Adsorption concentration

Reaction kinetics. The time-development of sorption processes often has been studied in connection with models of adsorption despite the well-known injunction that kinetics data, like thermodynamic data, cannot be used to infer molecular mechanisms (19). Experience with both cationic and anionic adsorptives has shown that sorption reactions typically are rapid initially, operating on time scales of minutes or hours, then diminish in rate gradually, on time scales of days or weeks (16,20-25). This decline in rate usually is not interpreted to be homogeneous The rapid stage of sorption kinetics is described by one rate law (e.g., the Elovich equation), whereas the slow stage is described by another (e.g., an expression of first order in the adsorptive concentration). There is, however, no profound significance to be attached to this observation, since a consensus does not exist as to which rate laws should be used to model either fast or slow sorption processes (16,21,22,24). If a sorption process is initiated from a state of supersaturation with respect to one or more possible solid phases involving an adsorptive, or if the... 

With an S-type isotherm, the slope initially increases with adsorptive concentrations, but eventually decreases and becomes zero as vacant adsorbent sites are filled. This type of isotherm indicates that at low concentrations the surface has a low affinity for the adsorbate which increases at high concentrations. 

The amount of solution in contact with colloidal particles is also an important attribute of a flow technique. Supplied with a solution of the same concentration, soil constituent particles with solution flowing past them will be exposed to a greater mass of adsorptive (concentration x flow rate x time) than the particles in a static system (concentration x solution volume) by the time an equilibrium is attained. More important, desorbed species that were originally on the adsorbent are constantly being removed (Akratanakul et al., 1983 Sparks, 1985, 1986). 

Table X. Comparison of Field Ionization and Low-Voltage Electron-Impact Mass Spectral Results for Two Adsorption Concentrates,...

In this chapter, we have so far discussed the adsorption of gases in solids. This section gives a brief description of the adsorption process from liquid solutions. This adsorption process has its own peculiarities compared with gas-solid adsorption, since the fundamental principles and methodology are different in almost all aspects [2,4,5], In the simplest situation, that is, a binary solution, the composition of the adsorbed phase is generally unknown. Additionally, adsorption in the liquid phase is affected by numerous factors, such as pH, type of adsorbent, solubility of adsorbate in the solvent, temperature, as well as adsorptive concentration [2,4,5,84], This is why, independently of the industrial importance of adsorption from liquid phase, it is less studied than adsorption from the gas phase [2],... 

For very dilute suspensions, a t and qn = (n + l/2)ir. A graph of M(t)/M plotted against the dimensionless time parameter Dt/f2 is shown in Fig. 4.539 for several values of a. Note that the uptake of adsorptive, for a given initial adsorptive concentration, is more rapid when the volume ratio of bulk aqueous solution to adsorbent pore space (i.e., a) is small. 

During the adsorption/concentration process, some fibres of the extraction disk were observed to be released from the sorbent surface upon stirring, and this was found to induce poor reproducibility of the signal. This problem was solved by prior treatment of the phase in which it was attached to its sampler holder and stirred vigorously in pure water for 30 min in order to eliminate the most fragile fibres from its surface. 

A persuasive argument for fractionating aquatic humic substances before concentration is the minimization of aggregation resulting from intermolecu-lar interactions discussed in the previous section. The resin-adsorption concentration procedures discussed by Aiken in Chapter 14 of this book also accomplish compound group fractionation at ambient concentrations. Most fractionation procedures can be performed at ambient concentrations if detection of the analyte is sufficiently sensitive or if the fractionating medium concentrates the analyte. However, preconcentration needs to be used if a preparative fractionation is desired where the analyte is not concentrated on the fractionating medium. 

When a polymer is added in a surfactant system, there are two critical concentrations CAC and CMC2. CAC is the critical adsorption concentration at which surfactant starts to adsorb on the polymer chains it is lower than the critical micelle concentration (CMC). CMC2 is the surfactant concentration at which micelles are formed when polymer is present it is higher than CMC (Li et al 2002). Both CAC and CMC2 are on the order of magnitude of CMC. 

When sulfate reduction occurs, sulhde can react with iron to form pyrite. This decreases the amount of phosphorus that is bound with iron and results in more available phosphorus. Inorganic phosphorus retention is regulated by pH, Eh, phosphate concentration (there is a limited amount of substrate for adsorption), concentrations of Ee, Al, and calcium carbonate, and temperature. 

According to the concepts of kinetics, the rate of adsorption is proportional to the product of the concentrations of the reaction partners. In this case, these are the adsorptive and the free sites on the surface. Partial pressure p or molar concentration c can be used as the measure of adsorptive concentration. The concentration of free sites, on the other hand, must be proportional to the fraction of surface that is not covered, 1 — 0. In summary, we have... 

When the mass transfer zone arrives at the bottom and an allowable (product or environmental standard) adsorptive concentration is surpassed the raw gas stream must be switched to the second adsorber which has been regenerated. It is understandable that the mass transfer zone should be short to utilize a maximum of the adsorption capacity. Therefore, the shape of the breakthrough eurve is economically very important. 

The volumetric flow rate density v (equal to the mean velocity referred to the empty bed) is high but the adsorptive concentration in the fluid is small... 

In fixed or fluidized beds, in pneumatic or hydrauhc transport systems, or in a stirred vessel, adsorbent particles are surrounded by a fluid phase. If the nticropore diffusion (for instance in cages of zeolite crystals) can be neglected the adsorptive concentration in the macropores of a pellet with the radius R is given by... 

The operation mode of fixed bed adsorbers can be isothermal (very small adsorptive concentration in the fluid and low heats of adsorption), nonisothermal, and adiabatic. The heat loss of large industrial adsorbers is often so small in comparison to the heat production by adsorption that the bed is nearly operated adiabatically. In such a case not only the mass balances but also the ener balances have to be taken into accoimt to get information on the operating mode and the fields of concentration and temperature in a fixed bed. These balances for the adsorbent (Index S = solid) and the fluid (Index G) are... 

Urochrome is determined by adsorptive concentration with aluminium hydroxide and photometric determination in a solution containing formic acid. 

The large difference in reactivity of the zeolites arises primarily from the large difference in micropore occupancy in the different zeolites. The large variation in micropore occupancy is deduced from the large differences in adsorption equilibrium constants shown in Table 4.5. The similarity of the elementary rate constants for isomerization implies that there is little variation in the corresponding activation energies. The overall rate of the reaction is found to be a maximum for the zeolite in which the adsorption concentration of the reactant is a maximum. 

At 1 kPa, for strongly adsorbing Cio alkanes in MEL-type zeolite, there is a large preference for adsorption of the linear alkane. This preference is much less than for the MFI zeolite. Differences appear at high micropore occupation. Competitive adsorption suppresses the formation of i-Cio in MEL owing to the difference in the channel cross-section geometry, where branched alkanes prefer to adsorb. As a consequence, the rate of n-Cio conversion is low towards i-Cio. The MFI zeolite, therefore has the superior rate since the rate of iCio formation is higher. The reaction products are the result of consecutive reactions of i-Cio. In contrast, as one notes from Fig. 4.39, in MEL at 10 kPa for C7 there is no such preference in adsorption for the n-C7 versus i-C7 molecule since under these conditions the adsorption concentration is still too low. 

E. Bouchaud and M. Daoud, Polymer adsorption concentration effects, J. Phys. (Paris) 48, 1991 (1987). 

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