Novel Ionic Grafts That Enhance Arsenic Removal via Forward Osmosis

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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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ACS Applied Materials & Interfaces

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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

285

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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295 296

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

297

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

298

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

299

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

300

and operated under the FO mode at room temperature.

301

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

302

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

303

S3).14,45-47

304

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

305

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

306

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


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Page 18 of 26

307

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

309

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

310

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

311

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

312

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

313

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

314

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

315

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

316

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

317

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

318

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

319

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

320

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

321

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

322

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

323

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

324

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

325

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

326

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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329

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

330

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

331

■ CONCLUSIONS

332

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

333

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

334

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

335

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

336

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

337

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

338

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

339

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

340

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

341

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

342

for future novel FO membrane exploration and FO application.

343

■ AUTHOR INFORMATION

344

Corresponding Authors

345

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

346

Authors Contributions

347

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

348

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

349

Notes

350

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

355

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

356

■ ASSOCIATED CONTENT

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Supporting Information Available

358

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

359

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

360

properties; FO experiments.

361

■ REFERENCES

362

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

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■ TOC Art

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Novel Ionic Grafts That Enhance Arsenic Removal via Forward Osmosis

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