Modelling Zn (II) sorption onto clayey sediments using a multi-site ion-exchange model

International audienceIn environmental studies, we need to be able to predict the behaviour of contaminants in more or less complex physico-chemical contexts. The improvement of this prediction partly depends on establishing thermodynamic models that can describe the behaviour of these contaminants and, in particular, the sorption reactions on mineral surfaces. In this way, based on the mass action law, it is possible to use surface complexation models and ion exchange models. Therefore, the aim of this study is i) to develop an ion-exchange model able to describe the sorption of transition metal onto pure clay minerals and ii) to test the ability of this approach to predict the sorption of these elements onto natural materials containing clay minerals (i.e. soils/sediments) under various chemical conditions. This study is focused on the behaviour of Zn(II) in the presence of clayey sediments. Considering that clay minerals are cation exchangers containing multiple sorption sites, it is possible to interpret the sorption of Zn(II), as well as competitor cations, by ion-exchange equilibria with the clay minerals. This approach is applied with success to interpret the experimental data obtained previously in the Zn(II)-H+-Na+-montmorillonite system [Baeyens, B., Bradbury, M.H., 1997. A mechanistic description of Ni and Zn sorption on Na-montmorillonite. Part I: Titration and sorption measurements. J. Contam. Hydrol. 27, 199–222]. Our research team has already studied the behaviour of Na+, K+, Ca2+ and Mg2+ versus pH in terms of ion exchange onto pure montmorillonite, leading us to develop a thermodynamic database including the exchange site concentrations associated with montmorillonite and the selectivity coefficients of Na+, K+, Ca2+, Mg2+, and Zn2+ versus H+. In the present study, we report experimental isotherms of Zn(II) on two different sediments in batch reactors at different pH and ionic strengths, using NaCl and CaSO4 as electrolytes. Assuming clay minerals are the main ion-exchanging phases, it is possible to predict Zn(II) sorption onto sediments under different experimental conditions, using the previously obtained data base on montmorillonite. Whatever the physico-chemical conditions tested, we observe a relatively good agreement between experimental results and the predicted sorption behaviour

Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands

http://www.radiochimacta.deInternational audienceHardened cement pastes (HCP) present a high affinity with a lot of radionuclides (RN) and can be used as waste confining materials in radioactive waste repository. Indeed, in cementitious media, RN can be removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms. In this study, the affinity of anionic RN (Cl−, I−, SeO32− and CO32− chemical forms) with a CEM-I HCP has been studied vs. the degradation of the HCP particles. These RN are considered as mobile in repository media and it is important to have a set of distribution ratio (Rd) in cement environment. The Rd values have been measured in batch experiments as a function of the pH, used as the degraded state parameter of the HCP suspensions. The Rd values increase in all cases, from the unaltered state (pH 13.3) to the altered state of HCP, i.e. until all portlandite is dissolved, corresponding to pH 12.6. Then, Rd values decrease until degraded states (pH 12.0), corresponding to the decalcification of the calcium silicate hydrate (C-S-H) phases. The behaviour of anionic RN seems to be correlated to the evolution of calcium concentration and is opposed to the evolution of sulphate concentration in solution which could have a competing effect. Comparison is done with the behaviour of caesium and uranium(VI), which is a cationic RN but has a major negative hydrolysed species at high pH. As awaited, the uranium(VI) behaviour is very different from purely anionic RN one in accord with spectroscopic analyses from literature works. The Rd values have also been measured for the organic ligands isosaccharinate (ISA) and EDTA. The uptake of ISA can be important and competing effect with the sorption of SeO32− has been evidenced in HCP suspensions as a function of the ISA concentration

Bound tritium: Preparation, measurement

The tritium activity in the ecosystem is measured on various matrices. The tritium bound to organic matter represents the tritium fixed by the operation of living objects. The sample should be prepared to preserve the tritium abundance with no isotopic fractionation. Transformation of the test sample should be conducted in order to obtain the highest possible combustion water yield, with no isotopic exchange. The test samples to be used should be sufficient to perform the measurements with the lowest possible detection threshold

Investigation of EDTA and gluconate migration into cementitious materials using carbon-14-labelled radiotracers

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Effect of organics on selenite uptake by cementitious materials

International audienceThe behaviour of three organic ligands in suspensions of fresh and degraded hydrated ordinary Portland cement pastes (HCP) has been investigated. EDTA arises as a decontamination product whilst ISA (isosaccharinic acid) is a main degradation product of cellulose. GLU (gluconic acid) is used as a retarding organic admixture in concrete. The affinity of EDTA, ISA and GLU with HCP increases with the degradation state. At long contact times, ISA and GLU desorbed from HCP, perhaps as a result of carbonation. Their influence on the uptake of selenium (as SeO32-) on HCP has been studied as a function of time, addition order and HCP degradation state. The sorption study of Se(IV) also shows a positive effect of the HCP degradation with Rd = 120 mL/g for fresh HCP and 1000 mL/g for degraded HCP. The addition order of Se(IV) and EDTA or ISA is important as pre-equilibration of HCP with either EDTA or ISA drastically decreases the uptake of Se(IV) to 10–30 mL/g. Mixing of cement with GLU seems to reduce the strong competitive effect of other organic compounds on Se(IV) sorption

Measurement and modeling of the surface potential evolution of hydrated cement pastes as a function of degradation

Hydrated cement pastes (HCP) have a high affinity with a lot of (radio)toxic products and can be used as waste confining materials. In cementitious media, elements are removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms as surface complexation equilibria. In this study, to improve the knowledge of the surface charge evolution vs the degradation of the HCP particles, two cements have been studied: CEM-I (ordinary Portland cement, OPC) and CEM-V (blast furnace slag and fly ash added to OPC). Zeta potential measurements showed that two isoelectric points exist vs HCP leaching, i.e., pH. Zeta potential increases from −17 to +20 mV for pH 13.3 to pH 12.65 (fresh HCP states) and decreases from 20 to −8 mV for pH 12.65 to 11 (degraded HCP states). The use of a simple surface complexation model of C-S-H, limited in comparison with the structural modeling of C-S-H in literature, allows a good prediction of the surface potential evolution of both HCP. Using this operational modeling, the surface charge is controlled by the deprotonation of surface sites (>SO−) and by the sorption of calcium (>SOCa+), which brings in addition a positive charge. The calcium concentration is controlled by portlandite or calcium silicate hydrate (C-S-H) solubilities

Mechanism of Europium Retention by Calcium Silicate Hydrates: An EXAFS Study

International audienceThe uptake of Eu by calcium silicate hydrate (C-S-H) phases as a function of Eu/sorbate ratio (from 37 to 450 µmol/g C-S-H), C-S-H Ca/Si mole ratio (1.3, 1.0, and 0.7), and initial supersaturating conditions was probed by solution kinetics experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy, to shed light on the retention mechanism of trivalent radionuclides under waste repository conditions. The rates of Eu (9.7 x 10-10 M) uptake in C-S-H suspensions and in solutions at equilibrium with C-S-H were rapid. Uptake of more than 90% of dissolved Eu was generally observed within 15 min. Europium LIII-edge EXAFS spectra collected on samples of Eu sorbed on, or coprecipitated in, C-S-H differed from that of Eu(OH)3(s) expected to precipitate under the pH conditions of C-S-H waters, ruling out compelling precipitation of pure hydroxide phases. Fourier transforms for EXAFS spectra for Eu in sorption/coprecipitation samples displayed comparable features at distances typical of neighboring cationic shells, pointing to similar crystallochemical environments. Optimal spectral simulations were obtained by assuming the presence of Si, Si/Ca, and Ca cationic shells surrounding Eu at distances of 3.2, 3.7-3.8, and 3.8-3.9 Å, respectively. The nearly continuous distribution of (Si, Ca) backscattering shells parallels the distribution in Ca-(Ca, Si) interatomic distances in structural models of C-S-H. Discernible effects of experimental parameters on the Eu local environment were observed by comparison of Fourier transforms, but could not be confirmed by EXAFS quantitative analysis. These results indicate that sorbed or coprecipitated Eu is located at Ca structural sites in a C-S-H-like environment. Kinetics and spectroscopic results are consistent with either Eu diffusion within C-S-H particles or precipitation of Eu with Ca and Si creating a C-S-H-like solid phase