Sorption equilibrium experiments

A series of experiments was also conducted by Bowman et al. [34] to ascertain the effects of differing environmental factors on the sediment-water interactions of natural estrogens (estradiol and estrone) under estuarine conditions. Sorption onto sediment particles was in this case relatively slow, with sorption equilibrium being reached in about 10 and 170 h for estrone and estradiol, respectively. On the other hand, true partition coefficients calculated on colloids were found to be around two orders of magnitude greater that those on sediment particles. Hence, it was concluded that under estuarine conditions, and in comparison to other more hydrophobic compounds, both estrone and estradiol... 

In many early experiments, hysteresis was observed for highly hydrophobic compounds such as PCBs (79, 80). Since the time to reach equilibrium can be quite long for strongly hydrophobic compounds, a solute may have never reached equilibrium during the sorption isotherm experiment. Consequently, Kj would be underestimated, which leads to the discrepancy between the sorption and desorption coefficients that was attributed to hysteresis. The case for hysteresis being an artifact is supported by recent data for tetrachlorobenzene (log K = 4.7), illustrating that sorption and desorption require approximately two days to reach equilibrium with approximately equal time constants (78). Finally, the diffusion model is consistent with the observation that the extent of hysteresis was inversely related to particle size (81). 

Mass fluxes of alkali elements transported across the solid-solution interfaces were calculated from measured decreases in solution and from known surface areas and mineral-to-solution weight-to-volume ratios. Relative rates of Cs uptake by feldspar and obsidian in the batch experiments are illustrated in Figure 1. After initial uptake due to surface sorption, little additional Cs is removed from solution in contact with the feldspars. In contrast, parabolic uptake of Cs by obsidian continues throughout the reaction period indicating a lack of sorption equilibrium and the possibility of Cs penetration into the glass surface. 

The concept of transport resistances localized in the outermost regions of NS crystals was introduced in order to explain the differences between intracrystalline self-diffusion coefficients obtained by n.m.r methods and diffusion coefficients derived from non-equilibrium experiments based on the assumption that Intracrystalline transport is rate-limiting. This concept has been discussed during the past decade, cf. the pioneering work [79-81] and the reviews [2,7,8,23,32,82]. Nowadays, one can state that surface barriers do not occur necessarily in sorption uptake by NS crystals, but they may occur if the cross-sections of the sorbing molecular species and the micropore openings become comparable. For indication of their significance, careful analysis of... 

The distribution coefficients evaluated for silver (at C) are also given in Figure 3 The silver coefficients determined in 0.68 N NaCl solutions are somewhat less than the corresponding coefficients for cesium and rubidium, and also for strontium and barium. Such results are probably due to either anionic complex formation (22) and/or a less favorable sorption equilibrium. (FurthermoreJ the experiments done using silver in sodium chloride solutions required equilibration times on the order of 90 days, as opposed to two to four days for most other experiments, and it appears that processes, which may or may not be important, are involved which are not understood.)... 

Most of these aspects of water-sorption equilibrium correspond to the equality of chemical potentials of water in the medium and in the polymer. The consequences of this principle are illustrated by the experiment of Fig. 14.2, where an interface is created between water and a nonmiscible liquid (oil, hydrocarbon, etc.), and a polymer sample is immersed into the organic liquid. It can be observed that, despite the hydrophobic character of the surrounding medium, the sample reaches the same level of water saturation as in direct water immersion or in a saturated atmosphere. What controls the water concentration in the polymer is the ratio C/Cs of water concentrations in the organic phase, where Cs is the equilibrium concentration, which can be very low but not zero. In other words, hydrophobic surface treatments can delay the time to reach sorption equilibrium but they cannot avoid the water absorption by the substrate. 

General agreement between batch and column results have been found, although significant differences also have been reported. For example, sorption coefficients determined from column experiments run at velocities of 10 3 cm s 1 were quite similar to those determined in 18-hour batch equilibrium experiments, while column experiments run at 10 2 cm s 1 showed the effect of slow sorption kinetics (Schwarzenbach and Westall, 1981 Maraga et al., 1998 Benker et al., 1998 Bayard et al., 1998). 

Sorption generally is regarded as a rapid process and, in many laboratory sorption experiments, equilibrium often is observed within several minutes or hours. An equilibration time of 24 hours often is used for convenience. True sorption equilibrium, however, may require weeks to months to achieve (Karickhoff, 1981 Karickhoff, 1984), depending on the chemical and environmental solid of interest. In many instances, an early period of rapid and extensive sorption, followed by a long slow period, is observed (Karickhoff, 1980 Wu and Gschwend, 1986 Brusseau and Rao, 1989 Brusseau et al., 1990 Ball and Roberts, 1991). 

Experiments show that when no change in ambient vapor pressure p occurs during the sorption process, M (f) approaches a limiting value as time increases. When this limiting value is reached, the film absorbs or desorbs no more penetrant and is at thermodynamic equilibrium with the ambient vapor. This is the state called sorption equilibrium. The value of M (t) for this state is denoted by Often the ratio M is... 

In a typical IINS experiment, a catalyst is characterized in a sealed reaction vessel or bypass can under vacuum, ambient conditions, equilibrium partial pressures of reactants, or higher gas pressures. The sample is quenched to liquid nitrogen temperature as fast as possible to maintain the conditions of a frozen sorption equilibrium as well as possible. Then the sample is loaded into a cryostat and cooled to a temperature below 30 K, and the spectrum is measured. 

Most experimental kinetic curves are rather smooth, i.e, the concentration of adsorbate in solution monotonically decreases, but some kinetic curves reported in the literature have multiple minima and maxima, which are rather unlikely to be reproducible. Such minima and maxima represent probably the scatter of results due to insufficient control over the experimental conditions. For instance use of a specific type of shaker or stirrer at constant speed and amplitude does not necessarily assure reproducible conditions of mass transfer. Some publications report only kinetic data—results of experiments aimed merely at establishing the sufficient equilibration time in equilibrium experiments. Other authors studied adherence of the experimentally observed kinetic behavior to theoretical kinetic equations derived from different models describing the transport of the adsorbate. Design of a kinetic experiment aimed at testing kinetic models is much more demanding, and full control over all parameters that potentially affect the sorption kinetics is hardly possible. 

Another point of interest was the difference in the velocity of ammonia sorption by analcite observed by Barrer(2) and Tiselius(6). The former found sorption rapid at temperatures as low as 200 C. the latter found that sorption equilibrium could not be established at 270 C., so slow was the ammonia uptake. These experiments all serve to show the complexity of behaviour of this very interesting series of diffusion systems. 

As the pressure measurements in oscillometric-manometric experiments only are needed to calculate the density of the sorptive gas = p (p, T)) in a sorption equilibrium state, it seems to be worthwhile to consider direct measurements of (p ), for example by using the buoyancy of a sinker at a magnetic suspension balance. Experiments of this type can be called oscillometric-densimetric measurements . They can be recommended to measure solubilities of gas mixtures in swelling (polymeric) sorbing materials. A scheme for such measurements is sketched in Figure 5.15. Equations to calculate the total mass of gas (m ) sorbed in the sorbent mass (m ) and the volume of the mass (m + m ) are the same as those given for oscillometric-manometric measurements in the next section (4.3). 

In the studies presented the model developed for sorption equilibrium [1] was extended also to sorption kinetics. Furthermore, an approximative procedure was outlined for the transformation of equilibrixim and kinetic sorption data from batch-experiments to transport processes in model soil columns. 

Metal sorption experiments were conducted with feed solutions of 1000 mg/L of Pb, Cu ", and Cd. The feed pH was adjusted to 5.5 for Pb and Cd and 5.0 for Cu. The permeate was recycled several times to allow the sorbent to reach its sorption equilibrium capacity. The extent of metal sorption was calculated from the volume of permeate collected and the permeate analysis. Sorption capacity of the membranes is given in Table 17.11 for Pb and Cd. The water permeability of the membrane after silane attachment was 0.4 x 10 mVm s bar. 

FTIR spectroscopy has been used to study hydrogen bonding in nylon as suggested in the literature (3). In each of our experiments 200-400 scans were averaged, processed, and compared against a previously ran background spectrum of the empty cell at the same temperature. The spectral resolution was set at 1.0 cm". Spectra were recorded at a number of temperatures between 23 and 280 °C. Prior to data collection, the cell was equilibrated for at least 5 minutes at each temperature. CO2 saturation time in the first measurement was 60 minutes to reach sorption equilibrium. Subsequently, once the test temperature was reached, an equilibration time of... 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

A typical sorption experiment involves exposing a polymer sample, initially at an equilibrium penetrant concentration of c to a bathing penetrant concentration of Ci. The weight gain or loss is then measured as a function of time. The term sorption used in this context includes both absorption and desorption. The sorption is of the integral type if c° = 0 in the case of absorption or if cf = 0 in the case of desorption. Details of the experimental setup for the sorption measurement are discussed elsewhere [4],... 

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