Solutes between Solid, Liquid and Gas Phases

This chapter is concerned with how ions and uncharged solutes in the water and soil solution in submerged soils interchange between the solid, liquid and gas phases present. This is a large topic. 1 give here the bare essentials needed to understand the transport and transformation processes discussed elsewhere in the book, and 1 give references to more detailed treatments where appropriate. The water and atmosphere overlying the soil are dealt with first and then the additional complexities in the soil. 

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Interchange of Solutes between Solid, Liquid and Gas Phases... 

This phase diagram shows how temperature and pressure affect the solid, liquid, and gas phases of a pure solvent (solid lines) and a solution (dashed lines). The difference between the solid and dashed lines corresponds to vapor pressure lowering (AP), boiling point elevation (AT[,), and freezing point depression (ATf). 

Although the solute and solvent can be any combination of solid, liquid, and gas phases, liquid water is indisputably the best known and most important solvent. Consequently, we emphasize aqueous solutions in this chapter, but you should always remember that dissolution also occurs in many other solvents. We describe formation of aqueous solutions by considering the intermolecular forces between the solute and water molecules. Because these forces can be quite different for molecular solutes and ionic solutes, we discuss these two cases separately. 

Synchroton radiation has been employed as a spectral source for a study of the absorption of HCN and DCN in the wavelength range 80—120nm. A vacuum-u.v. spectrophotometer for absorptions in the region 105—200 nm has been described. Solid-, liquid-, and gas-phase samples could be analysed at temperatures from —200 to 100 °C and at pressures between 0 and 150 atmospheres. The absorption spectrum of tra j-di-imide in the vacuum-u.v. has been measured. First-derivative u.v. spectroscopy has been employed in the analysis of Watts nickel plating solutions for trace amounts of saccharin. Impurity levels of 0.1 p.p.m. have been recorded. A wavelength modulated derivative spectrophotometer with a multi-pass absorption cell has been developed for the automatic analysis of atmospheric pollutants. Traces of SOj, NO, and NO2 were detected with limits of 15, 13, and Sp.p.b., respectively. A double-beam single-detector absorption spectrometer has been constructed. Independence... 

A numerical model was developed to simulate MeBr movement in soil and volatilization into the atmosphere. The model simultaneously solves partial differential equations for nonlinear transport of water, heat, and solute in a variably saturated porous medium. Henry s Law is used to express partitioning between the liquid and gas phases and both liquid and vapor diffusion are included in the simulation. Soil degradation is simulated using a first-order decay reaction and the rate coefficients may differ in each of the three phases (i.e., liquid, solid, or gaseous). 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

The mobile phase (solvent moving through the column) in chromatography is either a liquid or a gas. The stationary phase (the substance that stays fixed inside the column) is either a solid or a liquid that is usually covalently bonded to solid particles or to the inside wall of a hollow capillary column. Partitioning of solutes between the mobile and stationary phases gives rise to separation. In gas chromatography, the mobile phase is a gas and, in liquid chromatography, the mobile phase is a liquid. 

FIGURE 7.2 Schematic view of the interplay between gas, liquid, and solid phases in soil (a) soil can be viewed as an assortment of solid particles (often of colloidal sizes), with the pores between them partially filled by a liquid phase (the soil solution) and a gas phase (essentially air). The proportion of liquid and gas phases is dependent on rain and draught periods rain fills up the pores, whieh are afterward emptied by leaching toward lower horizons, (b) In a close-up, many proeesses (mainly ehemieal reactions) take place in the soil solution, including gas dissolution—separation, homogeneous chemical equilibria, and heterogeneous processes, including adsorption and surface chemical reactions. 

Note that all four properties are defined by an equilibrium between the liquid solution and a solid, liquid, or gas phase of the pure solvent. The properties called colligative (Latin tied together) have in common a dependence on the concentration of solute particles that affects the solvent chemical potential. 

The solid phase presents some fundamental differences from liquid and gas phases. First, the effect the solid has on the electronic structure of a sorbate can be profound (e.g., H2 chemidissociation on metals). Thus new processes may be energetically accessible in solid-state systems that are not important in liquid or gas phases. Second, dynamical processes in solid-state systems can be significantly different from those in liquid or gas phases. The average environment that a solute molecule encounters in gas and liquid phases is translationally invariant. This is not true for the solid with well-defined lattice sites e.g., the average environment a solute molecule sees near a lattice site is very different from that near an interstitial site. Therefore, diffusion of sorbates in or on a solid can often be treated as isolated jumps between well-defined sorption sites, and the diffusion constant can be approximated from the rate constants for isolated jumps. 

All chromatographic methods function on the same principle, which is the partitioning of components in a mixture between two phases (1) a stationary phase, which may be a solid, liquid, or gel, and (2) a mobile phase, which may be gas, liquid, solution, or a varying mixture of solvents. When a mixture is introduced into a chromatographic system, its components are alternately absorbed and desorbed, that is, partitioned between, the stationary and mobile phases. Partitioning is caused by different polarities of the stationary and mobile phases and the compounds being separated. Compounds in the mixture have different affinities for the phases and they will move at different rates in the chromatographic system and thus be separated. 

Dissolution and precipitation in the subsurface are controlled by the properties of the solid phases, by the chemistry of infiltrating water, by the presence of a gas phase, and by environmental conditions (e.g., temperature, pressure, microbiological activity). Rainwater, for example, may affect mineral dissolution paths differently than groundwater, due to different solution chemistry. When water comes in contact with a solid surface, a simultaneous process of weathering and dissolution may occur under favorable conditions. Dissolution of a mineral continues until equilibrium concentrations are reached in the solution (between solid and liquid phases) or until all the minerals are consumed. 

Chromatographic methods are divided into two types according to how solute molecules bind to or interact with the stationary phase. Partition chromatography is the distribution of a solute between two liquid phases. This may involve direct extraction using two liquids, or it may use a liquid immobilized on a solid support as in the case of paper, thin-layer, and gas-liquid chromatography. For partition chromatography, the stationary phase... 

In the solid state, the phenyl nuclei of biphenyl are coplanar (Dhar, 1932 Kitaigorodsky, 1946 Rizvi and Trotter, 1961). Bastiansen (1949) reports that the angle between the rings is 45° in the gas phase as determined by electron diffraction. The conformation of biphenyl in solution is not established (Wheland, 1955). It has been argued to be coplanar on the basis of the Kerr constant (Chau et al., 1959). Recent observations of the electronic spectrum of biphenyl and certain derivatives in the solid, liquid, and vapor states were interpreted as indicative of a 20° deviation from coplanarity in solution (Suzuki, 1959). 

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