Cations Adsorption

Adsorption of cations on iron oxides (Table 11.3) may be specific or non specific. With non specific adsorption, there is at least one water molecule between the adsorbing species and the surface functional group. Specific adsorption involves interaction with deprotonated surface hydroxyl groups to form mono- and bi-nuclear, inner sphere complexes, i.e. 

Cation adsorption is accompanied by release of protons with the number of protons released per cation being termed z. Ideally, z should be one for mononuclear complexes and two for binuclear complexes, but in fact, the value is often found to be between one and two. In addition, the value of z may also vary between these limits as the pH rises. The occurrence of intermediate values has been attributed to the simultaneous formation of binuclear complexes and mononuclear species and to the presence of different surface sites (Hohl Stumm, 1976 Benjamin Leckie, 1891 Schindler, 1984). 

The positive adsorption of metal cations by the solid phases in soil can involve the formation of either inner-sphere or outer-sphere surface complexes, or the simple accumulation of an ion swarm near the solid surface without complex formation. These adsorption mechanisms are implied in the development of the concept of surface charge balance (Eq. 3.3) and were illustrated, for the case of surface complex formation, in Figs. 1.8 and 1.10. The quantitative relationship between these mechanisms and measured surface excesses of metals on soil minerals is taken up in Chap. 5. In the present section, emphasis is placed on the qualitative 

LiCAND EFFECTS. The effccts of metal-complexing ligands in a soil solution on the adsorption of metal cations by soil constituents can be classified into four general categories 

The ligand has a high affinity for the metal and forms a soluble complex 

The ligand has a high affinity for the absorbent and is adsorbed, and the adsorbed ligand has a high affinity for the metal. 

This method has also been successfully applied for the preparation of gold on silica, conventional, or mesoporous (SBA-15). The particles appear smaller when they are reduced under H2 rather than when they are calcined (Table 17.4). However, Zhu et al. [71] showed that the gold particles still retain residual organic compounds after reduction, and that it is necessary to proceed to further calcination at 400°C to completely eliminate them. TEM showed that the gold particles were located inside the pores of SBA-15. The same paper 

Silica Surface area (m g- ) pH Au loading (wt%) Thermal treatment dAu (nm) Reference 

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Thus, it was established that adsorption of metal hydroxide species on the surface of the substrate provides a nucleation layer which is chemically converted to the metal chalcogenide. The forming metal chalcogenide layer acts then as a catalytic surface for subsequent anion and cation adsorption. 

Cation adsorption in binary solutions can be determined according to Eq. (10.34) from the relation between ESE and at a constant value of E ... 

Trujillano R., Villain R, Louis C., and Lambert J.-R 2007. Chemistry of silica-sup-ported cobalt catalysts prepared by cation adsorption. 1. Initial localised adsorption of cobalt precursors. J. Phys. Chem. C 111 7152-64. 

Figure 8.1. Schematic view of thin-film growth during one SILAR cycle, (a) cation adsorption cation O anion (b) rinsing (c) anion reaction anion o cation and (d) rinsing. Reprinted with permission from Lindroos 1997.

The uptake of a cation into a carbonate is thought to proceed via adsorption, eventually leading to surface precipitation and formation of a solid solution. Kinetics of cation adsorption occurs usually in subsequent steps the specific adsorption, i.e., the transfer at a carbonate surface from the solution phase into the adsorbed state must be assumed for most cations to be very fast. Most likely its rate is related to the rate of water exchange (cf. Chapter 4.4). Two examples of the rates of uptake or carbonate surfaces are given in Fig. 8.7. 

The right-hand side of Equation A-7 is the number of moles of H lost or gained by the solution per mole of B adsorbed. Thus, in terms of the general stoichiometric relationship for cation adsorption... 

Fig. 5-6. Electron energy levels of an adsorbed particle on adsorbent metal (a) cationic adsorption, (b) metallic adsorption. A = electron afGnity I = ionization energy 4> = work function. [From Benard, 1983.]...

Both cationic adsorption and anionic adsorption belong to what is called ionic adsorption. Covalent adsorption is due to the localized covalent bonding, and metallic adsorption is due to the delocalized covalent bonding. The distinction among these three modes of chemisorption, however, is not so definite that the transition from the covalent through the metallic to the ionic adsorption may not be discontinuous, but rather continuous, in the same way as the transition of the three-dimensional solid compounds between the covalent, metallic, and ionic bonding. 

Effects of Flooding and Redox Conditions onfs. I know of no published data on this. Bnt it is likely that the natnre of particle surfaces in intermittently flooded soils wonld restrict snrface mobility. For ions to diffuse freely on the surface there must be a continuous pathway of water molecules over the surface and uniform cation adsorption sites. But in intermittently flooded soils the surface typically contains discontinuous coatings of amorphous iron oxides on other clay minerals, and on flooding reduced iron is to a large extent re-precipitated as amorphons hydroxides and carbonates as discussed above, resulting in much microheterogeneity with adsorption sites with disparate cation affinities. 

Metal cation adsorption processes include exchange, Coulombic, and site-specific adsorption. Heavy metal cations exhibit exchange reactions with negatively charged surfaces of clay minerals. Cationic adsorption is affected by the pH and in an acid environment (pH < 5.5), and some heavy metals do not compete with Ca " (a ubiquitous constituent in the subsurface) for mineral adsorption sites. At a higher pH, heavy metal adsorption increases abruptly and becomes irreversible. 

Anion adsorption on iron oxides is frequently described by this equation whereas cation adsorption data is often fitted to the Freundlich equation, i.e. 

The presence of anions in solution may enhance cation adsorption by formation of mixed metal/ligand surface complexes (Schindler, 1990). This effect is termed ternary adsorption. Two forms of ternary adsorption have been identified ... 

The available nonaqueous reactivity data from Meyer and co-workers [13] and from Schmehl and co-workers [60] on injection and back ET to and from 2 appear to point, instead, to a special role for equilibrium cation adsorption. In the absence of initial alkali metal ion adsorption, injection from MLCT-type chromophores does not occur. In their presence, injection does occur, but in... 

Double-layer capacitance values are usually expressed as microfarads per square centimeter remember that practical electrical units, including the farad, are consistent with SI units. Comment on these results in terms of anion and cation adsorption. 

ION RETARDATION. A process hused on amphoteric (hifunclionalt ion-exchangc resins containing both anion and cation adsorption sites. These siies will associate with mohile anions and cations in solution and thus remove both kinds of ions from solutions. These ions may be eluted bv rinsing with water. This process can make elean separations of ionic-nonitmic mixtures It has also been suggested fur demineralization of salt solutions. 

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