A critical review of laboratory and in-situ hydraulic conductivity measurements for the Boom Clay in Belgium

The Boom Clay has been investigated for more than 30 years as a candidate host formation for the disposal of high-level and long-lived radioactive waste in Belgium. The very low hydraulic conductivity (on the order of 10^− 12 m/s) in combination with limited hydraulic gradients over the host formation (0.02–0.04) results in water flow in the Boom Formation being negligible and diffusion the dominant transport mechanism. The assessment of the long-term barrier function of the host clay formation in the framework of radioactive waste disposal requires rigorous quantitative characterization of key formation properties such as the hydraulic conductivity (K).Hydraulic conductivities of Boom Clay measured through various testing techniques in the laboratory, i.e. tracer percolation experiments, constant head permeameter experiments and isostatic experiments, exhibit similar K values in the order of 10^− 12 m/s. Based on a large set of test samples, the impact of sample scale, hydraulic gradient range adopted in the tests, stress controlled methods and pre-existing fissures in the sample on the K value is shown to be quite limited. In situ measurements obtained from both several-centimetre long piezometer filters and percolation into a 7-metre long gallery and 21-metre long shaft at the HADES underground research facility yield K values that are very similar to values measured in the laboratory on samples of a few centimetres. This indicates that the K measurements for the Boom Clay obtained through various techniques are very consistent. K values measured on a centimetre-scale are also representative at the metre-scale, which is often the size of grid cells used in numerical simulations for long-term safety assessments.Spatial analysis of K values across the Boom Clay at the Mol site reveals a typical profile with a very homogenous 61-m thick central part, i.e. the so-called Putte and Terhagen Members, which is also the least permeable part of the Boom Clay. The geometric mean of the vertical (K_v) and horizontal (K_h) hydraulic conductivities for the Putte and Terhagen Members at the Mol site are 1.7 × 10^− 12 and 4.4 × 10^− 12 m/s, respectively, with a vertical anisotropy K_h/K_v of about 2.5. Higher K values, but still low (10^− 12 to 10^− 10 m/s), are observed in the more silty zones above and below the Putte and Terhagen Members, i.e. the Belsele–Waas Member and the Boeretang Member, as well as in the double band of the lower Putte Member.A regional analysis of vertical K variability of the Boom Clay in the northeast of Belgium based on test results from five boreholes shows an increase in hydraulic conductivity from the east towards the west. Statistical analyses indicate that the effect of the samples' stratigraphic position on hydraulic conductivity is strongly related to different grain-size characteristics. However, a general K–grain-size model does not explain the geographical differences in K values satisfactorily. Geographical differences can be best explained by different K–grain-size relationships at the different boreholes. The regional variation in K could be attributed in part to porosity, which in turn is related to the burial depth of the clay.     

Effect of organo clay content on proton conductivity and methanol transport through crosslinked PVA hybrid membrane for direct methanol fuel cell

In the present study, crosslinked poly(vinyl alcohol) (PVA) membranes were prepared using poly(styrene sulfonic acid-co-maleic acid) (PSSA_MA) (PVA:PSSA_MA = 1:7). The PSSA_MA was used both as a crosslinking agent and as a donor of the hydrophilic group (–SO₃H and/or –COOH). The hybrid membranes were prepared by modified clay such as Clay Na⁺, Clay 30B, and Clay 15A. The thermal, water uptake, proton and methanol transport properties of the hybrid membrane were found to be sensitive to the clay type and content. The hybrid membrane with Clay 30B shows higher proton conductivity than other hybrid membranes due to hydroxyethyl group. The membrane with Clay 15A showed the lowest methanol permeability due to lower specific gravity than other clay. Compared to the membrane without modified, the PVA/PSSA_MA/Clay 15A containing 4 wt% of Clay 15A showed both high proton conductivity (0.023 S/cm) and low methanol permeability (2.19 × 10^?7 cm²/s).     

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd³⁺ cation diffusion into transparent polycrystalline YAG (Y₃Al₅O₁₂) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd³⁺ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies.The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol^?1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol^?1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol^?1.     

Electrogeneration of hydrogen peroxide in gas diffusion electrodes modified with tert-butyl-anthraquinone on carbon black support

Hydrogen peroxide (H₂O₂) is a versatile oxidizing agent that is synthesized commercially by the reduction of oxygen in organic medium. Electrochemical technology employing a modified gas diffusion electrode (MGDE) offers a viable alternative for the industrial-scale synthesis of the oxidant. Addition of 1% (w/w) of tert-butyl-anthraquinone (TBAQ) to carbon black deposited in the form of a microporous layer onto the disk of a rotating ring-disk electrode produced an increase in the ring current, which is directly related to H₂O₂ formation, and presented an efficiency of H₂O₂ generation of 89.6% compared with 76.6% for carbon black alone. No significant changes were detected in the number of electrons transferred in the presence of the catalyst suggesting an electrochemical/chemical mechanism for H₂O₂ formation. Analogous improvements in the generation of H₂O₂ were obtained with MGDEs comprising TBAQ on carbon black. The highest concentrations of H₂O₂ (301 mg L⁻¹) were produced at the fastest rate (5.9 mg L⁻¹ min⁻¹) with the lowest energy consumption (6.0 kWh kg⁻¹) when a potential of −1.0 V vs SCE was applied to a MGDE containing 1.0% of TBAQ on carbon black. It is concluded that the application of MGDEs comprising TBAQ on carbon black support offers considerable advantages in the electrogeneration of H₂O₂.     

Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–reduction of CH4–CO2 mixed gases

This paper shows the recent results on the development of layered composite promoting two types of electrochemical reactions (oxidation and reduction) in one cell. This cell consisted of porous Ni–Gd-doped (GDC) ceria cathode/thin porous GDC electrolyte (50 μm)/porous SrRuO₃–GDC anode. The external electric current was flowed in this cell at the electric field strength of 1.25 and 6.25 V/cm. The mixed gases of CH₄ (30–70%) and CO₂ (70–30%) were fed at the rate of 50 ml/min to the cell heated at 400–800 °C under the electric field. In the cathode, CO₂ was reduced to CO (CO₂ + 2e^? → CO + O^2?) and the formed CO and O^2? ions were transported to the anode through the pores and surface and interior of grains of GDC film. On the other hand, CH₄ was oxidized in the anode to form CO and H₂ through the reaction with diffusing O^2? ions (CH₄ + O^2? → CO + 2H₂ + 2e^?). As a result, H₂–CO mixed fuel was produced from the CH₄–CO₂ mixed gases (CH₄ + CO₂ → 2H₂ + 2CO). This electrochemical reaction proceeded completely at 800 °C and no blockage of gases was measured for long time (>10 h). Only H₂–CO fuel was generated in the wide gas compositions of starting CH₄–CO₂ gases.     

Isothermal sintering kinetic of 3YTZ and 8YSZ: Cation diffusion

We have studied the sintering kinetic of 3 and 8 mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450 °C. The sintering kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731 kJ mol^?1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400 °C in 4.05×10^?14 and 6.00×10^?11 cm² s^?1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.     

Physicochemical approach to nanobubble solutions

Small bubbles of nitrogen, methane, or argon with an average radius of 50 nm, as measured by scanning electron microscopy, were prepared under atmospheric conditions. The lifetime of the nanobubbles extended to more than two weeks. The total amount of gases in the nanobubble solutions reached 600 cm³ per 1 dm³ of water, and the liquid density was about 0.988 g/cm³. The internal pressure of the nanobubbles was estimated to be 6 MPa. The number of nanobubbles was 1.9×10¹⁶ bubbles per 1 dm³ of water. These findings show that almost no gas samples are dissolved homogeneously in the aqueous solution and that the vast majority is present in the form of nanobubbles, that is, nanobubbles should be thermodynamically unstable. Attenuated total reflectance infrared spectroscopy showed that the surfaces of the nanobubbles contain hard hydrogen bonds that may reduce the diffusivity of gases through the interfacial film.     

Giant effective liquid-self diffusion in stagnant liquids by magnetic nanomixing

Many chemical engineering applications require tools to intensify processes in regions where Fickian molecular diffusion is the dominant mechanism, such as in boundary layers, microporous catalysts or microfluidics. We demonstrate in this study that spinning magnetic nanoparticles (MNPs) by means of rotating magnetic fields (RMFs) gives rise to an intriguing nanomotion mechanism capable of triggering giant diffusion in stagnant liquids and thus able to stimulate transport beyond the molecular diffusion barrier especially in stagnant liquids. To evidence this mechanism, we report original water self-diffusion coefficients measured in aqueous media containing very low concentrations of ferrite MNPs that can be rotated in uniform RMF. The self-diffusion coefficient of distilled water (i.e., D₀ ≈ 3.5 × 10⁻⁹ m²/s) was enhanced up to 200 times by application of a rotating magnetic field in stagnant-liquid conditions. It was concluded that in absence of macroscopic convective flows, MNPs may prove to be efficient nanostirrers to enhance liquid transport properties at nanoscale. By delivering giant diffusion around them, rotating MNPs can constitute an appealing nanomixing process intensification tool.     

Removal of aqueous Fe2+ using MnO2–clinoptilolite in a batch slurry reactor: Catalyst synthesis,characterization and modeling of catalytic behavior

An Iranian clinoptilolite has been modified with MnO₂ for the catalytic removal of Fe²⁺ cations from water in a batch slurry reactor. The modified zeolite was subjected to FESEM, XRD, WDX, XRF and specific surface area analysis. A correlation for the intrinsic catalytic reaction rate incorporating both Fe²⁺ and dissolved oxygen concentration as a function of reaction temperature has been presented. The effect of the modified zeolite aggregate particle size on the iron removal kinetics has been investigated. It was shown that for particles larger than 150 μm, diffusion through the mesopores of the zeolite aggregate is rate controlling. The effective diffusion coefficient through the particles at RT has been calculated as 2.3 × 10^?6 cm² s^?1. It is shown that liquid phase molecular diffusion within the mesopores is the dominating mass transfer mechanism.     

Heterogeneous membranes based on a composite of a hypercrosslinked microparticle adsorbent and polyimide binder

This paper deals with the preparation and characterization of heterogeneous membranes based on microparticle hypercrosslinked polymeric adsorbents with a polyimide binder. The polyimide based membrane extension is hindered by their low permeability. We enhanced the permeability of polyimide membranes by changed chemical structure and by adding of the new type fillers. Hypercrosslinked polystyrene microparticles of diameter 1–5 μm were prepared by SnCl₄-catalyzed Friedel–Crafts reaction of polystyrene with chloromethyl methyl ether in 1,2-dichlorethane solution. The precursor polyamic acid (PAA) was synthesized by the reaction of equimolar amounts of 4,4′-oxy(bis(phthalic anhydride)) (ODPA) and bis(4,4′-oxydianiline) (ODA) or 4,4′-[(1,4-phenylene)dipropane-2,2-diyl]dianiline (BIS P) in N-methylpyrrolidone (content 10 wt.%). A PAA solution in N-methylpyrrolidone with the adsorbent was spread onto a glass substrate and kept at 60–240 °C for 12 h. Heterogeneous membranes were characterized by thermal, mechanical and separation measurements. The permeability for membrane ODPA–BIS P filled with 10 wt.% of hypercrosslinked adsorbent was 3.5 × 10⁻¹¹ cm³(STP) cm cm⁻² s⁻¹ cmHg⁻¹ for nitrogen and 4 × 10⁻⁹ cm³(STP) cm cm⁻² s⁻¹ cmHg⁻¹ for hydrogen. The permeability of homogeneous polyimide membranes is up to one order of magnitude lower. The diffusion coefficient of heterogeneous membranes increased in the order CH₄ < N₂ < O₂ < CO₂ < H₂. The selectivity of hydrogen–nitrogen separation with the amount of adsorbent decreased from 164 to 69. The prepared membranes are intended for separation of gases and low organic molecules even at enhanced temperature. The present paper aims at giving information on the influence of hypercrosslinked adsorbents and polyimide binding materials on the gas separation properties of membranes.     

Facile synthesis of AlOx dielectrics via mist-CVD based on aqueous solutions

Aluminum oxide (AlO_x) thin films were synthesized by mist-chemical vapor deposition (mist-CVD) using aluminum acetylacetonate (Al(acac)₃) dissolved in an aqueous solvent mixture of acetone and water. Nitrogen gas was used to purge the precursor solution and growth rates between 7.5–13.3 nm/min were achieved at substrate temperatures of 250–350 °C. The AlO_x layers deposited at temperatures below 350 °C exhibit 3–5 at% residual carbon levels, however those grown at 350 °C exhibit only 1–2 at% carbon impurity. Reasonable dielectric properties were obtained in the latter, with a dielectric constant (κ) of ~ 7.0, breakdown field of ~ 9 MV/cm and relatively low leakage current density of ~ 8.3×10⁻¹⁰ A/cm².     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.     

Environmental aspects of gasification of Korean municipal solid waste in a pilot plant

Municipal solid waste was gasified in a 3 ton/day capacity pilot plant based on thermoselect process with oxygen at a temperature of around 1200 °C. CO and H₂ content of synthesis gas were about 27–40 and 36–40%, respectively with high heating value 8.0–10.2 MJ/N m³. Chlorinated compounds like furan, dioxin, and other organics in gaseous and liquid phase were effectively destroyed due to high temperature of high temperature reactor and shock cooling. Exhaust gases were also found to be satisfying the Korean emission standard. Leachable concentration of heavy metals in the vitrified mineral aggregate were much less than the Korean regulatory limit values due to high melting temperature (1600 °C). The vitrified slag of dark brown color was found to be glassy and non-hazardous in nature and seems to have the possibility to be used as natural raw material in cement and construction industry.     

Selective esterification of tert-butanol by acetic acid anhydride over clay supported InCl3, GaCl3, FeCl3 and InCl2 catalysts

Clay (kaolin, mont-K10 and mont-KSF) supported InCl₃, GaCl₃, FeCl₃ and ZnCl₂ catalysts (metal chloride loading=1.1 mmol g⁻¹) show high selectivity (⩾98%) at high conversion in the esterification of tert-butanol by acetic anhydride to tert-butyl acetate (t-BA) and very low activity for the dehydration of tert-butanol at ⩽50°C. For all the catalysts, mont-K10 is the best support and the order of their esterification activity (at 26°C) is: InCl₃/mont-K10 (TOF=0.025 s⁻¹) > GaCl₃/mont-K10 (0.023 s⁻¹) > FeCl₃/mont-K10 (0.02 s⁻¹) ⩾ ZnCl₂/mont-K10 (0.019 s⁻¹). InCl₃/mont-K10 is highly active, selective and reusable catalyst for the esterification.     

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)³, ³⁶Cl⁻ and ¹³⁷Cs⁺ for these rocks. Effective diffusion coefficients D_e 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 D_e(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, D_e were equal to 1.3 · 10^− 12 m² s^− 1 for HTO, 3.5 · 10^− 13 m² s^− 1 for ³⁶Cl⁻ and 7 · 10^− 12 m² s^− 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.     

Optical evaluation of carrier lifetime and diffusion length in synthetic diamonds

The key electronic parameters of high-pressure-high-temperature and chemical-vapor-deposition grown diamonds have been determined at interband (hν = 5.82 eV) or below bandgap (hν = 4.68 eV) photoexcitation, using a picosecond transient grating (TG) technique. TG kinetics directly provided the values of ambipolar diffusion coefficient (6–10 cm²/s) and carrier lifetime (in a range from 0.17 to 2.8 ns) for crystals grown under different conditions. The carrier diffusion length was found to vary from 0.5 μm in CVD layers to 1.6 μm for IIa type HPHT diamond crystal. The carrier lifetimes correlated well with the nitrogen-related defect density in both types of diamonds.     

Cooperative adsorption of bisphenol-A and chromium(III) ions from water on activated carbons prepared from olive-mill waste

The aim of this study was to investigate the simultaneous adsorption of bisphenol A (BPA) and chromium ions from aqueous solution on activated carbons (both commercial and prepared from olive-mill waste), analyzing both kinetic and equilibrium adsorption data. The effects of solution pH and ionic strength on the adsorption processes were also studied, as well as the chemical interactions between the carbon surface and the pollutants. The activated carbon prepared from olive-mill waste showed: a large surface area (up to 1641 m² g⁻¹), a highly heterogeneous micropore distribution, and a basic chemical nature. The pore volume diffusion model was used to predict the adsorption kinetics of both pollutants. The effective diffusion coefficients ranged between 1.15 × 10⁻⁶ and 9.18 × 10⁻⁷ cm² s⁻¹ for the BPA–Cr(III) system and between 1.65 × 10⁻⁶ and 2.8 × 10⁻⁶ cm² s⁻¹ for the Cr(III)–BPA system. The presence of both Cr(III) and BPA in the binary systems increases the adsorption effective diffusion coefficients and therefore the overall adsorption rate of each pollutant. The increased adsorption of each adsorbate when both pollutants are present is due to the in situ formation of complex compounds between Cr(III), acting as central metallic cation, and BPA, acting as ligand, during adsorption of both adsorbates on activated carbon.     

96Zr diffusion in polycrystalline scandia stabilized zirconia

Diffusion of the stable tracer isotope ⁹⁶Zr in 12 mol% polycrystalline scandia stabilized zirconia was studied in air in the temperature range from 1200 to 1600 °C. Secondary ion mass spectroscopy (SIMS) was used to record the tracer diffusion profiles. The activation enthalpies for bulk and grain boundary diffusion were determined to be (5.0 ± 0.4) eV and (3.9 ± 0.5) eV, respectively, with the latter being six to seven orders in magnitude faster than the first. Using XRD, it was proved that the diffusion occurs only in the cubic phase.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.