Oxidation and recharge of reactive structural Fe(II) in titanomagnetite (Fe_3-xTi_xO₄) nanoparticles

Mixed-valent iron oxide minerals, such as magnetite (Fe(II)(Fe(III))₂O₄), are an important source of solid-state ferrous iron (Fe(II)) that can impact the speciation and transport of electron accepting contaminants in the Earth's subsurface, such as radioactive pertechnetate (⁹⁹Tc(VII)O₄^-). However, when oxidizing conditions are encountered, structural Fe(II) at the mineral surface is consumed yielding a maghemite (γ-Fe(III)₂O₃)-like layer that limits further electron transfer. This oxidized surface layer can be recharged back to the original Fe(II)/(III) ratio by re-exposure to reducing conditions, i.e., aqueous solutions containing Fe²⁺. However, for substituted magnetite (Fe_3-xM_xO₄, M = transition metal cation), the extent of this redox recyclability is unclear. Here, we examine oxidation and recharge for titanomagnetite (Fe_3-xTi_xO₄) nanoparticles, where the Fe(II)/Fe(III) ratio varies by the amount of Fe(II) required to charge balance the titanium (Ti(IV)) substituted into the structure. The nanoparticles were synthesized by aqueous precipitation from a solution containing ferrous, ferric and titanium chloride at room temperature. Transmission electron microscopy combined with electron energy loss spectroscopy revealed that rapid precipitation formed core-shell-like nanoparticles consisting of a hyperstoichiometric magnetite core, with Ti(IV) and charge balancing Fe(II) enriched at the surface. This surface enrichment made Fe(II) more available for electron transfer reactions with redox active solution species. Examination of oxidation by H₂O₂ followed by recharge with aqueous Fe²⁺ indicates recyclability of reducing equivalents in the nanoparticles, yielding a core recrystallized to stoichiometric magnetite and a shell bearing excess Fe(II) to charge balance the substituted Ti(IV). The recharged particles are shown to have restored redox reactivity with ⁹⁹Tc(VII)O₄^- resulting in reduction to ⁹⁹Tc(IV)O₂ and oxidation of the structural Fe(II) to Fe(III).