Anion Exclusion Research Articles

Remediation of hexavalent chromium from aqueous solution using clay mineral Fe(II)–montmorillonite: Encompassing anion exclusion impact

We have explored the highly efficient and environmentally benign clay mineral, Fe(II)–montmorillonite, for the reduction of Cr(VI) in aqueous solution. Fe(II)–Mt was treated with K2Cr2O7 solution at different pH, temperature and solid-to-liquid ratio. The [Cr2O7]2− was estimated by UV–vis spectra with a correction for anion exclusion impact. In general, the Cr(VI) reduction was rapid at acidic pH and increased with temperature up to 50°C. A complete reduction occurred in about 5min at pH 3–5. The time taken for complete reduction at 0°C, RT (30°C) and 40°C are 12min, 8min and 5min, respectively. The reduction followed by immobilization of Cr(III) on the spent clay mineral was well characterized by EDX and chemical extraction analysis. This remediation process could be easily scaled-up for real system applications.

Matrix diffusion and sorption of Cs+, Na+, I– and HTO in granodiorite: Laboratory-scale results and their extrapolation to the in situ condition

Matrix diffusion and sorption are important processes controlling radionuclide transport in crystalline rocks. Such processes are typically studied in the laboratory using borehole core samples however there is still much uncertainty on the changes to rock transport properties during coring and decompression. It is therefore important to show how such laboratory-based results compare with in situ conditions. This paper focuses on laboratory-scale mechanistic understanding and how this can be extrapolated to in situ conditions as part of the Long Term Diffusion (LTD) project at the Grimsel Test Site, Switzerland. Diffusion and sorption of 137Cs+, 22Na+, 125I– and tritiated water (HTO) in Grimsel granodiorite were studied using through-diffusion and batch sorption experiments. Effective diffusivities (De) of these tracers showed typical cation excess and anion exclusion effects and their salinity dependence, although the extent of these effects varied due to the heterogeneous pore networks in the crystalline rock samples. Rock capacity factors (α) and distribution coefficients (Kd) for Cs+ and Na+ were found to be sensitive to porewater salinity. Through-diffusion experiments indicated dual depth profiles for Cs+ and Na+ which could be explained by a near-surface Kd increment. A microscopic analysis indicated that this is caused by high porosity and sorption capacities in disturbed biotite minerals on the surface of the samples. The Kd values derived from the dual profiles are likely to correspond to Kd dependence on the grain sizes of crushed samples in the batch sorption experiments. The results of the in situ LTD experiments were interpreted reasonably well by using transport parameters derived from laboratory data and extrapolating them to in situ conditions. These comparative experimental and modelling studies provided a way to extrapolate from laboratory scale to in situ condition. It is well known that the difference in porosity between laboratory and in situ conditions is a key factor to scale laboratory-derived De to in situ conditions. We also show that cation excess diffusion is likely to be a key mechanism in crystalline rocks and that high Kd in the disturbed surfaces is critically important to evaluate transport in both laboratory and in situ tests.

Laboratory Investigation of Boundary Condition Impacts on Nitrate Anion Exclusion in an Unsaturated Soil

Transient unsaturated horizontal column experiments were conducted with a loam soil, under variable boundary conditions, to obtain added insight on nitrate (NO3−) anion exclusion processes. The boundary conditions evaluated were column inlet soil water content (θ0), initial soil water content (θi), column inlet soil solution NO3−–N concentration (C0), and the initial soil solution NO3− concentration (Ci). Results consistent with significant anion exclusion of NO3− were exhibited in all experiments. For tests with NO3− solution injected at the column inlet, the soil solution NO3− concentration at the inlet was found to be 12 to 19% less than the injected NO3− concentration. For these same tests, where initial soil conditions were dry with no NO3− originally present, the NO3− concentrations at the wetting front edge were substantially greater than the injected NO3− concentration, while for initially wetter conditions, with or without NO3− originally present, the NO3− front was found to extend well beyond the theoretical “piston front” separating the injected solution from displaced initial soil water. For displacement tests, in which water was injected at the inlet of a column initially wetted with a NO3− solution, the soil solution NO3− front also extended beyond the piston front. Inlet NO3− excluded water content ranged from 0.03 to 0.06 and was not affected by pore water velocity, but did have a modest positive correlation with total water content (r = 0.60 and r = 0.67). The findings of this investigation provide a better overall understanding of nitrate movement through the soil profile.

NO2 − and NO3 − leaching and solubilization of Al in variable charge soils treated with sewage sludge

The use of sewage sludge (SS) for agricultural purposes provides macronutrients and micronutrients and increases the soil organic matter. This study was conducted after 5 years of agricultural sludge application to a variable charge soil (clay loam-textured dark red dystroferric ox- isol) in an experimental field at the Brazilian Federal Agriculture Research Institute in Jaguariuna, Sao Paulo. The mineralization of SS organic matter forms sulfate (SO4 2- ) and ammonium (NH4 ? ) or ammonia (NH3). Subsequently, nitrification of NH4 ? or NH3 occurs, re- sulting in the formation of nitrite (NO2 - ) and NO3 - which release H ? and increases the redox potential of the solu- tion. In studied area, the SS organic matter decomposition favored the anion exclusion, which led to SO4 2- ,N O2 - and NO3 - leaching along the profile. At 2 m, the soil so- lution showed pH and Eh values of 4.2 and 600 mV, re- spectively. In addition, the highest soil NO2 - (27 mg/L), NO3 - (1220 mg/L) and Al (20 mg/L) concentrations were observed at 2 m. The low pH and high Eh at 2 m poten- tially resulted from the oxidation of leached NO2 - .F ur- thermore, the H ? released in solution due to nitrification was partially consumed during the solubilization of soil Al compounds, such as kaolinite and amorphous Al oxide. Geochemical modeling (GM) indicated that Al 3? in solution results from the solubilization of kaolinite and amorphous Al oxide. GM also highlights the importance of soil oxide protonation in buffering the soil solution acidity and in controlling the solubility of Al compounds and free Al in solution.

Chloride ion concentrations during the compression of rocks to extract pore water

AbstractPore water needs to be extracted from rocks with low permeabilities to allow the major ion concentrations in the pore water to be estimated. Compressing a rock is the most widely used method of extracting the pore water. However, ion concentrations have been found to change during compression in previous studies, and the mechanisms involved in such ion concentration changes have not yet been fully assessed. In this study, two natural rocks and four artificially prepared samples were compressed, and changes in the chloride ion (Cl−) concentrations as the compression pressure increased were investigated. Mechanisms that could have caused the changes observed were then assessed. The Cl− concentrations in squeezed water decreased as the pressure increased if the sample contained a significant amount of smectite. The strong dependence of Cl− concentration on the amount of smectite indicated that smectite played an important role in decreasing the Cl− concentration. The dilution of the pore water with interlayer water from the smectite appeared to be the dominant mechanism involved in the decrease in Cl− concentration found in a Kunigel‐V1 sample, because dilution of the pore water with interlayer water quantitatively explained the decrease in the Cl− concentration. The filter effect caused by the anion exclusion effect did not appear to be a dominant mechanisms in our case. However, Cl− concentration decrease found in natural rocks could not be fully explained by dilution with interlayer water, so other mechanisms must be involved the phenomenon in natural rocks.

Use of Fly Ash as Soil Amendment to Offset Anion Exclusion Effect on Nitrate Transport

This study investigated the direct effect of fly ash used as a soil amendment on anion exclusion in NO3− transport in a variably saturated soil. A sandy clay soil was amended with Class F fly ash (FA) at four levels: 0, 2, 10, and 20% FA. The soils were placed in a small soil column and kept at constant saturation using a steady‐state centrifugation unsaturated flow apparatus. Three saturation ranges were investigated, 0.75 to 0.9, 0.4 to 0.45, and 0.27 to 0.29, designated as high, medium, and low saturation, respectively. Ammonium nitrate (0.7 mmol L−1) solution was passed through the soil sample, and the leachate was collected at predetermined intervals up to a total volume equivalent to 12 effective pore volumes. The leachate samples were analyzed for NO3− concentration using ion chromatography, and breakthrough curves were constructed. Experimental and modeling results showed that the amount of fly ash and saturation level had a significant effect on the hydraulic properties of the soil, reducing the hydraulic conductivity and increasing water retention. Furthermore, it was shown that the fly ash treatment slowed the transport of NO3−, especially at low saturation levels, when the effect of anion exclusion is thought to be stronger.

Influence of Water Quality on Exchange Phase-Solution Phase Behavior of Texturally Different Salt-affected Soils

The influence of total electrolyte concentration (TEC) and sodium adsorption ratio (SAR) of water on ESR-SAR relationships of normal clay loam, saline silty loam and calcareous sodic loam soils was studied in a laboratory experiment. Twelve solutions, encompassing three TEC levels viz., 25, 50, and 100 me L−1 and four SAR levels viz., 5, 10, 20 and 30 mmol1/2 L−1/2 were prepared to equilibrate the soil samples using pure chloride salts of calcium, magnesium and sodium at Ca: Mg = 2: 1. The SAR of equilibrium solution decreased as compared to the equilibrating solution and more so in waters of low salt concentration and low SAR. At all electrolyte concentration, SAR values were not attained to the equilibrium solution because of addition of Ca and Mg from mineral dissolution or supply of Ca and Mg from exchange sites. At higher TEC levels, considerable increase in exchangeable sodium percentage (ESP) was observed when it was correlated for anion exclusion and more so in calcareous sodic loam followed by saline silty loam and normal clay loam soils. Irrespective of TEC, the exchangeable sodium in all the soils increased by 2.1 to 3.8-fold and irrespective of SAR, it increased by about 1.1 to 2.1-fold, respectively. So, there is a positive interaction of TEC and SAR in sodification of soils. A positive interaction of TEC and SAR influenced the ESP build-up and CEC and silt + clay content played a major role in the visual disparity in sodification of these soils. Gapon's selectivity coefficient values were perceptible in the order of calcareous sodic loam > saline silt loam > normal clay loam. This indicates the preference of normal clay loam soils for Ca2+ + Mg2+ than that of Na+ on the exchange complex, whereas loam soil exhibited high affinity for sodium. Regression coefficient of ESR-SAR relationships was maximum for sodic loam followed by saline silty loam and normal clay loam soil. The exchange equilibrium was strongly affected by TEC of the solution phase. Variation in soil pH was gradual with respect to TEC and SAR of equilibrating solution and a sharp change was observed. At a fixed TEC, critical ESP values were observed calcareous sodic loam followed by saline silt loam and normal clay loam. On the other hand, SAReq needed for critical ESP built-up was in the order of calcareous sodic loam > saline silt loam > normal clay loam. This may be because of dissolution of calcium bearing minerals in calcareous sodic loam; and wash out of calcium and magnesium from the exchange sites for saline silt loam soil.

Adsorption of novel insensitive munitions compounds at clay mineral and metal oxide surfaces

Environmental context Insensitive munitions compounds are increasingly used in the manufacture of military energetic materials because of their lower unintentional explosion risk during transport and handling. The current study was designed to better resolve the environmental chemistry of two of these insensitive munitions compounds. In particular, we investigated the solid–solution partitioning that occurs when aqueous solutions containing dissolved unexploded ordinances come into contact with soil mineral media. Abstract Insensitive munitions compounds (IMCs) are increasingly used for military energetic materials, yet their environmental fate is poorly understood. Prior work has shown that the nitroaromatic 2,4-dinitroanisole (DNAN) and the heterocyclic nitrogen compound 3-nitro-1,2,4-triazole-5-one (NTO), both newly introduced IMCs, can undergo microbially mediated reduction under anoxic conditions to form 2-methoxy-5-nitroaniline (MENA) and 3-amino-1,2,4,triazole-5-one (ATO) respectively. In the present work, DNAN, MENA, NTO and ATO were subjected to batch adsorption–desorption experiments with specimen soil mineral adsorbents that included montmorillonite, birnessite and goethite. DNAN and MENA exhibited high affinity, linear adsorption to montmorillonite, with enhanced surface excess at a given aqueous equilibrium concentration for K+-saturated relative to Na+-saturated forms, but negligible adsorption to the metal oxides. Powder X-ray diffraction data and surface occupancy calculations indicate interlayer intrusion by DNAN and MENA and adsorption at siloxane sites. Conversely, NTO and ATO exhibited low sorptive affinity and apparent anion exclusion upon reaction with the negatively charged layer silicate clays. However, both of the N-heterocycles showed positive adsorption affinities for goethite (Kd values of 11.1 and 3.1, and HI values of 1.8 and 0.50 respectively), consistent with anion adsorption to the positively charged goethite surface. Both ATO and MENA were subjected to apparent oxidative, abiotic chemical transformation during reaction with birnessite. The results indicate that the IMCs studied will exhibit adsorptive retardation – and their biodegradation products may undergo further abiotic transformation – upon reaction at soil mineral surfaces.

Polymer dynamics in thin p-type conducting films investigated by ac-electrogravimetry. Kinetics aspects on anion exclusion, free solvent transfer, and conformational changes in poly(o-toluidine)

A new transfer model is proposed to explain ac-electrogravimetry response of p-doped films. This model takes into account the exclusion effect occurring as a result of the anion transfer. The insertion/expulsion of anions inside a film involves simultaneously the expulsion/insertion of free solvent molecules. The number of solvent molecules excluded depends on the volume of anion transferred. Solvent transfer stimulated by the conformational changes of films constitutes the remaining electrogravimetric response when the exclusion process cannot explain by itself this response. Consequently, the kinetics of this free solvent transfer can be directly related to the kinetics of conformational changes. This model was validated in poly(o-toluidine) thin films where electrochemical kinetics were quantitatively investigated as a function of the applied potential. As far as we know, for the first time, charge transfer and conformational changes could be kinetically separated. This new transfer model may respond to fundamental questions on the electrochemistry of conducting polymers, which may have a significant impact on a large number of applications.

Transport properties of iodide in a sandy aquifer: Hydrogeological modelling and field tracer tests

Summary The release of radioactive iodine into geological media from nuclear waste disposal is an issue that has to be considered since iodine is a biophilic element. 129I is, with 99Tc, one of the two long-lived radionuclides that have the highest mobility in radioactive waste disposal. Within this context, iodide retardation is still a matter of debate. A low value of the retardation factor is generally accepted in soils without organic matter, but the possibility for sorption cannot be completely ruled out. Since isotopic exchange with naturally occurring iodine is one of the main potential sorption mechanisms, site-specific retention parameters are needed. In the present paper, we study iodide transport in a sandy aquifer. A hydrogeological model was built to fit deuterium, bromide and iodide breakthrough data from in situ tracer test experiments. Within the precision range of the fitting, iodide is excluded from 2.5% of the effective porosity by anionic exclusion and presents a field retention factor (Kd) lower than 0.025 L/kg.

Applying the squeezing technique to highly consolidated clayrocks for pore water characterisation: Lessons learned from experiments at the Mont Terri Rock Laboratory

Knowledge of the pore water chemistry in clayrock formations plays an important role in determining radionuclide chemical speciation and migration in the context of nuclear waste disposal. Among the different in situ and ex situ techniques for pore water sampling in clay sediments and soils, the squeezing technique dates back 115years. Although different studies have been conducted on the reliability and representativeness of squeezed pore waters, most of these involved high-porosity, high water content and unconsolidated clay sediments. Very few of them analysed squeezed pore water from low-porosity, low water content and highly consolidated clayrocks.In this paper, a specially designed and fabricated one-dimensional compression, two-directional fluid flow cell was used to extract and analyse the pore water composition of Opalinus Clay core samples from Mont Terri (Switzerland) with water contents between 6.2 and 7.8wt.%. A study of the influence of the squeezing pressure on the chemistry of the pore water was performed at pressures of up to 200MPa. To validate and demonstrate the validity of the squeezing technique for obtaining reliable pore water, different tests were performed at various pressures and the squeezed waters were compared with those obtained by other methods, such as in situ collected borehole waters. The reproducibility and quality of the squeezing method was also checked, as well as different artifacts which could affect the reliability of the pore water data.The results show that the reproducibility of the technique is good and no membrane effect (ionic ultrafiltration, chemical fractionation, anion exclusion) was found in Opalinus Clay at Mont Terri in the range of pressures analysed (70–200MPa). Pore waters extracted in this range of pressures do not decrease in concentration as a function of pressure, which indicates that a dilution of water by mixing of the free pore water and the outer layers of diffuse layer water does not occur and a threshold (safety) squeezing pressure of 175MPa could be established to avoid membrane effects. Furthermore, a direct comparison against in situ collected borehole waters shows that the pore waters extracted at these pressures are representative of the Opalinus Clay Formation at Mont Terri. Extraction artifacts such as temperature effect, oxidation and degassing can be avoided or minimised by taking special precautions when preparing and handling the core sample and extracting the pore water. Finally, we show that it is possible to obtain insightful information on anion-accessible porosity by combining squeezing data with aqueous leaching data to characterise the pore water composition and solute distribution for different rock porosities.

Diffusion and sorption of Cs+, Na+, I− and HTO in compacted sodium montmorillonite as a function of porewater salinity: Integrated sorption and diffusion model

Montmorillonite clay is an important component of barrier materials such as bentonites and argillaceous rocks for the safe geological disposal of radioactive waste. The diffusion and sorption behaviors of cationic Cs+ and Na+, anionic I− and neutral tritiated water (HTO) tracers in compacted montmorillonite were investigated as a function of porewater salinity by experimental and modeling approaches. The effective diffusivities of these tracers were measured by the through-diffusion method coupled with multiple curve analysis including depth profiles, and typical salinity-dependent cation excess and anion exclusion effects were shown. The distribution coefficients for Cs+ and Na+ obtained by through-diffusion tests were sensitive to salinity because of competitive ion exchange and were in good agreement with those obtained by batch sorption tests. However, trends in the apparent diffusivities of Cs+ and Na+ were mostly independent of salinity.A diffusion model based on a simplified pore structure and electrical double layer theory, describing the change of ionic concentration and viscoelectric effects caused by the electrostatic interaction with negatively charged clay surfaces, could quantitatively account for the cation excess and anion exclusion behaviors and their salinity dependence. The trends in the distribution coefficients predicted by the ion exchange sorption model show good agreement with the measured values as a function of salinity. Salinity-independent apparent diffusivities could be interpreted as results of the coupled effects of the drastic dependence of diffusion and sorption on salinity. The difference in the diffusion and sorption behaviors between Cs+ and Na+ can be explained by the difference in the sorption mechanism (inner- and outer-sphere) and their representation in the diffusion model (cation excess diffusion). This integrated sorption and diffusion model was further demonstrated as being applicable to the diffusion data of these monovalent cations and anions in compacted bentonites over a wide range of compaction.

Potential contribution of anion exclusion to hydroxide penetration in green liquor-modified kraft pulping

Abstract Partial substitution of green liquor for white liquor in kraft pulping reduces calcining energy and reportedly enhances product properties. However, a full-scale trial showed that pulper rejects increased when green liquor substitution levels approached 10%. Imaging experiments and laboratory pulping runs demonstrate that the slower diffusion of hydroxide in the presence of green liquor carbonate is likely responsible for the elevated rejects. Green liquor substitution becomes increasingly more viable with decreasing chip thickness. Hence, such an approach should be especially suited to sawdust pulping where diffusion is not rate determining.

Experimental characterization of cement–bentonite interaction using core infiltration techniques and 4D computed tomography

Deep geological storage of radioactive waste foresees cementitious materials as reinforcement of tunnels and as backfill. Bentonite is proposed to enclose spent fuel drums, and as drift seals. The emplacement of cementitious material next to clay material generates an enormous chemical gradient in pore water composition that drives diffusive solute transport. Laboratory studies and reactive transport modeling predict significant mineral alteration at and near interfaces, mainly resulting in a decrease of porosity in bentonite. The goal of this project is to characterize and quantify the cement/bentonite skin effects spatially and temporally in laboratory experiments. A newly developed mobile X-ray transparent core infiltration device was used, which allows performing X-ray computed tomography (CT) periodically without interrupting a running experiment. A pre-saturated cylindrical MX-80 bentonite sample (1920kg/m3 average wet density) is subjected to a confining pressure as a constant total pressure boundary condition. The infiltration of a hyperalkaline (pH 13.4), artificial OPC (ordinary Portland cement) pore water into the bentonite plug alters the mineral assemblage over time as an advancing reaction front. The related changes in X-ray attenuation values are related to changes in phase densities, porosity and local bulk density and are tracked over time periodically by non-destructive CT scans.Mineral precipitation is observed in the inflow filter. Mineral alteration in the first millimeters of the bentonite sample is clearly detected and the reaction front is presently progressing with an average linear velocity that is 8 times slower than that for anions. The reaction zone is characterized by a higher X-ray attenuation compared to the signal of the pre-existing mineralogy. Chemical analysis of the outflow fluid showed initially elevated anion and cation concentrations compared to the infiltration fluid due to anion exclusion effects related to compaction of the bentonite core that was adjusting to the experimental conditions. Subsequently, the OPC fluid is fully buffered, and a gradually decreasing ionic strength is observed as a result of progressive consumption of hydroxide at the mineral reaction front.

The influence of mineral variability of Callovo-Oxfordian clay rocks on radionuclide transfer properties

The upper part of the Callovo-Oxfordian clay-rich rock formation (C2c unit) (Meuse/Haute-Marne, France) displays large variations in mineralogical composition of quartz, carbonate and clay minerals. This study deals with the effects of this composition variability on the diffusion-dominated transport properties of HTO (tritiated water)33Tritiated water is a radioactive form of water where some usual hydrogen atoms are replaced with tritium., 36Cl− and 137Cs+ for these rocks. Effective diffusion coefficients De and accessible porosities ε were determined using the through-diffusion method for a set of C2c unit samples characterized by a contrasted mineralogy especially in terms of clay mineral content (4–29%).The relative variations of the effective diffusion coefficients measured for HTO, 36Cl− and 137Cs+ remain limited within a range of a factor 5 down to a clay mineral content of ~10%. In the range of clay mineral content higher than 15%, the effective diffusion coefficient tends to increase for 137Cs+ and decreases for 36Cl− whereas De(HTO) stays relatively stable. Anion exclusion and surface enhanced diffusion for caesium were quantified and can explain this behaviour. Below 10% of clay minerals, these effective coefficients and the accessible porosities decreased drastically. For 4% of clay minerals, De were equal to 1.3·10−12m2s−1 for HTO, 3.5·10−13m2s−1 for 36Cl− and 7·10−12m2s−1 for 137Cs+. One of the main findings is that the anion exclusion and the enhanced diffusion for caesium still occur for the samples characterized by the lowest clay mineral contents. Finally, the set of diffusion data has been analysed against the well-known Archie's relation linking the accessible porosity to the effective diffusion coefficient.

Retention and Transport of Nitrate and Ammonium in Loamy Sand Amended with Clinoptilolite Zeolite

Agricultural soils of southern New Mexico, especially sand and sandy loams receiving numerous nitrogen (N) fertilizations, are prone to leaching large amounts of N to shallow groundwater. Adsorption and leaching experiments were conducted to investigate the use of clinoptilolite zeolite (CZ) as an amendment to sandy soils to increase N retention and reduce nitrate (NO3−) leaching. Urea-ammonium-nitrate (UAN 32) fertilizer was applied to four soil treatments to simulate crop irrigation. The treatments were composed of 100% CZ, 100% loamy sand (LS), a mixture of 80∶20% (LS:CZ), and a mixture 60∶40% (LS:CZ) by mass, respectively. Results from the experiments showed an inverse relationship between nitrate-nitrogen (NO3−-N) adsorption and the amount of CZ added to soil caused by anion exclusion and a direct relationship between ammonium-nitrogen (NH4+-N) adsorption and the amount of CZ mixed with LS due to NH4+-N entrapment by the CZ molecules. It is therefore recommended to use other types of fertilizers that do not include NO3– to LS soils amended with CZ in order to reduce the risk of leaching. Otherwise, it is suggested to apply fertilizers containing ammonium nitrate in LS soils amended with CZ at reduced rates but at higher frequencies to meet plant demands.

Resolving Cl and SO4 Profiles in a Clay-Rich Rock Sequence

The chloride and sulfate concentration profiles in a 260 m thick clay-rich Mesozoic sediment sequence have been analyzed by various methods. Chloride data generally indicate a good consistency between different methods if anion exclusion is accounted for in leaching tests. For sulfate, however, there is an apparent inconsistency between leaching data and those obtained from the other methods, which points to the dissolution of a sulfur-bearing mineral. Traces of diagenetic gypsum seem to be a likely source, but other sulfur minerals cannot be ruled out.

Effect of dry density on 125I diffusion in GMZ bentonite

Gaomiaozi (GMZ) bentonite is regarded as the favorable candidate backfilling material for a potential repository in China. It is important to understand the diffusion behavior of 125I in GMZ bentonite and compare the diffusion behavior in GMZ and other types of bentonite like MX-80, Avonlea, etc. Therefore, through- and out-diffusion experiments were conducted to obtain the effective diffusion coefficient (De) and distribution coefficient (Kd). A computer code named Fitting for diffusion coefficient (FDP) was used for the experimental data processing and theoretical modeling. At the dry density of GMZ bentonite from 1600–2000 kg/m3, the De values of 125I were (2.4–20.4) ¢ 10−12 m2/s and Kd values were constants. At dry density above 1800 kg/m3, the diffusion behaviors were almost the same, indicating that the anion exclusion was ineffective. Out-diffusion results showed that the species of 125I may be changed during the diffusion processing. It was probably caused by some organic matters or reducing substances in GMZ bentonite. Since the main composition of bentonite is montmorillonite, similar diffusion parameters were obtained in GMZ and other types of bentonite. The relationship of De and accessible porosity (ɛacc) could be described by Archie’s law with exponent n = 1.2–2.8 for 125I diffusion in bentonite, whereas n = 2.0 in GMZ bentonite. Furthermore, bentonite with the dry density of 1800 kg/m3 was proposed as the backfilling materials used in the construction of high level radioactivity waste repository.

Prediction of the soil saturated paste extract salinity from extractable ions, cation exchange capacity, and anion exclusion

Process-based models could be used to predict the soil saturated extract salinity from extractable ion contents. However, a rigorous validation of such models for this purpose had not been carried out. A process-based model to predict the main inorganic ion composition, electrical conductivity, and pH of the saturated paste extract from extractable ion contents was developed step by step. The model development started from the principle of matter conservation in the soil solution as it concentrates from the 1 : 5 to the saturated paste extract. The need to include new hypotheses in the model was studied through calculation and analysis of standardised differences between measurements and model predictions. Therefore, best estimates of saturation paste extract properties occurred after taking into account the following: (i) free equilibration of the soil solution with the minerals calcite and gypsum under CO2 partial pressure of the saturated paste; (ii) further equilibration of the soil solution with the exchange complex; and (iii) determination of salt contents within the diffuse double layer (DDL) of the soil colloids, i.e. the anion exclusion. The last was necessary because the extracts where the determination of soil extractable anions (1 : 5 extracts), cations, and cation exchange capacity (CEC) were carried out were separated from their suspensions through centrifugation. Therefore, the reliable prediction of soil saturated paste extract salinity demands data on soil extractable ion contents, CEC, and the quantification of salts within the DDL.

Diffusion of HTO, 36Cl− and 125I− in Upper Toarcian argillite samples from Tournemire: Effects of initial iodide concentration and ionic strength

Diffusion parameters for HTO, 36Cl−, and 125I− were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I− interaction with Upper Toarcian argillite and to verify the effects of initial I− concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl− and I− diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers’ diffusive behaviour. The results show that the effective diffusion coefficient (De) and diffusion accessible porosity (εa) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01 eq L−1 are: De = 2.35–2.50 × 10−11 m2 s−1 and εa = 12.0–15.0% for HTO, and De = 14.5–15.5 × 10−13 m2 s−1 and εa = 2.5–2.9% for 36Cl−. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (De) and rock capacity factor (α) obtained for 125I are: De = 7.00–8.60 × 10−13 m2 s−1 and α = 4.3–7.2%. Such values make it possible to calculate low 125I distribution ratios (0.0057 < RD < 0.0192 mL g−1) which confirm the trend indicating that the 125I rock capacity factor increases with the decrease of the initial I− concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S. = 0.11 eq L−1). No evolution of HTO diffusion parameters was observed. The anionic tracers’ effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for 125I and 36Cl. Whatever the ionic strength of the synthetic solution used, 125I exhibited De values two times lower than those of 36Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying 125I sorption and postulating that 36Cl and 125I would diffuse in the same porosity. In other terms, at Tournemire, 125I sorption could be more pronounced than previously indicated.