Reverse Osmosis and Nanofiltration Membranes

Hirose et al. [1] suggested an approximately linear relationship between membrane surface roughness and permeate flux for TFC RO membranes with six different cross-Hnked aromatic polyamide skin layers. On a polysulfone (PSf) substrate, cross-linked aromatic polyamides were formed at a water/solvent interface using m-phenylene diamine (MPD) and 1,3,5-benzenetricarbonyl trichloride as monomers. Isopropyl alcohol content in the aqueous amine phase was changed from 0 to 60 wt.% to control the interfacial surface tension, which eventually led to different surface roughness values. 

The results of their work are given in Table 8.1. A linear relationship was found between the flux and the surface roughness, which was attributed to enlargement of the effective membrane area. 

Lu et al. [2] fabricated TFC NF membranes, in which the skin layer was either polyesters or polyamides. The monomers used in the polycondensation reactions are as follows (1) alcohol (bisphenol-A, BPA), (2) amine (metaphenylene diamine and piperazine), and (3) acid chloride (isophthaloyl chloride, terephthaloyl chloride [3,4], and trimesoyl chloride). They reported that the composite layer (active layer) was smoother than that of the substrate membrane. Upon formation of the active layer, the pore size decreased, which resulted in a flux decrease and a retention increase. Although it is unclear, they seem to maintain that their results confirm Hirose et al.s conclusion, i.e., the flux increases with an increase in surface roughness [Ij. 

Another kind of TFC NF membrane was studied by Hamza et al. [5] using AFM. Membranes were prepared by applying a thin coat of sulfonated poly(phenylene oxide) solution to a porous substrate poly(ether sulfone), followed by solvent evaporation. Mixtures of chloroform/methanol with different ratios were used as solvents. The authors reported that the nodule size decreased with an increase in chloroform concentration in the solvent mixture. In the separation experiment of sodium chloride solute, the flux decreased from 11 to less than 2 x 10 m m s as the chloroform concentration increased from 0 to 66%. Thus, the decrease in flux parallels the decrease in the nodule size. Although they did not report the surface roughness. 

Kwak and Ihm [7] used AFM and solid state NMR spectroscopy to characterize structure-property-performance correlations in high-flux RO membranes. The membranes were thin film composites, whose thin active layers were based on aromatic polyamide formed by the interfacial polymerization of MPD and trimesoyl chloride (TMC). These membranes, each coded as SH-I, SH-II, and SH-III, were provided by Saechan (Yongin-city, Korea). The variations among these commercial membranes are difficult to know. Most likely, they vary by the amount of catalyst or surfactant added to the aqueous MPD solution. Table 8.2 shows water flux, salt rejection, and the roughness parameter of those membranes, together with the data for another membrane, MPD/TMC, which was prepared at the laboratory of Kwak and Ihm [7]. 

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Peterson, R. ]., "Composite Reverse Osmosis and Nanofiltration Membranes," Journal of Membrane Science, 83,1993. 

Tang, C.Y., Criddle, Q.S., Eu, C.S., and Leckie, J.O. 2007. Effect of flux (transmembrane pressure) and membranes properties on fouling and rejection of reverse osmosis and nanofiltration membranes treating perfluorooctane sulfonate containing waste water. Environmental Science and Technology, 41 2008-14. 

Vrijenhoek EM, Hong S, and Elimelech M. Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes. J. Membr. Sci. 2001 188 155-128. 

This method, developed by Cadotte and coworkers of Film Tech in the 1970s, is currently most widely used to prepare high-performance reverse osmosis and nanofiltration membranes.A thin selective layer is deposited on top of a porous substrate membrane by interfacial in situ polycondensation. There are a number of modifications of this method primarily based on the choice of the monomers.However, for the sake of simplicity, the polycondensation procedure is described by a pair of diamine and diacid chloride monomers. 

Childress A.E. (1997), Characterization and performance of polymeric reverse osmosis and nanofiltration membranes. Dissertation, University of California. 

Childress A.E., Elimelech M. (1997), Effects of natural organic matter and surfactants on the surface characteristics of low pressure reverse osmosis and nanofiltration membranes, Proc. A WA Membrane Technology Conference, New Orleans, Feb. 97, 717-725. 

Petersen R.J. (1993), Review Composite reverse osmosis and nanofiltration membranes, Journal of Membrane Science, 83, 81-150. 

In contrast to the polymeric materials for reverse osmosis and nanofiltration membranes, for which the macromolecular structures have much to do with permeation properties such as salt rejection characteristics, the choice of membrane material for ultrafiltration does not depend on the material s influence on the permeation properties. 

Integrally skinned asymmetric reverse osmosis and nanofiltration membranes also feature small roughness parameters, ranging from 0.84 to 5.14 nm. 

Petersen, R.l. 1993. Composite reverse osmosis and nanofiltration membranes. 

Childress AE, Elimelech M (1996) Effect of solution chemistry on the surface charge of pol3rmeric reverse osmosis and nanofiltration membranes. J. Membrane Sci. 119 253-268. 

In addition, the ability to work in a wide range of operative conditions is another key aspect for the development of advanced membranes. Chemical stability is of particular importance when the membrane interfaces are exposed to aggressive solvents, such as in several organic solvent nanofiltration (OSN) applications [21]. Resistance to fouling is also important in water filtration because this phenomenon can threaten the continuous operability of the membrane module [22]. In high-temperature (eg, precombustion CO2 capture from syngas [23] or polymer electrolyte membranes for fuel cells [24]) and high-pressure (eg, reverse osmosis and nanofiltration membranes for... 

These various methods have been discussed in chapter III. Since reverse osmosis membranes may be considered as intermediate between porous ultrafiltration membranes and very dense nonporous pervaporation/gas separation membranes, it is not necessary that their structure to be as dense as for pervaporation/gas separatipn. Most composite reverse osmosis and nanofiltration membranes are prepared by interfacial polymerisation (see chapter in. 6) in which two very reactive bifunctional monomers (e.g. a di-acid chloride and a di-amine) or triiunctional monomers (e.g. trimesoyicbloride) are allowed to react with each other at a water/organic solvent interface and a typical rietwork structure is obtained. Another example of monomers used for interfacial polymerisation are given in table VI.6 (see also table m.1). 

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