Adsorptive forces, soil

Colloidal Stabilization. Surfactant adsorption reduces soil—substrate interactions and faciUtates soil removal. For a better understanding of these interactions, a consideration of coUoidal forces is required. 

Both adsorptive and capillary forces play an important part in soil-liquid interaction (see Figure 18.3). This is very important for unsaturated soil. The total force (i.e., the sum of capillary force and adsorptive force) is termed the matrix potential, which has a negative gage pressure relative to the external gas pressure on the soil water (more often the gage pressure is referred to as the atmospheric pressure). 

Water present in and moving through the unsaturated zone is subjected to several rules of physics in addition to those influencing the water below the water table. The presence of retained moisture above the water table is due to adsorptive forces between the water molecules and soil particles, in addition to surface tension of the water surface. 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

The Cl-, NOT, and SG anions are considered to be nonspecifically adsorbed. Table 9.1 shows typical data for Cl- and SO - adsorption by soils. The capacity of soils to adsorb anions increases with increasing acidity and is much greater for the kaolinitic soil, which has significant pH-dependent charge. At all pH values, the divalent SO - ion is adsorbed to a greater extent than the monovalent Cl- Ion, as would be expected on the basis of electrostatic attraction forces alone. 

Adsorption soil water — this water is bonded by adsorption forces to soil rock particles. The most strongly bonded is the layer of water molecules which is present immediately on the solid soil particles and provides hygroscopic water. The external layer of water molecules bonded to the solid particles of soil by intermolecular forces is called envelope water. Whereas hygroscopic water moves in the gaseous state only due to different vapour tension the envelope water moves also in the liquid state but only very slowly, and provides soil or rock moisture [1]. 

The adsorbing of bromacil by soil organic matter relative to several other herbicides is shown in Figure 11. The compound was adsorbed in low amounts, probably as a result of its high solubility. The pH of the system was approximately 5.7, so bromacil was present predominantly in the molecular form and was probably adsorbed through hydrogen bonding or other physical adsorption forces. 

The physical mechanisms underlying the effect of moisture on interparticle cohesion have received considerable attention in the soil physics literature and now are well understood. The total potential energy of soil water arises from both capillary and adsorptive forces and is represented as the matric potential (rlfm)- The relation of (Pa) to the humidity of the air within the soil voids is modeled by the Kelvin equation ... 

At the solid-liquid interface in soils, the specific distribution of cations and anions should be considered, as well as possible changes in the structure of water. The distribution of ions at the interface is determined by the electrical double layer. The structure and properties of water near the interface are governed by adsorption forces. The major contributor to interfacial effects in the soil system is the clay fraction because of the large total surface area per unit weight of clay particles. 

Since sorption is primarily a surface phenomenon, its activity is a direct function of the surface area of the solid as well as the electrical forces active on that surface. Most organic chemicals are nonionic and therefore associate more readily with organic rather than with mineral particles in soils. Dispersed organic carbon found in soils has a very high surface-to-volume ratio. A small percentage of organic carbon can have a larger adsorptive capacity than the total of the mineral components. 

The three types of adsorption are (1) physical, (2) chemical, and (3) exchange adsorption. Especially important to the success of in situ treatment by Fe° are the soil characteristics, which affect soil sorptive behavior such as mineralogy, permeability, porosity texture, surface qualities, and pH. Physical adsorption is due to van der Waal s forces between molecules where the adsorbed molecule is not fixed on the solid surface but is free to move over the surface and may condense and form several superimposed layers. An important characteristic of physical adsorption is its reversibility. On the other hand, chemical adsorption is a result of much stronger forces with a layer forming, usually of one molecule thickness, where the molecules do not move. It is normally not reversible and must be removed by heat. The exchange adsorption and ion exchange process involves adsorption by electrical attraction between the adsorbate and the surface (Rulkens, 1998). 

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