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

Endpoint summary

Additional information

This substance is an UVCB substance and can be described as a moist solid powder which is insoluble to water. The product consists primarily of sulphur (ca. 35 %), lead (ca. 25 %) and zinc (ca. 17 %) together with minor trace elements such as silver, silicon, aluminium, calcium and iron.

The transformation and dissolution study (OECD guidance 29) was conducted to determine the rate and extent to which metals or sparingly soluble metal compounds can produce toxic bioavailable forms and whether this rate and extent of formation is of concern and should lead to classification. Based on the screening test results (the 24-hour loading rate 100 mg/L), the most critical components for the assessment was lead, with release of 8282 µg/L. The other minor leachable metals were zinc (75.4 µg/L), silver (34.7 µg/L), cadmium (0.48 µg/L) and copper (17.2 µg/L). According to the 7-day and 28-day T/D study results, the most soluble and critical components of this substance are lead and zinc. Therefore, the chemical safety assessment focuses on these bioavailable constituents of the target substance.

The biodegradation and hydrolysis are not relevant fate processes for inorganic substances. The most critical fate properties of this substance is related to the bioaccumulation and adsorption potential of the critical constituents in water compartment, and the adsorption to soil compartment and exposure via air deposits related to air emissions from the manufacture and the end-use.

The following key values have been selected for the CSA from the read-across data of the critical constituents. According to the adsorption coefficients available from the literature, the adsorption potential of lead and zinc is quite similar. However, as discussed below their toxicity profile and bioaccumulation potential differs. The read-across justification with introductions on the properties of the source substances in comparison with the target substance is presented in the Annex I of the CSR.

Adsorption properties of the critical constituents of the target substance:

Partition coefficient in freshwater suspended matter:

• log Kpsusp (Zn) = 5.04 l/kg

• log Kpsusp (Pb)= 5.47 l/kg

Partition coefficient in freshwater sediment:

• log Kpsed (Zn)= 4.86 l/kg

• log Kpsed (Pb)= 5.19 l/kg

Partition coefficient in marine sediment:

• log Kd sed (Zn)= log 3.78 l/kg

• log Kpsed (Pb) = Log 5.66 l/kg

Partition coefficient in marine suspended matter:

• log Kp, susp (Pb) = Log 6.18 l/kg

Partition coefficient in soil compartment:

• log Kd value for soil (Zn) = 2.2 l/kg

• log Kd value soil (Pb) = 3.81 l/kg

General remarks on the most critical constituents of the target substance

Lead

The behaviour of Pb in the environment depends upon its chemical form: solid Pb, Pb oxides, halogenated compounds, soluble or insoluble salts and (in) organic complexes.

Lead enters the aquatic environment via community and industrial wastewater, runoff and leaching from natural and anthropogenically burdened soils, atmospheric deposition and corrosion and abrasion of lead containing materials. Natural weathering processes usually turn metallic lead and its compounds into compounds that are relatively stable and insoluble (e. g. carbonates, sulphates, sulphides and phosphates). In the aquatic environment the rate of dissolution of lead is strongly dependent on the composition of the water to which it is exposed. The hardness, for instance, is an important factor: solid lead slowly dissolves in soft waters whereas it generally does not dissolve in hard water.

Atmospheric releases of lead can occur mainly from manufacturing processes and thermal processes. Air emissions of lead can deposit to soil or aquatic environment. In assessing the ecotoxicity of metals in the various environmental compartments (aquatic, terrestrial and sediment), it is assumed that toxicity is not controlled by the total concentration of a metal, but by the bioavailable form. For metals, this bioavailable form is generally accepted to be the free metal-ion in solution. In the absence of speciation data and as a conservative approximation, it can also be assumed that the total soluble lead pool is bioavailable. All reliable data on ecotoxicity and environmental fate and behaviour of lead and lead substances were therefore selected based on soluble Pb salts or measured (dissolved) Pb concentration.

Zinc

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++). 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.

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). As such, it is shown that zinc (ions) brought into water will be rapidly removed from the water column (>70% removal within 28days).

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). Due to the insolubility of the ZnS (K=9.2 x 10-25) zinc will be sequestered in the (anaerobic) sediments, and the re-mobilisation of zinc ions into the water column will be prevented. 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).

Bioaccumulation of the target substance

Lead is classified as carcinogenic, reproduction toxic and it possess repeated dose toxicity via oral and inhalation routes. Since it is also the main constituent of the target substance, it triggers the target substance to be classified as carcinogenic, reproduction toxic and having repeated dose toxicity. Based on the BAF/BCF values of lead it has also bioaccumulation potential to water (1 553 L/kg (wet weight) and soil (0.39 kg/kg (dry weight). Therefore, the chemical safety assessment of the target substance focuses on lead emissions in relation to the exposure of man via environment, and the bioaccumulation factors of lead are used as key values when evaluating the bioaccumulation of the target substance in the environment.

In relation to bioaccumulation, the secondary poisoning was considered irrelevant for zinc which is an essential element that is actively regulated within the body of all organisms. Due to the general lack of increased whole body and tissue concentrations at higher exposure levels, the zinc BCF data show generally an inverse relationship to exposure concentrations (McGeer et al 2003). The physiological basis for the inverse relationship of BCF to zinc exposure concentration arises from Zn uptake and control mechanisms. At low environmental zinc levels, organisms are able to sequester and retain Zn in tissues for essential functions. When Zn exposure is higher, aquatic organisms are able to control uptake. There is clear evidence that many species actively regulate their body Zn concentrations, including crustaceans, oligochaetes, mussels, gastropods, fish, amphipods, chironomids by different mechanisms (McGeer et al 2003). The bioaccumulation potential in mammals is also considered low. Based on this, the chemical safety assessment concludes that bioaccumulation potential of zinc is not considered relevant to be evaluated.