Felix Brandt

Chlorite dissolution in the acid pH-range: A combined microscopic and macroscopic approach

Abstract The dissolution of chlorite with intermediate Fe-content was studied macroscopically via mixed flow experiments as well as microscopically via atomic force microscopy (AFM). BET surface area normalized steady state dissolution rates at 25 °C for pH 2 to 5 vary between 10−12 and 10−13 mol/m2.s. The order of the dissolution reaction with respect to protons was calculated to be about 0.29. For pH 2 to 4, chlorite was found to dissolve non-stoichiometrically, with a preferred release of the octahedrally coordinated cations. The additional release of octahedrally coordinated cations may be due to the transformation of chlorite to interstratified chlorite/vermiculite from the grain edges inward. In-situ atomic force microscopy performed on the basal surfaces of a chlorite sample, which has been preconditioned at pH 2 for several months, indicated a defect controlled dissolution mechanism. Molecular steps with height differences which correspond to the different subunits of chlorite, e.g. TOT sheet and brucite like layer, originated at surface defects such or compositional inhomogenities or cracks, which may be due to the deformation history of the chlorite sample. In contrast to other sheet silicates, at pH 2 nanoscale etch pits occur on the chlorite basal surfaces within flat terraces terminated by a TOT-sheet as well as within the brucite like layer. The chlorite basal surface dissolves layer by layer, because most of the surface defects are only expressed through single TOT or brucite-like layers. The defect controlled dissolution mechanism favours dissolution of molecular steps on the basal surfaces compared to dissolution of the grain edges. At pH 2 the dissolution of the chlorite basal surface is dominated by the retreat of 14 A steps, representing one chlorite unit cell. The macroscopic and microscopic chlorite dissolution rates can be linked via the reactive surface area as identified by AFM. The reactive surface area with respect to dissolution consists of only 0.2% of the BET-surface area. A dissolution rate of 2.5 × 10−9 mol/m2s was calculated from macroscopic and microscopic dissolution experiments at pH 2, when normalized to the reactive surface area.

Trivalent actinide coprecipitation with powellite (CaMoO4): Secondary solid solution formation during HLW borosilicate-glass dissolution

Summary Powellite (CaMoO4) is one of the various crystalline secondary alteration phases which form during the corrosion of high level waste (HLW) glasses. Due to their structural variability, powellite can accommodate considerable chemical substitutions including trivalent actinides. Batch adsorption and coprecipitation experiments in mixed flow reactors have been used to study quantitatively the uptake of Cm(III) and Eu(III) (as a nonradioactive chemical homologue for trivalent actinides) from aqueous solution under repository relevant conditions. A metal ion concentration-independent Kd of 1200 ml/g has been determined for adsorption above pH 4 up to 3 μmol/L Eu(III). The partition coefficient for coprecipitation varies between 10 and 800 depending on the precipitation rate. Time-resolved laser fluorescence spectroscopy has been used to study the aqueous complexation of Cm(III)/Eu(III) and MoO42- as well as their incorporation into the powellite crystal lattice during coprecipitation. A red shift of the Cm(III) fluorescence emission of the transition indicates the formation of aqueous CmMoO4+ (598 nm) and Cm(MoO4)22- (601 nm) complexes. The incorporation of Cm(III) is indicated by a significant red shift. The corresponding lifetime of 165 μs indicates quench effects. A similar behaviour has been observed for Eu(III). The life time of the 5D0-7F2 transition is 350 μs for an incorporated Eu(III) species, suggesting also intrinsic quench effects, due to the local crystal field or impurities.

Direct Measurement of Surface Dissolution Rates in Potential Nuclear Waste Forms: The Example of Pyrochlore

The long-term stability of ceramic materials that are considered as potential nuclear waste forms is governed by heterogeneous surface reactivity. Thus, instead of a mean rate, the identification of one or more dominant contributors to the overall dissolution rate is the key to predict the stability of waste forms quantitatively. Direct surface measurements by vertical scanning interferometry (VSI) and their analysis via material flux maps and resulting dissolution rate spectra provide data about dominant rate contributors and their variability over time. Using pyrochlore (Nd2Zr2O7) pellet dissolution under acidic conditions as an example, we demonstrate the identification and quantification of dissolution rate contributors, based on VSI data and rate spectrum analysis. Heterogeneous surface alteration of pyrochlore varies by a factor of about 5 and additional material loss by chemo-mechanical grain pull-out within the uppermost grain layer. We identified four different rate contributors that are responsible for the observed dissolution rate range of single grains. Our new concept offers the opportunity to increase our mechanistic understanding and to predict quantitatively the alteration of ceramic waste forms.

Uptake of Ra during the recrystallization of barite: a microscopic and time of flight-secondary ion mass spectrometry study.

A combined macroscopic and microanalytical approach was applied on two distinct barite samples from Ra uptake batch experiments using time of flight-secondary ion mass spectrometry (ToF-SIMS) and detailed scanning electron microscopy (SEM) investigations. The experiments were set up at near to equilibrium conditions to distinguish between two possible scenarios for the uptake of Ra by already existent barite: (1) formation of a Ba1-xRaxSO4 solid solution surface layer on the barite or (2) a complete recrystallization, leading to homogeneous Ba1-xRaxSO4 crystals. It could be clearly shown that Ra uptake in all barite particles analyzed within this study is not limited to the surface but extends to the entire solid. For most grains a homogeneous distribution of Ra could be determined, indicating a complete recrystallization of barite into a Ba1-xRaxSO4 solid solution. The maxima of the Ra/Ba intensity ratio distribution histograms calculated from ToF-SIMS are identical with the expected Ra/Ba ratios calculated from mass balance assuming a complete recrystallization. In addition, the role of Ra during the recrystallization of barite was examined via detailed SEM investigations. Depending on the type of barite used, an additional coarsening effect or a strong formation of oriented aggregates was observed compared to blank samples without Ra. In conclusion, the addition of Ra to a barite at close to equilibrium conditions has a major impact on the system leading to a fast re-equilibration of the solid to a Ba1-xRaxSO4 solid solution and visible effects on the particle size distribution, even at room temperature.

The Effect of Chlorite Dissolution on the Sorption Behavior of U(VI)

During the dissolution of chlorite the sorption of uranyl(VI) can be described by two different sorption sites. The first site is attributed to a surface site on the chlorite which is occupied after 138 hours of our mixed-flow experiment. Due to the fact that uranyl(VI) is continuously sorbed we conclude that as a second site the sorption sites of the newly forming ferrihydrite colloids are responsible for the additional sorption beyond the available sorption sites on chlorite.