In Belgium, deep geological disposal of radioactive waste is envisaged in poorly indurated clay formations like the Boom Clay (BC). In the present work we applied the Asymmetrical Flow Field-Flow Fractionation coupled to a UV-Vis spectrophotometer (AsFlFFF-UV) as well as the Liquid Chromatography coupled to Organic Carbon Detection, UV and ICP-MS (LC-OCD-UV-ICP-MS) to study dissolved organic matter (DOM) mobility. Small sized DOM fractions (<7 nm) are detected in all BC porewaters (BCPW) where the maximum of the species size distribution is at a hydrodynamic diameter of similar to 1.8-1.9 nm. Pore waters originating from or influenced by the double band (DB) structure exhibit a multimodal size distribution ranging to larger sized colloidal entities up to similar to 30 nm. The findings support the outcome of earlier investigations stating that the inter layer mobility of DOM in BC apparently is restricted to species with a diameter of <7 nm. Trace elements (Fe, Mn, Ni, lanthanides, Th, U) reveal a variable and complex association with colloidal species in BCPW and BC leachates where FeOOH/DOM aggregates play a role. An attempt is made to derive so-called in-situ K-d(OM)-values i.e. the distribution of naturally abundant lanthanides and actinides between dissolved OM in BCPW and OM being extractable from solid BC samples by a leaching step using 15 mM NaHCO3 solution. Results are compared with laboratory K-d-values described in the literature and point to the significant contribution of the immobile OM to lanthanide and actinide retention.

AbstractA research programme has been conducted jointly by GRS together with BRIUG and BGR to characterize GMZ bentonite as buffer material in comparison with the well-known MX80 bentonite.

UP2W is a polyacrylonitrile-based filter aid material used in nuclear power plants, which is disposed in repositories for low and intermediate level radioactive waste. The degradation of UP2W was investigated in a series of batch experiments in the presence and absence of portlandite, Fe(0) and NaOH, which simulate the hyperalkaline, reducing conditions expected in repositories with cementitious engineered barriers. Degradation experiments were performed under Ar atmosphere at T = 22 or 80 (± 2) °C. Aliquots of the supernatant solutions and retrieved solid phases were systematically characterized for ca. 5 years using a multi-method approach.The evolution of dissolved organic carbon shows a strong contrast between NaOH- and Ca(OH)2-buffered systems. In the absence of Ca, an early, sharp increase in the dissolved organic carbon is observed, and its magnitude correlates with the initial NaOH concentration.In Ca(OH)2-buffered systems, dissolved organic carbon remained very low (< 10 ppm) up to 600 days, but steadily increased afterwards reaching ∼ 50 ppm at t ≈ 1800 days. Size-exclusion / liquid chromatography coupled with organic carbon, ultra-violet and organic nitrogen detection (LC-OCD-UVD-OND) confirmed the presence of significantly smaller fragments in solution compared to NaOH-systems, which however were found to increase with time.Overall observations underline that hydrolysis of the nitrile functional groups, chain scission of the polymer backbone and cross-linking of polymer fragments in the degradation leachates play a key role in the progress of the degradation reaction. Chain scission is hindered in the presence of Ca, possibly due to the complexation of Ca with amide and carboxylate intermediates and the consequent decrease in electron density on the β-carbon atoms. This study improves the mechanistic understanding and quantitative description of UP2W degradation in cementitious environments relevant for L/ILW disposal.

The structure and size characterization of organic matter (OM) using flow field-flow fractionation (FFFF) is interesting due to the numerous interactions of OM in aquatic systems and water treatment processes. The estimation of hydrodynamic and electrostatic forces involved in the fractionation of OM over different molecular weight cut-off (MWCO) membranes is vital for a better understanding of the FFFF process. This work aims to understand the membrane-OM interactive forces with respect to membrane MWCO, solute molecular weight, flow rates, solution pH and ionic strength. Polystyrene sulfonate sodium salt (PSS) of molecular weights 10, 30 and 65 kDa were used as model organic solutes for fractionation over ultrafiltration (UF) membranes of MWCO 1-30 kDa. Maximum fractionation of PSS was achieved by using a tight membrane of 1 kDa MWCO at the conditions of high permeate flow rate (1.5-2.0 mL·min-1), low concentrate flow rate (0.2-0.3 mL·min-1) and low ionic strength (10 mM). The better fractionation corresponds to high permeate drag force and low concentrate drag force. A low membrane-solute DLVO interaction is favourable for the retention of a small solute. This study illustrated that FFFF characteristics can be analyzed based on membrane-solute interactive forces controlled by selected flow, size and charge parameters.

Bentonite is planned to be used as a backfill material in deep geological repositories (DGR) notably for those to be established in crystalline rock. Potential erosion processes have to be clearly understood in order to properly assess its long-term barrier function. A likely scenario over the lifetime of a DGR in certain geographical locations is the intrusion of low mineralized glacial melt water through low transmissivity bedrock fractures that could interact with bentonite and favor release of clay colloids. The objective of this work was to determine the dynamic of erosion under a glaciation scenario for Na-exchanged, Na-Ca-exchanged, Ca-exchanged and raw bentonite. The different bentonite types cover different potential materials used for backfill and different stages of cation loading via cation exchange processes upon contact with groundwater of different composition. To this end, erosion experiments were carried out with highly compacted bentonite confined by a porous filter (20 mu m) under dynamic conditions simulating the presence of a hydraulically active fracture filled with fault gauge and fracture filling minerals. The < 500 mu m size fractions of untreated MX80 bentonite and after Naor Ca-homoionization, were used. Two compacted clay pellets (1.6 +/- 0.1 g/cm(3)) of identical composition were placed in separate compartments of double-side reactors. Post-mortem visual inspection and the observed washout of water soluble MX80 components indicated full equilibration of the bentonite with simulated groundwater taking place during the experiment. The erosion was investigated by circulating a low ionic strength (1.6 10(-3) M) water at pH 8.4. Bentonite swelling was confined by stainless steel filter plates. The erosion (clay colloid detachment) via a low mineralized groundwater flow was quantified by determining an average eluted mass loss rate (AMLR). AMLR values integrated over 1466 days (4 years) erosion time for the raw MX80 bentonite, the Na-MX80 and the Na-Ca-MX80 are at 0.019 +/- 0.003 kg/y/m(2), 0.245 +/- 0.007 kg/y/ m(2) and 0.10 +/- 0.02 kg/y/m(2), respectively. Only by the sensitive ICP-MS analysis of clay-bound Th and U a tiny detachment of particles from Ca-MX80 could be identified, which shows the highest resistivity against erosion (0.004 +/- 0.002 kg/y/m(2)). The experiments pointed to the swelling pressure as main driving force for colloid generation. A possible inhibiting effect of accessory minerals on erosion can also be deduced from this study. Assuming erosion via a groundwater flow from a single fracture intersecting a deposition hole filled with raw MX80 bentonite at an angle of 90o and with a fracture aperture of up to 200 mu m, the present estimations indicated that under the given scenario the mass loss limit of 1200 kg bentonite will not be reached during the first 106 years.

Two-line ferrihydrite (2LFh) was aged for 12 years under ambient conditions and sheltered from light in the presence of Lu(III) used as surrogate for trivalent actinides. 2LFh aging produced hematite rhombohedra with overgrown acicular goethite particles. Analysis of the homogeneous suspension by asymmetrical flow field-flow fractionation (AsFlFFF) coupled to ICP-MS indicated that particles have a mean hydrodynamic diameter of about 140 nm and the strong correlation of the Fe and Lu fractograms hinted at a structural association of the lanthanide with the solid phase(s). Unfortunately, recoveries were low and thus results cannot be considered representative of the whole sample. The suspension was centrifuged and X-ray absorption spectroscopy (XAS) at the Lu L3-edge on the settled particles indicated that Lu(III) is sixfold coordinated by oxygen atoms, pointing to a retention by structural incorporation within particles. This result is consistent with AsFlFFF results on the same suspension without centrifugation. The detection of next nearest Fe and O atoms were consistent with the structure of goethite, ruling out incorporation within hematite. After centrifugation of the suspension, only nanoparticulate needle-like particles, very likely goethite, could be detected in the supernatant by ESEM. AsFlFFF data of the supernatant were comparable to that obtained for the homogeneous suspension, whereas XAS indicated that Lu(III) is predominantly present as dissolved species in the supernatant. Results from both techniques can be interpreted as a major fraction of Lu present as aqueous ions and a minor fraction as structurally incorporated. Findings from this study are corroborated by STEM-HAADF data and results from DFT calculations in a companion paper.

Transformation products of two-line ferrihydrite associated with Lu(III) were studied after 12years of aging using aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), high-efficiency energy-dispersive X-ray spectroscopy (EDXS), and density functional theory (DFT). The transformation products consisted of hematite nanoparticles with overgrown goethite needles. High-efficiency STEM-EDXS revealed that Lu is only associated with goethite needles, and atomic-resolution HAADF-STEM reveals structural incorporation of Lu within goethite, partially replacing structural Fe sites. This finding corroborates those recently obtained by AsFlFFF and EXAFS spectroscopy on the same sample (Finck et al. 2018). DFT calculations indicate that Lu incorporation within goethite or hematite are almost equally likely, suggesting that experimental parameters such as temperature and reaction time which affect reaction kinetics, play important roles in determining the Lu uptake. It seems likely that these results may be transferable to predict the behavior of chemically homologous trivalent actinides.

The Y(III) binding mechanism(s) by coprecipitation with or by adsorption on hectorite, a mineral phase frequently detected in nuclear waste glass alteration experiments, was investigated by polarized EXAFS (P-EXAFS) spectroscopy. The novelty in this study is the use of yttrium to take advantage of the higher angular dependence of the absorption coefficient at the K-edge for P-EXAFS measurements. In the coprecipitation experiment, a brucite precursor was prepared in the presence of Y and subsequently aged to produce hectorite. In the adsorption experiment, Y(III) ions contacted pre-formed hectorite in dispersion. The coprecipitated hectorite and brucite and the hectorite from adsorption experiment were each prepared as textured samples and the Y(III) local environment was probed by P-EXAFS spectroscopy. P-EXAFS analysis indicated that Y(III) is 6-fold coordinated by O atoms in both the coprecipitated brucite and the coprecipitated hectorite, and surrounded by next-nearest Mg/Si shells. The angular dependences of the coordination numbers strongly point to Y(III) substituting for Mg(II) in brucite layers. Upon hectorite crystallization the local environment evolved. Mg and Si shells were detected at distances suggesting an octahedral clay-like environment in the coprecipitated hectorite, and this finding was corroborated by the angular dependence of the coordination numbers. In the adsorption sample, Y(III) forms inner-sphere surface complexes at the platelet edges (i.e., (0 1 0) plane), slightly tilted off the median clay plane. The presence of such surface complexes in the coprecipitation sample could not be evidenced. Finally, the supernatant of the dispersion containing the coprecipitated hectorite was analyzed by the asymmetrical flow field-flow fractionation (AsFlFFF) technique coupled to ICP-MS to obtain information on the smallest sized particles. The AsFlFFF data indicate that nanoparticulate hectorite of various sizes (50–75nm, 125–140nm and >450nm) can be separated from the bulk dispersion and this finding was corroborated by TEM experiments on the same supernatant. Furthermore, AsFlFFF data also indicate that Y(III) behaves like Mg, used as fingerprint of the presence of hectorite. This finding suggests random substitution for octahedral cation within hectorite nanoparticles. Trivalent yttrium was used as proxy for trivalent actinides (An(III)). Consequently, this study supports the incorporation of An(III) into hectorite forming in the nuclear waste glass alteration layer in deep disposal sites, as already suggested in previous studies.

Bentonite is a strong radionuclide (RN) adsorbent. As a consequence, it is proposed as one of the engineered safety barriers in many nuclear waste disposal concepts in granite formations. Despite the many beneficial effects of bentonite, in contact with groundwater of low ionic strength montmorillonite colloids may be released from the bentonite buffer and transported towards the biosphere carrying the RNs bound to it. During the transport of colloids in bedrock fractures, size separation of clay colloids may occur, which may further affect RN mobility. In this work, RN adsorption (Th(IV), U(VI), Np(V), Tc(VII) and Pu(IV)) onto size fractionated montmorillonite colloids was studied in a synthetic, carbonated groundwater. Fractionation was done by simple settling procedures and sequential centrifugation. We combined batch adsorption experiments and geochemical modelling for the adsorption studies. U(VI), Np(V) and Tc(VII) did not adsorb to montmorillonite in the synthetic groundwater. Adsorption of Th(IV) and Pu(IV) is strong but, within experimental uncertainties, not significantly affected by the fractionation process. Montmorillonite colloids obtained by fractionation of the raw clay material but in the presence of organic matter during the initial separation step present significantly reduced uptake of Th and Pu. Based on the results, implementation of an “average log KD” (i.e. average distribution coefficients) for all colloidal sizes in reactive transport modelling codes would be acceptable.

The magnesian smectite hectorite is a corrosion product frequently detected in nuclear waste glass alteration experiments. The structural incorporation of a single trivalent lanthanide was previously demonstrated. Hectorite was presently synthesized, for the first time, in the presence of several lanthanides (La, Eu, Yb) following a multi-step synthesis protocol. The smallest-sized particles (nanoparticles, NPs) were isolated by centrifugation and analyzed by asymmetrical flow field-flow fractionation (AsFlFFF) coupled to ICP-MS, in order to obtain information on the elemental composition and distribution as a function of the size. Nanoparticles can be separated from the bulk smectite phase. The particles are able to accommodate even the larger-sized lanthanides such as La, however, with lower efficiency. We, therefore, assume that the incorporation proceeds by substitution for octahedral Mg accompanied by a concomitant lattice strain that increases with the size of the lanthanides. The presence of a mixture does not seem to affect the incorporation extent of any specific element. Furthermore, syntheses were performed where in addition the tetravalent zirconium or thorium elements were admixed, as this oxidation state may prevail for many actinide ions in a nuclear waste repository. The results show that they can be incorporated as well.

Bentonite clay is intended to form one of the barriers in most repositories of spent nuclear fuel located in granite. One important function of the bentonite barrier is to retard transport of radionuclides in the event of waste canister failure. Bentonite has a high sorption capacity of cations and its main constituent is montmorillonite. In contact with groundwater of low ionic strength, montmorillonite colloids can be released from bentonite and thereby control transport of radionuclides sorbed onto the colloids.In colloid transport in bedrock fractures, size separation of clay colloids may occur due to physical and chemical interactions with the bedrock fracture surface. This may enhance or retard the overall transport of radionuclides, depending on the sorption capacities and stability of the differently sized clay colloids. The bentonite barrier will be exposed to γ-radiation from the spent nuclear fuel. Irradiation affects surface-related properties of bentonite. If an average sorption capacity value cannot be used for all colloid sizes or if sorption is affected by exposure to γ-irradiation, corrected sorption capacity values would give higher resolution in current reactive transport models.In order to study the size separation process, a protocol was developed and successfully applied to fractionate montmorillonite into different-sized colloid suspensions by means of sequential or direct centrifugation. The stability and sorption capacity were studied using these fractions. Both stability and sorption capacity were found to be similar for all colloid sizes.Bentonite exposed to γ-radiation sorbed less divalent cations with increasing radiation dose. The effect was not large enough to have any impact on diffusion. The presence of bentonite enhanced irradiation-induced corrosion of copper under anaerobic atmosphere.An average sorption capacity value for montmorillonite can be used for all colloid sizes in reactive transport models. The effect of γ-irradiation on sorption capacity is sufficiently large to require consideration in transport modelling.

This study is focussed on the simulation of particle agglomeration at relatively high ionic strength using a refined stochastic algorithm developed in the context of parcel-tracking approaches. For that purpose, experimental data of both diffusion-limited and reaction-limited aggregation of latex particles were obtained using dynamic light scattering techniques for different initial particle sizes (diameters ranging from 24 to 495 nm) and at various chemical conditions (ionic strength between 0.5 and 2 M with NaCl or CaCl2 solutions). The experimental data collected have been compared to numerical results obtained with the refined parcel-tracking algorithm for particle agglomeration which has been developed. Results show that the evolution of the aggregate diameters over time can be properly captured by the present model with the value of the aggregate fractal dimension that is extracted from experimental data.

Bentonite clay is intended to form one of the barriers in most repositories of spent nuclear fuel located in granite. One important function of the bentonite barrier is to retard transport of radionuclides in the event of waste canister failure. Bentonite has a high sorption capacity of cations and its main constituent is montmorillonite. In contact with groundwater of low ionic strength, montmorillonite colloids can be released from bentonite and thereby control transport of radionuclides sorbed onto the colloids.In colloid transport in bedrock fractures, size separation of clay colloids may occur due to physical and chemical interactions with the bedrock fracture surface. This may enhance or retard the overall transport of radionuclides, depending on the sorption capacities and stability of the differently sized clay colloids. The bentonite barrier will be exposed to γ-radiation from the spent nuclear fuel. Irradiation affects surface-related properties of bentonite. If an average sorption capacity value cannot be used for all colloid sizes or if sorption is affected by exposure to γ-irradiation, corrected sorption capacity values would give higher resolution in current reactive transport models.In order to study the size separation process, a protocol was developed and successfully applied to fractionate montmorillonite into different-sized colloid suspensions by means of sequential or direct centrifugation. The stability and sorption capacity were studied using these fractions. Both stability and sorption capacity were found to be similar for all colloid sizes.Bentonite exposed to γ-radiation sorbed less divalent cations with increasing radiation dose. The effect was not large enough to have any impact on diffusion. The presence of bentonite enhanced irradiation-induced corrosion of copper under anaerobic atmosphere.An average sorption capacity value for montmorillonite can be used for all colloid sizes in reactive transport models. The effect of γ-irradiation on sorption capacity is sufficiently large to require consideration in transport modelling.

Bentonite is planned to be used as a technical barrier in the final storage of spent nuclear fuel and high level vitrified waste. In contact with ground water of low ionic strength, montmorillonite colloids may be released from the bentonite buffer and thereby enhance the transport of radionuclides (RNs) sorbed. In the present case, clay colloids represent aggregates of several clay mineral layers. It is of major importance to determine RN sorption properties for different sizes of montmorillonite aggregates, since size fractionation may occur during particle transport in natural media. In this study, a protocol for size fractionation of clay aggregates is developed, by sequential and direct centrifugation, in the presence and absence of organic matter. Seven colloidal fractions of different mean aggregate sizes are obtained ranging, when considering the mean equivalent hydrodynamic sphere diameter (ESD), from ~960nm down to ~85nm. Applying mathematical treatments (Jennings and Parslow, 1988) and approximating the clay aggregates to regular disc-shaped stacks of clay mineral sheets result in mean surface diameters varying from ~1.5μm down to ~190nm. All these colloidal fractions are characterized by XRD, IC and ICP-OES where they are found to have the same chemical composition. The number of edge sites (aluminol and silanol) is estimated (in mol/kg) for each colloidal fraction according to (Tournassat et al., 2003). It is calculated from the mean particle sizes obtained from AsFlFFF and PCS measurements, where the clay aggregates are approximated to regular disc-shaped stacks of clay mineral sheets. The estimated number of edge sites varies significantly for the different clay dispersions. In addition, stability studies using the various clay colloidal fractions are performed by the addition of NaCl, CaCl2 or MgCl2, in the presence or absence of organic matter, where no difference in stability is found.

Sorption/desorption kinetics for selected radionuclides (99Tc(VII), 232Th(IV), 233U(VI), 237Np(V), 242Pu and 243Am(III)) under Grimsel (Switzerland) ground water conditions (pH 9.7 and ionic strength of ∼1mM) in the presence of synthetic Zn or Ni containing montmorillonite nanoparticles and granodiorite fracture filling material (FFM) from Grimsel were examined in batch studies. The structurally bound Zn or Ni in the octahedral sheet of the synthetic colloids rendered them suitable as colloid markers. Only a weak interaction of the montmorillonite colloids with the fracture filling material occurs over the experimental duration of 10,000h (∼13months). The tri- and tetravalent radionuclides are initially strongly associated with nanoparticles in contrast to 99Tc(VII), 233U(VI) and 237Np(V) which showed no sorption to the montmorillonite colloids. Radionuclide desorption of the nanoparticles followed by sorption to the fracture filling material is observed for 232Th(IV), 242Pu and 243Am(III). Based on the conceptual model that the driving force for the kinetically controlled radionuclide desorption from nanoparticles and subsequent association to the FFM is the excess in surface area offered by the FFM, the observed desorption kinetics are related to the colloid/FFM surface area ratio. The observed decrease in concentration of the redox sensitive elements 99Tc(VII), 233U(VI) and 237Np(V) may be explained by reduction to lower oxidation states in line with Eh-pH conditions prevailing in the experiments and thermodynamic considerations leading to (i) precipitation of a sparingly soluble phase, (ii) sorption to the fracture filling material, (iii) possible formation of eigencolloids and/or (iv) sorption to the montmorillonite colloids. Subsequent to the sorption/desorption kinetics study, an additional experiment was conducted investigating the potential remobilization of radionuclides/colloids attached to the FFM used in the sorption/desorption kinetic experiments by contacting this FFM with pure Grimsel groundwater for 7days. A positive correlation of 242Pu, 232Th(IV) and 237Np was observed with the Zn and Ni concentrations in the desorption experiments indicating a remobilization of sorbed montmorillonite colloids. The results of the study in hand highlight (i) the novel use of structural labeled colloids to decrease the uncertainties in the determination of nanoparticle attachment providing more confidence in the derived radionuclide desorption rates. Moreover, the data illustrate (ii) the importance of radionuclide colloid desorption to be considered in the analysis and application of colloid facilitated transport both in laboratory or in-situ experiments and numerical model simulations and (iii) a possible remobilization of sorbed colloids and associated radionuclides by desorption from the matrix material (FFM) under non-equilibrium conditions.