PNNL

Abstract

Understanding the transport properties of chromium (VI) enhances the ability to model and predict chromium migration and outcomes of environmental remediation strategies. This work first estimates the limit of detection of Cr(VI) in the form of chromate (CrO42-) in multicomponent electrolytes representative of nuclear waste streams at the Hanford Site in Washington State using 53Cr Nuclear Magnetic Resonance (NMR) spectroscopy, followed by an investigation into CrO42- transport properties in an idealized alkaline solution. Transport properties were measured in two different ways, the first of which was from relaxation-based measurements that used saturation recovery and Carr-Purcell-Meiboom-Gill experiments to determine T_1 (spin-lattice relaxation time) and T_2 (spin-spin relaxation time). These were used in conjunction with the Bloembergen-Purcell-Pound (BPP) equation to estimate the rotational correlation time (t_c), and subsequently 53Cr self-diffusion coefficient (D_t) of CrO4 using the Stokes-Einstein-Debeye and Stokes-Einstein equations. These relaxation-derived D_t were compared against direct measurements obtained via Stesjkal-Tanner equation analysis of pulsed field gradient stimulated echo (PFGSTE) 53Cr NMR spectra. The latter of which, although feasible, is costly in terms of total acquisition time. Monte Carlo simulations then provided detailed estimates of uncertainty propagation and highlighted predictive opportunities to improve data collection efficiency. These findings enhance our understanding of chromate transport and demonstrate the potential description of element- and isotope-specific transport properties of NMR-active nuclei traditionally deemed out of reach.