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Environmental fate & pathways

Endpoint summary

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Bioconcentration factors (BCF) are preferentially based on kinetic parameters (uptake and depuration rates), but also BCF values derived as the ratio of Ag concentration in biota over Ag concentration in water are accepted in case there is evidence for equilibrium. All bioconcentration factors are expressed on a wet weight (ww) basis and are preferentially derived for whole body or soft tissues (ECHA, 2017). In case only dry weight (dw) based concentrations were reported, a default water content of 90% is assumed for calculation of the wet-weight based bioconcentration factors.

There are many studies in the literature relevant to the bioaccumulation of silver. However, many of these studies do not report sufficient information for BCF values to be calculated. In addition, many of the data were obtained from studies of short duration which did not achieve steady-state conditions. The available reliable studies suggest low BCF values for fish, although studies of the accumulation of silver by molluscs, such as Hedouin et al. (2006), indicate the potential for higher BAF values for these organisms.

Fish are the preferred species for bioaccumulation assessment (ECHA, 2017) and high-quality data are available for 4 freshwater and 2 marine fish species. Bioconcentrations factors for fish vary between 0.4 and 327 L/kg ww and are all well below the threshold of 500 L/kg ww for the potential to bioconcentrate for classification purposes (ECHA, 2017). There is no significant difference between results for freshwater and marine fish species. The study selected as key study in IUCLID for this endpoint was Baudin et al. (1993) in which carp were exposed to silver in water for a six-week period, followed by a depuration phase. A steady state concentration in fish was achieved after 30 days exposure and a BCF value of 70 was calculated for the whole fish on a wet weight basis.

The available data for BCF derived at different exposure concentrations (Zimmermann et al., 2017; Kuehr et al., 2020; Ribeiro et al., 2017 and Hu et al., 2018) show higher BCF at lower exposure concentrations. This supports the inverse relation between BCF and water concentrations observed for inorganic metal compounds (McGeer et al., 2003). Although Ag is not an essential element, this inverse relation is explained by the uptake mechanisms for ionic substances (active transport or facilitated diffusion), which are regulated by carriers in the cell membrane causing saturation properties of the uptake kinetics. Due to their dependence on external factors (exposure concentrations), bioaccumulation factors are not an intrinsic property for metals and the magnitude of the BCF or BAF is not an indication for potential hazards. This is in contrast with non-polar organic chemicals, which are generally taken up by passive transport and where bioaccumulation can be estimated from the octanol-water partition coefficient (Kow). Internal Ag concentrations further depend on internal storage and depuration, all regulated processes. Internal storage of silver is moreover often in forms that are biologically unavailable (e.g., granular forms or bound to metallothionein). It must be noted that the largest BCF values observed (27500 L/kg ww for the freshwater snailLymnaea stagnalisand 7500 L/kg ww for the marine bivalve Gafrarium tumidum) were both derived at the lowest exposure concentrations (<0.1 μg Ag/L), below toxicity thresholds for these species (Arijs et al., 2021).

The observation that BCF values for fish are generally lower than BCF values for organisms belonging to lower trophic levels is a strong indication that there is no biomagnification of silver in the foodchain. This observation is confirmed by the available biomagnification factors derived for the algae-daphnia foodchain.

Read-across from the dissolved silver ion is applied to fulfil information requirements for silver compounds. Supporting information for this read-across is summarised in endpoint summaries and here below.