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  • 1.
    Holmboe, Michael
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    The bentonite barrier: microstructural aspects on colloid filtration and radiation effects on bentonite colloid stability2009Licentiate thesis, comprehensive summary (Other academic)
  • 2.
    Holmboe, Michael
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    The Bentonite Barrier: Microstructural properties and the influence of γ-radiation2011Doctoral thesis, comprehensive summary (Other academic)
  • 3.
    Holmboe, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Bourg, Ian C.
    Interlayer diffusion in hydrated smectites - a molecular dynamics study2013In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 245, p. 163-GEOC-Article in journal (Other academic)
  • 4.
    Holmboe, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Bourg, Ian C.
    Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 2, p. 1001-1013Article in journal (Refereed)
    Abstract [en]

    The diffusion coefficients (D) of water and solutes in nanoporous Na-smectite clay barriers have been widely studied because of their importance in high-level radioactive waste (HLRW) management and in the isolation of contaminated sites. However, few measurements have been carried out at the high temperatures that are expected to occur in HLRW repositories. We address this knowledge gap by using molecular dynamics (MD) simulations to predict the temperature dependence of diffusion in clay interlayer nanopores, expressed as a pore scale activation energy of diffusion (E-a). Our sensitivity analysis shows that accurate prediction of pore scale D and E-a values requires careful consideration of the influence of pore size, simulation cell size, and clay structure flexibility on MD simulation results. We find that predicted D values in clay interlayer nanopores are insensitive to the size of the simulation cell (contrary to the behavior observed in simulation of bulk liquid water) but sensitive to the vibrational motions of clay atoms (particularly in the smallest pores investigated here, the one-, two-, and three-layer hydrates). Our predicted D and E-a values are consistent with experimental data. They reveal, for both water and Na+, that E-a increases by similar to 6 kJ mol(-1) with increasing confinement, when going from bulk liquid water to the one-layer hydrate of Na-montmorillonite.

  • 5.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Influence of γ-radiation on the reactivity of Montmorillonite towards H2O22012In: Radiation Physics and Chemistry, ISSN 0969-806X, E-ISSN 1879-0895, Vol. 81, no 2, p. 190-194Article in journal (Refereed)
    Abstract [en]

    Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH•, e-(aq), H•, H202, H2, H02•, H30+), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe(III) content in montmorillonite and its reactivity towards hydrogen peroxide (H2O2) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/FeTot ratio of dispersed montmorillonite are increased from ≤ 3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/FeTot ratio and the H2O2 decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H2O2 additions, since the structural Fe(II)/FeTot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H2O2.

  • 6.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Norrfors, Karin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry (closed 20110630).
    Effect of γ-radiation on Radionuclide Retention in Compacted Bentonite2011In: Radiation Physics and Chemistry, ISSN 0969-806X, E-ISSN 1879-0895, Vol. 80, no 12, p. 1371-1377Article in journal (Refereed)
    Abstract [en]

    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 gamma) under anoxic conditions. Because of this, the effects of gamma-radiation on the apparent diffusivity values and sorption coefficients in bentonite for Cs(+) and Co(2+) were tested under different experimental conditions. Radiation induced effects on sorption were in general more noticeable for Co(2+) than for Cs(+), which generally showed no significant differences between irradiated and unirradiated clay samples. For Co(2+) however, the sorption to irradiated MX80 was significantly lower than to the unirradiated clay samples regardless of the experimental conditions. This implies that gamma-radiation may alter the surface characteristics contributing to surface complexation of Co(2+). With the experimental conditions used, however, the effect of decreasing sorption was not large enough to be reflected on the obtained D. values.

  • 7.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Colloid diffusion in compacted bentonite: microstructural constraints2010In: Clays and clay minerals, ISSN 0009-8604, E-ISSN 1552-8367, Vol. 58, no 4, p. 532-541Article in journal (Refereed)
    Abstract [en]

    In Sweden and in many other countries, a bentonite barrier will be used in the repository for spent nuclear fuel. In the event of canister failure, colloidal diffusion is a potential, but scarcely studied mechanism of radionuclide migration through the bentonite barrier. Column and in situ experiments are vital in understanding colloid diffusion and in providing information about the microstructure of compacted bentonite and identifying cut-off limits for colloid filtration. This study examined diffusion of negatively charged 2-, 5-, and 15-nm gold colloids in 4-month diffusion experiments using MX-80 Wyoming bentonite compacted to dry densities of 0.6–2.0 g/cm3. Breakthrough of gold colloids was not observed in any of the three diffusion experiments. In a gold-concentration profile analysis, colloid diffusion was only observed for the smallest gold colloids at the lowest dry density used (estimated apparent diffusivity Da 5x10–13 m2/s). The results from a microstructure investigation using low-angle X-ray diffraction suggest that at the lowest dry density used, interlayer transport of the smallest colloids cannot be ruled out as a potential diffusion pathway, in addition to the expected interparticle transport. In all other cases, with either greater dry densities or larger gold colloids, compacted bentonite will effectively prevent diffusion of negatively charged colloids due to filtration.

  • 8.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Free porosity investigation of compacted Bentonite using XRDIn: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053Article in journal (Other academic)
    Abstract [en]

    Many countries intend to use compacted bentonite as abarrier in their deep geological repositories for nuclearwaste. In order to describe and predict hydraulicconductivity or radionuclide transport through thebentonite barrier, fundamental understanding of themicrostructure of compacted bentonite is needed. Thisstudy examined the interlayer swelling and overallmicrostructure of Wyoming Bentonite MX-80 and thecorresponding homo-ionic Na+ and Ca2+ forms, usingXRD with samples saturated under confined swellingconditions and free swelling conditions. For thesamples saturated under confined conditions, theinterparticle, or so-called free porosity was estimatedby comparing the experimental interlayer distancesobtained from one-dimensional XRD profile fittingagainst the maximum interlayer distances possible forthe corresponding water content. The results showedthat interlayer porosity dominated total porosity,irrespective of water content, and that free porositywas lower than previously reported in the literature. Atcompactions relevant for the saturated bentonitebarrier (1.4-1.8 g/cm3), the free porosity was estimatedto ≤ 3%.

  • 9.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Free Porosity of Compacted Saturated MX-80 Bentonite and Na-montmorillonite as a Function of Dry Density2009Article in journal (Refereed)
  • 10.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Porosity investigation of compacted bentonite using XRD profile modeling2012In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 128, no 1-4, p. 19-32Article in journal (Refereed)
    Abstract [en]

    Many countries intend to use compacted bentonite as a barrier in their deep geological repositories for nuclear waste. In order to describe and predict hydraulic conductivity or radionuclide transport through the bentonite barrier, fundamental understanding of the microstructure of compacted bentonite is needed. This study examined the interlayer swelling and overall microstructure of Wyoming Bentonite MX-80 and the corresponding homo-ionic Na+ and Ca2+ forms, using XRD with samples saturated under confined swelling conditions and free swelling conditions. For the samples saturated under confined conditions, the interparticle, or so-called free or external porosity was estimated by comparing the experimental interlayer distances obtained from one-dimensional XRD profile fitting against the maximum interlayer distances possible for the corresponding water content. The results showed that interlayer porosity dominated total porosity, irrespective of water content, and that the interparticle porosity was lower than previously reported in the literature. At compactions relevant for the saturated bentonite barrier (1.4-1.8 g/cm(3)), the interparticle porosity was estimated to <= 3%.

  • 11.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    García García, Sandra
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Effects of gamma-irradiation on the stability of colloidal Na+-Montmorillonite dispersions2009In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 43, no 1, p. 86-90Article in journal (Refereed)
    Abstract [en]

    In many concepts for final storage of spent nuclear fuel bentonite will be used as an engineered barrier, mainly due to its inertness, plasticity and ability to retard transport of radionuclides by adsorption. In the event of water-bearing fractures making contact with the bentonite barrier, generation and transport of colloidal particles will strongly depend on groundwater composition and the surface properties of the colloidal particles. The bentonite barrier will unavoidably be exposed to ionizing radiation from the spent nuclear fuel but very little is known about effects of ionizing radiation on bentonite concerning colloidal stability. In this work we have studied the effect of gamma-radiation on the stability of dilute colloidal Na+-montmorillonite dispersions using a Cs-137 gamma-source (doses of 0-53.2 kGy). Aggregation kinetics and sedimentation experiments revealed significant radiation effects, evident as increased colloid stability. The only rationale for this is a gamma-radiation induced increase in surface potential. The effects appeared to depend on the Na+-montmorillonite concentration in the irradiated dispersions, indicating that the change in surface potential is induced by aqueous radiolysis products.

  • 12.
    Holmboe, Michael
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Wold, Susanna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Petterson, Torbjörn
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
    Effects of the injection grout Silica sol on Bentonite2011In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 36, no 17/18, p. 1580-1589Article in journal (Refereed)
    Abstract [en]

    Silica sol, i.e., colloidal SiO2, may be used as a low-pH injection grout for very fine fractures in the construction of deep geological repositories for radioactive waste in Sweden and in Finland. If the bentonite barrier encounters SiO2-colloid particles under conditions favorable for aggregation, there is concern that it will modify the bentonite barrier at the bentonite/bedrock interface. In this study qualitative experiments were performed with mixed dispersions of SiO2-colloids and bentonite or homo-ionic Na/Ca-montmorillonite. Samples were prepared at different colloid concentrations and treated under various conditions such as low and high ionic strength (0.3 M NaCl), as well as dehydration and redispersing. Free swelling and settling experiments were performed in order to qualitatively compare the conditions in which SiO2-colloids affect the bulk/macro properties of bentonite. In order to study specific SiO2-colloid/montmorillonite interactions and preferred type of initial aggregation, dilute dispersions of homo-ionic montmorillonite dispersions mixed with varying concentrations of SiO2-colloids were prepared and selected samples were characterized by PCS, SEM/EDS, AFM and PXRD. The results from this study show that bentonite and montmorillonite particles can be modified by SiO2-colloids when mixed in comparable amounts, due to dehydration or high ionic strength. Some indications for increased colloidal stability for the SiO2-colloid modified clay particles were also found. From the AFM investigation it was found that initial attachment of the SiO2-colloids in Na+ dominated samples seemed to occur on the edges of the montmorillonite layers. In Ca2+ dominated samples not subjected to excess NaCl, SiO2-colloid sorption onto the faces of the montmorillonite layers was also found. In all, contact between the bentonite barrier and ungelled Silica sol should preferably be avoided.

  • 13. Tinnacher, Ruth M.
    et al.
    Holmboe, Michael
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Davis, James A.
    Tournassat, Christophe
    Bourg, Ian
    Impacts of pore structure and diffusion-accessible porosity for calcium-bromide diffusion in sodium-montmorillonite2014In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 248Article in journal (Other academic)
  • 14.
    Yang, Guomin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Neretnieks, Ivars
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.
    Holmboe, Michael
    Umeå University.
    Atomistic simulations of cation hydration in sodium and calcium montmorillonite nanopores2017In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 147, no 8, article id 084705Article in journal (Refereed)
    Abstract [en]

    facile synthesis of 3d-metal based electro-catalysts directly incorporated into a carbon support was carried out by.-radiation. Transition metals of period 4, i.e. Ni and Co, were precipitated and reduced from their respective salt solutions. The obtained materials were characterized by XRD, SEM, SQUID and the BET methods. Thereafter, the electrodes for fuel cells were fabricated out of synthesized material and their electrochemical performance for the oxygen reduction reaction in 6 M KOH was measured. Although the concentrations of Co and Ni in the electrode material were low (3.4% Co and 0.4% Ni) after reduction by irradiation, both the Ni and Co-based gas diffusion electrodes showed high catalytic activity for oxygen reduction both at room temperature and at 60 degrees C.

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