Impact of ancient iron smelting wastes on current soils: Legacy contamination, environmental availability and fractionation of metals

Highlights: • The study area is considered as highly contaminated with some metals (Co, Fe, Mn). • Slag weathering (fragmentation and dissolution) contributes to this contamination. • Soil OM and clay control the partitioning of metals released in soils. • The availability of metals (Mn, Ni, Zn) denote low to medium environmental risks. Past and present metallurgical activity is the origin of the metallic contamination of some current soils. The purpose of this research is to assess the environmental risk of ancient Fe smelting wastes to the terrestrial compartment. For this purpose, two study sites were investigated in Bourgogne-Franche Comté (France). For each site, the soil contamination (Co, Cu, Fe, Mn, Ni and Zn) and the mobility of each metal from the slag to the topsoils were assessed. The principal results show that the topsoils are particularly enriched in Fe and Mn compared to the reference soils. The bulk chemistry of the slag showed high Fe and Mn content related to the mineralogy of slags, in which the minerals include fayalite, spinel, wustite and glass. In the topsoils, we also observed newly formed minerals (clay minerals, goethite and hematite), which were absent in the reference soils. The presence of slag microfragments in soils and the partial weathering of slags, which contributed to the release of metals in the soils, can explain the contribution of slags to the current contamination of soils. The extensive study of a depth profile from Puisaye showed a low vertical diffusion of the released metal in the heap substratum. We also investigated the fractionation of metals in soils and their environmental availability. The results showed that Mn is generally present in reducible forms or associated with the residual fraction but is less adsorbed to the organic matter (OM) or present in easily exchangeable forms. In contrast, the low extractability of Fe indicates that it is mostly bound to the residual (i.e., mineral) fraction. Based on the easily exchangeable metal concentrations measured in soils, low to medium ecological risks were identified at the sites investigated.