Sorption complexes

Sorption complex Electron pair donor Hydrogen bond Ion pair (hydrogen... 

Sylwester, E. R., Hudson, E. A. Allen, P. G. 2000. The structure of uranium(VI) sorption complexes on silica, alumina and montmorillo-nite. Geochimica et Cosmochimica Acta, 64, 2431-2438. 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

One approach commonly used to study how strongly an aqueous ion binds to a sohd surface is measurement of adsorption isotherms as a function of pH, ionic strength, and total metal-ion concentration, in the presence or absence of other ions or organic coadsorbents, ft is often assumed that significant inhibition of adsorption with increasing ionic strength at a given pH indicates that the sorption complexes are dominantly of the weakly bound, outer-sphere type [123]. In contrast, when there is... 

Stable adsorption complexes are characterized by local minima on the potential energy hypersurface. The reaction pathway between two stable minima is determined by computation of a transition state structure, a saddle point on the potential energy hypersurface, characterized by a single imaginary vibrational mode. The Cartesian displacements of atoms that participate in this vibration characterize movements of these atoms along the reaction coordinate between sorption complexes. 

The transition state that leads to the formation of the surface methoxy species is similar to that of Sinclair and Catlow (241) insofar as it exhibits little strain around the planar CH3 component. However, the activation energy for methoxide formation (referenced to the initial sorption complex of two methanol molecules) is lower in the calculations of Blaszkowski et al. (245). The value is 160 kJ/mol, compared with the value of Sinclair and Catlow (241) of 180-190 kJ/mol. The reason for this difference lies in the mode of interaction of the methanol molecules. The twofold interaction modeled by Sinclair and Catlow results in a costly rotation to allow formation of the transition state. Such a rotation is not required in the case of a threefold interaction. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

Pfeifer et al. (263) conclude from their measurements of T, and T2 versus temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x 10-9 sec at 50°C with nonlocalized cations and at - 10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/Al ratios, which agrees with model calculations by Dempsey (282) of the electrostatic fields around cations. At higher coverages the mobility of H20 is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. 

The presence of either HEDTA or EDTA resulted in significantly lower neptunium and plutonium sorption. Complexation of the neptunium and plutonium by HEDTA and EDTA may have caused the reduced sorption. However, this evidence for complex formation was not consistent with the observations made in the solubility studies (HEDTA increased and EDTA decreased neptunium solubility neither affected plutonium solubility). Thus, HEDTA and EDTA may have decreased neptunium and plutonium sorption through some undetermined effect on the sediment minerals. 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

The EXAFS results reported for the untreated samples (see Section 8.3.4) led to the conclusion that Zn may form highly ordered inner-sphere sorption complexes with gibbsite surfaces or substitute into an octahedral Al-hydroxide layer of some sort. The use of sequential extraction enabled more concrete conclusions to be made. For the nonextracted soil samples (bulk and coarse), second-shell Al coordination numbers did not exceed four, in fine with the dioctahedral structure of gibbsite sheets (only two out of three metal positions are occupied). Elsewhere, a gradual increase was observed in Al coordination up to six with each extraction step, indicating that Zn is part of a fully occupied, trioctahedral Al-Zn2+ layer and not part of gibbsite or another dioctahedral Al compound.67 While dioctahedral Al-hydroxide layers are... 

Fig. 9.37. Typical model for sorption complexes of proline enantiomers on (.S )-proline- or (S)-hydroxyproline-derived poly.styrene-type sorbents. Retention of (5)-Pro is diminished by the steric interaction with the water molecule crdinatcd in the axial position of the Cu(ll) ion. Retention of (R)-Pro is enhanced by the (favourable in the aqueous mobile phase) hydrophobic interaction with the non polar polystyrene chain (reprinted with permis.sion from Ref. 1403]).

Semiempirical quantum chemical calculations (MNDO, AMI and PM3) have been performed for determining the possible positions of exchangeable cations and geometries of sorption complexes of cyclopropane and propene in zeolite A. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

An example of the use of ab initio XANES calculations to determine nanoparticle structure is the Zn/ferrihydrite sorption system examined by Waychunas et al. (2001). In the case of sorption complexes the XAI S spectrum of the sorbed species will contain information about the local structure of the substrate, and thus the structural nature of the full sorption complex. In the Zn/ferrihydrite system it was observed via EXAFS that the number of Fe next nearest neighbors about the sorbed Zn ion decreased as the Zn sorption density increased. Direct calculation of the XANES structure identified MS paths that changed in number as a function of cluster size (and thus number of neighbor Fe atoms), and gave rise to XANES features that changed in intensity (Fig. 32). These changes agreed well with the structural interpretation of the EXAFS and the crystal chemistry of Zn-Fe hydroxides. 

Figure I. Schematic representation of possibie arsenic sorption complexes on mineral surfaces (Modifiedfrom Brown, 1990). Outer-sphere (physisorbed) complexes, in which As is fully coordinated by water molecules, are bound to the mineral surface by weak electrostatic forces. Inner-sphere (chemisorbed) complexes are characterized by the formation of one or more chemical bonds between the sorbing As oxoanion and the mineral surface. Surface precipitation refers to the formation of a new phase on the mineral surface. Reprinted with permission.

Chisholm-Brause, C. J., and D. E. Morris. 1992. Specia-tion of uranium(VI) sorption complexes on montmo-rillonite. Proc. 7th inti. symp. on water-rock interaction, ed Y. K. Kharaka and A. S. Maest, pp. 137-40. Rotterdam A. A. Balkema Publ. 

Cheah, S. F., Brown, G. E., and Parks, G. A. (1998). XAFS spectroscopy study of Cu(II) sorption on amorphous SiO2 and y-ALOs effect of substrate and time on sorption complexes. J. Colloid Interface Sci. 208, 110-128. 

Chisholm-Brause, C.J. 1991. Spectroscopic and equilibrium study of cobalt (II) sorption complexes at oxide/water interfaces. Ph.D. diss. Stanford Univ., Stanford, CA. 

O Day, P.A., CJ. Chisholm-Brause, S.N. Towle, G.A. Parks, and G.E. Brown, Jr. 1996. X-ray absorption spectroscopy of Co(II) sorption complexes on quartz (-Si02) and rutile (Ti02). Geochim. Cosmochim. Acta 60 2515-2532. 

Contact of polysaccharides with halogens may produce sorption complexes. The action of chlorine on cereals has been used to improve baking properties,2253 but the observed effect could be due to the oxidative effect of chlorine on flour polysaccharides (consult Section VII) and other reactions of nonsaccharide flour components. Chlorination of flours increases their viscosity2254,2255 and hydrophobicity.2256 Chlorinated flours have also been reported as adhesives2257 and as oil collectors.2258,2259 Their stability can be increased by neutralization with ammonia.2260... 

---