Zinc didocosanoate

Endpoint summary

Description of key information

In the assessment of environmental fate of zinc didocosanoate, read-across to analogue substances and the assessment entities soluble zinc substances and docosanoic acid is applied since the ions of zinc didocosanoate determine the environmental fate of zinc didocosanoate.

 

Zinc didocosanoate is produced by the oleochemistry sector, starting from natural fatty materials, i.e. docosanoic/behenic acid (C22), and consists of up to 91.5% natural fatty acid and 9.9 % zinc. It is a zinc salt of a fatty acid containing 22 C-atoms. Thus, read-across of data available for zinc salts of other fatty acids (C16 -18) based on structural similarity, water solubility and zinc content in is assumed to support conclusions on the biodegradability of zinc didocosanoate.

 

In the Closed Bottle test according to OECD 301D, 93% of zinc stearate (i.e. Fatty acids, C16 -18, zinc salts; CAS 91051-01-3) was biodegraded after 28 days. Zinc stearate is readily biodegradable as the 60% level was passed within 28 days but failed the 10 day window criteria. Another zinc salt of a C-18 fatty acid (i.e. zinc-12-hydroxystearate, CAS 35674 -68 -1) biodegraded up to 71% after 28 days in the OECD 301B test with 14.5 % after 4 days and 53.5% after 13 days. Zinc-12-hydroxystearate is also readily biodegradable as the 60% level was passed within 28 days but failed the 10-day window criteria. Thus, similar biodegradation rates are observed in different tests with zinc salts of other fatty acids (C16 -18).

 

Fatty acids biodegrade readily according to the β-oxidation pathway including three major steps (activation in the cytosol, transport into mitochondria, β-Oxidation until the entire C-chain is cleaved into acetyl CoA units). Thus, stable degradation products are not formed in the environment. According to the QSAR-based prediction of BIOWIN (v 4.10), docosanoic acid is readily biodegradable. Docosanoic acid is within the applicability domain of “BIOWIN v.4.10”. The conditions for ready biodegradability are satisfied since the Biowin5 (MITI linear model) probability is > 0.5 and the Biowin3 (ultimate survey model) result is "weeks".

 

However, only the fatty acid moiety (i.e. docosanoate) is biodegradable in the proper sense. Fatty acids, including docosanoate i, are not persistent in water, and transformation products of environmental concern are also not expected. The concept of “biodegradability” has been developed for organic substances and is not applicable to inorganic substances, including zinc. As a surrogate approach for assessing “degradability”, the concept of “removal from the water column” has been developed to assess whether or not a respective metal ion would remain present in the water column upon addition (and thus be able to exert a chronic effect) or would be rapidly removed from the water column. In this concept, “rapid removal” (defined as >70% removal within 28 days) can be considered equivalent to “rapid degradation”. For zinc cations, information is available on the removal of zinc from the water column. The removal from the water column was modelled referring to the EUSES model parameters and different conditions of pH. Zinc is removed by > 70% under the reference conditions for the EU regional water (EUSES) (see Mutch Associates, LLC, 2010a,b). Consequently, zinc is considered as equivalent to being ‘rapidly degradable in the context of classification for chronic aquatic effects.

 

In sum, the moieties of zinc didocosanoate are biodegraded or removed from the water column.