[Show abstract][Hide abstract] ABSTRACT: To avoid impact on the environment, facilities for permanent disposal of hazardous waste adopt multi-barrier design schemes. As the primary barrier very often consists of cement-based materials, two distinct aspects are essential for the selection of a suitable complementary barriers: 1) selective sorption of the contaminants in the repository and 2) long-term chemical stability in hyperalkaline concrete derived media. A multidisciplinary approach combining experimental strategies from environmental chemistry and materials science is therefore essential to provide a reliable assessment of potential candidate materials. Chabazite is typically synthesized in 1 M KOH solutions, but also crystallises in simulated young cement porewater, a pH 13 aqueous solution mainly containing K+ and Na+ cations. Its formation and stability in this medium was evaluated as function of temperature (60 and 85°C) over a timeframe of more than 2 years and was also asessed from a mechanistic point of view. Chabazite demonstra
[Show abstract][Hide abstract] ABSTRACT: Construction of multibarrier concrete based waste disposal sites and management of alkaline mine drainage water requires cation exchangers combining excellent sorption properties with a high stability and predictable performance in hyper alkaline media. Though highly selective organic cation exchange resins have been developed for most pollutants, they can serve as a growth medium for bacterial proliferation, impairing their long-term stability and introducing unpredictable parameters into the evolution of the system. Zeolites represent a family of inorganic cation exchangers, which naturally occur in hyper alkaline conditions and cannot serve as an electron donor or carbon source for microbial proliferation. Despite their successful application as industrial cation exchangers under near neutral conditions, their performance in hyper alkaline, saline water remains highly undocumented. Using Cs+ as a benchmark element, this study aims to assess the long-term cation exchange performance of zeolites in concrete derived aqueous solutions. Comparison of their exchange properties in alkaline media with data obtained in near neutral solutions demonstrated that the cation exchange selectivity remains unaffected by the increased hydroxyl concentration; the cation exchange capacity did however show an unexpected increase in hyper alkaline media.
[Show abstract][Hide abstract] ABSTRACT: Within the 6th EC FP Integrated Project “Fundamental Processes of Radionuclide Migration” (FUNMIG), progress has been made to improve knowledge about the phases and reaction mechanisms involved in complex reduction processes of radionuclide contaminants in natural subsurface environments. This review paper gives an overview of the achievements made by the research groups involved in this project, and puts the scope and results of the studies in a more global context. Firstly, both thermodynamic and experimental evidence show that green rust is present and reactive in subsurface groundwater with a composition that spans the Fe(II)/Fe(II) redox boundary. Green rust has been shown to reduce Np(V), Se(VI) and Se(IV), but the pathways for the redox processes and the reaction products that result are complicated, and change as a function of the reaction parameters. Secondly, considerable evidence has emerged that Se(IV) is reduced on Fe(II)-bearing minerals which are ubiquitous in subsurface environments
[Show abstract][Hide abstract] ABSTRACT: The addition of H3PW12O40 to an aqueous solution of K2CrO4 led to the hydrolysis of [PW12O40]3– with its quantitative conversion into K13[KP2W20O72]. K13[KP2W20O72] in these chromate-containing solutions was characterized by 183W and 31P NMR, EXAFS, SAXS, and EPR spectroscopy.
European Journal of Inorganic Chemistry 08/2012; 2012(24). DOI:10.1002/ejic.201200440 · 2.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: HKUST-1 is one of the popular metal–organic frameworks (MOFs). The formation of this MOF is significantly accelerated by adding Keggin polyoxometalate anions to the synthesis solution. In this paper the chemistry behind this observation was investigated. Upon addition of Keggin type H3PW12O40 heteropolyacid the speciation of Cu2+ cations in ethanol:H2O mixture drastically changes. Combining EPR and XANES measurements with accurate pH measurements and prediction of Cu2+ hydrolysis provides strong evidence for surface induced hydrolysis and consequent dimerisation of monomeric Cu2+ species on Keggin ions in acidic conditions. This enables paddle wheel formation, hence explaining the instantaneous precipitation of Cu3(BTC)2 at room temperature and the systematic encapsulation of Keggin ions in its pores.
[Show abstract][Hide abstract] ABSTRACT: The geochemical fate of selenium is of key importance for today's society due to its role as a highly toxic essential micronutrient and as a significant component of high level radioactive waste (HLRW) originating from the operation of nuclear reactors. Understanding and prediction of the long-term behavior of Se in natural environments requires identification of the in situ speciation of selenium. This article describes an XAS-based investigation into the solid phase speciation of Se upon interaction of Se(IV) with Boom Clay, a reducing, complex sediment selected as model host rock for clay-based deep geological disposal of HLRW in Belgium and Europe. Using a combination of long-term batch sorption experiments, linear combination XANES analysis and ITFA-based EXAFS analysis allowed for the first time to identify Se0 as the dominant solid phase speciation of Se in Boom Clay systems equilibrated with Se(IV).
[Show abstract][Hide abstract] ABSTRACT: 79Se is considered as a key mobile fission product for nuclear waste disposal in the Boom Clay, the reference host formation presently studied in Belgium. Therefore, a good understanding of selenium migration behaviour through the clay barrier is essential to underpin its transport parameters selected for the performance assessment of a deep repository for spent fuel and high-level waste. Under the reducing conditions prevailing in Boom Clay, selenide, [Se(-II)], is the predominant thermodynamically stable chemical form of selenium. However, selenium may suffer of severe redox disequilibrium and selenate is very reluctant to reduction while a kinetically controlled sorption-reduction-precipitation behaviour is observed for selenite. Due to the large uncertainty on the speciation of selenium in the waste form, and taking the stability of selenate versus reduction into account, it is necessary to consider the non-solubility limited SeO42– as a possible migrating species. However, when a lower oxidation state [Se(IV), Se(0), and Se( II)] is present in the waste form, a solubility limit may also contribute to delay and attenuate the 79Se release. Therefore, two sets of transport parameters are proposed to PA: a first set for selenate (without solubility limit) and a second set for selenide (with solubility limit).
[Show abstract][Hide abstract] ABSTRACT: For more than 30 years the Boom Clay formation is studied as a reference host formation for methodological research concerning clay-based geological disposal of HLRW in Belgium and Europe. Boom Clay provides good sorption capacity, very low permeability and chemically reducing conditions due to the anoxic conditions and the presence of pyrite and siderite. Performance Assessment calculations have indicated Se79 (t1/2 = 2.95×105 y) to be one of the critical radionuclides for the geological disposal of HLRW . Aqueous selenite [Se(+IV)] and selenate [Se(+VI)] are the dominant species in mildly and strongly oxidizing environments. Under reducing conditions the solubility of Se is theoretically controlled by the formation of sparsely soluble selenium phases such as elemental Se or transition metalselenide salts (e.g. FeSe or FeSe2) [2, 3]. Slow kinetic reactions between the different redox states have been observed  and proposed to explain different redox phases observed within a single reducing envi
Geochimica et Cosmochimica Acta 01/2010; 74(12):A122-A122. · 4.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A structure elucidation strategy combining X-ray Absorption Spectroscopy (XAS) and Density Functional Theory (DFT) is demonstrated and reveals the existence of the [Tc=O](2+) moiety.
[Show abstract][Hide abstract] ABSTRACT: A new and quick method for direct speciation of Tc(IV) in humic rich solutions, based on the induced aggregation of humic substances in the presence of the trivalent cation La3+, is presented. This method (the "La-precipitation method") allows flocculating all the humic substances and also the Tc(IV) associated with humic substances. The method is tested on solutions containing Tc(IV) and Gorleben humic substances. The influence of different parameters (humic substance concentration, Tc concentration, reaction time and pH) is investigated on the observed free Tc(IV) concentration after precipitation of all humic substances. None of these parameters had a (significant) influence on the observed Tc(IV) concentration in solution after addition of La3+ to Tc(IV)-HS containing solutions. It is therefore proposed that the method can be used to separate the Tc(IV) bound to humic substances from the free inorganic Tc species in solution.
[Show abstract][Hide abstract] ABSTRACT: The interaction between colloidal Tc(IV) species and colloidal Gorleben humic substances (HS) was quantified after application of the La-precipitation method on supernatant solutions obtained under various experimental conditions but at constant ionic strength of the Gorleben groundwater (0.04M). The determined interaction constant LogKHS (2.3+/-0.3) remained unchanged over a large range of Tc(IV) and HS concentrations and was independent of the pH of the original supernatant solution (pH range 6-10), Tc(IV)-HS loading (10(-3)-10(-6)molTcg(-1) HS) and the nature of the reducing surface (Magnetite, Pyrite and Gorleben sand) used for the pertechnetate reduction. The LogKHS value determined by the La-precipitation method is lower than the LogK value obtained from a previous study where the interaction between colloidal Tc(IV) species and Gorleben humic substances was quantified using a modified Schubert approach (2.6+/-0.3). The La-precipitation method allows to accurately determine the amount of Tc(IV) associated with HS but leads to a (small) overestimation of the free inorganic Tc(IV) species.
[Show abstract][Hide abstract] ABSTRACT: The solid-phase Se speciation after short-term (3 weeks) contact of selenite [Se(IV)] oxyanions with pyrite (FeS2) and troilite (FeS) was investigated using X-ray absorption spectroscopy (XAS; X-ray absorption near-edge spectroscopy-extended X-ray absorption fine structure (XANES-EXAFS)). It was found that the nature of the sulfide mineral dictates the final speciation since respectively Se(0) and FeSe(x) were formed, meaning that the reaction mechanism is different and that these phases cannot be regarded as geochemically similar. The experimental results support the previously proposed sorption/ reduction mechanism for the reaction of selenite with pyrite. In the presence of troilite the reduction proceeds through the intermediate formation of Se(0) by reduction of selenite with dissolved sulfide. XAS data recorded for the FeS2 and FeS were compared with different Se reference phases, ranging in oxidation state from -II to +IV, used for validation of the XAS analysis methodology. This methodology can in principle be used to analyze Se phases formed in "in situ" geochemical conditions such as high-level radioactive waste disposal facilities.
[Show abstract][Hide abstract] ABSTRACT: Varying pertechnetate (Tc(VII)) doses were reduced to Tc(IV) in the presence and absence of Gorleben humic substances with the aid of magnetite, a reducing Fe(II)-containing surface. In absence of humic substances dissolved Tc(IV) concentrations are over-saturated with respect to the known TcO2 · nH2O solubility and increase with increasing Tc(VII) dose due to the formation of a range of mononuclear to colloidal Tc(IV) species. In presence of dissolved humic substances, the Tc solubility is enhanced due to the additional interaction of dissolved Tc(IV) species with humic substances. Both in the absence and the presence of dissolved humic substances a sorption mechanism controls the distribution of the range of mononuclear to colloidal Tc(IV) species between the solid and the liquid phase. The proposed reaction mechanism between Tc(IV) and HS is represented by Σ[TcO(OH)2]n+HS = [TcO(OH)2]n − HS in which Σ[TcO(OH)2]n stands for the sum of monomeric and polynuclear (colloidal) Tc(IV) species present in the equilibrium solution. A log K-value of 2.9 (±0.3) was quantified from a modified Schubert approach which is based on the competition of HS and magnetite for all dissolved Tc(IV) species and was found independent of Tc–HS loading, Tc–magnetite loading and pH.
[Show abstract][Hide abstract] ABSTRACT: Several commercially available adsorbents were screened for their ability to remove Cu, Zn and organotin compounds from both artificial contaminated and real dockyard wastewater. An adsorption--flocculation process using a mixture of two adsorbents (a clay based adsorbent and a powdered activated carbon) was optimized for an optimal adsorbent and pollutant removal. At the optimal conditions the process was evaluated with both artificial and real shipyard wastewater, and the cost of the adsorption-flocculation process with relation to different influent concentrations and discharge limits was estimated.