Solute Transfer Rate Research Articles

Saline groundwater in Mono Basin, California: 2. Long‐term control of lake salinity by groundwater

Mono Lake is a perennial, closed‐basin lake that has existed for at least 700 kyr. This 46‐m‐deep alkaline lake's present concentration is about 90,000 ppm total dissolved solids. Well logs show that beneath part of the lake, saline groundwater of concentration > 18,000 ppm extends to the bottom of the basin fill aquifer (Rogers and Dreiss, this issue). Numerical simulations indicate that redistribution of the basin's solutes between the lake and underlying saline groundwater body, driven by late Quaternary lake level changes, may be responsible for the high present‐day solute content of Mono Lake. At low lake levels, the high lake concentration causes solute loss via free convection; this might occur more rapidly through faults or fractures. The saline‐fresh groundwater interface position reflects a balance between the saline groundwater density and the force of inflowing fresh groundwater discharging near the shoreline. At higher lake stages, the shoreline discharge zone moves toward the basin edge, and the unrestrained saline groundwater mass subsides, drawing solutes from the lake into the basin sediments. Falling lake levels again constrict the saline groundwater beneath the lake, forcing saline water into the lake and increasing its solute content. The sediment permeability below the lake is the major control on the solute transfer rate between the lake and the groundwater reservoir. Only the larger, longer‐term lake stage changes cause saline groundwater movement; depending on the assumed basin fill aquifer permeability, equilibration of the saline groundwater and lake solute content with lake stage changes requires hundreds to thousands of years. Simulations suggest that the historical conditions, where a more saline Mono Lake (50,000–90,000 ppm) overlies less concentrated groundwater (apparently ∼ 18,000 ppm), could be due to the still present impact of late Pleistocene lake high stands. Mono Lake's historical salinity data have a large scatter, but suggest a 5% decrease in the lake's solute content over the last 50 years. We estimated the diffusive solute flux into lacustrine sediments for this period from core data. The salinity data are consistent with a combination of solute diffusion into sediments and an equal, or greater, model‐predicted advective solute loss to groundwater.

Field Method for Measuring Mobile/Immobile Water Content and Solute Transfer Rate Coefficient

AbstractNumerous field and laboratory studies have documented the occurrence of preferential transport of solutes due to a fraction of the soil water being immobile and not taking part in the transport process. Domain models have been developed that describe these processes, but before we can apply them routinely, we need methods for measuring the required model parameters, particularly the fraction of immobile water to total water θlm/θ and the exchange coefficient between the mobile and immobile domains, α. We developed a field method for measuring both θlm/θ and α. The method uses a sequence of conservative anionic tracers consisting of Br−, pentafluorobenzoate, o‐trifluoromethylbenzoate, and 2,6‐difluorobenzoate infiltrated with time through a tension infiltrometer. Previous studies have confirmed that these tracers have very similar transport properties in a wide range of soils. The method was applied to an undisturbed loam and a greenhouse soil as an initial test of the approach. Calculated θim/θ fractions averaged 0.69 and ranged from 0.25 to 0.98, while calculated α values averaged 0.0081 h−1 and ranged from 0.0030 to 0.021 h−1. These values compare well with values reported earlier by other investigators. The method is simple and allows routine measurement of transport properties of field soils. The method can also be used to validate the applicability of domain models to specific soils.

Rotational dispersion in porous media due to fluctuating flows

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.

Clinical value of standardized equilibration tests in CAPD patients.

Peritoneal transport rates, a critical determinant of peritoneal dialysis efficiency, vary widely among patients and may be easily categorized by standardized peritoneal equilibration test. Measurements of creatinine and glucose transfer are particularly useful in selecting optimal dialysis prescription. Patients with high-average peritoneal solute transport do well on standard CAPD even after losing residual renal function. Patients with high peritoneal solute transfer rates are likely to have inadequate ultrafiltration on standard CAPD. These patients do much better on dialysis regimens with short-dwell exchanges, such as nightly peritoneal dialysis or daytime ambulatory peritoneal dialysis. Patients with low-average and particularly with low peritoneal transport rates are likely to develop symptoms and signs of inadequate dialysis on standard CAPD as residual renal function becomes negligible, and may require high-dose peritoneal dialysis prescriptions.

Peritoneal Equilibration Test

Peritoneal transfer rates of urea, creatinine, glucose, protein potassium, and sodium as well as drain and residual volumes were measured during 103 equilibration tests performed in 18 diabetic and 68 nondiabetic patients. Equilibration test was performed over a 4-hour dwell exchange with 2 L of 2.5% Dianeal solution. Excellent reproducibility was seen after tests were standardized for length of preceding exchange, times of inflow and drainage, patient position, methods of obtaining and processing samples and laboratory assays. Diabetics did not have lower peritoneal solute transfers than nondiabetics. Wide variations were found in the study population. Measurements of creatinine, glucose and sodium transfer were particularly useful in predicting the patient's response to the standard CAPO. The patients with highaverage peritoneal solute transport did well on standard CAPO even after losing residual renal function. Patients with high peritoneal solute transfer rates were likely to have inadequate ultrafiltration on standard CAPO. These patients did much better on dialysis modalities with short dwell exchanges, i.e. nightly peritoneal dialysis (NPO) or daytime ambulatory peritoneal dialysis (OAPO). Patients with low average, and particularly low peritoneal transport rates were likely to develop symptoms and signs of inadequate dialysis as their residual renal function became negligible, particularly in individuals with high body surface area. Repeated tests were helpful in evaluating causes of insufficient ultrafiltration and/or inadequate dialysis.

Blood flow dependence of postglomerular fluid transfer and glomerulotubular balance.

The rate of blood flow entering a capillary network can, in some vascular systems, regulate capillary surface area and the rate of fluid and solute transfer. To determine whether such a mechanism exists in the renal peritubular capillary, we performed micropuncture studies in 28 rats during relatively low and high efferent arteriolar blood flow (EABF). High EABF was achieved by intravenous infusion of isoncotic plasma (group 1: from 120 +/- 11 to 301 +/- 49 nl/min [+/- SE]); whole blood with high hematocrit (approximately 75 vol %) (group 2: from 141 +/- 14 to 252 +/- 31 nl/min); or acetylcholine (group 3: from 193 +/- 20 to 266 +/- 26 nl/min). In group 1 rats, plasma infusion caused an increase in single nephron glomerular filtration rate (SNGFR), on average, from 23.2 +/- 2.4 to 45.2 +/- 3.9 nl/min, owing primarily to increased glomerular plasma flow rate (from 63 +/- 5 to 210 +/- 21 nl/min). The rate of fluid uptake by the peritubular capillary, assessed by the absolute rate of proximal fluid reabsorption (APR), also rose significantly, on average from 10.5 +/- 1.2 to 17.5 +/- 2.4 nl/min. This rise in APR was associated with near constancy in mean transcapillary hydraulic (delta Pc) and oncotic (delta IIc) pressure differences, and was therefore attributed to a significant increase in peritubular capillary reabsorption coefficient (Kr), with the mean from 0.017 +/- 0.003 to 0.030 +/- 0.005 nl/(s . mmHg). In group 2 rats, high hematocrit blood infusion led to a significant rise in APR; on average, from 10.7 +/- 0.7 to 15.0 +/- 1.2 nl/min, without changing SNGFR. This rise in APR occurred despite unfavorable changes in the physical forces, namely a significant increase in delta Pc and constancy in delta IIc. Instead, an increase in EABF was again associated with a significant rise in Kr (on average, from 0.016 +/- 0.002 to 0.030 +/- 0.06 nl/[s . mmHg]), which accounted entirely for the rise in APR, independently of SNGFR. In group 3 rats, in which an increase of EABF was induced pharmacologically with acetylcholine, a rise in EABF was also accompanied by a significant increase in Kr, on average, from 0.019 +/- 0.002 to 0.026 +/- 0.004 nl/(s . mmHg). The results indicate that: (a) Kr is modulated by EABF. (b) In view of plasma flow dependence of GFR, blood flow dependence of Kr and APR provides an important basis for glomerulotubular balance.

The effect of inhibitors of folic acid absorption on the transfer rate constants in the rat everted proximal jejunum: a method for their evaluation from a three-compartment model.

When solute transfer through the intestinal in vitro everted sac preparation is described by a three-compartment system, solute transfer rate constants can be derived for the mucosal and serosal permeability barriers. A catenary variant has been presented as well as a mammillary one where paracellular movement of solute is additionally allowed for. The first order differential rate equations governing the change in solute concentration in all three compartments with respect to time have been solved and the explicit analytical solutions provided. Since these solutions are cumbersome to use in the estimation of the required rate constants, a least squares procedure has been applied directly to the differential form of the rate equations in order to derive the rate constants without recourse to the analytical solutions. Verification of the solutions and of the estimated rate constants was by substitution of the latter into the former to test the goodness of fit for folic acid absorption data. Both variants take into account the simultaneous fluid movement which occurs during absorption experiments and which complicates the interpretation of absorption data. The mammillary model showed that only 10% of folic acid movement could pass directly through the paracellular pathways and that the bulk of folate movement is probably through the epithelial cells. However the catenary model without paracellular movement gave just as good fit to the data and was used subsequently. Experiments investigating the effect of substances implicated in folate malabsorption were analyzed in terms of the catenary model for folic acid absorption, in order to investigate their effects on the transfer rate constants free from the complicating effects on fluid movement. When pronounced inhibition took place, as with methotrexate, the mucosal rate constants were reduced, whereas the serosal rate constants were elevated. Also, the forward (k12) mucosal rate constant correlated significantly with the overall folate transfer in contrast to the other rate constants. These observations are consonant with the concept of a mucosally sited entry step exerting a controlling influence over the transfer rate of folic acid rather than a serosally sited exit process and with the conclusion that this may be the site of action of substances causing folate malabsorption.

Solute Transport in Aggregated Porous Media: Theoretical and Experimental Evaluation

AbstractBreakthrough curves (BTC's) for 36Cl‐ and 3H2O displacement through water‐saturated columns of aggregated and nonaggregated porous media were measured at pore‐water velocities varying over an order of magnitude (2 to 96 cm/hour). These BTC's were used to verify two conceptual solute transport models in which the pore‐water was partitioned into inter‐ and intra‐aggregate regions. In both models, convective‐dispersive solute transport was assumed to be limited to the inter‐aggregate pore‐water region, while the intra‐aggregate pore‐water was assumed to behave as a diffusion sink/source for solute. The rate of solute transfer between the two pore‐water regions was described either by Fick's second law of diffusion written in spherical coordinates (Model I) or was assumed proportional to the concentraion difference between the two regions (Model II). Values of all input parameters in each model were measured in independent experiments rather than by curvefitting to the measured BTC's. Agreement between calculated and measured BTC's at all velocities was good for both models. The value of the mass transfer rate coefficient in Model II was found to vary with aggregate radius, time, and pore‐water velocity. This result was predicted based on theory and experimental results presented in an earlier paper. Conditions under which solute diffusion in aggregates leads to tailing or asymmetry in measured BTC's (during saturated water flow) were identified from a sensitivity analysis using Model I. For certain aggregate radii and pore‐water velocities, the diffusion sink/source effects of the aggregates could be incorporated into the dispersion coefficient, and this lumped‐parameter approach was used to successfully describe the measured BTC's.

Modeling of Carrier-Mediated Extraction

Abstract A two parametric mathematical model of carrier-mediated extraction is proposed for the case when emulsion technique is applied. Its simplified version, valid for diluted solutions, is solved analytically, and an implicit expression relating both parameters—rate of solute transfer and rate of emulsion destruction—is derived. Model parameters were evaluated from the experimental data obtained with two different three-phase systems: oil-in-water-in-oil and water-in-oil-in-water. In the first case, toluene and heptene were separated using water as an intermediate phase, and parameter evaluation was performed by means of an analog computer. In the second case, acetic, nitric, and hydrochloric acids were separated using a solution of tributylphosphate in carbon tetrachloride as a carrier phase. In this case the rate coefficients were found from the proposed implicit equation.

Solute transfer between intramolecular polar and non-polar phases as a possible rate-limiting step in chymotrypsin deacylation

Equations predicting the relative rate of solute transfer from a polar to a non-polar phase and from a non-polar to a polar phase are derived on the basis of a model which assumes that the solute becomes partially dissociated from both phases during transfer. It is further assumed that the rate-limiting step in the deacylation of α-chymotrypsin is a transfer between phases rather than the hydrolysis itself, or alternatively, that the rate of hydrolysis depends on the rate of transfer and the equations are applied to the rates of deacylation of the acetyl through the caproyl derivatives measured by Cane & Wetlaufer (1966) at pH 8 · 1. For a rate-limiting transfer from a non-polar to a polar phase the following relationship would be expected: log ( KC) = γ log ( PC) + constant; whereas for a transfer from a polar to a non-polar phase: log ( K C ) = γ log ( P C ) + constant ; where K is the first-order constant for deacylation, Ci s the non-polar: polar partition coefficient for the free acid derived from the acyl group, P is the vapor pressure of the free acid, and γ is a fraction between 0 and 1 · 0 which is related to the extent of dissociation of the acid from both phases during transfer. (A γ of 1 · 0 implies complete dissociation from both phases.) Using the benzene: water partition coefficient for C, the rate constants are found to be consistent with a rate-limiting transfer from a polar to a non-polar phase; for the C 2 through C 4 acids or acyl groups, γ = 0 · 5, whereas for the C 5 and C 6 groups γ is 0 · 1 to 0 · 2, indicating that a definite change in the mode of transfer occurs between C 4 and C 5 such that the shorter-chain groups lose contact with the two phases to a greater extent during transfer. Indole is found to increase γ for all derivatives, suggesting that indole tends to pull the two phases apart. Since the experimental procedure used to determine the rates of deacylation would measure the total hydrolysis rather than the release of the free acids from the enzyme, the foregoing deductions would imply that a rate-limiting transfer of the attached acyl group occurs prior to hydrolysis, and that the hydrolysis would ensue relatively rapidly once the configuration of the acyl group had been altered by the transfer. Inasmuch as the acyl groups considered represent poor substrates for chymotrypsin, it is also possible that the hydrolysis rather than the transfer becomes rate-limiting for the good substrates. The increase in the ΔS ∗ for deacylation between C 4 and C 5 found by Cane & Wetlaufer (1966) as well as the “better binding: better reaction” postulate of Knowles (1965) are considered in light of these findings.

Augmentation of tissue-blood transfer of solutes by transcapillary filtration and absorption.

AbstractThe effect of net transcapillary fluid movement on the rate of transfer of 131I, 24Na, 86Rb and 133Xe from tissue to blood was studied in cat skeletal muscle. The technique used permitted control, directly or indirectly, of other factors of importance for exchange, such as blood flow, flow velocity. capillary‐ flow distribution. size of capillary surface area and capillary permeability. Net fluid filtration was produced by raising mean hydrostatic capillary pressure and net fluid absorption by increasing plasma osmotic pressure. It was shown that a net transcapillary fluid movement per se (filtration or absorption) increased the transport rate from tissue to blood considerably above the control level when no net fluid movement occurred (Fig. 3). The rate of solute transfer seemed to increase progressively with the rate of net fluid movement, at least in the lower range. The extent to which transfer increased was roughly the same for all water‐soluble substances studied. While for 133Xe net transcapillary filtration appeared to produce a less marked increase in transport rate. Reduction in nutritional blood flow was followed by a decrease in rate of isotope transfer. This decrease proved much smaller when the flow reduction was associated with net transcapillary fluid filtration. The possible physiological significance of transcapillary fluid movements for control of tissue‐blood exchange is tentatively discussed.