Nanoparticle decoration with surfactants: Molecular interactions, assembly, and applications

Emerging natural and tailored materials for uranium-contaminated water treatment and environmental remediation

Geological disposal is the preferred option for the final storage of high-level nuclear waste and spent nuclear fuel in most countries. The selected host rock may be different in individual national programs for radioactive-waste management and the engineered barrier systems that protect and isolate the waste may also differ, but almost all programs are considering an engineered barrier. Clay is used as a buffer that surrounds and protects the individual waste packages and/or as tunnel seal that seals off the disposal galleries from the shafts leading to the surface.

Bentonite and bentonite/sand mixtures are selected primarily because of their low hydraulic permeability in a saturated state. This ensures that diffusion will be the dominant transport mechanism in the barrier. Another key advantage is the swelling pressure, which ensures a self-sealing ability and closes gaps in the installed barrier and the excavation-damaged zone around the emplacement tunnels. Bentonite is a natural geological material that has been stable over timescales of millions of years and this is important as the barriers need to retain their properties for up to 106 y.

In order to be able to license a final repository for high-level radioactive waste, a solid understanding of how the barriers evolve with time is needed. This understanding is based on scientific knowledge about the processes and boundary conditions acting on the barriers in the repository. These are often divided into thermal, hydraulic, mechanical, and (bio)chemical processes. Examples of areas that need to be evaluated are the evolution of temperature in the repository during the early stage due to the decay heat in the waste, re-saturation of the bentonite blocks installed, build-up of swelling pressure on the containers and the surrounding rock, and degradation of the montmorillonite component in the bentonite. Another important area of development is the engineering aspects: how can the barriers be manufactured, subjected to quality control, and installed?

Geological disposal programs for radioactive waste have generated a large body of information on the safety-relevant properties of clays used as engineered barriers. The major relevant findings of the past 35 y are reviewed here.

The Use of Clay as an Engineered Barrier in Radioactive-Waste Management a Review

Natural and man-made materials often rely on functional interfaces between inorganic and organic compounds. Examples include skeletal tissues and biominerals, drug delivery systems, catalysts, sensors, separation media, energy conversion devices, and polymer nanocomposites. Current laboratory techniques are limited to monitor and manipulate assembly on the 1 to 100 nm scale, time-consuming, and costly. Computational methods have become increasingly reliable to understand materials assembly and performance. This review explores the merit of simulations in comparison to experiment at the 1 to 100 nm scale, including connections to smaller length scales of quantum mechanics and larger length scales of coarse-grain models. First, current simulation methods, advances in the understanding of chemical bonding, in the development of force fields, and in the development of chemically realistic models are described. Then, the recognition mechanisms of biomolecules on nanostructured metals, semimetals, oxides, phosphates, carbonates, sulfides, and other inorganic materials are explained, including extensive comparisons between modeling and laboratory measurements. Depending on the substrate, the role of soft epitaxial binding mechanisms, ion pairing, hydrogen bonds, hydrophobic interactions, and conformation effects is described. Applications of the knowledge from simulation to predict binding of ligands and drug molecules to the inorganic surfaces, crystal growth and shape development, catalyst performance, as well as electrical properties at interfaces are examined. The quality of estimates from molecular dynamics and Monte Carlo simulations is validated in comparison to measurements and design rules described where available. The review further describes applications of simulation methods to polymer composite materials, surface modification of nanofillers, and interfacial interactions in building materials. The complexity of functional multiphase materials creates opportunities to further develop accurate force fields, including reactive force fields, and chemically realistic surface models, to enable materials discovery at a million times lower computational cost compared to quantum mechanical methods. The impact of modeling and simulation could further be increased by the advancement of a uniform simulation platform for organic and inorganic compounds across the periodic table and new simulation methods to evaluate system performance in silico.

Simulations of inorganic–bioorganic interfaces to discover new materials: insights, comparisons to experiment, challenges, and opportunities

This paper discusses the mechanism of montmorillonite structural alteration and modification of bentonites’ properties (based on samples from clay deposits Taganskoye, Kazakhstan and Mukhortala, Buriatia) under thermochemical treatment (treatment with inorganic acid solutions at different temperatures, concentrations and reaction times). Treatment conditions were chosen according to those accepted in chemical industry for obtaining acid modified clays as catalysts or sorbents. Also, more intense treatment was carried out to simulate possible influence at the liquid radioactive site repositories. A series of methods was used: XRD, FTIR, ICP-AES, TEM, nitrogen adsorption, and particle size analysis. It allowed revealing certain processes: transformation of montmorillonite structure which appears in the leaching of interlayer and octahedral cations and protonation of the interlayer and –OH groups at octahedral sheets. In turn, changes in the structure of the 2:1 layer of montmorillonite and its interlayer result in significant alterations in the properties: reduction of cation exchange capacity and an increase of specific surface area. Acid treatment also leads to a redistribution of particle sizes and changes the pore system. The results of the work showed that bentonite clays retain a significant portion of their adsorption properties even after a prolonged and intense thermochemical treatment (1 M HNO3, 60 °C, 108 h).

https://www.mdpi.com/2075-163X/7/4/49/pdf

Experimental Study of Montmorillonite Structure and Transformation of Its Properties under Treatment with Inorganic Acid Solutions

The estimated quantity of cement for construction and sealing purposes is around 9E5 kg in the planned Swedish KBS3 repository for nuclear waste. The highly alkaline cement pore fluid (pH > 12) may affect other components in the repository, and especially the bentonite buffer is of concern. In this study, we simulated possible interactions between cement and bentonite by contacting highly compacted bentonite with high molar hydroxide solutions in a series of laboratory experiments. Wyoming bentonite (MX-80) and purified homo-ionic Na- and Ca-montmorillonite were used for tests with 0.1, 0.3 and 1.0 M NaOH and saturated Ca(OH)2 solutions. Pressure cells with permeable filters were loaded with compacted discs of bentonite at the proposed buffer density (2000 kg/m(3) at full water saturation). A hydroxide solution was circulated on one side of the cell and an isotonic chloride solution on the other during a minimum of 45 days. Swelling pressure and solution pH were monitored during the tests and the change in the solution composition and bentonite mineralogy were determined after completed tests. No effect on swelling pressure was observed in tests with 0.1 M NaOH (pH 12.9) or saturated Ca(OH)2 solutions (pH 12.4) and the mineralogical/chemical changes of the clay were minimal. The bentonite swelling pressure was significantly reduced in the tests with 0.3 (pH 13.3) and 1.0 M (pH 13.8) NaOH solutions. The reduction seems to be due to an instant osmotic effect, and to a continuous dissolution of silica minerals, resulting in mass loss and, consequently, a decrease in density. At these high pH, the release of silica was dominating and the CEC of the clay increased by 20-25%. The structural formula of the smectite and X-ray diffraction tests for nonexpandability (Greene-Kelly test) provided strong evidence that the dissolution of montmorillonite proceeds incongruently through an initial step of beidellitization. The calculated rate of silica release from montmorillonite is 1.6E-9 g g(-1) clay s(-1) for 1.0 M and 5E-10 g g-1 clay s-1 for 0.3 M NaOH solutions. The Si release rate is, however, not a straightforward measure of the montmorillonite dissolution rate due to the non-stoichiometric dissolution. Upon contact between bentonite and NaCl solutions, ion-equilibrium is established between the external solution and the exchangeable cations of the clay. A similar initial pressure response on exposure of bentonite to NaOH solutions indicates that such equilibrium may establish also with an external NaOH solution. If so, the OH-concentration of the clay pore water will be lower than that of the external solution, which would explain that dissolution rates in our experiments with highly compacted bentonite are lower than those reported for batch experiments with hydroxide solutions of the same concentration. (C) 2006 Published by Elsevier Ltd. (Less)

Experimentally determined swelling pressures and geochemical interactions of compacted Wyoming bentonite with highly alkaline solutions

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proved to be one of the most sensitive parameters for detecting changes of mineral properties such as swelling capacity and illitization in alteration experiments. Whether measured differences in CEC values of bentonite buffer samples before and after an experiment are (1) actual differences caused by clay structural changes such as illitization or (2) simply data scatter due to the different methods used by international research teams is an open question. The aim of this study was to measure the CEC of clay samples in five different laboratories using the same method and to evaluate the precision of the values measured. The Cu-trien method and four reference materials of the Alternative Buffer Material (ABM) test project in Aspo, Sweden, were chosen for this interlaboratory study. The precision of the Cu-trien method, which uses visible spectroscopy, was very good with a standard deviation of ±0.7–2.1 meq/100 g for CECs that ranged from 11 to 87 meq/100 g. For the same CEC range, analysis of Cu-trien index cations using inductively coupled plasma (mass spectrometry) and atomic absorption spectroscopy were less precise with a standard deviation of ±2.8–3.9 meq/100 g. Based on the measured precision, greater measured differences in Cu-trien CEC and exchangeable cation values of bentonite buffer samples, before and after an experiment, might be actual differences. Great care must be taken when interpreting measured CEC differences, and analytical characterization of any structural changes may be needed. Compared with results from the ‘International Soil-Analytical Exchange’ (iSE) program for soils, most absolute concentrations were much larger for the clays studied; however, for the two parameters exchangeable Ca2+ and CEC the range was similar to the iSE ring test and, most importantly, the precision was comparable. Future studies should discuss the accuracy of CEC and exchangeable cation values and compare them to alternative CEC methods in which care is taken to prevent dissolution of soluble minerals, such as calcite and gypsum.

Interlaboratory CEC and Exchangeable Cation Study of Bentonite Buffer Materials: I. Cu(II)-triethylenetetramine Method

In the Swedish KBS-3 repository concept, there is potential for gas to be generated from corrosion of ferrous materials under anoxic conditions, combined with the radioactive decay of the waste and radiolysis of water. A full understanding of the probable behaviour of this gas phase within the engineered barrier system (EBS) is therefore required for performance assessment. We demonstrate key features from gas transport experiments on pre-compacted Mx80 bentonite, under laboratory and field conditions, and discuss their implications in terms of a conceptual model for gas migration behaviour. On both scales, major gas entry is seen to occur close to the sum of the porewater and swelling pressures of the bentonite. In addition, gas pressure at breakthrough is profoundly sensitive to the number and location of available sinks for gas escape. Observations of breakthrough can be explained by the creation of dilatational pathways, resulting in localized changes in the monitored porewater pressures and total stresses. These pathways are highly unstable, evolving spatially and temporally, and must consequently influence the gas permeability as their distribution/geometry develops. Such observations are poorly embodied by conventional concepts of two-phase flow, which do not fully represent the key processes involved. Although dilatancy based models provide a better description of these processes, the paucity of data limits further development and validation of these models at present.

/pdf/gas-migration-experiments-in-bentonite-implications-for-2tetdx6a23.pdf

Gas migration experiments in bentonite: implications for numerical modelling

Bentonite, which is envisaged as a promising engineered barrier material for the safe disposal of highly radioactive waste, was and is investigated in different large scale tests. The main focus was and is on the stability (or durability) of the bentonite. However, most countries concentrated on one or a few different bentonites only, regardless of the fact that bentonite performance in different applications is highly variable. Therefore, SKB (Svensk Karnbranslehantering) set up the first large scale test which aimed at a direct comparison of different bentonites. This test was termed the ‘alternative buffer material test’ and considers eleven different clays which were either compacted (blocks) or put into cages to keep the material together. One so-called package consisted of thirty different blocks placed on top of each other. These blocks surrounded a heated iron tube 10 cm in diameter. Altogether three packages were installed in the underground test laboratory Aspo, Sweden. The first package was terminated 28 months after installation and the bentonite had been exposed for the maximum temperature (130°C) for about one year. Almost all geochemical and mineralogical alterations of the different bentonites (apart from exchangeable cations) were restricted to the contact between iron and bentonite. The increase of the Fe 2 O 3 content was attributed to corrosion of the tube. However, the typical 7 or 14 A smectite alteration product was not found. At the contact of one sample, siderite was precipitated. Some samples showed anhydrite and organic carbon accumulation and some showed dissolution of clinoptilolite and cristobalite. IR spectroscopy, XRD, and XRF data indicated the formation of trioctahedral minerals/domains in the case of some bentonites. Even more data has to be collected before unambiguous conclusions concerning both alteration mechanisms and bentonite differences can be drawn.

Patrik Sellin

Papers

The Use of Clay as an Engineered Barrier in Radioactive-Waste Management a Review

Experimentally determined swelling pressures and geochemical interactions of compacted Wyoming bentonite with highly alkaline solutions

Interlaboratory CEC and Exchangeable Cation Study of Bentonite Buffer Materials: I. Cu(II)-triethylenetetramine Method

Gas migration experiments in bentonite: implications for numerical modelling

Mineralogical investigations of the first package of the alternative buffer material test – I. Alteration of bentonites