Bentonite Colloids Research Articles

Graft Copolymerization of N-Isopropylacrylamide and Acrylic Acid on Bentonite Colloids for In-Depth Fluid Diversion

A new cost-effective in-depth fluid diversion has been developed and reported. In this paper, the diverting agent is prepared successfully from natural bentonite particles that were modified with N-isopropylacrylamide (NIPAM) and acrylic acid (AA) copolymers. First, bentonite particles were intercalated with small precursor molecules that contained functional groups. These precursors were used to reduce the bentonite particle size and introduce a vinyl group for subsequent polymerization. Then, poly(NIPAM-co-AA) was grafted onto hydrophilic bentonite through a free radical polymerization process. The grafted bentonite morphology, microstructure, and thermal stability were investigated using FTIR spectroscopy, dynamic light scattering, XRD, and TGA measurements. The particle dispersion stability and rheological properties have been investigated by using a turbidimeter and rheometer. Sand packs and core flooding tests were conducted to investigate the injectivity and determine permeability reductions. Exper...

Fractal model for erosion mass of bentonite colloids

Compacted bentonite has been considered as a candidate buffer material in underground repository for the disposal of high-level radioactive waste. Bentonite erosion caused by groundwater flow may take place at the interface of compacted bentonite and fractured granite. Bentonite colloids formed at the outer part of a bentonite buffer could expand and release into rock fractures during erosion by seeping groundwater. A novel idea is proposed to estimate bentonite erosion mass in consideration of fractal structure of bentonite colloids in this paper. The erosion model for accumulated bentonite erosion mass under various geochemical conditions is presented on the basis of fractal model for bentonite colloids. The erosion model presents the relation between accumulated volume of water flow and accumulated erosion mass of bentonite colloids in terms of a power law. The exponent parameter is related to fractal dimension (D) of bentonite colloids, and expressed as D/(6 − D). A number of data from different tests are used to verify the erosion model for bentonite erosion mass, and the results from all tests were well matched the prediction of the proposed erosion model.

Study of 85 Sr transport through a column filled with crushed granite in the presence of bentonite colloids

Abstract The present work is focused on the study of strontium transport through crushed granite in the presence of bentonite colloids under dynamic arrangement. The aim of the experiments was to investigate the effect of bentonite colloids on strontium migration in crushed granite. The tracer behaviour was studied in a column set-up under aerobic conditions with a continuous inlet of the liquid phase of a constant tracer concentration (activity) and flow rate. Defined volumes of liquid phase were sampled at periodic time intervals at the column outlet for the measurements of tracer concentrations (activity). The transport was described by breakthrough curves. The stepwise approach included these steps: (1) an evaluation of the hydrodynamic column properties by the non-sorbing tracer 3 H; (2) a column experiment with bentonite colloids in deionized water was performed; (3) migration of 85 Sr solution in two liquid phases (deionized and synthetic granitic water); and (4) the transport of a radiocolloid suspension in deionized water was studied. Results showed different behaviour of bentonite colloids and strontium in the column. Bentonite colloids behaved as a non-sorbing tracer: conversely, strontium showed strong sorption on granitic material. The strontium transport in the presence of bentonite colloids differed from strontium transport itself. The strontium transport in the presence of colloids was faster than transport without the bentonite colloids. The observed retention of strontium on granite suggests a higher affinity of strontium towards granitic rock than towards bentonite colloids, and showed the reversibility of the sorption of strontium on bentonite colloids.

Influence of heteroaggregation processes between intrinsic colloids and carrier colloids on cerium(III) mobility through fractured carbonate rocks

Colloid facilitated transport of radionuclides has been implicated as a major transport vector for leaked nuclear waste in the subsurface. Sorption of radionuclides onto mobile carrier colloids such as bentonite and humic acid often accelerates their transport through saturated rock fractures. Here, we employ column studies to investigate the impact of intrinsic, bentonite and humic acid colloids on the transport and recovery of Ce(III) through a fractured chalk core. Ce(III) recovery where either bentonite or humic colloids were added was 7.7–26.9% Ce for all experiments. Greater Ce(III) recovery was observed when both types of carrier colloids were present (25.4–37.4%). When only bentonite colloids were present, Ce(III) appeared to be fractionated between chemical sorption to the bentonite colloid surfaces and heteroaggregation of bentonite colloids with intrinsic carbonate colloids, precipitated naturally in solution. However, scanning electron microscope (SEM) images and colloid stability experiments reveal that in suspensions of humic acid colloids, colloid-facilitated Ce(III) migration results only from the latter attachment mechanism rather than from chemical sorption. This observed heteroaggregation of different colloid types may be an important factor to consider when predicting potential mobility of leaked radionuclides from geological repositories for spent fuel located in carbonate rocks.

Size distribution of FEBEX bentonite colloids upon fast disaggregation in low-ionic strength water

AbstractBentonite colloids generated from the backfill barrier in nuclear waste repositories may act as radionuclide carriers, if they are stable and mobile. Repository scenarios with highly saline groundwater inhibit colloid stability as particles tend to aggregate but, in the time frame of repositories, groundwater conditions may evolve, promoting particle disaggregation and stabilization. The disaggregation of FEBEX bentonite colloids by fast dilution to lower ionic strength was analysed in this study. Time-resolved dynamic light-scattering experiments were carried out to evaluate the kinetics of bentonite colloid aggregation and disaggregation processes in Na+ and Na+-Ca2+ mixed electrolytes of low ionic strength. Attachment and detachment efficiencies were determined.Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations. Once bentonite colloids are aggregated, a decrease in ionic strength facilitates disaggregation, but the process is not fully reversible as the initial size of the stable bentonite colloids at low ionic strength is not fully recovered. Particle-size distribution and concentration in suspension were analysed on disaggregated samples by single particle-counting measurements. Small colloids were measured in the disaggregated samples but their population was smaller than in the initial stable sample, especially in the presence of Ca2+.

A simplified method for the purification of an intrinsically disordered coagulant protein from defatted Moringa oleifera seeds

Coagulant proteins from Moringa oleifera (MO) seeds were successfully purified by removing seed oil followed by a protein salting out method at 40% (NH4)2SO4 combined with subsequent dialysis and heat treatment. A microtiter plate-based coagulation activity assay was then performed using colloidal bentonite solution and alum as positive control. The results show an oil recovery of 40±2% while the water-soluble protein fraction in the defatted MO seed was 12.5±0.5mg/g or 4.4% of the total seed protein content. Heating the desalted protein fraction to 121°C helped to obtain a 94% pure protein of approximately 7kDa. Computational analyses support the hypothesis that the coagulating MO protein is an intrinsically disordered protein, providing reason for its stability at extreme temperatures (121°C). The purified, thermo-stable coagulating protein retained its coagulating activity paving the way to economically produce a sterile natural coagulant from MO seeds.

Reversibility in radionuclide/bentonite bulk and colloidal ternary systems

AbstractTernary systems of 152Eu(III), bulk bentonite and ethylenediaminetetraacetic acid (EDTA) ([Eu] = 7.9 × 10–10 M; pH = 6.0–7.0) have been studied. Without EDTA, there was slow uptake in a two-stage process, with initial rapid sorption of Eu(III) (96%), followed by slower uptake of a much smaller fraction (3.0% over a period of one month). The reversibility of Eu(III) binding was tested by allowing Eu(III) to sorb to bentonite for 1–322 days. EDTA was added to the pre-equilibrated Eu bentonite systems at 0.01 M, a concentration that was sufficient to suppress sorption in a system where EDTA was present prior to the contact of Eu(III) with bentonite. A fraction of the Eu was released instantaneously (30–50%), but a significant amount remained bound. With time, the amount of Eu(III) retained by the bentonite reduced, with a slow fraction dissociation rate constant of approximately 4.3 × 10–8 s–1 (values in the range 2.2 × 10–8 – 1.0 × 10–7 s–1) for pre-equilibration times ≥7 days. Eventually, the amount of Eu(III) remaining bound to the bentonite was within error of that when EDTA was present prior to contact (4.5% ± 0.6), although in systems with pre-equilibration times >100 days, full release took up to 500 days. Europium interactions with colloidal bentonite were also studied, and the dissociation rate constant measured by a resin competition method. For the colloids, more Eu was found in the slowly dissociating fraction (60–70%), but the first-order dissociation rate constant was faster, with an average rate constant of 8.8 × 10–7 s–1 and a range of 7.7 × 10–7 –9.5 × 10–7 s–1. For both bulk and colloidal bentonite, although slow dissociation was observed for Eu(III), there was no convincing evidence for 'irreversible' binding.

Laboratory investigation of the role of desorption kinetics on americium transport associated with bentonite colloids

Understanding the parameters that control colloid-mediated transport of radionuclides is important for the safe disposal of used nuclear fuel. We report an experimental and reactive transport modeling examination of americium transport in a groundwater–bentonite–fracture fill material system. A series of batch sorption and column transport experiments were conducted to determine the role of desorption kinetics from bentonite colloids in the transport of americium through fracture materials. We used fracture fill material from a shear zone in altered granodiorite collected from the Grimsel Test Site (GTS) in Switzerland and colloidal suspensions generated from FEBEX bentonite, a potential repository backfill material. The colloidal suspension (100 mg L−1) was prepared in synthetic groundwater that matched the natural water chemistry at GTS and was spiked with 5.5 × 10−10 M 241Am. Batch characterizations indicated that 97% of the americium in the stock suspension was adsorbed to the colloids. Breakthrough experiments conducted by injecting the americium colloidal suspension through three identical columns in series, each with mean residence times of 6 h, show that more than 95% of the bentonite colloids were transported through each of the columns, with modeled colloid filtration rates (kf) of 0.01–0.02 h−1. Am recoveries in each column were 55–60%, and Am desorption rate constants from the colloids, determined from 1-D transport modeling, were 0.96, 0.98, and 0.91 h−1 in the three columns, respectively. The consistency in Am recoveries and desorption rate constants in each column indicates that the Am was not associated with binding sites of widely-varying strengths on the colloids, as one binding site with fast kinetics represented the system accurately for all three sequential columns. Our data suggest that colloid-mediated transport of Am in a bentonite-fracture fill material system is unlikely to result in transport over long distance scales because of the ability of the fracture materials to rapidly strip Am from the bentonite colloids and the apparent lack of a strong binding site that would keep a fraction of the Am strongly-associated with the colloids.

Size distribution analysis of colloid generated from compacted bentonite in low ionic strength aqueous solutions

An experimental methodology is presented, with the objective of investigating erosion and colloid generation from compacted bentonite, to evaluate its possible effects on radionuclide migration in a deep geological repository (DGR) in crystalline rock.Bentonite erosion studies were performed using a natural bentonite and considering two possible scenarios existing in a DGR in crystalline rock: the first one, where the water hydrating the bentonite is stagnant (static scenario) and the second one where the water can flow at the bentonite surface, simulating the existence of a water conducting fracture (dynamic scenario).In particular, the size of the generated colloids was analysed, as it is a very important parameter for transport and retention/filtration processes in crystalline rock and, ultimately, for colloid-driven radionuclide transport. The analysis of the size distribution of colloids, detected in erosion experiments, was carried out by the very sensitive single particle counting (SPC) technique.As bentonite colloids are particularly stable and mobile in low ionic strength (I) solutions, these chemical conditions were considered of particular interest.Results showed that the quantity of colloids generated from compacted bentonite depends on the chemistry of the water, and that the presence of calcium in the electrolyte inhibits the maximum generation of colloidal particles. The maximum concentration of bentonite colloid in solution corresponds to particles with a size around 50–100nm.The size distribution of bentonite colloids was similar in all the analysed cases, but the concentration of particles of smaller size (50–100nm) was always higher in the absence of calcium, indicating the clear effect of this element on their aggregation.Results also evidenced the importance of water/clay interactions: these can be of extreme importance for the evaluation of erosion and stability of colloids in a DGR at a long term.

Influences of different environmental parameters on the sorption of trivalent metal ions on bentonite: batch sorption, fluorescence, EXAFS and EPR studies

The presence of long-lived radionuclides in natural aquatic systems is of great environmental concern in view of their possible migration into biospheres of mankind. Trivalent actinides such as (241/243)Am can contribute a great deal to radioactivity for several thousand years. This migration is significantly influenced by various factors such as pH, complexing ions present in aquatic environments, and the sorption of species involving radionuclides by sediments around water bodies. Clay minerals such as bentonite are known to be highly efficient in radionuclide retention and hence are suitable candidates for backfill materials. This study presents experimental results on the interaction of Eu(iii) and Gd(iii) (chemical analogs of Am(iii) and Cm(iii)) with bentonite clay under varying experimental conditions of contact time, pH, and the presence of complexing anions such as humic acid (HA) and citric acid (cit). The sorption of HA on bentonite decreased with increasing the pH from 2 to 8, which was attributed to electrostatic interactions between HA and the bentonite surfaces. The sorption of Eu(iii) on bentonite colloids showed marginal variation with pH (>95%). However, a decrease in Eu(iii) sorption was observed in the presence of HA beyond pH 5 due to the increased aqueous complexation of Eu(iii) with deprotonated HA in the aqueous phase. The complexation of Eu(iii) with citrate ions was studied using Time Resolved Laser induced Fluorescence Spectroscopy (TRLFS) to explain the sorption data. Extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) investigations were carried out to understand the local chemical environment surrounding Eu(iii) and Gd(iii) (EPR probe) sorbed on bentonite under different experimental conditions. Surface complexation modelling shows the predominant formation of ≡XOEu(+2) (silanol) up to pH < 7, and beyond which ≡YOEu(OH)(+) (aluminol) is responsible for the quantitative sorption of Eu(iii) onto bentonite in the studied pH range.

Addition of Al2O3 nanoparticles to bentonite: effects on surface charge and Cd sorption properties

ABSTRACTCompacted bentonite barrier in radioactive waste repositories is expected to prevent radionuclide migration, due to its high sorption capability for many radionuclides. This study analyses whether the addition of Al2O3 nanoparticles (NPs) enhances the sorption properties of bentonite. The study was carried out with 109Cd, highly pollutant heavy metal and divalent fission product. Sorption experiments were conducted in NaClO4 at different ionic strengths (5·10-4 to 10-1 M) and pH (2 to 10), using mixtures of sodium homoionised bentonite and Al2O3 in different proportions.It has been probed that addition of Al2O3 NPs to bentonite enhances Cd sorption at pH higher than 6. The effect of Al2O3 NPs addition on the surface properties of bentonite colloids was also analyzed by measuring particle size and surface charge in all studied systems.

Ultrasound irradiation combined with hydraulic cleaning on fouled polyethersulfone and polyvinylidene fluoride membranes

In this study, an ultrasonic irradiation technique was utilized to mitigate the fouling of polyethersulfone (PES) and polyvinylidene fluoride (PVDF) membranes. The use of ultrasound at 20 kHz was applied to a dead-end microfiltration cell in order to mitigate fouling caused by the presence of colloidal bentonite particles. The effect of ultrasonic power and pulse duration on the permeate flux recovery was examined. Measurements indicate that an increase in ultrasonic power and longer pulse duration results to a higher permeate flux recovery. In order to reduce power consumption, a low to high power shift (LHPS) and pulsation method, were investigated. Methods of cleaning such as ultrasonic irradiation, ultrasonic cleaning with forward flushing and ultrasonic cleaning with backwashing were utilized and their cleaning efficiencies were examined. The cleaning performance was assessed using the clean water flux method and scanning electron microscope analysis of the cleaned membranes. Results showed that LHPS and pulsation method both improve the permeate flux recovery but were not able to attain the 93.97 and 74.88% flux recovery for PES and PVDF that was achieved by constant-15 W ultrasonic cleaning. In addition, forward flushing and backwashing may enhance the performance of ultrasonic cleaning at 9 W but could become disadvantageous at 15 W.

Sorption reversibility kinetics in the ternary system radionuclide–bentonite colloids/nanoparticles–granite fracture filling material

Abstract The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, 232Th(IV), 242Pu(IV) and 243Am(III) are clearly colloid associated in contrast to 233U(VI), 237Np(V) and 99Tc(VII). Concentrations of colloid-borne 232Th(IV), 242Pu(IV) and 243Am(III) decrease after ∼100 h showing desorption from bentonite colloids while 233U(VI) and 99Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. 232Th(IV) and 242Pu(IV) data yield a slower dissociation from colloids compared to 243Am(III) indicating stronger RN–colloid interaction. In the case of 237Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).

Interaction of bentonite colloids with Cs, Eu, Th and U in presence of humic acid: A flow field-flow fractionation study

The interaction of Cs(I), Eu(III), Th(IV) and U(VI) with montmorillonite colloids was investigated in natural Grimsel Test Site groundwater over a 3 years period. The asymmetric flow field-flow fractionation combined with various detectors was applied to study size variations of colloids and to monitor colloid association of trace metals. The colloids suspended directly in the low ionic strength ( I), slightly alkaline granitic groundwater ( I = 10 −3 mol/L, pH 9.6) showed a gradual agglomeration with a size distribution shift from initially 10–200 nm to 50–400 nm within over 3 years. The Ca 2+ concentration of 2.1 × 10 −4 mol/L in the ground water is believed to be responsible for the slow agglomeration due to Ca 2+ ion exchange against Li + and Na + at the permanently charged basal clay planes. Furthermore, the Ca 2+ concentration lies close to the critical coagulation concentration (CCC) of 10 −3 mol L −1 for clay colloids. Slow destabilization may delimit clay colloid migration in this specific groundwater over long time scales. Eu(III) and Th(IV) are found predominantly bound to clay colloids, while U(VI) prevails as the UO 2(OH) 3 − complex and Cs(I) remains mainly as aquo ion under our experimental conditions. Speciation calculations qualitatively represent the experimental data. A focus was set on the reversibility of metal ion-colloid binding. Addition of humic acid as a competing ligand induces rapid metal ion dissociation from clay colloids in the case of Eu(III) even after previous aging for about 3 years. Interestingly only partial dissociation occurs in the case of Th(IV). Experiments and calculations prove that the humate complexes dominate the speciation of all metal ions under given conditions. The partial irreversibility of clay bound Th(IV) is presently not understood but might play an important role for the colloid-mediated transport of polyvalent actinides over wide distances in natural groundwater.

Analysis of colloids erosion from the bentonite barrier of a high level radioactive waste repository and implications in safety assessment

To investigate the dominant mechanisms of colloid formation from compacted and confined bentonite innovative experiments were conducted. Chemical or physical processes that can affect the erosion of the bentonite surface were analyzed (ionic strength of the water, Ca in the water and in the exchange complex of the clay, dry density of the clay and presence of a water flow rate at the bentonite surface). Hydration, swelling and extrusion of clay into pores or fractures are primary steps for the formation of free colloidal particles in the aqueous phase, and the chemistry of the clay/water system is the most important parameter controlling the generation and stability of colloids. Ca-bentonite formed colloids quantities below the detection limit of our techniques, even in deionised water, but a percentage of Na approximately 20–30% in the clay exchange complex, as that present in the FEBEX bentonite, is enough to allow the formation of colloidal particles in quantities very similar to those produced by the Na-bentonite. The results for bentonite colloid generation obtained at a laboratory scale allowed the estimation of a range of colloid generation rates under different chemical conditions. Results were compared with in situ experimental investigations carried out at the FEBEX gallery emplaced in a granite massif at the Grimsel Test Site (Switzerland). The quantitative analysis of laboratory and in situ data can be used as input for models and performance assessment (PA) of high level radioactive waste (HLRW) repositories.

Strontium migration in a crystalline medium: effects of the presence of bentonite colloids

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10 − 6 m/s–224 m/yr. Experiments with bentonite colloids alone showed that – at the lowest water flow rate used in our experiments (7.1·10 − 6 m/s) – approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R f ~ 6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded ( R f = 1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20.

Distribution of Cs and Am in the solution-bentonite colloids-granite ternary system: effect of addition order and sorption reversibility

Abstract Bentonite colloids may be generated from bentonite in the geological disposal system planned in Japan, and sorb radionuclides (RNs) thus facilitating migration. In this context, distribution coefficients and reversibility of the sorption of RNs onto colloids are sensitive factors in safety assessment. In this study, distribution behavior of Cs and Am in the synthetic groundwater-bentonite colloids-granite ternary system was investigated by batch methods. To evaluate the sorption reversibility, three series with different addition order of components were prepared, such as CR+G corresponding to the series beginning with a colloid (C)–RN (R) solution binary system followed by addition of granite (G) after around 100 d, GR+C and CG+R. Distribution coefficients for the bentonite colloids (R d,col) interaction with Cs obtained in CR+G and CG+R are similar to calculated value (6.7–15 m3 kg−1) based on a previously reported sorption model, while that of GR+C is higher. Considering the contribution of micaceous colloids generated from granite, high R d,col in GR+C can be explained. The experimental R d,col data for Am in each series show similar values and agree well with calculation results (7.6×103–8.7×103 m3 kg−1) by applying the sorption models. Therefore, it is concluded that the sorption behavior of Cs and Am onto the bentonite colloids are reversible and the sorption models developed in the binary system are applicable to the ternary system. Interaction of Cs with granite shows slow kinetics both in sorption and desorption process. The K d,gra values for Am in CR+G are lower than those of other series just after addition of granite, however, they gradually increase with time and finally congruent values are attained. The concentration of free Am in CR+G is one order of magnitude higher than that in GR+C in spite of the ternary system under similar chemical conditions of the solution, suggesting temporal generation of species such as colloidal Am which is not sorbed on granite in CR+G.

Colloidal stability of bentonite clay considering surface charge properties as a function of pH and ionic strength

Surface charge properties of bentonite colloids were investigated to study their colloidal stability in a solution as a function of the pH and ionic strength. Potentiometric titrations and zeta potential measurements for the bentonite colloids depending upon the pH and ionic strength were performed to investigate the surface charge properties. It was observed from the potentiometric titrations that a zero net proton adsorption occurred at about pH 8.2 (pH PNZPC ∼ 8.2). Surface charges of the bentonite colloids mainly carrying structural negative charges revealed a very small dependency on the pH. The same behavior was also observed in the zeta potential measurements. The zeta potential measurements for the bentonite colloid showed that the bentonite colloids were stable at lower ionic strengths of 0.01 and 0.001 M NaClO 4 but unstable at a higher ionic strength of 0.1 M NaClO 4 within the whole pH range studied. Therefore, it can be concluded that bentonite colloids would be stable in most of the considered fresh groundwater conditions, i.e., pH from 6 to 10 and an ionic strength from 0.001 to 0.01 M, and then they can be mobilized along a flowing groundwater.

Modeling and characterization of piezoelectric transducers by means of scattering parameters. Part II: Application on colloidal suspension monitoring

This paper illustrates the application of the S-parameter concept for the analysis of the response of a piezoelectric sensor exploited for the discrimination of bentonite colloids of interest in construction engineering. The device is a millimeter-sized cantilever beam fully immersed in the suspension under examination. The beam is energized by a sweeping sinusoidal voltage: the electrical admittance is measured and the corresponding S-parameter is calculated. A procedure for the extraction of the lumped element circuital model is presented and the identification process clearly improves when the S-parameter response is used. Dealing with variable loading conditions, any change on density and/or viscosity of the interacting fluid determines a well-behaved variation of the frequency response associated with the principal modes of mechanical vibration. Inspection of response variations, especially in the S-parameter Smith chart and in the neighborhoods of resonance, allows the discrimination of different loading mixtures. Experimentation was carried out with 2.5%, 5.0%, 7.5% and 10.0% bentonite mixtures and the sensor has been able to fully discriminate between these different bentonite colloidal suspensions.

Characterisation of Granite Fractures From the In-Situ FEBEX Experiment (Grimsel, Switzerland): ossible Effects on Bentonite Colloid and Radionuclide Transport

AbstractThe FEBEX in-situ experiment, installed in 1997 at the Grimsel Test Site (GTS, Switzerland) 400 m depth under the Swiss Alps, simulates a high level radioactive waste repository (HLWR) emplaced in granite. Its initial aim was to study the performance of a bentonite engineered barrier but recently, two new boreholes were drilled in the granite to study the possible bentonite colloid formation and their migration in the granite.This study presents the characterization performed, at the micrometer scale, of the threemain water conductive fractures that were identified on the granite cores extracted from the newboreholes. These fractures are possible pathways for bentonite colloid transport (or retention),may be source of natural colloids and may condition colloid stability. The nuclear ion beamtechniques µ-Particle X-Ray Emission (µPIXE) and Rutherford Backscattering Spectrometry(RBS) were applied for visualizing and quantifying the elemental composition of the fracturessurface and of the surrounding micro-fractures, as support of the bentonite colloid analyses.