Cadmium isotope systematics for source apportionment in an urban-rural region

Cadmium (Cd) is a highly carcinogenic metal that can cause toxic reactions, even at low concentrations. In the urban-rural region of Chengdu (China), the Cd concentration of soil ranged from 0.02 mg kg(-1) to 5.03 mg kg(-1). To decipher Cd source apportionment, samples were collected from topsoil, weathered material, and bedrocks (profiles reaching 200 m in depth) and from local anthropogenic endmembers (pigment, fertilizer, and urban dust). Geogenic Cd weathering processes were determined using the alteration index and exchangeable fraction of Cd in soil. Simultaneously, the Cd isotope composition was measured to determine the geogenic isotopic trend of Cd during weathering and the impacts of anthropogenic inputs on the soil surface. Overall, we observed a high variability in the mass transfer of Cd, indicating the presence of multiple Cd sources. Chemical weathering led to the preferential depletion of heavy Cd isotopes through mineral dissolution and the preferential enrichment of light Cd isotopes during Cd sorption onto newly formed soil phases (-0.35 parts per thousand & nbsp; < delta Cd-114/110(soil-bedrock) < 0.01 parts per thousand). Anthropogenic inputs were associated with a wide range of delta Cd-114/110 values (-0.50 +/- 0.02 parts per thousand to 0.08 +/- 0.02 parts per thousand). It can be inferred that anthropogenic inputs alter the typical Rayleigh fractionation observed during chemical weathering and modify the Cd isotopic trend in the soil surface of urban-agricultural areas.