Zirconium praseodymium yellow zircon

Bioaccumulation: aquatic / sediment

Administrative data

Endpoint:

bioaccumulation in aquatic species: fish

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

the study does not need to be conducted because the substance has a low potential to cross biological membranes

Justification for type of information:

JUSTIFICATION FOR DATA WAIVING
According to Column 2 of Information Requirement 9.3.2., Annex IX, Commission Regulation (EU) 1907/2006, ”The study need not be conducted if: the substance has a low potential for bioaccumulation (for instance a log Kow ≤ 3) and/or a low potential to cross biological membranes.”

Zirconium praseodymium silicon zircon can be considered environmentally and biologically inert due to the characteristics of the synthetic process (calcination at a high temperature of approximately 1000°C), rendering the substance to be of a unique, stable crystalline structure in which all atoms are tightly bound and not prone to dissolution in environmental and physiological media. This assumption is supported by available transformation/dissolution data (Grane, 2010) that indicate a very low release of pigment components. Transformation/dissolution of zirconium praseodymium silicon zircon (24-screening test according to OECD Series 29, loading of 100 mg/L) resulted in mean dissolved praseodymium concentrations of 3.05 µg/L Pr and 21.66 µg/L Pr, silicon concentrations of 0.13 µg/L Si and 0.02 µg/L Si at pH 6 and 8, respectively, whereas dissolved zirconium concentrations remained below the LOD (< 0.08 µg/L Zr). Since silicon does not have an ecotoxic potential, as confirmed by the absence of respective ecotoxicity reference values in the Metals classification tool (MeClas) database, and the dissolution of praseodymium is highest at pH 6, pH 6 is considered as pH that maximised metal release. Metal release at the 1 mg/L loading and pH 6 resulted in dissolved concentrations of 2.10 µg/L Pr and 0.17 µg/L Zr after 7 days and 0.79 µg/L Pr and < 0.08 µg/L Zr (< LOD) after 28 days whereas silicon concentrations remained below the LOD (< 0.07 µg/L Si) during the test. Thus, the rate and extent to which zirconium praseodymium silicon zircon produces soluble (bio)available ionic and other praseodymium-, silicon- or zirconium-bearing species in environmental media is limited. Hence, the pigment can be considered as environmentally and biologically inert during short- and long-term exposure. The poor solubility of zirconium praseodymium silicon zircon is expected to determine its behaviour and fate in the environment, including its low potential for bioaccumulation.

Further, “for naturally occurring substances such as metals, bioaccumulation is more complex, and many processes are available to modulate both accumulation and potential toxic impact. Many biota for example, tend to regulate internal concentrations of metals through (1) active regulation, (2) storage, or (3) a combination of active regulation and storage over a wide range of environmental exposure conditions. Although these homeostatic control mechanisms have evolved largely for essential metals, it should be noted that non-essential metals are also often regulated to varying degrees because the mechanisms for regulating essential metals are not entirely metal-specific (ECHA, 2008).”

Regarding the potential of bioaccumulation, the study of marine (seaweed, zooplankton, bivalves and fish) and terrestrial (Plants, fruits, liver of 15 wild avian and mammalian species) matrices by Squadrone et al. (2019) confirmed that rare earth elements, including praseodymium, “have a low potential for biomagnification, but instead are subject to trophic dilution”.

According to OECD (2004), “the bioavailable forms of silica (SiO2) are dissolved silica [Si(OH)4] almost all of which is of natural origin. The ocean contains a huge sink of silica and silicates where a variety of the marine habitat (diatoms, radiolarians, and sponges) is able to exploit this resource as a construction material to build up their skeletons”. Most organisms contain silicon at least at trace levels. Whereas silicon is essential for some organisms, including diatom algae, gastropods and mammals, and actively taken up, others take it up passively and excrete it.

“Due to the known inherent physico-chemical properties, absence of acute toxic effects as well as the ubiquitous presence of silica/silicates in the environment, there is no evidence of harmful long-term effects arising from exposure to synthetic amorphous silica/silicates (OECD, 2004).” Thus, given the ubiquitous presence of silica and silicates in the environment, silicon is regarded as element without or with a very low potential for bioconcentration and bioaccumulation.

Regarding the bioaccumulation potential of zirconium, Souza et al. (2020) evaluated abiotic and biotic matrices across six trophic levels (plankton, oyster, shrimp, mangrove trees, crabs and fish) in two neotropical mangrove estuarine ecosystems. Whereas biodilution of zirconium was observed at one location, a significant transfer was not be observed at the other. The lack of a potential for bioaccumulation may be explained (at least in part) with its very low solubility and mobility under most environmental conditions, mainly due to the stability of the principal host mineral zircon and the low solubility of the hydroxide Zr(OH)4 (Salminen et al. 2005).

Thus, based on the poor solubility of zirconium praseodymium silicon zircon in aquatic environments, the potential of zirconium praseodymium silicon zircon for bioaccumulation can safely be expected to be low. Consequently, the study on bioaccumulation does not need to be conducted based on low solubility, bioavailability and a corresponding low bioaccumulation potential of zirconium praseodymium silicon zircon in accordance with Column 2 of Information Requirement 9.3.2., Annex IX, Commission Regulation (EU) 1907/2006.

References:

ECHA (2008) Guidance on IR & CSA, Appendix R.7.13-2: Environmental risk assessment for metals and metal compounds. July 2008.

OECD (2004) SIDS Initial Assessment Profile Silicon dioxide, Silicic acid, aluminum sodium salt, Silicic acid, calcium salt. SIAM 19, 19-22 October 2004.

Souza et al. (2020) Trophic transfer of emerging metallic contaminants in a neotropical mangrove ecosystem food web. Journal of hazardous materials 408: 124424.

Squadrone et al. (2019) Rare earth elements in marine and terrestrial matrices of Northwestern Italy: Implications for food safety and human health. Science of the Total Environment 660: 1383–1391.