Nanofiltration membranes synthesized from hyperbranched polyethyleneimine

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Abstract

Four nanofiltration membranes, two negatively and two positively charged, were fabricated by interfacial polymerization. Three different amines, ethylenediamine (EDA), diethylenetriamine (DETA), and hyperbranched polyethyleneimine (PEI) were selected to react with two acyl chlorides, trimesoyl chloride (TMC) and terephthaloyl chloride (TPC). The two membranes containing hyperbranched PEI, PEI/TPC and PEI/TMC, are positively charged at the operational pH. But the other two membranes, EDA/TMC and DETA/TMC, are negatively charged. It is found that the two PEI membranes own special rejection characters during nanofiltration. The PEI/TPC membrane has a similar pore size to the EDA/TMC membrane but owns simultaneously the higher salt rejection and permeation flux. The PEI/TMC has a pore size as large as 1.5 nm and still has a higher NaCl rejection than the EDA/TMC membrane of which the pore size as small as 0.43 nm. We consider that the special rejection characters are derived from the special structure of PEI. The hyperbranched structure allows some of the charged amine groups drifting inside the pores and interacting with the ions in the pathway. The drifting amines increase salt rejection but have little effect on water permeation. It implies that a high flux and high rejection membrane for desalting can be obtained by attaching freely rotating charged groups.

Introduction

Nanofiltration (NF) is a relatively new membrane separation technique developed in the 1980s based on reverse osmosis. It is a pressure-driven membrane process normally applicable for separating dissolved components having a molecular weight range from 200 to 1000. One special application of nanofiltration is water softening. The process is regarded as an innovative and promising water treatment technique, and a potential alternative to conventional water treatment approaches [1], [2].

Most NF membranes developed to date are composite in nature, with a selective layer on top of the microporous substrate. Several methods have been developed to prepare the selective layer, including plasma-initiated polymerization [3], [4], photo-initiated polymerization [5], [6], [7], [8], and interfacial polymerization [9], [10], [11], [12], [13]. Interfacial polymerization, however, is the main method in producing the commercial NF membrane such as the NF series from Filmtec Corporation, the NTR series from Nitto Denko Company, the UTC series from Toray Industries, and so forth [9]. The interfacial polymerized NF membranes are generally synthesized by condensing water-soluble diamines and water-insoluble trimesoyl chloride (TMC) in a water–organic interface [10], [11], [12]. Thus produced nanofiltration membranes usually have thin, highly crosslinked, and negatively charged separating layers [13].

To reject ions that are much smaller than the pore size, Donnan exclusion caused by charge repulsion is one of the major separation mechanisms in addition to size exclusion [14], [15]. Tight NF membrane has a smaller pore size, which is usually adopted for water softening. It rejects ions by both size and Donnan exclusion. Subsequently, a high rejection ratio to divalent ions can usually be obtained. But a higher operating pressure is needed to achieve an adequate permeation flux. Loose NF membrane has a larger pore size, which rejects ions predominately by Donnan exclusion. High permeation flux and low operating pressure can usually be obtained. But the effect of Donnan exclusion is easily hindered and the salt rejection is greatly reduced when the feed solution contains divalent counter-ions which have charges opposite to those on the NF membrane.

This study attempts to investigate the possibility to obtain a membrane simultaneously owning high rejection to salt and high water permeation. We investigate the pore size effect by comparing two similar membranes of different pore sizes. The two membranes are synthesized by interfacially polymerizing trimesoyl chloride with ethylenediamine (EDA) or diethylenetriamine (DETA). We also try to investigate membranes of a special structure which allows ionizable functional groups freely drifting inside the pores. Trimesoyl chloride or terephthaloyl chloride (TPC) is used to react with hyperbranched polyethyleneimine (PEI) by interfacial polymerization. The unreacted primary amines on the side chains are protonized at near neutral pHs and swing inside the network. The molecular weight cut-off (MWCO), the surface zeta potential, and pure water permeate flux of these membranes are measured. The rejections to various salts are compared. The possible rejection mechanisms are discussed.

Section snippets

Reagents and solvents

The membrane materials used, EDA and toluene, were purchased from Aldrich (USA). Hyperbranched PEI (M.W. = 2000), N-methyl-2-pyrrolidinon (NMP), polyvinylpyrrolidone (PVP, M.W. = 10,000), polyethylene glycol (PEG, M.W. = 200, 1000, 3400, and 8000), and polyacrylnitrile (PAN) are supplied by Sigma. WAKO Pure Chemical Industries Ltd. (Japan) supplied DETA and Fluka supplied TPC and TMC. The chemical structures of PEI, EDA, and DETA are shown in Fig. 1. All reagents and solvents were used without

Surface zeta potential of membranes

Fig. 3 shows the zeta potentials of membranes measured at various pHs. A commercialized NF membrane, Osmotic DK type membrane, is used in this study as a reference. It can be observed that the EDA/TMC and DETA/TMC membranes have isoelectric points at 4.8 and 5.4, respectively. Both the isoelectric points of PEI/TPC and PEI/TMC membranes are near 6.7. It is also found that the charge density on the surface of the PEI/TMC membrane is obviously higher than that of the PEI/TPC one. The operating pH

Conclusion

We have found four special characters of the nanofiltration membrane made of highly branched polyethyleneimine. The first character is the relatively higher flux achieved at the same MWCO. The second is their relatively high salt rejection. The third is their relatively high rejection to sodium chloride. The fourth is their special salt rejection order. One of the possible explanations to their special characters is their flexible pendant amine groups. The pendant amine groups may easily

Acknowledgments

The authors wish to express sincere gratitude to the Ministry of Economic Affairs (MOEA), R.O.C. for the grant of the Technology Development Program for Academia (TDPA) project and the National Science Council (NSC) and the Center-of-Excellence (COE) Program on Membrane Technology from the Ministry of Education (MOE), R.O.C. for their financial support.

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