Removal by A Multi-Charged Hydroacid Complex Draw Solute

Jan 29, 2016 - Draw Solute Facilitated Forward Osmosis-Membrane Distillation (FO- ... ppm As(III) solution as the feed and 1.0 M Na−Cr−OA as the d...
0 downloads 0 Views 1MB Size
Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 22

Environmental Science & Technology

1

Effective As(III) Removal by A Multi-Charged

2

Hydroacid Complex Draw Solute Facilitated

3

Forward Osmosis-Membrane Distillation (FO-MD)

4

Processes

5

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

6

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

7

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

8

4 Engineering Drive 4, Singapore 117576, Singapore

9

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

10

Technology, Saudi Arabia 23955-6900

11

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

12

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

13

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

14

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

15

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

ACS Paragon Plus Environment

1

Environmental Science & Technology

Page 2 of 22

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

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

27

■ TOC Art

28 29

■ INTRODUCTION

30

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

31

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

32

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

33

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

ACS Paragon Plus Environment

2

Page 3 of 22

Environmental Science & Technology

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

risk of severe membrane fouling.10

51

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

52

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

53

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

54

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

55

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

56

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

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 22

57

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

58

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

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

arsenic removal.

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

system for As(III) removal.

78

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

79

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

ACS Paragon Plus Environment

4

Page 5 of 22

Environmental Science & Technology

80

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

81

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

82

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

83

and sustainably remove As(III) from water.

84

■ MATERIALS AND METHODS

85

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

86

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

87

disclosed in the supporting information (SI).

88

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

89

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

90

analyses are included in the SI.

91

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

92

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

93

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

94

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

95

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

96

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

97

■ RESULTS AND DISCUSSION

98

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

99

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

100

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

101

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

ACS Paragon Plus Environment

5

Environmental Science & Technology

Page 6 of 22

102

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

103

different temperatures is also presented in Figure S2.

104

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

105

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

106

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

107

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

108

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

109

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

110

experiments and discussed in the subsequent sections.

111

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

112

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

113

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

114

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

115

membranes. (a)

(b)

116

ACS Paragon Plus Environment

6

Page 7 of 22

Environmental Science & Technology

(c)

(d)

117 118

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

119

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

120

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

121

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

122

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

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

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

131

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

132

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

133

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

134

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

ACS Paragon Plus Environment

7

Environmental Science & Technology

Page 8 of 22

135

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

136

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

137

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

138

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

139

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

140

temperatures.

141

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

142

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

143

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

144

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

145

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

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

ACS Paragon Plus Environment

8

Page 9 of 22

Environmental Science & Technology

158

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

159

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

160

polymeric network in solutions account for the negligible reverse fluxes.

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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

178

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

179

under neutral conditions.8,29

ACS Paragon Plus Environment

9

Environmental Science & Technology

(a)

Page 10 of 22

(b)

180 (c)

(d)

181 182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

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

190

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

191

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

ACS Paragon Plus Environment

10

Page 11 of 22

Environmental Science & Technology

192

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

193

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

194

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

195

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

196

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

197

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

198

majority of As(III) efficiently. (a)

(b)

199 200

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

201

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

202

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

203

analyses (n = 6).

204

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

205

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

206

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

207

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

208

and As(III) rejection.

ACS Paragon Plus Environment

11

Environmental Science & Technology

(a)

Page 12 of 22

(b)

209 210

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

211

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

212

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

213

duration: 30 min, room temperature.

214

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

215

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

216

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

217

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

218

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

219

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

220

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

221

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

222

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

223

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

224

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

225

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

ACS Paragon Plus Environment

12

Page 13 of 22

Environmental Science & Technology

226

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

227

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

228

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

229

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

230

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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

238

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

239

As(III) rejection in very alkaline conditions.

240

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

241

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

242

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

243

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

244

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

245

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

246

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

247

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

248

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

ACS Paragon Plus Environment

13

Environmental Science & Technology

Page 14 of 22

249

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

250

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

251

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

252

passage of the neutral substance.

253

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

254

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

255

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

256

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

257

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

258

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

259

feed solution due to the addition of anions.

260

(a)

(b)

261 262

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

263

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

264

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

265

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

ACS Paragon Plus Environment

14

Page 15 of 22

Environmental Science & Technology

266

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

267

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

268

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

271

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

272

duration: 30 min.

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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

282

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

283

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

ACS Paragon Plus Environment

15

Environmental Science & Technology

Page 16 of 22

284

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

285

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

286

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

287

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

288

respectively.

289

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

290

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

291

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

302

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

ACS Paragon Plus Environment

16

Page 17 of 22

Environmental Science & Technology

(a)

(b)

303 304

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

305

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

306

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

307

mode in the FO process, 60 °C.

308

■ AUTHOR INFORMATION

309

Corresponding Author

310

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

311

■ ACKNOWLEDGMENT

312

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

313

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

314

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

315

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

316

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

317

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

318

■ ASSOCIATED CONTENT

319

Supporting Information Available

ACS Paragon Plus Environment

17

Environmental Science & Technology

Page 18 of 22

320

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

321

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

322

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

323

and MD hollow fiber membranes.

324 325

■ REFERENCES

326

(1) Choong, T. S. Y.; Chuah, T. G.; Robiah, Y.; Koay, F. L. G.; Azni, I. Arsenic toxicity,

327

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

328

166.

329

(2) Mascher, R.; Lippmann, B.; Holzinger, S.; Bergmann, H. Arsenate toxicity: effects on

330

oxidative stress response molecules and enzymes in red clover plants. Plant Sci. 2002, 163, 961–

331

969.

332

(3) World Health Organisation (WHO), Guidelines for Drinking Water Quality, 1993, p. 41.

333

(4) Tanaka, M.; Takahashi, Y.; Yamaguchi, N.; Kim, K.-W.; Zheng, G.; Sakamitsu, M. The

334

difference of diffusion coefficients in water for arsenic compounds at various pH and its

335

dominant factors implied by molecular simulations. Geochim. Cosmochim. Acta 2013, 105, 360–

336

371.

337

(5) Uddin, M. T.; Mozumder, M. S. I.; Figobli, A.; Islam, M. A.; Drioli, E. Arsenic removal by

338

conventional and membrane technology: An overview. Indian J. Chem. Technol. 2007, 14, 441–

339

450.

ACS Paragon Plus Environment

18

Page 19 of 22

Environmental Science & Technology

340

(6) Brandhube, P.; Amy, G. Arsenic removal by a charged ultrafiltration membrane influences

341

of membrane operating conditions and water quality on arsenic rejection. Desalination 2001,

342

140, 1−14.

343 344

(7) Sato, Y.; Kang, M.; Kamei, T.; Magara, Y. Performance of nanofiltration for arsenic removal. Water Res. 2002, 36, 3371–3377.

345

(8) Zhu, W. P.; Gao, J.; Sun, S. P.; Zhang, S.; Chung, T. S. Poly(amidoamine) dendrimer

346

(PAMAM) grafted on thin film composite (TFC) nanofiltration (NF) hollow fiber membranes for

347

heavy metal removal. J. Membr. Sci. 2015, 487, 117–126.

348 349 350 351

(9) Xu, P.; Capito, M.; Cath, T. Y. Selective removal of arsenic and monovalent ions from brackish water reverse osmosis concentrate. J. Hazard.Mater. 2013, 260, 885– 891. (10) Qin, J. J.; Liberman, B.; Kekre, K. A. Direct osmosis for reverse osmosis fouling control: principles, applications and recent developments. Open Chem. Eng. J. 2009, 3, 8–16.

352

(11) Jin, X.; She, Q.; Ang, X.; Tang, C. Y. Removal of boron and arsenic by forward osmosis

353

membrane: influence of membrane orientation and organic fouling. J. Membr. Sci. 2012, 389,

354

182–187.

355

(12) Mondal, P.; Tran, A. T. K.; der Bruggen, B. V. Removal of As (V) from simulated

356

groundwater using forward osmosis: Effect of competing and coexisting solutes. Desalination

357

2014, 348, 33–38.

358 359

(13) Cui, Y.; Ge, Q.; Liu, X. Y.; Chung, T. S. Novel forward osmosis process to effectively remove heavy metal ions. J. Membr. Sci. 2014, 467, 188–194.

ACS Paragon Plus Environment

19

Environmental Science & Technology

Page 20 of 22

360

(14) Chung, T. S.; Li, X.; Ong, R. C.; Ge, Q.; Wang, H. L.; Han, G. Emerging forward osmosis

361

(FO) technologies and challenges ahead for clean water and clean energy applications. Curr.

362

Opin. Chem. Eng. 2012, 1, 246–257.

363 364

(15) Cath, T. Y.; Childress, A. E.; Elimelech, M. Forward osmosis: principles, applications, and recent developments. J. Membr. Sci. 2006, 281, 70–87.

365

(16) Li, Z.Y.; Yangali-Quintanilla, V.; Valladares-Linares, R.; Li, Q.; Zhan, T.; Amy, G. Flux

366

patterns and membrane fouling propensity during desalination of seawater by forward osmosis.

367

Water Res. 2012, 46, 195–204.

368 369 370 371 372 373

(17) Mi, B. X.; Elimelech, M. Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents. J. Membr. Sci. 2010, 348, 337–345. (18) Ge, Q.; Fu, F. J.; Chung, T. S. Ferric and cobaltous hydroacid complexes for forward osmosis (FO) processes. Water Res. 2014, 58, 230–238. (19) Ge, Q.; Chung, T. S. Oxalic acid complexes: promising draw solutes for forward osmosis (FO) in protein enrichment. Chem. Commun. 2015, 51, 4854–4857.

374

(20) Su, J.; Ong, R. C.; Wang, P.; Chung, T. S. Advanced FO membranes from newly

375

synthesized CAP polymer for wastewater reclamation through an integrated FO-MD hybrid

376

system. AIChE J. 2013, 59, 1245–1254.

377

(21) Ge, Q.; Wang, P.; Wan, C.; Chung, T. S.; Amy, G. Polyelectrolyte-promoted forward

378

osmosis-membrane distillation (FO-MD) hybrid process for dye wastewater treatment. Environ.

379

Sci. Technol. 2012, 46, 6236–6243.

380

ACS Paragon Plus Environment

20

Page 21 of 22

381 382

Environmental Science & Technology

(22) der Bruggen, B. V.; Luis, P. Forward osmosis: understanding the hype. Rev. Chem. Eng. 2015, 31, 1–12.

383

(23) Sukitpaneenit, P.; Chung, T. S. High performance thin-film composite forward osmosis

384

hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water

385

production. Environ. Sci. Technol. 2012, 46, 7358−7365.

386 387

(24) Ge, Q.; Su, J.; Amy, G. L.; Chung, T. S. Exploration of polyelectrolytes as draw solutes in forward osmosis processes. Water Res. 2012, 46, 1318−1326.

388

(25) http://en.wikipedia.org/wiki/Osmotic_pressure.

389

(26) McCutcheon, J. R.; Elimelech, M. Influence of concentrative and dilutive internal

390

concentration polarization on flux behavior in forward osmosis. J. Membr. Sci. 2006, 284, 237–

391

247.

392 393

(27) Zhao, S.; Zou, L.; Tang, C. Y.; Mulcahy, D. Recent developments in forward osmosis: opportunities and challenges. J. Membr. Sci. 2012, 396, 1–21.

394

(28) Yang, Q.; Wang, K. Y.; Chung, T. S. Dual-layer hollow fibers with enhanced flux as

395

novel forward osmosis membranes for water reclamation. Environ. Sci. Technol. 2009, 43, 2800–

396

2805.

397

(29) Mondal, P.; Hermans, N.; Tran, A. T. K.; Zhang, Y.; Fang, Y.; Wang, X.; der Bruggen, B.

398

V. Effect of physico-chemical parameters on inorganic arsenic removal from aqueous solution

399

using a forward osmosis membrane. J. Environ. Chem. Eng. 2014, 2, 1309–1316.

400 401

(30) Ng, K. S.; Ujang, Z.; Le-Clech, P. Arsenic Removal Technologies for Drinking Water Treatment. Rev. Environ. Sci. Biotechnol. 2004, 3, 43−53.

ACS Paragon Plus Environment

21

Environmental Science & Technology

402 403

Page 22 of 22

(31) Baig, S. A.; Sheng, T.; Hu, Y.; Xu, J.; Xu, X. Arsenic removal from natural water using low cost granulated adsorbents: a review. CLEAN–Soil, Air, Water 2015, 43, 13−26.

404

(32) Bonyadi, S.; Chung, T. S.; Rajagopalan, R. A novel approach to fabricate macrovoid-free

405

and highly permeable PVDF hollow fiber membranes for membrane distillation. AIChE J. 2009,

406

55, 828–833.

407

(33) Lee, S. H.; Kim, K. W.; Choi, H.; Takahashi, Y. Simultaneous photooxidation and

408

sorptive removal of As(III) by TiO2 supported layered double hydroxide. J. Environ. Manage.

409

2015, 161, 228−236.

ACS Paragon Plus Environment

22