Magnetic separation coupled with Pb-Cd stable isotopes to unravel geochemical background in metal(loid)-polluted...

A common approach to assess soil pollution is to compare elemental concentrations with geochemical background levels. However, in contaminated areas, these values are often difficult to define due to the superposition of geogenic contributions and multiple historical anthropogenic inputs. Here, we propose an innovative approach in which magnetic separation is applied as a pretreatment prior to lead (Pb) and cadmium (Cd) stable isotope analyses to identify the geochemical background in polluted soils. Soils affected by historical smelting activities were collected at a highly contaminated site. The geochemical background was inferred from the characterization of the parent material and multivariate statistical analyses reported in previous studies, while potential pollution sources were identified through smelter-derived fly ashes and slags. Soil samples were subjected to magnetic separation using a wet high-intensity magnetic separator at different magnetic field intensities (10, 30, and 50%), aiming to concentrate anthropogenic particles in the magnetic fraction. Marked differences between magnetic and non-magnetic fractions were observed, particularly at the lowest magnetic intensity. At 10%, Pb in the non-magnetic fraction showed an isotopic signature consistent with the parent material (²⁰⁶Pb/²⁰⁴Pb = 18.43), whereas the magnetic fraction matched the pollution source (²⁰⁶Pb/²⁰⁴Pb = 17.97). Similarly, the magnetic fraction was enriched in lighter Cd isotopes (δ¹¹⁴Cd = 0.14 ± 0.02‰), consistent with fly ash signatures, while the non-magnetic fraction was heavier (δ¹¹⁴Cd = -0.34 ± 0.05‰), reflecting the geogenic background. Overall, our results demonstrate that magnetic separation constitutes an effective and relatively low-cost pre-treatment for disentangling geogenic and anthropogenic contributions in metal-contaminated soils.