Enriched isotope tracing to reveal the fractionation and lability of legacy and newly introduced cadmium under different amendments

Abstract The newly introduced Cd (CdN) has different environmental fates than legacy Cd (CdL) and how to distinguish them in soil under different amendments is crucial for understanding natural aging and engineered remediation of Cd pollution in soil. In this study, enriched stable isotope tracer (112Cd) was introduced to distinguish the fate of CdN and CdL in paddy soil under pH adjustment and quicklime, slaked lime, and biochar amendments. The behaviors of CdN and CdL were studied during 56 days of flooding incubation through overlying water analysis, sequential extraction fractionation and lability (exchangeable pool probed by 110Cd isotopic spike) assessment. The results showed that soil pH is the main driving factor controlling the partition of both CdN and CdL in overlying water. During the incubation, CdN transformed quickly from soluble fraction to residual fraction under all treatments. In addition, at the end of the incubation, CdN concentrations in residual fraction were much higher than that of CdL, suggesting a more thorough aging of CdN than CdL. The labile CdN (ECdN) under pH adjustment and biochar amendment decreased during incubation and ECdN% was essentially the same with that of ECdL% after 28 days, indicating the aging equilibrium of exchangeable pool of CdN.