Remarkable effect of Co substitution in magnetite on the reduction removal of Cr(VI) coupled with aqueous Fe(II): Improvement mechanism and Cr fate

The interaction between magnetite and aqueous Fe(II) profoundly impacts the mineral recrystallization, trace-metal sequestration, and contaminant reduction. The iron ions in natural magnetite are extensively substituted by other cations. It is still unclear whether the substitution with thermodynamically favorable redox repairs (e.g., Co²⁺/Co³⁺) plays a vital role in the reducing capability of the coupled system. Herein, a series of Co-substituted magnetite samples (Fe_3−xCo_xO₄, 0.00 ≤ x ≤ 1.00) were synthesized and tested for the reductive removal of Cr(VI) in the presence of Fe(II). Fe_3−xCo_xO₄ had a spinel structure with the preferential occupancy of Co²⁺ on octahedral sites. No visible variation in the BET surface area was observed, whereas the surface site density increased gradually with Co substitution. Cr(VI) was found first adsorbed on the Fe_3−xCo_xO₄ surface and then reduced to Cr(III) by the structural Fe²⁺ and the absorbed Fe(II), accompanied by the oxidation of bulk Fe²⁺ and surface Fe(II) in Fe_3−xCo_xO₄ without phase transformation. The Cr(III) was precipitated on the Fe_3−xCo_xO₄ surface with Fe(III), or substituted octahedral Fe in Fe_3−xCo_xO₄. Both the reaction kinetics and the electron transfer efficiency revealed that Co substitution significantly improved the reactivity of Fe_3−xCo_xO₄/Fe(II) towards Cr(VI) reduction. This was ascribed to the presence of the redox pairs Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺ accelerating electron transfer from the Fe_3−xCo_xO₄ interface to Cr(VI).