Shewanella oneidensis MR-1 for enhanced the reactivity of FA-stabilized nZVI toward Cr(VI) removal

In present work, nanoscale zero-valent iron (nZVI) was successfully stabilized using fulvic acid (nZVI-FA). Shewanella oneidensis MR-1 (MR-1) was used to enhance the reactivity of the nZVI-FA. The results demonstrated that nZVI-FA combined with the MR-1 (nZVI-FA/MR-1) system could effectively solve the aggregation and surface passivation of nZVI, thereby significantly enhancing the reactivity of nZVI for Cr(VI) removal. The Cr(VI) removal efficiency of the nZVI-FA/MR-1 system (100 %) within 24 h was apparently higher than that of the ZVI-FA system (64.43 %). The enhancement of Cr(VI) removal is because fulvic acid (FA) FA can stabilize nZVI against aggregation; MR-1 could rapidly reduce the Fe(III) (hydr)oxides (FeHOs) passivation layers on the surface of nZVI-FA to biogenic Fe(II) [bio-Fe(II)], which in turn reduced Cr(VI). X-ray photoelectron spectroscopy (XPS) analysis revealed that the Fe(II)/Fe(III) ratio (1.20) and Cr(III)/Cr(VI) ratio (2.53) of the precipitates in the nZVI-FA/MR-1 system were much higher than that those in the nZVI-FA system (0.55 and 1.40, respectively). Three-dimensional excitation-emission matrix (3D-EEM) spectral analysis confirmed that tyrosine, tryptophan, and protein-like substances in extracellular polymeric substances (EPSs) from MR-1 are vital for the removal of Cr(VI). The mechanism analysis demonstrated that most Cr(VI) was reduced to Cr(III); Cr(III) existed as Cr2O3, FeCr2O4 and CrxFe1-x(OH)3 precipitates, and some of the Cr(VI) was removed from the solution by adsorption and complexation. This study offers a low-cost, efficient, and environmentally friendly strategy to enhance the reactivity of nZVI for Cr(VI) removal, which has broad prospective applications.