Anion and cation binding

The binding of cations and anions by Fe oxides through surface adsorption (see Chap. 11) and/or incorporation (see Chap. 3) makes soils important sinks for a range of compounds such as heavy metals, phosphate and sulphate. This can be derived from significant correlations between such compounds and the Fe oxide content of the soils. 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Element Laterites from basalts (n SI) Accumulation factor with respect to parent rock Lateritic soils of Western Australia (n 39) Range of Correlation coefficient with concentration Fe concentration  

West Australian soils. The lower left graph shows gruency curves II, III and V intermediate situa-the plots that could be obtained for different dis- tions (Singh, Gilkes, 1992 with permission). 

As in pure systems, the adsorption of anions and cations on iron oxides is strongly pH dependent. This has to be kept in mind when an optimum pH is to be obtained with liming. The adsorption of phosphate, arsenate etc. increases as the pH falls below 7, whereas the adsorption of heavy metal cations rises as pH goes up (see eq. 11.18 11.19). Therefore, as soils become more acidic, heavy metals will be released into the soil solution. Conversely, liming soils has the opposite effect. 

---

Channa, A., Steed, J. W., Anion and cation binding by a pendant arm cyclam and its macrobicyclic derivatives. Dalton Trans. 2005, 2455-2461. 

Faced with the challenge of ion pair binding, there are three basic ion pair receptor designs contact ion pair receptors (including cascade complexes) in which the anion and cation are bound as a contact ion pair, ditopic receptors with individual, well-separated anion and cation binding sites, and zwitterion receptors, Figure 5.2. We will discuss some examples of each type of complex in the following sections. 

We have already seen in Section 2.2 how the transport of both anions and cations is a vital part of biochemistry. We will examine supramolecular models of biological ion channels in detail in Chapter 12 but here we focus on some simple ion transport systems (ionophores) relevant to simultaneous anion and cation binding. Because of the need to maintain overall and local charge neutrality during any transport process the transport of individual ions across a biological or artificial membrane never occurs in isolation. There are two kinds of primary transport processes. Ion exchange or antiport, occurs when chemically different ions of like charge such as Na+ and K+ are simultaneously transported in... 

The anion- and cation-binding portions of an ion pair binding system do not necessarily have to be covalently linked together and indeed it is synthetically usually much more straightforward and cheaper... 

Figure 15 A schematic representation of [cG 9] 2- MX (H2O ) that shows the anion and cation binding sites...

Another way to view salt binding is to accept the fact that salts prefer to form ion pairs and. thus, develop ditopic receptors that bind salts as associated ion pairs (Fig. 2). Two examples are the macrobicycles 14 and 15 with juxtaposed anion- and cation-binding sites. In the case of 14, the presence of Na cation enhances Cl binding by about a factor of 10. An x-ray crystal structure of the 14 NaCl complex shows that the salt is bound as a solvent-separated ion pair. The Na sits in the crown ring, and... 

The next phase of our research is the design of bifunctional neutral receptors which contain both anionic and cationic binding sites and therefore are capable to complex simultaneously anionic and cationic species in apolar solvents. There are only a few examples known in the literature related to this subject. 

---