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 adsoprtion to soil compartment and exposure via air deposits related to air emissions from the manufacture and the end-use.

In water compartment, both lead and zinc can occur in both suspended and dissolved form and is partitioned over a number of chemical species. The speciation and solubility of both metals depend from abiotic factors e.g. organic matter content, redox potential, etc. In low pH (≤ 6.5), most of the dissolved lead and zinc are in form of free ions. In higher pH (≥ 6.5), lead is in the form of PbOH+ and PbCO₃ (aq) and zinc in Zn(OH)2, Zn(OH)₃ and ZnCO₃. The amount of dissolved zinc and lead in solution also depends from the organic complexing material present in the water. (LDAI 2008, ECB 2008)

Both lead and zinc have adsorption potential to sediments. The efficiency of adsorption from solution to sediment varies on metal concentration, pH, redox potential (Eh), salinity and concentrations of complexing ligand. Both metals can also be extracted from sediments by desorption. For zinc, desorption can occur under aerobic conditions with high salinity.

In soil, lead and zinc can be present as free ions in solution or adsorbed to reactive soil surfaces (e.g. soil organic matter, clay minerals, Fe and Mn oxides). The distribution between various forms depends on soil properties (e.g. pH, % organic matter, parent material). The mobility of metals in soil depends from the solubility of the metal species and from abiotic parameters, for example higher pH decreases the mobility of metals in the soil. (LDAI 2008, ECB 2008)

In the atmosphere, lead and zinc exist primarily in the aerosols in particulate form. Upon release to the atmosphere, lead and zinc particles are dispersed and ultimately removed from the atmosphere by wet or dry deposition. An important factor in determining the atmospheric transport is particle size distribution and determination of the amount of respirable particles (Lannefors et al. 1983, LDAI 2008).