Cadmium distribution in Pb-Zn slags from Upper Silesia, Poland: implications for cadmium mobility from slag phases to the environment

Cadmium is a toxic element, generally highly mobile and easily transferred from Cd-bearing ore minerals to the environment as shown by numerous leaching experiments and studies of biota. However, the behaviour of Cd is less understood during smelting activities, when it is transferred from Cd-bearing ore to the metal as well as to the smelting by-product - slag. Slag material, with Cd concentrations over 500 mg/kg, occurs at an abandoned slag pile after Pb-Zn metal smelting in Swiętochłowice (Upper Silesia). Such material, extremely enriched in Cd (15 mg/kg is a legislative guideline for industrial soils), offers a possibility to study the distribution of Cd within slag fragments composed of silicates and oxides, gaining insight into Cd mobility. Electron microprobe analyses of primary slag show that Cd partitioning between different phases is not controlled by site preference and Cd does not substitute for elements with similar radius, such as Zn in Zn-spinel or Ca in Pb-hardystonite. Cadmium is strongly incompatible and enriched in late melts that, in the case of the studied slag, crystallize Pb-hardystonite rims, late Pb-silicates, probably alamosite and phosphates. These phases contain most of the Cd budget, but in a small volume (5–10 μm phases). As such, Cd behaviour and distribution in silicate slags may contrast that of other potentially toxic elements, which are generally selectively partitioned into one or more major phases. The study of another slag fragment, composed of secondary Fe-(hydro)oxides, formed after sulphide weathering, shows low Cd contents suggesting that either Cd was mobilized from the sulphide during weathering or lost in the smelting process. General distribution of Cd within slag in easily accessible parts (enrichment at rims and in late phases occurring at grain boundaries) and not in secondary phases, suggests that Cd have a high potential mobility from the slag to the environment.