Persistently high Cr6+ removal rate of centi-sized iron turning owing to tribocatalysis

Fe0 has been found a good material for removing heavy metal from waste water due to high efficiency, low cost, facile operation. But its application has been impeded by its rapidly decreasing reduction rate with increasing treatment time owing to easily oxidization during reduction. This study found that a centi-sized commercial iron turning with a thin layer of amorphous iron oxides (AIO) possesses persistently high Cr6+ removal rate of 98% in the testing time (36 h). The tribocatalysis arising from the friction between the solution and above-said AIO could account for above-mentioned results. During the first tribocatalysis stage, the electrons of Fe0 in iron turning were stimulated to higher energy and more easily went through the interface Fe0/AIO, resulting in increasing Cr6+ reduction efficiency, and the reaction from the Fe0 inside AIO to Fe2+ and Fe3+. During the next tribocatalysis stage, Since there is weak bonding between Fe0 and FeO due to lattice mismatch, the surface AIO dropped from iron turning in case of friction. Thus, much more Fe0 was exposed to Cr6+ solution and much higher Cr6+ removal rate has been obtained. More importantly, the high Cr6+ removal rate can be persistently kept. It has also been found that iron with larger friction area favors in tribocatalysis than one with less friction area. All in all, this work has found that an effective way-tribocatalysis-to lift the possibility that Fe0 go through potential barrier to react with heavy metal ions, and to timely remove the covered materials on the surface of Fe0, and therefore resulting in persistently high Cr6+ removal rate. This work has also given a clear picture for tribocatalysis, facilitating people to understand the mechanism of tribocatalysis.