Preferential sorption-capillary flow

Reverse osmosis models can be divided into three types irreversible thermodynamics models, such as Kedem-Katchalsky and Spiegler-Kedem models nonporous or homogeneous membrane models, such as the solution—diffusion (SD), solution—diffusion—imperfection, and extended solution—diffusion models and pore models, such as the finely porous, preferential sorption—capillary flow, and surface force—pore flow models. Charged RO membrane theories can be used to describe nanofiltration membranes, which are often negatively charged. Models such as Dorman exclusion and the... 

Figure 1. Schematic of preferential sorption-capillary flow mechanism for reverse-osmosis separations of sodium chloride from aqueous solutions...

The preferential sorption-capillary flow mechanism of reverse osmosis does that. In the NaCl-H20-cellulose acetate membrane system, water is preferentially sorbed at the membrane-solution Interface due to electrostatic repulsion of ions in the vicinity of materials of low dielectric constant (13) and also due to the polar character of the cellulose acetate membrane material. In the p-chlorophenol-water-cellulose acetate membrane system, solute is preferentially sorbed at the interface due to higher acidity (proton donating ability) of p-chlorophenol compared to that of water and the net proton acceptor (basic) character of the polar part of cellulose acetate membrane material. In the benzene-water-cellulose acetate membrane, and cumene-water-cellulose acetate membrane systems, again solute is preferentially sorbed at the interface due to nonpolar... 

Preferential Sorption - Capillary Flow Model (porous model)... 

FIGURE 8.6 (a) schematic representation of preferential sorption-capillary flow theory (b) critical pore diameter for separation (c) flnx decline with time (d) correction factor for surface area of cellulose acetate and (e) solnte rejection as a function of operating time. 

A number of models have been developed over the years to describe reverse osmosis. These models Include the solution-diffusion model, the finely porous model, and the preferential sorption - capillary flow model. In each case, the model was originally developed based on the separation of aqueous,salt solutions. The application of each of these models to systems which exhibit anomalous behavior will be discussed in this section. 

Preferential Sorption-Capillary Flow Model. An alternative approach to those mentioned above has been presented by... 

The advantage of the preferential sorption-capillary flow approach to reverse osmosis lies in its emphasis on the mechanism of separation at a molecular level. This knowledge is useful when it becomes necessary to predict membrane performance for unknown systems. Also, the approach is not restricted to the so-called "perfect", defect-free membranes, but encompasses the whole range of membrane pore size. Until recently, the application of a quantitative model to the case of solute preferential sorption has been missing. Attempts to change this situation have been made by Matsuura and Sourirajan (21) by using a modified finely porous model. In addition to the usual features of this model (9-12), a Lennard-Jones type of potential function is Incorporated to describe the membrane-solute interaction. This model is discussed elsewhere in this book. 

Another concept of water and salt transport in reverse osmosis is the preferential sorption-capillary flow mechanism. In this model, the surface of a membrane is microporous and heterogeneous at all levels of solute separation. It is hypothesized that, due to the chemical nature of the membrane skin layer in contact with the aqueous solution, a preferential sorption for the water causes a sorbed water layer to be formed at the skin. This layer of purified water is then forced through the capillary pores by pressure. 

The Preferential Sorption/Capillary Flow Model (Sourirajan and Matsuufa (1985)) is based on the assumption that a layer of water sorbs at the membrane surface, creating a deficit of solute at the surface. The membrane is viewed as a microporous medium, and transport is controlled by the surface chemistry of the membrane and water transport through the membrane. Ions with large hydrated radii are retained better, since they also have to overcome more energy to strip off the water. Ions diffuse through the laj et of strucmred water at the membrane surface and through water cluster channels in the membrane (Staude (1992)), where B is the pure water permeability of the membrane. 

Several mechanisms have been proposed to explain reverse osmosis. According to the preferential sorption-capillary flow mechanism of Sourirajan [114], reverse osmosis separation is the combined result of an interfacial phenomenon and fluid transport under pressure through capillary pores. Figure 5.58a is a conceptual model of this mechanism for recovery of fresh water from aqueous salt solutions. The surface of the membrane in contact with the solution has a preferential sorption for water and/or preferential repulsion for the solute, while a continuous removal of the preferentially sorbed interfacial water, which is of a monomolecular nature, is effected by flow under pressure through the membrane capillaries. According to this model, the critical pore diameter for a maximum separation and permeability is equal to twice the thickness of the preferentially sorbed interfacial layer (Figure 5.58b). 

FIGURE 5.58 (a) Schematic representation of preferential sorption capillary flow mechanism, (b) Critical pore... 

FIGURE 3.1 Preferential sorption-capillary flow mechanism of RO. (From Matsuura, T., Synthetic Membranes and Membrane Separation Processes, CRC Press, Boca Raton, FL, 1994.)... 

The preferential sorption-capillary flow model starts from the consideration of the solid-liquid interface. For example, aqueous sodium chloride solution is in contact with a solid surface. Sodium chloride solution represents the reverse osmosis system where the separation of solute (sodium chloride) flrom solvent (water) occurs. This system also represents one of the most important applications of reverse osmosis, i.e., seawater desalination. A concentration gradient should inevitably appear at the solution-solid interface, as shown in Figure 6.1. The Gibbs adsorption isotherm... 

Figure 6 2. Preferential sorption-capillary flow model. (Reproduced from [51 ] with permission.)...

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