Terrestrial compartment Terrestrial toxicity

Table 3.3.3.a (toxicity of zinc to soil microbe-mediated processes) and Table 3.3.3.b (chronic toxicity of zinc to soil invertebrates) in Annex 3.3.3.A of the Risk Assessment Report on Zinc metal include data on some tests in which ZnO was used as test compound, in addition to the majority of tests in which a soluble zinc salt was used as test compound. The results suggest that ZnO may be somewhat less toxic than soluble zinc, but the data for ZnO are much too limited for a firm conclusion. Based on differences in water solubility and, hence, most likely in bioavailability, it can be predicted that soluble zinc compounds will be more toxic to soil organisms than insoluble zinc compounds, at least shortly after the addition to soil. After a certain period of time, however, the toxicity will be less dependent on the zinc species that is added, because of transformations into other species. Ultimately, the resulting zinc speciation and bioavailability will mainly depend on the soil characteristics, and less on the original chemical form in which zinc was added to the soil.

The Tables 3.3.3.a and 3.3.3.b in Annex 3.3.3.A. of the Risk Assessment Report on Zinc metal include also the results from some tests that show that, in the same test system, the addition of soluble zinc compounds resulted in the same NOEC (expressed as zinc) as the addition of less soluble zinc carbonate. These results support the earlier assumption that the toxicity of zinc ultimately will mainly depend on the soil characteristics.

Test on fate and effects of ZnO containing tyre debris in soil

A one-year experiment was set-up to quantify the fate and effects of Zn from tyre debris in soil (Smolders et al., 2001). Two soils (an acid sandy soil and a silt loam soil) were mixed with the <100-µm fraction of car and truck tyre debris (25 g kg-1 soil). These application rates correspond to 282 mg Zn kg-1soil (car) and 595 mg Zn kg-1 soil (truck). In other treatments, soils were spiked with 300 mg Zn kg-1soil from either pure ZnO or ZnSO4. Soils

were transferred to soil columns with free drainage and placed outdoors (28/10/99- 6/10/2000). Additional treatments included surface application of tyre debris at soil column average rates that were identical as in treatments where the debris was mixed in the soil. The release of Zn in soil is measured based on Zn concentrations in pore waters and leachates of soil columns. The potential toxic effect of tyre debris is measured with a nitrification test in soil.

Eleven months outdoor weathering of the tyre debris in soil resulted in much smaller Zn release in soil than in treatments where ZnO or ZnSO4 was applied. Zinc leaching was only

significantly increased compared to the control in the acid sandy soil in which the car tyre debris was homogeneously mixed. Truck tyre debris in this soil did not increase pore water Zn or Zn in leachates compared to the control. In the silt loam soil there were no effects of tyre debris on Zn concentration in leachates but pore water Zn was increased at the final harvest. This increase was however only 1.4 % (truck) or 4.6 %(car) of the increase due to ZnSO4 application. The quantity of Zn leached from the car tyre debris in the acid soil is

4.6% of the Zn leached from the ZnSO4 treated soil and is 20.2% of the Zn leached from the

ZnO treated soil (nominal Zn rates are all about equal in these 3 treatments).

Tyre debris application increased nitrification rate whereas ZnSO4 application, at

corresponding or smaller Zn rates, decreased nitrification rate. The authors explained this stimulation of nitrification by the increased soil pH in the tyre debris applied soils.

CAS No. 1314-13-2 53

The labile Zn content in the soils was measured at the end of the weathering period using isotope dilution with a 65Zn2+ salt. This assessment showed that 10-40% of the Zn from tyres transforms in 1 year to a Zn species that behaves as a Zn2+ salt added to that soil. However, the increased soil pH in the soils treated with tyre debris counteracts the increased quantity of labile Zn in soil, hence resulting in minor increases in Zn in leachates and even a stimulation of the nitrification rate.

3.3.1.2 Zinc

Although zinc oxide is much less water soluble than zinc salts such as zinc sulphate and zinc chloride, zinc may be dissolved from zinc oxide solutions to a level that may result in toxic effects to aquatic organisms, see section 3.3.1.1.1. Once emitted into the environment, zinc oxide will (partly) be transformed into other zinc species. The further speciation of zinc, which includes complexation, precipitation and sorption, depends on the environmental conditions. Therefore, emitted zinc oxide and other emitted zinc species (e.g. zinc sulphate) will contribute to the effect of the total amount of zinc in the environment, regardless of the original source or chemical form. For this reason the risk characterisation is based on zinc (regarding zinc as the causative factor for toxicity), not on zinc oxide as such. Thus, in the local risk characterisation for zinc oxide, the PNECadd values for zinc (see Table 3.3.2) have

been compared with the local PECadd values which are also expressed as zinc, but derived

from the local emissions due to the production or use of zinc oxide. In the regional risk characterisation, which is not for zinc oxide specifically but for zinc from “all” anthopogenic sources, the PNECadd values for zinc have been compared with PECadd values for zinc, the

latter values derived from the sum of the regional emissions due to industrial and non- industrial sources, diffuse sources included (see also earlier in section 3.2 for further explanation). For the regional risk characterisation the reader is referred to the Risk Assessment Report on Zinc metal (RAR Zinc metal).

In the RAR Zinc metal, PNECadd values have been derived for zinc, on the basis of tests with

soluble zinc salts (especially zinc sulphate or zinc chloride), using the “added risk approach” (see also earlier in section 3.1 of the present report for an explanation of the added risk approach). These PNECadd values for zinc are listed in Table 3.3.2 and used in the risk

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Table 3.3.2 PNECadd values for zinc (from RAR Zinc metal) Environmental

compartment PNECadd PNECas Zn add value, Remark

Freshwater

(Hardness > 24 mg/L) (1) PNECadd, aquatic 7.8 µg/l 21 µg /l Dissolved zinc Total zinc (2) Freshwater

(Hardness <24 mg/L) (1)

PNECadd, aquatic softwater

3.1 µg/l Dissolved zinc Freshwater sediment PNECadd, sediment 49 mg/kg dwt

11 mg/kg wwt

Dry weight of sediment (3) Wet weight of sediment (3) STP effluent PNECadd, microorganisms 52 µg/l Dissolved zinc

Soil PNECadd, terrestrial 26 mg/kg dwt

23 mg/kg wwt

Dry weight of soil (4) Wet weight of soil (4) (1) Total hardness (mg/l), as CaCO3.

(2) Total-Zn concentration: calculated from the PNECadd, aquatic of 7.8 µg/l for dissolved zinc, a Csusp of 15 mg/l (according to the TGD,

2003) and a Kpsusp of 110,000 l/kg.

(3) For the dry to wet weight normalisation of the PNECadd, sediment it is assumed that wet sediment contains 10% solids (density 2500

kg/m3_{) and 90% water (density 1000 kg/m}3_{) by volume, i.e. 22% solids by weight. These properties are set equal to those of}

suspended matter, thus the PNECadd, suspended matter equals the PNECadd, sediment (according to the TGD, 2003).

(4) For the dry to wet weight normalisation of the PNECadd, terrestrial it is assumed that wet soil contains 60% solids (density 2500 kg/m3)

and 20% water (density 1000 kg/m3_{) by volume, i.e. 88% solids by weight.}