Ionic liquid regulated interfacial polymerization process to improve acid-resistant nanofiltration membrane permeance

Highlights

• A new ILs regulated IP diffusion process strategy was proposed.

• The IL regulated membrane permeance is 1.36 times of the pristine membrane.

• The membrane showed good acid stability in the 30 days long-term test.

Abstract

In order to enhance the permeance of the acid resistance membrane, an ionic liquid (IL) regulating strategy was proposed to rearrange the interfacial polymerization process. Herein, we revisited polyethylenimine (PEI) and cyanuric chloride (CC) as the pristine acid-resistant membrane. 1-aminopropyl-3-methylimidazolium chloride ([AEMIm][Cl]) and 1-aminopropyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([AEMIm][Tf₂N]) ILs were used to regulate the interfacial polymerization process. Molecular dynamic (MD) simulation was used to reveal IL and PEI diffusion behavior in aqueous solution, membrane pore size distribution, and porosity. The effects of ILs on membrane surface morphology, surface zeta potential, chemical composition, and separation property were analyzed. The IL regulating strategy endow the membrane with uniform smaller pore and higher porosity, thus improved membrane permeance and selectivity simultaneously. For the [AEMIm][Cl] IL, the corresponding AEMIC-PEI-CC membrane showed high permeance of 79.1 L m⁻²h⁻¹bar⁻¹, which is 1.36 times the pristine PEI-CC membrane, combined with high Y³⁺ rejection of 97.5%, low H⁺ rejection of 1.35%. In addition, this membrane showed good acid stability in 30 days long-term test.

Introduction

Nanofiltration (NF) is a pressure-driven membrane process technology with the characteristic pore size in the nanometer range, which is between that of reverse osmosis (RO) and ultrafiltration (UF). Its applications have been developed in extensive fields such as water softening [1], water purification [2], wastewater treatment [3], food processing [4], solvent recovery [5] et al. While many processes involve extreme low pH conditions, such as recycling rare-earth from the acidic extraction solution, recovery of sulfuric acid and separation of noble metals from the gold mining effluent [6]; deacidification and nutritional enrichments of certain food products including fruit-juice and lactic acid [7]; purification of phosphoric acid solutions [8]; concentration of acetic acid and furfural from the condensate of eucalyptus spent sulphite liquor [9]. These are not exhaustive, implying a broad potential application landscape for pH-stable NF membrane. Unfortunately, the most prevalent commercial NF membrane is made of polyamide, in which the carbonyl group was inherently susceptible to nucleophilic attack and hydrolysis in acid conditions (Fig. 1a) [10]. Thus the search for acid-resistant NF membrane is ongoing.

The acid resistance of nanofiltration membranes depends mainly on the chemical structure of the material. The most commonly used resistant materials are sulfonated polymers (like sulfonated polysulfone, sulfonated poly(ether ether ketone) (SPEEK), polysulfoamide), and polymers with s-triazine rings (such as amine-cyanuric chloride (CC), Fig. 1b). The acid-resistant ability of the sulfonated polymers was based on the smaller bond angle of SO (109.5°) (Fig. 1c) in sulfonated polymer than CO (120°) (Fig. 1d) in polyamide, and the higher steric hindrance effect helps SO less likely to be attacked by H⁺. Dalwani et al. spin coating SPEEK on polyethersulfone support, and the membrane showed good stability against chemical attack in the entire pH range from 0 to 14. Under the optimal conditions, the membrane with a permeance of 0.8 L m⁻²h⁻¹bar⁻¹ and NaCl rejection of 63% [11]. Liu et al. prepared the piperazine-naphthalene-1,3,6-trisulfonylchloride (PIP-NTSC) based acid-resistant polysulfonamide membrane; it showed high acid stability in static acid soaking test and pure water permeance of 5.8 L m⁻²h⁻¹bar⁻¹, Na₂SO₄ rejection of higher than 86.5% [12]. Many commercial acid-resistant NF membranes made by Microdyn-Nadir [13,14] and NTR-7400 series by Hydranautics/Nitto Denko [[15], [16], [17]], including NP010, NP030, and NF-PES-10 membranes, are made of the above sulfonated aromatic polymers. Unfortunately, these membranes have low permeance and/or high MWCO [11]. Also, s-triazine rings as part of the polymer main chain have been confirmed as an acid-resistant membrane material. The alternating nitrogen and carbon atoms linked by single and double bonds form the stable resonance structure in acid solution. Lee et al. prepared acid-resistant membranes based on CC-series monomeric amines. Diethylene triamine (DETA)/CC exhibits the best permeance of 1.5 L m⁻²h⁻¹bar⁻¹, NaCl rejection of 85.2%, and the superior resistance towards nucleophilic attack induced by extreme pH conditions [18,19]. Zeng et al. developed a reactive s-triazine-amine precursor 1,3,5-(tris-piperazine)-triazine (TPT) and then reacted with TMC to form a poly(amide-s-triazine-amine) NF membrane. This membrane with water permeance of 8.68 L m⁻²h⁻¹bar⁻¹ and a rejection of 97% for divalent ions [20]. In addition, Advanced Membrane System Technologies (AMS) has acid-stable membranes designed as the A3012 or 3014 series, which is claimed to be stable at highly acidic conditions due to the trazine ring [21,22]. Unfortunately, due to the stepwise low reactivity of chlorine atom in CC [23], the formed membrane always showed lower water permeance than TMC [18]. In conclusion, both sulfonated polymer and s-triazine rings-based polyamide acid-stable membrane showed low permeance [18,19], limiting its large-scale application.

Interfacial polymerization (IP) is the most commonly used method to prepare the selectivity layer of the NF membrane [[24], [25], [26]]. Usually, the IP is an amine molecule diffusion-controlled process far from thermodynamic equilibrium [27]. The amine monomer in an aqueous solution diffused across the water-organic interface to react with the organic monomer instantaneously [28]. In brief, membrane structure and performance were determined by amine's diffusion behavior to a great extent. Some aqueous additives were most recently employed to assist in constructing the fine membrane structure like polyvinyl alcohol was used to construct the Turing structure [29], surfactant successfully assistant forming an active polyamide layer with more uniform sub-nanometer pores [30]. Ionic liquids (ILs) are molten salts at ambient temperature, composed of cations and anions [[31], [32], [33]]. ILs have been used in the membrane post-treatment process to improve membrane performance. Xiao et al. post-modified the prepared PIP-TMC NF membranes by grafting amino acid ionic liquid (AAIL) on its surface. AAIL improved membrane surface wettability and negatively charged property. Thus the membrane showed high permeance of 12.2 L m⁻²h⁻¹bar⁻¹ and Na₂SO₄/NaCl selectivity of 973.9 [34]. He et al. anchored the 1-aminoethyl-3-methylimidazolium bromide (AMIB) to the prepared PIP-TMC NF membrane surface. The obtained membranes showed permeance of 22.5 L m⁻²h⁻¹bar⁻¹ (4 times of the pristine PIP-TMC membrane), Na₂SO₄ rejection of 95%, and also exhibited excellent antibacterial properties [35]. Wu et al. also grafted amine-functionalized IL on the PIP-TMC NF membrane surface. The quaternary ammonium group of IL endowed the membrane with a high positive charged surface and thus improved its Mg²⁺/Li⁺ selectivity from 1.92 to 8.12 [36]. So far, ILs have been used in the membrane post-treatment process. In contrast, due to the regulable functional groups and specific ion diffusion behavior in aqueous solutions, ILs can be an additive in amine-aqueous solutions to control the amine-diffusion behavior. The amine group of the cation diffused into the organic phase and reacted with the organic monomer first. In contrast, the hydrophilic part of the cation still stayed in the aqueous phase, forming a row of channels at the aqueous-organic interface, facilitate the oriented diffusion of amine monomer (Scheme 1).

In this paper, two kinds of ILs 1-aminopropyl-3-methylimidazolium chloride ([AEMIm][Cl]) and 1-aminopropyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([AEMIm][Tf₂N]) were individually used to obtain the IL-PEI mixed aqueous solution and reacted with CC by in situ IP on porous polysulfone support (Scheme 2). The formed membrane was used to recycle rare-earth ions (REⁿ⁺) from the acid extract solution. The molecular dynamic (MD) simulation illustrated the IL and PEI diffusion behavior in the aqueous phase, membrane pore size distribution, and porosity. The effects of ILs on membrane surface morphology, surface zeta potential, chemical composition, and separation property were analyzed. The [AEMIm][Cl] regulated membrane showed permeance is 1.36 times of the pristine PEI-CC membrane, combined with good H⁺/RE^n + selectivity and long-term acid stability.