Membrane polymer solute rejection

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

The composite membrane was subjected to the permeation experiments in which the volume flux of water and the rejection of polymer solutes, defined by... 

Figure 24 shows the rejections of polymer solutes, polyethylene glycols) (PEG) with monodispersed molecular weights. From Fig. 24, it is apparent that the composite membrane can find application for ultrafiltration. The molecular weight cut-off drastically decreased by more than 10 fold from the swollen state at 25 °C to the shrunken state at 45 °C. Thus the switching ability of the gel was demonstrated in the permeation experiments. 

The polyamide-hydrazide 7 was prepared by solution polymerization in anhydrous dimethylacetamide from terephthaloyl chloride and p-amino-benzhydrazide at ca. 10 °C. The polyamide 8 resulted from the polycondensation of m-phenylenediamine with isophthaloyl chloride at —20 °C, whereas 9 was prepared by the reaction of terephthaloyl chloride with the complex diamine l,3-bis(3-aminobenzamide)benzene at —20 °C. The water flux and salt rejection through these membranes were summarized in Table 5. The polyamide-hydrazide (7) membranes were prepared from polymer solutions containing 6 7% polymer (Mv 3 34,000) by casting on glass plates. The material was placed in an oven for 30 60 min and coagulated in deionized... 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

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. 

Particularly, the nonsolvent immersion, that is, the Loeb-Sourirajan preparation method is an important methodology. In this method, a polymer solution is cast into a film and the polymer precipitated by immersion into water [10,144], The nonsolvent (water) quickly precipitates the polymer on the surface of the cast film, producing an extremely thin, dense-skin layer of the membrane [10,144], The polymer under the skin layer precipitates gradually, ensuing in a more porous polymer sublayer [145], Following polymer precipitation, the membrane is usually annealed in order to improve solute rejection [10,144]. 

Pertinent to the understanding of the operation of an RO system is the fundamental knowledge of various theoretical models describing movement of solutes and water through an RO membrane. By understanding how solutes and water are transported through membranes, appropriate modifications can be made to the membrane polymers to improve performance (flux and rejection). See the book by Richard Baker, Membrane Technology and Applications, 2nd edition (John Wiley Sons, 2004) for more detail about the history and development of membrane and transport models. 

Rejection of the solute (or dispersed colloid) is, together with permeate flux, one of the two key performance parameters of any ultrafiltration membrane. The values of rejection coefficients are of crucial Importance in many applications of ultrafiltration. The objective of this contribution is to consider and analyze the individual factors affecting rejection of polymer solutes by ultrafiltration membranes. The factors that will be considered include, sterlc rejection (sieving), solute velocity lag and solute-membrane Interaction. 

Example 30.5. Ultrafiltration tests with a 1.5-cm tubular membrane at — 25,000 gave a permeate flux of 40 L/m -h and 75 percent rejection for a 5 percent polymer solution. The polymer has an average molecular weight of 30,000, and the estimated diffusivity is 5 x 10 cm s. (fl) Neglecting the effect of molecular diffusion in the pores, predict the fraction rejected for a flux of 20 L/m -h, and predict the maximum rejection, (b) Estimate the fraction rejected for the low-molecular-weight fraction of the pol3mier with M 10,000. (c) If the selective layer thickness is 0.2 fan, does molecular diffusion have a significant effect on the rejection for case (a) ... 

The flux also depends on the physical make-up of the membrane and the pore size. The pure water flux vs. hydrauhc pressure difference curve is linear. Liquid flux for membrane processes is shown in F ure 1.5. The data show how the permeation rate varies with the size of the species and pore size (implicit in type of membrane). The ordinate represents the flux of water per unit pressure gradient. Since the pore radius of an RO membrane is 0.6 nm, water molecules whose radius is about 0.1 nm can pass through the membrane freely while dissolved ions and organic solutes (e.g., sucrose) cannot. These solutes are either rejected at the membrane surface, or are more strongly attracted to the solvent water phase than to the membrane surface. The preferential sorption of water molecules at the solvent—membrane interface, which is caused by the interaction force working between the membrane, solvent, and solute responsible for the separation [8]. As the pore size decreases and tends toward a non-porous skin structure, the transport mecharusm changes from convective flow through pores to SD in the membrane polymer. The latter is the transport mechanism in GS and PV. 

---