Neglected readsorption and redistribution of heavy metals during the BCR sequential extraction procedure - form swine manure to biochar.

The BCR®sequential extraction process (SEP) is widely applied to assess the heavy metals (HMs) fractions, i.e. F1 (exchangeable/acid‑soluble), F2 (reducible), F3 (oxidizable) and F4 (residual), in diverse biomass and their biochars. However, no study has systematically investigated how readsorption and redistribution during BCR®SEP affect the interpretation of HMs speciation evolution from biomass to biochar. In this study, the standard addition method, which was performed by spiking samples with no greater than 100% of the natural HMs content in each step of BCR®SEP, was applied to quantify readsorption and redistribution artifacts during BCR®SEP for swine manure (SM) and biochars produced at 300 °C, 500 °C, and 700 °C. Results revealed that copper (Cu) was predominantly readsorbed during F1(59.91-97.78%) and F2 (54.09-80.78%) steps; zinc (Zn) was primarily readsorbed during F1 (64.85-87.90%) step, which were significantly higher than those for SM, except for SMB300 in F1. Following readsorption, Cu in biochar was redistributed mainly into the F3 (30.91-71.24%) and F4 (27.82-68.05%) fractions, whereas Zn redistribution occurred predominantly into F2 (27.78-33.46%) and F4 (42.21-66.56%). After quantitative correction, the apparent proportions of HMs assigned to F1 and F2 increased markedly, while those assigned to F3 and F4 decreased correspondingly. Consequently, the modified potential ecological risk index (RI) based on corrected fractions was 59.3-329.0% higher than the RI calculated from measured fractions. These artifacts of conventional BCR®SEP could overestimate the stability of HMs in animal manures and their biochars upon soil amendment, potentially affecting fractionation-based assessments of environmental risk.