Cadmium isotope compositions of Fe-Mn nodules and surrounding soils: Implications for tracing Cd sources

Cadmium (Cd) pollution in agricultural soils has become a severe threat to food security and human health in recent years. Stable Cd isotopes are a potentially powerful tool for identifying the sources of Cd in soils. However, many Earth surface processes, including adsorption, leaching, and biogeochemical cycles in plants, may generate Cd isotope fractionation, which can complicate the potential application of Cd isotopes in tracing the sources of Cd pollution in soils. In this work, the Cd isotope compositions of typical Fe-Mn nodules (FMNs) and surrounding soils in two different soil profiles are investigated. Our results show that the FMNs in lower layers (i.e., C and W horizons) are isotopically lighter than the surrounding soils by –0.114‰ to –0.156‰ (Δ114/110CdFMN-soil). We interpret this fractionation as the result of preferential adsorption of isotopically light Cd onto the surface of goethite. In the upper layers (i.e., P and A horizons), the Δ114/110CdFMN-soil values are more negative in the P horizon (–0.213‰ to –0.388‰) but more positive in the A horizon (0.061‰ to 0.204‰). We interpret these fractionations as the result of natural biogeochemical processes (i.e., leaching and biological cycling) during soil development. Soil leaching preferentially releases isotopically heavy Cd into the underlying soil (i.e., P horizon), shifting the topsoil towards lower δ114/110Cd values but the underlying soils towards higher δ114/110Cd values. Moreover, biological cycling contributes isotopically heavy Cd to the topsoil, probably shifting the topsoil towards higher δ114/110Cd values. Our study demonstrates that the formation of Fe oxyhydroxides, leaching, and biological cycling can considerably modify the soil Cd isotope signature, highlighting the need to consider natural biogeochemical processes when using Cd isotopes to trace heavy metal pollution in soils.

Two-step calculation method to enable the ecological and human health risk assessment of remediated soil treated through thermal curing

The centralized utilization of heavy-metal-contaminated soil has become the main strategy to remediate brownfield-site pollution.However,few studies have evaluated the ecological and human health risks of reusing these remediated soils.Considering Zn as the target metal,systematic pHdependent leaching and the Community Bureau of Reference(BCR)extraction were conducted at six pH values(pH=2,4,6,8,10,12)for the remediated soil treated through thermal curing.The pHdependent leaching results showed that with the formation of ZnCr2O4 spinel phases,the remediated soil exhibited strong inherent resistance to acidic attack over longer leaching periods.Furthermore,the BCR extraction results showed that the leaching agent pH value mainly affected the acid-soluble fraction content.Moreover,a strong complementary relationship was noted between the leaching and acid-soluble fraction contents,indicating that the sum of these two parameters is representative of the remediated soil risk value.Therefore,we proposed a two-step calculation method to determine the sum of the two heavy metal parameters as the risk value of remediated soil.In contrast to the traditional one-step calculation method,which only uses the leaching content as the risk value,this two-step calculation method can effectively avoid underestimating the risk of remediated soil.