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Environmental fate & pathways

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The data for the assessment of the environmental fate properties of C16-C18 TMAC is based on the followingalkyltrimethylquaternary ammonium compounds (TMACs) with differing alkyl chain lengths and benzyl alkyldimethyl quarternary ammonium compound (ADBAC):

 

Alkyltrimethylquaternary ammonium compounds (TMACs) used for read-across:

  • quaternary ammonium compounds, coco alkyltrimethyl chlorides (R=C12-C18)
  • quaternary ammonium compounds, cetrimonium chloride (R= C16)
  • quaternary ammonium compounds, tallow alkyl trimethyl ammonium chloride (R=C16-C18 and C18 and unsaturated)
  • quaternary ammonium compounds, octadecyl trimethyl ammonium chloride (R=C18)

 

Benzyl alkyldimethyl quaternary ammonium compounds (ADBACs) used for read-across:

·        quaternary ammonium compounds, benzyl-C12-C16-alkyldimethyl, chloride (R=C12-C16)

 

 

The read-across is justified since both TMACs and ADBAC compounds have comparable core chemical structures. They are characterized by a positively charged ammonium group. In the case of TMACs, the nitrogen is bound to three methyl groups and the longer alkyl chain and in the ADBAC, one methyl group is replaced with abenzyl substituent. The positively charged quaternary ammonium group is balanced by a negatively charged chloride anion. The structural similarity results in similar physicochemical properties which predominantly determine the environmental fate of this class of substances.

Hydrolysis in combination with biodegradation is considered to be the main removal mechanisms of the quaternary ammonium salts (QAS). As there are no important releases into the atmosphere and due to their low vapour pressure; negligible volatilisation, these removal mechanisms are not considered relevant.

From an environmental fate perspective, an important property of the substances is its cationic nature. This results in a strong adsorption to negatively charged surfaces present in soil or sediments. This is reflected by the mean Kd value for 13,630 L/kg for 3 different types of soil that have been measured for the read-across substance C12-C16 ADBAC which covers the large portion of the C-chain distribution in this category. Hence this value can be read across to the other QAS substances which have not been specifically tested for this purpose.

As stated before, hydrolysis in combination with biodegradation is considered to be the main removal mechanism of the QAS substances. Representing the QAS category, hydrolysis testing has been conducted for quaternary ammonium compounds, C12-C18 (even numbered)-alkyltrimethyl chloride (C12-C18 TMAC) and C12-C16 ADBAC which have shown half-lives at pH 5, 7, and 9 and pH 4, 7, 9 of > 30 days and 1 year respectively.

The route of degradation by microorganisms has been well studied for various quaternary ammonium compounds. The cleavage of the C alkyl-N bond in TMAC is the initial step in degradation. Also studies on C12-16-ADBAC indicate cleavage of the C alkyl-N bonds as the first step. The long alkyl chains are used sources for microbial growth. After being split-off, the alkyl chains are channelled into the ß-oxidation cycle. All available data indicate that QAS substances are readily biodegradable and share the same degradation pathways. All adsorb strongly to soils thus limiting their biodegradation and preventing toxicity to organisms. The DT50in soil is about 40 days.

QAS members exhibit good hydrolytic stability in water and are easily soluble in water. Their ready biodegradability in water, makes these substances neither persistent nor do they produce problematic metabolites. In natural circumstances QAS will adsorb strongly onto soil and will not desorb very easily, therefore the bioavailability will be low for terrestrial organisms. No leaching further into soils or ground water is anticipated; QAS members can be considered to be immobile in soil.

The low octanol/water partition coefficients and the result of the estimated and experimental BCF values of ≤79 indicate low bioavailability in the environment. Furthermore, due to the positively charged structure, these types of substances easily adsorb to any available negatively charged material which also results in a lowered bioavailability in the environment.