Solute preferential sorption

It is to this topic of solute preferential sorption in reverse osmosis that this paper is dedicated. Specifically, this discussion will involve a description of solute preferential sorption, an overview of the literature in the area, and finally a presentation of some recent work on the removal of aromatic hydrocarbons from water. The significance of this work is at least two-fold. From a practical point of view the classes of solutes which demonstrate preferential attraction to the membrane material tend to be organic compounds and the removal and recovery of these solutes from water is environmentally and economically important. From a theoretical point of view an understanding of the phenomena involved is essential to the achievement of a fundamental description of the RO process. Although this paper deals solely with aqueous solutions and cellulose acetate membranes, it Is important to recognize that the concepts discussed can be extended to Include other membrane materials and non-aqueous systems. 

At this point, it is important to describe exactly what is meant by solute preferential sorption and the consequences that result from this situation. Consider first the classical case of the separation of aqueous NaCl solutions by cellulose acetate membranes. In this instance the membrane material has a stronger affinity for the solvent than it has for the solute. The result... 

The list of solutes that are known to be preferentially sorbed by cellulose acetate membranes includes many alcohols, phenols, un-lonlzed carboxylic acids and hydrocarbons ( ). Although solute preferential sorption is a common occurrence with a number of important aqueous organic systems little experimental or quantitative work has appeared in the literature. The purpose of the current work is to rectify this situation. 

Finely Porous Model. In this model, solute and solvent permeate the membrane via pores which connect the high pressure and low pressure faces of the membrane. The finely porous model, which combines a viscous flow model eind a friction model (7, ), has been developed in detail and applied to RO data by Jonsson (9-12). The most recent work of Jonsson (12) treated several organic solutes including phenol and octanol, both of which exhibit solute preferential sorption. In his paper, Jonsson compared several models including that developed by Spiegler eind Kedem (13) (which is essentially an irreversible thermodynamics treatment), the finely porous model, the solution-diffusion Imperfection model (14), and a model developed by Pusch (15). Jonsson illustrated that the finely porous model is similar in form to the Spiegler-Kedem relationship. Both models fit the data equally well, although not with total accuracy. The Pusch model has a similar form and proves to be less accurate, while the solution-diffusion imperfection model is even less accurate. 

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. 

The trends in Figures 2 to 6 are consistent with the qualitative features of solute preferential sorption discussed earlier in this paper. The permeate flux is lower than the pure water flux due to pore blocking. This effect is enhanced by either decreasing the pore size or increasing the feed concentration. 

In conclusion, several Important points of this work should be reiterated. An understanding and quantitative description of solute preferential sorption Is Imperative to the advancement of a fundamental knowledge of the separation mechanism and to the application of reverse osmosis. For the systems studied,... 

Increasing the feed concentration was found to Increase separation and decrease permeate flux. This behavior can be contrasted to the case of water preferential sorption where both separation and permeate flux would remain constant for these dilute concentrations. The results for the benzene studies and the toluene studies were similar In that separation Increased and flux decreased with Increasing feed concentration or decreasing pore size. The benzene studies showed a minimum in separation with Increasing pressure. At similar experimental conditions the toluene system showed higher separation and lower flux than the benzene system. This observation Is consistent with the difference in the nonpolar character of the solutes as expressed by the Small s number. Further work Is needed In order to improve the quantitative understanding of systems which exhibit solute preferential sorption. 

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...

According to the above mechanism, reverse osmosis separation is governed by two distinct factors, namely (i) an equilibrium effect which is concerned with the details of preferential sorption in the vicinity of the membrane surface, and (ii) a kinetic effect which is concerned with the mobilities of solute and solvent through membrane pores. While the former (equilibrium effect) is governed by repulsive and attractive potential gradients in the vicinity of the membrane surface, the latter (mobility effect) is governed both by the potential gradients (equilibrium effect) and the steric effects associated with the structure and size of molecules relative to those of pores on the membrane surface. 

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 at Membrane-Solution Interfaces and Solute Separation In Reverse Osmosis... 

With particular reference to reverse osmosis systems involving cellulose acetate membranes and aqueous solutions, the membrane material has both polar and nonpolar character, and the solvent, of course, is polar. When these two components of the reverse osmosis system are kept constant, preferential sorption at the membrane-solution interface, and, in turn, solute separation in reverse osmosis, may be expected to be controlled by the chemical nature of the solute. If the latter can be expressed by appropriate quantitative physicochemical parameters representing polar-, steric-, nonpolar-, and/or ionic-character of the solutes, then one can expect unique correlations to exist between such parameters and reverse osmosis data on solute separations for each membrane. Experimental results confirm that such is indeed the case (18). 

Gibbs adsorption equation is an expression of the natural phenomenon that surface forces can give rise to concentration gradients at Interfaces. Such concentration gradient at a membrane-solution Interface constitutes preferential sorption of one of the constituents of the solution at the interface. By letting the preferentially sorbed Interfacial fluid under the Influence of surface forces, flow out under pressure through suitably created pores in an appropriate membrane material, a new and versatile physicochemical separation process unfolds itself. That was how "reverse osmosis" was conceived in 1956. 

In the performance data of various polyamide and related membranes published to date there should be valuable information for molecular design of more excellent barrier materials. But at present a means for their evaluation and optimization is still not clear. One of the reasons may at least come from the competitive flood of proposals for the detailed mechanisms of reverse osmosis, e.g. the solution-diffusion model, the sieve model, the preferential sorption model and so on. 109)... 

The case of infinite dilution of the polymer is the one for which total and preferential sorption have been most extensively studied. In this dilute solution limit the interpretation of the preferential adsorption coefficient,, requires knowledge of the g interpretation parameters at infinite dilution, g°, for the polymer in each one of the pure liquids [9],... 

Good quality RO membranes can reject >95-99% of the NaCl from aqueous feed streams (Baker, Cussler, Eykamp et al., 1991 Scott, 1981). The morphologies of these membranes are typically asymmetric with a thin highly selective polymer layer on top of an open support structure. Two rather different approaches have been used to describe the transport processes in such membranes the solution-diffusion (Merten, 1966) and surface force capillary flow model (Matsuura and Sourirajan, 1981). In the solution-diffusion model, the solute moves within the essentially homogeneously solvent swollen polymer matrix. The solute has a mobility that is dependent upon the free volume of the solvent, solute, and polymer. In the capillary pore diffusion model, it is assumed that separation occurs due to surface and fluid transport phenomena within an actual nanopore. The pore surface is seen as promoting preferential sorption of the solvent and repulsion of the solutes. The model envisions a more or less pure solvent layer on the pore walls that is forced through the membrane capillary pores under pressure. 

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