Impact of Cr and Co on 99Tc retention in magnetite: A combined study of ab initio molecular dynamics and experiments

The effect of co-mingled dopants, Co(II) and Cr(III), on Tc(IV) incorporation and retention in magnetite under varying temperatures (75-700 degrees C) was explored using ab initio molecular dynamics simulations, batch experiments, and solid phase characterization. Tc(IV) stabilization was achieved with a magnetite surface over-saturated with or containing an equal number of Tc and Cr. Under oversaturation conditions, the forced formation of a Cr2O3 phase on the magnetite surface may help prevent Tc release. Upon Co addition, and depending on the relative concentration of Tc, Cr, and Co at the magnetite surface, Co was found to preferentially stabilize Cr rather than Tc and suppress the formation of the protective Cr2O3 surface phase. Only systems with similar Cr/Co concentrations or relatively high Cr concentrations stabilized Tc within magnetite. As such, the relative concentration of Tc, Cr, and Co was identified as a critical parameter for maximizing dopant efficacy towards Tc stabilization in magnetite.