Polydopamine as electron shuttle and dispersant: Unlocking nZVI reactivity coupled with Shewanella oneidensis MR-1 for efficient Cr(VI) removal

Abstract

The inherent limitations of nano zero-valent iron (nZVI) severely limit its widespread application in heavy metal pollution remediation. To address this, Shewanella oneidenis MR-1 (MR-1) was utilized to counteract the iron oxide passivation layer on nZVI surface, however, this approach suffered from low electron transfer rate between MR-1 and the passivation layer. In this research, a novel PDA-modified nZVI composite (PDA-nZVI) was synthesized. Compared to unmodified nZVI, the proportions of Fe⁰ and Fe(II) on the PDA-nZVI surface increased 2.06 % and 21.63 %, respectively. The electrochemical tests (e.g., a more negative corrosion potential, a lower charge-transfer resistance, a higher current density, and a 3.4-times larger electrochemical active surface area for PDA-nZVI compared to nZVI) confirm that PDA enhances electron transfer from iron core to the outside, thereby promoting Fe⁰ availability and Fe(II) generation on the PDA-nZVI surface. Meanwhile, PDA modification successfully enhanced the material's dispersibility and hydrophilicity in aqueous solutions. Results demonstrate that the Cr(VI) elimination capacity of the PDA-nZVI and MR-1 combined system (PDA-nZVI/MR-1) was markedly superior to that of the nZVI/MR-1 system. Mechanism analyses demonstrate that PDA can act as an electron shuttle to accelerate electron transfer between MR-1 and the passivation layer, thereby promoting more Fe(III) transformation to Fe(II). This process enables more Cr(VI) to be reduced to Cr(III), which is subsequently adsorbed or complexed on the material surface as Cr₂O₃/FeCr₂O₄ and Cr_xFe_1-x(OH)₃, ultimately being removed from the solution via co-precipitation. The PDA-nZVI/MR-1 system successfully overcomes the inherent limitations and exhibits an exceptional Cr(VI) elimination capability