Perrhenate sorption kinetics in zerovalent iron in high pH and nitrate media

Technetium(Tc)-99 is one of major risk drivers in low level radioactive liquid waste at the U.S. Department of Energy sites. Cementitious waste technology (CWT) has been considered immobilizing pertechnetate, Tc(VII)O-4(-), in brine and alkaline waste solutions, as Tc(IV) oxides and/or sulfides with the use of reducing agents like slag. In this study, zero valent iron (ZVI) was evaluated as a potential reducing agent in CWT as a function of pH and [nitrate] (0-0.1 M) using perrhenate, Re(VII)O-4(-), as an analogue for Tc(VII)O-4(-). Batch Re(VII)O-4(-) sorption experiments in conjunction with X-ray absorption spectroscopic analysis showed that the Re(VII) sorption occurred via the reductive precipitation of Re(IV)O-2(s) and the extent of sorption decreased with increasing pH from 8 to 10. Interestingly, pseudo 2nd order kinetic rates increased with increasing [nitrate] which was attributed to co-adsorption of NH4* (i.e., a reaction product of reduced nitrate by ZVI), facilitating electrostatic attraction towards Real-under alkaline conditions. Considering the thermodynamically favorable reduction ofTc(VII) over Re(VII), ZVI might have potential for improving the reduction capacity of the current CWT. (C) 2016 Elsevier B.V. All rights reserved.