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Environmental fate & pathways

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Diquat C16 -18 sorbs strongly to negatively charged surfaces like glassware, soil and sediment constituents. For three different soils, Kd values were observed ranging from: 1.6 * 10^4 to 1.9 * 10^5 L/kg. Biodegradation is considered to be the main removal mechanism of this substance. Most ready biodegradability tests are however hampered by the biocidal activity of these substances. In a SCAS (inherent ) test only partial degradation of the diquat C16 -18 was observed. Based on mass balance calculations (based on Carbon and Nitrogen consumption) considering the degradation of impurities and the ready biodegradability of the monoquat, the most likely metabolite structures are a diquat with an alkyl chain length of 4 or 2 carbons and an carboxylic acid at the end. Effluent tests with about 15 mg/L of this persistent metabolite present showed no effects in an acute daphnia test (Kean , 2010a) and an algae test (Kean, 2010b).

In summary: The parent diquat C16 -18 is quickly degraded into a metabolite which is considered to be persistent as a worst-case. The metabolite formed is more water soluble and where the parent is very toxic to aquatic organisms the metabolite formed has a low observed toxicity to aquatic organisms.

The half-life of the parent in the different environmental compartments is estimated to be strongly influenced by the bioavailability of the substance. No data is available for the determination of the half-life of diquat C16 -18 in soil or sediment. These values are therefore as a worst-case based on the readily biodegradability of the available fraction and the sorption data as determined in a sorption desorption test.

Table Summary of degradation rate constants of the parent compound, N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3 -propanediammonium chloride in various (eco)systems.




Surface water (fresh)

TGD default value

15 days half-life

Surface water (fresh) sediment

TGD default value

30000 days half-lifea

Marine water

TGD default value

50 days half-life

Marine water sediment

TGD default value

30000 days half-lifea


TGD default value

30000 days half-lifea

Degradation in sewage treatment plants

Determined in bioreactors

>99.9% removal primarily by biodegradation

aHalf-life of the fraction dissolved in the water phase is expected to be in the order of a few days.

Diquat C16 -18 has a short predicted half-life in air but because there are no important releases into the atmosphere and volatilisation is expected to be negligible, this removal mechanism is thought to be of low relevance.

Diquat C16 -18 does not contain hydrolysable covalent bonds. Cleavage of a carbon-nitrogen bond under environmental conditions is only possible with a carbonyl group adjacent to the nitrogen atom. Degradation of diquat C16 -18 through hydrolysis is therefore not considered.

Direct photolysis of diquat C16 -18 in air/water/soil will not occur, because it does not absorb UV radiation above 290 nm. Photo transformation in air/water/soil is therefore assumed to be negligible.

Standard OECD 305 tests are technically not feasible with these strongly sorbing easily degradable substances. In addition is the route of exposure in a standard OECD 305 test unrealistic for these substances because the substance will either be sorbed or biodegraded. The bioaccumulation potential of diquat C16 -18 was therefore assessed based on a measured log Kow. This log Kow of 0 is measured in an OECD 123 slow-stirring test. This low measured Log Kow indicates a low bioaccumulation potential.

The predicted low bioaccumulation potential is supported by the low acute to chronic ratio observed in the long-term daphnia test.

The daphnia reproduction test result shows that at 810 μg/L all parental daphnids were immobile within two days, without reproduction, while at the next concentration of 270 µg/L there is no detrimental effect on reproduction for the surviving daphnids when compared to the control. These observations result in the derivation of a NOEC of 270 µg/L for reproduction resulting in a low acute-to-chronic ratio. A low acute-to-chronic ratio is indicative of a non-specific mode of action and is often associated with not systemic effects. This observation is consistent with the known effects of cationic surfactants on aquatic organisms, where toxicity is associated with physical binding to respiratory membranes. This explains the steep concentration curves seen and the lack of intermediate chronic effects on reproduction.