Enhancement of Cr(VI) removal by mechanically activated micron-scale zero-valent aluminum (MA-mZVAl): Performance and mechanism especially at near-neutral pH

The dense oxide film greatly limits the strong reducibility of micron-scale zero-valent aluminum (mZVAl), especially at near-neutral pH. In this study, we found that the removal efficiency of Cr(VI) can be greatly enhanced over a wide pH range (3.00-11.00) by mechanically activated mZVAl (MA-mZVAl), which was prepared from mZVAl by ball milling (BM) in the presence of additive NaCl. Compared with mZVAl, at initial pH 7.00, MA-mZVAl removed Cr(VI) at a roughly 292-fold faster rate. The effects of BM parameters including the amount of additive NaCl, BM speed, BM time, and BM atmosphere on Cr(VI) removal were first evaluated. Then, the Cr(VI) removal mechanism was explored by measuring the concentration of different valent chromium, monitoring the changes of water chemical parameters including DO, pH, ORP, Al ions concentration, and by ascertaining the role of Cl- in solution. Furthermore, the structure evolution of mZVAl after BM and the surface corrosion mechanism of MA-mZVAl during reaction with Cr(VI) were unveiled through the characterization by means of SEM-EDS, BET, XRD, and XPS. Based on above tests, we found that Cr(VI) was reductively removed by MA-mZVAl to form Cr(OH) 3(s) precipitated on the surface of MA-mZVAl. NaCl promoted the reactivity of MA-mZVAl not in corrosion process but in BM process. The dense oxide film was destroyed by BM, and changed into a rough Al-(hydr) oxide film after corrosion. The reusability of MA-mZVAl was hindered by the formation of Al and Cr (hydr) oxide, but can be easily and completely rejuvenated by BM. Overall, BM can not only break through the bottleneck of oxide film but recover the passivated MA-mZVAl. Therefore, MA-mZVAl particles are of considerable potential for pollutants removal.