[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 07/2012; · 3.12 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.
Journal of Materials Chemistry 01/2011; 21:9768-9771. · 6.63 Impact Factor
[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: 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. · 3.88 Impact Factor
[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: 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.
Environmental Science and Technology 06/2008; 42(10):3595-601. · 5.48 Impact Factor
[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.
[Show abstract][Hide abstract] ABSTRACT: The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur.
[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: In this contribution, the formation and immobilisation of chromium(iii) hydroxyoxide colloids is investigated. Nano-sized Cr(iii) colloids are generated in situ upon reduction of Cr(vi), dosed to a stirred reactor. The growth of the elementary colloids by the consumption of solved Cr is kinetically unfavorable compared to their aggregation to larger secondary particles, the size of which depends on the concentration of the building block colloids. This aggregation process can be steered by simple process parameters such as dosing rate and concentration of the Cr(vi). The Cr(iii) colloids are immobilised in situ on a support material via precipitation chromatography. Upon drying, the initially amorphose hydroxyoxides are gradually transformed into crystalline Cr(2)O(3) nanoparticles, mainly located at the external surface of the support. This approach opens new opportunities for the synthesis of supported metal oxide catalysts.
Physical Chemistry Chemical Physics 11/2007; 9(39):5382-6. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In [Maes, A., Bruggeman, C., Geraedts, K., Vancluysen, J., 2003. Quantification of the interaction of Tc with dissolved boom clay humic substances. Environ. Sci. Technol., 37, 747–753] the interaction of Tc(IV) with dissolved boom clay humic substances was quantified by the interpretation of the solid–liquid distribution of Tc in boom clay batch suspensions using a modified Schubert approach. During this procedure it was established that multiple sorption sinks for Tc(IV) were present within the boom clay. By combining different initial Tc concentrations and different added amounts of ground FeS2 to a boom clay suspension at a single solid-to-liquid ratio, the same Schubert approach was used here to identify FeS2 as one of the sorption sinks present within the boom clay. The amount of Tc(IV) adsorbed by FeS2 could be described by a simple Langmuir adsorption isotherm. The interaction of Tc(IV) with dissolved boom clay humic substances in these systems was also investigated and could be quantified by an identical interaction constant (logKTcO(OH)2=5.30) as the one calculated in Maes et al. (2003).
Physics and Chemistry of the Earth, Parts A/B/C. 01/2007;
[Show abstract][Hide abstract] ABSTRACT: Batch experiments were performed in which the interaction of selenite () and selenate () with dissolved Boom Clay and Gorleben humic substances (HS) was followed as a function of time. Particulate HS (>25 nm) was first removed by centrifugation. The supernatant solutions were analyzed by a combination of Ion Chromatography, Gel Permeation Chromatography and Ultrafiltration to determine the Se solution speciation. A first experiment in which 1 × 10−6 M was in contact with 128 ppm Boom Clay HS or 160 ppm Gorleben HS in a synthetic clay H2O background electrolyte, showed that the total Se solution concentration and Se speciation remained unchanged, indicating that there was no interaction whatsoever between and HS during the three months’ observation time. Upon contact of 1 × 10−6 M with the same HS-containing solution however, a decrease of ∼95% in total Se solution concentration after centrifugation was noted and final Se solution concentrations of 4.4 × 10−8 M (Boom Clay HS) and 1.3 × 10−7 M (Gorleben HS) were measured after six months observation. Together with the decrease in total solution concentration, a change in the Se solution speciation was observed. After one month, the solution speciation in both systems was dominated by a colloidal Se species that was associated with HS. The chemical nature of this Se species could not be determined. The concentration decreased steadily to a final concentration of 1.9 × 10−8 M (Boom Clay HS) and 1.5 × 10−8 M (Gorleben HS). The detailed understanding of the mechanisms at work in these systems will necessitate spectroscopic measurements of the Se species present in the pellet and of Se associated with the humic substance fraction in solution.