Activation of zero-valent iron through ball-milling synthesis of hybrid Fe 0 /Fe 3 O 4 /FeCl 2 microcomposite for enhanced nitrobenzene reduction.

To activate zero-valent iron (ZVI) for efficient nitrobenzene (NB) reduction, a hybrid Fe0 /Fe3 O4 /FeCl2 microcomposite (hZVIbm ) was synthesized via simple ball-milling of the ternary mixture of ZVI, Fe3 O4 , and FeCl2 ·4H2 O (hZVI). SEM-EDX and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) indicated the hZVIbm microcomposite (10-20 μm) consisted of Fe0 core covered by ∼3.3 μm-thick shell decorated with Fe3 O4 /FeCl2 fine particles (0.1-2 μm). Efficient removal (>95%) of NB (200 mg/L) was achieved by hZVIbm (2.0 g Fe/L) in 30 min over a wide pH range from 3 to 9. Notably, the NB removal efficiency of hZVIbm 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 Fe3 O4 /FeCl2 -inlaid shell and the Fe(II) components played significant activation roles, as observed from the comparative experiments in their absence via pretreatments of hZVIbm 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.