Activation of zero-valent iron through ball-milling synthesis of hybrid Fe⁰/Fe₃O₄/FeCl₂ microcomposite for enhanced nitrobenzene reduction

To activate zero-valent iron (ZVI) for efficient nitrobenzene (NB) reduction, a hybrid Fe⁰/Fe₃O₄/FeCl₂ microcomposite (hZVI^bm) was synthesized via simple ball-milling of the ternary mixture of ZVI, Fe₃O₄, and FeCl₂·4H₂O (hZVI). SEM-EDX and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) indicated the hZVI^bm microcomposite (10–20 μm) consisted of Fe⁰ core covered by ∼3.3 μm-thick shell decorated with Fe₃O₄/FeCl₂ fine particles (0.1–2 μm). Efficient removal (>95%) of NB (200 mg/L) was achieved by hZVI^bm (2.0 g Fe/L) in 30 min over a wide pH range from 3 to 9. Notably, the NB removal efficiency of hZVI^bm was over 30 times higher than the virgin ZVI or over three times higher than hZVI. The enhanced reactivity synergistically resulted from both chemical and physical aspects. Chemically, the Fe₃O₄/FeCl₂-inlaid shell and the Fe(II) components played significant activation roles, as observed from the comparative experiments in their absence via pretreatments of hZVI^bm by sonication and rinsing, respectively, with direct evidence of depassivation effect by XRD analysis. Physically, the ball-milling-induced inter-particle compaction effect was considered crucial to facilitate the interfacial mass/electron transfer processes during the reduction. The reduction pathway from NB to aniline via two intermediates was analyzed by liquid chromatography.