Gibbsite and boehmite are not only important precursor minerals for aluminum and aluminum oxide, broadly used in many industrial areas, they are also the primary non-radioactive constituents of the high-level waste sludge at various former nuclear material production sites. For the latter, the dissolution characteristics of these aluminum mineral phases will significantly affect the reprocessing strategies and economies. In this work, nanoparticulate gibbsite and boehmite were synthesized by hydrothermal method and resulting solids were treated with i) metal ion (Cr3+, Eu3+ and UO22+) adsorption; ii) organic acid (oxalic acid and acetic acid) adsorption; and iii) 60Co γ-irradiation. The treated solids are then subjected to dissolution in highly basic NaOH solution at 80 °C. While the solid dissolution was quantified by the aqueous Al(III) concentrations, the particle morphology, structure and interfacial chemistry were analyzed by an array of microscopy and spectroscopy techniques. Scanning electron imaging and X-ray diffraction analysis show negligible changes on boehmite upon metal ion or organic adsorption, or by γ-irradiation. Temperature-dependent dissolution rates of pure versus Cr(III)-adsorbed boehmite showed that the initial rate for the latter is consistently several times lower. Solid characterization reveal a phyicochemical armoring effect by adsorbed Cr(III) that substantially reduces the number of dissolution-active sites on particle surfaces. Similarly, organic acid-adsorption also reduced boehmite dissolution, likely due to blocking of dissolution-active sites. However, their effects were much less pronounced compared with that with metal ion (Cr) adsorption. We also found that 60Co γ-irradiation generally enhanced boehmite dissolution while the extent of the effect was also dependent on the presence of electrolytes, such as nitrate anion.

Dissolution chemistry of boehmite and gibbsite under conditions relevant to high-level tank waste

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