Synergizing carbon sequestration mechanisms during the remediation of Cr(VI) by nano zero-valent iron loaded biochar (nZVI-BC)

Abstract

Applying nano zero-valent iron loaded biochar (nZVI-BC) to remediate soil heavy metal pollution has been frequently tested in current studies. However, little knowledge has been disclosed regarding the effect of nZVI-BC on soil organic carbon (SOC) mineralization/sequestration. In this study, the dual effect of nZVI-BC addition on Cr(VI) removal and SOC mineralization/sequestration and the underlying mechanisms were explored, based on a 90 days laboratory soil incubation. The results confirmed that nZVI-BC significantly improved the Cr(VI) removal rate while markedly reducing the cumulative CO2 emissions. Following the 90 days incubation, nZVI-BC addition notably increased the contents of Fe-oxides, Fe-bound organic carbon (Fe-OC), and SOC. Pearson correlation and PLS-SEM analyses indicated that the enhanced Fe-OC content, resulting from increased Fe-oxides, was a critical abiotic mechanism for improved SOC preservation. Additionally, the observed decrease in enzymatic activity and the shift in bacterial community from unstable carbon-dominant taxa (e.g., Proteobacteria and Actinobacteria) to stable carbon-dominant taxa (e.g., Firmicutes) in the nZVI-BC treatments suggested that SOC bio-mineralization was also inhibited. Overall, this study demonstrates that nZVI-BC not only aids in heavy metal remediation but also promotes long-term SOC preservation.