Specific sorption

Both types of xyloglucans exhibit monolayer sorption onto cellulose (116) and tamarind xyloglucan exhibits maximum specific sorption onto cellulose less than that of coniferous xylan. By inference with other data, this is also less than that of glucomannan and hardwood xylan, but similar to many additives used in the paper industry. 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

Equation (9.15) was written for macro-pore diffusion. Recognize that the diffusion of sorbates in the zeoHte crystals has a similar or even identical form. The substitution of an appropriate diffusion model can be made at either the macropore, the micro-pore or at both scales. The analytical solutions that can be derived can become so complex that they yield Httle understanding of the underlying phenomena. In a seminal work that sought to bridge the gap between tractabUity and clarity, the work of Haynes and Sarma [10] stands out They took the approach of formulating the equations of continuity for the column, the macro-pores of the sorbent and the specific sorption sites in the sorbent. Each formulation was a pde with its appropriate initial and boundary conditions. They used the method of moments to derive the contributions of the three distinct mass transfer mechanisms to the overall mass transfer coefficient. The method of moments employs the solutions to all relevant pde s by use of a Laplace transform. While the solutions in Laplace domain are actually easy to obtain, those same solutions cannot be readily inverted to obtain a complete description of the system. The moments of the solutions in the Laplace domain can however be derived with relative ease. 

Garcia-Miragaya, J. Specific sorption of trace amounts of cadminmby soils. Comm. Soil Sci. Plant Anal, 11 1157-1166,1980. 

Description of polymerization kinetics in heterogeneous systems is complicated, even more so given that the structure of complex formed is not very well defined. In template polymerization we can expect that local concentration of the monomer (and/or initiator) can be different when compared with polymerization in the blank system. Specific sorption of the monomer by macromolecular coil leads to the increase in the concentration of the monomer inside the coil, changing the rate of polymerization. It is a problem of definition as to whether we can call such a polymerization a template reaction, if monomer is randomly distributed in the coil on the molecular level but not ordered by the template. 

Equilibrium thermodynamics is one of the pillars supporting the safety analyses of radioactive waste repositories. Thermodynamic constants are used for modelling reference porewaters, calculating radionuclide solubility limits, deriving case-specific sorption coefficients, and analysing experimental results. It is essential to use the same data base in all instances of the modelling chain in order to ensure internally consistent results. 

The sorption of a penetrant by a class (a) membrane can be treated most simply, if the processes of sorption in the polymer matrix and into each kind of specific sorption site within it can be treated as mutually independent and hence additive. Under these conditions, each sorbed species may similarly be expected to contribute additively to the total diffusion flux. Hence, if there are N — 1 kinds of specific sorption sites, the overall sorption and permeability or diffusion coefficients can be written as... 

Here, S and P, or DXj characterize the ith sorbed penetrant species (i = 1 refers to the polymer matrix and i = 2, 3,. .., N identify each kind of specific sorption site). 

Earlier work on the application of the concept of dual mode sorption and diffusion to glassy polymer-gas systems has been reviewed in detail 6) and important aspects of more recent work have been dealt with in more recent reviews 7 10). Eq. (5) was first applied by Michaels et al U). Sorption in the polymer matrix and in the specific sorption sites was represented by linear (Henry s law) and Langmuir isotherms respectively so that Sj in Eq. (5) is given by... 

Tiller, K.G., Hodgson, 3.F. and Peech, M., 1963. Specific sorption of Cobalt by soil clays. Soil Science, 95 392-399. 

However, more recently competitive sorption has been reported and is thought to be the result of site-specific sorption occurring in soil organic matter (Xing et al., 1996 Xing and Pignatello, 1998). 

Estimate the site-specific sorption coefficient (Kd or Kf) from the estimated K(k and the organic carbon content of the soil or sediment, using Equation (3). 

The pronounced differences between Equations 16.2 to 16.5 indicate that it might not be possible to obtain good soil sorption QSARs for diverse data sets Nendza (1998) has suggested that the differences may reflect class-specific sorption mechanisms. However, some sorption QSARs based on diverse chemicals have been developed. Baker et al. (1997) found a remarkably good correlation ... 

Despite the poorly defined nature of the shear plane, EM has been commonly used as a noninvasive technique for evaluating various surface-surface and surface-solute reactions, such as evaluating the PZC for amphoteric minerals specific sorption reactions for inorganics and organic compounds on mineral surfaces ... 

To determine the electrosorption capacity of this ACF, a long-term test was conducted with a 3S0mg/L U(VI) feed at - 0.9V. As shown in Fig. 2, the effluent concentration is rapidly reached at less than Img/L and it corresponds to a specific sorption rate of 662/IB/(g min). The cumulative amount of uranium within ISh is about 5 32mggnniunySaCF ... 

Surface Complex Formation, Ion Exchange, and Transport in Ground-water and Soil Systems The retardation equation can also be applied to inorganic soluble substances (ions, radionuclides, metals). But here we have to consider, in addition to the sorption or ion exchange process, that the speciation of metal ions or ligands in a multicomponent system influences the specific sorption process and varies during the pollutant transport in the groundwater chemistry then becomes an important part of the transport. 

Because the activities of species in the exchanger phase are not well defined in equation 2, a simplified model—that of an ideal mixture—is usually employed to calculate these activities according to the approach introduced bv Vanselow (20). Because of the approximate nature of this assumption and the fact that the mechanisms involved in ion exchange are influenced by factors (such as specific sorption) not represented by an ideal mixture, ion-exchange constants are strongly dependent on solution- and solid-phase characteristics. Thus, they are actually conditional equilibrium constants, more commonly referred to as selectivity coefficients. Both mole and equivalent fractions of cations have been used to represent the activities of species in the exchanger phase. Townsend (21) demonstrated that both the mole and equivalent fraction conventions are thermodynamically valid and that their use leads to solid-phase activity coefficients that differ but are entirely symmetrical and complementary. 

Substantial efforts have been made to develop physicochemical models for ion exchange based on the Gouy-Chapman diffuse-layer theory (e.g., 9, 10). This work not only has provided insight into the role of diffuse-layer sorption in the ion-exchange process but also has pointed to the need to consider other factors, especially specific sorption at the surface. Consideration of specific sorption enables description of the different tendencies of ions to... 

Similar reactions can be written for the specific sorption of cations and inions at other kinds of surface sites. Here, we represent ion binding at permanent charge sites by the reaction... 

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