Experimental simulations of ZnPb slag weathering and its impact on the environment: Effects of acid rain, soil solution, and microbial activity

Slag heaps are far from stable and evolve continuously from the time they are deposited. In the studied site (Świętochłowice, Upper Silesian Industrial Region, Poland), the major change in the slag heap has involved the formation of fine-grained strongly weathered material. Possible conditions of slag weathering, as well as any impacts of weathered material on the environment, have been evaluated by weathering experiments. The experiment simulating acid rain at adjusted pH = 4 precisely reproduced many features of the weathering transformations observed in nature, such as strong disintegration of slag material, formation of abundant Fe(hydro)oxides, silica and alumina removal, and Pb precipitation. Conversely, bioweathering with Acidithiobacillus thiooxidans often showed the opposite direction of transformations (Si and Al precipitation, Pb removal, transformations at the rims of larger slag particles), probably due to the too-aggressive low pH of the simulations. There are environmental consequences as more and more slag in the heap weathers, with fresh slag releasing mostly Zn and Ba. On the other hand, weathered slag material may release high contents of Pb, Cd, As, but only if the pH remains at 4–6. In present-day slag heap conditions, precipitation of Fe(hydro)oxides and other secondary phases diminishes toxic element mobility. However, soils and sediments surrounding the heap show high levels of contamination (up to 2500 mg/kg for Pb, up to 11,300 mg/kg for Zn, up to 120 mg/kg for Cd), which is perhaps due to leaching of weathered slag or its mechanical removal. Other contaminants include smelter emissions and high Ba and Hg components that are tentatively linked to the episode of early weathering of fresh slag.