Enhancing surface gully erosion of micron-sized zero-valent aluminum (mZVAl) for Cr(VI) removal: Performance and mechanism in the presence of carbonate buffer

As a unique amphoteric metal, zero-valent aluminum (ZVAl) possesses clear superiority in water treatment under alkaline condition. However, the research on corrosion mechanism of micron-sized zero-valent aluminum (mZVAl) to remove pollutants in alkaline condition remains obscure. In this work, the reductive performance, mZVAl surface corrosion mechanism and Cr(VI) removal process in the presence of carbonate (CO32−) buffer under anaerobic condition were deeply investigated. The results indicated that efficient Cr(VI) removal can be achieved in the CO32− buffer system, and the removal rate constant (kobs,Cr(VI)) was the highest among all three buffer systems (CO32−, B4O72− and HPO42− buffers), which was 17.6 times that of OH− control system. Meanwhile, Cr(VI) removal performance after multiple uses of mZVAl was still fully maintained. Based on the characterization of SEM-EDS, BET, XRD and XPS during the anaerobic corrosion process, we found that surface cracking of mZVAl appeared “gully”, in contrast to the mZVAl with new secondary passivation film Al-(hydr)oxide in the form of small fragments covering on the surface in the OH− control system. Furthermore, with the increase of reuse times, the gullies became larger and deeper and were present as a scene of “criss-cross ravines and gullies”, which incredibly increased the specific surface area (SBET enlarged 242.2 times) and total pore volume of mZVAl, and simultaneously exposed more Al0. Therefore, Cr(VI) was far more likely to obtain electrons released from mZVAl surface erosion, which then reduced to Cr(III) and combined with OH− to form Cr(OH)3 precipitation on the surface. Through solution parameters mensuration, chromium balance determination and Cr(VI) desorption experiment, Cr(VI) reduction reaction rather than adsorption was verified. And the electron utilization efficiency of Cr(VI) removal and mZVAl corrosion degree were quantitatively calculated via H2 generation experiments.