Systematic cadmium isotope fractionation between the organic and mineral fractions of soils

Cadmium (Cd) isotope signatures in natural soil organic matter and fractionation during Cd partitioning among soil organic and mineral components remain unclear, yet they are critical for tracing Cd fate in soils. This study focused on particulate organic matter (POM), a labile fraction of organic matter with significant Cd enrichment, and POM (2000250 and 25053 μm), mineral (2000250 and 25053 μm) and organo-mineral (< 53 μm) fractions were physically separated from six contaminated soils for Cd concentration and isotope analyses. Cadmium concentrations in the POM fractions were 0.8451.1 fold higher than those in the mineral fractions. Carboxylic groups drove Cd enrichment in POM whereas iron (Fe) oxides dominated Cd sequestration in the mineral fractions, with POM systematically enriched in heavy Cd isotopes relative to the mineral fractions (Δ^114/110Cd_POM-mineral = 0.090.50‰). The observed Cd isotope fractionation from mineral to POM here was different from that for humic acid (HA) complexation preferring light Cd isotopes (Δ^114/110Cd_HA-solution = -0.15 ± 0.01‰). Cadmium-carboxyl complexation appeared to be a major mechanism controlling Cd isotope signatures in POM and POM with more hydroxylic groups was likely to enrich heavier Cd isotopes. The enrichment of Fe oxides likely contributed to the lighter Cd isotope compositions in the coarser mineral fractions. Cadmium isotope fractionation between the POM and mineral fractions is in accord with an equilibrium-like isotope fractionation pattern, indicating reversible Cd exchange between the mineral and POM fractions via soil solutions. This study provides the first systematic assessment of Cd isotope fractionation associated with the distribution of Cd in different soil pools. As such, it advances mechanistic understanding of Cd interaction with the solid organic and mineral phases of soils.