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Paraquat

DESIGNATIONS

CAS No.: 1910-42-5; 4685-14-7 (Paraquat dichloride)
Registry names: Paraquat
Chemical name: 1,1'-Dimethyl-(4,4'-bipyridinium) cation
Synonyms, Trade names: 1,1'-Dimethyl-(4,4'-bipyridinium) [dichloride], N,N'-dimethyl-4,4'-bipyridinium chloride, methylviologen, Gramoxone, Gramoxon, Terraklene, Weedol, Dextrone X and many others
Chemical name (German): Paraquat [Dichlorid];1,1'-Dimethyl-(4,4'-bipyridinium) [Dichlorid]
Chemical names (French): [Dichlorure de] paraquat, [dichlorure de] 1,1'-dimthyl-(4,4'-dipyridinium)
Appearance: pure paraquat salt is a white, crystalline, odourless powder; the technical product has a yellowish appearance.

BASIC CHEMICAL AND PHYSICAL DATA

Note:

All data refer to the dichloride.

Empirical formula:

C12H14N2 [ C12H14Cl2N2 ]

Rel. molecular mass:

257.2 g

Density:

1.25 g/cm3 at 20C

Relative gas density:

8.88

Boiling point:

decomposition

Melting point:

above 300C decomposition

Vapour pressure:

very low (<10-3 Pa at 20C; also 10-5 Pa at 20C cited)

Ignition temperature:

(may be marketed in flammable mixtures)

Solvolysis/solubility:

in water 700 g/l at 20C; soluble in alcohol; insoluble in organic solvents

Conversion factors:

1 ppm = 10.7 mg/m3

1 mg/m3 = 0.094 ppm

ORIGIN AND USE

Usage:
Paraquat is a non-selective contact herbicide used in particular in the cultivation of fruit and vines to combat weeds directly before cultivating useful plants. The sulphate or chloride is normally used. There is a ban in Germany on substances which have only paraquat as active substance for treating plants.

Origin/derivation:
Paraquat is produced synthetically. There are no known natural sources. Paraquat is part of the bipyridinium derivative group. Two technical products are manufactured: 1,1'-dimethyl-4,4'-bipyridinium dichloride and 1,1'-dimethyl-4,4'-bipyridinium dimethyl sulphate. The product is synthesised from pyridine, by treatment with sodium in liquid ammonia, followed by oxidation to form 4,4'-bipyridine, and subsequent methylation.

Production figures:
Paraquat is produced in a number of countries (e.g. People's Republic of China, Taiwan, Italy, Great Britain, USA). It is applied in more than 130 countries (normally as paraquat dichloride, in the (former) USSR as paraquat dimethyl phosphate). Production figures are not available (WHO, 1984).

Toxicity

Humans:

DTA 0.008 mg/kg

acc. DFG, 1985

Mammals:

Mouse:

LD50 100-120 mg/kg, oral

acc. DVGW, 1988

Mouse:

LD50 62 mg/kg, dermal

acc. WHO, 1984

Rat:

LD50 100-150 mg/kg, oral

acc. DVGW, 1988

Rat:

LD50 80-90 mg/kg, dermal

acc. DVGW, 1988

Rat:

LC50 1-10 mg/m3, inhalation

acc. WHO, 1984

Dog:

LD50 25-50 mg/kg, oral

acc. DVGW, 1988

Cat:

LD50 30-50 mg/kg, oral

acc. DVGW, 1988

Rabbit:

LD50 120-130 mg/kg, oral

acc. DVGW, 1988

Rabbit:

LD50 236-500 mg/kg, dermal

acc. WHO, 1984

Guinea pig:

LD50 20-40 mg/kg, oral

acc. DVGW, 1988

Guinea pig:

LD50 319 mg/kg, dermal

acc. WHO, 1984

Guinea pig:

LC50 4 mg/m3, inhalation

acc. WHO, 1984

Monkey:

LD50 50 mg/kg, oral

acc. WHO, 1984

Aquatic organisms:

Guppy:

LC50 21.8-46.4 mg/l (96h)

acc. DVGW, 1988

Sun perch:

LC50 100 mg/l (48h)

acc. DVGW, 1988

Sun perch:

LC50 12 mg/l (96h)

acc. DVGW, 1988

Trout:

LC50 4.5-32 mg/l (96h)

acc. DVGW, 1988

Crawfish:

LC50 11 mg/l (96h)

acc. DVGW, 1988

Characteristic effects:

Humans/mammals: Paraquat is toxic for humans and damages the kidneys, liver and lungs. In aqueous form the herbicide has a caustic effect on the skin, mucous membranes and conjunctiva; it is absorbed through the skin. There is also good absorption of paraquat via the lungs where the substance is stored. However, resorption is usually minimal (5% of the amount absorbed) (DVGW, 1988). Metabolisation in the body is likewise very limited; most of the substance is excreted without undergoing change.

Animal experiments have not revealed any increase in the tumour rate due to paraquat. Genetic toxicity and teratogenic effects have been substantiated with certain species of animal.

Plants: Paraquat is absorbed by plant leaves only from the aqueous solutions of its salts. Absorption takes place more readily and more effectively in the dark than in daylight. It is transported with the perspiration flow in the xylem. Phytotoxicity is only encountered in daylight and with sufficient oxygen content. There is a sustained

ENVIRONMENTAL BEHAVIOUR

Water:
Paraquat is adsorbed on suspended matter and on sediment in water or absorbed by plants. The herbicide finds its way into the water either following direct usage or by erosion from neighbouring areas. If paraquat is not subject to sorption, it is rapidly degraded by microorganisms and thus no longer effective. Because of this, there is only a low risk of groundwater pollution.

Air:
Small quantities of paraquat are released into the air as a function of the type of application (as spray or mixed with liquid). The crucial factor is, however, whether paraquat is inhaled during application or resorbed through the skin. Passage through the skin represents the principal way of assimilation. The half-life in air is reported between a period of several hours up to 64 days.

Soil:
Paraquat is subject to considerable sorption in clay soils. In topsoils with a high content of organic matter, it is more or less immobile and then it may not be absorbed by plants. Depending on the type of soil, paraquat remains in the uppermost soil horizon. Because of the high sorption rate, there is no elution by seepage water and no metabolisation by microorganisms.

Half-life:
When used in water, paraquat could not be detected after 1-4 days; residues were still found in sludge after more than 400 days. The half-life in sandy clays is about 7 years.

Degradation, decomposition products:
Photochemical degradation results in less toxic decomposition products. Biological degradation in the soil leads to rapid decline of the amount applied, but also to a reduction in the population density of the microorganisms.

Food chain:
No accumulation by way of food chains.

ENVIRONMENTAL STANDARDS

Medium/acceptor Sector Country/organ. Status Value Cat. Remarks Source
Water: Drinkw D

L

0.1 g/l

    acc. DVGW, 1988
Drinkw EC

L

0.1 g/l

    acc. DVGW, 1988
Surface EC

L

0.001 mg/l

  1) acc. DVGW, 1988
Surface EC

L

0.0025 mg/l

  2) acc. DVGW, 1988
Surface EC

L

0.005 mg/l

  3) acc. DVGW, 1988
Air: Workp BG

(L)

0.01 mg/m3

    acc. WHO, 1984
Workp D

L

0.1 mg/m3

MAK   acc. DFG, 1994
Workp H

(L)

0.02 mg/m3

    acc. WHO, 1984
Workp USA

(L)

0.1 mg/m3

TWA   ACGIH, 1986

Notes:
The use of paraquat in the cultivation of cereals is banned in the Federal Republic of Germany (Order Governing Use of Pesticides, as at 1988).
1) Limit value for physical treatment and sterilisation of drinking water
2) Limit value for physical and chemical treatment/sterilisation of drinking water
3) Limit value for physical and refined chemical treatment of drinking water

Assessment/comments

Paraquat is a strong contact herbicide; inhalation and skin contact should be avoided when using it. Use of the substance is not recommended because of the high toxicity for humans and animals. This recommendation is further strengthened by the high persistence in soil.


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