Geogenic pollution, fractionation and potential risks of Cd and Zn in soils from a mountainous region underlain by black shale

This study investigated the pollution, fractionation and potential risks of heavy metals in soils from a mountainous area with black shale outcropping, with emphasis on Cd and Zn. Elevated concentrations of heavy metals in black shales reflected high geochemical baseline in the study area. Metals released from oxidative weathering were redistributed and retained during pedogenic processes, leading to enrichment of Cd, Cu, Cr, Ni, V and Zn in soils. As a highly concerned metal pollutant, the content of Cd in all collected samples (N = 112, Cd: 0.44–16.1 mg/kg) exceeded the risk screening value for Chinese agricultural land. Metals in local soils were retained through adsorption onto soil constituents clay minerals and FeMn oxides and/or precipitation within secondary minerals. Based on sequential extraction, Cd and Zn fractionated in soils, with Cd mainly occurring in the mobile fraction (47.9% to 78.7%) as adsorbed, FeMn oxides and carbonate associated phases, whereas residual fraction (67.1%) dominated for Zn, followed by FeMn oxides coagulation (17.9%). Metals in residual soils from slopping location were highly related to metals in the parent rocks distributed more heterogeneously than metals in alluvial soils from flat location. High levels of heavy metal pollution in soils posed potential risks to the local eco-environmental systems and community, and Cd was highly concerned due to its high mobility. Cultivation in alluvial soils from valleys poses less risk than in residual soils, but appropriate approaches to reduce the risk in local soils is still necessary. The findings from this study provide basic knowledge and insight for risk control and targeted management of soils with geogenic heavy metal pollution in black shale outcropped mountainous areas.