Mobility and environmental impact of cadmium (Cd) during weathering of carbonaceous black shales in western Hunan, China

Carbonaceous black shale generally contains high concentration of Cd, with weathering leading to Cd release to environment. In this study, the mobility of Cd during weathering was quantified using geochemical assessment on black shale from western Hunan, China. Results suggested that Cd was heterogeneously distributed in shale profiles with concentrations ranging from 0.16 to 109.9 (mg/kg). Cd distribution was heterogeneous resulting from the parent shale inheritance and the mobility of Cd during weathering. Black shales weathered to a moderate degree with Cd mobility characterized by both enrichment in and release from weathered shales. Cd enrichment in weathered shales resulted from the re-enrichment of Cd in secondary minerals formed during the initial stage of carbonate (and phosphorite) dissolution, and the secondary stage of sulfide oxidation. The release of Cd was caused by decomposition of the secondary Cd-bearing minerals. Cadmium was extensively released during pedogenesis, and Cd release mass flux was estimated to range from 1.26 to 9.50 (g/m2) with a mean of 6.60g/m2. Thus, black shale weathering may lead to the releasing of large amount of Cd resulting in Cd contamination to local environments. Environmental Implication The carbonaceous black shales with phosphorites all over the world are crustal rocks that generally contain high concentration of Cd and other metals. Weathering of these rocks can lead to the liberation of Cd, and thus these shales represent a type of hazardous materials that can be the source of Cd in environment. Thus, it is important to quantify the mobility of Cd during weathering to understand Cd contamination in environment.