Metal Substitution-Induced Reducing Capacity of Magnetite Coupled with Aqueous Fe(II)

Aqueous Fe(II) (Fe(II)aq) effectively magnifies the reducibility of magnetite toward environmental substances. In natural magnetite, isomorphous substitution by foreign metals is ubiquitous, and Zn2+ and Co2+ have been reported to positively improve the reducing capacity of magnetite coupled with Fe(II)aq. Though most metal ions significantly alter the surface properties of magnetite, their effects on the reactivity of magnetite coupled with Fe(II)aq have rarely been systematically compared, resulting in the ambiguity of constraint mechanism and controlling factors. Herein, magnetites (Fe3-xMxO4, M = Co2+, Mn2+, Zn2+, Mg2+, Cr3+, and Al 3+) with similar substitution level (x ≈ 0.5) were synthesized, characterized, and tested for the reduction of nitrobenzene (NB) in the presence of Fe(II)aq. Both the reduction kinetics and the extent of electron transfer illustrated the positive effect of divalent metals but the negative effect of trivalent ones. Such distinct effects were further correlated to the physiochemical properties and microstructure of magnetite by the Pearson analysis. The active-site density and electrical conductivity of magnetite were critical factors determining the reduction performance of the coupled system. Specifically, Co, Mn, Zn, and Mg increased the active-site density and accordingly the adsorption capacity of Fe(II)aq. Moreover, the octahedral Mn and Co with thermodynamically favorable redox pairs, i.e., Co2+/Co3+ and Mn2+/Mn3+, accelerated electron exchange, giving rise to the increase of electrical conductivity. The tetrahedral Zn2+ induced the oxidation of octahedral Fe2+ to Fe3+, which also promoted the electron transfer. These results shed light on the role of natural magnetite and its impact on the fate of nitroaromatic compounds in anoxic environments.