Metatorbernite

Abstract: Nous avons synthetise la torbernite, Cu[(UO 2 )(PO 4 )] 2 (H 2 O) 1 2 , la zeunerite, Cu[(UO 2 )(AsO 4 )] 2 (H 2 O) 1 2 , la metatorbernite, Cu[(UO 2 )(PO 4 )] 2 (H 2 O) 8 , et la metazeunerite, Cu[(UO 2 )(AsO 4 )] 2 (H 2 O) 8 , par diffusion dans des gels ou bien par voie hydrothermale Les intensites des raies en diffraction X sur monocristal ont ete mesurees a temperature ambiante en utilisant un rayonnement MoKa et un detecteur de type CCD Nous avons resolu la structure des quatre composes par methodes directes, avec affinement sur matrice entiere par techniques de moindres carres en utilisant les facteurs F2 pour toutes les reflexions uniques jusqu'a un residu wR 2 (torbernite, zeunerite, metatorbernite et metazeunerite) de 56, 35, 48 et 51% pour toutes les donnees, et R1 de 24, 16, 20 et 23%, calcule pour 764, 468, 794 et 1447 reflexions uniques observees (‖F o ‖ ≥ 4σ F ), respectivement La torbernite est tetragonale, groupe spatial P4/nnc, a 70267(4), c 20807(2) A, V 10273(1) A 3 , Z = 2, D c a l c 3264(1) g/cm 3 La zeunerite est isostructurale avec la torbernite, a 71797(3), c 20857(1) A, V 10751(1) A 3 , Z= 2, D c a l c 3391(1) g/cm 3 La structure de la metatorbernite a ete determinee a partir d'un cristal macle par meroedrie Elle est tetragonale, groupe spatial P4/n, a 69756(5), c 17349(2) A, V 8442(1) A 3 , Z = 2, D c a l c 3689(1) g/cm 3 La metazeunerite est isostructurale avec la metatorbernite, a 71094(1), c 17416(1) A, V 8803(1) A 3 , Z = 2, D c a l c 3869(1) g/cm 3 ; le cristal qui a servi a l'ebauche aussi est macle Ces mineraux contiennent un feuillet de type autunite, de composition [(UO 2 )(PO 4 )] - , dans lequel il y a partage des coins equatoriaux des bipyramides carrees a uranyle avec des tetraedres de phosphate Dans chacune des structures, les cations Cu 2 + sont situes entre les feuillets dans des octaedres rendus difformes (4 + 2) a cause de l'effet de Jahn-Teller, avec des liaisons courtes a quatre groupes H 2 O dans un agencement carre planaire, et deux liaisons plus longues aux atomes d'oxygene des ions uranyle Un groupe H 2 O symetriquement independant est present dans chaque structure, lie seulement par liaisons hydrogene, et dans la torbernite (et la zeunerite) forme des agencements planaires carres de groupes interstitiels de H 2 O par-dessus et par-dessous les plans contenant les cations Cu 2 + L'affinement des quatre structures avec certaines contraintes a propos des longueurs des liaisons impliquant l'hydrogene a mene a des descriptions cristallochimiquement raisonnables des liaisons hydrogene

Abstract: A key difficulty in developing accurate, science-based conceptual models for remediation of contaminated field sites is the proper accounting of multiple coupled geochemical and hydrologic processes. An example of such a difficulty is the separation of desorption and dissolution processes in releasing contaminants from sediments to groundwaters; very few studies are found in the literature that attempt to quantify contaminant release by these two processes. In this study, the results from several extraction techniques, isotopic exchange experiments, and published spectroscopic studies were combined to estimate the contributions of desorption and dissolution to U(VI) release from contaminated sediments collected from the vadose zone beneath former waste disposal ponds in the Hanford 300-Area (Washington state). Vertical profiles of sediments were collected at four locations from secondary pond surfaces down to, and slightly below, the water table. In three of the four profiles, uranium concentration gradients were observed in the sediments, with the highest U concentrations at the top of the profile. One of the vertical profiles contained sediments with U concentrations up to 4.2x10-7 mol/g (100 ppm). U(VI) release to artificial groundwater solutions and extracts from these high-U concentrations sediments occurred primarily from dissolution of precipitated U(VI) minerals, including the mineral metatorbernite,more » [Cu(UO2PO4)2∙8H2O]. At the bottom of this profile, beneath the water table, and in all three of the other profiles, U concentrations were <5.88x10-8 mol/g (14 ppm), and U(VI) release to artificial groundwater solutions occurred primarily due to desorption of U(VI). When reacted in batch experiments with artificial groundwater solutions with compositions representative of the range of chemical conditions in the 2 underlying aquifer, all samples released U(VI) at concentrations greater than regulatory limits within a few hours. A semi-mechanistic surface complexation model was developed to describe U(VI) adsorption on sediments collected from near the water table, as a function of pH, alkalinity, and Ca and U(VI) concentrations, using ranges in these variables relevant to groundwater conditions in the aquifer. Dilute (bi)carbonate solution extractions and uranium isotopic exchange methods were capable of estimating adsorbed U(VI) in samples where U(VI) release was predominantly due to U(VI) desorption; these techniques were not effective at estimating adsorbed U(VI) where U(VI) release was affected by dissolution of U(VI) minerals. The combination of extraction and isotopic exchange results, spectroscopic studies, and surface complexation modeling allow an adequate understanding for the development of a geochemical conceptual model for U(VI) release to the aquifer. The overall approach has generic value for evaluating the potential for release of metals and radionuclides from sediments that contain both precipitated and adsorbed contaminant speciation.« less

Abstract: Long-term historic spills of uranium at the 300 Area fuel fabrication site (58,000 kg of disposed uranium over 32 yr) and at the 200 East Area BX tank farm (7000 kg of spilled uranium in one event), both within the Hanford formation in the Hanford Site, Washington State, were investigated by subsurface sampling and subsequent microscale investigations of excavated samples. The 200 Area sediments contained uranyl silicate mineralization (sodium boltwoodite) in restrictive microfractures in granitic clasts, in the vadose zone over a narrow range in depth. Well logging and column experiments indicated that tank wastes migrated deeper than observed in core samples. The 300 Area sediments included metatorbernite and uranium at low concentrations associated with detrital aluminosilicates, along with other mineral phases that could accommodate uranyl, such as uranophane and calcium carbonate. The association of contaminant uranyl with Hanford formation sediments provided a persistent source of uranium to groundwater. The results of both studies suggest that the formation of secondary solid uranyl-bearing phases infl uences the subsequent release of uranium to the environment and that our understanding of these processes and individual waste sites is incomplete.