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Water Flux (water + flux)

Distribution by Scientific Domains

Engineering	35%
Life Sciences	25%
Chemistry	21%
Polymers and Materials Science	9%
Earth and Environmental Science	3%

Selected Abstracts

Water fluxes at a fluctuating water table and groundwater contributions to wheat water use in the lower Yellow River flood plain, China

HYDROLOGICAL PROCESSES, Issue 6 2007
Jianfeng Yang
Abstract Capillary upflow from and deep percolation to a water table may be important in crop water supply in irrigated areas of the lower Yellow River flood plain, north China. These fluxes at the water table and the variations of the capillary upflow in relation to crop evapotranspiration need to be investigated to quantify the effect of a water table on soil water balance and to improve agricultural water management. A large weighing lysimeter was used to determine daily crop evapotranspiration, daily capillary upflow from and daily percolation to a fluctuating water table during a rotation period with wheat growing in a dry season and maize in a rainy season. The water table depth varied in the range 0·7,2·3 m during the maize growth period and 1·6,2·4 m during the wheat growth period. Experimental results showed that the capillary upflow and the percolation were significant components of the soil water balance. Three distinctly different phases for the water fluxes at the water table were observed through the rotation period: water downward period, the period of no or small water fluxes, and water upward period. It implied that the temporal pattern of these water fluxes at the water table was intimately associated with the temporal distribution of rainfall through the rotation period. An empirical equation was determined to estimate the capillary upflow in relation to wheat evapotranspiration and root zone soil water content for local irrigation scheduling. Coupled with the FAO-Penman,Monteith equation, the equation offers a fast and low cost solution to assess the effect of capillary upflow from a water table on wheat water use. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Ericaceous shrubs on abandoned block-cut peatlands: implications for soil water availability and Sphagnum restoration

ECOHYDROLOGY, Issue 4 2009
Kegan K Farrick
Abstract Following harvesting by manual block-cut methods and subsequent abandonment, Cacouna bog has undergone a natural vegetation succession, with ericaceous shrubs covering more than 90% of the surface. The abundance of shrubs plays a major role in the soil water flux and availability at the site, impacting Sphagnum regeneration. From June 1 to August 22, 2007, field measurements indicate that transpiration represented the largest water loss from the shrubs at 1·7 mm day,1, comprising 142 mm (42%) of rainfall, compared to 93 mm of evaporation (28%) from bare soil. The rainfall interception from the canopy (62 mm) and litter (15 mm) accounted for 23% of seasonal rainfall. Thus after transpiration and interception losses are accounted for, only 115 mm of the 334 mm of rain (34%) remained available for other processes (recharge/soil evaporation). In the field, the litter layer prevented 17 mm from being lost over the summer as it reduced evaporation by 18%. Laboratory experiments using intact soil monoliths with and without shrubs and litter indicate that at depths below 10 cm the water content from the shrub monoliths decreased 27% versus 20% in the bare peat monoliths because of root water uptake. As a management prescription, raising the water table within 20 cm of the surface would provide water to the most active root uptake zones, reducing the need for extraction from the upper 10 cm of the peat. At this level sufficient water can be supplied to the surface through capillary rise, providing adequate water for the reestablishment/survival of Sphagnum. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Analysis of Direct Methanol Fuel Cell (DMFC)-Performance via FTIR Spectroscopy of Cathode Exhaust

FUEL CELLS, Issue 4 2003
F. Meier
Abstract Water and methanol flux through NafionÔ and polyaryl-blend membranes prepared at ICVT were studied under DMFC operation. The water, methanol, and CO2 content in the cathode exhaust were measured by FTIR spectroscopy. Both the water and methanol flux turned out to be strongly dependent on the operating temperature and thus on membrane swelling. Apart from this, water flux through the membrane is primarily affected by the gas volume flux on the cathode side. A coupling between water flux and methanol flux was observed, which leads to the conclusion that methanol is transported both by diffusion and by convection caused by the superimposed water flux. Polyaryl-blend membranes showed a reduced diffusive methanol transport when compared to NafionÔ due to their different internal microstructure. The impact of methanol cross-over on cathode losses at high current density needs further clarification with respect to the prevailing mechanism of methanol oxidation at the cathode. [source]

C3,C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide

FUNCTIONAL ECOLOGY, Issue 1 2007
H. W. POLLEY
Summary 1Plants usually respond to carbon dioxide (CO2) enrichment by increasing photosynthesis and reducing transpiration, but these initial responses to CO2 may not be sustained. 2During May, July and October 2000, we measured the effects of temporarily increasing or decreasing CO2 concentration by 150,200 µmol mol,1 on daytime net ecosystem CO2 exchange (NEE) and water flux (evapotranspiration, ET) of C3,C4 grassland in central Texas, USA that had been exposed for three growing seasons to a CO2 gradient from 200 to 560 µmol mol,1. Grassland grown at subambient CO2 (< 365 µmol mol,1) was exposed for 2 days to an elevated CO2 gradient (> 365 µmol mol,1). Grassland grown at elevated CO2 was exposed for 2 days to a subambient gradient. Our objective was to determine whether growth CO2 affected the amount by which grassland NEE and ET responded to CO2 switching (sensitivity to CO2). 3The NEE per unit of leaf area was greater (16,20%) and ET was smaller (9,20%), on average, at the higher CO2 concentration during CO2 switching in May and July. The amount by which NEE increased at the higher CO2 level was smaller at elevated than subambient growth concentrations on both dates, but relationships between NEE response and growth CO2 were weak. Conversely, the effect of temporary CO2 change on ET did not depend on growth CO2. 4The ratio of NEE at high CO2 to NEE at low CO2 during CO2 change in July increased from 1·0 to 1·26 as the contribution of C3 cover to total cover increased from 26% to 96%. Conversely, in May, temporary CO2 enrichment reduced ET more in C4 - than C3 -dominated grassland. 5For this mesic grassland, sensitivity of NEE and ET to brief change in CO2 depended as much on the C3,C4 composition of vegetation as on physiological adjustments related to prior CO2 exposure. [source]

Self-Supporting, Double Stimuli-Responsive Porous Membranes From Polystyrene- block -poly(N,N -dimethylaminoethyl methacrylate) Diblock Copolymers

ADVANCED FUNCTIONAL MATERIALS, Issue 7 2009
Felix Schacher
Abstract Asymmetric membranes are prepared via the non-solvent-induced phase separation (NIPS) process from a polystyrene- block -poly(N,N -dimethylaminoethyl methacrylate) (PS- b -PDMAEMA) block copolymer. The polymer is prepared via sequential living anionic polymerization. Membrane surface and volume structures are characterized by scanning electron microscopy. Due to their asymmetric character, resulting in a thin separation layer with pores below 100,nm on top and a macroporous volume structure, the membranes are self-supporting. Furthermore, they exhibit a defect-free surface over several 100,µm2. Polystyrene serves as the membrane matrix, whereas the pH- and temperature-sensitive minority block, PDMAEMA, renders the material double stimuli-responsive. Therefore, in terms of water flux, the membranes are able to react on two independently applicable stimuli, pH and temperature. Compared to the conditions where the lowest water flux is obtained, low temperature and pH, activation of both triggers results in a seven-fold permeability increase. The pore size distribution and the separation properties of the obtained membranes were tested through the pH-dependent filtration of silica particles with sizes of 12,100,nm. [source]

Deficiency of electroneutral K+,Cl, cotransporter 3 causes a disruption in impulse propagation along peripheral nerves

GLIA, Issue 13 2010
Yuan-Ting Sun
Abstract Nerve conduction requires the fine tuning of ionic currents through delicate interactions between axons and Schwann cells. The K+,Cl, cotransporter (KCC) family includes four isoforms (KCC1,4) that play an important role in the maintenance of cellular osmotic homeostasis via the coupled electroneutral movement of K+ and Cl, with concurrent water flux. Mutation in SLC12A6 gene encoding KCC3 results in an autosomal recessive disease, known as agenesis of the corpus callosum associated with peripheral neuropathy. Nevertheless, the role of KCC3 in nerve function remains a puzzle. In this study, the microscopic examination of KCC isoforms expressed in peripheral nerves showed high expression of KCC2,4 in nodal segments of the axons and in the perinucleus and microvilli of Schwann cells. The KCC inhibitor [[(dihydroindenyl)oxy]alkanoic acid] but not the Na+,K+,2Cl, -cotransport inhibitor (bumetanide) dose-dependently suppressed the amplitude and area of compound muscle action potential, indicating the involvement of KCC activity in peripheral nerve conduction. Furthermore, the amplitude and area under the curve were smaller, and the nerve conduction velocity was slower in nerves from KCC3,/, mice than in nerves from wild-type mice, while the expression pattern of KCC2 and KCC4 was similar in KCC3 kockout and wild-type strains. KCC3,/, mice also manifested a prominent motor deficit in the beam-walking test. This is the first study to demonstrate that the K+,Cl, cotransporter activity of KCC3 contributes to the propagation of action potentials along peripheral nerves. © 2010 Wiley-Liss, Inc. [source]

Transient Leakance and Infiltration Characteristics during Lake Bank Filtration

GROUND WATER, Issue 1 2009
B. Wiese
Infiltration capacity of bank filtration systems depends on water extraction and hydraulic resistance of the bed sediments. Lakebed hydraulics may be especially affected by clogging, which is dependent on settlement of fine particles, redox potential, and other factors. In the field, most of these processes are difficult to quantify, and thus, when calculating response to pumping the water flux across the sediment surface is assumed to be linearly dependent on the hydraulic gradient. However, this assumption was not adequate to describe conditions at a bank filtration site located at Lake Tegel, Berlin, Germany. Hence, we first assumed the leakage coefficient (or leakance) is spatially distributed and also temporally variant. Furthermore, observations show that the leakance is considerably higher in shallow than in deeper areas; hence, leakance was assumed to be dependent on the existence and thickness of an unsaturated zone below the lake. The proposed explanation of spatial and temporal variability in leakance involves a hypothesis for redox dependent and reversible biogeochemical clogging, supported by geochemical observations in surface water and ground water. Four leakance approaches are implemented in the ground water flow code MODFLOW2000 and calibrated by inverse modeling using the parameter estimation software PEST. These concepts are evaluated by examining the fit to the hydraulic heads, to infiltration measurements, transport modeling results, and considering the degrees of freedom due to the number of calibration parameters. The leakage concept based on the assumption of the influence of an unsaturated zone on clogging processes best explains the field data. [source]

Utilization of Decadal Tritium Variation for Assessing the Residence Time of Base Flow

GROUND WATER, Issue 3 2007
S. Rose
An iterative algorithm is presented that allows the user to model the subsurface residence time of shallow ground water comprising stream base flow based on decadal scale variation of tritium concentrations. The algorithm accounts for the effects of radioactive decay, the shallow subsurface mixing of ground water with precipitation, and ground water flux. The inverse of the best-fitting modeled flux through the saturated zone is equivalent to the residence time. The data required for this model include at least two measurements of tritium in base flow for a given stream location made at least a decade apart and the long-term tritium input in precipitation for the region of interest. The model is sensitive to relatively small changes in tritium concentrations and is limited by analytic uncertainties to an accuracy of approximately ±5 years. The algorithm was applied to stream base flow for several basins in the Piedmont Province of Georgia in which tritium concentrations were measured during the early 1990s and again in the 2000s. The model results produced highly concordant residence times for three hydrogeologically similar basins in the Upper Ocmulgee Basin in North Central Georgia. The best estimate of the average residence time for ground water comprising base flow in this Piedmont basin using this new method is between approximately 14 and 18 years. These results are generally consistent with calculations made in previous studies, and these relatively long residence times can be attributed to the storage of water in the clay soils that dominate Piedmont Province watersheds. [source]

Estimating Ground Water Recharge from Topography, Hydrogeology, and Land Cover

GROUND WATER, Issue 1 2005
Douglas S. Cherkauer
Proper management of ground water resources requires knowledge of the rates and spatial distribution of recharge to aquifers. This information is needed at scales ranging from that of individual communities to regional. This paper presents a methodology to calculate recharge from readily available ground surface information without long-term monitoring. The method is viewed as providing a reasonable, but conservative, first approximation of recharge, which can then be fine-tuned with other methods as time permits. Stream baseflow was measured as a surrogate for recharge in small watersheds in southeastern Wisconsin. It is equated to recharge (R) and then normalized to observed annual precipitation (P). Regression analysis was constrained by requiring that the independent and dependent variables be dimensionally consistent. It shows that R/P is controlled by three dimensionless ratios: (1) infiltrating to overland water flux, (2) vertical to lateral distance water must travel, and (3) percentage of land cover in the natural state. The individual watershed properties that comprise these ratios are now commonly available in GIS data bases. The empirical relationship for predicting R/P developed for the study watersheds is shown to be statistically viable and is then tested outside the study area and against other methods of calculating recharge. The method produces values that agree with baseflow separation from streamflow hydrographs (to within 15% to 20%), ground water budget analysis (4%), well hydrograph analysis (12%), and a distributed-parameter watershed model calibrated to total streamflow (18%). It has also reproduced the temporal variation over 5 yr observed at a well site with an average error < 12%. [source]

Variability and Comparison of Hyporheic Water Temperatures and Seepage Fluxes in a Small Atlantic Salmon Stream,

GROUND WATER, Issue 1 2003
Matthew D. Alexander
Ground water discharge is often a significant factor in the quality of fish spawning and rearing habitat and for highly biologically productive streams. In the present study, water temperatures (stream and hyporheic) and seepage fluxes were used to characterize shallow ground water discharge and recharge within the streambed of Catamaran Brook, a small Atlantic salmon (Salmo salar) stream in central New Brunswick, Canada. Three study sites were instrumented using a total of 10 temperature sensors and 18 seepage meters. Highly variable mean seepage fluxes, ranging from 1.7 × 10,4 to 2.5 cm3 m,2 sec,1, and mean hyporheic water temperatures, ranging from 10.5° to 18.0°C, at depths of 20 to 30 cm in the streambed were dependent on streambed location (left versus right stream bank and site location) and time during the summer sampling season. Temperature data were useful for determining if an area of the streambed was under discharge (positive flux), recharge (negative flux), or parallel flow (no flux) conditions and seepage meters were used to directly measure the quantity of water flux. Hyporheic water temperature measurements and specific conductance measurements of the seepage meter sample water, mean values ranging from 68.8 to 157.9 ,S/cm, provided additional data for determining flux sources. Three stream banks were consistently under discharge conditions, while the other three stream banks showed reversal from discharge to recharge conditions over the sampling season. Results indicate that the majority of the water collected in the seepage meters was composed of surface water. The data obtained suggests that even though a positive seepage flux is often interpreted as ground water discharge, this discharging water may be of stream water origin that has recently entered the hyporheic zone. The measurement of seepage flux in conjunction with hyporheic water temperature or other indicators of water origin should be considered when attempting to quantify the magnitude of exchange and the source of hyporheic water. [source]

Field and laboratory estimates of pore size properties and hydraulic characteristics for subarctic organic soils

HYDROLOGICAL PROCESSES, Issue 19 2007
Sean K. Carey
Abstract Characterizing active and water-conducting porosity in organic soils in both saturated and unsaturated zones is required for models of water and solute transport. There is a limitation, largely due to lack of data, on the hydraulic properties of unsaturated organic soils in permafrost regions, and in particular, the relationship between hydraulic conductivity and pressure head. Additionally, there is uncertainty as to what fraction of the matrix and what pores conduct water at different pressure heads, as closed and dead-end pores are common features in organic soil. The objectives of this study were to determine the water-conducting porosity of organic soils for different pore radii ranges using the method proposed by Bodhinayake et al. (2004) [Soil Sci. Soc. Am. J. 68:760,769] and compare these values to active pore size distributions from resin-impregnated laboratory thin sections and pressure plate analysis. Field experiments and soil samples were completed in the Wolf Creek Research Basin, Yukon. Water infiltration rates were measured 16 times using a tension infiltrometer (TI) at 5 different pressure heads from , 150 to 0 mm. This data was combined with Gardiner's (1958) exponential unsaturated hydraulic conductivity function to provide water-conducting porosity for different pore-size ranges. Total water-conducting porosity was 1·1 × 10,4, which accounted for only 0·01% of the total soil volume. Active pore areas obtained from 2-D image analysis ranged from 0·45 to 0·60, declining with depth. Macropores accounted for approximately 65% of the water flux at saturation, yet all methods suggest macropores account for only a small fraction of the total porosity. Results among the methods are highly equivocal, and more research is required to reconcile field and laboratory methods of pore and hydraulic characteristics. However, this information is of significant value as organic soils in permafrost regions are poorly characterized in the literature. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Improvement of the hydrological component of an urban soil,vegetation,atmosphere,transfer model

HYDROLOGICAL PROCESSES, Issue 16 2007
A. Lemonsu
Abstract A numerical study was conducted on the Rezé suburban catchment (Nantes, France) to evaluate the hydrological component of the town energy balance (TEB) scheme, which simulates in a coupled way the water and energy balances for the urban covers. The catchment is a residential area where hydrological data were continuously collected from 1993 to 1998 by the Laboratoire Central des Ponts et Chaussées (LCPC), notably the runoff in the stormwater drainage network. A 6-year simulation with the TEB and interaction soil,biosphere,atmosphere (ISBA) schemes in off-line mode enabled the comparison of modelled and observed runoff. Some weaknesses of the TEB were uncovered and led to improved parameterization of water exchanges: (1) calibration of the maximum capacity of the rainfall interception reservoir on roads and roofs and (2) inclusion of water infiltration through the roads, according to a simple formulation. The calibration of this water flux gives results that are consistent with direct measurements of water infiltration performed on the Rezé site and from the literature. The new parameterization produces better runoff in terms of timing and magnitude, which are comparable to those obtained by the LCPC with other hydrological models. It shows also the impact of the water infiltration through the roads, corresponding to a water transfer from the TEB to ISBA, on the water balance: the water contents of road, roof and soil reservoirs being modified, the evaporation from artificial surfaces decreases, while the evapotranspiration from natural covers increases. Through the evaporative flux, such a modification of the water balance induces large repercussions on the surface energy balance. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Moss beneath a leafless larch canopy: influence on water and energy balances in the southern mountainous taiga of eastern Siberia

HYDROLOGICAL PROCESSES, Issue 15 2007
Kazuyoshi Suzuki
Abstract The southern mountainous taiga of eastern Siberia has a sparse larch canopy and an understory dominated by a thick moss layer. The physiology of moss is very different from that of other plants, as mosses lack roots and vascular systems and take up water directly. During May 2002, we conducted hydrological and meteorological measurements in the taiga of eastern Siberia to investigate the role of understory moss on water and energy balances within a leafless larch forest. We found that below-leafless canopy net all-wave radiation partitions into 39% latent heat flux and 39% sensible heat flux, while the mean daily Bowen ratio is about 1. Ground heat flux on the moss surface is also an important factor, as it comprises 22% of net all-wave radiation. Evaporation from moss beneath the leafless canopy was 24 mm during the 1-month observation period, representing 23% of the water flux into the larch forest. This finding implies that moss intercepted 23% of the water flux into the larch forest. In addition, evaporation from the moss understory during May 2002 comprised 22% of total evapotranspiration previously estimated above the canopy (April,October 2001). We conclude that moss is an important component of the water and energy balance in larch forests in the taiga region. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

HYDROLOGICAL PROCESSES, Issue 13 2006
F. Holwerda
Abstract The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain-free conditions (0·11 ± 0·05 mm h,1) and rainy conditions (0·24 ± 0·13 mm h,1) were about three to six times the eddy-covariance-based estimate (0·04 ± 0·002 mm h,1). Under rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ,40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ,770 mm year,1 for the summit cloud forest just below the ridge top (according to the water budget method) and ,785 mm year,1 for the cloud forest on the lower windward slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence). Copyright © 2006 John Wiley & Sons, Ltd. [source]

Transport mechanisms and performance simulation of a PEM fuel cell

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 6 2008
Geng-Po Ren
Abstract A three-dimensional, gas,liquid two-phase flow and transport model has been developed and utilized to simulate the multi-dimensional, multi-phase flow and transport phenomena in both the anode and cathode sides in a proton exchange membrane (PEM) fuel cell and the cell performance with different influencing operational and geometric parameters. The simulations are presented with an emphasis on the physical insight and fundamental understanding afforded by the detailed distributions of velocity vector, oxygen concentration, water vapor concentration, liquid water concentration, water content in the PEM, net water flux per proton flux, local current density, and overpotential. Cell performances with different influencing factors are also presented and discussed. The comparison of the model prediction and experimental data shows a good agreement. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Effect of the preparation conditions on the permeation of ultrahigh-molecular-weight polyethylene/silicon dioxide hybrid membranes

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010
Nana Li
Abstract Porous ultrahigh-molecular-weight polyethylene/SiO2 membranes were prepared by thermally induced phase separation (TIPS) with white mineral oil as the diluent and SiO2 as an additive. Influential factors, including extraction method, SiO2 content, and cooling rate, were investigated. The results suggest that the both porosity and pure water flux of the membranes by extraction of the solvent naphtha in the tension state with alcohol were the best among our research. With increasing SiO2 content, the porosity, pure water flux, and pore diameter increased. However, with excessive SiO2 content, defects formed easily. Moreover, SiO2 improved the pressure resistance of the membranes. The cooling rate directly effected the crystal structure. A slow cooling rate was good for crystal growth and the integration of the diluent. Therefore, the porosity, pure water flux, and bubble-point pore diameter increased with decreasing cooling rate. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

Effect of molecular structure of hybrid precursors on the performances of novel hybrid zwitterionic membranes

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007
Junsheng Liu
Abstract Three types of novel hybrid zwitterionic membranes were prepared via a coupling reaction between two silane-coupling agents in a nonaqueous system and a subsequent reaction with 1,4-butyrolactone to create ion pairs in the hybrid precursors. FT-IR spectra corroborated the corresponding reactions. The synthesized membranes were characterized by thermal analyses, ion-exchange capacities, streaming potentials, and pure water flux. Thermal analyses exhibited that the degradation temperature of the hybrid precursors decreased with an increase in zwitterionic extent because of the introduction of ion pairs. Ion-exchange capacity measurements revealed that the anion-exchange capacities and cation-exchange capacities were in the range of 0.023,0.05 and 0.32,0.58 mmol g,1, respectively. Streaming potentials displayed that when the membranes coated for one or three times, the isoelectric points were in the pH range of 6.6,7.58 and 6.58,7.7, respectively. The pure water flux showed that it could be affected by the coating times and the ingredients of these zwitterionic membranes. This difference in membrane's characteristics can be ascribed to the effect of molecular structure of the hybrid precursors. Both the Coulombic interactions of ion pairs between the polymer chains and the hydrophilicity of these membranes were proposed to clarify the above phenomena. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 [source]

Pervaporation of tertiary butanol/water mixtures through chitosan membranes cross-linked with toluylene diisocyanate,

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 12 2005
Smitha Biduru
Abstract Membranes made from 84% deacetylated chitosan biopolymer were cross-linked by a novel method using 2,4-toluylene diisocyanate (TDI) and tested for the separation of t -butanol/water mixtures by pervaporation. The unmodified and cross-linked membranes were characterized by Fourier transform infra red (FTIR) spectroscopy, X-ray diffraction (XRD) studies and sorption studies in order to understand the polymer,liquid interactions and separation mechanisms. Thermal stability was analyzed by differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) while tensile strength measurement was carried out to assess mechanical strength. The membrane appears to have good potential for breaking the aqueous azeotrope of 88.2 wt% t -butanol by giving a high selectivity of 620 and substantial water flux (0.38 kg m,2 hr,1). The effects of operating parameters such as feed composition, membrane thickness and permeate pressure on membrane performance were evaluated. Copyright © 2005 Society of Chemical Industry [source]

Theoretical studies on structural and electrical properties of PES/SPEEK blend nanofiltration membrane

AICHE JOURNAL, Issue 8 2009
A. F. Ismail
Abstract Polyethersulfone (PES) nanofiltration membranes were prepared using a simple dry-jet wet spinning technique with different contents of sulfonated poly(ether ether ketone) (SPEEK) ranging from 0 to 4 wt %. The structural parameters (rp and Ak/,x) and electrostatic properties (, and X) of the blend membranes were deduced by employing the combination of irreversible thermodynamic model, steric hindrance pore (SHP) model, and Teorell-Meyer-Sievers (TMS) model. The modeling results obtained have been analyzed and discussed. The mean pore radius and pore size distribution of the blends were also determined based on the theoretical models. The results showed that pore radius increased with increasing the concentration of SPEEK from 0 to 2 wt % but decreased with a further increase in SPEEK content. The water flux, however, showed a systematically increase with increasing SPEEK content. The SPEEK also showed significant effect on membrane electrical properties. Both effective charge density and ratio of effective charge density to electrolyte solution increased with increasing concentration of SPEEK in the dope solution, reaching a value of ,21.02 and ,2.29, respectively. The pore radius which was determined by using different transport models has also been analyzed and discussed. It is found that the addition of SPEEK into dope solution is one of the paramount parameters in developing the negatively charged nanofiltration membrane with enhanced water flux while retaining the pore radius in the nanometer range. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

The influence of temporal cake moisture content on a discontinuous washing process in the centrifugal field

AICHE JOURNAL, Issue 3 2009
Franky Ruslim
Abstract In solid/liquid separation processes, filter cake washing is an essential step in improving the quality of particulate products by elimination of impurities. During cake washing and dewatering, the cake saturation changes depending on the flow conditions and it cannot always be measured and controlled accurately. This article deals with investigations on the influence of the initial and temporal cake saturation on washing PVC and silica sand particles in the centrifugal field. It was found, that high initial saturation levels and high maximum saturations during the washing process had a positive impact in inducing a high hydrostatic pressure for advective flow and enabling a homogeneous distribution of the wash water inside the cake. This was achieved by increasing the wash water flux and/or decreasing the g-factor. A good method to obtain low final impurity quantity is the combination of washing at a low g-factor and dewatering at a high one. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Modeling water flux in forward osmosis: Implications for improved membrane design

AICHE JOURNAL, Issue 7 2007
Jeffrey R. Mccutcheon
Abstract Osmotically-driven membrane processes, such as forward osmosis and pressure retarded osmosis, operate on the principle of osmotic transport of water across a semipermeable membrane from a dilute feed solution into a concentrated draw solution. The major hindrance to permeate water flux performance is the prevalence of concentration polarization on both sides of the membrane. This article evaluates the external and internal boundary layers, which decrease the effective osmotic driving force. By modeling permeate flux performance, the role that feed and draw concentrations, membrane orientation, and membrane structural properties play in overall permeate flux performance are elucidated and linked to prevalence of external and internal concentration polarization. External concentration polarization is found to play a significant role in the reduction of driving force, though internal concentration polarization has a far more pronounced effect for the chosen system conditions. Reduction of internal concentration polarization by way of membrane modification was found to improve the predicted flux performance significantly, suggesting that alteration of membrane design will lead to improved performance of osmotically driven membrane processes. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source]

Investigation of nail permeation enhancement by chemical modification using water as a probe

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2002
Gouri G. Malhotra
Abstract Our objective was to screen molecules that could interact with keratin in the human nail and thereby improve the topical penetration of actives into and through the nail plate. We used specialized Franz-type diffusion cells for our permeation experiments and water as a marker molecule. Aqueous/hydroalcoholic gels containing the enhancers were spiked with tritiated water and compared with a control (without enhancer). We computed the normalized water flux (defined as a product of flux and nail thickness) for each gel. We defined an enhancement factor for water as the ratio of the normalized water flux from a gel containing enhancer to that of the control. Our results indicate that the chemical structure of the modifier is most important in determining its ability to enhance penetration. The best enhancement effect was obtained using N-(2-mercaptopropionyl) glycine, a mercaptan derivative of an amino acid, in combination with urea. The concentration of each chemical modifier was linearly related to normalized water flux and mercaptan levels were more important that urea levels in penetration enhancement. Barrier integrity of nails was compromised after treatment with effective chemical modifiers. Thus, we have developed a suitable technique to screen nail penetration enhancers using water as a probe. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:312,323, 2002 [source]

Attempt of a flux-based evaluation of field capacity

JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 3 2008
Steffen Zacharias
Abstract Field capacity is one of the most frequently used soil parameters. According to the original definition, the field capacity is a site-specific parameter, which is closely connected to soil water flux. Many of the existing procedures for field-capacity estimation neglect its flux-based character. In this study, a method and selected results are presented. This allows a better reflection of the unsteady behavior of unsaturated water flow in estimating field capacity. Based on the velocity of the relative internal drainage, a threshold value is defined (field-capacity threshold) which enables a flux-based estimation of the field-capacity value. Furthermore, the direct consideration of the drainage time allows the soil water contents to be estimated for defined internal drainage durations. [source]

Brain edema in liver failure: Basic physiologic principles and management

LIVER TRANSPLANTATION, Issue 11 2002
Fin Stolze Larsen MD
In patients with severe liver failure, brain edema is a frequent and serious complication that may result in high intracranial pressure and brain damage. This short article focuses on basic physiologic principles that determine water flux across the blood-brain barrier. Using the Starling equation, it is evident that both the osmotic and hydrostatic pressure gradients are imbalanced across the blood-brain barrier in patients with acute liver failure. This combination will tend to favor cerebral capillary water influx to the brain. In contrast, the disequilibration of the Starling forces seems to be less pronounced in patients with cirrhosis because the regulation of cerebral blood flow is preserved and the arterial ammonia concentration is lower compared with that of patients with acute liver failure. Treatments that are known to reverse high intracranial pressure tend to decrease the osmotic pressure gradients across the blood-brain barrier. Recent studies indicate that interventions that restrict cerebral blood flow, such as hyperventilation, hypothermia, and indomethacin, are also efficient in preventing edema and high intracranial pressure, probably by decreasing the transcapillary hydrostatic pressure gradient. In our opinion, it is important to recall that rational fluid therapy, adequate ventilation, and temperature control are of direct importance to controlling cerebral capillary water flux in patients with acute liver failure. These simple interventions should be secured before more advanced experimental technologies are instituted to treat these patients. [source]

Coconut water as a potential resource for cellulose acetate membrane preparation

POLYMER INTERNATIONAL, Issue 3 2008
Cynthia Radiman
Abstract BACKGROUND: Cellulose acetate membranes are frequently used for pressure-driven membrane processes. The aim of this work was to prepare cellulose acetate membranes from nata-de-coco using coconut water as starting material. The use of this lignin-free material will certainly minimize the use of chemicals usually needed in the traditional pulps and substitute for the use of wood, which helps prevent global warming and preserves nature as well. RESULTS: Coconut water was fermented by Acetobacter xylinum for 6 days to produce nata-de-coco, which was then acetylated to produce cellulose diacetate with an acetyl content of 39.6%. Fourier transform infrared analysis showed characteristic peaks for the acetyl group at 1748 and 1236 cm,1. The resulting membranes made from the hydrolysis product showed a water flux of 210.5 L m,2 h,1 under an applied pressure of 2 kg cm,2 while the rejection coefficients of dextran T-500 and T-2000 solutions were 78 and 93.7%, respectively. CONCLUSION: Coconut water has a potential to be used in the fabrication of membranes by converting it to nata-de-coco and then to cellulose diacetate which gives an added value to its original nature. It is also highly competitive compared to the traditional pulps, by which acetylation decreases the degree of crystallinity of nata-de-coco resulting in higher membrane permeability. Copyright © 2007 Society of Chemical Industry [source]

Studies on cellulose acetate,carboxylated polysulfone blend ultrafiltration membranes,Part II

POLYMER INTERNATIONAL, Issue 1 2003
J Sajitha
Abstract Hydrophilic polysulfone ultrafiltration membranes were prepared from blends of cellulose acetate and carboxylated polysulfones of 0.43 and 0.75 degrees of carboxylation. The effects of degree of carboxylation on membrane characteristics such as compaction, pure water flux, water content and membrane hydraulic resistance, have been investigated to evaluate the performance of the membranes. The influence of the degree of carboxylation on the performance of the blend membranes of various blend polymer compositions has been investigated and also compared with earlier reports on blend membranes prepared from cellulose acetate and polysulfone or carboxylated polysulfone of 0.14 degree of carboxylation. © 2003 Society of Chemical Industry [source]

A heuristic approach of calculating spray water flux needed to avert fire-induced runaway reactions,,

PROCESS SAFETY PROGRESS, Issue 3 2006
BChE (Honors), Dilip K. Das BSc (Honors), MSChE
In general all reactions have some heat effects. When the ability of the equipment to remove the heat is exceeded by the heat generated by a reaction, a hazardous situation called a runaway reaction may take place. Sometimes the exothermicity of runaway reactions is so high that the size of an emergency vent becomes impractical to install. A water spray system can sometimes be used to avert a fire-induced runaway reaction. Because the water spray system has a finite activation time, insulation helps to prolong the time required to reach the decomposition temperature. This article concludes that the required water flux to avert the fire-induced runaway reaction may be conservatively estimated by adding the water flux necessary to maintain an unbroken water film on the external surface of the equipment and the water flux necessary to absorb the fire heat after allowing for the splash loss and the in-flight loss. When adequate spray water is used, the metal temperature of the insulation jacket cannot theoretically exceed the boiling point of water thereby ensuring the avoidance of fire-induced runaway reactions whose adjusted onset decomposition temperature exceeds 100 ° C. Fire-induced runaway reactions with lower onset temperature can also be avoided depending on the initial temperature of the contents, mass of the contents and equipment, insulation thickness, and fire duration, for example, but a detailed calculation including dynamic simulation is necessary and the burden of proof lies with the designer. The reliability of the spray water system must be maintained high to include its credit as an environmental factor defined according to NFPA 30 to avoid the fire-induced runaway reaction as a scenario. Although API RP 521 does not allow any credit for sprinkler water, it allows credit, unlike NFPA 30, for insulation thickness and thus a runaway reaction can be avoided by using insulation alone according to API RP 521. © 2006 American Institute of Chemical Engineers Process Saf Prog, 2006 [source]

Accumulation of polycyclic aromatic hydrocarbons in rural soils based on mass balances at the catchment scale

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2007
Tilman Gocht
Abstract Polycyclic aromatic hydrocarbons (PAHs) are hydrophobic organic pollutants that are ubiquitously distributed in the environment at relatively high concentrations. In our study we investigated the long-term fate of atmospheric PAHs in soils of rural areas, resulting from diffuse pollution based on mass balances at the catchment scale. By determining PAHs in several environmental compartments, estimates of soil storages and water fluxes were made and compared with atmospheric deposition. The results indicate that more than 90% of the incoming PAHs remain in the catchments and accumulate in the topsoils. Furthermore, revolatilization of PAHs from soils and degradation in the soils is very limited, resulting in ongoing accumulation in topsoils, in particular for low-volatile PAHs. Combustion-derived carbonaceous particles were detected in atmospheric deposition as well as in the soil samples. Since these particles are very strong adsorbents, they are suspected to play a key role in the environmental fate of the diffuse distributed PAHs. [source]

Below-ground hydraulic conductance is a function of environmental conditions and tree size in Scots pine

FUNCTIONAL ECOLOGY, Issue 6 2007
J. MARTÍNEZ-VILALTA
Summary 1Variations in water tension in a transpiring tree cause elastic changes in stem diameter. To better understand the dynamics of these variations, stem diameter changes and sap flow rates were monitored simultaneously in trees from two Scots pine chronosequences in Scotland. 2Tree below-ground hydraulic conductance (kbg) was estimated from the relationship between leaf-specific sap flow rates and the difference between stem and soil water potentials estimated from diameter variations in the stem. 3In a given tree, kbg varied both within and among days, with conductance increasing as a function of sap flow and evaporative demand. These patterns could be explained in terms of a composite model of root water transport and possible changes in the gating of aquaporins. 4We interpreted these trends of increasing kbg with evaporative demand as a mechanism to enhance the ability of trees to control leaf water potential and keep it within physiologically acceptable limits, with potential implications for our general understanding of plant water relations, and for the estimation and modelling of ecosystem water fluxes. 5Across trees, kbg declined with increasing tree age/size, but the proportional contribution of below-ground to whole-tree hydraulic resistance also declined. This is consistent with an increase in below-ground carbon allocation in old/tall trees and a partial acclimation of tall trees to hydraulic limitations. It is argued that these trends have to be considered when discussing the importance of tree height for water transport and growth. [source]