ABSTRACT
The binding of ions to soil organic matter (SOM), and particularly soil humic substances (HSs), is, at rst sight, akin to metal ion complexation to small organic molecules, where cations such as Cu(II), Cd(II), and Fe(II/III) are coordinated to Lewis basic groups (electron-pair donors), such as –COO− and −NH2. However, HSs are relatively large molecules (or molecular associations, see Section 10.3.5) with colloidal properties; consequently they also retain cations electrostatically, showing cation exchange properties. As discussed in Chapter 10, HSs, being anionic, show little interaction with other anions. In HSs, groups able to complexate (i.e., to bind specically) are mostly oxygen-containing ones, given the typical composition of HSs; most of such groups are carboxylates; some others are also considered usually, mostly phenolic moieties (Section 10.3.5). Despite the exact nature of these groups, the approach is basically that introduced in Chapter 11, considering both specic and electrostatic interactions. At variance with mineral colloids, the range of binding groups and binding constants is higher and is also poorly known (see Section 10.4), so that detailed treatments, such as the CD-MUSIC model discussed in Section 12.3, are not possible. Thus, the assortment of different coordinating groups (which can be regarded as adsorption sites) is generally modeled through some sort of distribution of interaction free energies (or, equivalently, of binding equilibrium constants), that is, an afnity spectrum, as often termed (Section 11.2). For the electrostatic contribution, diverse approaches can be found; some of them borrow concepts from models developed for mineral particles, presented in Chapter 12. However, as HSs are not really solid, hard particles but softer molecular aggregates (Duval et al. 2005), several more specic treatments have been developed.
