Reaction mass of zinc oxide and zinc peroxide

Endpoint summary

Additional information

No environmental fate data are available for the target substance Zinc peroxide. However, information is available on (ionic) Zinc and hydrogen peroxide. A justification for read-across is attached to IUCLID section 13.

Zinc is a natural element, which is essential for all living organisms. It occurs in the metallic state, or as zinc compound, with one valency state (Zn2+). All environmental concentration data are expressed as “Zn”, while toxicity is caused by the Zn2+ ion. For this reason, the sections on human toxicity and ecotoxicity are applicable to all zinc compounds, from which zinc ions are released into the environment. Some zinc compounds have however very low solubility and will therefore not release zinc ions; this strongly decreases their potential (eco-)toxicity. As a consequence, distinction for toxicity and ecotoxicity is being made between zinc compounds, as a function of their solubility.

For checking the potential of metal substances to release ions in the environment, a specific test, the transformation/dissolution (T/D) test is applied.

The issue of degradation is not applicable to inorganic compounds. However, the speciation of zinc in the environment compartments is relevant.

When zinc ions are formed in the environment, they will further interact with the environmental matrix and biota. As such, the concentration of zinc ions that is available to organisms, the bioavailable fraction, will depend on processes like dissolution, absorption, precipitation, complexation, inclusion into (soil) matrix, etc. These processes are defining the fate of zinc in the environment and, ultimately, its ecotoxic potential. This has been recognised e.g. in the guidance to the CLP regulation 1272/2008 (metals annex): “Environmental transformation of one species of a metal to another species of the same does not constitute degradation as applied to organic compounds and may increase or decrease the availability and bioavailability of the toxic species. However as a result of naturally occurring geochemical processes metal ions can partition from the water column. Data on water column residence time, the processes involved at the water – sediment interface (i. e. deposition and re-mobilisation) are fairly extensive, but have not been integrated into a meaningful database. Nevertheless, using the principles and assumptions discussed above in Section IV.1, it maybe possible to incorporate this approach into classification.“

 

In the water, the bioavailability of zinc through interaction with components of the water and biota has been studied in detail in the zinc RA (ECB 2008). This has resulted in an approach for quantifying zinc bioavailability into risk assessment. The ultimate fate of zinc in water (in the water column) is assessed via the “unit world model”, that can quantify the “removal from the water column” of the zinc species. As such, it is shown that zinc (ions) brought into water will be rapidly removed from the water column (>70% removal within 28 days).

 

In sediment, zinc binds to the sulphide fraction to form insoluble ZnS. As such, zinc is not bioavailable anymore to organisms. This has been discussed in the EU RA (ECB 2008), and has resulted in an approach for quantifying zinc bioavailability into risk assessment. Based on experimental data, a default conservative bioavailability factor of 0.5 was proposed in the RA. This approach can be refined when the relevant data on sulphide and Zn in sediment are available. Due to the insolubility of the ZnS (K=9.2E-25) zinc will be sequestered in the (anaerobioc) sediments, and the re-mobilisation of zinc ions into the water column will be prevented.

Hydrogen peroxide, which is released in small amounts from Zinc peroxide as demonstrated in the transformation/dissolution test, is a quite reactive substance in the presence of other substances, elements, radiation, materials or cells. Both biotic and abiotic degradation processes are important routes in the removal of hydrogen peroxide in the environment.

Biological degradation of hydrogen peroxide is an enzyme-mediated process. Abiotic degradation of H2O2 is due to reaction with itself (disproportionation), reaction with transition metals, organic compounds capable to react with H2O2, reaction with free radicals, heat or light.

Hydrogen peroxide is normally a short-lived substance in the environment. Rapid degradation will occur due to many alternative and competitive degradation pathways.

Due to the high water solubility of hydrogen peroxide,no bioaccumulation is expected. The estimated log Kow of about -1.5 indicates negligible potential of bioconcentration in aquatic organisms. BCFs calculated according to the TGD for fish and earthworm are low, 1.4 and 3.3, respectively. (EU RAR, 2003)