Pore water residence time

Finally, track-etched MF membranes are made from polymers, such as polycarbonate and polyester, wherein electrons are bombarded onto the polymeric surface. This bombardment results in sensitized tracks, where chemical bonds in the polymeric backbone are broken. Subsequently, the irradiated film is placed in an etching bath (such as a basic solution), in which the damaged polymer in the tracks is preferentially etched from the film, thereby forming cylindrical pores. The residence time in the irradiator determines pore density, and residence time in the etching bath determines pore size. Membranes made by this process generally have cylindrical pores with very narrow pore-size distribution, albeit with low overall porosity. Furthermore, there always is the risk of a double hit, i.e., the etched pore becomes wider and could result in particulate penetration. Such filter membranes are often used in the electronic industry to filter high-purity water. 

Figure 6. Dissolution and transport in the weathering zone (W is net weathering rate) C is concentration in discharge, Q is volume discharge, A is land surface area, h is thickness of reactive zone, is its porosity, and t is water residence time within the reactive zone space, where S is the reactive surface area of minerals. For a weathering rate consisting of contributions from individual minerals, R, is mineral dissolution rate, Sgi is mineral surface area per gram, and pt is mineral density. Depth of the reactive zone and volume of water in pore space (f> may be functions of the water discharge volume /ib/), v(r/).

Franklin (1991) found that both acetate and oxalate significantly increased albite solubility at temperatures, pressures, and pH values (4.6-4.7) that may exist under diagenetic conditions. For example, aluminum reached a steady-state concentration of 13 ppm in the acetate buffer solution, essentially identical to that observed by Reed (1990) in acetic acid at the same pH (4.7) and total acetate concentration. Dissolved aluminum was even higher in the acetate-oxalate solution (pHf = 4.6) reaching a steady-state concentration of 48 ppm (Fig. 2), whereas aluminum concentrations in distilled water and the C02-water solution were high concentrations of aluminum in the acetate-oxalate solution did not vary substantially with flow rate and were maintained for pore-fluid residence times of up to 3... 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

Schiesser and Lapidus (S3), in later studies, measured the liquid residencetime distribution for a column of 4-in. diameter and 4-ft height packed with spherical particles of varying porosity and nominal diameters of in. and in. The liquid medium was water, and as tracers sodium chloride or methyl orange were employed. The specific purposes of this study were to determine radial variations in liquid flow rate and to demonstrate how pore diffusivity and pore structure may be estimated and characterized on the basis of tracer experiments. Significant radial variations in flow rate were observed methods are discussed for separating the hydrodynamic and diffusional contributions to the residence-time curves. 

Adsorption Isotherms. The adsorption isotherms were determined using the serum-replacement adsorption or desorption methods (7). For the adsorption method, the latex samples (50 or 100 cm 2% solids) containing varying amounts of PVA were equilibrated for 36 hours at 25°C, placed in the serum replacement cell equipped with a Nuclepore membrane of the appropriate pore size, and pressurized to separate a small sample of the serum from the latex. For the desorption method, the latex samples (250 cm 2.5% solids) were equilibrated for 36 hours at 25°C and subjected to serum replacement with DDI water at a constant 9-10 cm /hour. The exit stream was monitored using a differential refractometer. The mean residence time of the feed stream was ca. 25 hours. It was assumed that equilibrium between the adsorbed and solute PVA was maintained throughout the serum replacement. For both methods, the PVA concentration was determined using a An-C calibration curve. 

The above behavior of narrow-pore supported cobalt catalysts toward co-fed water can also be explained in terms of relative size of cobalt clusters, pore network of support, expected location of cobalt clusters within the pore network, and relative differences in the residence time of water vapor within and outside the... 

The results described in the above sections were obtained in batch studies, which generally run for less than one day. Long-term effects of water quality on catalyst activity are better observed through continuous flow columns which can operate for months or years. These column experiments generally use packed bed reactors (as in the field) and provide better simulations of field conditions. However, because the Pd technology is relatively new, few column studies have been conducted thus far results of published studies are discussed in the following section in more detail. Note that for both the column and field studies, the most relevant parameters are residence time, conversion data and pore volumes treated. The residence... 

Because the application of Pd catalysts to the treatment of contaminated water is relatively new, only one major field study (at Lawrence Livermore National Laboratories) has been conducted and published thus far. (McNab et al. 2000) Other studies, such as that in Bitterfeld, Germany, are currently underway. The Bitterfeld site operates at a residence time of 15 minutes, with a flow of approximately 100 pore volumes/day and uses a zeolite-supported Pd catalyst, which was optimized in laboratory experiments. In the initial tests in the field, the catalyst was deactivated, apparently by sulfide-producing bacteria. Treating the column with 10 g/L of hydrogen peroxide for 2 hours each week (approximately 8 pore volumes of peroxide solution per 700 pore volumes of water treated) resulted in column operation for 15 weeks with 90-99% removal of chlorobenzene and without any apparent catalyst deactivation. (Weiss et al. 1999) As the Pd technology develops further, more field tests are expected. 

Selim et al. (1976b) developed a simplified two-site model to simulate sorption-desorption of reactive solutes applied to soil undergoing steady water flow. The sorption sites were assumed to support either instantaneous (equilibrium sites) or slow (kinetic sites) first-order reactions. As pore-water velocity increased, the residence time of the solute decreased and less time was allowed for kinetic sorption sites to interact (Selim et al., 1976b). The sorption-desorption process was dominated by the equilib-... 

Experiments conducted on the solubility behavior of the odlitic sediments indicated that they were not in metastable equilibrium with their pore waters. On the basis of equilibration rates, Bernstein and Morse (1985) estimated that the residence time of the pore waters in these sediments was only a few days or less. 

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

One can illustrate the relationships among bioturbation, dissolution kinetics and the asymptotic concentration in sediment pore waters rather simply in the following way. If we assume that the reactive portion of the opal rain to the sediment-water interface dissolves in the top hem of the sediments, then the residence times for the reactive opal concentration, [Si02]sed (mol cm7 ), with respect to in situ dissolution is ... 

Aeolianites that become submerged below the groundwater table may become cemented in the phreatic environment. In this zone, pore spaces between sand grains are completely filled with water and any cements derived from the interstitial waters are often isopachous in nature (Muller, 1971). Large solution volumes and longer residence times in the phreatic zone can result in coarser spar compared with the vadose zone. Increases in temperature, degree of supersaturation and NaCl content appear to result in larger cement crystal sizes (Badiozamani et al., 1977). 

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