Merits of using cellulose triacetate as a substrate in producing thin-film composite nanofiltration polyamide membranes with ultra-high performance

Highlights

• Ultra-high performance polyamide thin-film composite membrane was fabricated.

• Polyamide layer was deposited on top of cellulose triacetate (CTA) support.

• Using CTA support delivered higher performance compared to polysulfone support.

Abstract

Choosing the property of the supporting membrane is crucial in preparing high performing nanofiltration membranes through interfacial polymerization. In this study, an oxygen rich membrane – cellulose triacetate (CTA) – was used to fabricate the support membrane. Polyamide was deposited onto the CTA support using interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC). The concentration of the monomers was optimized. Furthermore, the polyamide layer prepared on CTA support exhibited higher separation efficiency for sodium sulfates and dyes compared to using traditional polysulfone (PSf) support. The oxygen groups of CTA facilitate better adsorption of amines on the surface; thus, using low concentration of PIP could still provide a defect-free polyamide layer. Utilizing the optimum condition, the polyamide/CTA membrane delivered a high pure water flux (operating at 6 bar) of 179.5 L/m²h with the following rejections: Na₂SO₄ = 98.4%; MgSO₄ = 60.3%; MgCl₂ = 15.0%; NaCl = 3.7%; Rose Bengal = 95.5%; Brilliant Blue R = 99.9%; Amido Black 10B = 90.6%; Orange G = 67.3%. Moreover, the polyamide/CTA membrane had excellent stability at a wide range operating conditions.

Introduction

Chemical industries, such as petroleum, tannery, and pharmaceutical industries, contribute to water pollution by producing high saline wastewater [1]. One of its major pollutants is sodium sulfate. For example, the pharmaceutical industry generates sodium sulfate as a byproduct from producing of magnesium stearate—an effective lubricant for tablet formulations [2]. Ingesting a high concentration of sulfates can cause a laxative effect that could lead to diarrhea [3]. United Nations reported that, for each day, there are 1000 children dying because of waterborne diseases. In addition, an excess amount of sulfate could lead to severe soil damage and disruption of aquatic ecosystems. Wastewater treatment plants can eliminate sulfate through chemical, biological, and physical processes. For chemical treatment, calcium or barium salt can precipitate sulfates [4, 5]. Biological processes are used if toxic metals are present in water. However, the typical biological treatment does not have a significant effect on salts [6]. Physical treatment includes ion exchange process, nanofiltration (NF) and reverse osmosis (RO). Among membrane separation techniques mentioned, RO consumes higher energy than NF because it operates at high pressure. Hence it is only considered if monovalent salts are present in the waste stream [4].

Nanofiltration (NF) is an effective technique in separating sodium sulfate from industrial wastewater. It is also preferred for sodium sulfate separation over reverse osmosis and ultrafiltration. NF membranes attains higher rejection compared to ultrafiltration because of its smaller pore size, while it produces lesser fouling problem compared to reverse osmosis. It can separate low molecular weight molecules and divalent salts [7]. It has several advantages such as energy-efficient, space-efficient, environmentally friendly, and simplicity of operations. NF membranes follow different transport mechanisms which expand its range of application. The transport of molecules through the membrane depends on size, charge, and the dielectric constant of compounds. Thus, NF membranes can follow any of the following transport mechanisms: solution diffusion; size sieving; Donnan exclusion; and dielectric exclusion [[8], [9], [10]]. Nevertheless, the challenge of producing high quality and effective NF membranes remain.

Interfacial polymerization (IP) is a coating technique that fabricates high performance nanofiltration membranes [11]. Interfacial polymerization requires two phases: an aqueous phase, where amines are dissolved in water; and an organic phase, where acyl chlorides are dissolved in organic solvent that is immiscible with water. The polymerization reaction occurs in the interface between the aqueous and organic phase, where the amines and acyl chlorides react to produce polyamide (PA) layer [12, 13]. Several methods have been developed to improve the separation performance of the PA layer. Commonly used methods include: adding additives, such as comonomers [14,15], particles [[16], [17], [18], [19], [20]], surfactants [[21], [22], [23]], and cosolvent [24, 25]; optimizing interfacial polymerization conditions [[26], [27], [28]]; and altering the property of membrane support [[29], [30], [31]]. Surface property of the supporting membrane plays an important role in interfacial polymerization reaction. In choosing the supporting membrane, its physicochemical properties must be taken into consideration. The characteristics of the supporting membrane is vital in fabricating a high performing, thin selective PA layer [30]. Studies have shown that its physio-chemical properties affect both the formation of the PA layer and its NF performance [30, 32, 33]. For example, Misdan et al. [32] explored three kinds of polymer as membrane support for PA thin-film, which are polysulfone (PSf), polyethersulfone (PES), and polyetherimide. They concluded that performances of the PA composite membranes were affected by surface hydrophilicity and pore size. Wang et al. [34] chose poly(m-phenylene isophthalamide) as their supporting membrane for PA, because of its excellent thermal property. They acquired a high pure water flux of over 100 L/m²h at 6 bar with an excellent rejection of divalent salts. Zhu et al. [35] blended PES and polyaniline to prepare their membrane support for IP. They enhanced the flux and salt rejection of NF membrane at the optimum composition of PES and polyaniline in the membrane support.

In this study, cellulose triacetate (CTA) was used as membrane support. It has excellent physio-chemical properties such as high mechanical strength, high hydrolytic stability, outstanding resistance to free chlorine, and superior resistance to biodegradation [36, 37]. It contains oxygen functional groups that reduces the amount of amines required to come in contact with the membrane to produce a defect free PA layer, which results to high flux and high sodium sulfate rejection. Therefore, this study aims to optimize the IP conditions of piperazine (PIP) and trimesoyl chloride (TMC) to improve NF performance of PA/CTA membranes. Characterizations involve membrane morphology, membrane functional groups, and overall membrane performance (Dye variation and four salts (Na₂SO₄, MgSO₄, NaCl, and MgCl)). We also highlighted the effect of active chlorine on membrane performance.