
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P.add % meq NH4+/200ml
Soil (meq) SAC 0.2 2.0 20.0 200 400 1000
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Mont. 0.391 65 0.391 0.391 0.391 0.391 0.391 0.390
Kaol. 0.0117 58.5 0.0117 0.0117 0.0117 0.0095 0.0099 0.0076
Loam 0.117 57.5 0.117 0.117 0.117 0.105 0.0915 0.0839
Loam 0.0391 19.1 0.0391 0.0391 0.0391 0.0387 0.0387 0.0342
Loam 0.0156 7.6 0.0156 0.0156 0.0156 0.0156 0.0156 0.0143
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Maximum paraquat adsorbed on original and organic matter free soils of
differing SAC (all values in meq/100g)(12)
Original Soil O.M.Free Soil
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Max.P. Max P.
Soil SAC adsorbed CEC SAC adsorbed CEC
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LS 0.19 2.8 5.9 0.08 1.1 1.4
LS 0.78 3.6 5.7 0.78 2.8 4.9
L 4.1 9.8 13.8 3.6 7.0 9.4
CL 4.7 15.6 21.1 4.1 9.3 17.4
C 13.7 25.7 33.7 7.8 17.1 27.9
SL 0.78 2.02 2.88 0.78 2.18 2.88
C 1.20 4.4 10.1 1.17 2.7 6.0
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IV. References (*denotes key reference)
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1.Best, J.A., J.B. Weber and S.B.Weed. 1972. Soil Science. 114(6).
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444-50.
*2.Burns, R.G. and L.J. Audus. 1970. Weed Res. 10. 49-58.
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3.Burns, I.G., M.H.B. Hayes. 1974. Residue Reviews. 52. 117-46.
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*4.Burns, I.G., M.H.B. Hayes, M. Stacey. 1973. Weed Res. 13.
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67-78(I).
*5.Burns, I.G., M.H.B. Hayes and M. Stacey. 1973. Weed Res.
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79-90(II).
*6.Calderbank, A. and T.E. Tomlinson. 1968. Outlook on Agriculture.
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5(6). 252.
*7.Damanakis, M., D.S.H. Drennan, J.D. Fryer, and K. Holly. 1970.
Weed Res. 10. 264-77.
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*8.Juo. A.S.R. and O.O. Oginni. 1978. JEQ. 7(1). 9-12.
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9.Kahn, S.U. Pesticides in the Soil Environment. Amsterdam:Elsevier.
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1980.
*10.Karickhoff, S.W. and D.S. Brown. 1978. JEQ. 7(2). 246-52.
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11.Knight, A.G. and P.J. Denny. 1970. Weed Res. 10. 40-48.
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12.Knight, B.A.B. and T.E. Tomlinson. 1967. J.SoilSci. 18(2).
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233-43.
13.Watkin, E.M. and G.R. Sagar. 1971. Weed Res. 11. 247-56.
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14.Weed, S.B. and J.B. Weber. 1969. SSSAP. 33. 379-82.
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