Smectite Colloids Research Articles

The chemical stabilization of methylene blue in colloidal dispersions of smectites

The model reaction of methylene blue (MB) with ascorbic acid (AA) to give the reduced leuco form of MB and dehydroascorbic acid was chosen to evaluate the chemical reactivity of the dye in aqueous solution and smectite colloidal dispersions. Saponite Sumecton (Sap) and montmorillonite Kunipia (Mt) from Kunimine Ind. were used. To obtain a large series of spectral data, UV–Vis absorption spectrophotometer equipped with a diode-array detector was used. The spectral components of the reaction were analyzed by chemometric methods. MB decomposition was evident only in aqueous solutions without smectites. The reaction kinetics was dependent on the AA concentration and the acidity of the solution but was not significantly affected by the concentration of MB. The half-life of the reaction without addition of HCl was in the range 69 to 23 s and further reduced in the presence of HCl. In the systems based on smectite colloidal dispersions, no similar reaction was observed. MB formed molecular aggregates depending on the surface properties of the smectite particles. Relatively complex spectral forms indicated the presence of monomers (672, 690 nm), J-aggregates (761 nm) and H-aggregates absorbing at a wavelength lower than 500 nm. Nothing indicated the bleaching of MB in smectite colloids as a consequence of the reaction with AA. The importance of this phenomenon to stabilize reactive dyes from the point of view of application and to interpret some of the earlier results of MB antimicrobial efficacy is discussed.

Synthetic Smectite Colloids: Characterization of Nanoparticles after Co-Precipitation in the Presence of Lanthanides and Tetravalent Elements (Zr, Th)

The magnesian smectite hectorite is a corrosion product frequently detected in nuclear waste glass alteration experiments. The structural incorporation of a single trivalent lanthanide was previously demonstrated. Hectorite was presently synthesized, for the first time, in the presence of several lanthanides (La, Eu, Yb) following a multi-step synthesis protocol. The smallest-sized particles (nanoparticles, NPs) were isolated by centrifugation and analyzed by asymmetrical flow field-flow fractionation (AsFlFFF) coupled to ICP-MS, in order to obtain information on the elemental composition and distribution as a function of the size. Nanoparticles can be separated from the bulk smectite phase. The particles are able to accommodate even the larger-sized lanthanides such as La, however, with lower efficiency. We, therefore, assume that the incorporation proceeds by substitution for octahedral Mg accompanied by a concomitant lattice strain that increases with the size of the lanthanides. The presence of a mixture does not seem to affect the incorporation extent of any specific element. Furthermore, syntheses were performed where in addition the tetravalent zirconium or thorium elements were admixed, as this oxidation state may prevail for many actinide ions in a nuclear waste repository. The results show that they can be incorporated as well.

Photophysical and antibacterial properties of complex systems based on smectite, a cationic surfactant and methylene blue

Solid or colloidal materials with embedded photosensitizers are promising agents from the medical or environmental perspective, where the direct use of photoactive solutions appears to be problematic. Colloids based on layered silicates of the saponite (Sap) and montmorillonite (Mon) type, including those modified with dodecylammonium cations (C12) and photosensitizer – methylene blue (MB) were studied. Two representatives of bacteria, namely Enterobacter cloacae and Escherichia coli, were selected for this work. A spectral study showed that MB solutions and also colloids with Sap including C12 exhibited the highest photoactivities. The antimicrobial properties of the smectite colloids were not directly linked to the photoactivity of the adsorbed MB cations. They were also influenced by other parameters, such as light vs. dark conditions, the spectrum, power and duration of the light used for the irradiation; growth phases, and the pre-treatment of microorganisms. Both the photoactivity and antimicrobial properties of the colloids were improved upon pre-modification with C12. Significantly higher antimicrobial properties were observed for the colloids based on Mon with MB in the form of molecular aggregates without significant photoactivities. The MB/Mon colloids, both modified and non-modified with C12 cations, exhibited higher antimicrobial effects than pure MB solution. Besides the direct effect of photosensitization, the surface properties of the silicate particles likely played a crucial role in the interactions with microorganisms.

Plant Root Exudates Decrease Mobility of Smectite Colloids in Porous Media in Contrast to Humic Acid

Plant root exudates are primarily composed of carbohydrates (CHs), amino acids (AAs), and organic acids (OAs). Little is known about how plant root exudates influence the stability and mobility of clay colloids in the soil profile. In this study, transport behaviors of K+–saturated smectite colloids dispersed in maize (Zea mays L.) artificial root exudate (ARE) solution, humic acid (HA) solution, and deionized water through water-saturated sand columns were investigated with solution ionic strengths of 0.1 and 10 mmol L−1 KCl and pH of 5, 7, and 9. Results showed that smectite colloids were more aggregated and transported less in ARE solution, followed by water and HA solution. This trend became less apparent with increasing pH but more pronounced with increasing ionic strength, suggesting that enhanced stability and mobility of HA-dispersed smectite colloids probably resulted from increased electrostatic and/or steric repulsions. The results for CH-, AA-, or OA-dispersed smectite colloids revealed that the AA fraction was primarily responsible for enhanced colloid retention because positively charged amine groups in AAs (especially lysine) might neutralize the negative surface charge of colloids and promote intersurface bridging. The residual colloids, after flushing with deionized water (thus eliminating secondary energy minima), decreased in the order of ARE-, water-, and HA-dispersed colloids, suggesting greater retention by primary energy minima and pore straining for the ARE-dispersed smectite colloids. Overall, in contrast to HA, plant root exudates decreased the stability and mobility of smectite colloids, thus facilitating the retention of clay colloids in root zones during water percolation events.

Analysis of latex, gold and smectite colloid transport and retention in artificial fractures in crystalline rock

The transport behavior of artificial (gold and latex) and natural (smectite) colloids within artificial fractures in natural crystalline rock was analyzed. The selected colloids presented different size, shape, density and surface charge: the effects of these parameters on their transport and retention in the fractures were evaluated in conjunction with the effects of the hydrodynamic characteristics of the system (water velocity/residence time in the fracture and fracture width). Resultshowed that, in crystalline rock fractures, all the analyzed colloids travel as fast as or faster than water (retardation factors, Rf≤1). Different parameters were identified to simultaneously control the velocity of colloids in the fracture, and an efficient analysis of Rf values could be performed relating them to the Taylor dispersion. This parameter accounts for the Brownian diffusion (colloid size), the mean water velocity and the fracture width.All the experiments were carried out under unfavorable electrostatic conditions (rock and colloids both negatively charged) and, in spite of clear exclusion effects observed, colloids were retained on the fracture surface. Under similar water velocities, the recovery of smectite colloids was always higher than that of latex and gold colloids, most probably due to the different morphology of the particles.Colloid retention was observed mainly in zone of the rock with defects, micro-fractures and grain boundaries and increased significantly as the water flow rate decreased. The retention behavior observed could not be totally explained considering only sedimentation and Brownian motion effects.

Colloids from the aqueous corrosion of uranium nuclear fuel

Colloids may enhance the subsurface transport of radionuclides and potentially compromise the long-term safe operation of the proposed radioactive waste repository at Yucca Mountain. Little data is available on colloid formation for the many different waste forms expected to be buried in the repository. This work expands the sparse database on colloids formed during the corrosion of metallic uranium nuclear fuel. We characterized spherical UO 2 and nickel-rich montmorilonite smectite-clay colloids formed during the corrosion of uranium metal fuel under bathtub conditions at 90 °C. Iron and chromium oxides and calcium carbonate colloids were present but were a minor population. The estimated upper concentration of the UO 2 and clays was 4 × 10 11 and 7 × 10 11–3 × 10 12 particles/L, respectively. However, oxygen eventually oxidized the UO 2 colloids, forming long filaments of weeksite K 2(UO 2) 2Si 6O 15 · 4H 2O that settled from solution, reducing the UO 2 colloid population and leaving predominantly clay colloids. The smectite colloids were not affected by oxygen. Plutonium was not directly observed within the UO 2 colloids but partitioned completely to the colloid size fraction. The plutonium concentration in the colloidal fraction was slightly higher than the value used in the viability assessment model, and does not change in concentration with exposure to oxygen. This paper provides conclusive evidence for single-phase radioactive colloids composed of UO 2. However, its impact on repository safety is probably small since oxygen and silica availability will oxidize and effectively precipitate the UO 2 colloids from concentrated solutions.

Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland – impact of reaction kinetics and speciation on radionuclide migration

SummaryThe influence of smectite colloids on the migration behaviour of U(VI), Th(IV), Pu(IV), Am(III), Np(V), Sr(II) and Cs(I) is investigated within the Colloid and Radionuclide Retardation experiment (CRR). Twoin situexperiments in a well-characterized granitic fracture zone are carried out in presence and absence of bentonite colloids. Radionuclide retardation observed in the field studies increases in the sequence Np(V)∼U(VI)<Sr(II)<Cs(I), where a small fraction of colloid borne breakthrough is only stated for Cs(I) in presence of bentonite colloids. Am(III) and Th/Pu(IV) mainly migrate as colloids without retardation in the presence and absence of smectitic colloids. The radionuclide migration behaviour is discussed on the basis of results obtained in laboratory batch sorption experiments and spectroscopic studies. Consistent with the field observation, laboratory derivedKdvalues increase in the order Np(V)∼U(VI)<Sr(II)<Cs(I). Significant kinetic hindrance for the sorption to fault gauge minerals is observed for Sr(II) and Cs(I), but notably for Am(III) and Pu(IV). The slow sorption reaction of tri- and tetravalent actinide ions is explained by their kinetically hindered dissociation from colloidal species. In order to explain the colloidal behaviour of tri- and tetravalent actinides even in absence of bentonite colloids, ultracentrifugation and spectroscopic experiments are performed. It is found that up to 60% of Pu(IV) and Am(III) species can be centrifuged off. Adding Cm(III) (5×10-8mol L-1) into both injection solutions instead of Am(III) allows for a spectroscopic study by using the time resolved laser fluorescence spectroscopy (TRLFS). Peak position and fluorescence lifetimes (λ=604 nm, τ=110-114 μs) together with the fact that Cm(III) can be widely separated by ultracentrifugation, suggest the existence of inner-sphere surface complexes on groundwater and bentonite colloids. CarbonK-edge XANES analysis of the bentonite colloids reveal the presence of natural organic constituents. They are mainly of aliphatic nature containing high fractions of carboxylate groups. A contribution of these organic species towards the bentonite colloid stability and sorption of actinides is assumed to be likely.

Functional group analysis of natural organic colloids and clay association kinetics using C(1s) spectromicroscopy

The quantification of natural humic colloid functional group content and chemical association of humic substances with clay minerals exerts a crucial role in the colloid-borne mobility of actinides due to the functional group dependent strong interaction with multivalent metal ions. Functional group quantification in isolated fulvic acids of the Gorleben groundwater (Lower Saxony, NW Germany) by comparison of high resolution C(ls) NEXAFS spectra deconvolution with 3 C-NMR measurements showed good correlation (r 2 > 0.9) and gives a potential quantification tool in complex natural groundwater systems. Time resolved soft X-ray spectromicroscopy on dissolved organic carbon stabilized SWy-2 smectite colloids revealed an enrichment of carboxyl groups on broken edges (silanol/aluminol groups) at short contact times (Ih). With longer contact times (7d, 6 months) the clay associated organic carbon increases and significantly higher aromatic content associated with basal surfaces were detected. The enhanced sorption of aromatic compounds can be related to an increase in mineral surface hydrophobicity and/or preferential sorption on charged siloxane surfaces.