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Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three-dimensional numerical model of a natural in-stream gravel bar (20 m x 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater-borne nitrate when mixed with stream-sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log-space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.
Polyglycolide (PGA) is a biodegradable polymer with multiple applications in the medical sector. Here the synthesis of high molecular weight polyglycolide by ring-opening polymerization of diglycolide is reported. For the first time stabilizer free supercritical carbon dioxide (scCO(2)) was used as a reaction medium. scCO(2) allowed for a reduction in reaction temperature compared to conventional processes. Together with the lowering of monomer concentration and consequently reduced heat generation compared to bulk reactions thermal decomposition of the product occurring already during polymerization is strongly reduced. The reaction temperatures and pressures were varied between 120 and 150 degrees C and 145 to 1400 bar. Tin(II) ethyl hexanoate and 1-dodecanol were used as catalyst and initiator, respectively. The highest number average molecular weight of 31 200 g mol(-1) was obtained in 5 hours from polymerization at 120 degrees C and 530 bar. In all cases the products were obtained as a dry white powder. Remarkably, independent of molecular weight the melting temperatures were always at (219 +/- 2)degrees C.
Polyglycolide (PGA) is a biodegradable polymer with multiple applications in the medical sector. Here the synthesis of high molecular weight polyglycolide by ring-opening polymerization of diglycolide is reported. For the first time stabilizer free supercritical carbon dioxide (scCO2) was used as a reaction medium. scCO2 allowed for a reduction in reaction temperature compared to conventional processes. Together with the lowering of monomer concentration and consequently reduced heat generation compared to bulk reactions thermal decomposition of the product occurring already during polymerization is strongly reduced. The reaction temperatures and pressures were varied between 120 and 150 °C and 145 to 1400 bar. Tin(II) ethyl hexanoate and 1-dodecanol were used as catalyst and initiator, respectively. The highest number average molecular weight of 31 200 g mol−1 was obtained in 5 hours from polymerization at 120 °C and 530 bar. In all cases the products were obtained as a dry white powder. Remarkably, independent of molecular weight the melting temperatures were always at (219 ± 2) °C.
A confocal set-up is presented that improves micro-XRF and XAFS experiment with high-pressure e diamond-anvil cells (DACs) In this experiment a probing volume is defined by the focus of the incoming synchrotron radiation beam and that of a polycapillary X-ray half-lens with a very long working distance, which is placed in front of the fluorescence detector This set-up enhances the quality of the fluorescence and XAFS spectra, and thus the sensitivity for detecting elements at low concentrations. It efficiently suppresses signal from outside the sample chamber, which stems from elastic and inelastic scattering of the incoming beam by the diamond anvils as well as from excitation of fluorescence from the body of the DAC
Barium, lanthanum, ytterbium, and yttrium partitioning experiments between fluid-saturated haplogranitic melts and aqueous solutions were conducted at 750 to 950 degrees C and 0.2 to 1 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. Partition coefficients were determined using different experimental methods. On one hand quenched experiments were performed, and on the other hand, trace element contents in the aqueous fluid were determined directly using a hydrothermal diamond-anvil cell and synchrotron radiation X-ray fluorescence microanalysis of K-lines. The latter required a high excitation energy of 50 key due to the high energies necessary to excite the K-lines of the studied elements. The data from these two techniques showed good agreement for chloridic solutions, whereas quenching had a significant effect on results of the experiments with only water in the case of Ba. In Cl-free experiments, lanthanum and yttrium, trace element contents were even below detection limit in the quenched fluids, whereas small concentrations were detected in comparable in-situ experiments. This distinct difference is likely due to back reactions between fluid and melt upon cooling. The partitioning data of all elements show no dependence on the temperature and only small dependence on pressure. In contrast, the partitioning is strongly influenced by the composition of the starting fluid and melt. For chloridic fluids, there was a sharp increase in the Ba, La, Y and Yb partition coefficients with the alumina saturation index (ASI). The Ba partition coefficient increased from 0.002 at an ASI of 0.8 to 0.55 at an ASI of 1.07. At higher ASI, it decreased slightly to 0.2 at an ASI of similar to 1.3. Likewise, it was one to two orders of magnitude higher in chloridic fluids compared to those found in H2O experiments. Fluid-melt partition coefficients of La and Y increased from 0.002 at an ASI of similar to 0.8 to similar to 0.1 at an ASI of 1.2. In the same ASI range, the Yb partition coefficient increased to a maximum value of 0.02. Even at high salinities all elements fractionate into the melt. The compositional dependence of the partitioning data imply that both melt composition and fluid composition have a strong influence on trace element behavior and that complexation of Ba. REE and Y tin the fluid is not only controlled by the presence of Cl- in the fluid. Instead, interaction of these elements with major melt components dissolved in the fluid is very likely.
Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16- 3.56 m (NaCl + KCl) +/- HCl) were conducted at 750-950 degrees C and 0.2-1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and Melt upon cooling have a significant effect on results from the experiments with water. The Rb fluid-melt partition coefficient shows no recognizable dependence on melt composition and temperature. For chloridic Solutions, it is similar to 0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid-melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H2O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity. The obtained fluid-melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid-melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.
Trace element concentrations in aqueous fluids in equilibrium with haplogranitic melt were determined in situ at elevated P-T conditions using hydrothermal diamond-anvil cells and synchrotron-radiation XRF microanalyses. Time- resolved analyses showed that the Rb and Sr concentrations in the fluids became constant in less than 2000 s at all temperatures (500 to 780 degrees C). Although fluid-melt equilibration was very rapid, the change in the concentration of both elements in the fluid with temperature was fairly small (a slight increase for Rb and a slight decrease for Sr). This permitted partitioning data for Rb and Sr between haplogranitic melt and H2O or NaCl+KCl+HCl aqueous solutions at 750 degrees C and 200 to 700 MPa to be obtained from EMP analyses of the quenched melt and the in situ SR-XRF analyses of the equilibrated fluid. The resulting D-Rb(f/m) and D-Sr(f/m) were 0.01 +/- 0.002 and 0.006 +/- 0.001 for water as starting fluid, and increased to 0.47 +/- 0.08 and 0.23 +/- 0.03 for 3.56 m (NaCl+KCl)+0.04 in HCl at pressures of 224 to 360 MPa. In the experiments with H2O as starting fluid, the partition coefficients increased with pressure, i.e. D- Rb(f/m) from 0.01 +/- 0.002 to 0.22 +/- 0.02 and D-Sr(f/m) from 0.006 0.001 to 0.02 +/- 0.005 with a change in pressure from 360 to 700 MPa. At pressures to 360 MPa, the Rb/Sr ratio in the fluid was found to be independent of the initial salt concentration (Rb/Sr = 1.45 +/- 0.6). This ratio increased to 7.89 +/- 1.95 at 700 MPa in experiments with chloride free fluids, which indicates different changes in the Rb and Sr speciation with pressure.
The iron speciation in hydrous haplotonalitic and haplogranitic silicate glasses was studied using XAFS spectroscopy and transmission electron microscopy (TEM). Spectral features occurring at the main crest of the XANES at the iron K-edge of hydrous glasses indicate contributions to the spectra by iron-moieties present in a more ordered structural environment than found in the dry glass. These differences are also suggested by analysis of the EXAFS. These effects are not completely suppressed even for those samples that were quenched with a higher cooling rate. Strongest differences to the dry glass are observed for a sample that was quenched slowly through the temperature of glass transformation. Crystals (60 to 1500 nm in size) of magnetite, maghemite and another unidentified phase were observed in this sample by TEM, whereas no crystals were found in samples quenched with regular or high cooling rates. In-situ XANES measurements up to 700 degrees C and 500 MPa were performed to reveal the origin (i.e., during synthesis or quench) of the structural differences for those hydrous glasses that do not display any detectable crystallization. The comparison of XANES spectra collected on Fe2+ in water-saturated haplogranitic melt at 700 degrees C and 500 MPa and on Fe2+ in dry melt at 1150 degrees C shows that the local structural environment of Fe2+ in both systems is similar. This indicates that there is no detectable and direct influence of water on the local structure around iron in this type of melt. Hence, the differences observed between hydrous and dry glasses can only be related to artefacts formed during the quench process. (c) 2006 Elsevier B.V. All rights reserved