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Based on studies with analogous substances, i.e. zinc salts of fatty acids with alkyl chain length ranging from C8 to C18, Fatty acids, C14-18 and C16-18-unsatd., zinc salts can be considered readily biodegradable: In a Closed Bottle test according to OECD 301D, 93% of the analogue substance Fatty acids, C16-18, zinc salts was biodegraded after 28 days. This value was calculated from the BOD, the biological oxygen demand, and the ThOD, the theoretical oxygen demand. Fatty acids, C16-18, zinc salts is readily biodegradable as the 60% level was passed within 28 days but failed the 10 day window criteria. Therefore, a similar biodegradability is assumed for Fatty acids, C14-18 and C16-18-unsatd., zinc salts.

A similar zinc salt of a fatty acid (i. e. zinc bis[12-hydroxyoctadecanoate]) biodegraded up to 71% after 28 days as determined in the OECD 301B test with 14.5% after 4 days and 53.5% after 13 days. Zinc bis[12-hydroxyoctadecanoate] 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 similar zinc compounds.

In addition, data are available on the biodegradation of the shorter-chained (C8) octanoic acid, zinc salt, basic (CAS 90480-58-3; consisting of approximately 77% natural fatty acids and 23% zinc). In a static Manometric Respirometry Test (OECD 301 F), the biodegradation of octanoic acid, zinc salt was determined with 80% and 86% at 30 mg/L and 100 mg/L after 28 days, respectively, meeting the 10-day window criteria. Thus, octanoic acid, zinc salt, basic, i.e. a zinc salt of a shorter-chained (C8) fatty acid, is also readily biodegradable.

Substance Fatty acids, C14-18 and C16-18-unsatd., zinc salts is produced by the oleochemistry sector, starting from natural fatty materials, and constitutes a mixture of stearic (C18), palmitic (C16) and myristic acid (C14) and unsaturated C16-18 fatty acids. Fatty acids, C14-18 and C16-18-unsatd., zinc salts consist of 88.8% natural fatty acids and 11.2% zinc. 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 being formed in the environment.

The risk assessment of a structural analogue (i.e. similar fatty acid zinc salt) concluded (EU RAR Zinc distearate (CAS-No.: 557-05-1 & 91051-01-3 EINECS-No.: 209-151-9 & 293-049-4) Part 1 - Environment (Final report R074_0805_env, May 2008): "Once emitted into the environment, zinc distearate will partly dissociate into the zinc cation and the stearic [CH3(CH2)16COO-] and palmitic [CH3(CH2)14COO-] anions, especially in an acidic environment. The further speciation of zinc, which includes complexation, precipitation and sorption, and the environmental fate of the fatty acids depend on the environmental conditions. In the presence of other cations such as calcium and magnesium, zinc distearate will partly form other “insoluble” stearates (e. g. Schmets, 1996). Stearic and palmitic acid as such are readily biodegradable, although the degradability can be inhibited by the formation of insoluble salts (e. g. calcium, magnesium and zinc distearates), that are not readily biodegradable (BKH, 1994; Schmets, 1996)3. This is confirmed in a OECD 301D study. "

Thus, similar biodegradation rates are observed in different tests with zinc salts of shorter-chained (C8) and similar alkyl chain lengths (C16-18). Therefore, by read-across from structural analogue substances (i.e. similar fatty acid salts), it is concluded that Fatty acids, C14-18 and C16-18-unsatd., zinc salts is readily biodegradable.

However, only the fatty acid moiety is biodegradable in the proper sense. 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 in water, 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 waters (EUSES). Consequently, zinc is considered as equivalent to being ‘rapidly degradable in the context of classification for chronic aquatic effects. For further details, please refer to the respective endpoint summary of the assessment entity "Zinc" and reports by Mutch Associates, LLC, 2010 a, b.

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