Surface modified zero-valent aluminum for Cr(VI) removal at neutral pH

Zero-valent aluminum has attracted considerable attention in degradation of organic contaminants and reduction of heavy metal ions due to its very low redox potential (E0(Al3+/Al0) = −1.662 V). Usually there is a passive oxide film on Al surface, inhibiting its electron transfer and leading to low reaction efficiency. In this work, metal Al surface was modified by loose fine Al2O3 (SM-Al) using a procedure of soaking Al powder in water for a time period and then heat-treating in vacuum. It was found that surface modification remarkably enhanced the reaction efficiency of zero-valent Al in reduction of Cr(VI) to Cr(III) ions in aqueous solution. The longer the soaking time of Al powder in water is, the higher the efficiency in removing Cr(VI) is. The effects of Al particle size and dosage, initial Cr(VI) concentration and reaction temperature on Cr(VI) removal were investigated as well. XPS analyses revealed that the decrease in thickness of Al surface passive oxide films and the promotion of surface covered loose Al2O3 phases on hydration of passive oxide films are two main reasons responsible for the rapid removal of Cr(VI) by SM-Al. Meanwhile, SM-Al exhibited an excellent recyclability and the reused SM-Al has a very fast reaction rate relative to as-prepared SM-Al, 80% of Cr(VI) can be removed even it was reused up to five cycles. The present study provides a new route to activate Al in removal of contaminants in aqueous solution.