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Removal of Sulfonated Humic Acid through A Hybrid Electrocoagulation-Ultrafiltration Process Nana Han, Guohe Huang, Chunjiang An, Shan Zhao, Yao Yao, Haiyan Fu, and Wei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00949 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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Industrial & Engineering Chemistry Research

246x136mm (150 x 150 DPI)

ACS Paragon Plus Environment


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1

Removal of Sulfonated Humic Acid through A Hybrid

2

Electrocoagulation-Ultrafiltration Process

3 4

Nana Han1, Guohe Huang1,2,*, Chunjiang An1,2, Shan Zhao2, Yao Yao2,

5

Haiyan Fu3, and Wei Li1

6 7

1

8

Sino-Canada Resources and Environmental Research Academy, North China Electric

9

Power University, Beijing 102206, China

MOE Key Laboratory of Regional Energy and Environmental Systems Optimization,

10

2

11

Regina S4S 0A2, Canada

12

3

13

Xiamen 361024, China

Institute for Energy, Environment and Sustainable Communities, University of Regina,

College of Environmental Science and Engineering, Xiamen University of Technology,

14 15

KEYWORDS: Sulfonated humic acid; Electrocoagulation; Ultrafiltration; Process

16

optimization


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17


Abstract

18 19

This study investigated the removal of sulfonated humic acid (SHA) from water through

20

a hybrid electrocoagulation-ultrafiltration treatment process. The effects of major

21

operating parameters including electrocoagulation time, current density and initial pH on

22

the electrocoagulation performance were evaluated. The increase in current density and

23

operating time as well as decrease of pH improved the SHA removal efficiency. The

24

operating conditions of electrocoagulation process were optimized through Box-Behnken

25

design to maximize SHA removal. The optimum conditions for electrocoagulation

26

included time of 7 min, current density of 10 mA/cm2 and pH of 5. Effective SHA

27

removal was furhter achieved in the hybrid electrocoagulation-ultrafiltration treatment

28

process. The performances of three molecular weight cut-off membranes were examined.

29

The results showed that the SHA removal efficiency increased with the increasing initial

30

concentration of SHA, and decreased with the increasing transmembrane pressure. The

31

SHA removal efficiency was more than 95% by 5 kD-membrane. The SHA removal

32

efficiency by different membranes from high to low in turn was: 5 kDa > 8 kDa > 10 kDa.

33

The results will have significant implications for the treatment of complex drilling and

34

hydraulic fracturing wastewater through electrocoagulation-ultrafiltration process.



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1. INTRODUCTION

36 37

Drilling and hydraulic fracturing of wells have been used in many industrial activities

38

such as oil production and mineral exploration. A large amount of water is required in

39

these production processes and the general water cycle may include water acquisition,

40

chemical mixing, well injection, flowback and wastewater disposal. In 2012, an estimated

41

280 billion gallons of wastewater was generated in the activities needed to bring a shale

42

gas or oil well into production in the United States.1 Recently, there has been an

43

increasing concern for the environmental impacts of drilling and hydraulic fracturing at

44

each stage of the water cycle.2 On the one hand, the huge volumes of water utilized in

45

drilling and hydraulic fracturing can greatly intensify the pressure on water supplies. This

46

results in increasing competition for scarce water resources among industrial, agricultural

47

and municipal sectors.3,4 It is necessary to reduce the demand for water by appropriate

48

re-using of flowback wastewater. On the other hand, a wide range of chemicals are used

49

in drilling and hydraulic fracturing. For example, sulfonated humic acid (SHA) has been

50

applied as an important additive to reduce viscosity, gel strength and filtrate loss during

51

hydraulic fracturing process. Such additives can present a significant risk when they enter

52

environment through leaks and spills, well blowouts and improper disposal of wastewater.

53

Consequently, effective technologies are needed to treat drilling wastewater and

54

minimize their impacts on human health and the environment.

55 56

Despite the prevalence of literature describing the treatment of various industrial

57

wastewater, few efforts have been made to investigate the disposal of drilling and


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hydraulic fracturing wastewater. Wang et al.5 evaluated the possibility of improving the

59

biodegradability of drilling wastewater using ozone following coagulation pretreatment.

60

It was found that biological treatment following short-term ozonation was efficient in the

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removal of total organic canbon. Hickenbottom et al.2 applied forward osmosis for

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treatment and reclamation of water from drilling wastewater. That process was able to

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recover more than 80% of the water from the drilling waste. Further study is necessary to

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develop appropriate regulation and wastewater treatment technology, as well as define the

65

role of different factors along with their interactive characteristics.

66 67

Among various physical-chemical techniques, electrocoagulation has been studied for

68

treatment of wastewater from oil, leather and dye industry.6-10 Oncel et al.11 conducted a

69

quantitative comparison between chemical precipitation and electrocoagulation for

70

removal of heavy metals. The results showed that the electrocoagulation process was

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more effective than the chemical precipitation with respect to the removal efficiency,

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amount of sludge generated and operating cost. Alinsafiet et al.12 investigated the effects

73

of key operational parameters on the performance of electrocoagulation for COD and

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color removals and found 30% and 90% of COD and color could be removed from water.

75

In these applications, iron and aluminium were commonly used as electrodes.

76

Electrocoagulation showed advantages over chemical coagulation in terms of wide

77

pollutant applicability, minute chemical usage, and less secondary pollution.13 It is

78

regarded as an effective, low-cost and eco-friendly alternative for the removal of various

79

recalcitrant contaminants from wastewater.14

80



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81

Recently, electrocoagulation has also been applied with other treatment technologies in

82

the removal of contaminants from wastewater. Ouaissa et al. reported15 the removal of

83

hexavalent chromium from synthetic effluents through electrocoagulation with aluminum

84

electrodes coupled with a sorption process using red onion skin adsorbent. Nguyen et al.16

85

evaluated a hybrid treatment system combining bioreactor and electrocoagulation to treat

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organic and nutrient pollutants from municipal wastewater. Daghrir et al.17 investigated

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the use of electrocoagulation and electro-oxidation process for the treatment of restaurant

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wastewater containing oil, grease and suspensions solids. There is also an increasing

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interest in the combined using of electrocoagulation and membrane filtration treatment.

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Membrane filtration can be well applied with electrocoagulation due to its high selectivity,

91

high-surface area, and potential to control the contact and mixing of two phases.18 Moshe

92

Ben-Sasson al.19,20 reported that the contaminant removal efficiency observed in

93

combined electrocoagulation-membrane process was higher than that in individual

94

electrocoagulation or membrane process. It was also observed that electrocoagulation

95

treatment followed by nanofiltration processes were effective in the treatment of textile

96

wastewater effluent.21 In addition, electrocoagulation can act as a suitable pretreatment

97

approach prior to membrane filtration to decrease improve feed water quality and reduce

98

membrane fouling.22,23 Although the combined electrocoagulation and membrane

99

filtration technologies have been reported previously, investigations about the

100

electrocoagulation-ultrafiltration approach in the treatment of drilling and hydraulic

101

fracturing wastewater are still limited. A well understanding of various factors involved in

102

this process is challenging in many respects.

103


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Drilling and hydraulic fracturing wastewater is a complex mixture of various organic and

105

inorganic substances. The present study will focus on the removal of SHA, which is a

106

representative pollutant of drilling and hydraulic fracturing wastewater. The performance

107

of a hybrid electrocoagulation-ultrafiltration treatment process will be evaluated. The

108

effects of key operating parameters including electrolysis time, current density, and

109

solution pH on the removal of SHA will be investigated to explore the optimum

110

conditions

111

electrocoagulation and ultrafiltration process will be also examined. The results of this

112

study can provide theoretical basis and synthetic applications for technologies used to

113

remove pollutants in drilling and hydraulic fracturing wastewater.

for

electrocoagulation

process.

The

combined

treatment

through

114 115

2. EXPERIMENTAL SECTION

116 117

2.1. Chemicals

118 119

SHA for this experiment was obtained from Renqiu Chemical Reagent Company, China.

120

The properties of SHA are listed in Table 1. All other chemicals used were of reagent

121

grade quality or higher.

122 123

--------------------------

124

Place Table 1 here

125

--------------------------

126



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127

2.2. Electrocoagulation-Ultrafiltration System

128 129

A bench-scale electrocoagulation-ultrafiltration system was applied in this study. The

130

corresponding schematic diagram is illustrated in Figure 1. The electrocoagulation unit

131

included two anodes made of iron and two cathodes made of graphite with a total surface

132

area of 353.6 cm2 (Figure S1). The iron anodes were made from plates with dimensions

133

of 15 cm × 12 cm × 0.3 cm and the graphite cathodes were 15 cm × 12 cm × 1 cm. The

134

electrode pads were firmly assembled parallel to each other and the interelectrode

135

distance of each electrode pair is 1.5 cm. The electrodes were physically connected to

136

either the positive or the negative outlet of the electric control module. Polyvinylidene

137

fluoride (PVDF) flat sheet ultrafiltration membranes were purchased from Xiamen

138

Starmem Membrane Technology Co. Ltd, China (Figure S2). Three molecular weight

139

cut-off membranes (MWCO) (5, 8 and 10 kDa) were used and the surface area of each

140

membrane is 176.0 cm2.

141 142

---------------------------

143

Place Figure 1 here

144

---------------------------

145 146

2.3. Treatment Experiments

147 148

The SHA wastewater was prepared by dissolving the required amounts of SHA in

149

deionized water. The treated water flowed through outlet and then was recycled to the


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150

feed tank for retreatment. The sedimentation flocs were removed through a drainage

151

valve at the bottom of the electrocoagulation reaction tank. At the end of the

152

electrocoagulation experiment, the clean and treated water was collected at outlet for

153

testing. The current density was maintained constant by the electric control module. After

154

electrocoagulation, the pretreated effluent flowed through a bag filter and then settled for

155

30 min in the feed water tank. Filtration experiments were performed without

156

recirculating the permeate in the feed tank. Before each experiment, the anode was

157

soaked in 5% HCl for 30 min to clean the passivation layer and then rinsed with distilled

158

water. The membrane system was washed twice with distilled water before each

159

experiment. After each round of experiment, 0.4% NaOH was used for membrane

160

cleaning and 0.1% NaHSO3 was used for membrane storage.24,25 Prior to the test, pH of

161

SHA solution was adjusted with appropriate HCl or NaOH, respectively. All experiments

162

were performed at a room temperature of 25 °C.

163 164

Various optimization approaches have been applied in environmental studies.26,27 The

165

Box-Behnken design (BBD) was used to explore the optimal parameters of

166

electrocoagulation process in the present study. The number of experiments (N) required

167

for the development of BBD is defined as N=2k(k−1)+C (where k is the number of

168

factors and C is the number of central points). Figure S3 illustrates a Box-Behnken design

169

for three factors. Each of the experimental points is taken at the midpoint of the cube

170

edges. The BBD model consists of 12 factorial design runs and 3 replicates at the central

171

point, for a total of 15 experiments.28 The polynomial equation generated by this

172

experimental design is shown as follows:



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173

Y = b0 + b1 X 1 + b2 X 2 + b3 X 3 + b12 X 1 X 2 + b13 X 1 X 3 + b23 X 2 X 3 + b11 X 12 + b22 X 22 + b33 X 32 (1)

174

where Y is a measured response associated with each factor level combination; X1, X2 and

175

X3 are independent variables; b0 is model constant; b1, b2 and b3 are linear coefficients;

176

b12, b13 and b23 are cross product coefficients and b11, b22 and b33 are the quadratic

177

coefficients.

178 179

2.4. Analytical Methods

180 181

Concentrations of SHA were determined by using TOC analyzer (TOC-VCPH, Shimadzu,

182

Japan) and UV-VIS spectrophotometer (Cary 50, Varian, USA) at 294 nm. Both of these

183

two methods have been widely used in the analysis of organic matter in solution.29 The

184

results of two methods have good consistency in this study. The removal efficiency of

185

SHA was calculated by the following equation:

186

Removal efficiency (%) =

187

where C0 is the SHA concentration in solution before treatment and Ca is the SHA

188

concentration in solution after treatment.

C0 − C a ×100 C0

(2)

189 190

Membrane flux was calculated using the following equation:

191

J=

192

where J is membrane flux (L m-2 h-1), A is the area of membrane (m2), V is the filtrate

193

volume (L) and t is time (h). The transmembrane pressure (TMP) is calculated using the

194

following equation:

1 dV A dt

(3)


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Pin + Pout + Pp 2

195

TMP =

196

where Pin is the feed pressure (bar), Pout is the retentate pressure (bar) and Pp is the

197

permeate pressure (bar). The experimental design and statistical analyses were conducted

198

using Design-Expert 9 (Stat-Ease, USA).

(4)

199 200

3. RESULTS AND DISCUSSION

201 202

3.1. Electrocoagulation Treatment for the Removal of SHA

203 204

Electrocoagulation is a complex process involving a multitude of mechanisms that

205

contribute to the synergistic removal of pollutants from wastewater.30 For iron electrode

206

used in this study, the electrolytic dissolution of the iron anode produced Fen+, which

207

could be further transformed into Fe(OH)2 and Fe(OH)3 as effective flocculants. Two

208

mechanisms as follows have been proposed for the reactions occurring in the electrode

209

compartment.31,32

210 211

Mechanism 1:

212

Anode:

213

4Fe → 4Fe2+ + 8e−

(5)

214

4Fe2+ + 10H2O + O2 → 4Fe(OH)3 + 8H+

(6)

215

Cathode:

216

8H+ + 8e− → 4H2

217

Overall reaction:

(7)



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218

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4Fe + 10H2O + O2 → 4Fe(OH)3 + 4H2

(8)

219 220

Mechanism 2:

221

Anode:

222

Fe → Fe2+ + 2e−

223

Fe2+ + 2OH− → Fe(OH)2

224

Cathode:

225

2H2O + 2e− → H2 + 2OH−

226

Overall reaction:

227

Fe + 2H2O → Fe(OH)2 + H2

(9) (10)

(11)

(12)

228 229

The contaminant removal efficiency can often be influenced by different factors.33,34

230

Some important factors such as the characteristics of electrolytic flow and solution

231

chemistry can play an important role during electrocoagulation.35 It is therefore essential

232

to elucidate the influence of such factors on the removal efficiency of SHA and to

233

optimize the electrocoagulation process. In the present study, this was accomplished by

234

investigating

235

electrocoagulation system while maintaining other parameters fixed. Parameters

236

investigated in this study include electrolysis time, current density and initial pH in

237

aqueous solution. These factors often play an important role in the electrocoagulation

238

process.14

the

effects

of

single

parameter

on

239 240

3.1.1. Effect of Electrocoagulation Time on SHA Removal


treatment

performance

of

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241 242

In the electrocoagulation process, the insoluble metal hydroxide of iron can be produced

243

in the aqueous phase as a suspension. It will facilitate the treatment of wastewater

244

through precipitation and adsorption of SHA. Current density and electrocoagulation time

245

have been recognized as two most important parameters for controlling the reaction

246

rate.36 To better understand the influencing factors for electrocoagulation process, the

247

effect of electrocoagulation time on SHA removal was studied and the results are shown

248

in Figure 2. When a current density (8.5 mA/cm2) and an initial SHA concentration of

249

120 mg/L were applied, the SHA removal efficiency increased dramatically as time

250

passed. Within first 6 min, the SHA removal efficiency showed a rapid increase from

251

12.12% at 2 min to 79.04% at 6 min. There was a slow increase of SHA removal

252

efficiency after 6 min.

253 254

---------------------------

255

Place Figure 2 here

256

---------------------------

257 258

For a particular electrical current flow in an electrolytic cell, the amount of iron generated

259

can be calculated using Faraday’s Law:30

260

m=

261

where m is the mass in grams of Fe generated at a specific current (I, amperes) over a

262

time interval (t, seconds), Z is the number of electrons transferred per Fe atom, MW is the

263

molecular weight of Fe (55.85 g/mol), and F is Faraday’s constant (96486 C/eq). It can be

I × t × MW ZF

(13)



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264

seen that the amount of Fe generated in solution is proportional to reaction time and

265

current. At the beginning of electrocoagulation, the metal ions and gas were not fully

266

produced and diffused. There was a small quantity of flocculants and thus SHA removal

267

efficiency was not significantly improved. This is similar with previous study on the

268

removal of Reactive Blue 140 and Disperse Red 1 through electrocoagulation.37 As time

269

went by, a larger quantity of OH- and Fe3+ were produced. There were enhanced

270

flocculation and diffusion, which could facilitate the removal of SHA. In the late stage of

271

electrocoagulation reaction, the major part of SHA has been removed and the

272

concentration was low. Moreover, the metal passivation on electrodes would also has

273

negative influence on the iron anodic dissolution.38 Therefore, the removal efficiency was

274

shown to approach a limit at the end of reaction. Taking into account of both power

275

consumption and removal efficiency, 6 to 8 min could be considered as an appropriate

276

reaction time range for this electrocoagulation system.

277 278

3.1.2. Effect of Current Density on SHA Removal

279 280

Current density is the only operational parameter that can be controlled directly. Figure 3

281

illustrates the results of SHA removal efficiency at different current densities in a range

282

from 4 to 10 mA/cm2, with a reaction time of 6 min and an initial SHA concentration of

283

120 mg/L. The SHA removal efficiency increased dramatically from 37.96% to 75.02%

284

when the current density varied from 4 to 7 mA/cm2. With the further change of current

285

density from 7 to 10 mA/cm2, the removal efficiency showed a relatively slow increasing

286

from 75.02% to 84.32%. The supply of current to electrocoagulation system can


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287

determine the amount of metal ions released from electrode to solution. Anode current

288

density can reflect the anodic oxidation reaction rate. When the other parameters are fixed,

289

a higher anodic current density is corresponding with a faster electrochemical reaction. At

290

a higher level of current density, electrolysis can produce more Fe2+ or Fe3+ on anode and

291

then a greater amount of iron hydroxide can be generated. It will faciliate the formation

292

of flocs and improve SHA removal efficiency. However, a high current density can be

293

associated with a reduced utilization efficiency of electrical energy because current can

294

be partially over-consumed in heating up solution.39 Electrode passivation might also

295

come up with the increased current density. In the present study, the appropriate current

296

density range was from 7 to 10 mA/cm2.

297 298

---------------------------

299

Place Figure 3 here

300

---------------------------

301 302

3.1.3. Effect of Initial pH on SHA Removal

303 304

Aqueous characteristic can also play an important role in electrocoagulation process.

305

Among different parameters for aqueous solution, pH value has been identified as a key

306

factor affecting the performance of electrochemical process.40,41 In order to investigate

307

the effect of solution pH on SHA removal, a series of experiments were performed by

308

adjusting the initial solution pH within a range of 3 to 11 and the results are shown in

309

Figure 4. The current density and initial SHA concentration were fixed at 8.5 mA/cm2



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310

and 120 mg/L, respectively. At low pH level ranging from 3 to 7, SHA removal efficiency

311

could change from 83.92% to 79.04%. In comparison with the results observed at low pH

312

level, SHA removal efficiency decreased when the pH varied from 7 to 11. At pH 11, the

313

SHA removal efficiency reached 71.29%, which was the lowest one in testing range. In a

314

previous study about the electrocoagulation of COD, oil and grease, the pH effect was not

315

very significant in the range 3-10.42 Song et al.43 observed that the efficiency for the

316

decolorization of C.I. Reactive Blue 19 increased with the increase of pH from 2 to 10.

317

However, the results in our study indicated that lower pH level led to better SHA removal

318

efficiency. Solution pH is correlated with the characteristic of SHA molecules and metal

319

hydroxides, and thus it may have an impact on the mechanism of SHA removal. SHA

320

molecule is a reticular macromolecule polymer including many active carboxyl and

321

phenolic hydroxyl groups. At low pH level, carboxyl and hydroxyl radicals of SHA exist

322

in the chemical form of -COOH and -OH, respectively; at high pH level, they exist in the

323

form of -COO¯ and -O¯. Under alkaline conditions, SHA can show a negative charge and

324

more Fe2+ is consumed to neutralize the negative charge. Therefore, the treatment

325

efficiency would decrease under such scenario due to the competitive loss of Fe2+ and

326

produced flocculant during electrocoagulation. At higher pH level, the surfaces of

327

hydroxides precipitates generated in electrocoagulation are negatively charged and would

328

tend to repulse the anionic SHA in solution.

329 330

---------------------------

331

Place Figure 4 here

332

---------------------------


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333 334

3.2. Box-Behnken Response Surface Optimization of the Electrocoagulation Process

335 336

3.2.1. Model Development

337 338

It has been demonstrated that SHA removal efficiency during electrocoagulation can be

339

influenced by some important factors including electrolysis time, current density, and

340

solution pH. To better reveal the correlated influence of different factors, the low,

341

medium and high levels of each independent factor were selected based on the results

342

from the single-factor experiments. A 3-factor, 3-level and 17-run BBD approach was

343

applied to derive a quadratic polynomial equation which can predict the optimal

344

combination of influencing factors. The selected range of each variable, coded as -1, 0,

345

and +1, is given in Table 2. The dependent variable is the SHA removal efficiecny (Y).

346

The response values as well as the codified and actual values of three important factors

347

under different experimental conditions are also shown.

348 349

-------------------------

350

Place Table 2 here

351

-------------------------

352 353

3.2.2. Statistical Analysis

354



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355

The quadratic model was employed to investigate the responses of SHA removal. The

356

final empirical regression model in terms of coded factors for SHA removal was

357

described as follows:

358 359

Y = 79.33 + 28.87X1 + 6.04X2 – 3.92X3 – 2.19X1X2 + 2.46X1X3 + 1.22X2X3 – 26.75X12

360

+ 0.58X22 + 1.87X32

(14)

361 362

As can be seen in Figure 5, the experimental results for SHA removal were in good

363

agreement with those predicted by the proposed model. The adjusted determination

364

coefficient (R2) values for the model was 0.9945. It indicated the high reliability for the

365

developed regression model in explaining experimental data. The analysis of variance

366

(ANOVA) was further applied to evaluate the significance and adequacy of the model

367

and identify the complex relationship between variables and responses.44 The calculated

368

statistical results are summarized in Table 3. The F-value of 320.59 and a low P-value

369

lower than 0.0001 indicate that the model is significant. The calculated lack of fit value,

370

along with corresponding F-value and P value for responses indicated the lack of fit of

371

model is not significant. Equation 14 can reasonably reflect the relationship between

372

various factors (X) and SHA removal efficiency (Y), as well as analyze and predict the

373

removal of SHA.

374 375

---------------------------

376

Place Figure 5 here

377

---------------------------


Page 19 of 43

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378 379

-------------------------

380

Place Table 3 here

381

-------------------------

382 383

Significance for effect of each factor (X) on response (Y) can be reflected by F-value. The

384

results implied that the impact of various factors on the response of SHA removal follows

385

a sequence of electrolysis time (X1) > current density (X2) > pH (X3). P values for X1, X2,

386

X3, and X12 are all less than 0.05, which were considered to be statistically significant.

387

Electrocoagulation time, current density, solution pH and quadratic electrocoagulation

388

time have significant effect on SHA removal efficiency. Three-dimensional (3D)

389

response surface plots of the predictive quadratic model for the SHA removal are shown

390

in Figures 6. For the removal of SHA, electrocoagulation time is found to be the most

391

pronounced factor compared with other factors, current density and solution pH. Both of

392

Figures 6(a) and (b) demonstrate that longer electrocoagulation time is favorable for the

393

enhancement of SHA removal efficiency. According to Figure 6(c), interaction between

394

current density and solution pH has no significant influence on the SHA removal

395

efficiency.

396 397

---------------------------

398

Place Figure 6 here

399

---------------------------

400



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

401

3.2.3. Optimal Conditions

402 403

To identify the optimal conditions for maximizing SHA removal, the desirable point

404

prediction function in the Design-Expert software was applied. The developed model is

405

adequate for the prediction of SHA removal using electrocoagulation. The predicted

406

optimal results include electrocoagultion time of 6.9 min, current density of 10 mA/cm2

407

and solution pH of 5. The electrocoagulation process could be effective for the removal

408

of SHA. For practical operating process, the adjusted optimum conditions are as follows:

409

7 min, 10 mA/cm2 and pH of 5.

410 411

3.3. Performance of the Electrocoagulation-Ultrafiltration Process

412 413

The above results suggest that electrocoagulation can be used to decrease the SHA

414

concentration in water. A sequent ultrafiltration process will be necessary for the

415

complete removal of SHA contaminants. In this study, the performance of a hybrid

416

electrocoagulation-ultrafiltration process was further investigated under the optimal

417

conditions of electrocoagulation process. The effects of initial SHA concentration in

418

feeding water, membrane pore size and transmembrane pressure on the removal of SHA

419

and membrane permeate flux were studied.

420 421

3.3.1. Effect of Initial Feed Concentration and Membrane Pore Size

422 423

The results of SHA removal efficiency using different initial feed concentrations (80, 120


Page 20 of 43

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424

and 160 mg/L) and membrane pore sizes (5, 8 and 10 kDa) are shown in Figure 7. It can

425

be seen that high removal efficiency (>90%) during the electrocoagulation-ultrafiltration

426

treatment of SHA can be achieved at different initial feed concentrations. For each

427

membrane, the SHA removal efficiency slightly increased with the increasing in initial

428

SHA concentration. When the initial feed concentration changed from 80 to 160 mg/L,

429

the SHA removal efficiency could vary from 95.5% to 97.8%, from 92.2% to 96.6%, and

430

from 91.3% to 96.0% by using 5, 8 and 10 kDa membranes, respectively. The increasing

431

of SHA concentration in feed water could lead to low retentate flow rate. When

432

transmembrane pressure (TMP) was 2 bar, the retentate flow rate with 5 kDa membrane

433

was as low as 17.7 L/h at a feed SHA concentration of 160 mg/L, compared to 21 L/h at a

434

feed concentration of 80 mg/L. When different membrane pore sizes were employed,

435

SHA removal efficiency decreased in a sequence of 5 kDa >8 kDa >10 kDa. The highest

436

removal efficiency which was greater than 95% was observed when 5 kDa membrane

437

was used.

438 439

Permeate flux and retentate flow can be used to reflect the fouling characteristics during

440

membrane filtration.45 According to the results of permeate flux in this study, it was

441

found that all three membranes (5, 8 and 10 kDa) experienced some degree of fouling.

442

For 10 kDa membrane, an approximate 50% decrease in retentate flow from 143 to 70

443

L/h was observed after 6 runs. The filtration with 8 and 10 kDa membranes experienced

444

rapid fouling during operation and it was necessary to recover both permeate and

445

retentate flux during operation. However, the membrane performance could not be

446

completely recovered even after membrane cleaning with special detergent, indicating



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

447

that permanent fouling occurred during the ultrafiltration of SHA. The molecule size of

448

SHA are similar with that of pores in these membranes, especially in the case of using 5

449

kDa membrane. Membrane fouling can occur in two ways: adsorption of foulant

450

(irreversible, cannot be removed by physical cleaning) and cake formation (generally

451

reversible by water washing or back flush).46,47 The excess flux would foul the surface

452

and form cake layers. Such fouling could lead to a low retentate flow rate which was

453

below the critical value required for forced membrane cleaning. When the retentate flow

454

rate was maximized to make SHA molecules deposited in the pore, it was found retentate

455

flow rate with 10 kDa membrane decreased quickly. The results showed that this

456

membrane pore size was not suitable for stable SHA removal through ultrafiltration. The

457

5 kDa membrane therefore can be used as membrane with low fouling potential.

458 459

---------------------------

460

Place Figure 7 here

461

---------------------------

462 463

3.3.2. Effect of Transmembrane Pressure

464 465

Transmembrane pressure has been regarded as an important operating parameter of

466

ultrafiltration system.48 The effects of TMP on SHA removal efficiency for the three

467

membranes (5, 8 and 10 kDa) are shown in Figure 8. The initial SHA concentration of

468

160 mg/L was used in these tests. When transmembrane pressure changed from

469

approximately 4.5 to 1 bar, the SHA removal efficiency varied from 96.5% to 98.1%,


Page 22 of 43

Page 23 of 43

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470

from 95.7% to 96.8%, and from 94.4% to 96.3% for the 5, 8, and 10 kDa membranes,

471

respectively. It can be seen that the SHA removal efficiency decreased with the increase

472

in transmembrane pressure. Low transmembrane pressure implies low driving force

473

across membrane surface, which can reduce the risk of SHA sorption and fouling at

474

membrane surface. At the same transmembrane pressure, the SHA removal efficiency for

475

10 kDa membrane was less than those for membranes with smaller pore size.

476 477

---------------------------

478

Place Figure 8 here

479

---------------------------

480 481

4. CONCLUSIONS

482 483

The present study investigated the removal of SHA from water through hybrid

484

electrocoagulation-ultrafiltration treatment process. Electrocoagulation time, current

485

density and solution pH could influence the performance of electrocoagulation process. A

486

BBD approach was applied to develop a quadratic model which can predict the optimal

487

combination of these influencing factors. The optimum conditions for electrocoagulation

488

include the electrocoagulation time of 7 min, current density of 10 mA/cm2 and pH of 5,

489

while taking into account both theoretical and practical considerations. During the

490

electrocoagulation-ultrafiltration treatment process, enhancement of initial SHA

491

concentration in feed water resulted in an increase in the SHA removal. Compared with

492

the 8 and 10 kDa membranes, the 5 kDa membrane provided the highest removal



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

493

efficiency with minimum fouling. The SHA removal efficiency decreased with increasing

494

transmembrane pressure. The hybrid electrocoagulation-ultrafiltration process can be a

495

feasible alternative for the treatment of drilling and hydraulic fracturing wastewater

496

containing SHA. The overall efficiency of such treatment process can be influenced by

497

various factors. The results of this study have important implications for investigating the

498

interactive parameters and optimal conditions in hybrid electrocoagulation-ultrafiltration

499

process. This process has the potential to be used for in situ treatment of contaminated

500

water. The influencing parameters gained from batch tests can be useful for the parameter

501

determination and experimental design of future pilot system. Further studies are desired

502

to obtain more theoretical foundation for reaction mechanisms related to a variety of

503

surface and internal phenomena in hybrid electrocoagulation-ultrafiltration process.

504

Different electrode types and optimal design for treatment system will be investigated for

505

scale-up application.

506 507 508

AUTHOR INFORMATION

509 510

CORRESPONDING AUTHOR

511

E-mail: [email protected]

512 513

ACKNOWLEDGMENTS

514 515

This research was supported by the Natural Science Foundation (51309096), the Program


Page 24 of 43

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516

for Innovative Research Team in University (IRT1127), the 111 Project (B14008), the

517

Natural Science and Engineering Research Council of Canada and Petroleum Technology

518

Research Centre of Canada. The authors are also grateful to the editors and the

519

anonymous reviewers for their insightful comments and suggestions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

520

References

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electrocoagulation and electro-oxidation processes for the treatment of restaurant

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wastewaters. J. Environ. Eng. 2012, 138, 1146-1156.

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mining impacted water by an electrocoagulation-ultrafi ltration hybrid process. Desalin.

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electrocoagulation with ultrafiltration membrane processes. Desalination 2012, 300,

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hybrid electrocoagulation/nanofiltration process. J. Hazard Mater. 2009, 168, 868-874.

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thin film nanocomposite reverse osmosis membranes by incorporating functionalized

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G. C.; da Silva, E. G. P.; Portugal, L. A.; dos Reis, P. S.; Souza, A. S.; dos Santos, W. N.

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Chim. Acta 2007, 597, 179-186.

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Performance and Cost. Technomic Publishing Co.Inc., Pennsylvania, USA, 2001.

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removals from produced water by electrocoagulation pretreatment prior to Reverse



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electrocoagulation using iron and aluminum electrodes. J. Hazard. Mater. 2003, 100,

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163-178.

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Electrocoagulation of blue reactive, red disperse and mixed dyes, and application in

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table and Figure Caption List

Table 1 Properties and major constituents of SHA Table 2 Experimental design matrix and dependent variables attributed to the factors of Box-Behnken design

Table 3 Analysis of variance (ANOVA) for response surface quadratic models on SHA removal

Figure 1. Schematic diagram of the hybrid electrocoagulation-ultrafiltration treatment system.

Figure 2. Effect of electrocoagulation time on SHA removal. Figure 3. Effect of current density on SHA removal. Figure 4. Effect of initial pH on SHA removal. Figure 5 Predicted and experimental values of SHA removal efficiency. Figure 6. 3D response surface plots for combined effects on SHA removal: (a) electrocoagulation time and current density, pH=7; (b) electrocoagultion time and pH, current density=8.5 mA/cm2; (c) current density and pH, electrocoagultion time=6min.

Figure 7. Removal of SHA through ultrafiltration using different initial SHA feed concentrations and membrane pore sizes.

Figure 8. Effect of TMP on SHA removal.


Page 32 of 43

Page 33 of 43

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Table 1 Properties and major constituents of SHA Properties

Descriptions

Physical form

Powder

Appearance

Black

Density (g/cm3)

1.14

Organic carbon (%)

43.94

Total sulfur (%)

0.45

Hydrogen (%)

1.69

Nitrogen (%)

0.92



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

Table 2 Experimental design matrix and dependent variables attributed to the factors of Box-Behnken design

Run no.

Reaction time (min)

Current density

X1

X2

2

(mA/cm )

pH

SHA removal efficiency (%)

X3

Y

Coded

Actual

Coded

Actual

Coded

Actual

Observed Predicted

1

1

8

-1

7

0

7

78.31

78.19

2

-1

4

0

8.5

-1

5

33.5

31.96

3

1

8

0

8.5

1

9

80.32

81.86

4

-1

4

1

10

0

7

32.41

32.54

5

-1

4

-1

7

0

7

15.63

16.06

6

0

6

0

8.5

0

7

77.56

79.33

7

-1

4

0

8.5

1

9

18.23

19.21

8

1

8

1

10

0

7

86.32

85.89

9

0

6

0

8.5

0

7

81.74

79.33

10

0

6

0

8.5

0

7

78.15

79.33

11

0

6

0

8.5

0

7

80.16

79.33

12

0

6

-1

7

-1

5

79.77

80.88

13

0

6

1

10

-1

5

89.12

90.53

14

1

8

0

8.5

-1

5

85.77

84.79

15

0

6

0

8.5

0

7

79.04

79.33

16

0

6

-1

7

1

9

72.02

70.61

17

0

6

1

10

1

9

86.23

85.12


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Analysis of variance (ANOVA) for response surface quadratic models on SHA removal P-value

Source

Sum of squares

Degree of Mean freedom square

F-value

Model

10147.28

9

1127.48

320.59

< 0.0001

X1

6667.24

1

6667.24

1895.80

< 0.0001

X2

292.22

1

292.22

83.09

< 0.0001

X3

122.93

1

122.93

34.95

0.0006

X1 X2

19.23

1

19.23

5.47

0.0520

X1 X3

24.11

1

24.11

6.86

0.0345

X2 X3

5.90

1

5.90

1.68

0.2361

X1 2

3012.05

1

3012.05

856.46

< 0.0001

X2 2

1.43

1

1.43

0.41

0.5433

X3 2

14.74

1

14.74

4.19

0.0798

Residual

24.62

7

3.52

Lack of fit

13.51

3

4.50

1.62

0.3182

Pure error

11.11

4

2.78

Cor total

10171.90

16

R2 = 0.9976

R2 (adj) = 0.9945

Pred R2 = 0.9770


(Prob>F)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DC power supply Pump

+

Flow meter

-

Prefiltration

Ultrafiltration unit Electrocoagulation unit Electrodes

Feed tank

Figure 1. Schematic diagram of the hybrid electrocoagulation-ultrafiltration treatment system.


Page 37 of 43

90 80

SHA Removal Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 1

2

3

4

5

6

7

8

9

10

11

12

Time (min)

Figure 2. Effect of electrocoagulation time on SHA removal.


13


90

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70

60

50

40

30 3

4

5

6

7

8

9

10

2

Currency Density (mA/cm )

Figure 3. Effect of current density on SHA removal.


11

Page 39 of 43

100 95


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 85 80 75 70 65 60 2

3

4

5

6

7

8

9

10

pH

Figure 4. Effect of initial pH on SHA removal.


11

12


Predicted vs. Actual 100

Predicted Removal Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20

0

0

20

40

60

80

100

Actual Removal Efficiency (%) Figure 5. Predicted and experimental values of SHA removal efficiency.


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Figure 6. 3D response surface plots for combined effects on SHA removal: (a) electrocoagulation time and current density, pH=7; (b) electrocoagultion time and pH, current density=8.5 mA/cm2; (c) current density and pH, electrocoagultion time=6min.



5 KDa

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8 KDa

10 KDa

95

90

85

80

75

80

120 160 Initial SHA Concentration (mg/L)

Figure 7. Removal of SHA through ultrafiltration using different initial SHA feed concentrations and membrane pore sizes


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Page 43 of 43

100 99


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


98 97 96 95 94 93 92

5 kDa

91

8 kDa

10 kDa

90 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

TMP(bar)

Figure 8. Effect of TMP on SHA removal


4.5

5.0

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