Environmental impacts and risk assessment in the re-use of Cr-bearing pyrolyzed tannery wastes: a case study in a residential area

Increasing concern has been raised on the environmental impacts of chromium-tanning wastes recycling. In particular, the pyrolytic conversion of leather industry sludges into Cr(VI)-free carbonized residues is believed to represent a viable route for a sustainable re-use of this type of wastes. The aim of this study was to determine the impact of recycled Cr-bearing pyrolyzed char (named KEU) as backfilling material for road construction in an urban area in Tuscany (Italy). Geochemical and chromium-isotope data, together with microstructural analyses (HR-TEM and XAS), indicate that the presence of KEU results in a significant enhancement of the natural Cr background. The results support the hypothesis that, in environmental conditions, the Cr(III) hosted in KEU is converted into Cr(VI), which is leached out by rainwater. Indeed, Cr(VI) is dispersed in surface water, reaching concentrations up to 18 mg/L and it also occurs in coatings on the surface gravel of unpaved roads. The 53Cr/52Cr ratio measured in KEU, reported in δ53Cr notation, was in the restricted range δ53Cr = -0.031±0.057 ‰; on the contrary, the δ53Cr in water varied between +1.581±0.038 ‰ and +3.261±0.191 ‰, indicating the reduction of Cr(VI) after the pristine oxidative mobilization. The risk-based soil screening levels (SSLs) for total Cr are well above the concentration measured in soil for all the exposure pathways. On the contrary, the SSL for Cr(VI)indicates that soil contamination poses health hazards for residents.