Nanoscale zero-valent iron/biochar composites containing persistent free radicals (PFRs) for degradation of p-nitrophenol.

Despite the vital roles of Fe/biochar composites in the Fenton-like systems for eliminating pollutants that have been recognized, the contributions of persistent free radicals (PFRs) of carbon-based materials are typically overlooked. In this study, the high-PFR-containing biochar nanoiron composites were prepared (nZVI/500), and the in situ generation of hydroxyl radicals (·OH) and degradation of p-nitrophenol (PNP) were investigated. The results showed that nZVI/500 could effectively remove PNP in solution within the pH range of 3-8. Quantitative experiments of ·OH presented that, compared with low PFRs-containing composites, nZVI/500 could generate 64.6 µM ·OH in 60 min without any extra energy consumption. Mechanistic studies revealed that (1) both PFRs and Fe are able to utilize dissolved oxygen to generate HO in situ; (2) PFRs can promote the cycling of Fe/Fe in the system due to their strong electron exchange ability; and (3) PFRs directly transfer electrons to HO; therefore, the presence of PFRs accelerates the generation of ·OH in the system and facilitates the removal of PNP. This study provides an important theoretical basis and technical reference for expanding the application of PFR-rich carbon-based materials to remove environmental pollutants.