Tracing industrial heavy metal inputs to topsoils using using cadmium isotopes

Abstract:

Anthropogenic activities have dominated heavy metal (such as Cd, Pb, and Zn) cycling in many environments. The extent and fate of these metal depositions in topsoils, however, have not been adequately evaluated. Here, we utilize an innovative Cadmium (Cd) isotope tool to trace the sources of metal pollutants in topsoils collected from surrounding a Vanadium Titanium Magnetite smelting plant in Sichuan, China.

Topsoil samples and possible pollution end-members such as fly ashes, bottom ashes, ore materials, and coal were also collected from the region surrounding the smelting plant and were analyzed for Cd isotope ratios (d¹¹⁴Cd relative to Cd NIST 3108). Large Cd isotope fractionation (up to 3 ‰) was observed in these industrial end-members: fly ashes possessed higher δ¹¹⁴Cd values ranging from +0.03 to +0.19‰; bottom fly ashes have lower δ¹¹⁴Cd values ranging from -0.35 to -2.46‰; and unprocessed ore and coal samples has δ¹¹⁴Cd value of -0.40‰. This fractionation can be attributed to the smelting processes during which bottom ashes acquired lighter Cd isotope signatures while fly ashes were mainly characterized by heavy isotope ratios, in comparison to the unprocessed ore and coal samples. Indeed, δ¹¹⁴Cd values of topsoils in the smelting area range from 0.29 to -0.56‰, and more than half of the soils analyzed have distinct δ¹¹⁴Cd values > 0‰. Cd isotopes and concentrations measured in topsoils suggested that processed materials (fly and bottom ashes from ore and coal actually used by the smelting plant) were the major source of Cd in soils. In a δ¹¹⁴Cd vs 1/Cd mixing diagram, the soils represent a mixture of three identified end members (fly ash, bottom ash and deep unaffected soil) with distinct Cd isotopic compositions and concentrations. Deep soils have the same δ¹¹⁴Cd values range as the unprocessed ore and coal, which indicated the Cd isotope fractionation did occur during evaporation and condensation processes inside the smelting plant. The signatures of fly ash end member might be even higher according to the δ¹¹⁴Cd increasing trend of topsoils with the increasing of Cd concentration of the topsoils. Our study suggested that δ¹¹⁴Cd values can be used to distinguish sources of anthropogenic Cd and to construct metal budgets in in this studying area.