Sorbed species

The spillover effect can be described as the mobility of sorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). In this way a seemingly inert material can acquire catalytic activity. In some cases, the acceptor can remain active even after separation from the donor. Also, quite often, as shown by Delmon and coworkers,65 67 simple mechanical mixing of the donor and acceptor phases is sufficient for spillover to occur and influence catalytic kinetics leading to a Remote Control mechanism, a term first introduced by Delmon.65 Spillover may lead, not only to an improvement of catalytic activity and selectivity but also to an increase in lifetime and regenerability of catalysts. 

The reaction on the catalyst surface was followed by in situ i.r. spectroscopy using a Bruker IFS88 FTIR spectrometer for the characterisation of sorbed species and mass spectroscopy for the analysis of gas phase. The state of Pt was further investigated by in situ X-ray absorption spectroscopy (Daresbury, UK, beamline 9.1, transmission mode, Si(220) monochromator, Pt-Lj, edge). Details of catalyst characterisation techniques are reported elsewhere [13,14]. 

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

The mass balance equations for a system including sorbing species are given as,... 

These equations are the same as those already considered (3.28-3.31), with the addition of the summations over the masses of the sorbed species. 

The Freundlich isotherm (or Freundlich model) is an empirical description of species sorption similar to the K, approach, but differing in how the ratio of sorbed to dissolved mass is computed. In the model, dissolved mass, the denominator in the ratio, is raised to an exponent less than one. The ratio, represented by the Freundlich coefficient Kf, is taken to be constant, as is the exponent, denoted f, where 0< f <1. As before, the masses of dissolved and sorbed species are entered, respectively, in units such as moles per gram of dry sediment and moles per cm3 fluid. Since the denominator is raised to an arbitrary exponent rif, the units for Kf are not commonly reported, and care must be taken to note the units in which the ratio was determined. 

To cast the model in general form, we begin with the basis shown in Equation 9.5 and write each sorption reaction in the form of Equation 9.7. The mass action equation corresponding to the reaction for each sorbed species Aq is... 

When more than one sorption reaction is considered, of course, the summation includes the other sorbed species. 

As a final note, a variant of the calculation is useful in many cases. Suppose a chemical analysis of a groundwater is available, giving the amount of a component in solution, and we wish to compute how much of the component is sorbed to the sediment. We can solve this problem by eliminating the summations over the sorbed species (the over q terms) from each of the mass balance equations,... 

The iteration step, however, is complicated by the need to account for the electrostatic state of the sorbing surface when setting values for mq. The surface potential T affects the sorption reactions, according to the mass action equation (Eqn. 10.13). In turn, according to Equation 10.5, the concentrations mq of the sorbed species control the surface charge and hence (by Eqn. 10.6) potential. Since the relationships are nonlinear, we must solve numerically (e.g., Westall, 1980) for a consistent set of values for the potential and species concentrations. 

In the flow-through model, any mineral mass present at the end of a reaction step is sequestered from the equilibrium system to avoid back-reaction. At the end of each step, the model eliminates the mineral mass (including any sorbed species) from the equilibrium system, keeping track of the total amount removed. To do so, it applies Equation 13.11 for each mineral component and sets each nk to a vanishingly small number. It is best to avoid setting nk to exactly zero in order to maintain the mineral entries Ak in the basis. The model then updates the system composition according to Equations 13.5-13.7 and takes another reaction step. 

As before, mq are the molal concentrations of the sorbing species Aq, and vwq, etc., are the coefficients of the reaction forming Aq from the basis. In these equations, we have taken minerals and sorbed species as being immobile, although this assumption might be relaxed to account, for example, for the migration of colloids or suspended sediment. 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

Clearly an unambiguous examination of the chemical nature of sorbed complexes using SIMS in these measurements is complicated by the presence of manganese and manganese-cobalt containing ions in the spectra. The greater relative ion intensities and the intensity distribution differences for the pH 10 sample compared to the pH 7 material, may arise due to the presence of different surface amine species. Alternatively, the difference may be related to different secondary ion formation processes for sorbed species. 

The immobilization of dissolved chemical species by adsorption and ion exchange onto mineral surfaces is an important process affecting both natural and environmentally perturbed geochemical systems. However, sorption of even chemically simple alkali elements such as Cs and Sr onto common rocks often does not achieve equilibrium nor is experimentally reversible (l). Penetration or diffusion of sorbed species into the underlying matrix has been proposed as a concurrent non-equilibration process (2). However, matrix or solid state diffusion is most often considered extremely slow at ambient temperature based on extrapolated data from high tem-... 

The increase in time resolution of advanced sorption uptake methods and the joint use of sorption and radio-spectroscopic techniques allow for a more detailed analysis of the so-called "non-Fickian" behaviour of sorbing species in the intracrystalline bulk phase [18,28,29,76]. Correspondingly, information on molecular dynamics has been obtained for n-butane and 2-but ne in NFI zeolites by means of the single step frequency response method and C n.m.r. line-shape analysis [29]. As can be seen from Figures 4 and 5, the ad- / desorption for both sorbates proceeds very quickly, but with a... 

The development of mixture sorption kinetics becomes increasingly Important since a number of purification and separation processes involves sorption at the condition of thermodynamic non-equilibrium. For their optimization, the kinetics of multicomponent sorption are to be modelled and the rate parameters have to be identified. Especially, in microporous sorbents, due to the high density of the sorption phase and, therefore, the mutual Influences of sorbing species, a knowledge of the matrix of diffusion coefficients is needed [6]. The complexity of the phenomena demands combined experimental and theoretical research. Actual directions of the development in this field are as follows ... 

Ion exchange is similar to adsorption, since mass transfer from a fluid to a solid phase is common in both processes, i.e. they are basically diffusion processes. Ion exchange is also a sorption process, but ions are the sorbed species in contrast to adsorption, where electrically neutral species are sorbed (Noble and Terry, 2004 Perry and Green, 1999). It is generally accepted that adsorption and ion exchange can be grouped together as sorption for a unified treatment in practical applications. 

Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase there are, however, some significant differences. Specifically, although both processes can be characterized as sorption processes, the sorbed species are ions in ion exchange, whereas electrically neutral substances are sorbed hi adsorption. Moreover, in ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal... 

For any species adsorbed on the surface of a zone, the fraction of the sorbed species which desorbs during the time of a cycle can be measured (or estimated). Another parameter, G, is therefore defined... 

Figures 1 through 5 should, therefore, be considered as representative values which (especially at values of Cj less than 10 mg-atom/ml) might be subject to variations on the order of 0.5 log units. Furthermore, the assumption that the solid-phase concentrations of rubidium, strontium and barium given in Table I represented the concentrations of sorbed species introduces further...

Figure 9.1 Illustration of some processes in which sorbed species behave differently from dissolved molecules of the same substance. (a) Dissolved species may participate directly in air-water exchange while sorbed species may settle with solids. (b) Dissolved species may react at different rates as compared with their sorbed counterparts due to differential access of other dissolved and solid-phase reactants. ...

For the case at hand, it is easy to see that the amount of = MOH2 and sMO" species on the solid surface control the surface s charge. The concentration of this charge, specifically sorbed species) by the difference between positive and negative site concentrations ... 

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