Novel Ionic Grafts That Enhance Arsenic Removal via Forward Osmosis

Apr 19, 2019 - For example, designer ILs containing functional groups can be .... Designing and synthesizing a series of imidazolium-based ILs with di...
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Novel Ionic Grafts that Enhance Arsenic Removal via Forward Osmosis Qiaoli Yang, Cher Hon Lau, and Qingchun Ge ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03991 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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Novel Ionic Grafts that Enhance Arsenic Removal

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via Forward Osmosis

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Qiaoli Yang,† Cher Hon Lau *,‡, Qingchun Ge *,†

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† College of Environment and Resources, Fuzhou University, Fujian 350116, China.

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‡ School of Engineering, The University of Edinburgh, Robert Stevenson Road, The King’s

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Buildings, Edinburgh, EH9 3FB, Scotland, UK

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Correspondence to: Q. C. Ge (E-mail: [email protected]), Tel: (86)591-22866219;

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C. H. Lau (email: [email protected]).

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ABSTRACT: Current forward osmosis (FO) membranes are unsuitable for arsenic removal from

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water because of their poor arsenic selectivity. In this study, we designed and synthesized a series

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of novel imidazolium-based ionic liquids via one-step quaternization reactions and grafted these

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novel compounds on to conventional thin-film composite FO membranes for treatment of arsenic-

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containing water. The newly developed ionic membranes contained a functionalized selective

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polyamide layer grafted with either carboxylic acid/carboxylate or sulfonate groups that drastically

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enhanced membrane hydrophilicity and thus FO water permeation. Ionic membranes modified

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with sodium 1-ethanesulfonate-3-(3-aminopropyl) imidazolium bromide (NH2-IM-(CH2)2-

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SO3Na) outperformed pristine membranes with higher water recovery efficiency. Exceptional

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performance was achieved with this ionic membrane in FO arsenic removal with a water flux of

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11.0 LMH and a rejection higher than 99.5% when 1000 ppm arsenic (HAsO42-) as the feed with

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a dilute NaCl solution (0.5 M) as the draw solution under the FO mode. Ionic membranes

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developed here in this work facilitated FO for the treatment of arsenic-containing water whilst

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demonstrating its superiority over incumbent technologies with more efficient arsenic removal.

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KEYWORDS: forward osmosis, FO membrane, membrane modification, ionic liquid, water

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treatment, arsenic removal.

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■ INTRODUCTION

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Arsenic contamination of groundwater is a high-profile problem that is exacerbated by industrial

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wastewater discharge impacts on 202 million people around the world, and can be overcome with

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water treatment.1 Technologies such as oxidation,2 adsorption,1 coagulation-flocculation3 and ion

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exchange1 have been deployed to purify arsenic-containing water. Unfortunately, most of these

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technologies are insufficient to deliver high purity water matching the standards of the World

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Health Organization (WHO). Hence secondary treatments are required to further reduce arsenic

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concentration. Some of these methods require large amounts of chemicals, or generate a high

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volume of arsenic-containing sludge; creating secondary pollution.1,4 In contrast, membrane

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technologies such as nanofiltration1,5 and reverse osmosis1,6 can be highly effective for arsenic

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removal but will require large operating pressures to provide a driving force to drive separations.

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The low-pressure requirement of forward osmosis (FO),7-9 high rejection rates towards a broad

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range of contaminants and low fouling tendencies8,9 of FO membranes can be harnessed for

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wastewater treatment including heavy metal removal,10 oil-water separation,11 dye reclamation

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from water.12 Although FO has been deployed to remove arsenic from water,13,14 this approach

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remains insufficient to reach the stringent targets of WHO. This is due to a lack of effective FO

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membranes that typically consist of a thin selective layer deposited on to a porous support – the

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core technology of FO processes.13,14

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Chemical modification to the thin selective layer can enhance membrane separation

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performances.7,15 For example, thin polyamide selective layers modified with neutral amines,

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alcohols, zwitterions and ionic compounds7 become more selective at the cost of lower water

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permeation. Chemical modifications with these compounds increase mass transfer resistance that

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reduce flux; limiting the impact of modification. Modified membranes also remain prone to fouling,

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hence are ineffective for wastewater treatment. Moreover, these onerous modification processes

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may require the use of corrosive compounds.7 Incomplete reactions requiring toxic starting

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materials may also arise from polymer modification protocols and reagents.16 Ideally, FO

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membranes should be modified with simple protocols that enhances both water recovery efficiency

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and selectivity whilst demonstrating a high fouling resistance alongside excellent chemical

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stability, and low concentration polarization.17 Clearly, existing modification protocols or

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compounds are inadequate to deliver such a membrane. Hence, a new modification compound

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synthesized using a facile approach is required to simultaneously enhance water flux and rejection

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towards contaminants, in this case, arsenic, whilst overcome fouling.

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Ionic liquids (ILs) are considered as environmental friendly compounds for a range of

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applications, including carbon capture, biofuel purification, ion exchange and desalination.18-21

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Composed of organic cationic and organic/inorganic anionic components, ILs can be tailored to

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yield desirable properties that benefit targeted applications. For example, designer ILs containing

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functional groups can be polymerized to form novel membranes for carbon capture,22,23 biofuel

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purification,24 proton transfer in fuel cell25 and desalination through RO.26 Meanwhile, membrane

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modification with ILs18,19 are typically achieved via physical blending, ion exchange, direct

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crosslinking or by combining with porous fillers.27,28 However, these approaches remain

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insufficient to overcome the key problem of ILs leaching into the liquid feed. This is due to the

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lack of compatibility or affinity between membrane and ILs. Moreover, the high viscosities of

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these ILs exacerbated the trade-off relationship between flux and selectivity membrane separation

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performance as higher permeability was achieved with lower selectivities.18,19,29 As such, IL-

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modified membranes have never been deployed in FO.

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To harness the benefits of FO processes for arsenic removal from contaminated water source,

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we synthesized and grafted a series of bifunctional imidazole-based ILs on to thin-film composite

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(TFC) membranes to repel most arsenic species present in oxyanions,1-6 whilst enhancing water

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flux. Amine functional groups were incorporated into our bespoke ILs to facilitate grafting on to

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the thin polyamide selective layers through a simple amidation grafting procedure. Here, we also

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studied the impact of chain length and functional groups in the ILs on water recovery and the

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rejection of arsenic species. Our IL-modified membranes generated water fluxes of up to 13.5

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LMH (FO mode) at room temperature with a dilute NaCl solution (0.5 M) as the draw solution

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and were used to reject 93 – 100 % of arsenic at different pH values, arsenic concentrations and in

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the presence of other acidic ions through FO processes (Figure 1).

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Figure 1. (A) Water permeation across the pristine thin-film composite (TFC) membrane comprising a

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dense polyamide (PA) selective layer supported on porous supports is slow while the rejection of As ions

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is poor. (B) The separation performance of these TFC membranes can be drastically enhanced with a series

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of designed ionic liquids.

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■ MATERIALS AND METHODS

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Synthesis of bifunctional ionic liquids. The novel imidazole-based ILs used for membrane

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modification were synthesized by a one-step quaternization reaction. The experimental details

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were given in the supporting information (SI).

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Fabrication of thin-film composite (TFC) membranes. A dope solution was prepared and

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deployed to fabricate polyethersulfone (PES) substrates using an established method.30 More

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detailed information was provided in the SI. A thin-film composite (TFC) FO membrane with a

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dense polyamide (PA) layer was synthesized through interfacial polymerization on the dense skin

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layer of the PES substrate (TFC-PES membrane). Briefly, the dense layer of the PES substrate was

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immersed in an aqueous solution containing 2.0 wt. % of MPD for 2 mins and subsequently

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immobilized in a rectangular frame. The dense surface was then exposed to a n-hexane solution

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containing 0.15 wt. % of TMC for 1 min. The excessive TMC solution was removed. An ultrathin

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PA layer was formed on the PES substrate via acylation reactions between MPD and TMC.

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Grafting ILs on to TFC membranes. ILs produced in this work were grafted on to the PA

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selective layer of the TFC membranes via an amidation reaction between the PA acyl chloride

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groups and primary amine groups in the ILs. Specifically, solutions containing 0.5 wt. %, 1.0 wt.

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%, 2.0 wt. % of ILs were exposed to the PA layer of TFC-PES membranes for different periods

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between 5 mins, 10 mins, 20 mins and 40 mins. Optimum conditions can be obtained by the

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systematic experiments to synthesize FO membranes with optimal performance.

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Table 1 shows the nomenclature of the IL-grafted membranes fabricated here.

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Table 1 Nomenclature of the ILs and membranes with IL grafts studied here in this work

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Ionic Liquid

Name of ionic membrane

NH2-IM-CH2COOH

CH2COOH-membrane

NH2-IM-CH2COONa

CH2COONa-membrane

NH2-IM-(CH2)2COOH

(CH2)2COOH-membrane

NH2-IM-(CH2)2COONa

(CH2)2COONa-membrane

NH2-IM-(CH2)2-SO3Na

(CH2)2-SO3Na-membrane

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Characterization of membrane structure. The pore size, pore size distribution and MWCO of

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both the PES substrate and FO membranes were determined based on solute separation

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experiments, as established elsewhere.17 Details were provided in the SI.

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Mass transfer properties. The water permeability coefficient (A), salt permeability coefficient

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(B) and salt rejection (R) of membranes were determined by using a bench-scale RO set-up. The

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detailed experimental conditions were provided in the SI.

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Membrane surface properties. Membrane surface properties were determined by analyzing

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functional groups on the surfaces of membranes, water contact angles, morphologies of membrane

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surface and surface roughness of membranes. Details were provided in the SI.

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FO experiments. The FO performances of membranes studied here were evaluated through a

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lab-scale FO set-up. The experimental details were disclosed in the SI.

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■ RESULTS AND DISCUSSION

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Synthesis of novel ionic liquids. To ensure widespread application of membrane modifiers,

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such compounds must be synthesized easily, simultaneously enhance membrane flux and

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selectivity, and improve membrane hydrophilicity to overcome the fouling effects on membrane

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separation performances. Guided by these requirements, we designed and synthesized a series of

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novel imidazole-based ILs via a one-step quaternization reaction between 1-(3-aminopropopyl)

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imidazole and functionalized halogenated hydrocarbon derivatives (Figure 2). NMR and FTIR

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characterizations revealed that all the ILs synthesized here contained bifunctional amine and

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carboxylic acid/carboxylate or sulfonate groups (Figure S1).

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Figure 2. Designing and synthesizing a series of imidazolium-based ionic liquids with different chain

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lengths, and functional groups.

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Carrying multiple strong polar groups including a positively charged imidazole ring, amine and

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carboxylate/sulfonate, all of the ILs synthesized in this study have good water solubility (> 1.0

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g/mL) which ensures all the modification experiments to be conducted in water. The 1H NMR

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resonances for protons from the imidazole ring (δ = 7.44 - 8.97 ppm) and the side chain of propyl

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amine (δ = 2.24 - 4.29 ppm) were similar for all ILs. This was attributed to the same chemistry

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environment for these protons because of insignificant reactions. The resonance of protons from

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the methylene connecting the imidazole ring on the acid substituted side chain was shifted upfield

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from NH2-IM-CH2COOH (δ = 4.84 ppm) to NH2-IM-(CH2)2COOH (δ = 4.36 ppm) due to the

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electron donor effect of the adjacent methylene. Likewise, resonance of protons connecting to the

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carboxyl groups was downfield shifted from the acid ILs to their corresponding sodium salt

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analogues resulting from the stronger electron withdrawing properties of the latter. The resonance

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of protons from the methylene adjacent to -COONa was further downfield shifted from NH2-IM-

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(CH2)2COONa (δ = 3.34 ppm) to NH2-IM-(CH2)2SO3Na (δ = 3.58 ppm) due to the stronger

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electron-withdrawing ability of -SO3Na. The characteristic peaks centered at 3400 cm-1 and 650

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cm-1 present in all ILs corresponded to the N-H or O-H bonds and imidazole ring, respectively.

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The signals at 1300-1680 cm-1 were ascribed to the stretching vibration of carboxyl/carboxylic

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groups of all the ILs except NH2-IM-(CH2)2-SO3Na. The signals at 1150 and 1250 cm-1 were

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caused by the stretching vibration of sulfonate group from NH2-IM-(CH2)2-SO3Na (Figure S1).

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The spectroscopic data were in line with the proposed chemical formulae of all ILs. These ILs

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were grafted on to the dense PA selective layers of TFC-PES membranes.

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Grafting ionic liquids onto membranes. TFC-PES FO membranes with a dense PA selective

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layer were produced using a two-step approach. Asymmetric PES substrates were first produced

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via the well-established method of phase inversion.30,31 A subsequent interfacial polymerization

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process7,17 between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) deposited thin PA

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selective layers on to the PES supports. Compared to other FO membrane chemical modification

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approaches,7,16,32,33 our proposed technique is underpinned by a highly reactive acylation that

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increased modification efficiency without involving additional chemicals such as intermediate

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reagents, catalyst and initiators, whilst reducing the risk of membrane damage that is typical of

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multistep chemical modifications (Figure 3).7,16,33 More importantly, our proposed technique of

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ionic grafting altered the chemical structure and properties of TFC-PES membranes to benefit

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arsenic removal during the FO mode.

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Figure 3. The synthesis scheme of the IL-modified TFC-PES membranes

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Impact of ionic grafting on membrane structure. FTIR analysis validated the PA deposition

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process where shoulder peaks centered at 1670 cm-1 observed in the FTIR spectra of both the

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pristine and modified membranes were absent from the PES substrate (Figure 4A). These peaks

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are correlated to amide groups produced by the amidation reaction between the PA acyl chloride

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groups and IL primary amine groups.17,32 The intensities of these peaks were drastically enhanced

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upon ionic grafting. This was ascribed to the presence of a higher concentration of amide groups

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on the surfaces of ionic membranes. Acyl chloride groups on within the PA selective layer were

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crucial for grafting ILs on to the membranes via one-step amidation reactions with primary amines

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located on the imidazolium ILs synthesized here. Peaks centered at 1780 cm-1, corresponding to

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acyl chlorides disappeared, while the one at 1670 cm-1 corresponding to amides was intensified.

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Here it is important to point out that peaks at 700-800 cm-1 and 1480-1590 cm-1, 1150 and 1250

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cm-1 correlated to benzene ring and sulfonate groups remained unchanged. The FTIR spectra

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analysis indicated that these ionic groups grafted onto the membrane surfaces firmly via covalent

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bonds between amines and acyl chloride functional groups. We also observed that the presence of

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ionic grafts on the membrane surface also reduced pore sizes.

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Figure 4. The characterizations of the pristine and modified membranes: (A) FTIR spectra; (B) Pore size

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distribution; (C) water contact angle as a function of reacting time (left; concentration: 1.0 wt%) and NH2-

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IM-(CH2)2-SO3Na concentration (right; time: 20 min) of the (CH2)2-SO3Na-membrane); and (D) water

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contact angles of modified membranes under the optimal conditions.

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Mean pore sizes within the membrane shifted from the ultrafiltration range (17 nm) to FO range

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(0.3 nm) upon deposition of a dense PA layer on to the surface of an asymmetric PES substrate.

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Ionic grafting further reduced the mean pore sizes to 0.29 nm, as exemplified by (CH2)2-SO3Na-

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membranes (Figure 4B). This was also validated by the successive decline in MWCO when the

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PES substrate was coated with a dense layer of PA, followed by ionic grafting modification.

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Likewise, higher salt rejections were achieved with smaller pore sizes (Table S1). Surprisingly,

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the notable pore size reduction did not impact on water permeability. Different from other works,7

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ionic grafting enhanced water permeability; even with smaller pore sizes. This could be attributed

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to the presence of a high concentration of free ionic sulfonate groups that enhanced membrane

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hydrophilicity. SEM micrographs also revealed that ionic grafting did not impact on the bottom

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membrane surface but significantly altered the topology of the dense PA selective layer (Figure

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S2A). Similar to other TFC membranes,7,32 the PA selective layer of membranes studied here

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resembled a leaf-like structure with ridge-valley features. Ionic grafting compacted these leaf-like

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structures, yielding a smoother surface when compared to pristine PA surfaces (Figure S2B).

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Smoother membrane surfaces are preferred for FO as they are more hydrophilic i.e. enhance water

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permeation whilst providing resistance towards fouling caused by hydrophobic compounds.7,34

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The water contact angles of polymer surfaces are a benchmark to describe membrane

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hydrophilicity, where smaller water contact angles are typically observed in membranes that are

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more hydrophilic, and vice versa.7,35,36 Here we observed that ionic grafting drastically reduced

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the water contact angles of TFC membranes as a function of modification time and concentration

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(Figure 4C). The water contact angles of TFC membranes fabricated here were reduced in the

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following trend where (CH2)2COOH > CH2COOH > (CH2)2COONa > CH2COONa > (CH2)2-

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SO3Na (Figure 4D). Conductivity experiments revealed that the ionization degree could be

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enhanced by replacing carboxylate groups with sulfonates, producing the most hydrophilic

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membrane studied here. The water contact angle of a TFC membranes modified with NH2-IM-

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(CH2)2-SO3Na was 38.7˚; significantly lower than the water contact angles of state-of-the-art FO

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membranes modified by 2-[(2-aminoethyl) amino]-ethane sulfonic acid monosodium salt

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(49.5˚),32 ethylene diamine (55.0˚),32 amino-poly(ethylene glycol) diglycidyl ether) (78.1˚),37 thin

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film nanocomposites (58.4˚) and N-[3-(trimethoxysilyl) propyl] ethylenediamine (50.5˚).38,39

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Other than altering surface topology to enhance water flux, ionic grafting also increased the

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hydrophilicity of the TFC membranes studied here. Consequently, the water recovery efficiency

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was further improved.

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Impact of ionic grafting on FO operation. Modification material and experimental conditions

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both have a remarkable impact on the FO performance of TFC membranes. Here we systematically

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examined these effects by tailoring IL chain length, ionization factor, reaction time and

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concentration to achieve the best FO performance for ionic membranes. Screening tests revealed

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that the reaction time and IL concentration had the largest impacts on FO performance for each

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IL-modified membrane. This was exemplified by the separation performance of (CH2)2-SO3Na-

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membranes (Figure 5A and Figure 5B). Regardless of the membrane orientation, the same trend

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was observed in water flux as a function of different reaction time and IL concentration. At a fixed

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time, water flux increased first and decreased as a function of NH2-IM-(CH2)2-SO3Na

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concentration. Membrane hydrophilicity and density increased with higher IL concentrations on

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membrane surfaces. However, the presence of more IL grafts on the surface will reduce FO water

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flux. This trade-off between ionic grafting and membrane performance was abated using a

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modification solution containing 1.0 wt. % of NH2-IM-(CH2)2-SO3Na. The optimal modification

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time was elucidated to be at 20 min. Variations in reaction time and IL concentration did not impact

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salt flux.

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Figure 5. The NH2-IM-(CH2)2-SO3Na modified membrane performance as a function of (A) modification

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time (concentration: 1.0 wt%), and (B) NH2-IM-(CH2)2-SO3Na concentration (time: 20 min). FO

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performance comparison of the pristine and IL-modified membranes under the optimal modification

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conditions: (C) water flux, (D) the ratio of Js/Jw. Experimental conditions: DI water as the feed, 0.5 M

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NaCl as the draw solution.

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Here it is important to mention that the distinctive characteristic of each ILs led to different sets

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of optimal modification conditions. However, it was clear that ionic membranes outperformed the

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original TFC-PES membrane with higher water transfer rates and comparable solute loss in

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treating the same amount of feed water (Figure 5C and Figure 5D). The FO performance of these

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novel ionic membranes modified at individual optimal conditions follows an uptrend of

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(CH2)2COOH < CH2COOH < (CH2)2COONa < CH2COONa < (CH2)2-SO3Na. This sequence is

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the exact opposite of the trends observed in water contact angle measurements, but is consistent

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with the degree of membrane hydrophilicity. Hydrophilic -COOH, -COONa and -SO3Na

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functional groups covered ionic membrane surfaces enhanced both hydrophilicity and fouling

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resistance, resulting in improved FO separation performance. NH2-IM-(CH2)2-SO3Na exhibited

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the highest ionization degree amongst all ILs synthesized here, leading to the most hydrophilic and

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best performing FO membrane.

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The FO performance of (CH2)2-SO3Na membranes was fully evaluated through experiments

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involving different membrane orientation and draw solution concentration and compared to the

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performance of the original TFC-PES membrane (Figure 6).

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Figure 6. FO performance comparison of the pristine and (NH2-IM-(CH2)2-SO3Na) modified TFC-PES

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membranes. NaCl with varying concentrations as the draw solutions. DI water as the feed in all experiments.

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Regardless of membrane type, the separation performance of membranes studied here operated

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in the PRO mode constantly surpassed those under the FO mode. This was attributed to the adverse

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effect of internal concentration polarization (ICP) on the latter.7,40 Water transfer rates were

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enhanced with higher draw solution concentrations due to an increased trans-membrane pressure

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according to the Van’t Hoff Law.41 The higher transmembrane pressure enabled a larger net

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driving force that would consequently enhance water permeation. Membranes grafted with NH2-

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IM-(CH2)2-SO3Na outperformed the pristine TFC-PES membranes under these conditions. The

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water fluxes of (CH2)2-SO3Na-membranes were 50 % higher than pristine TFC-PES FO

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membranes when NaCl concentration was increased from 0.5 to 2.0 M. Remarkably, this increase

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in draw solution concentration did not impact on solute loss, yet recovering the same amount of

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feed water. Although higher hydrophilicity in membrane usually couples with a lower salt rejection

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in membrane separation, the (CH2)2-SO3Na-membrane exhibited a comparable high salt rejection

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with the pristine membrane because of the increased electrostatic repulsion between the salt ions

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and charged membrane surface. (CH2)2-SO3Na)-membranes also outperformed state-of-the-art

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modified FO membranes7,32,38,42-44 where our ionic membranes exhibited higher water permeation

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with lower reverse salt diffusion under the same conditions (Table S2). This was due to the

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presence of both an ionic sulfonate group and an imidazole ring in NH2-IM-(CH2)2-SO3Na. The

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combination of these chemical moieties is more efficient for producing the hydrophilic membranes

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with excellent FO performances than neutral modification materials such as ethylene diamine,32

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porphyrin,42 graphene oxide44 and glutaraldehyde43 or aliphatic ionic 2-[(2-aminoethyl) amino]-

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ethane sulfonic acid monosodium salt.32

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Impact of ionic grafts on arsenic removal. (CH2)2-SO3Na-membranes developed here were

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deployed to treat arsenic-containing water in FO as a function of As5+ concentration, acid-base

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properties of the As5+ feed solution, and the presence of co-solutes. These factors are frequently

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encountered in arsenic wastewater reclamation and are essential for the characterization of the FO

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performance of these membranes. Water permeation in both the ionic and pristine membranes was

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reduced when DI water feed was replaced with aqueous solutions containing As5+ ions (Figure

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7A). The As5+ concentrations used in this study are higher than those in real wastewater for purpose

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of accurate detection since the permeate in FO is diluted by the bulky draw solution. With As5+

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concentration increased from 0 to 1000 ppm, the water fluxes of (CH2)2-SO3Na-modified and

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pristine TFC-PES membranes were reduced by 15% and 19 %, respectively. This was due to the

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drop in osmotic pressure differential across the membrane as feed osmotic pressure increased with

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higher solute concentrations that lowered the net driving force and consequently reduced water

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permeation across the membranes. Concentration polarization could have contributed towards the

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decline in water permeation. More importantly, as As5+ feed concentration increased from 0 to

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1000 ppm, ionic grafting enhanced water fluxes by 50 – 67 % when compared to the original TFC-

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PES membrane. This could be ascribed to stronger repulsion between the negatively-charged ionic

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membrane and arsenic oxyanions, leading to less severe concentration polarization and higher

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water permeation. Similar charges between the (CH2)2-SO3Na-modified membranes and As5+ ions

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led to a near-100 % rejection rate that was significantly higher than the rejection rates of pristine

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membranes (Figure 7A).

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Figure 7. Comparison of FO performance in removing As5+ from water using both the pristine and (NH2-

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IM-(CH2)2-SO3Na) modified membranes under various experimental conditions: (A) 0 – 1000 ppm As5+

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solutions; (B) pH 3 – 11 (As5+ at 1000 ppm); (C) (0 – 150 ppm NO3-) + 1000 ppm As5+ solutions, and (D)

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(0 – 150 ppm SO42-) + 1000 ppm As5+ solutions. Experimental conditions: 0.5 M NaCl as the draw solution

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and operated under the FO mode at room temperature.

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The FO process via (CH2)2-SO3Na-membranes outperformed existing membrane technologies

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that have been studied for arsenic removal from water (including FO and nanofiltration) (Table

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S3).14,45-47

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Arsenic species can appear in various forms at different pH values.6,7 Hence the impact of pH

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effects on membrane FO performance in As5+ removal were also studied here over a broad pH

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range from acidity (pH = 3) to basicity (pH = 11) (Figure 7B). Better FO performance with

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increased water fluxes and higher As5+ rejections was achieved for both the ionic and pristine

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membranes as pH changed from 3 to 11. The initial mixture of neutral H3AsO4 and monovalent

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H2AsO4- was gradually converted into higher valence arsenic oxyanions with the pH ranging from

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acidity to alkalinity,1,7 enhancing the mutual repulsion between the negatively-charged ionic

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membrane and arsenic oxyanions, hence increasing As5+ rejections. Having higher

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electronegativity and stronger electrostatic repulsion with arsenic oxyanions, the NH2-IM-(CH2)2-

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SO3Na- modified membranes were less prone to fouling and were more water-permeable with

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higher As5+ rejections (> 99.5 %) than the original TFC-PES membrane throughout the studied pH

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range. The presence of co-solutes of NO3- or SO42- impacted slightly on the FO performance of

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the (CH2)2-SO3Na-modified ionic membrane (Figure 7C and Figure 7D). A small decline in both

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water permeation and As5+ rejection rates occurred when the concentration of NO3- or SO42-

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increased from 0 to 150 ppm. Addition of NO3- or SO42- increased the feed osmotic pressure,

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reducing the net driving force across the membrane that reduced water flux. Meanwhile, the

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repulsion of larger NO3- (hydrated radius = 0.34 nm)48 or SO42- (hydrated radius = 0.38 nm)48 ions

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by the negatively charged (CH2)2-SO3Na-membrane could enable smaller HAsO42- (hydrated

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radius = 0.20 nm)49 to pass through the membrane interior; aggravating the impact of concentration

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polarization and fouling that led to a decrease in water flux and As5+ rejection. This was also

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observed elsewhere during the treatment of arsenic-containing water.49 Intriguingly, compared to

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the pristine TFC membrane, the decline in water transfer rates and As5+ rejections were smaller

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for the (CH2)2-SO3Na-modifed ionic membrane. This was because the smaller HAsO42- ions could

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be easily transported across the pristine TFC-PES membrane without repulsion effects from

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negatively-charged ionic grafts, leading to a larger decline in both water flux and As5+ rejection.

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These results clearly demonstrated the advantages of our novel ionic grafting approach for the

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treatment of arsenic-containing water using membranes that are widely reported for FO.

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■ CONCLUSIONS

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A series of bifunctional novel imidazolium ILs were synthesized via a simple one-step

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quaternization reaction and grafted onto conventional TFC-PES FO membranes through a facile

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and efficient amidation reaction. The presence of ionized carboxylate or sulfonate groups

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drastically enhanced the hydrophilicity of resultant membranes. All ionic membranes studied here

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recovered feed water efficiently as compared to pristine TFC membranes. TFC membranes grafted

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with NH2-IM-(CH2)2-SO3Na ionic liquids outperformed all membranes studied here, and state-of-

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the-art membranes for arsenic removal from water, with a good water flux of 11.0 LMH and 

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100% As5+ rejection rates with a dilute NaCl solution (0.5 M) under the FO mode. The negatively-

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charged nature of NH2-IM-(CH2)2-SO3Na-modified ionic membrane can be potentially deployed

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to treat other types of wastewater containing anionic components. This study provides inspiration

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for future novel FO membrane exploration and FO application.

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■ AUTHOR INFORMATION

344

Corresponding Authors

345

E-mail: [email protected]; Tel: (86)591-22866219; [email protected]

346

Authors Contributions

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All the authors contributed equally to this work. The article was written through contributions of

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all authors have given approval to the final version of the manuscript.

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Notes

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The authors declare no competing financial interest.

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■ ACKNOWLEDGMENT

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QLY and QCG thank the financial supports from the National Natural Science Foundation of

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China (NSFC) (grant number: 21677035), the Natural Science Foundation of Fujian Province

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(grant number: 2016J01056), and Fuzhou University Testing Fund of precious apparatus (grant

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number: 2019T008). CHL thank the University of Edinburgh Chancellor’s Fellowship for support.

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■ ASSOCIATED CONTENT

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Supporting Information Available

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Materials; Synthesis of bifunctional ionic liquids; Fabrication of thin-film composite (TFC)

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membranes; Characterization of membrane structure; Mass transfer properties; Membrane surface

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properties; FO experiments.

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■ REFERENCES

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health hazards and removal techniques from water: an overview. Desalination 2007, 217 (1), 139

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