Determining hexavalent chromium transport properties in alkaline nuclear waste using nuclear magnetic resonance spectroscopy

Abstract

This study focuses on the transport properties of hexavalent chromium, specifically the chromate anion, to improve predictive models and environmental remediation strategies for Cr(VI) migration. Using ⁵³Cr Nuclear Magnetic Resonance (NMR) spectroscopy, the research quantifies chromate in multicomponent electrolytes replicating nuclear waste conditions at the Hanford Site in Washington State. The consistency of the ⁵³Cr NMR signal integral with chromate concentration, despite varying matrix compositions, establishes it as a reliable concentration indicator. The transport properties of chromate in an alkaline solution were assessed using relaxation-based measurements via saturation recovery and Carr-Purcell-Meiboom-Gill experiments, determining spin-lattice and spin-spin relaxation times. These measurements, combined with the Bloembergen-Purcell-Pound equation, helped estimate the rotational correlation time and the ⁵³Cr self-diffusion coefficient using Stokes-Einstein-Debye and Stokes-Einstein equations. Direct measurements were obtained through pulsed field gradient stimulated echo ⁵³Cr NMR spectroscopy. Monte Carlo simulations further estimated uncertainty propagation. The results enhance comprehension of chromate transport and highlight prospects for identifying transport properties of NMR-active nuclei, traditionally considered unreachable.

Fig. 1. Analysis of Hanford tank waste simulants.

A Concentrations of species of interest in five different Hanford tank waste simulants to show trends in composition. The lines in the parallel coordinates diagram are drawn only to guide the eyes. B ⁵³Cr resonance of CrO₄²⁻ in the five different Hanford tank waste simulants. Spectra correspond to ~1 day of signal averaging. C Integral of ⁵³Cr NMR data in (B) versus Cr(VI) concentration determined by ICP-OES with a linear fit to the integrated ⁵³Cr NMR resonance. Note the units of concentration are millimoles/L (mM).

Fig. 2. NMR Relaxometry of 1.47 m KOH and 1.75 m K₂CrO₄.

A ⁵³Cr saturation recovery spectra at 20 °C. B ⁵³Cr CPMG spectra at 20 °C. C Data fit to determine $T_1$ and (D) $T_2$ as a function of temperature. E Diagram to show Arrhenius relationship to determine the temperature dependence of the $T_1$, $T_2$ and (F) ${\tau }_c$. The error bars (+/- σ) in this figure were determined via analysis of Monte Carlo simulations as later described. Tables of $T_1$, $T_2$ and ${\tau }_c$ are provided in Tables S4–S6 of the Supplementary Information. Note, the units of concentration are in molality.

Fig. 3. Diffusion analysis of 1.47 m KOH and 1.75 m K₂CrO₄ solutions using ⁵³Cr NMR.

A ⁵³Cr PFGSTE NMR spectra of 1.47 m KOH and 1.75 m K₂CrO₄ at 20 °C. B Stejskal Tanner plot in which the logarithm of the normalized signal intensity is related to b, for instrument parameters contributing to beta, see the methodology section. C Temperature dependence of the $D_T$ estimated with BPP, Stokes-Einstein-Debye and Stokes-Einstein Equations with comparison the direct measurement of $D_T$ at 20 °C with PFGSTE NMR. A table of $D_T$ is provided in Table S7 of the Supplementary Information. The error bars (+/- σ) in this figure were determined via analysis of Monte Carlo simulations as later described. Note, the units of concentration are in molality.

Fig. 4. Monte Carlo uncertainty analysis of NMR relaxometry and diffusion experiments.

Analysis (A) Inversion recovery experiments where 20 of 10,000 simulations are shown (B) CPMG experiments, where 20 of 10,000 simulations are shown (C) PFGSTE NMR experiments, where 20 of 14,000 simulations are shown. Resulting distributions of (D) $T_1$, (E) $T_2$, (F) ${\tau }_c$, (G) $D_T$ estimated from relaxometry (H) $D_t$ measured from ⁵³Cr PFGSTE NMR, where P is probability. Skewed Gaussian fits are in orange and symmetric gaussian fits are marked with a black dashed line. Uncertainties (+/− σ) are likewise annotated.