Effective As(III) Removal by A Multi-Charged Hydroacid Complex

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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

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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

ACS Paragon Plus Environment

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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

28 29

■ 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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(c)

(d)

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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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(b)

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(d)

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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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(b)

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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)

261 262

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)

269 270

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

292

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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(a)

(b)

303 304

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

309

Corresponding Author

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

311

■ 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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Effective As(III) Removal by A Multi-Charged Hydroacid Complex

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