The effect of ionic strength on the diffusion of 125I in Gaomiaozi bentonite

The diffusion of ¹²⁵I^? in compacted Gaomiaozi (GMZ) bentonite was investigated by capillary in-diffusion method. Apparent and effective diffusion coefficients and accessible porosity of iodide in GMZ bentonite were obtained, and the effect of ionic strength on diffusion parameters was studied. The apparent diffusion coefficients of iodide in compacted GMZ bentonite are in the range of 1.0–6.0 × 10^?10 m² s^?1 under the conditions of dry bulk density 1,500 kg m^?3 and temperature 298 K, and increase with increasing ionic strength. This effect was explained through the analysis of microscopic structure of compacted bentonite. The iodide can only diffuse in unbound interparticle pore solution of compacted bentonite. The apparent diffusion coefficient is a function of accessible porosity which is decided by the thickness of diffusion double layer, and the thickness is in turn controlled by ionic strength.     

Thermal behavior and immersion heats of selected clays from Turkey

Four clays (two bentonites and two kaolinites) from Turkey were investigated by X-ray diffraction (XRD), thermal analysis (DTA/TG-DSC) and surface area measurement techniques. Mineralogically bentonite samples were characterized low concentration of montmorillonite and high level of impurities. Both kaolinite samples mainly contained kaolinite and quartz as major mineral. TG-DTA curves of all clay samples were measured in the temperature range 30–1200 °C. The total % weight losses for the bentonite samples (B1 and B2) and the kaolinite samples (K1 and K2) were determined as 14.50, 13.42, 5.55 and 11.85%, respectively. Differential Scanning Calorimeter (DSC) analyses of samples were carried out by heating the samples from 30 to 550 °C. The immersion heats of clay samples were measured using with a Calvet-type C-80 calorimeter. The higher exothermic Q_imm values were determined for bentonite samples compared to kaolinite samples.     

A new timescale dimension for migration experiments in clay: proof of principle for the application of miniaturized clay column experiments (MCCE)

Geological clay formations are often considered as a host rock for a future nuclear waste repository. Many studies concerning sorption or desorption experiments with metal ions like radionuclides or other relevant substances (e.g. metal complexing ligands) onto/from geological clay samples are performed with the batch techniques where only small amounts of the homogenized clay is in contact with the appropriate metals diluted in high volumes of aqueous solutions. This unnatural contact of clay with water can lead to high bias or not transferable results for a risk assessment study of a future repository. Diffusion experiments as an alternative and more natural experimental tool have the lack of huge time consuming when the migration of higher valent metal ions is considered. With the herein described new miniaturized clay column setup a linker between the unnatural batch techniques and the time consuming diffusion experiments is installed. The presented miniaturized clay column experiments (MCCE) derived and modified from high performance liquid chromatography can be applied in a lot of geochemical studies. Using MCCE, migration experiments of inert tracers (iodide), natural organic matter as complexing ligands (lactate and salicylate) as well as trivalent metal ions (europium) in compacted clay can be performed within a short time span of a few minutes or hours only in contrast to several months by use of classical diffusion or column methods. As preliminary results, typical migration times through miniaturized clay columns (20 × 3.5 mm, L × ID) of iodide as inert tracer are in the range of 145 min, meanwhile increasing retention times of salicylate from about 390 min in the absence of Eu to migration times in the range of 420–470 min in the presence of different Eu concentration can be observed.     

Barrier properties of polylactic acid/layered silicate nanocomposites for food contact applications

Polylactic acid/layered silicate nanocomposite films were prepared by solution casting technique. Four types of organo modified montmorillonite and an unmodified bentonite were used as inorganic fillers. The structural characterizations were done by FTIR/ATR and dispersion of the layered silicates was determined by XRD. XRD results showed that the prepared nanocomposites showed flocculated, intercalated and exfoliated structure. The highest crystallinity degree obtained was 28. Overall migration tests were studied with food simulants included distilled water, 3% acetic acid, 95% ethanol. The migration values of all the prepared films were found to be below the allowed limit (10 mg/dm²). The best result in oxygen gas transmission and water vapor transmission rates were 233.4 cm³ mm/m² day MPa and 98.3 g/m² day, respectively. Consequently the oxygen barrier property has increased by 34% and water vapor barrier property increment was 65% when compared to pure PLA film.     

Preparation and characterization of poly(ethyl acrylate)/bentonite nanocomposites by in situ emulsion polymerization

Transparent poly(ethyl acrylate) (PEA)/bentonite nanocomposites containing intercalated–exfoliated combinatory structures of clay were synthesized by in situ emulsion polymerizations in aqueous dispersions containing bentonite. The samples for characterization were prepared through direct‐forming films of the resulting emulsions without coagulation and separation. An examination with X‐ray diffraction and transmission electron microscopy showed that intercalated and exfoliated structures of clay coexisted in the PEA/bentonite nanocomposites. The measurements of mechanical properties showed that PEA properties were greatly improved, with the tensile strength and modulus increasing from 0.65 and 0.24 to 11.16 and 88.41 MPa, respectively. Dynamic mechanical analysis revealed a very marked improvement of the storage modulus above the glass‐transition temperature. In addition, because of the uniform dispersion of silicate layers in the PEA matrix, the barrier properties of the materials were dramatically improved. The permeability coefficient of water vapor decreased from 30.8 × 10^?6 to 8.3 × 10^?6 g cm/cm² s cmHg. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1706–1711, 2002     

Formability and properties of self-standing clay film by montmorillonite with different interlayer cations

Influences of exchangeable interlayer cations were investigated on self-standing film formability, film morphology, and properties of the clay films such as flexibility and gas barrier property. Ion-exchanged montmorillonite samples were prepared by a cation exchange from naturally bearing cation, mostly Na⁺, to Li⁺, Mg²⁺, Ca²⁺, Al³⁺, and Fe^{2+, 3+}. Self-standing films were prepared from aqueous colloidal dispersions of these montmorillonite samples with no additives. The montmorillonite samples with monovalent or divalent cation formed flat self-standing films while the Al-montmorillonite sample produced a distorted film. The Fe-montmorillonite sample formed many separated reddish-brown rod-shaped pieces. Clay film microstructures were different with interlayer cations. The films with monovalent interlayer cations were constructed by the stacking of units with delicately waved thin clay sheets in the whole film, but other films show different morphologies between the upper side and lower side; the upper side is laminated with thin sheets; the lower side is laminated with large thick sheets.The self-standing films’ flexibility and gas barrier property differed according to the interlayer cations. These properties were good in cases of samples with monovalent cations. The innumerable short wave and sheet thinness are considered to foster good flexibility and gas barrier properties. The differences in film formability and properties of the films are attributable to different swellability among samples with different interlayer cations. The montmorillonite samples with monovalent cations swell sufficiently by water, but those with polyvalent cations swell poorly. In the latter case, clay crystals aggregate in water, then the aggregate grows into large particles, creating a film with large particles.     

Application of simplified desorption method to study on sorption of americium(III) on bentonite

To elucidate the sorption behavior of americium(III) on bentonite, which is a mixture of montmorillonite clay, quartz and other minerals, simplified desorption experiments were applied to the solid phases collected after the sorption experiments. The sorption–desorption behavior was examined in the final pH range from 2 to 8. The desorption experiments revealed that most of the Am was sorbed on the montmorillonite moiety of the bentonite. The sorption of Am on montmorillonite was divided into two types: one was the “exchangeable” sorption, in which the sorbed Am was desorbed with a 1 M KCl aqueous solution, and the rest was the “unexchangeable” sorption. The exchangeable sorption was ion exchange of mostly Am³⁺. The unexchangeable sorption was the strong sorption of Am hydroxides. An accessory iron mineral, pyrite, might be involved in the Am sorption on bentonite at neutral pH.     

Hydrophobically modified polyelectrolyte for improved oxygen barrier in nanobrick wall multilayer thin films

The influence of attaching hydrophobic side groups to a polyelectrolyte, used for deposition of a multilayer oxygen gas barrier thin film, was investigated. Polyethyleneimine (PEI) was labeled with pyrene and deposited in “quadlayers” of PEI, poly(acrylic acid), PEI, and sodium montmorillonite clay using layer‐by‐layer assembly. Thin films made of three repeating quadlayers using unmodified PEI had much lower density (1.24 g/cm³) than pyrene‐labeled PEI‐based films (1.45 g/cm³), which is believed to be the result of greater chain coiling from the increased hydrophobicity of pendant pyrene groups. This increased density in pyrene‐labeled PEI layers allowed three quadlayers to match the oxygen transmission rate of a four quadlayer film made with unmodified PEI. This discovery provides an additional tool for tailoring the barrier behavior of clay‐based multilayer thin films that could prove useful for a variety of packaging applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1153–1156     

Effect of γ-radiation on radionuclide retention in compacted bentonite

Compacted bentonite is proposed as an engineered barrier in many concepts for disposal of high level nuclear waste. After the initial deposition however, the bentonite barrier will inevitably be exposed to ionizing radiation (mainly γ) under anoxic conditions. Because of this, the effects of γ-radiation on the apparent diffusivity values and sorption coefficients in bentonite for Cs⁺ and Co²⁺ were tested under different experimental conditions. Radiation induced effects on sorption were in general more noticeable for Co²⁺ than for Cs⁺, which generally showed no significant differences between irradiated and unirradiated clay samples. For Co²⁺ however, the sorption to irradiated MX80 was significantly lower than to the unirradiated clay samples regardless of the experimental conditions. This implies that γ-radiation may alter the surface characteristics contributing to surface complexation of Co²⁺. With the experimental conditions used, however, the effect of decreasing sorption was not large enough to be reflected on the obtained D_a values.     

Interactions between C.I. Basic Blue 41 and aluminosilicate sorbents

Four aluminosilicate sorbents (montmorillonite, bentonite, raw perlite, and expanded perlite) were employed for retention of the cationic dye C.I. Basic Blue 41. Interactions between the clay and the dyestuff were investigated at several temperatures and clay:dye ratios. The mechanism behind the adsorption involves the formation of H-aggregates of the dye on both clays, followed by dye migration into the interlayer in the case of montmorillonite. Time-dependent absorbance spectra revealed the presence of various dye species in montmorillonite. Introduction of the dye molecules into the interlamellar space occurs more rapidly in bentonite than in montmorillonite. The dye molecules inserted between the clay leaves adopt different orientations and, eventually, stack in layers at increased dye loadings for both montmorillonite and bentonite. Higher dye aggregates are then present as suggested by diffuse reflectance spectroscopy. Dye sorption on both raw and expanded perlite proceeds via H-aggregate formation as well.     

Adsorption of polyethylene glycol from aqueous solution on montmorillonite clays

Adsorption rates and capacities of polyethylene glycol (PEG) were investigated for five montmorillonite clays. The adsorption of PEG for all the montmorillonite clays was rapid, and equilibrium was attained within 30 min. The adsorption isotherms of PEG for all the montmorillonites conformed to the Freundlich equation. The adsorption heats were 7.3 and 11.6 kJ · mol^–1(mw.:2000), and 8.7 and 14.2 kJ · mol^–1(mw.:20000) for the montmorillonite and the bentonite II-Ca, respectively. Adsorption capacities for all the clay samples approached constants for the molecular weight of PEG over 2000, though they increased with the increase of molecular weight under 2000. The adsorption capacities were slightly influenced by a nearly neutral pH. The montmorillonite clays which had different interlayer cations showed quite different adsorption capacities. The bentonite II-Ca, the acid clay, and the activated clay showed large adsorption capacities that were 30–50 % of that of an activated carbon.     

Gas barrier properties of poly(ε‐caprolactone)/clay nanocomposites: Influence of the morphology and polymer/clay interactions

Poly(ε‐caprolactone)/montmorillonite nanocomposites were prepared maintaining a constant inorganic content with three means: melt blending of poly(ε‐caprolactone) with natural or organomodified clays, in situ polymerization of ε‐caprolactone in the presence of organomodified clays, and initiation of ε‐caprolactone polymerization from the silicate layer with appropriate organomodified montmorillonites and activator. In this last case, the polymer chains were grafted to the silicate layers and it was possible to tune up the grafting density. The presence of clays did not modify the polymer crystallinity. It was shown that the in situ polymerization process from the clay surface improved the clay dispersion. The gas barrier properties of the different composite systems were discussed both as a function of the clay dispersion and of the matrix/clay interactions. The highest barrier properties were obtained for an exfoliated morphology and the highest grafting density. Similar evolution of the permeability and the diffusion coefficients was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 205–214, 2005     

Effects of compression molding on meltability of uranium dendrites for ingot consolidation in a pyroprocess

Compression molding was carried out to enhance the meltability of uranium dendrites. Uranium dendrites were successfully compressed, increasing their bulk density more than 9 times from 1.1 to 10 g/cm³. The average bulk density was about 8.7 g/cm³ which was almost half of the material density. The compressed dendrites were favorably melted at a temperature of 1,400 °C in an induction furnace. 3 kg of the compressed dendrites were consolidated into an ingot form, showing 96.7 % yield. About 3 % of dross was formed during the melting test in the form of fine powder which was characterized as a uranium oxide. This compression molding method was compared to the supplemental charge method in which uranium dendrites were poured into a molten metal pool produced from a uranium ingot. The capacity of dendrite melting was higher in the compression molding method than in the supplemental charge method. We consider that the higher capacity can be attributed to enhanced thermal conductivity as the bulk density was increased by the compression. These results suggest the high feasibility of the compression molding method for uranium melting in a pyroprocess at an engineering scale.     

Saturation ratio of poly(ethylene oxide) to silicate in melt intercalated nanocomposites

Melt intercalation has been found to be a very successful approach for preparation of polymer-clay nanocomposites. An aspect of this area that has been little investigated is the amount of polymer required to fill the interlayer galleries of the clay. This paper reports experiments which determine the amount of poly(ethylene oxide) (PEO) required to saturate the spacing between montmorillonite (MMT) or organically-modified bentonite (B34) layers. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to determine the saturation ratios of PEO to silicates, which are then compared to theoretical calculations. The deduced saturation ratio of PEO to MMT is 28:72, and PEO to B34 15:85 by XRD and DSC, whilst ratios of PEO to MMT of 21:79 and PEO to B34 10:90 were obtained via TGA. The density of intercalated PEO in the silicate galleries is estimated to be 0.82 g/cm³, which suggests that PEO in the silicate galleries is far less efficiently packed than in the amorphous region of the bulk polymer.     

Stripping voltammetry method for determination of manganese as complex with oxine at the carbon paste electrode with and without modification with montmorillonite clay

Oxine (8-hydroxyquinoline) was used as an efficient and selective ligand for stripping voltammetry trace determination of Mn(II). A validated square-wave adsorptive cathodic stripping voltammetry method has been developed for determination of Mn(II) selectively as oxine complex using both the bare carbon paste electrode (CPE) and the modified CPE with 7 % (w/w) montmorillonite-Na clay. Modification of carbon paste with montmorillonite clay was found to greatly enhance its adsorption capacity. Limits of detection of 45 ng l^?1 (8.19?×?10^?10 mol L^?1) and 1.8 ng l^?1 (3.28?×?10^?11 mol L^?1) Mn(II) were achieved using the bare and modified CP electrodes, respectively. The achieved limits of detection of Mn(II) as oxine complex using the modified CPE are much sensitive than the detection limits obtained by most of the reported electrochemical methods. The developed stripping voltammetry method using both electrodes was successfully applied for trace determination of Mn(II) in various water samples without interferences from various organic and inorganic species.     

A new method for the determination of low-level actinium-227 in geological samples

We developed a new method for the determination of ²²⁷Ac in geological samples. The method uses extraction chromatographic techniques and alpha-spectrometry and is applicable for a range of natural matrices. Here we report on the procedure and results of the analysis of water (fresh and seawater) and rock samples. Water samples were acidified and rock samples underwent total dissolution via acid leaching. A DGA (N,N,N′,N′-tetra-n-octyldiglycolamide) extraction chromatographic column was used for the separation of actinium. The actinium fraction was prepared for alpha spectrometric measurement via cerium fluoride micro-precipitation. Recoveries of actinium in water samples were 80 ± 8 % (number of analyses n = 14) and in rock samples 70 ± 12 % (n = 30). The minimum detectable activities (MDA) were 0.017–0.5 Bq kg^?1 for both matrices. Rock sample ²²⁷Ac activities ranged from 0.17 to 8.3 Bq kg^?1 and water sample activities ranged from below MDA values to 14 Bq kg^?1of ²²⁷Ac. From the analysis of several standard rock and water samples with the method we found very good agreement between our results and certified values.     

Removal of natural uranium from water produced in the oil industry using Algerian bentonite

Batch sorption experiments have been carried out to remove natural uranium (NORM) from water obtained together with crude oil and natural gas, using Algerian bentonites. The effect of some important factors such as S/L ratio, pH, initial concentration, particle size was evaluated and a kinetic study performed. The value of the distribution coefficient (K _d) at equilibrium for natural uranium varied from 30 to 600 cm³·g⁻¹ and 50 to 1100 cm³·g⁻¹ (∼10% margin error) using natural bentonite and drilling bentonite, respectively. The isotherms showed that the data are consistent with both Freundlich and Langmuir models.     

Salinity dependence of 226Ra adsorption on montmorillonite and kaolinite

The effect of NaCl concentration (10.0–1,000 mM) on ²²⁶Ra adsorption was investigated in the presence of montmorillonite and kaolinite. A positive correlation was observed between the dissolved ²²⁶Ra and NaCl concentrations in the presence of these adsorbents. Distribution coefficients decreased from the order of 10⁴ to 10⁰ (mL g^?1) with an increase in NaCl concentration. Although the coefficients were higher for montmorillonite than kaolinite at lower NaCl concentrations, the trend was reversed at higher NaCl concentrations (≥500 mM) owing to the sharper reduction of the coefficient for montmorillonite with the increase in NaCl concentration. The rapid reduction was ascribed to higher negative charge density of montmorillonite, which leads the Ra²⁺ adsorption mechanism to approach charge-compensating ion exchange.     

A characterization study of some aspects of the adsorption of aqueous Co2+ ions on a natural bentonite clay

The natural bentonite used in this study contained montmorillonite in addition to low cristobalite. The uptake of aqueous Co(2+) ions was investigated as a function of time, concentration, and temperature. In addition, the change in the interlayer space of montmorillonite was analyzed using XRPD, and the distribution of fixed Co(2+) ions on the heterogeneous clay surface was recorded using EDS mapping. The sorbed amount of Co(2+) appeared to closely follow Freundlich isotherm, with the sorption process showing apparent endothermic behavior. The relevance of the apparent DeltaH(o) values is briefly discussed. Analysis of the Co-sorbed bentonite samples using SEM/EDS showed that the montmorillonite fraction in the mineral was more effective in Co(2+) fixation than the cristobalite fraction. XRPD analysis demonstrated that the interlayer space of montmorillonite was slightly modified at the end of sorption.     

Thermal stability of ionene polymers

Two different cationic polymers of the same chemical type and with very similar chemical structures were reacted with a natural bentonite over a wide range of polymer/clay ratios. This study involved the synthesis of cationic aliphatic ammonium polyionenes, specifically 3,6-ionene and 3,6-dodecylionene. Ionenes are ion-containing polymers that contain quaternary nitrogen atoms in the main macromolecular chain as opposed to a pendant chain. The CHN content, basal spacing, and elemental composition of each of the polymer–clay complexes were analyzed by X-ray diffraction, X-ray fluorescence, and thermogravimetry. All the polycations reacted to form interlayer complexes with clay, which displaced more Na⁺ and little Ca²⁺. Sodium and calcium were both present as interlayer cations in the clay and its complexes. The TG/DTG curves show that both polymers underwent thermal degradation in more than one stage. Specifically, 3,6-ionene was found to undergo two stages of decomposition and 3,6-dodecylionene undergo three stages. The behavior of the TG/DTG curves and the activation energy values suggest that 3,6-dodecylionene (E = 174,85 kJ mol^?1) complexes have greater thermal stability than 3,6-ionene (E = 115,52 kJ mol^?1) complexes. The mechanism of degradation suggests a direct interaction with the dodecyl chain containing 12 carbons, which are present in 3,6-dodecylionene but not in 3,6-ionene.