Rust triggers rapid reduction of Cr by red phosphorus: The importance of electronic transfer medium of Fe.

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 Cr is limited. Due to electrostatic repulsion, it is difficult for the electrons on the red P to transfer to the chromate anion (Cr). Interestingly, we found that Fe derived from rust, waste iron or Fe reagents can be used as the electron transport medium to solve the electron transport obstacles between red P and Cr. As a result, the reduction of Cr by red P/rust system takes only 20 min, which is far lower than the 140 min of red P. The reduction rate of Cr 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 Fe. The adsorbed Fe will be reduced to Fe by red P, and the regenerated Fe will diffuse into the solution to rapidly reduce Cr. Therefore, this work provides an alternative waste iron reuse pathway and also sheds light on the important role of electron medium in reduction reaction.