Titanomagnetite is a common naturally occurring iron oxide in sediments with basaltic provenance,having the potential to react with important environmental contaminants including Tc.The Ti(Ⅳ)content(x)in titanomagnetite(Fe3-xTixO4)determines the structural Fe(Ⅱ)/Fe(Ⅲ)ratio,and thus affects the electron transfer kinetics to contaminants.Here,the characteristics of natural Fe(Ⅱ)/Fe(Ⅲ)-bearing microparticles from sediments at the Hanford nuclear reservation,Washington,U.S.A,were systematically studied.Inspired by the ability of the ground titanomagnetite microparticles to reduce Tc(Ⅶ),Titanomagnetite(Fe3-xTixO4)nanoparticles(10-12 nm)with varying Ti content(0 ≤ x ≤ 0.53),as synthetic analogues,were made.Their batch redox reactivity with aqueous Tc(Ⅶ)were studied as a function of Ti content.The results of reactions with 10 and 30μM Tc(Ⅶ)solutions,respectively,indicate that increasing structural Fe(Ⅱ)/Fe(Ⅲ)ratio in the nanoparticles yielded fast exponentially decaying reduction kinetics.Nanoparticles before and after reduction experiments and surface-associated products of Tc(Ⅶ)reduction were characterized using transmission electron microscopy(TEM),X-ray absorption near-edge spectroscopy(XANES),extended X-ray absorption fine structure spectroscopy(EXAFS),micro X-ray diffraction(μ-XRD),X-ray absorption(XA)and X-ray magnetic circular dichroism(XMCD).Based on the concentrations of soluble Fe(Ⅱ),structural Fe(Ⅱ),as well as soluble pertechnetate anion [Tc(Ⅶ)O4-],the reaction process was quantitatively linked to mass and electron balanced changes in the Fe3-xTixO4 nanoparticles,and the accessibility of structural Fe(Ⅱ)from these phases was determined.This study implies that titanium substitution is an elegant way to adjust the structural Fe(Ⅱ)/Fe(Ⅲ)ratio in titanomagnetite and increase the redox reactivity towards key environmental contaminants.