Mineralogy and weathering processes in historical smelting slags and their effect on the mobilisation of lead

The mineralogy and weathering of historical lead slags and contaminated soils from five historical smelting sites in Britain were examined using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). Slags were found to be highly heterogeneous materials, dominated by calcium and barium compounds and a variety of lead phases. In this preliminary study, evidence was found which indicates that an unstable di-calcium silicate phase (Ca2SiO4:H2O) may be one of the first to breakdown during weathering. Air and water will then penetrate the slag structure, facilitating further weathering and the release of lead into the soil. The weathering of this di-calcium silicate, and other Ca compounds present in the slags, may be responsible for elevating the pH in the upper soil layers at the study sites. This appears to have promoted the precipitation of Pb mainly as cerussite (PbCO3), although other weathering products have been noted at particular sites, such as the adsorption of Pb onto Fe/Mn oxides. Formation of these weathering products may have slowed the downward migration of the lead at the study sites.