Stabilization of composite hollow fiber nanofiltration membranes with a sulfonated poly(ether ether ketone) coating

Highlights

• Origin of instability of SPEEK nanofiltration membranes is revealed.

• Reptation rate of polymer chains determines membrane rejection.

• Stable performance for SPEEK-PVA cross-linked membrane over 2000 h was obtained.

• Constraints help to reduce the loss rate of SPEEK.

Abstract

Composite hollow fiber nanofiltration membrane coated with a thin layer of sulfonated poly(ether ether ketone) (SPEEK) can separate monovalent ions from multivalent ions with a low energy demand. It has found applications in desalination of glyphosate wastewater and removal of arsenic from drinking water. Before large scale application of such membranes, one of the main issues is the long time performance stability. This paper reports, for the first time, systematic investigations on the origins of the instability of the SPEEK composite membrane in aqueous phase. Membrane preparation parameters, the coating polymer concentration, the coating time, and the post-treatment temperature were correlated to the membrane stability. A reptation mechanism was proposed to explain the rejection decline and flux increase with time, based on swelling and dissolution of SPEEK. To reduce the swelling and loss of SPEEK, a PVA/SPEEK blend was used as the coating layer, which after cross-linking by glutaraldehyde, yielded a NF membrane with constant rejection during 2000 h of operation.

Introduction

Nanofiltration (NF) is a pressure driven membrane process that removes pathogens, multivalent ions, and organic compounds of molecular weight larger than 200 Da [1], [2]. NF has found applications in water softening as well as in the food industry for concentration and purification of polysaccharides, protein and fruit juice [3], [4], [5], dye waste water reuse, leachate treatment, etc. Most of the existing commercial nanofiltration membranes are spiral-wound polyamide membranes by interfacial polymerization [6]. Fouling is a major drawback of such type of module configuration. To avoid membrane fouling, heavy demands on pre-treatment are required, resulting in high costs in installation and operation. Hollow fiber NF membranes, on the other hand, have well-defined fluid flow channel geometry, allowing relatively higher content of suspended solids in the feed solution. Moreover, the higher packing density of hollow fiber membrane modules lead to a compact system, bringing extra merits to the promise of practical usage [7].

Several approaches have been utilized in the preparation of hollow fiber NF membranes, including direct spinning [8], co-extrusion [9], [10], interfacial polymerization [7], UV-photografting [11] and direct dip-coating [12], [13]. Among these methods, dip-coating appears to be the most versatile and facile process to apply a selective layer onto a support membrane. We have reported the preparation of hollow fiber nanofiltration membranes by dip-coating a sulfonated polyether ether ketone (SPEEK) ultrathin layer onto various support membranes [12]. The resulting composite membrane showed high rejection of sodium sulfate, and low rejection of magnesium chloride. The membrane could separate glyphosate from sodium chloride as well [13]. It was characterized by high rejection of multivalent anions, low operating pressure, and high permeability.

The performance stability of NF membranes determines the operation stability as well as the economy of the NF process. Cross-linking of the sodium carboxy-methyl cellulose (CMCNa) on the outside of the polypropylene (PP) microporous support hollow fiber has yielded a NF membrane with a Congo red retention of over 99.8% [14]. During 100 h of operation it showed almost constant rejection but slightly reduced permeability which confirmed that cross-linking is an effective means to make stable NF membranes. Cross-linking of polyvinyl alcohol (PVA) with glutaraldehyde (GA) [15], [16] has been demonstrated in the coating layer on top of PP hollow fiber membranes. After PVA cross-linking, the NF membrane showed a stable performance of up to 150 h with no apparent decline in permeation and rejection. Cross-linking found also applications in stabilizing thin film composite membranes prepared by interfacial polymerization [17]. Other cross-linking approaches via thermally activated bridging of the polymer chains with polyatomic alcohols were also successful, however, reduced the effectiveness of sulfonic acid groups and polymer swelling [18]; alternatively, thermal cross-linking at high temperatures up to 160 °C involves sulfonic group bridging between macromolecular chains [19]. In summary, the literature has demonstrated that cross-linking is an effective means to improve the performance stability of the NF membranes. But the origin of the instability mechanism of the coating layer has not been studied thoroughly.

The stability of the SPEEK composite membrane was investigated over a wide feed pH range [20]. In-depth research to correlate the microscopic behavior of the coating polymer in the working environment with the membrane performance has not been extensively reported. We aimed to investigate the instability of SPEEK coated nanofiltration membranes via a systematic study of the time-dependent behavior of the SPEEK polymer chains. The effects of the preparation parameters, such as SPEEK concentration, the coating steps, and the post-treatment temperature on the permeation and rejection of the SPEEK NF membranes during prolonged operation time were studied. A hypothesis of the instability of NF membranes was proposed based on the reptation of SPEEK chains in the aqueous phase. Finally, by applying a coating layer of a SPEEK-PVA network, a stable NF membrane was obtained. This work illustrates for the first time the origin of the instability of the coating layer, and thereby may provide a foundation for an in-depth understanding of the long term behavior of the SPEEK composite NF membrane and a theoretical/technical basis for the improvement of membrane performance.