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 Fe 0 /Fe 3 O 4 /FeCl 2 microcomposite (hZVI bm ) was synthesized via simple ball-milling of the ternary mixture of ZVI, Fe 3 O 4 , and FeCl 2 ·4H 2 O (hZVI). SEM-EDX and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) indicated the hZVI bm microcomposite (10–20 μm) consisted of Fe 0 core covered by ∼3.3 μm-thick shell decorated with Fe 3 O 4 /FeCl 2 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 Fe 3 O 4 /FeCl 2 -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.