Reversible and Irreversible Retention

Reversible and irreversible retention of contaminants on the subsurface solid phase is a major process in determining pollutant concentrations and controlling their redistribution from the land surface to groundwater. After being retained in the solid, contaminants may be released into the subsurface liquid phase, displaced as water-immiscible liquids, or transported into the subsurface gaseous phase or from the near surface into the atmosphere. The form and the rate of release are governed by the properties of both contaminant and solid phase, as well as by the subsurface environmental conditions. We consider here contaminants adsorbed on the solid phase. 

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In this chapter we present a general-purpose transport model of the multireaction type. The model was successfully used to predict the adsorption as well as transport of several heavy metals in soils (Selim, 1992 Hinz and Selim, 1994 Selim and Amacher, 2001). Multireaction models are empirical and include linear and nonlinear equilibrium and reversible and irreversible retention reactions. A major feature of... 

Other possible reasons for the high experimental capacity values are the high surface area and highly organized ID structure of titania nanotube layers. When a rate of 100 pA cm (2.5C) is used, the reversible and irreversible capacities are lower than at a rate of 5 pA cm, but the capacity retention for crystalline is around 96% (close and star-shaped symbols in Fig. 5.13c Table 5.1). 

Retention Rejection and Reflection Retention and rejection are used almost interchangeably. A third term, reflection, includes a measure of solute-solvent coupling, and is the term used in irreversible thermodynamic descriptions of membrane separations. It is important in only a few practical cases. Rejection is the term of trade in reverse osmosis (RO) and NF, and retention is usually used in UF and MF. 

The multireaction approach, often referred to as the multisite model, acknowledges that the soil solid phase is made up of different constituents (clay minerals, organic matter, iron, and aluminum oxides). Moreover, a heavy metal species is likely to react with various constituents (sites) via different mechanisms (Amacher et al 1988). As reported by Hinz et al. (1994), heavy metals are assumed to react at different rates with different sites on matrix surfaces. Therefore, a multireaction kinetic approach is used to describe heavy metal retention kinetics in soils. The multireaction model used here considers several interactions of one reactive solute species with soil matrix surfaces. Specifically, the model assumes that a fraction of the total sites reacts rapidly or instantaneously with solute in the soil solution, whereas the remaining fraction reacts more slowly with the solute. As shown in Figure 12.1, the model includes reversible as well as irreversible retention reactions that occur concurrently and consecutively. We assumed that a heavy metal species is present in the soil solution phase, C (mg/L), and in several phases representing metal species retained by the soil matrix designated as Se, S, S2, Ss, and Sirr (mg/kg of soil). We further considered that the sorbed phases Se, S, and S2 are in direct contact with the solution phase (C) and are governed by concurrent reactions. Specifically, C is assumed to react rapidly and reversibly with the equilibrium phase (Se) such that... 

As illustrated in Fig. 6-1, the model accounts for reversible as well as irreversible retention reactions of the concurrent and consecutive type. We assumed Cu to be present in the soil solution phase, C (mg L-1), and in several phases representing Cu retained by the soil matrix as Se, S2, Ssand Sm (mg kg-1 soil). We further considered the sorbed phases Se, and S2 are in direct contact with the solution phase (Q and are governed by concurrent reactions. Specifically C is assumed to react rapidly and reversibly with the equilibrium phase (Se) such that... 

Ion-pair HPLC mode is a superposition of two competitive processes ion-exchange and reversed-phase. Component retention is strongly dependent on the type of ionpairing agent, its concentration, and most of all, on the history of the used column. The virgin reversed-phase (RP) column does show the hydrophobic selectivity in the ion-pair mode. However, with time, the adsorbent surface can become covered with a dense layer of adsorbed surfactant. This may irreversibly transform the RP column into an ion-exchange one. 

K. Biittner, C. Pinilla, J.A. Appel, and R.A. Houghten, J Chromatogr. 625, 191 (1992). P.C. Sadek, Elucidation of the Factors Responsible for Small Solute Retention and Irreversible Protein Binding in Reversed-Phase High Performance Liquid Chromatography, Ph.D. dissertation. University of Minnesota, Minneapolis, 1985,... 

The general phenomenon of polymer adsorption/retention is discussed in some detail in Chapter 5. In that chapter, the various mechanisms of polymer retention in porous media were reviewed, including surface adsorption, retention/trapping mechanisms and hydrodynamic retention. This section is more concerned with the inclusion of the appropriate mathematical terms in the transport equation and their effects on dynamic displacement effluent profiles, rather than the details of the basic adsorption/retention mechanisms. However, important considerations such as whether the retention is reversible or irreversible, whether the adsorption isotherm is linear or non-linear and whether the process is taken to be at equilibrium or not are of more concern here. These considerations dictate how the transport equations are solved (either analytically or numerically) and how they should be applied to given experimental effluent profile data. 

Kloubek [S3] considers the concept of pore dimension to be erroneous because of the above errors and recommended that the results be presented using the actual values of p instead of calculated radii. He suggested that the dependence of net re-intrusion and retention volumes on mercury pressure should be evaluated [54], In this way pores can be separate into two groups, one in which mercury is retained reversibly and the other where retention is irreversible. This method of mercury porosimetry evaluation offers a valuable contribution to understating porous structures and their properties. 

Because there are three separate retention equations (eqns. 12, 17, 18) to account for the three cationic metal species in solution, one might expect to find three peaks in the final chromatogram. In reality, however, only one peak is usually evident. The number of peaks that appear is a function of the kinetics of the equilibrium processes occurring in solution. One peak is seen if the rate of all reversible and/or irreversible chemical equilibria associated with an eluite as it migrates through the column is fast relative to the elution time of the eluite. If interconversion between the metal-ligand species is slow, however, aqrmmetric or multiple peaks may result. 

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

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