Experimental and theoretical investigations for mitigating NaAlH4 reactivity risks during postulated accident scenarios involving exposure to air or water

Experimental and theoretical studies were conducted to investigate the pyrophoricity and water-reactivity risks associated with employing sodium alanate (NaAlH₄) complex metal hydride in on-board vehicular hydrogen (H₂) storage systems. The ignition and explosivity of NaAlH₄ upon exposure to oxidizers in air or water were attributed to the spontaneous formation of stable hydroperoxyl intermediates on the NaAlH₄ surface and/or H₂ production, as well as the large driving force for NaAlH₄ conversion to favorable hydroxide products predicted by atomic and thermodynamic modeling. The major products from NaAlH₄ exposure to air: NaAl(OH)₄, gibbsite and bayerite Al(OH)₃, and Na₂CO₃ observed by XRD, were identified to be formed by surface-controlled reactions. The reactivity risks were significantly minimized, without compromising de-/re-hydrogenation cyclability, by compacting NaAlH₄ powder into wafers to reduce the available surface area. These core findings are of significance to risk mitigation and H₂ safety code and standard development for the safe use of NaAlH₄ for on-board H₂ storage in light-duty vehicles.     

Framing the search for a theory of land use

ABSTRACT

Land system science and affiliated research linked to sustainability require improved understanding and theorization of land and its change as a social-ecological system (SES). The absence of a general land-use theory, anchored in the social subsystem but with explicit links to the environmental subsystem, hampers this effort. Drawing on land-use explanations, meta-analyses, and associated frameworks, we advance a broad framework structure of eight elements – aggregations of explanatory variables – with links to the biophysical subsystem, for systematic comparisons of extant explanations. Tests and models can be employed to identify which set of variables and their configurations provide robust explanations of across land uses, identifying the potential for theory development. The framework and its application are applicable to both top-down and bottom-up explanatory approaches employed in the social sciences. Links to the environmental subsystem invite future exploration of SES explanations that reach across the different dimensions of global change and sustainability science.     

Solid-liquid distribution of selected concrete admixtures in hardened cement pastes

The distribution between hardened cement paste and cement pore water of selected concrete admixtures (BZMs), i.e., sulfonated naphthalene-formaldehyde condensate (NS), lignosulfonate (LS) and a gluconate-containing plasticiser used at the Paul Scherrer Institute for waste conditioning, was measured. Sorption data were fitted to a single-site Langmuir isotherm with affinity constants K=(19+/-4)dm(3)g(-1) for NS, K=(2.1+/-0.6) dm(3)g(-1) for LS and sorption capacities q=(81+/-16)g kg(-1) for NS, q=(43+/-8)g kg(-1) for LS. In the case of gluconate, a two-site Langmuir sorption model was necessary to fit the data satisfactorily. Sorption parameters for gluconate were K(1)=(2+/-1)x10(6)dm(3)mol(-1) and q(1)=(0.04+/-0.02)mol kg(-1) for the stronger binding site and K(2)=(2.6+/-1.1)x10(3)dm(3)mol(-1) and q(2)=(0.7+/-0.3)mol kg(-1) for the weaker binding site. Desorption of these BZMs from cement pastes and pore water in cement specimens prepared in the presence of the BZMs were then used to test the model. It was found that only minor parts of NS and LS could be mobilised as long as the cement composition was intact, whereas the sorption of gluconate was found to be reversible. The Langmuir model makes valuable predictions in the qualitative sense in that the pore water concentration of the BZMs is reduced by several orders of magnitude as compared to the initial concentrations. In view of the necessity for conservative predictions used in the safety analysis for disposal of radioactive waste, however, the predictions are unsatisfactory in that the measured pore water concentrations of NS and LS were considerably larger than the predicted values. This conclusion does not apply for gluconate, because its concentration in cement pore water was below the detection limit of approximately 50 nM.     

Degradation of Cellulosic Materials Under the Alkaline Conditions of a Cementitious Repository for Low- and Intermediate-Level Radioactive Waste. II. Degradation Kinetics

The degradation of cellulosic materials, differing mainly in the degree of polymerization and the number of reducing end groups, was studied under the alkaline conditions similar to those existing in a cementitious repository for low- and intermediate-level radioactive waste (pH 13.3, T = 25°C). The kinetics of alkaline degradation (peeling-off reaction) were studied and the data analyzed by the model of Haas et al. [13]. The observed kinetic parameters for the propagation reaction and overall stopping reaction were compared with literature data. Although measured under different experimental conditions, literature data and data from this study show a consistent picture. Differences in the extent of degradation observed for the different cellulosic materials could be satisfactorily explained by differences in reducing end group content and, consequently, by differences in the degrees of polymerization. Besides the number of reducing end groups, the degree of amorphousness also plays an important role. The main degradation products formed under the experimental conditions used are - and -(gluco)isosaccharinic acid. This is in agreement with many other studies on alkaline degradation of cellulose. The two isomers are formed in roughly equal amounts.