Adsorption of gluconate and uranyl on C-S-H phases: Combination of wet chemistry experiments and molecular dynamics simulations for the binary systems

Abstract

International audienceAs a first step in developing better molecular scale understanding of the effects of organic additives on the adsorption and mobility of radionuclides in cement under conditions of geological nuclear waste repositories, two complementary approaches, wet chemistry experiments and molecular dynamics (MD) computer simulations, were applied to study the sorption behaviour of two simple model systems: gluconate and uranyl on calcium silicate hydrate phases (C-S-H) – the principal mineral component of hardened cement paste (HCP). Experimental data on sorption and desorption kinetics and isotherms of adsorption for gluconate/C-S-H and U(VI)/C-S-H binary systems were collected and quantitatively analysed for C-S-H samples synthesised with various Ca/Si ratios (0.83, 1.0, 1.4) corresponding to various stages of HCP aging and degradation. Gluconate labelled with 14C isotope was used in order to improve the sensitivity of analytical detection technique (LSC) at particularly low concentrations (10-8 - 10-5 mol/L). There is a noticeable effect of Ca/Si ratio on the gluconate sorption on C-S-H, with stronger sorption at higher Ca/Si ratios. Sorption of organic anions on C-S-H is mediated by the presence of Ca2+ at the interface and strongly depends on the surface charge and Ca2+ concentration. In parallel, classical MD simulations of the same model systems were performed in order to identify specific surface sorption sites most actively involved in the sorption of gluconate and uranyl on C-S-H and to clarify molecular mechanisms of adsorption

Molecular dynamics simulation of the interaction of uranium (VI) with the C–S–H phase of cement in the presence of gluconate

“…They provide a compromise between the necessity to realistically represent the true compositional and structural complexity of the C-S-H phase and still maintain relative simplicity, which should facilitate better quantitative investigation of the specific molecular mechanisms of interaction among different chemical species near the surface (Androniuk et al, 2017).…”

Effect of Chloride and Sulfate in the Immobilization of Cs-137 in C-S-H Gel

“…This is not trivial since affects the retention and migration of the confined species: a cation located close to the C-S-H surface that establishes strong interactions with their oxygen atoms is well retained and its mobility is restricted, while the cations in the pore that do not interact with the surface are not retained at all and they can move freely through the pore. Many authors have classified the cations confined in the C-S-H gel into three categories according to their proximity to the surface and their coordination shell: inner-sphere complexes, outer-sphere-complexes and non-adsorbed cations (Payne et al 2013;Androniuk et al 2017;Jiang et al 2017;Duque-Redondo et al 2018b). Non-adsorbed cations are far from the C-S-H, so they do not interact with the surface's atoms and their coordination shell is mainly composed of water molecules and complete it with the counterions.…”

: A force field database for cementitious materials including validations, applications and opportunities

“…Dissolution of the anhydrous clinker and growth of the solid hydrous cement phases [9][10][11][12][13][14][15][16] ii) Interaction of chemical additives with anhydrous and hydrous cement phases [17][18][19][20] iii) Influence of replacing clinker with locally available materials (clays, waste materials and recycled materials) which influences the chemistry, hydration dynamics, effect on microstructure development [4][5][6][7]21] iv) Degradation mechanisms in cement and concrete (corrosion of steel reinforcements, freeze thaw and sulphate attack) [22][23][24] v) Mechanism of waste/cement interactions (storage of radioactive and nonradioactive waste) [25][26][27][28] vi) Understanding the dependence of mechanical and fracture characteristics due to anisotropy, hardness, dislocation dynamics, porosity and impurities, etc. [29][30][31][32][33][34][35] All of these phenomena deal with chemistry at interfaces at the nanoscale level -the mechanisms behind the above mentioned points are poorly understood even if some knowledge of the microscopic level chemistry is well documented.…”