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Effective As(III) Removal by A Multi-Charged Hydroacid Complex Draw Solute Facilitated Forward Osmosis-Membrane Distillation (FO-MD) Processes Qingchun Ge, Gang Han, and Tai-Shung Chung Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05402 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 10, 2016

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Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Effective As(III) Removal by A Multi-Charged

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Hydroacid Complex Draw Solute Facilitated

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Forward Osmosis-Membrane Distillation (FO-MD)

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Processes

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Qingchun Ge,† Gang Han, ‡ Tai-Shung Chung*,‡§

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

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‡Department of Chemical & Biomolecular Engineering, National University of Singapore,

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4 Engineering Drive 4, Singapore 117576, Singapore

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§Water Desalination & Reuse (WDR) Center King Abdullah University of Science and

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Technology, Saudi Arabia 23955-6900

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Correspondence to: T. S. Chung (E-mail: [email protected])

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Tel: (65) 6516-6645; Fax: 65-6779-1936

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ABSTRACT: Effective removal of As(III) from water by an oxalic acid complex with the

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formula of Na3[Cr(C2O4)3] (Na-Cr-OA) is demonstrated via an forward osmosis-membrane

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distillation (FO-MD) hybrid system in this study. Na-Cr-OA firstly proved its superiority as a

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draw solute with high water fluxes and negligible reverse fluxes in FO, then a systematic

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investigation of the Na-Cr-OA promoted FO process was conducted to ascertain the factors in

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As(III) removal. Relatively high water fluxes of 28 LMH under the FO mode and 74 LMH under

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the pressure retarded osmosis (PRO) mode were achieved when using a 1000 ppm As(III)

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solution as the feed and 1.0 M Na-Cr-OA as the draw solution at 60 °C. As(III) removal with a

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water recovery up to 21.6% (FO mode) and 48.3% (PRO mode) were also achieved in 2 hours.

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An outstanding As(III) rejection with 30 ∼ 3000 µg/L As(III) in the permeate was accomplished

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when As(III) feed solutions varied from 5x104 to 1x106 µg/L, superior to the best FO

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performance reported for As(III) removal. Incorporating MD into FO not only makes As(III)

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removal sustainable by re-concentrating the Na-Cr-OA solution simultaneously, but also reduces

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the As(III) concentration below 10 µg/L in the product water, meeting the WHO standard.

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■ TOC Art

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■ INTRODUCTION

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Arsenic contamination in groundwater is a serious problem due to its high toxicity and

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ubiquitous presence.1 It mainly exists in inorganic forms with oxidation states of As(III) and

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As(V) in different proportions. The toxicity and mobility of arsenite species (As(III)) are higher

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than those of their arsenate (As(V)) analogues.2 According to the drinking water guideline set by

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World Health Organization (WHO), the maximum arsenic concentration in drinking water is 10

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µg L-1.3 Arsenic poisoning is one of major environmental problems in the world with millions of

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people being exposed to arsenic-contaminated drinking water.4 Hence, arsenic removal from

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contaminated water is an important issue that needs to be solved urgently.

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Many technologies including coagulation, adsorption, ion exchange and bacterial treatment

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have been employed for arsenic removal from water.1 Most of them require secondary treatments

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to further reduce arsenic concentration in order to meet the WHO standard. Additionally, a large

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amount of chemicals is involved and a high volume of sludge is produced. In some processes,

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arsenic leaches out from the sludge. Thus, the maintenance of these conventional technologies is

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usually costly. In contrast, membrane technology shows unique advantages of little chemical

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involvement, small sludge and relatively low maintenance cost when removing arsenic from

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water.5 Hence, membrane processes such as microfiltration (MF), ultrafiltration (UF),

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nanofiltration (NF) and reverse osmosis (RO) have been extensively used in arsenic-containing

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water purification.5,6 Among them, NF and RO have manifested their effectiveness in removing

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arsenic from water.7-9 However, both processes are operated at high pressures and have relatively

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low water recovery rates. Therefore, comparatively high costs are required and they suffer the

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risk of severe membrane fouling.10

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Recently, forward osmosis (FO) has been applied to arsenic removal from water.11-13 FO is

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more promising than other membrane processes due to its low pressure operation, high water

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recovery and low membrane fouling.14-17 As FO membrane and draw solute are the two

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determining factors in FO technology, a powerful draw solute with a minimal reverse flux favors

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the improvement of FO performance. NaCl,11 MgCl2 and glucose12 were the draw solutes in the

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previous studies of arsenic removal. These conventional draw solutes, however, were not

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efficient in feed water recovery for arsenic removal due to low water permeate fluxes and/or

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relatively high reverse solute fluxes. Hydroacid complexes have proven their superiority as draw

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solutes in FO processes.18,19 The oxalic acid-Cr complex with a molecular formula of

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Na3[Cr(C2O4)3] (Na-Cr-OA) possessing a Cr-centered octahedral structure and multi-charged

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feature (Figure S1) demonstrated its suitability as a draw solute in FO with a very high water

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flux and a negligible reverse solute flux.19 Besides the advantages of having high water

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solubility, multi-charges and negligible reverse flux, Na-Cr-OA has good thermal stability and

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only decomposes when temperature is up to 240 °C. It has lower viscosity compared to other

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hydroacid complexes.19 Na-Cr-OA was therefore used as the draw solute in this FO study for the

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arsenic removal.

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Since the current arsenic removal technologies are less effective to remove As(III) than As(V),

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pre-oxidation of As(III) to As(V) is an indispensable step in most treatment technologies for

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better removal.5 To mitigate the oxidation step and to achieve an efficient and effective removal

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of As(III) from water, a Na-Cr-OA facilitated forward osmosis-membrane distillation (FO-MD)

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hybrid system was applied to the As(III) removal. FO-MD has been proven workable as an

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integrated membrane process.20-22 To our best knowledge, the hybrid FO-MD system has not yet

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been employed in As(III) removal from water. The hybrid FO-MD system was chosen because

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(1) the Na-Cr-OA draw solution would perform better at temperatures higher than ambient

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conditions with a lower viscosity, (2) the permeate from MD could meet the stringent WHO

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standard on As(III) of less than 10 µg/L and (3) we aim to develop a sustainable hybrid FO-MD

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system for As(III) removal.

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Therefore, the objectives of this study are (1) to systematically study the effects of

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experimental factors including membrane orientation, temperature, co-existing solutes, pH and

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As(III) feed concentration on As(III) removal in FO processes and (2) to investigate the MD

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process and to identify the most suitable conditions for a sustainable FO-MD process. This study

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may provide inspiration for future studies in novel draw solutes and hybrid systems to effectively

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and sustainably remove As(III) from water.

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■ MATERIALS AND METHODS

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Synthesis of the Na-Cr-OA Draw Solute. Na-Cr-OA was synthesized through a modified

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method19 from Na2C2O4, H2C2O4 and Na2Cr2O7⋅2H2O. The detailed experimental procedures are

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disclosed in the supporting information (SI).

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As(III) Solution Preparation and Analyses. The As(III) feed solutions were prepared from

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As2O3 with the aid of NaOH or HCl solutions. The details of the As(III) solution preparation and

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analyses are included in the SI.

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Individual FO, MD and hybrid FO-MD processes. The FO-MD experiments were

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conducted via a bench-scale hybrid FO-MD system as depicted elsewhere.20,21 The system was

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also used for individual FO and MD experiments. Lab-made thin-film composite FO membranes

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fabricated on polyethersulfone hollow fiber substrates (TFC-PES)23 and PVDF MD hollow fiber

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membranes21 with characteristics shown in Table S1 were employed as FO and MD membranes,

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respectively. The detailed experimental conditions are described in the SI.

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■ RESULTS AND DISCUSSION

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Effects of Temperature on Relative Viscosity of Na-Cr-OA solutions. As the viscosity of a

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draw solution has notable effects on its FO performance,24 the impact of temperature on relative

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viscosity of the Na-Cr-OA solution is studied prior to investigating its FO performance (Figure

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S2). Since NaCl has been extensively used as the draw solute in FO, it is served as a benchmark

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for performance comparison with the Na-Cr-OA draw solute in this study. Its relative viscosity at

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different temperatures is also presented in Figure S2.

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The relative viscosity of the Na-Cr-OA solution declines with increasing temperature. The

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decrement is greater at a higher temperature. In contrast, the NaCl solution has insignificant

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changes in its relative viscosity with temperature. In view of the inverse relationship between

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viscosity and water flux in FO, a higher water flux is expected for the Na-Cr-OA solution at a

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higher temperature. However, the increment in water flux with temperature may not be so

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significant when using NaCl as the draw solute. This will be verified by the results of FO

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experiments and discussed in the subsequent sections.

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FO processes via TFC-PES hollow fiber membranes. As membrane orientation and

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operation temperature have remarkable effects on FO performance, FO processes under different

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conditions were evaluated prior to studying the As(III) removal. Figure 1 summarizes their

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effects on water flux and reverse solute flux as a function of time using TFC-PES hollow fiber

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membranes. (a)

(b)

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Figure 1. Effects of temperature and membrane orientation on FO performance using TFC-PES

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hollow fiber membranes: (a) water flux of Na-Cr-OA, (b) water flux of NaCl, (c) reverse salt

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flux of Na-Cr-OA, and (d) reverse salt flux of NaCl. Initial conditions: 1.0 M Na-Cr-OA and

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NaCl as the draw solutions, DI water as the feed solution.

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Both Na-Cr-OA and NaCl draw solutions exhibit higher water fluxes at higher temperatures

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because the hydration sphere of the hydrated solute molecules changes with temperature. A

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higher temperature conduces to a smaller hydration radius of the solute molecules which results

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in lower viscosity, as confirmed by Figure S2. In addition, a higher temperature elevates their

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osmotic pressures according to the van’t Hoff equation ( π = iMRT, where T is the

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thermodynamic temperature).25 Both changes lead to an enhancement in FO water fluxes. The

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decreasing trend in water flux as a function of time is possibly caused by the dilution of draw

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solutions and the effects of concentration polarization.20-24 Nevertheless, an average water flux of

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around 56 ∼ 76 LMH and 19 ∼ 29 LMH at 25 ∼ 60 °C were acquired for Na-Cr-OA in the 2-h

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duration test under the pressure retarded osmosis (PRO) and FO modes, respectively. The water

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fluxes under the PRO mode consistently surpass those under the FO mode in all experiments

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because the former has less internal concentration polarization (ICP) than the latter.24,26-28 Na-Cr-

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OA outperforms NaCl in terms of water flux under the PRO mode, especially at a higher

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temperature because the former has more disassociated ionic species than the latter at the same

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molar concentration. In contrast, the water flux difference between Na-Cr-OA and NaCl under

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the FO mode is not as big as that under the PRO mode. This is due to the fact that Na-Cr-OA has

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a larger structure which possibly causes a lower diffusion coefficient and hence severer ICP than

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NaCl. Nevertheless, Na-Cr-OA still outperforms NaCl in terms of water flux especially at higher

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temperatures.

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The reverse salt fluxes of both Na-Cr-OA and NaCl as a function of temperature exhibit the

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same trends as their water fluxes (Figures 1(c) and 1(d)). The Na-Cr-OA draw solute has an

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insignificant reverse salt flux of less than 0.7 gMH for all experiments. In contrast, NaCl has a

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much higher reverse salt flux (Figure 1(d)). Moreover, its reverse solute flux increases

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significantly with an increase in temperature. Hence, Na-Cr-OA shows its superiority to NaCl in

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terms of higher water flux and insignificant reverse solute flux in the studied temperature range.

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The specific flux, defined as Js/Jv (g L-1), is smaller than 0.01 g L-1 for Na-Cr-OA, indicating a

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negligible loss of Na-Cr-OA in the FO process. The negligible reverse flux is primarily attributed

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to the two factors: 1) Donnan exclusion effect. Na-Cr-OA can ionize and form a trivalent

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oxyanion of [Cr(C2O4)3]3- in solutions which is repulsive with the negatively-charged TFC-PES

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membrane. Thus, it lowers the Na-Cr-OA diffusion across the membrane to the feed side; 2) size

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exclusion effect. Unlike NaCl, the Na-Cr-OA molecule has a Cr-centered octahedral structure, as

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proven by its single crystal chromatography.19 In addition, its carboxylic groups can form

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abundant H-bonds with the surrounding water molecules. Furthermore, its sodium ions can

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interact with the oxygen atoms of water molecules and grow the Na-Cr-OA complex to a

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polymeric network in the aqueous solution. Figure S3(a) displays the proposed polymeric

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network of Na-Cr-OA in the aqueous solution, while Figure S3(b) shows its solid-state

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polymeric structure generated from the cif file in reference 19 using the software of “the Bruker

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SHELXTL Software Package”. Therefore, both the inherent structure of Na-Cr-OA and its

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polymeric network in solutions account for the negligible reverse fluxes.

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As(III) removal through FO processes. Figure 2 shows the As(III) removal and water

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recovery as functions of temperature and operational mode by using 1000 ppm As(III) as the

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feed solution and 1.0 M Na-Cr-OA as the draw solution. The water flux declines slightly but

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varies with temperature similar to the trend when using DI water as the feed (Figure 1). The

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slight decline in water flux is ascribed to the effect of simultaneous concentration change with an

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increase in the As(III) feed solution and a decrease in the Na-Cr-OA draw solution when water

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transports from the feed side to the draw solution. As a result, the effective driving force across

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the membrane decreases and results in a decline in water flux. However, relatively high water

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fluxes of 17 ∼ 28 LMH (FO mode) and 54 ∼ 74 LMH (PRO mode) were still achieved at 25 ∼ 60

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°C. Encouragingly, the As(III) solutions are rapidly concentrated to 1276 ppm and 1935 ppm

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with water recovery up to 21.6 % and 48.3 % within 2 hours at 60 °C under FO and PRO modes,

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respectively, as illustrated in Figure 2(b). These performances are superior to the best FO

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performance reported for As(III) removal.29 In addition, comparatively high As(III) rejections

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were achieved under neutral conditions. As displayed in Figures 2(c) and 2(d), the rejections of

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As(III) were almost constant during the 2-h experiments with values higher than 80% under both

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PRO and FO modes. Interestingly, the FO mode has a better rejection of above 90% than the

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PRO mode. In addition, a higher rejection is obtained at room temperature. These rejection

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values are much higher than those reported in the literature for As(III) removal via FO processes

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under neutral conditions.8,29

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Figure 2. Effects of temperature and membrane orientation on FO performance in As(III)

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removal: (a) water fluxes under both PRO and FO modes, (b) water recovery of the feed solution

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and As(III) feed concentration after 2-h FO processes, (c) rejection of As(III) under the PRO

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mode, and (d) rejection of As(III) under the FO mode. Initial conditions: 1.0 M Na-Cr-OA as the

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draw solution, 1000 ppm As(III) as the feed solution.

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The FO performance as a function of As(III) concentration and temperatures was also studied.

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As illustrated in Figure 3, water flux is more sensitive to temperature variation than to the

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change of As(III) concentration. A slight change in water flux is observed with an increase in

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As(III) concentration at a certain temperature, especially at a higher temperature. In contrast, the

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feed As(III) concentration has noticeable impact on rejection. A lower As(III) concentration led

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to a higher As(III) rejection in the studied temperature range. As a result, the As(III)

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concentration in the final draw solution after FO is in the range of 0.03 ∼ 3 ppm when the As(III)

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feed concentration varies from 50 to 1000 ppm under neutral conditions. Even though the

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rejection becomes lower at higher feed concentrations, the As(III) concentration in the final draw

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solution is still much lower than the best value reported for As(III) removal from water via a

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single method/process.30,31 Hence, FO is a facile and reliable technology that can remove the vast

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majority of As(III) efficiently. (a)

(b)

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Figure 3. Effects of As (III) concentration and temperature on FO performance in As (III)

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removal: (a) water flux, and (b) As (III) rejection. Experimental conditions: 1.0 M Na-Cr-OA as

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the draw solution, FO mode. Test duration: 30 min. Error bars were obtained by repeated

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analyses (n = 6).

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pH effect on As(III) removal. The pH effect on As(III) removal through the FO process was

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further explored in this study. As(III) solutions with pH varying in the range of 3 ∼ 11 were

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evaluated because the dominant species would gradually change from the neutral H3AsO3 to the

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charged H2AsO3- under such conditions. Figure 4 displays the effects of pH on both water flux

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and As(III) rejection.

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Figure 4. Effects of pH on FO performance in As(III) removal: (a) water flux and As(III)

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rejection under the PRO mode, (b) water flux and As(III) rejection under the FO mode. Initial

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conditions: 1.0 M Na-Cr-OA as the draw solution, 1000 ppm As(III) as the feed solution. Test

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duration: 30 min, room temperature.

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Regardless of membrane orientation, water flux decreases slightly when the pH increases from

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weakly acidic to weakly alkaline (pH 3 ∼ 9). An obvious decline in water flux is observed when

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the As(III) feed solution becomes very basic (pH ∼ 11) because the amount of neutral species

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(H3AsO3) decreases whilst that of charged species (H2AsO3-) increases in the feed solution. The

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higher the pH value, the higher the amount of charged species is present. Meanwhile, the more

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the charged species in the feed solution, the greater the osmotic pressure is. As a consequence,

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the osmotic pressure difference between the draw solution and feed solution decreases with an

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increase in feed pH value. Accordingly, the net driving force across the FO membrane declines

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and results in a reduced water flux. It should be noted that no obvious pH change was observed

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in the Na-Cr-OA draw solutions with the pH variation in the As(III) feed solution under the

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entire studied conditions. This may be due to the high As(III) rejection, as depicted in Figure 4,

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which cause no effect on the pH of the Na-Cr-OA draw solutions.

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The changes of As(III) rejection with pH under both PRO and FO modes show a similar trend.

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The As(III) rejection undergoes a slight increase from pH 3 to 9 but increases substantially at pH

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11 because the fraction of H2AsO3- at this pH may reach as high as about 91% according to the

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study conducted by Tanaka et al..4 As H2AsO3- has a larger hydrated size than H3AsO3 due to

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more interactions with surrounding water molecules for the former than the latter,4 size exclusion

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is more prominent for H2AsO3- removal at a higher pH. Meanwhile, separation due to Donnan

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exclusion also becomes more significant because of the repulsion between the negatively-

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charged TFC-PES FO membrane and H2AsO3-. Consequently, the combined effects of size

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exclusion and Donnan exclusion enable the rejection to reach the highest at pH = 11. In addition,

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the difference in As(III) rejection between PRO and FO modes becomes smaller with an increase

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in pH value. They are almost the same at pH = 11. In brief, pH plays an important role in

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determining both water flux and As(III) rejection due to the state variation of the feed As(III)

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species under different pH values. It can decrease the water flux but significantly increase the

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As(III) rejection in very alkaline conditions.

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Effect of co-existing solutes on As(III) removal. The influences of co-solutes on FO

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performance in terms of As(III) rejection and water flux are represented in Figure 5. The cations

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(Na+ and Mg2+) in the presence of a common counter-ion (Cl-) and the anions (Cl- and SO42-)

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paired with a counter-ion (Na+) were added to the As(III) aqueous solutions in separate

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experiments. As shown in Figure 5(a), the existence of cations results in a declined water flux.

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The flux decline is greater in the presence of divalent Mg2+ than monovalent Na+. This is

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possibly because of the interaction between the cations and the negatively-charged FO

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membrane, resulting in membrane fouling and hence a drop in water flux. An increase in cation

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concentration may also contribute to the decrease in water flux. Unlike the variation trends in

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water fluxes, the changes in the As(III) rejection are insignificant upon the addition of cations.

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This may be attributed to the presence of the dominant species of H3AsO3 in solution under the

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studied conditions; hence the charge changes in the membrane surface have little effect on the

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passage of the neutral substance.

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Compared to Figure 5(a), the influence of anions (Cl- and SO42-) on water flux and As(III)

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rejection is insignificant in the presence of either monovalent Cl- or divalent SO42- (Figure 5(b)).

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This is understandable because despite the existence of electrostatic repulsion between the anions

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and the negatively-charged FO membrane, the neutral status of H3AsO3 and membrane

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properties remain the same. Therefore, the influence of these anions on FO performance is

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negligible. The slight change in water flux may result from the osmotic pressure change of the

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feed solution due to the addition of anions.

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(a)

(b)

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Figure 5. Effects of co-solutes on FO performance in As(III) removal: (a) cation effect on water

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flux and As(III) rejection, (b) anion effect on water flux and As(III) rejection. Initial conditions:

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1.0 M Na-Cr-OA as the draw solution, 1000 ppm As(III) as the feed solution, under the FO mode

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and neutral conditions. Test duration: 30 min, room temperature.

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Hybrid FO-MD process. Prior to combining MD with FO to re-concentrate the diluted Na-

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Cr-OA solution from FO, the MD process was investigated to ascertain how the effects of

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temperature and Na-Cr-OA concentration on water flux and water transfer rate. (a)

(b)

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Figure 6. Effects of temperature and Na-Cr-OA concentration on: (a) water flux, and (b) water

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transfer rate. Initial conditions: MD processes, DI water at 20 °C in the permeate side, Test

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duration: 30 min.

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The experiments studying the effects of temperature change on water flux were carried out using

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Na-Cr-OA solutions at different concentrations flowing against the shell side of the MD module

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at 40 °C, 50 °C and 60 °C, respectively. Meanwhile, DI water at 20 °C was circulating in the

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permeate side of the MD module under all conditions. A 30-min test duration was used in the

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individual MD processes. As illustrated in Figure 6, temperature shows the dominant effect on

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water flux in the MD process since it is a thermal driven process. When increasing temperature,

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an accelerated increment in water flux is observed as the water vapor pressure follows an

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exponential function of temperature.32 In contrast, water flux changes insignificantly with Na-Cr-

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OA concentration. Since water transfer rate is a product of water flux and membrane surface

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area, a MD module packed with 10 pieces of PVDF hollow fibers with 15 cm in length has the

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similar water transfer rate as the FO process under the FO mode in the first 30-min run at 60 °C.

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Therefore, the hybrid FO-MD experiments were conducted at 60 °C using a 1000 ppm As(III)

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feed solution and a 1.0 M Na-Cr-OA draw solution circulating in the lumen and shell sides of the

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FO membrane module, respectively, while the diluted Na-Cr-OA solution from the FO side and

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DI water at 20 °C were circulated in the shell and lumen sides of the MD membrane module,

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respectively.

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The hybrid FO-MD system is more efficient than the individual FO process in the

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concentration of the As(III) feed solution since Na-Cr-OA solution was concentrated

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instantaneously by the MD process once it was diluted in FO. As depicted in Figure 7, a more

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concentrated As(III) feed solution is obtained and a higher recovery rate is achieved due to a

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steady water flux in the hybrid FO-MD process compared to the single FO process. It is

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encouraging to observe that the As(III) concentration in the MD permeate is lower than 0.01ppm

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(10 µg/L), satisfying the standard set by WHO. In addition, no variation in conductivity is

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detected at the permeate side of MD under all experimental conditions, indicative of an almost

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complete rejection of Na-Cr-OA in the MD process. In conclusion, the Na-Cr-OA facilitated FO-

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MD system shows its efficiency and effectiveness in the As(III) removal from water,

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demonstrating its practicability in As(III) removal under neutral conditions. The newly

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developed system does not need the oxidation pretreatment step by converting As(III) to As(V),

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which is essential for conventional technologies.29,33 This study may provide useful insights for

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novel draw solute exploration and inspire future studies for effective As(III) removal from water.

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(b)

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Figure 7. Performance comparison between single FO and hybrid FO-MD processes: (a) As (III)

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feed concentration with time, (b) As (III) feed recovery with time. Initial conditions: 1.0 M Na-

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Cr-OA as the draw solution and 1000 ppm As(III) as the feed solution in the FO process, FO

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mode in the FO process, 60 °C.

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■ AUTHOR INFORMATION

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Corresponding Author

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E-mail: [email protected]; Tel: (65) 6516-6645; Fax: 65-6779-1936

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■ ACKNOWLEDGMENT

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This research is supported by both the National Research Foundation- Prime Minister's office,

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Republic of Singapore under its Competitive Research Program entitled “Advanced FO

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Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater

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Desalination” (Grant numbers: R-279-000-336-281 & R-279-000-339-281), and Fuzhou

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University (Grant numbers: 510160 & 521142). Special thanks are given to Ms. Jie Gao, Mr.

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Zhenlei Cheng and Mr. Zhiwei Thong for their valuable help.

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■ ASSOCIATED CONTENT

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Supporting Information Available

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Materials; synthesis of the Na-Cr-OA draw solute; relative viscosity of Na-Cr-OA and NaCl

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draw solutions; As(III) solution preparation and analyses; individual FO, MD and hybrid FO-

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MD processes; structures of Na-Cr-OA in both solution and solid state; the properties of the FO

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and MD hollow fiber membranes.

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