A procedure to assess the importance of chemical kinetics in the humic-mediated transport of radionuclides in radiological performance assessment calculations

Previous work has shown that humic substances can bind metal ions in two fractions: the exchangeable, where it is available instantaneously for reaction with other sinks (such as mineral surfaces); and the non-exchangeable, from which it may only dissociate slowly. In the absence of metal ion/humic/mineral surface ternary complexes, if the dissociation rate is slow compared to the solution residence time in the groundwater column, then metal in the non-exchangeable will have a significantly higher mobility than that in the exchangeable. The critical factor is the ratio of the non-exchangeable first order dissociation rate constant and the residence time in the groundwater column, metal ion mobility increasing with decreasing rate constant. Sorption of humic/metal complexes at mineral surfaces may reduce mobility. In addition to direct retardation, sorption also increases the residence time of the non-exchangeable fraction, giving more time for dissociation and immobilisation. The magnitude of the effect depends upon the concentrations of the mineral surface humic binding sites and the humic in solution, along with the magnitudes of the equilibrium constant and the forward and backward rate constants. The non-exchangeable dissociation reaction and the sorption reaction may be classified in terms of two Damkohler numbers, which can be used to determine the importance of chemical kinetics during transport calculations. These numbers could be used to determine when full chemical kinetic calculations are required for a reliable prediction, and when equilibrium may be assumed, or when the reactions are sufficiently slow that they may be ignored completely

The role of humic non-exchangeable binding in the promotion of metal ion

Metal ions form strong complexes with humic substances. When the metal ion is first complexed by humic material, it is bound in an ‘exchangeable’ mode. The metal ion in this fraction is strongly bound, however, if the metal–humic complex encounters a stronger binding site on a surface, then the metal ion may dissociate from the humic substance and be immobilised. However, over time, exchangeably-bound metal may transfer to a ‘non-exchangeable’ mode. Transfer into this mode and dissociation from it are slow, regardless of the strength of the competing sink, and so immobilisation may be hindered. A series of coupled chemical transport calculations has been performed to investigate the likely effects of non-exchangeable binding upon the transport of metal ions in the environment. The calculations show that metal in the nonexchangeable mode will have a significantly higher mobility than that in the exchangeable mode. The critical factor is the ratio of the non-exchangeable first-order dissociation rate constant and the residence time in the groundwater column, metal ion mobility increasing with decreasing rate constant. A second series of calculations has investigated the effect of the sorption to surfaces of humic/metal complexes on the transport of the non-exchangeably bound metal. It was found that such sorption may reduce mobility, depending upon the humic fraction to which the metal ion is bound. For the more weakly sorbing humic fractions, under ambient conditions (humic concentration etc.) the non-exchangeable fraction may still transport significantly. However, for the more strongly sorbed fractions, the non-exchangeable fraction has little effect upon mobility. In addition to direct retardation, sorption also increases the residence time of the nonexchangeable fraction, giving more time for dissociation and immobilisation. The nonexchangeable dissociation reaction, and the sorption reaction have been classified in terms of two Damkohler numbers, which can be used to determine the importance of chemical kinetics during transport calculations. These numbers have been used to develop a set of rules that determine when full chemical kinetic calculations are required for a reliable prediction, and when equilibrium may be assumed, or when the reactions are sufficiently slow that they may be ignored completely

XLF and APLF bind Ku80 at two remote sites to ensure DNA repair by non-homologous end joining

International audienceThe Ku70-Ku80 (Ku) heterodimer binds rapidly and tightly to the ends of DNA double-strand breaks and recruits factors of the non-homologous end-joining (NHEJ) repair pathway through molecular interactions that remain unclear. We have determined crystal structures of the Ku-binding motifs (KBM) of the NHEJ proteins APLF (A-KBM) and XLF (X-KBM) bound to a Ku-DNA complex. The two KBM motifs bind remote sites of the Ku80 alpha/beta domain. The X-KBM occupies an internal pocket formed by an unprecedented large outward rotation of the Ku80 alpha/beta domain. We observe independent recruitment of the APLF-interacting protein XRCC4 and of XLF to laser-irradiated sites via binding of A- and X-KBMs, respectively, to Ku80. Finally, we show that mutation of the X-KBM and A-KBM binding sites in Ku80 compromises both the efficiency and accuracy of end joining and cellular radiosensitivity. A- and X-KBMs may represent two initial anchor points to build the intricate interaction network required for NHEJ

CO Desorption Kinetics at Concentrations and Temperatures Relevant to PEM Fuel Cells Operating with Reformate Gas and PtRu/C Anodes

The kinetics of the CO desorption process have been determined using isotopic exchange experiments at concentrations and temperatures relevant to PEM fuel cells operating with reformate gas and commercial carbon supported platinum-ruthenium alloy anodes. The CO desorption kinetics have been studied as a function of CO concentration, temperature and flow rate. Desorption rate constants have been determined experimentally for a wide range of concentrations (100-500ppm) and temperatures (25-150°C) and have been extrapolated to one order of magnitude lower CO concentration range between 10 and 100ppm, which is directly relevant to PEM fuel cells operating with reformate gas. The desorption rates measured for the 100-500ppm CO concentration range appear to be significantly larger than previously published CO oxidation data, suggesting that the CO desorption process plays a more significant role in determining the equilibrium CO coverage at the fuel cell anode than the electrochemical CO oxidation process. The proposed desorption rate values at the lower 10-100ppm CO concentration range and at relevant temperatures are believed to be of added value for the modelling of PEM fuel cells operating with reformate gas and PtRu/C anodes, since significantly different empirical values have been used up to now for the modelling of the CO desorption process. The variation of the apparent Arrhenius parameters as a function of CO concentration provides also some insight into the CO poisoning effect and the underlying adsorption/desorption processes.JRC.DG.F.2-Cleaner energ

FCANODE - Non-Noble Catalysts for Proton Exchange Membrane Fuel Cell Anodes

The FCANODE project is funded by the EC FP6, under the Specific Targeted Research Project Action. Its aim is to take up the materials development for Proton Exchange Membrane Fuel Cells (PEMFC) by replacing the expensive platinum based catalysts by cheaper non-noble metal based nanoparticulate catalysts. This could reduce significantly the cost of the fuel cells and bring them closer towards full commercialisation.JRC.F.2-Cleaner energ

CO Desorption Kinetics under Conditions of Relevance to PEM Fuel Cells Operating with Reformate Gas

The kinetics of the CO desorption process have been investigated on a PtRu/C anode using isotopic exchange experiments under conditions of relevance to PEM fuel cells operating with reformate gas. Desorption rate constants have been determined experimentally for a wide range of concentrations (100-1000 ppm) and temperatures (25-150°C) and have been extrapolated to one order of magnitude lower CO concentration range between 10 and 100 ppm, which is directly relevant to PEMFC operating with reformate gas. The results are discussed with relation to the CO tolerance issue at the PEM fuel cell anode and to the development of more accurate models for PEM fuel cells operating with reformate gas.JRC.DDG.F.2-Cleaner energ

Non-noble Catalysts Cut Fuel Cell Costs (2007-2010) - FCAnode project

The cost of noble-metal catalyst systems for proton exchange membrane fuel cells (PEMFC) is driving research to find less expensive non-noble alternatives.JRC.F.2-Cleaner energ

Investigation of PEM Fuel Cell Electrodes by Transmission Electron Microscopy, Scanning Electron Microscopy and -Ray Diffraction

PEM fuel cell electrodes were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and x-ray diffraction (XRD). As one example of the application of these techniques, catalyst nanoparticles were characterized before and after desorption experiments up to 150°C, in the frame of temperature-dependent CO desorption kinetics experiments, which are of importance for a better understanding of the fundamental physicochemical processes involved in improving CO tolerance. Particle diameter distributions of a novel Pt-on-Au based catalyst nanomaterial were determined from TEM data and compared with average sizes calculated from XRD data.JRC.DDG.F.2-Cleaner energ

Effect of Humidity on Carbon Monoxide Desorption Kinetics

The kinetics of carbon monoxide desorption on a platinum catalyst under humidified conditions were investigated with the steady state isotropic transient kinetic analysis (SSITKA) method. The effect of the humidity level on desorption kinetics was quantified. The carbon monoxide (CO) desorption kinetic constant was calculated regardless of the gas flow rate. The kinetic constant dropped up to 58% with the increasing relative humidity. The negative effect of humidity in terms of CO poisoning for PEM fuel cells was determined.JRC.F.2-Energy Conversion and Storage Technologie

Optimization of Capillary Electrophoresis Separation for Lanthanides (La-Lu) and their Detection by both Direct UV Spectrophotometry and Electrospray Mass Spectrometry.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe