Equilibrium washout

Equilibrium washout of tri-butylphosphate between the aqueous phase and the TVEX matrix (Figure 8.8) shows S-shaped curves typical for physical adsorption with formation of an extractant polymolecular adsorbed layer. The increase of temperature leads to a decrease of equilibrium distribution (Figure 8.9) due to the decrease of TBP solubility in water with temperature increase. Thus, TBP losses from TVEX matrix are small and are caused by tri-butylphosphate solubility in the aqueous phase, depending on solution acidity, ionic strength, and temperature. For example, during the pilot tests, TBP leakage from TVEX-TBP resins after 100... 

The bioreactor has two equilibrium points within the physically realizable domain. Such equilibrium points correspond to washout and operation conditions. For the operation condition (i.e., when degradation of the organic... 

In some studies it is desirable to do constant infusion to achieve a steady state or equilibrium condition which is a function of input, extraction rate, tissue washout, and radioactive decay (23). Figure 6 shows the yield of Rb-82 at various elution rates to a steady-state condition. At the faster flow rate of 5.33 ml/min, there is 24% yield of Rb-82 and at the slower flow rate of 2.15 ml/min there is about 1% yield of Rb-82. The lower yield at the slower flow rate is mostly accounted for in decay during transit through the line to the patient. 

On a reverse-phase column, separation occurs because each compound has different partition rates between the solvent and the packing material. Left alone, each compound would reach its own equilibrium concentration in the solvent and on the solid support. However, we upset conditions by pumping fresh solvent down the column. The result is that components with the highest affinity for the column packing stick the longest and wash out last. This differential washout or elution of compounds is the basis for the HPLC separation. The separated, or partially separated, discs of each component dissolved in solvent move down the column, slowly moving farther apart, and elute in turn from the column into the detector flow cell. These separated compounds appear in the detector as peaks that rise and fall when the detector signal is sent to a recorder or computer. This peak data can be used either to quantitate, with standard calibration, the amounts of each material present or to control the collection of purified material in a fraction collector. 

The value of H provides the means to describe vapour phase equilibrium between water and air. This can be used to help define vapour phase washout by rain, and also the oceanic gas phase exchange between marine air masses and associated surface water. 

In the case of washout of sulfur dioxide (S02), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous S02 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for... 

The time course of action of PPADS at 5 pM, a concentration which inhibited the effects of endogenous ATP by about 70%, was examined in the field-stimulated rabbit vas deferens. In these experiments, PPADS acted as a slowly-equilibrating and a slowly-reversible P2X purinoceptor antagonist. It was found to reach apparent equilibrium in about 120 min, and the tissue took about 90 min to regain its control twitch height by continual washout of PPADS. Importantly, these findings indicate that PPADS is not an irreversible antagonist at P2X Purinoceptors in rabbit vas deferens. 

The entire scheme for a deuterium washout equilibrium perturbation is described in Fig. 7.7A. The two reactants initially present are boxed. The starting substrates for the perturbation are ] H]-D-Ala (lower manifold) and ] H]-L-Ala (upper manifold). All hydrons on the upper manifold are considered to have the same identity as solvent. An equilibrium perturbation-type washout of the ] H]-D-Ala in H2O proceeds by abstraction of Ca deuteron by a protiated enzyme (lower manifold), followed by donation of a proton, to yield the protiated L-isomer. The enzyme rapidly and irreversibly exchanges the deuteron for proton, moving from the lower to the upper manifold. The contemporaneous racemization of the r-isomer on the upper manifold occurs more rapidly than the racemization from the lower manifold. This transient accumulation of the slower species (in this case o-isomer) produces the perturbation in the optical signal, from which (V/K) for the d l direction may be determined. 

Figure 7.7. Schematic representation of a D-hydron washout perturbation. The upper panel describes the washout of deuterated D-alanine in H2O (equal starting concentrations of[ H]-D-alanine and H]-L-alanine). Upon initiation of the perturbation, H]-D-alanine-enzyme complex (lower manifold) and the H]-l-alanine-enzyme complex (upper manifold) dominate, with transient accumulation of the former, due to its slower racemization. Upon racemization of the [ H]-D-alanine-enzyme complex, the deuteron is washed out into the solvent pool. At equilibrium only the upper manifold exists, in which forward and reverse racemization rates are equivalent. The...

Knowles and coworkers also performed competitive deuterium washouts (i.e., an equilibrium perturbation-type washout experiments), using deuterated substrates in H2O solutions, which yielded the (V/K) values for both directions [85]. Further confirmation of these KIE values was validated by a double competitive deuterium washout experiment, in which both substrates are Ca deuterated, which yielded a ratio of the two (V/K) values. The authors were also able to perform competitive deuterium washout experiments where direct proton exchange between free enzyme forms is rate-limiting (i.e., at high substrate concentration the lower manifold of Fig. 7.16 is dominant). This experiment indicated that interconversion of free enzyme forms is very similar to the racemization manifold, in that loss of proton from one form yields the other free enzyme form, with water acting as the catalyst. Fig. 7.16. 

Wet deposition encompasses the removal of gases and particles from the atmosphere by precipitation events, through incorporation into rain, snow, cloud, and fog water, followed by precipitation (Hales, 1986). As in the case of dry deposition, wet deposition is a complex phenomenon which in this particular case involves transport to the surface of a droplet, absorption, and possible aqueous-phase chemical conversion. Wet removal of gases is frequently approximated by assuming that the species is in equilibrium between the gas and aqueous phases. The equilibrium partitioning is represented in terms of a washout ratio, Wg = [C]drop/[C]air, where [C]drop and [C]ajr are the concentrations of the chemical in the aqueous and gas phases (Mackay, 1991). 

Most of the experimental applications of the ZLC technique have been with gaseous systems, and for these systems the technique may now be regarded as a standard method. Based on our experience it is possible to suggest some guidelines as to how the experiments should be carried out. The key parameter is L, which from its definition (Eq. 17) can be considered the ratio of the diffusional and washout time constants R /D and KVs/F. This parameter is also equal to the dimensionless adsorbed phase concentration gradient at the surface of the solid at time zero. From either of these definitions it is evident that L gives an indication of how far removed the system is from equilibrium control. This parameter is proportional to the flow rate, so it can be easily varied, and to extract a reliable time constant, it is necessary to run the experiment at at least two different flow rates. 

While the major removal of radon gas from air is by radioactive decay, the chemically radioactive decay products (formerly called radon daughters) may additionally be removed by processes such as washout and plating-out on surfaces. In air inside buildings, these radioactive decay products can attach themselves to walls, floors, people, or airborne particles that are inhaled into the Ivmgs. Unattached radon decay products can also be inhaled and, subsequently, can become deposited on the Ivmg tissue. As a consequence, radon decay products are seldom in radioactive equilibrium with radon in the lower atmosphere (near the earth s sruface) or indoors. 

Containment spray systems are used in some types of PWR plants, resulting not only in a condensation of steam but also in a washout of airborne radioiodine and other fission products from the containment atmosphere. Normtilly, an alkaline borate spray solution is used, resulting in a shift of the I2 disproportionation equilibrium to the iodate side and a suppression of revolatilization of iodine previously trapped by the spray droplets. In some cases, solutions containing sodium thiosulphate have been used to decompose volatile organic iodides present in the containment atmosphere, but because of its corrosivity this reagent has been largely abandoned. By the addition of boric acid to the spray solution, subcriticality of the reactor core is guaranteed when, after the injection phase, the sump water is recirculated for removal of decay heat from the core. 

In the case of washout of sulfur dioxide (SO2), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous SO2 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for the SO2 concentration in air beneath the rain-forming cloud. These models lump together all processes affecting SO2 removal from air (i.e., dissolution into water droplets, hydration, oxidation, and ionization). A first-order decay constant for the SO2 concentration. A, varies with the rainfall characteristics (rainfall rate and size of raindrops). Boubel et al. (1994) suggest a value of A equal to... 

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