1.1 History of Nanofiltration Membrane and Applications
Nanofiltration (NF) is the fourth class of pressure-driven membranes born after microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO). It was first developed in the late 1970s as a variant of RO membrane with reduced separation efficiency for smaller and less charged ions such as sodium and chloride. As the term NF was not known in the 1970s, such membrane was initially categorized as either loose/open RO, intermediate RO/UF, or tight UF membrane. The term NF appears to have been first used commercially by the FilmTec Corporation (now The Dow Chemical Company) in the mid-1980s to describe a new line of membrane products having properties between UF and RO membrane. Owing to the uniqueness and meaningfulness of the word NF, other membrane scientists have begun using it. The widespread use of this word today is testament to the need for just such a descriptor in the membrane lexicon (Schafer et al., 2005).
Figure 1.1 shows the distinction between the pressure-driven membrane processes with respect to solute rejection capability. NF membrane that is positioned in the lower end of UF and upper end of RO has a molecular weight cutoff (MWCO) of about 200â1000 Dalton (Da), which corresponds to pore sizes between 0.5 and 2 nm. Unlike UF and RO membranes which generally carry no charge on their surface, NF membrane often carries positive or negative electrical charges (Strathmann, 2011). In most cases, NF membranes are negatively charged in neutral or alkaline conditions and positively charged in highly acidic condition. In view of this, the separation of NF membrane is governed by three distinct mechanisms, namely the steric hindrance (or size sieving), electrostatic (Donnan) exclusion, and dielectric exclusion.
The first generation of NF membranes in the early 1970s was made of cellulose acetate (CA) or its derivatives. These membranes were synthesized based on the well-known LoebâSourirajanâs dryâwet phase inversion Âtechnique in which a homogenous polymeric solution was cast on a glass plate followed by partial evaporation of solvent before immersing in a bath containing a non-solvent coagulant. Originally, this phase inversion technique was used to produce a dense membrane structure for RO application, but it was realized that by varying the condition of post treatment (heat treatment), a membrane with relatively larger pores could be tailored and used for specific applications, particularly those requiring a lower degree of NaCl rejection and lower operating pressure (Loeb and Sourirajan, 1964; Cohen and Loeb, 1977). Nevertheless, it soon became evident that the poor biological and chemical stability of cellulose-based membranes have limited the range of industrial applications as these membranes always suffered from continual changes in water flux and solute rejection during operation.
FIGURE 1.1
Separation spectrum of membrane.
Because of these reasons, the second generation of NF membrane based on noncellulosic materials was developed. This membrane is a thin film composite (TFC) membrane consisting of three different layers: an ultrathin polyamide (PA) selective layer on the top surface, a microporous interlayer, and a nonwoven polyester bottom layer, as illustrated in Figure 1.2. The advantage of a TFC membrane is that each layer can be independently optimized to achieve desirable membrane performance with respect to water permeability, solute selectivity, and chemical and thermal stability (Lau et al., 2012). Compared to the asymmetric CA membrane which can be synthesized via a single-step fabrication process, the PA composite membrane is typically fabricated through a two-step process. The top PA thin selective layer is synthesized by interfacial polymerization (IP) of amine monomer (e.g., m-phenylenediamine [MPD]) with acyl chloride monomer (e.g., trimesoyl Âchloride [TMC]) while the microporous interlayer is fabricated via the dryâwet phase inversion technique. Even though the TFC membrane was reported in the literature in the 1970s, commercial TFC membranes for NF application were only available in the second half of the 1980s after years of research and development. Currently, the TFC membrane is regarded as the most Âpopular and reliable material in the membrane market. Permeate flow rate and its quality have been improved 10 times more than that at the beginning (Li et al., 2008).
FIGURE 1.2
(a) Illustration of typical structure of TFC NF membrane consisting of (i) top selective layer, (ii) microporous substrate, and (iii) nonwoven fabric and (b) structure of polymers that are commonly used for each layer.
The earliest documented NF application was a potable water application in Florida in the late 1970s (Paulson, 2015). It was probably the first commercial and intentional use of NF membranes (made of CA) for removing color molecules while allowing monovalent ions like sodium to pass through. This membrane partially demineralized the feed water, removing 10%â90% of dissolved solids, compared with up to 99.5% for typical RO. Low-level calcium hardness remaining in the NF permeate imparts a sweet taste to the water. Statistics revealed that NF ...
