Concentrates of lead and zinc compounds with sulfur resulting from hydrometallurgy (hot acid leaching, super-hot acid leaching and flotation)

Toxicity to soil microorganisms

Description of key information

According to the transformation/dissolution study (OECD guidance 29) conducted for the substance, the most critical and bioavailable constituents of the target substance are lead and zinc. Therefore, the chemical safety assessment focuses on the properties of these constituents. However, the ecological relevance of reduced enzymatic activities is unclear and, therefore the studies on soil microorganisms is not used for PNEC calculations for lead, and the data is only presented as supporting evidence for CSA. The key value for this endpoint is selected based on the data available for zinc as the toxicity of zinc to soil microorganisms is also taken into account for PNEC soil (Zn) derivation.

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

17 mg/kg soil dw

Additional information

Toxicity to terrestrial soil micro-organisms was evaluated based on the toxicity data on the most bioavailable constituents of the target substance. According to the T/D study results conducted for the target substance, the most soluble and critical components of this substance are lead and zinc which appear in the substance in sulphide and sulphate forms. Because the target substance is an inorganic solid UVCB substance and insoluble to water, the environmental fate and toxicity is related to the soluble (bioavailable) constituents of the substance. The read-across data for toxicity to terrestrial organisms is not only focusing on the sulphate but also on the other bioavailable lead and zinc compounds. The read-across justification is presented in CSR annex I. The weight of evidence approach was used to make conclusions on the key values for CSA.

Based on the exposure considerations both lead and zinc emissions are relevant for the environmental exposure assessment and to evaluate the terrestrial toxicity of the target substance. For ES&RC, the PNECs were derived separately for both critical constituents of the target substance. In case of lead, the ecological relevance of reduced enzymatic activities is unclear and, therefore the toxicity data on soil micro-organisms is not used for PNEC calculations, but only presented as supporting evidence. In PNEC soil derivation for zinc, the toxicity to soil micro-organisms is taken into account for the extrapolation of NEC soil. For further information on PNEC derivation, see CSR section 7.6.

Toxicity of lead and its compounds

There are reliable NOEC or EC10 values selected for 5 different functional parameters for microbial processes. The functional parameters comprise C- and N-mineralization. The selected NOEC and EC10 values on functional parameters vary from 96 to 4144 mg Pb/kg (both values observed for respiration). Additionally, several NOEC or EC10 values are selected for 8 different enzyme assays, ranging from 132 mg Pb/kg for dehydrogenase to 16576 mg Pb/kg for phosphatase. The ecological relevance of reduced enzymatic activities is unclear and, therefore this data is not used for PNEC calculations, but only presented as supporting evidence. Therefore, no key value is either selected for the CSA of the target substance in relation to lead.

Toxicity of zinc and its compounds

For microbial assays, in total 108 individual high quality NOEC/EC10s are selected for the PNEC derivation. These values represent 4 nitrogen transformation processes, 5 carbon transformation processes and 8 enzymatic processes and range from 17 mg Zn/kg dw for respiration (Chang and Broadbent, 1981 and Lighthart et al., 1983) to 2623 mg Zn/kg dw for phosphatase (Doelman and Haanstra, 1989).

Information on the background Zn concentration, allowing correction for differences in bioavailability among soils is only available for 76 NOEC or EC10 values, representing 13 microbial processes (4 for N cycle, 5 for C cycle and 4 enzymatic processes). The total range in NOEC/EC10 values for the dataset with results for background Zn concentration is the same as for the total dataset for micro-organisms.

All toxicity data are expressed as spiked Zn concentration in soil, based either on the nominal or measured concentrations, and the studies from field studies take into account the background corrected Zn concentrations in soil. The lowest chronic toxicity (NOEC 12-wk) value of 17 mg/kg (no guideline) was selected as a key value for CSA.

Conclusions for CSA:

Toxicity is evaluated by using the read-across data from the two critical and bioavailable constituents of the target substance (zinc and lead). However, the ecological relevance of reduced enzymatic activities is unclear and, therefore the studies on soil microorganisms is not used for PNEC calculations for lead, but only presented as supporting evidence. For zinc, the toxicity data on soil microorganisms is taken into account for the PNEC calculations and the key value for CSA is selected for zinc based on the lowest chronic toxicity response. See the CSR section 7.6 for further information on the PNEC derivation.