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Environmental fate & pathways

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The extent to which bioaccumulation may occur depends on the bioavailability of cerium for uptake. Bioavailability is affected by the substance’s water solubility as well as by the characteristics of the receiving environmental compartment, determining cerium speciation. From the Pourbaix diagram for cerium (see more detailed explanation in the read across justification attached to IUCLID Section 13) it is clear that at environmentally relevant conditions Ce+4 is not stable in aqueous media, i.e., there will be no dissolved Ce+4 species. Depending on the conditions, Ce+4 can be (partly) reduced to Ce+3, of which certain species can exist in solution. Because only Ce+3 can exist in solution, bioaccumulation is expected to be solely or predominantly due to Ce+3 uptake. Therefore, field data and data from laboratory studies using soluble trivalent cerium compounds were considered relevant for evaluation of the environmental fate of cerium from tetravalent cerium compounds too (although cerium bioavailability (and hence uptake and bioaccumulation) may be relatively lower after release to the environment of a tetravalent cerium compound compared to after release of a similar amount (Ce-based) of a trivalent cerium compound).

The available information on bioaccumulation of cerium in the aquatic and terrestrial foodchain was thoroughly studied, yielding the following conclusions:

- In the aquatic foodchain, cerium has clearly the potential to bioconcentrate/bioaccumulate in organisms from lower trophic levels, such as aquatic plants and invertebrates. However, the available information on fish, including valuable information from a microcosm experiment and trophic transfer factors for organisms in several foodchains calculated based on field data, indicate that bioaccumulation of cerium decreases when ascending the foodchain. The values obtained for fish were very low and indicate that cerium does certainly not biomagnify and has a limited potential for bioaccumulation at higher levels in the foodchain. There is evidence that cerium promotes certain processes in living organisms. As for many other metals (even non-essential metals), organisms are able to regulate metal uptake and retainment, and bioaccumulation is expected to be limited. Also, a concentration dependency may exist for bioaccumulation of cerium in aquatic organisms, showing increasing BCF/BAF values with decreasing environmental concentrations (down to background levels). Indeed, for several field studies in which cerium exposure was very low, quite high BCF/BAF values were observed for aquatic plants or invertebrates.

- Based on a selection of reviews on the transfer of cerium and other rare earths from soil to plants, it could further be concluded that there is no potential for bioaccumulation of cerium in the terrestrial foodchain, as all BSAF values were < 1. BSAF values for a sediment dwelling amphipod and water spinach rooted in sediment were also consistently below 1.