Oxidative Transformations of Contaminants in Natural and in Technical Systems

In this paper, we present case studies of oxidative transformations of contaminants by oxidants which are relevant in natural and in technical systems. These oxidants are reactive oxygen species (ROS), manganese(III,IV), iron(III), and molecular dioxygen (O2). Regarding ROS, we discuss i) their concentrations and their efficiencies as oxidants in natural waters and in water treatment, ii) reactions occurring in bromide-containing waters in the presence of ROS, iii) role of iron in the formation of ROS, and iv) quantitative structure-activity relationships (QSAR) of reactions of ROS with contaminants. Concerning MnIII and MnIV as oxidants, we present experimental studies on the oxidation of anilines by δ-MnO2. With respect to oxidative transformation of the hydrophilic organic contaminants ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA), we show that these organic complexing agents are efficiently oxidized by FeIII and O2, respectively, if catalyzed by light (for the oxidation of EDTA by FeIII and by enzymes (for the oxidation of NTA by O2)

Insights from an extensive triaxial testing campaign on a shale for comparative site characterization of a deep geological repository

Several boreholes were drilled for site comparison of a deep geological repository (DGR) in Northern Switzerland. The main target of the exploration program was the >100m thick Opalinus Clay, the designated host rock encountered at approximately 450 to 1000 m depth in three different sites. This contribution focuses on the evaluation of geomechanical properties and the deformation behavior from the triaxial testing campaign, both aspects relevant to construction and the assessment of the long-term safety of a DGR. Some 140 triaxial tests were performed on cores from seven different boreholes to evaluate potential differences in material properties by depth and geographic location. Core sampling, preparation chain, and testing protocols were validated before the campaign, and three laboratories were commissioned to perform the tests. A comparison of basic properties from cores used for triaxial testing with a much larger database of complementary core analyses and geophysical logging demonstrates that the performed tests cover the range of expected material properties. Limited to no differences in strength and stiffness are detected from cores at different depths and sites. Despite a relatively large variation in bulk mineralogy of the formation (e.g. clay-mineral content varying between 35 and 75 wt%), the strength values of Opalinus Clay vary only moderately, with equivalent (calculated) unconfined compressive strengths of 21 ± 5 MPa, for loading directions parallel or perpendicular to bedding. This contrasts with the results of Opalinus Clay from the Rock Laboratory at Mont Terri, where the effect of material composition was more relevant. Assuming a Mohr-Coulomb-type failure law, the transition from peak to post-peak strength comes at the expense of cohesion, and only a small reduction of the shear strength angle. Hence the burial history, tectonic overprint, and current depth mainly control the intact properties by additional cohesion, whereas the post-peak behavior is mainly controlled by bulk mineralogy.publishedVersio

Petrophysical properties of opalinus clay drill cores determined from Med-XCT images

Erworben im Rahmen der Schweizer Nationallizenzen (http://www.nationallizenzen.ch)The determination of petrophysical properties such as density, porosity and mineralogical composition of the rock are key objectives in cored sections of drilling campaigns. In view of the large amount of sample material that accumulates during a drilling campaign, a seamless determination of the properties along the cores is not feasible if only direct methods are used. Therefore, fast, non-destructive and affordable methods have been developed. Three-dimensional images of Opalinus Clay drill cores were acquired by using a medical X-ray computed tomographic scanner (med-XCT). The CT numbers of the images were density calibrated, which allowed to determine bulk density variations along drill cores. Then, a relationship between rock composition and bulk density was built in form of linear regression models to predict the porosity or the contents of major components from density calibrated image data. This relationship was established on the basis of rock samples, of which mineralogical compositions and porosities were measured in the laboratory. It turned out that the bulk density of Opalinus Clay is systematically related to porosity and the contents of clay minerals, quartz and calcite. With increasing density, porosity and clay minerals content decrease. This is because the pores and clay minerals together form the porous clay matrix and are thus structurally connected. The density of the porous clay matrix is comparatively low, and its content therefore controls the bulk density of Opalinus Clay. With a decrease in the content of the porous clay matrix, the calcite and quartz contents both increase, which is associated with an increase in bulk density. No systematic behavior was found for the accessories. Thus, their influence on bulk density is considered to be small. Med-XCT in combination with reference samples allows the determination of the rock composition and porosity along drill cores. In the case of Opalinus Clay, a larger number of reference samples (>~50) are required to predict the properties with confidence

Slip localization and fault weakening as a consequence of fault gouge strenthening - Insights from laboratory experiments

A laboratory study of simulated quartz gouges was conducted to investigate how solution transfer processes influence the mechanical behaviour of fault wear products at high temperature, hydrothermal conditions. Experiments were performed under nominally dry conditions, as well as in the presence of an aqueous pore fluid, at elevated temperatures (500 to 927 °C), and at effective confining pressure conditions (σ2′ = σ3′ = 100 MPa) to simulate, on a laboratory timescale, processes that may be important in fluid-active fault zones at depth in the continental crust. The mechanical data and microstructural analysis indicate that the kinetics of solution transfer processes can exert a fundamental control on the mechanical behaviour of fault wear products. It is found that, at nominally dry conditions, gouges deform by cataclastic creep and distributed shear, with strength and microstructures being relatively unaffected by temperature. At moderately chemically reactive, hydrothermal conditions (500-600 °C, coarse grain size, or fast deformation rate), the presence of a reactive pore fluid slightly reduces the shear strength with respect to dry conditions. However, at highly chemically reactive, hydrothermal conditions (600-927 °C, small grain size, and slow deformation rate), rapid porosity reduction is accommodated by dissolution-precipitation processes. Deformation under such conditions results in a fast increase of grain contact area and the development of cohesive bonds between adjacent particles, which in turn inhibits cataclastic granular flow. With increasing displacement and compaction of the quartz gouge, there is a sudden transition from distributed cataclastic flow, to slip localization at the interface between the gouge and one of the forcing blocks. This deformation mode switch is associated with dramatic weakening (up to 50% drop in shear resistance, and changes in the apparent coefficient of friction from > 0.7 to ≈ 0.4). Stress drop occurs over many minutes in the laboratory. It is speculated that solution-assisted gouge compaction, and consequent slip localization with associated slow, yet dramatic stress drop, could provide a mechanism for the occurrence of slow earthquakes

On the swelling behavior of shallow Opalinus Clay shale

The Opalinus Clay shale is the selected geological formation for the construction of a deep geological repository in Switzerland. The geomaterial will experience cyclic drying and wetting paths during different stages of the repository lifetime. The shale, characterized by transversely isotropic behaviour, has the capacity to swell/shrink upon suction variation. The impact of suction variation on the shale behavior is therefore of interest. This paper presents the results on a shallow sourced sample, subjected to a complete hydric cycle in a wide range of applied suction (from ≈0 to 300 MPa). The results highlight the anisotropy in the shale strains and its evolution during the cycle, while slight irreversible strains cumulate

Effect of the mineralogical composition on the elastoplastic hydromechanical response of Opalinus Clay shale

Opalinus Clay is the shale currently under investigation as the host formation for geological radioactive waste disposal in Switzerland. Its hydromechanical response has been widely studied, and the experimental results show a range of values whose dispersion needs to be clarified. This work aims to explain the dispersion in the literature results by correlating the hydro-mechanical response to the mineralogical variability of the tested specimens. Based on published microstructural studies, the Opalinus Clay shale is herein schematised as a sequence of two kinds of layers: the shaly (high in clay-mineral content) and the sandy (low in clay-mineral content) layers. The mineralogical composition, porosity and hydromechanical parameters are assigned to each layer type. The one-dimensional compressibility and the elastic response of combinations of layers are derived adopting an analytical solution for the stratified, transversely isotropic medium. The drained elastic properties are used to calibrate the approach, and the undrained elastic properties are derived and compared to literature data. Empirical correlations between the layer composition and the geomaterial strength are also drawn. It is shown that by adopting a layered structure with an alternation of two kinds of layers, most of the variability in the studied geomechanical properties of Opalinus Clay can be captured