Surface site density, silicic acid retention and transport properties of compacted magnetite powder

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Modeling the complexation properties of mineral-bound organic polyelectrolyte: An attempt at comprehension using the model system alumina/polyacrylic acid/M (M = Eu, Cm, Gd)

International audienceThis paper contributes to the comprehension of kinetic and equilibrium phenomena governing metal ion sorption on organic-matter-coated mineral particles. Sorption and desorption experiments were carried out with Eu ion and polyacrylic acid (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4 as a function of the metal ion loading. Under these conditions, M interaction with the solid is governed by sorbed PAA (PAAads). The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS) with Cm and Gd. The interaction between M and PAAads was characterized by a kinetically controlled process: after rapid metal adsorption within less than 1 min, the speciation of complexed M changed at the particle surface till an equilibrium was reached after about 4 days. At equilibrium, one part of complexed M was shown to be not exchangeable. This process was strongly dependent on the ligand-to-metal ratio. Two models were tested to explain the data. In model 1, the kinetically controlled process was described through successive kinetically controlled reactions that follow the rapid metal ion adsorption. In model 2, the organic layer was considered as a porous medium: the kinetic process was explained by the diffusion of M from the surface into the organic layer. Model 1 allowed a very good description of equilibrium and kinetic experimental data. Model 2 could describe the data at equilibrium but could not explain the kinetic data accurately. In spite of this disagreement, model 2 appeared more realistic considering the results of the TRLFS measurements

Sorption of selenite in a multi-component system using the "dialysis membrane" method

Abstract 79Se is a potentially mobile long-lived fission product, which may make a dominant contribution to the long-term radiation exposure resulting from deep geological disposal of radioactive waste. Its mobility is affected by sorption on minerals. Selenium sorption processes have been studied mainly by considering interaction with a single mineral surface. In the case of multi-component systems (e.g. soils), it is difficult to predict the radioelement behaviour only from the mineral constituents. This study contributes to the understanding of multi-component controls of Se concentrations towards predicting Se behaviour in soils after migration from a disposal site. This goal was approached by measuring selenite sorption on mono and multi-phase systems physically separated by dialysis membranes. To the best of the authors' knowledge, very few studies have used dialysis membranes to study the sorption competition of selenite between several mineral phases. Other workers have used this method to study the sorption of pesticides on montmorillonite in the presence of dissolved organic matter. Indeed, this method allows measurement of individual Kd in a system composed of several mineral phases. Dialysis membranes allowed (i) determination of the competition of two mineral phases for selenite sorption (ii) and determination of the role of humic acids (HAs) on selenite sorption in oxidising conditions. Experimental results at pH 7.0 show an average Se(IV) sorption distribution coefficient (Kd) of approximately 125 and 9410 L kg−1 for bentonite and goethite, respectively. The average Kd for goethite decreases to 613 L kg−1 or 3215 L kg−1 in the presence of bentonite or HA, respectively. For bentonite, the average Kd decreases slightly in the presence of goethite (60 L kg−1) and remains unchanged in the presence of HA. The experimental data were successfully modelled with a surface complexation model using the PHREEQC geochemical code. The drastic decrease in Se(IV) sorption on goethite in a multi-phase system is attributed to competition with dissolved silica released by bentonite. As with Si the HA compete with Se for sorption sites on goethite

Adsorption and transport of polymaleic acid on Callovo-Oxfordian clay stone: Batch and transport experiments

Dissolved Organic Matter (DOM) can affect the mobility of radionuclides in pore water of clay-rich geological formations, such as those intended to be used for nuclear waste disposal. The present work studies the adsorption and transport properties of a polycarboxylic acid, polymaleic acid (PMA, Mw = 1.9 kDa), on Callovo-Oxfordian argillite samples (COx). Even though this molecule is rather different from the natural organic matter found in clay rock, the study of its retention properties on both dispersed and intact samples allows assessing to which extent organic acids may undergo sorption under natural conditions (pH 7) and what could be the impact on their mobility. PMA sorption and desorption were investigated in dispersed systems. The degree of sorption was measured after 1, 8 and 21 days and for a range of PMA initial concentrations from 4.5 × 10− 7 to 1.4 × 10− 3 mol.L− 1. The reversibility of the sorption process was estimated by desorption experiments performed after the sorption experiments. At the sorption steady state, the sorption was described by a two-site Langmuir model. A total sorption capacity of COx for PMA was found to be 1.01×10− 2 mol.kg− 1 distributed on two sorption sites, one weak and one strong. The desorption of PMA was incomplete, independently of the duration of the sorption phase. The amount of desorbable PMA even appeared to decrease for sorption phases from 1 to 21 days. To describe the apparent desorption hysteresis, two conceptual models were applied. The two-box diffusion model accounted for intraparticle diffusion and more generally for nonequilibrium processes. The two-box first-order non-reversible model accounted for a first-order non-reversible sorption and more generally for kinetically-controlled irreversible sorption processes. The use of the two models revealed that desorption hysteresis was not the result of nonequilibrium processes but was due to irreversible sorption. Irreversible sorption on the strong site was completed after 1 day and represented 96% of the total sorption on this site. On the weak site the irreversible uptake was slower and completed only after 16 days but it also dominated the sorption. 85% of the PMA sorbed on the weak site was not desorbable after 21 days of sorption. The migration of PMA was studied by applying a hydraulic gradient to a clay core inserted in a stainless steel cell. Breakthrough of polymaleic acid, simulated with a 1D transport model including the two-box first-order non-reversible model, revealed that the mobility of PMA was limited by the same set of reversible/irreversible interactions as observed in the dispersed system. However, to describe efficiently the transport, the total sorption capacity had to be reduced to 33% of the capacity estimated in batch experiments. The irreversible sorption on the weak site was also slower in the intact sample than in the crushed sample. Geometrical constraints would therefore affect both the accessibility to the sorption sites and the kinetics of the irreversible sorption process

Determination of dissolution and precipitation rates of clayey materials by 29Si/28Si isotopic exchange. Effect of Temperature

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Tritiated Water Diffusion in Fresh and Carbonated Cement Pastes: Effect of a drying/resaturation cycle

International audienceIn the French radioactive waste management context, cementitious materials will be widely used as engineered barriers, backfill materials or confinement matrices for some Intermediate-Level Long-Lives waste. In a waste disposal facility, during the construction and the operation phases, the ventilation of underground galleries/shafts will impose specific conditions (relative humidity, presence of CO2) that will modify the degree of water saturation (Sw) of cement materials (surfacial drying) and induce a potential carbonation. The aim of this study is to investigate the effect of a drying/resaturation cycle on the effective diffusion coefficient of tritiated water (HTO) in fresh and carbonated cement pastes.Samples used are disks (2 mm thick) of a CEM V/A hardened cement paste (HCP) prepared with a water to cement ratio of 0.43 and cured at least 27 months in an artificial cement pore water (ACW ; pH~13.5) [1]. The drying stage consisted of storing HCP samples (during several months at 20±1°C under Ar atmosphere) at fixed relative humidity (RH) imposed by saturated saline solutions. Thus, initial Sw values obtained range from 0.63 to 0.85. Afterward, HCP samples were resaturated with ACW (under vaccum). Additionally, a fully saturated sample was used as a reference. Carbonated samples were prepared in conditions representative of atmospheric carbonation [2]. An accelerated carbonation process was performed in a climatic chamber (PCO2 = 3% vol ; RH=55 % in order to obtain a maximal carbonation rate). Through-diffusion experiments were performed, in at least duplicate, using standard two-reservoir set-ups. The effective diffusion coefficient, De(HTO) and material capacity factor, were obtained by modelling the HTO cumulative activity curves (in the downstream reservoir) with a routine based on an analytical solution of Fick laws (with fixed initial and boundary conditions). Deref(HTO) value, obtained for the reference sample is 3±1 10-13 m2.s-1 which is in agreement with literature data for this materials [1,3]. For fresh HCP samples, values are similar to water accessible porosity (0.33±0.02). No significant HTO retention is then assumed in HCP samples.For fresh HCP, the drying/resaturation cycle results in a slight non-linear increase of De(HTO) values as initial Sw values diminish. This result is consistent with the reported description of the drying effect on HCP microstructure [4]. For high initial Sw (0.85), water is mostly drained from the capillary porosity with no specific effect on HCP microstructure. As this process is reversible the final De(HTO) value (after resaturation) is similar to Deref(HTO). For intermediate initial Sw (0.63), the water molecules, distributed as multilayers at C-S-H particles surfaces, partly desorb and cause small changes in the mesopore distribution by opening/closing pores (MIP measurements) which ease HTO diffusion through the pore network. The impact on De(HTO) value is an increase by a factor of 2 (5.4±0.4 10-13 m2.s-1).For carbonated HCP samples, the drying/resaturation cycle results in decreasing the water accessible porosity (0.21±0.01) and increasing of De(HTO) value (12±2 10-13 m2.s-1). These results are consistent with a combination of porosity clogging (precipitation of CaCO3 in pores) and presence of microcracks (decalcification of C-S-H) as described in Auroy et al study [2]. The increase of De(HTO) value supports the conclusion that for CEM V HCP, microcracks could be the main pathway for diffusion of HTO in carbonated samples.Comparing to the reference sample (fully saturated non-carbonated HCP), these results suggest that HTO diffusion is favoured during a drying/resaturation cycle. For a blended cement, this effect may be enhanced if coupled with carbonation. AcknowlegmentThe research leading to these results has received funding from the European Union’s Horizon 2020 Research and Training Programme of the European Atomic Energy Community (EURATOM) (H2020-NFRP-2014/2015) under grant agreement n°662147 (CEBAMA)[1] Savoye S., Macé N., Lefèvre S., Spir G., Robinet J.C., Applied Geochemistry 96 (2018) 78-86[2] Auroy M., Poyet S., Le Bescop P., Torrenti J.M., Charpentier T., Moskura M., Bourbon X., Cement and Concrete Research, 74 (2015) 44-58[3] Landesman C, Macé, N., Radwan J., Ribet S., Bessaguet N., David K., Page J., Henocq P., 3rd International Symposium on Cement-based Materials for Nuclear Wastes (NUWCEM), October 24-26, 2018 (Avignon, France)[4] Roosz C., Gaboreau S., Grangeon S., Prêt D., Montouillout V., Maubec N., Ory S., Blanc P., Vieillard P., Henocq P., Langmuir, 2016, 32, 6794-680

Temperature effect of U(VI) retention on the Callovo-Oxfordian clay rock

International audienceIn the context of management of the radioactive waste in deep geological formations, the effect of temperature (20–80 °C) on U(VI) adsorption by Callovo-Oxfordian claystone (COx) was studied. A step-by-step approach was followed, starting with the single mineral, illite, followed by an increase in the complexity of the system, through the analysis of the clay fraction and the natural samples of the Callovo-Oxfordian formation. Depending on the study conditions, and the speciation of U(VI) in solution (hydrolysed species, carbonate species and presence of ternary U(VI)-Ca(Mg)‑carbonate complexes), the temperature effect was either negligible, or positive (where the increase in temperature favours retention). The most important positive effect was observed for the U(VI)/COx system in the presence of ternary complexes. The data were modelled considering an existing sorption model at 20 °C and the thermodynamic data available to describe the evolution of the speciation of U(VI) in solution in function of temperature. The enthalpy values associated with the surface complexes were fitted from the experimental data following a stepwise approach based on the van't Hoff equation

Evaluation of thermodynamic data for aqueous Ca-U(VI)-CO3 species under conditions characteristic of geological clay formation

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Assessment of surface reactivity of thorium oxide in conditions close to chemical equilibrium by isotope exchange 229Th/232Th method

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U(VI) retention in compact Callovo-Oxfordian clay stone at temperature (20–80 °C); What is the applicability of adsorption models?

International audienceIn the context of the radioactive waste management in deep geological formations, U(VI) retention by intact Callovo-Oxfordian claystone (COx) was studied by percolation-type experiments at 20 and 80 degrees C. The experimental results were confronted with modelling prediction based on a published adsorption model developed from dispersed media in the 20-80 degrees C temperature range. For the experiments at 20 degrees C, the adsorption model allowed to explain the results for the intact system; the retention was weak (R-d similar to 10 L.kg(-1)) and the analysis of the COx phases at the end of the experiment confirmed a retention of U by the clay fraction. The adsorption model in temperature also explained the observed trend of increasing retention with increasing temperature. However, it underestimated the temperature effect on the adsorption of U(VI) by the COx clay fraction, and other phases contributed to the retention. Solid-state analysis of the percolation-doped samples indicated a reactivity in the order pyrite>clay>calcite phases. The transposition of the knowledge at 20 degrees C from the dispersed system to the intact medium was therefore not possible at 80 degrees C for the studied U(VI)/COx system