PNNL

Abstract

Sodium aluminate electrolytes are of industrial and geochemical importance as intermediates in the Bayer process for aluminum refining and high-level nuclear waste processing. We utilized quasi-elastic neutron scattering (QENS) and proton nuclear magnetic resonance spectroscopy (1H NMR) to investigate the picosecond (ps) to microsecond (ms) timescale motions of H-bearing species (Al(OH)4- monomers and Al-clusters, OH- and H2O) in extremely concentrated sodium aluminate solutions. Variable temperature, elastic window scans demonstrated that the addition of Al solutes to NaOH solutions resulted in more gradual changes in the elastic line profiles when compared to a pure (Al-free) NaOH solution, where a sharp liquidus-solidus transition was observed. The QENS signal was analyzed by applying a two-Lorentzian fitting function. No short-time translational diffusion was observed but two types of stochastic localized motions were found: (A) local backbone tumbling of large hydrated ion clusters on the order of 40-100 ps at 293 K; and (B) much slower, complex, collective dynamics of the ensemble of H-bearing species on the order of 350 – 1400 ps at 293 K. Variable temperature, pulsed field gradient, diffusion ordered NMR was used to quantify the ensemble translation of the H-species, along with NMR relaxometry to calculate rotational correlation coefficients and complementary molecular dynamics to consider the overall solution structure. The ensemble rotational correlation times were on the order of 184-300 ps from 1H NMR, which is in agreement with the timescale of the fast and slow QENS components. Understanding the mobility of the H-bearing entities in these industrially-relevant, concentrated sodium aluminate electrolytes is critical to predict their behavior during processing for aluminum recovery and nuclear waste retrieval. These results can also be extended to other important concentrated electrolytes.