Enhanced reactivity of zero-valent aluminum with ball milling for phenol oxidative degradation

Micron-sized zero-valent aluminum (ZVAl), a heterogeneous Fenton-like catalyst in organic wastewater treatment, whose catalytic activity is limited by the dense and stable oxide layer coating on its surface. In this paper, a simple method of ball milling was exploited to pretreat inert aluminum particles with the addition of low-cost and non-toxic sodium chloride (NaCl) grains. Then the pretreated ZVAl (marked as ZVAl^bm) was employed to activate molecular oxygen catalytically for phenol oxidative degradation. No induction period was observed in ZVAl^bm/Air system. Meanwhile, the reaction rate and mineralization efficiency of phenol degradation had improved in contrast with the original ZVAl. The characterization results of SEM-EDS, BET, XRD and XPS revealed that the native oxide layer of ZVAl^bm was destroyed and became rougher, where its surface was embedded in NaCl grains. Thus the dissolution of NaCl in aqueous solution was imagined to expose the fresh surface of ZVAl^bm, facilitating the electron transfer at the interface of ZVAl^bm/H₂O. Moreover, the specific surface area of ZVAl^bm increased for ball milling improved its surface roughness, resulting in the enhanced reactivity of ZVAl^bm. The interfacial reaction mechanism was revealed that more dissolved oxygen (DO) was activated by the exposed surface of ZVAl^bm to form large amounts of hydrogen peroxide (H₂O₂). Then in-situ production of H₂O₂ was catalyzed by the active-surface of ZVAl^bm via a Fenton-like process to generate massive OH, which was detected as the predominant active species for phenol degradation. Finally, the reusability experiment indicated that ball milling could rejuvenate the main catalytic activity of used ZVAl^bm easily. In summary, ball milling provides a green and easily-operated method to promote the reactivity of inert ZVAl for its application in organic wastewater treatment.