Cysteine induced cascade electron transfer by forming a unique ternary complex with Fe(II) on goethite

Abstract

The interfacial electron transfer on iron (Fe) minerals with electron shuttles may play a great significance to the fate of pollutants in soils, while the exact surface chemistry is barely understood. This work systematically investigated cysteine induced the reductive transformation of nitrobenzene (NB) on goethite by means of electrochemistry and kinetic modeling. Results show that NB can be efficiently reduced by cysteine on goethite with a rate constant of 0.72 h−1 that is remarkably higher than that of 0.05 h−1 by cysteine. The concomitant production of surface bound Fe(II) has also observed contributing to NB reduction, which cascaded transferring electrons from cysteine to surface Fe(III), and towards NB. Raman spectra and electrochemical results support the formation of a unique ternary FeOOFe(II)-Cyt complex (Emid = − 0.26 V) that is more thermodynamically favorable than surface bound Fe(II) (Emid = − 0.12 V) for electron transfer. Meanwhile, solid state analysis demonstrates that no apparent secondary Fe minerals form, ruling out the effects of the change of crystalline structure on the efficiency of NB reduction. Accordingly, a kinetics model has been developed by combining elementary reactions, finely describing the interfacial reduction of NB on induced by cysteine. Results show that the relative contribution of the ternary complex FeOOFe(II)-Cyt complexes account for more than 60% of NB reductions. Our findings provide new insights into understanding the surface chemistry of electron shuttles on Fe minerals and its significance for the transformation and fate of pollutants in the environment.