Rust triggers rapid reduction of Cr6+ by red phosphorus: The importance of electronic transfer medium of Fe3+

Red phosphorus (P) is one of the metalloid materials, with five external electrons, it should be an excellent electron donor. However, the activity of red P to reduce Cr6+ is limited. Due to electrostatic repulsion, it is difficult for the electrons on the red P to transfer to the chromate anion (Cr6+). Interestingly, we found that Fe3+ derived from rust, waste iron or Fe3+ reagents can be used as the electron transport medium to solve the electron transport obstacles between red P and Cr6+. As a result, the reduction of Cr6+ by red P/rust system takes only 20 min, which is far lower than the 140 min of red P. The reduction rate of Cr6+ in the red P/rust system is about 12.3 times that of red P. The reaction mechanism is that red P is not easy to access chromate anions but can easily adsorb Fe3+. The adsorbed Fe3+ will be reduced to Fe2+ by red P, and the regenerated Fe2+ will diffuse into the solution to rapidly reduce Cr6+. Therefore, this work provides an alternative waste iron reuse pathway and also sheds light on the important role of electron medium in reduction reaction.