Cation Exchange Outer-Sphere Complexation

As usual, hydrogen ion plays a special role in the interfacial reactions of geological systems. The characteristic pH range of groundwater ranges from 6 to 8, that is, the solution is close to neutral. Water molecules, however, provide an unlimited source of hydrogen ions, and, of course, hydroxide ions, which have an effect on the solid and solution phases, as well as on the interface. 

Hydrogen ions participate in the cation-exchange processes of the interlayer space. As will be seen later (Section 2.7.1), they have a very large affinity for the layer charge. Hydrogen and hydroxide ions are potential-determining ions of the external surfaces via the protonation and deprotonation processes of aluminol and silanol sites. In acidic media, the degradation of aluminosilicates can be observed. 

In addition, water molecules may cause the hydrolysis of some cations, and the formation of oxide, hydroxide colloids, and precipitates. Consequently, the cations in the solution precipitate, forming an independent solid phase. In this case, the solubility of the precipitate will determine the metal ion concentration in the solution. Thus, caution should be exercised in the so-called adsorption edges at pH values when the hydrolysis of dissolved cations takes place (Schindler et al. 1976 McKenzie 1980) since the process is not related to adsorption but to hydrolysis. 

Since the cation-exchange processes of the main cations (sodium, potassium, magnesium, calcium) have a significant role in the nutrient cycle of soils, the classical literature has discussed their cation-exchange processes in detail (e.g., Boyd et al. 1947 Gaines and Thomas 1953 Howery and Thomas 1965 Sposito 1981 Filep 1999). As described earlier, cation exchange is characterized by the selectivity coefficients and 

Another characteristic of some of these cations compared to those of the main element is that they can be sorbed on the deprotonated edge sites by inner-sphere complexation. Some examples will be discussed in detail in Section 2.5. 

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Interaction 1 denotes electrostatic forces between humic substances (negatively charged) and metal ions (positively charged). It is a relatively weak interaction (outer-sphere complex) and the cation can be readily exchanged by other weakly bonding cations,... 

During the exchange in the interlayer space, the cations keep their hydrate shell this process is called outer-sphere complexation. It is directed by electrostatic forces that, the greater the charge and smaller the size of the hydrated cation, the more favorable is the ion exchange. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Figure 1. Part a Surface complex formation of an ion (e.g., cation) on a hydrous oxide surface. The ion may form an inner-sphere complex (chemical bond), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer. (Reproduced with permission from reference 2. Copyright 1984.) Part h Schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups (s), inner-sphere complexes (a), outer-sphere complexes ( 3), and the diffuse ion swarm (d). In the case of an inner-sphere complex with a ligand (e.g., F or HPOfi ), the surface hydroxyl groups are replaced by the ligand (ligand exchange). (Modified from reference 3.)...

In the next paper (Jensen et al., 2002), EXAFS data were performed for one IL and 1-octanol, demonstrating that two nitrate ions are present in the Sr first coordination sphere when 1-octanol is the solvent, while no nitrate can be detected in the first coordination sphere when CiC5imTf2N is the solvent. However, the authors noted that it does not prove unambiguously that nitrate ions are not coextracted with because EXAFS cannot detect NO3 in an outer-sphere complex. This is perfectly right, but we would like to stress that it does not prove either that nitrates are coextracted with in an outer-sphere complex. In fact, in this case, EXAFS does not help identifying the extracted species. Consequently, Dietz and coworkers performed nitrate titration of the IL phase and wrote that the amounts of anion coextracted into the IL are vastly insufficient to produce neutral Sr complexes, so they concluded that the phase transfer reaction proceeds primarily through cation exchange as described by Eq. (17). As can be clearly seen, this model is identical to the IX model. 

Complex formation of lanthanides is a rapid process. This is very obvious from the high rates for water exchange in lanthanide ions. Thus rapid motion of molecules of water in and around the lanthanide ion can be envisaged. Most of the time the relative positions with respect to one another and the cation are the same. The complexes have dynamic structures. Inner sphere complexation will certainly affect the water molecules in the outer spheres. Both dissociative and associative pathways of complex formation in aqueous media for lanthanides are possible. 

Rapid exchange reactions between Mn04 and Mn04- have been studied, and cation effects were interpreted in terms of an outer-sphere bridged activated complex. 

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