Applied Geochemistry (v.18, #8)

Heat-induced changes in speciation and extraction of uranium associated with sheet silicate minerals by Benjamin Ritherdon; Colin R. Hughes; Charles D. Curtis; Francis R. Livens; J.Fred W. Mosselmans; Steve Richardson; Anna Braithwaite (1121-1135).
Three samples of gouge from a U-mineralised fault, and two model samples, montmorillonite and muscovite, spiked with U, were heat-treated at a range of temperatures up to 1100 °C. Mineralogical changes were followed by thermal analysis, powder XRD and electron microscopy, and U extractability was measured by extraction with NH4 +. Changes in U speciation in the montmorillonite sample were followed using EXAFS spectroscopy. On heating, the minerals progressively dehydrate, dehydroxylate and eventually decompose to form new phases in a glassy matrix. In the case of montmorillonite (90% of U extractable from unheated material), U extractability increased slightly on heating to temperatures around 400 °C. Almost 50% of U was extracted from unheated muscovite, and this increased slightly by 450 °C. Above 500–600 °C, U extractability from both montmorillonite and muscovite declined to very low levels, reflecting dehydration of the uranyl ion and trapping in the new phases and glassy matrix. Uranium extractability from the natural samples was much lower in all cases (0.25–5% of the total before heating). In 2 samples, a significant increase in U extraction was associated with dehydroxylation at around 600 °C, followed by a decrease to very low levels at higher temperatures. Uranium extraction from the third natural sample, which contained X-ray amorphous U minerals, decreased steadily on heating. The results show that changes in U extraction can be related to structural and morphological changes in sheet silicate minerals. Heat treatment has potential to fix U but only if temperatures above 800 °C are reached. If only lower temperatures, in the range 400–600 °C, are used, then U extraction may increase.

During the cooling of the geothermal water of Chott El-Fejjej (Tunisia), considerable scaling occurred in the inlet to the tower. To attempt to resolve this problem, decarbonation (CaCO3 precipitation) induced by seeding, with aragonite crystals recovered on site, coupled with air bubbling has been studied. The advantage of this new technique is that no chemical addition is needed and a large volume of sludge is avoided. Attention is paid to seeding time, airflow rates and amount of seed added. Experimental results indicate that crystallisation takes place on seed crystals; consequently, precipitation on the wall of the reactor is prevented. The seeding which accelerates the precipitation, that is dependent on the physico-chemical properties of the water, should be curried out at the earliest possible stage for instantaneous precipitation. A threshold for air bubbling, beyond which no improvement appeared is observed. The kinetics of the secondary germination are dependent on supersaturation of water, the airflow rate and the amount of added seed.

A core collected in the Guaymas Basin contained an organic-poor, Mn oxide-rich and (relatively) Fe oxide-rich turbidite layer that affected the distribution of Fe, Mn, C, S and trace metals. Results indicate that sediments not influenced by the turbidite layer achieved a 100% degree of pyritization and, by extension, that pyrite production is Fe-limited in these sediments. In contrast, the mud slide layer apparently supplied enough reactive Fe to transfer essentially 98% of the total S present at the base of the turbidite (17–19 cm) to the pyrite reservoir. C/S ratios showed rapid decreases with depth, from a high of 38 close to the sediment-water interface, to minimum values of 2.8 at the lower limit of the turbidite layer, a ratio equal to the average C/S value of normal marine modern sediments, where concentrations of organic C and pyrite supposedly have attained quasi-steady values. A significant part of the reactive Mn was associated with carbonates (41±12%) and, to a much lower degree, with pyrite (2.7±1.2%). The turbidite layer is currently showing a depletion of Mn relative to the host sediment. It is possible that Mn, a major metal constituent in these sediments, was initially present in high concentrations in the mud slide, but was eventually mobilized and transferred either to the water column or to the sediments immediately below the turbidite layer. Metals associated with this element probably followed the same path, affecting their incorporation into pyrite. The turbidite layer apparently affected the distribution of most of the trace metals associated with pyrite, except maybe Cd, Pb and, to a certain, extent Cr. However, Cu, Cr, Zn, Ni and Co were all found to be highly pyritized (>80%) in the sediments of the Guaymas Basin.

Thermodynamic stability of waste glasses compared to leaching behaviour by Didier Perret; Jean-Louis Crovisier; Peter Stille; Graham Shields; Urs Mäder; Thierry Advocat; Kaarina Schenk; Marc Chardonnens (1165-1184).
The thermodynamic stability of products obtained from the high-temperature treatment of municipal solid wastes and their associated residues (bottom ash, fly ash, filter cake, optional additives) can be estimated by calculation of their free energy of hydration ΔG hydr by a polyhedral approach. This approach has been applied on a series of 23 samples originating from high-temperature treatment processes operated under a range of conditions, and 3 thoroughly characterised standards. For vitreous or vitrocrystalline samples, it is demonstrated that Si and Ca contents clearly control their thermodynamic stability, and that the type of incineration process plays only a minor role. Silicon directly influences the durability of the samples, while Ca governs the pH during corrosion, which in turn affects the thermodynamic stability. It is also shown that there is a tight inverse relationship between the calculated thermodynamic stability of the samples and their rates of dissolution under aggressive conditions of corrosion. Attempts to compare the results to the large literature database of results obtained from nuclear high-level waste glasses, their proxies and other analogs (ancient and commercial glasses) are limited by sample preparation constraints. It is however concluded that the calculated thermodynamic stability of these “waste glasses” offers a valid estimate for their relative quality and, in turn, for their durability.

Exploratory data analysis (EDA) techniques based on the boxplot and robust-class selection were applied to the analysis of single-element stream sediment data in the Collo area (N-E Algeria). The area is characterised by many factors that affect data variability: variation of lithology, combined existence of permanent and ephemeral streams, flash floods, rugged terrain, and climate. The boxplot proved to be very useful in capturing the empirical data distribution, the skewness, and in defining outliers. No data transformation was needed prior to the analysis of single element distributions (Cr, Pb, Zn, Cu, As, and Fe) as is the case in classical statistics. Geochemical mapping of these elements was based on resistant class selection as defined by the boxplot. Results showed the close spatial correlation of outlier data for Cr with a plagioclase-lherzolite intrusion and known chromite pods. The geochemical maps of Pb, Zn, Cu and As concentrations showed an association of these elements coincident with known base-metal sulphides and arsenopyrite mineralisation, delineating a northeastern anomaly spreading well over known mineralisation. The choice of the robust classes based on the boxplot also showed the close spatial distribution of Fe with known hematite and magnetite mineralization. EDA techniques proved to be very useful in delineating known mineralisation in the Collo area where stream sediment data is subject to variability owing to many factors (geologic, physiographic and climatic). EDA proved to be a simple and very useful tool in analysing single-element geochemical data. EDA could be used in similar terrains as an alternative to the classical methods where a normality precondition is needed prior to analysis and where class selection may be affected by the presence of “wild” data.

This study constitutes a first stage of the elaboration of a concept for setting up capillary barriers composed of reactive tailings and alkaline industrial wastes. The aim of this barrier is (1) short-term mitigation by neutralising acid produced by Fe sulfides and (2) long-term mitigation by stabilisation of secondary products. The alkaline industrial wastes, cement kiln dust (CKD) and red mud bauxite (RMB), are used to produce neutral conditions. A series of time-controlled static leaching experiments were performed. Samples of reactive tailings were prepared containing 0, 2, 5, and 10% alkaline material. The water:solid ratios were also varied (3:1, 6:1, and 20:1) in order to characterise the geochemical balance between liquid and solid phases. The data are used to discern whether mineral solubility controls exist for particular ions. The pH results show that the use of 5% CKD, 10% CKD and 10% (CKD+RMB) allows neutral pH conditions to be maintained in the reactive layer. At these percentages, the concentrations of Al, Fe, Cu, Zn and SO4 in solution are significantly reduced compared with those obtained with the reactive tailings only. Aluminum concentrations are principally controlled by secondary phases like boehmite and gibbsite; Fe, by goethite and ferrihydrite; Cu and Zn, by hydroxides. Calcium and SO4 concentrations are controlled by precipitation of gypsum. The concentrations of Na and K are found not to be controlled by mineral solubility.

In this paper, the concentration of labile Cu measured by differential pulse anodic stripping voltammetry with a double acidification method (DAM-DPASV) was used to predict the growth inhibition of Cu on the alga Scenedesmus oblignus (S. oblignus). The influences of alkalinity, Cl, and different concentrations of ethylene diamine tetraacetic acid (EDTA) and fulvic acid (FA) on the labile concentration and toxicity of Cu were investigated in reconstituted water and natural water. The results showed that Cl and alkalinity did not affect labile Cu concentration, but decreased the toxicity of Cu to S. oblignus in the Cu concentration range observed in fresh water. The influences of alkalinity and Cl could be modeled if Cu-carbonate complexes and Cu-chloride complexes were considered as non-toxic species. Organic complexes of Cu with EDTA and FA did not contribute either to the labile concentration of Cu or to the toxicity of Cu on S. oblignus. The growth inhibition of Cu on S. oblignus in natural water could be predicted by using the bioavailable concentration of Cu, or [Cu*], which could be calculated from the labile Cu concentration using the DAM-DPASV method and the concentrations of Cu-carbonate and Cu-chloride complexes obtained from the MINTEQ A2 model.

The kinetics of carbonate and Mn oxide dissolution under acidic conditions were examined through the in situ exposure of pure phase samples to acidic ground water in Pinal Creek Basin, Arizona. The average long-term calculated in situ dissolution rates for calcite and dolomite were 1.65×10−7 and 3.64×10−10 mmol/(cm2 s), respectively, which were about 3 orders of magnitude slower than rates derived in laboratory experiments by other investigators. Application of both in situ and lab-derived calcite and dolomite dissolution rates to equilibrium reactive transport simulations of a column experiment did not improve the fit to measured outflow chemistry: at the spatial and temporal scales of the column experiment, the use of an equilibrium model adequately simulated carbonate dissolution in the column. Pyrolusite (MnO2) exposed to acidic ground water for 595 days increased slightly in weight despite thermodynamic conditions that favored dissolution. This result might be related to a recent finding by another investigator that the reductive dissolution of pyrolusite is accompanied by the precipitation of a mixed Mn–Fe oxide species. In PHREEQC reactive transport simulations, the incorporation of Mn kinetics improved the fit between observed and simulated behavior at the column and field scales, although the column-fitted rate for Mn-oxide dissolution was about 4 orders of magnitude greater than the field-fitted rate. Remaining differences between observed and simulated contaminant transport trends at the Pinal Creek site were likely related to factors other than the Mn oxide dissolution rate, such as the concentration of Fe oxide surface sites available for adsorption, the effects of competition among dissolved species for available surface sites, or reactions not included in the model.

Carbon isotope and C/N ratios of suspended matter in rivers by Liu Weiguo; An Zisheng; Zhou Weijian; M.J Head; Cai Delin (1241-1249).
A combination of δ 13C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C4 and C3 vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ 13C values, low C content and low C/N ratios; (2) plant litter with varying δ 13C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C4/C3 vegetation within the area studied, as part of the Loess Plateau. The abundance of C4 grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ 13C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C4/C3 vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.

U-series in Fe-U-rich fracture fillings from the oxidised cap of the “Mina Fe” uranium deposit (Spain): implications for processes in a radwaste repository by M.T Crespo; L Pérez del Villar; A.J Quejido; M Sánchez; J.S Cózar; M Fernández-Dı́az (1251-1266).
Within the framework of the ENRESA (Spain) natural analogue programme, the U-ore deposit of “Mina Fe” is being studied as a natural analogue of radioactive spent fuel behaviour. In this context, the knowledge of the role played by fracture minerals as scavengers of certain analogue elements, mainly U, and the establishment of the time scale of the rock-water interaction processes controlling the uptakes or losses of U in the system are two relevant objectives. Fracture-infill materials from the site have first been mineralogically characterised, then the upper part of the U-series determined in both bulk samples and U-rich leachates obtained by sequential leaching. Uranium-series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1.6 Ma, while in other fractures water/rock interaction processes affecting the upper part of the U-series have been identified. These processes indicate recent or rapid U accumulation or losses (<102 ka), old 234U accumulation (> 102 ka) or 234U+230Th recoil gain. The apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures, as a result of their varying degree of sealing. Uranium concentrations and 234U/238U ARs of the leachates obtained with Morgan's solution and 6N HCl indicate that minerals dissolved with these reagents (U(IV/VI) oxyhydroxides and goethite+clays, respectively) are responsible for the retention of almost all of the U in the bulk samples. Furthermore, the 234U lost by the U minerals dissolved with Morgan's solution seems to be fixed onto goethite–clay mixtures, the intersticial water being the vehicle for the isotopic transfer, which in turn is a recent or recent-past process.

A natural attenuation of arsenic in drainage from an abandoned arsenic mine dump by Keisuke Fukushi; Miwa Sasaki; Tsutomu Sato; Nobuyuki Yanase; Hikaru Amano; Hodaka Ikeda (1267-1278).
At the abandoned As mine in Nishinomaki, Japan, discharged water from the mining and waste dump area is acidic and rich in As. However, the As concentration in the drainage has been decreased to below the maximum contaminant level (0.01 mg/l for drinking water, Japan) without any artificial treatments before mixing with a tributary to populated areas. This implies that the As concentration in water from the waste dump area has been naturally attenuated. To elucidate the reaction mechanisms of the natural attenuation, analysis of water quality and characterization of the precipitates from the stream floor were performed by measuring pH, ORP and electric conductivity on-site, as well as X-ray diffraction, ICP-mass spectrometry and ion-chromatography. Selective extractions and mineral alteration experiments were also conducted to estimate the distribution of As in constituent phases of the precipitates and to understand the stability of As-bearing phases, respectively. The water contamination resulted from oxidation of sulfide minerals in the waste rocks, i.e., the oxidation of pyrite and realgar and subsequent release of Fe, SO4, As(V) and proton. The released Fe(II) transformed to Fe(III) by bacterial oxidation; schwertmannite then formed immediately. While the As concentrations in the stream were lowered nearly to background level downstream, those in the ochreous precipitates were up to several tens of mg/g. The As(V) was effectively removed by the formed schwertmannite and had been naturally attenuated. Although schwertmannite is metastable with respect to goethite, the experiments show that the transformation of schwertmannite to goethite may be retarded by the presence of absorbed As(V) in the structure. Therefore, the attenuation of As in the drainage and the retention of As by schwertmannite are expected to be maintained for the long term.