A quantitative theoretical analysis of the enthaplic effects accompanying ion adsorption at the oxide/electrolyte interface, based on a model of energetically heterogeneous surface oxygens, is presented. The triple layer complexation model is accepted, along with the 2-pK charging mechanism. For the purpose of illustration a set of experimental data is subjected to that quantitative analysis including titration curves, radiometrically measured individual iostherms of ions, and calorimetric titration data for the alumina/NaCl electrolyte system. Two models of energetic heterogeneity were taken into consideration. One of them assumes that the binding-to-oxygen energies of the surface complexes vary but are highly correlated when going from one to another surface oxygen. The other model of surface heterogeneity assumes that these correlations are very small. Our numerical simultaneous analysis of the titration data, of the individual isotherms of Na+ and Cl- adsorption, and of the accompanying heat effects advocates strongly for the model of surface heterogeneity assuming small correlations to exist. A good simultaneous fit of all three kinds of experimental data is obtained, with a small uncertainty as for the values of the estimated adsorption parameters. A simultaneous fit of the measured enthalpic effects appears to be an especially strong criterion for a proper choice of adsorption parameters.

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In the context of depollution and textile wastewater treatment, the sorption-based processes are good candidates to achieve the efficient removal of such toxics substances as dyes. In the present study, the exchange−adsorption from aqueous solutions of three azoic dyes, Methyl Orange (MO), Orange II (OII), and Orange G (OG), onto Mg−Al−LDH−NO 3 layered double hydroxides (LDH, molar Mg:Al ratio of 2) was investigated through monitoring all retained and removed species in combination with direct calorimetry and X-ray diffraction measurements. Kinetic curves, determined for several initial concentrations of the three dyes, indicated that the process was fast (between 60 and 100 min) and followed the pseudo-second order model in line with the passage of the removed dye through a chemisorption stage, thus constituting the ratelimiting step. Dye adsorption isotherms (H2-type) showed some differences in the maximum adsorption quantity (5.5 mmol g −1 , MO; 2.7 mmol g −1 , OII; 1.7 mmol g −1 , OG), consistent with anionic exchange capacity and adsorption on the external surface, depending on the cross-sectional area of the dye species and with their hydrophobic− hydrophilic character. The uptake of sodium cations as a function of the dye type and the surface coverage ratio pointed that the counterions can either stay in solution or be adsorbed to neutralize the free −SO 3 − moieties or other anionic species in the interlayer space. The cumulative enthalpy of displacement was negative in conformity with the exothermic character of the overall process. The intercalation of dye anions into the interlayer space of LDH materials led to its expansion with various distances being dependent both on the dye type and on the overall exchange balance. The latter included also the desorption of nitrates as well as the presence of carbonate species within the interlayer space, due to exchange in open systems exposed to the ambient atmosphere.

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International audienceSwelling clays act as ion-exchange membranes owing to the presence of extra-framework ions surrounded by water molecules. A better understanding of the motion of the ions present in the pore space is required to predict the diffusion properties in these solids. Since water molecules tend to adsorb both on ions and clay surfaces, they moderate the interactions between the ions and the framework. The hydration of ions therefore has an impact on their diffusion properties. In this paper, the MX-80 montmorillonite considered for nuclear waste disposal applications has been selected and studied using an approach combining measurements of complex impedance spectroscopy and water adsorption isotherms. The number of charge carriers has been estimated, and the diffusion coefficient for interlayer cations at various hydration states was determined. The evolution of the diffusion coefficient is subsequently correlated with the effect of the opening of the interlayer space and its hydration state. The resulting picture sheds light on the reversible or irreversible character of ion exchange. Finally, our results obtained from the difference between the interlayer diffusion coefficients extracted from conductivity measurements and those obtained at the macroscopic scale is discussed in terms of the textural parameters

The incorporation of cerium and manganese ions in perfluorosulfonic acid (PFSA) membranes strongly decreases the fluoride ion emission rate from fuel cell membrane electrode assemblies through their scavenging of reactive oxygen species generated during fuel cell operation. Concentration gradients of these ions and water fluxes lead to their migration and even loss from the fuel cell, but little is known about these phenomena that nevertheless impact proton exchange membrane fuel cell durability. We have determined the diffusion behavior of manganese and cerium ions in perfluorosulfonic acid membranes and find the diffusion coefficient of divalent manganese ions to be higher than that of trivalent cerium. We relate this observation to their relative efficacy in mitigating membrane degradation in a proton exchange membrane single cell, where ionic HPLC analysis of effluent water, technique used here for the first time, shows that three times greater loss of manganese ions than cerium ions after 200 hours at open circuit. Fluoride emission from PFSA membranes degraded ex situ and in situ are corroborated by results from detailed infrared, 19 F MAS NMR and X-ray photoelectron spectroscopic analysis.

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