Electrostatic Adsorption of Anions

Electrostatic adsorption of anions is one of the important characteristics of variable charge soils. This is caused by the fundamental feature that these soils carry a large quantity of positive surface charge. However, because these soils carry positive as well as negative surface charges, they may exert both attractive and repulsive forces on anions. Therefore, the situation in the adsorption of anions by these soils may be quite complex. There may also be the occurrence of negative adsorption of anions. Besides, for some anion species both electrostatic force and specific force may be involved during their interactions with variable charge soils. As shall be seen in this chapter, such specific force may be operative even for some anion species such as chloride that are generally considered as solely electrostatic in nature during adsorption. Because of historical reasons, the literature on electrostatic adsorption of anions by soils is very limited. Nevertheless, as shall be seen in this chapter, the topic is of interest in both theory and practice. In the present chapter, adsorption of anions shall be discussed mainly from the viewpoint of electrostatic adsorption. The other type of adsorption, specific adsorption or coordination adsorption, shall be dealt with in Chapter 6. The radius of anions is generally much larger than that of cations. Thus, the charge density on anions would be low. When hydrated, because of the smaller ion-dipole force exerted on water molecules, anions are less hydrated than cations. This can be seen in Table 4.1. The rH/rc ratio for cations ranges from 2.22 to 6.37, while that for anions is smaller than 2 except for F-. The orientation of water molecules around anions, especially in the primary hydration region, is also different from that around cations (Conway, 1981). Because of the small rH/rc ratio, hydration does not induce the change in order of size when anions of the same valency are compared. For example, the crystal radii of Cl-, NO3-, and ClO4- are 0.181, 0.264, and 0.292 nm, respectively, while the hydrated radii of these ions are 0.332, 0.335, and 0.338, respectively.