Soaked Al powder for efficient reduction of hexavalent chromium in neutral solution

Zero-valent Al (ZVAl) is a good electron donor, however its surface passive film impedes the electron release from inside to outside. In this work, a soaking procedure was adopted to activate ZVAl powder, which was used to remove hexavalent chromium (Cr(VI)) in neutral solution. The results indicated that the soaking process can greatly enhance Cr(VI) reduction due to a layer of loose and fine flocculent Al(OH)3 grains covering on Al particles, which adsorb Cr(VI) ions and speed up the electron release from inner Al. There is an optimal soaking time, which can promote ZVAl to reduce Cr(VI) to Cr(III) in neutral solution up to 90% within 2 min. Mechanism analyses revealed that adsorption of Cr(VI) ions on soaked Al surfaces are beneficial for them to receive more electrons from inner Al, leading to reduction of Cr(VI) ions favorable in competition with Al-water reaction, because these two processes consume electrons simultaneously. It was also found that the soaked Al powder has a good reusability and Cr(VI) removal is >70% when it was recycled for 5 times. This study provides an effective way for activating Al to reduce heavy metal ions in water.