Rejection in NF

Similar to the approach for solvents, both diffusive and convective transport of solutes can be modeled separately. For dense membranes, a solution-diffusion model can be used [14], where the flux / of a solute is calculated as  

The transport equations of Spiegler and Kedem combine both diffusion and convection  

The permeability Ps is a measure of the transport of a molecule by diffusion. The reflection coefficient a of a given component is the maximal possible rejection for that component (at infinite solvent flux). Various models have been proposed for the reflection coefficient [75-77]. In the lognormal model [78], a lognormal distribution is assumed for the pore size. No steric hindrance in the pores or hydrodynamic lag is taken into account, but it is assumed that a molecule permeates through every pore that is larger than the diameter of the molecule. Moreover, the diffusion contribution to the transport through the membrane is considered to be negligible. Therefore, the reflection curve can be expressed as  

These effects were observed for both polymeric and ceramic NF-membranes, showing that differences in rejection are not due to swelling. Nevertheless, swelling effects have been demonstrated by Tarleton et al. [82, 83] and are known to affect transport in polymeric membranes. 

Models lor PV from Solution-Diffusion to Maxwell-Stefan 

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In the other pressure-driven separations in Table I, the difference in size between the permeating component A and rejected components B, C, etc., is progressively reduced in NF vs RO vs GS. This shift in size discrimination requirements is illustrated in Table II. 

Nanofiltration is a rapidly advancing membrane separation technique for concentration/separation of important fine chemicals as well as treatment of effluents in pharmaceutical industry due to its unique charge-based repulsion property [5]. Nanofiltration, also termed as loose reverse osmosis, is capable of solving a wide variety of separation problems associated with bulk drug industry. It is a pressure-driven membrane process and indicates a specific domain of membrane technology that hes between ultrafiltration and reverse osmosis [6]. The process uses a membrane that selectively restricts flow of solutes while permitting flow of the solvent. It is closely related to reverse osmosis and is called loose RO as the pores in NF are more open than those in RO and compounds with molecular weight 150-300 Da are rejected. NF is a kinetic process and not equilibrium driven [7]. 

In aqueous solutions, NF membranes become charged, allowing separation of specific ionic species. It is believed that sieving (steric hindrance) is the dominant rejection mechanism in NF for colloids and large molecules, while the physicochemical interactions of solute and membrane become increasingly important for ions and low-molecular-weight organics [13]. 

The osmotic pressure difference can usually be neglected in MF and UF, since the rejected solutes are large and their osmotic pressure small. However, even polymeric solutes can develop a significant osmotic pressure at boundary layer concentrations (Ho and Sirkar (1992)). This naturally implies that the resistance in series model (equation (3.4)) would be more appropriate in MF, while the osmotic pressure model (equation (3.6)) may be more useful in NF and RO. Both models have been applied to UF. 

Both, charge and size are important in NF rejection. At a neutral pH most NF membranes are negatively charged, while they might be positively charged at low pH (Zhu et al (1995), Peeters (1997)). The principal transport mechanisms of NF are depicted in Figure 3.2. 

The MWCO of NF and RO is in the 100 to 1000 Da range with pores < 1 nm in diameter. Organics rejection is therefore expected to be high. According to van der Bruggen et al. (1999), differences in rejection between membranes are clearly visible for compounds which exhibit about 50% rejection. Taylor and Mulford (1995) found TOC removal in NF to be sieving-controlled, and, thus independent of pressure and recover. The rejection of inorganic solutes was diffusion limited. 

Rejection for the 100 kDa membrane is shown in Figure 6.44. For all calcium concentrations below 2.5 mM the organic rejection is low. As in the absence of inorganic colloids (Figure 6.30), the DOC rejection increases dramatically for calcium concentrations above 2.5 mM. UV rejection is initially higher than the DOC rejection due to the colloids. UV rejection at 2.5 mM calcium was not measured. Overall the organic rejection is lower than in the absence of coUoids. This is most likely due to the formation of a thick unstirred deposit layer in which organics accumulate and rejection decreases due to a concentration polarisation effect. A similar effect was observed in NF (see Chapter 7). 

Concentration polarisation may be of particular importance in NF because of the relatively high flux (compared to RO) and high rejection (compared to UF). Due to low concentrations of solute in surface water treatment applications, concentration polarisation is often neglected. In this study the extent of concentration polarisation is estimated for conditions typical of surface water membrane processing. 

Rejection and fouling anal3 sis in NF is more difficult than that of more open membranes such as MF and UF due to an increased interplay of colloids and natural organics with salt, which is retained to a greater extent. Osmotic effects become important. 

Inorganic colloids did not cause flux decline as they adsorb the organics, which are likely to deposit on the membrane, on their surface. However, the build-up of a colloidal cake-layer results in a reduced solute rejection, but flux decline is fully reversible. This shows that pore fouling is not important, and it is surface deposit which causes detrimental flux decline in NF. 

In NF, it is also charge and sieving that are important. While in MF colloid interactions and in UF organic interactions are important, in NF it is the speciation, ion dissociation, organic-cation complexation, as well as the structure and size of compound that have a significant impact on rejection. Solute-solute interactions of decreasing size scale are determining rejection as membrane pore size decreases. 

Fouling also affects rejection. In MF and UF pores gradually fill up, while in NF a deposit of a higher charge than the membrane may form and enhance rejection. Colloid fouling can also increase the thickness of the unstirred boundary layer and increase concentration polarisation, which adversely affects rejection. 

Ferric chloride is used in MF and UF to increase rejection. In Chapter 7 it has been demonstrated that ferric chloride can also be used to reduce fouling in NF. In this section the effect on rejection, flux, and the cost of such a pretreatment are compared. 

Table 8.8 Rejection in percent of various processes with two ferric chloride dosages. Solutions contain 5 mglj IHSS FA and 0.5 mM CaCl, (MF and UFJ and 12.5 mgL IHSS FLA and 2.5 mM CaCf (NF) and colloidal systems with 10 mgL hematite I (75 nm). pH adjusted to pH 7-8 prior to ferric chloride addition.

Ferric chloride pretreatment leads to a higher WQP due to the higher organic and colloid rejection. While the value is stable in NF (although minor changes can occur as shown in Chapter 7), a correction of WQP due to the improved rejection is carried out for MF and UF. 

These models have been applied in the prediction of electrolyte rejection by NF membranes in aqueous solutions, successfully for specific cases. Bowen et al. (1997) reported the observation of distinctive pores by atomic force microscopy (AFM) in the surface of commercial membranes prepared by IP process. The research supported the hypothesis that a dense coating layer swells at working conditions, resulting in water passages and NF properties. 

Though the flux in NF process is higher, its rejection is much lower than RO. However, NF can be applied wherever the ground water contains low fluoride concentrations (< 2 mg/L). In specific rural areas where ground water is characterized by low TDS (< 700 mg/L) and moderate fluoride, NF can be employed. 

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