Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

General introduction to chapters 4, 5, 6 and 7.

 

Under Regulation 793/93/CEE, an extensive risk assessment on Zinc and 5 zinc compounds (ZnO, ZnCl2, ZnSO4, Zn orthophosphate and zinc distearate) has been recently prepared by the Dutch authorities for the EU. The risk assessment report (RAR) on these 6 zinc substances has been recently published (ECB 2008).

Since these RARs were the result of intensive discussions between all stakeholders, and were approved by experts from all the member states; since they provide a recent review of the available evidence on zinc and zinc compounds (the file was closed in September 2006), they will be used as the main reference for this chemical safety report.

In this chemical safety report, the information, data and conclusions of the RAR will be summarised, focusing on the principles applied, the assumptions made and the conclusions. Where available and relevant, new information and data will be included and discussed.

 

General remarks on the chapter on environmental fate properties.

 

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 (Zn++). All environmental concentration data are expressed as “Zn”, while toxicity is caused by the Zn++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 (see chapters 5 and 7).

For checking the potential of metal substances to release ions in the environment, a specific test, the transformation/dissolution (T/D) test is used. For metallic zinc and some of the zinc compounds, this test has been performed. If applicable, the results of such T/D test are discussed in section 4.6. (data in IUCLID section 5.6.).

The issue of degradation (section 4.1.) is not applicable to inorganic compounds. However, the speciation of zinc in the environment compartments is relevant and is discussed under section 4.2.

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 28days). This phenomenon is described in section 4.6. (data in IUCLID 5.6), and is considered for classification.

 

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=9.2 x 10-25) zinc will be sequestered in the (anaerobioc) sediments, and the re-mobilisation of zinc ions into the water column will be prevented. This is also quantified in the unit world model, see section 4.6.

 

In soil, short-term interaction of zinc ions upon spiking, and long term interactions (“ageing”) have been extensively discussed in the zinc RA (ECB 2008). This has resulted in an approach for quantifying zinc bioavailability into risk assessment. Based on experimental data, a general ageing factor of 3 was derived in the RA; according to soil type, the bio-availability of zinc can be further determined, when the relevant data on e.g. pH, CEC are available.