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Environmental fate & pathways

Endpoint summary

Administrative data

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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 (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 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 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 (

European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 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 (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, European Chemicals Bureau, Risk Assessment Report Zinc metal, Final report, 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.

The environmental fate and release of zinc and zinc compounds has been discussed extensively in the RAR (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008)

Environmental distribution in water

Zinc in freshwater or seawater can occur in both suspended and dissolved forms and is partitioned over a number of chemical species. Depending on the concentration of suspended matter, about 25-40 % of the zinc entering the surface water is in dissolved form, the remaining part is bound to the suspended matter. For toxicity, only the fraction not bound is important.

Dissolved forms of zinc in freshwater are e.g.: hydrated zinc ions, zinc ions complexed by inorganic or organic ligands (humic and fulvic acids), zinc oxy ions and zinc adsorbed to solid matter (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008).

Possible chemical forms of zinc in seawater are presented in the table below. In this table the variation in the percentages of total zinc can for instance be explained by analytical differences or by the different ion strengths of the examined seawaters.

Table: (taken from the RA zinc, (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008) Possible chemical forms (speciation) of dissolved zinc in seawater, (Cleven RFMJ, Janus JA, Annema JA and Slooff W (1993). Integrated criteria document zinc. RIVM Report No.710401028, Bilthoven, The Netherlands.[Cited from

EU (2004 a) Zinc metal. Risk assessment report, 2nd priority list, Volume 42, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy 

EU (2004 b) Zinc oxide. Risk assessment report, 2nd priority list, Volume 43, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy 

EU (2004 c) Zinc chloride. Risk assessment report, 2nd priority list, Volume 45, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy 

EU (2004 d) Trizinc bis(orthophosphate).Risk assessment report, 2nd priority list, Volume 47, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy 

EU (2004 e) Zinc sulphate. Risk assessment report, 2nd priority list, Volume 46, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy 

EU (2004 f) Zinc distearate. Risk assessment report, 2nd priority list, Volume 44, European Commission, Joint Research Center, Institute for Health and Consumer Protection, European Chemicals Bureau, Ispra, Italy])

 

 Percentage of total zinc            
 Zn species  Reference 1  Reference 2  Reference 3  Reference 4
 Zn2+  17  16.1  12.5  5.7
 ZnCln2 -n(n:1 -4)  11.4 63.7  79  17.8
 ZnOH+, Zn(OH)2  62.2  2.3  0.6  71.8
 ZnCO3  6  3.3  1.6  2.4
 ZnHCO3+  0.7  0.3  -  0.2
 ZnOHCl  -  12.5  -  -
 ZnSO4  4  1.9  1.6

 2.2

The speciation of zinc in the aquatic compartment is of high complexity and depends highly on abiotic factors, such as pH, (dissolved) organic matter content, redox potential, etc. It is assumed that speciation is very relevant for the migration of zinc through sediment, for the distribution of zinc among its truly dissolved and non-dissolved forms, and for the uptake of zinc by some aquatic and sediment organisms. The relationship between physicochemical factors driving the speciation of zinc in water, and the bioavailability, and consequently, the toxicity of Zinc has been experimentally elucidated and has been quantified in the biotic ligand model (BLM) (see further).

Environmental distribution in soil; adsorption/desorption of zinc in soil

Speciation of zinc in soil.

The speciation of zinc in soils has been extensively reviewed in the EU risk assessment on zinc (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008). The following is being summarised from the risk assessment: in soils, zinc interacts with various reactive soil surfaces. The most important in this respect are soil organic matter, amorphous soil oxides (Al, Fe, Mn) and clay minerals. The major process by which metals are bound to these surfaces is adsorption. Other processes including precipitation of carbonate type minerals can occur but are, in non- and moderately polluted soils, unlikely to control the solubility of metals in soils. An exception to this is the formation of sulphide minerals that are formed, in the presence of sulphate under reducing conditions.

Zinc in soil is distributed between the following fractions (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008):

1. Dissolved in pore water (which includes many species)

2. Exchangeable, bound to soil particles

3. Exchangeable, bound to organic ligands (of which a small part in the dissolved fraction and the major part in the solid fraction)

4. Present in secondary clay minerals and metal oxides/hydroxides

5. Present in primary minerals

So, zinc is present in the soil in various forms, that have varying degree of extractability. The soil pH is an important parameter that affects the speciation and the distribution of the zinc species over the soil and the solution. Zinc tends to be more sorbed and complexed at higher pH (pH > 7) than at lower pH. Below pH 7, the amount of zinc in solution was reported to be inversely related to soil pH (Janssen, RPT et al.(1997). Equilibrium partitioning of heavy metals in Dutch field soils. I. Relationship between metal partition coefficients and soil characteristics. Environ. Toxicol. Chem. 16, 2470-2478).

The pH of the soil not only determines the degree of complexation and adsorption of zinc, but also the solubility of the various zinc minerals. The solubility of zinc in soil decreases with increasing pH (Cleven RFMJ, Janus JA, Annema JA and Slooff W (1993). Integrated criteria document zinc. RIVM Report No.710401028, Bilthoven, The Netherlands.[Cited from EU 2004, a-f]).

After addition of a metal to a soil, often a slow decrease in the soil solution concentration, or the available fraction as determined in an extraction solution (e.g. by CaCl2) decreases as a result of (presumably) slow diffusion processes of metals into the matrix of the reactive surfaces. It is this process, or sum of as of yet poorly defined slow processes, that can be defined as ‘ageing’

(European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008).

The challenge is to develop models that scale from the theoretical and laboratory level to the field scale. Following an extensive discussion in the risk assessment process, an integrative research program has been conducted aiming to reveal the relevant information required for using bioavailability corrections within the framework of the terrestrial risk assessment. The various relationships between on the one hand abiotic soil parameters and on the other hand the toxicity of zinc to plants, invertebrates and microbial endpoints were used to develop “soil sensitivity” functions i.e. relationships that express the potential toxicity of zinc in various soil types as a function of soil characteristics. Longterm distribution of zinc in soil was also recognised as an important process that affects the distribution of zinc and bioavailability in soil and toxicity towards soil species. Based on recent studies and a recent evaluation of older studies, the ‘ageing’ phenomenon was also quantitatively taken into account in the EU RAR (European Commission – Joint Research Centre, Institute for Health and Consumer Protection, Risk Assessment Report Zinc metal, Final report, 2008).