Sorbent materials, surface model

The sorbent materials are supplied as finely dispersed colloidal particles, whose surfaces are smooth. Some of their properties are presented in Table 3. The sorbents cover different combinations of hydrophobicity and sign of the surface charge. Thus, the model systems presented allow systematic investigation of the influences of hydrophobicity, electric charge, and protein structural stability on protein adsorption. 

Figure 2.28 Surface model for common sorbent materials (Supelco). (a) Carbotrap, surface area about lOOm /g, uniform charge distribution over all carbon atom centres.

Studies on sorption of triazines by individual soil constituents and by model sorbents have been very helpful in evaluating sorption mechanisms and in assessing the potential contribution of various constituents to triazine sorption by soils. However, intimate associations between organic substances, silicate clays, and oxyhydroxide materials modify the sorptive properties of the individual constituents. Associations between soil constituents influence soil properties - such as pH, specific surface area, and functional group availability - which in turn influence triazine sorption behavior. For instance, atrazine and simazine sorption behavior is different for synthetic mixtures of model soil... 

Porous texture of the different materials was all characterized using nitrogen (N2) physisorption at 77 K and up to a pressure of 0.1 MPa. From the nitrogen physisorption data, obtained with the High Speed Gas Sorption Analyser NOVA 1200, the BET-surface area, total pore volume, microporous volume and t-volume were derived. The BET surface area (SBet) is the surface area of the sorbent according to the model formulated by Brunauer et al. [8] for planar surfaces. 

Finally, finely divided hydrous oxides of iron, aluminum, manganese, and silicon are the dominant sorbents in nature because they are common in soils and rivers, where they tend to coat other particles. This is the reason why numerous laboratory researchers have been studying the uptake of trace elements by adsorption on hydrous oxides (Dzomback and Morel, 1990). Partition coefficients (concentration in solid/concentration in the solution) for a number of trace elements and a great variety of surfaces have been determined. The comparison of these experimental with natural values should give information on the nature of the material on which trace elements adsorb in namral systems and allow quantitative modeling. 

Crystals of microporous materials must be formed into pellets of siutable dimensions, porosity and mechanical strength, or be formed into a membrane on the surface of support materials when used in practice. Such composite pellets or membranes offer a bidispersed porous structure, with macro-or mesopores between the crystals and micropores permeating the crystals. The overall rate of the transport in such systems depends on the interplay of various processes occurring within the pellets or membranes. Jordi and Do [24,46] have developed a general theoretical model and seven relevant degenerate models to analyse the frequency response spectra of a system containing bidispersed pore structure materials for slab, cylindrical and spherical macro- and micropore geometry. Sun et al. [47] also reported the theoretical models of the FR for non-isothermal adsorption in biporous sorbents. 

The Kelvin-equation-based methods were found to perform reasonably well for macro-porus and some mesoporous materials. However, it was foimd that the classical approach does not hold trae for micropores, in which case the intermolecular attractive forces between the sorbate and sorbent molecules predominate over bulk fluid forces such as surface tension. The potential energy fields of neighboring sorbent surfaces are known to overlap when the pores are only a few molecular dimensions wide. This results in a substantial increase in the interaction energy of an adsorbed molecule [12], which is not accounted for by simple classical thermodynamic models such as the Kelvin equation. 

The density of the sorbate (Ar) and the sorbent (oxide ion) surface densities, as well as the magnetic susceptibility of the oxide ion, also seems to have a moderate influence. The predicted pore size was the least sensitive to polarizability of the oxide ion. These results emphasize the need for standardizing physical parameters before discriminating between different models for their merit for predicting the PSD of microporous materials. 

MIL-53 materials provide a convenient model for comparison with traditional sorbents with similar geometric characteristics, such as activated charcoals and zeoUtes. The specific surface area estimated for MIL-53,1100 mVg (BET method), is similar to the average value for nanocarbon materials and exceeds that for zeolites. The framework of MIL-53 comprises unidimensional channels of 8.5 A in diameter, which is similar to the pore diameter in zeolites (6-12 A) and smaller than that in the IRMOF series, including MOF-5 (12-15 A). At the same time, the hydrogen adsorption capacity of MIL-53 is somewhat higher than that of the zeolite CaX (2.19wt.%, Table 2) and activated charcoals (2.15 wt.%) [179]. It is possible that this parameter is affected by the channel geometry, because zeolites more often have a 3D channel system, as opposed to the unbranched unidimensional channels in MIL-53. However, additional experiments on the adsorption mechanism are required to draw more definitive conclusions. 

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