Biochar compost associations affect the absorption, translocation and bioavailability of legacy and newly introduced cadmium in lettuce

Biochar is an environment-friendly immobilizing agent and performs better in remediating soil cadmium (Cd) along with compost. However, little attention has been paid to the effects of the intrinsic properties of biochar in combination with compost on the phytoavailability and translocation of legacy Cd (CdL) and newly introduced Cd (CdN) in the soil-lettuce-human system. Therefore, in this study, four biochars originated from pig manure, sewage sludge, wood chip, and rice husk were produced into mm/μm/nm-particles, which was combined with compost respectively. The impact of the different particle-size biochar compost blends (BCCM) on the translocation, bioaccessibility and bioavailability of CdL and CdN was investigated though applying enriched isotope tracing and Caco-2 cell model. The results showed that the lowest labile CdL/N (ECdL/N) values were found in nm-BCCM by converting exchangeable Cd into other unusable Cd forms. nm-BCCM has a significant reduction in CdL/N uptake, accumulation, and translocation by lettuce as nm-BCCM increased the cell wall percentages and decreased the solution components of CdL/N in the root. Despite almost immobilizing agents did not decrease the bioaccessibility and bioavailability of CdL/N, the lower bioaccessible and bioavailable CdL/N contents imply nm-BCCM can guarantee the public was minimally exposed to the risk of Cd. These results suggested that combining nm-biochar and compost had greater potential to reduce CdL and CdN translocation and absorption in complex environmental systems.