Field-scale evaluation of nanoscale zero-valent iron and biochar coupled with phytoremediation for in situ stabilization of polluted soil

Biochar and zero-valent iron nanoparticles (nZVI) are promising amendments for stabilizing polluted soils due to their cost-effectiveness and environmental benefits. However, their field-scale application and impact on the soil-plant system remain underexplored. This work evaluated a pilot-scale experiment using As- and metal-polluted soil, where 7% wt biochar and 1% wt nZVI were applied individually and combined over 60 days. Soil properties, pollutant mobility, and the physiological response of Brassica juncea were assessed. The amendments significantly reduced metal mobility in the soil (up to 73%, 63%, 59% and 81% for Cu, Zn, Cd and Pb, respectively), which correlated with a decrease accumulation in roots. However, biochar-treated soil resulted in an elevated As concentration in the aerial part of the plants (37% increase), while the nZVI application led to a notable reduction (41% decrease). Plants grown in biochar-treated soil exhibited reduced levels of H 2 O 2 and MDA (34% and 21% decrease), whereas leaves of plants growing in nZVI-treated soil revealed a higher H 2 O 2 concentration (27% increase). Interestingly, the combined application of biochar and nZVI mitigated the oxidative stress induced by the Fe-based amendment. Furthermore, the highest biomass production was observed in plants grown in biochar-treated soils, both alone and combined with nZVI, which coincided with the highest proline accumulation in leaves. In conclusion, these findings suggest that biochar and nZVI co-application emerges as a reliable strategy to achieve an adequate balance between the simultaneous immobilization of As and metals and promoting satisfactory plant growth without relevant symptoms of stress. • Biochar and nZVI combination boosts in situ phytoremediation efficiency. • Enhanced Pb immobilization in soil shows strong synergy between both amendments. • Biochar improves As uptake in plants, but nZVI reduces this effect. • Biochar mitigates oxidative stress in birch caused by metal(loid) exposure. • Simultaneous biochar application reduces nZVI-induced oxidative stress effects.