Filtration and Electrochemical Disinfection ... - ACS Publications

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Filtration and electrochemical disinfection performance of PAN/PANI/AgNWs-CC composite nanofiber membrane Junjie Wen, Xiaojun Tan, Yongyou Hu, Qian Guo, and Xuesen Hong Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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 26

Environmental Science & Technology

1

Filtration and electrochemical disinfection performance of PAN/PANI/AgNWs-CC composite

2

nanofiber membrane

3

Junjie Wenb, c, Xiaojun Tanb, c, Yongyou Hua,b, * , Qian Guoa , Xuesen Hongb, c

4 5

a

6

Guangzhou 510006, PR China

7

b

8

University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China

9

c

School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre,

The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China

School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, 510640, PR China

10 11

*Corresponding author: Yongyou Hu

12

Tel: +86 13602746125; Fax: +86-20-39380508; Email: [email protected]

13

Address: School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China

14 15

Other authors

16

First author：Junjie Wen

17

Second author：Xiaojun Tan

(Tel: +86 15914336545; Email: [email protected]) (Tel: +86 18613170205; Email: [email protected])

ACS Paragon Plus Environment


18 19

Page 2 of 26

Abstract The removal and inactivation of waterborne pathogens from drinking water are important for

20

human

health.

Here,

a

polyacrylonitrile/polyaniline/silver

21

(PAN/PANI/AgNWs-CC) composite nanofiber membrane was fabricated using a simple and rapid

22

co-electrospinning process, and an electrical device was applied with the PAN/PANI/AgNWs-CC

23

filter for water electrochemical disinfection. The effects of voltage, flow rate and microbial

24

concentration on the filtration and electrochemical disinfection performance of the nanocomposite

25

membrane were investigated. The characterization results show that PAN/PANI/AgNWs with

26

uniform diameters and without beads were successfully fabricated on CC. AgNWs were uniformly

27

distributed in the PAN/PANI/AgNWs. The PAN/PANI/AgNWs-CC filter was an effective sieve for

28

completely removing both Escherichia coli and Staphylococcus aureus in the absence of applied

29

voltage, and the sieved bacteria were completely inactivated by the released silver within 8 h. Over

30

99.999% inactivation of the sieved bacteria was achieved within a few seconds by concurrent

31

filtration and electrochemical disinfection under a voltage of 3 V. This high performance is enabled

32

by means of an electrical mechanism, and an extremely high electric field induces sharp AgNWs tips

33

to generate electroporated pores in the bacteria. The electrochemical PAN/PANI/AgNWs-CC

34

membrane is an excellent material with potential application value in point-of-use drinking water

35

treatment.

36


nanowires-carbon

fiber

cloth

Page 3 of 26

37


TOC/Abstract art

38 39



40

Page 4 of 26

Introduction

41

The diseases and death caused by waterborne pathogens in developing countries are receiving

42

increasing attention as primary public health concerns 1. According to the 2014 World Health

43

Organization report, more than 700 million people, particularly in developing and rural areas, lack

44

adequate safe drinking water, which results in 2 million deaths from diarrhea annually

45

insufficient funds, a centralized water supply is limited in dispersed rural areas. Additionally, owing

46

to the generation of toxic disinfection by-products (DBPs), ineffectiveness against resistant

47

microorganisms and difficulty in managing equipment, conventional disinfection methods are not

48

suitable for developing and rural areas that lack water utilities. Developing innovative point-of-use

49

(POU) methods for the removal and inactivation of waterborne pathogens is urgently needed.

2, 3

. Due to

50

Although membrane filtration is a preferred disinfection method for POU 4, pathogens can only be

51

removed but not inactivated by traditional membrane filtration, and the consequent membrane

52

fouling problem and potential environmental risks cannot be ignored. Developing a new type of

53

membrane that simultaneously removes and inactivates water pathogens is thus important in

54

membrane application.

55

Electrospinning has emerged as a powerful technique for fabricating polymer nanofiber membrane

56

because of its adaptability and utility 5. Electrospun polymer fibers possess characteristics such as a

57

high specific surface area, lightweight and flexibility that make them applicable to such fields as

58

substrates or scaffolds 5, 6. Because the electrospun nanofibrous membrane porosity is adjustable 7, it

59

can overcome the low-flux and high-fouling performance of porous polymeric membranes that are

60

manufactured by conventional methods. Moreover, by incorporating superior electrical conducting

61

materials into the conducting polymer, electrospun conducting fibers can possess better electrical


Page 5 of 26

62 63


properties and can be used extensively in electronic devices 8. Silver nanomaterial is known for its high electrical conductivity, highest antibacterial activity, 9, 10

64

biocompatibility and infrequent release of harmful DBPs into water

. Although it has

65

disadvantages, such as difficulty in separation, recovery and reuse, immobilizing nanosilver on

66

electrospun nanofibers offers a new way to achieve the separation and reuse of nanosilver both

67

simply and effectively.

68

Through the inactivation mechanisms of electroporation and reactive oxygen species (ROS),

69

electrochemical disinfection is effective for both viral and bacterial inactivation in water in a short

70

time at relatively low voltage and without DBPs 11-15. Electrochemical devices can be designed to be

71

portable and compact 13, 16, 17. Therefore, it is expected that electrochemical disinfection devices with

72

polymer/nanosilver electrospun nanofiber membranes would be effective and promising for POU

73

water disinfection.

74

In this study, membrane filtration and an electrochemical disinfection process were combined.

75

PAN nanofibers were fabricated by electrospinning using N,N-Dimethylformamide (DMF) as a

76

solvent. The obtained PAN nanofibers, with their large specific surface areas and good solvent

77

resistance properties, can act as an effective substrate. But the poor electrical conductivity of PAN

78

nanofiber is a setback in our electrochemical system, which can be enhanced by doping conducting

79

materials. Given PANI’s special conduction mechanism, unique electrical property, relative

80

environmental stability and controllable electrochemical property and processability

81

PANI into PAN nanofibers to fabricate PAN/PANI composite nanofibers with a higher electrical

82

conductivity. Silver nanowires (AgNWs) were added to the electrospun solution to compose

83

PAN/PANI/AgNWs, thus ensuring the antibacterial and conductive performance of the composite


18

, we doped


84

nanofibers. Allowing for the not high stiffness and mechanical strength of the nanofiber membrane,

85

carbon fiber cloth (CC) was chosen as the collected electrospinning nanofiber substrate, which would

86

improve its strength and thus expand the application of nanofiber membrane. Importantly, the porous

87

and conductive carbon fiber cloth can be well run in our filtration/electrochemical disinfection

88

system. Then, the antimicrobial and conductive PAN/PANI/AgNWs-CC nanocomposite membrane

89

was fabricated through a simple electrospinning process. Field emission scanning electron

90

microscopy (FESEM), x-ray diffraction (XRD), transmission electron microscope (TEM), energy

91

dispersive x-ray (EDS) and graphite furnace atomic absorption spectrometry (GFAAS) were used to

92

analyze the physicochemical properties of the synthesized nanocomposites. Then, an electrochemical

93

filter device with a PAN/PANI/AgNWs-CC nanocomposite membrane was structured for the

94

simultaneous removal and inactivation of both Gram-negative Escherichia coli (E. coli) and

95

Gram-positive Staphylococcus aureus (S. aureus). The effects of microbial concentration, voltage

96

and flow rate were investigated to evaluate the membrane filtration and electrochemical disinfection

97

performance. The filtration and electrochemical mechanism was investigated by measuring the silver

98

concentration of the effluent and observing SEM images of bacteria after electrolysis at different

99

voltages. This work offers insights for new silver nanocomposites synthesis strategies for

100

electrochemical POU drinking water disinfection.

101

2. Materials and Methods

102

2.1 Materials and chemicals

103

PAN with an average Mw of 150,000 was obtained from Shanghai Macklin Biochemical Co. Ltd

104

(Shanghai, China). PANI (Mw ~ 20,000) was obtained from Sigma–Aldrich (USA). AgNWs with a

105

diameter of approximately 50 nm and length of approximately 20 µm were acquired from Changsha


Page 6 of 26

Page 7 of 26


106

Wei Xi New Materials & Technology Company (Changsha, China). CC with a thickness of 0.5 mm

107

was purchased from Tianxiang Textile Technology Company (Shanghai, China). All chemicals used

108

in the study were of analytical grade.

109

2.2 Preparation of electrospun solution

110

First, 1.8 g PAN was dissolved in 18.2 g DMF at 80 °C and magnetically stirred for 50 min to

111

obtain the PAN electrospun solution (9 wt %). Then, 0.4 g PANI was added to the PAN solution (9

112

wt %) and magnetically stirred at room temperature for 12 h to acquire the PAN/PANI electrospun

113

solution (PAN 9 wt %, PANI 2 wt %). To prepare the PAN/PANI/AgNWs electrospun solution, first,

114

the mixture of 1.8 g PAN powder and 12.85 g DMF was magnetically stirred for 50 min at 80 °C.

115

After PAN was dissolved completely in DMF, 5 ml AgNWs solution (dispersed in DMF, 20 mg/ml)

116

was added to the solution and magnetically stirred at room temperature (~ 28 °C) for 4 h. Ultimately,

117

0.4 g PANI was added to the solution and magnetically stirred at room temperature (~ 28 °C) for 12

118

h followed by probe sonication (JY92-IIDN, Scientz biotechnology Co. Ltd, Ningbo, China) for 1 h.

119

2.3 Electrospinning nanofibers

120

Electrospinning machines, DT-200 from Dalian Dingtong Science and Technology Development

121

Co. Ltd. (China), were utilized to prepare the nanofibers. The obtained PAN electrospun solution was

122

fed into a 10 mL 19 gage stainless steel needle syringe. The stainless steel needle was linked to the

123

positive electrode, which was applied at 21 kV. The solution flow rate in the syringe was maintained

124

at 0.3 mL/h by a syringe pump. The stainless needle and collection cylinder were separated by 16 cm.

125

Aluminum foil or a CC (prior to use, CC was immersed in acetone overnight to remove the residual

126

organics, rinsed with deionized (DI) water and dried) was taped on the collected cylinder. The PAN

127

nanofibers were collected on the collective cylinder to produce an electrospun PAN nanofiber film



128

on aluminum foil or an electrospun PAN-CC composite film. The preparation of PAN/PANI-CC and

129

PAN/PANI/AgNWs-CC resembled that of PAN-CC.

130

2.4 Characterization

131

The surface and cross section morphologies of the as-obtained composites were investigated using

132

a field emission scanning electron microscope (FESEM, Zeiss Merlin) and an accelerating voltage of

133

5 kV. Transmission electron microscope (TEM) observations were made using a Hitachi TEM

134

system at 80 kV. The structure phase was analyzed by XRD (Bruker D8 Advance). Energy dispersive

135

X-ray spectra (EDS) were obtained on a JEOL-2010 microscope operating at an accelerating voltage

136

of 200 kV.

137

2.5 Disinfection experiments

138 139 140

The disinfection performance of PAN/PANI/AgNWs-CC nanofiber membrane was evaluated using both Gram-negative E. coli (ATCC 25922) and Gram-positive S. aureus (ATCC 6538). For the filtration removal and inactivation tests, the same nanocomposite membrane filtration 19

141

device described in our previous study

was applied. First, 10 mL of bacteria suspensions at

142

different concentrations (105, 107 CFU/mL) was passed through PAN/PANI/AgNWs-CC (PAN-CC,

143

PAN/PANI-CC) membranes. Effluents were collected in autoclaved containers. Aliquots (100 µL) of

144

effluents were spread over an agar plate and incubated at 37 °C for 24 h. To assess the viability of the

145

bacteria adsorbed on nanocomposite membrane, the membrane was removed from the device and

146

transferred into an autoclaved glass vial containing 10 mL of PBS after filtration. The vial was bath

147

sonicated for 8 min to remove bacteria from the membrane. Then, 100 µL of suspension was added

148

to a standard agar culture medium and incubated at 37 °C for 24 h. After cultivation, the numbers of

149

colony cells were counted to identify the cell concentrations. Bacterial inactivation efficiency was


Page 8 of 26

Page 9 of 26

150


calculated using the following equation: Bacterial inactivation efficiency = Log10 (C0/C)

151

(1)

152

where C0 and C were the number of activated colony cells before and after each experiment,

153

respectively.

154

For static bactericidal tests, 1 x 2.5 cm nanofiber composite membranes were positioned in petri

155

dishes and sterilized on both sides by exposure to UV for 30 min. The solutions containing bacteria

156

(50 µL, 107 and 105 CFU/mL) were introduced onto the membranes. Samples with the bacterial

157

solutions were incubated at 37 °C for 8 h. To resolve the potential problem of water evaporation from

158

the bacterial solution, petri dishes were sealed with sealing rubber. At the same time, a beaker filled

159

with DI water was put at the bottom of the incubator. To dissociate bacteria that had adhered to

160

membrane surfaces, we positioned membranes in 10 mL centrifuge tubes and added 3 mL phosphate

161

buffer solution (PBS), followed by ultrasonication for 8 min. Finally, 100 µL of suspension was

162

added to a standard agar culture medium and incubated at 37 °C for 24 h.

163

For the electrochemical disinfection test, an electrochemical composite membrane filtration device 19

164

was used as described in our previous study

. NaCl, the background electrolytic throughout the

165

experiment, is a common electrolytic water treatment and is ubiquitous in aquatic systems. All

166

electrolysis experiments were completed at 10 mM NaCl. The electrochemistry was driven by a

167

PS-302DM DC power supply (Longwei Instruments Co. Ltd, HK). In all cases, electrolysis was

168

completed at constant voltages of 1.5, 3, 4.5 and 6 V. Here, 10 mL of bacteria suspension (107

169

CFU/mL) was pumped through composite film filters under different voltages (0, 1.5, 3, 4.5, 6 V) at

170

different flow rates (1, 2, 4 mL/min) using a peristaltic pump. In addition, to investigate the

171

reusability and stability of the composite nanofiber membrane, a 5-recycles and a 10 h continuous



172

operating electrochemical disinfection tests were carried out. In the 5-recycles test, 10 mL of bacteria

173

suspension (107 CFU/mL) was pumped through composite filters under different voltages (3, 6 V) at

174

the flow rate of 1 mL/min. And membrane was washed by DI water before each recycle. For the 10 h

175

continuous operating test, at a flow rate of 1 mL/min, bacteria suspension (107 CFU/mL) was

176

continuously pumped through composite filters under different voltages (3, 6 V) for 10 h. The

177

viability of bacteria adsorbed on nanocomposite membrane was assessed in the same manner as the

178

filtration test.

179

FESEM was performed to examine the effects of electrochemistry on E. coli cell morphology on

180

PAN/PANI/AgNWs-CC membrane. After the completion of electrolysis, membranes were fixed with

181

2.5% glutaraldehyde for 2 h and cleaned three times with 0.2 M phosphate buffer. Then, samples

182

were dehydrated in a series of ethanol solutions (30, 50, 70, 85, 95 v/v%) for 10 min sequentially,

183

with the final dehydration completed by immersion twice in pure ethanol for 20 min. Samples were

184

freeze dried and sputtered with gold for SEM imaging.

185

Three parallel experiments were run throughout the study, and every culture was prepared in

186

triplicate.

187

2.6 Released silver concentrations from nanocomposite membranes

188

To probe the inactivation mechanism and determine potential for composite nanofibers health risks,

189

the silver concentrations in the effluents were scaled. The total silver (ion and metal) concentrations

190

in effluents were analyzed by graphite furnace atomic absorption spectrometry (GFAAS,

191

PerkinElmer, PinAAcle 900T). The test procedures were described previously in detail 19.


Page 10 of 26

Page 11 of 26


192

3. Results and Discussion

193

3.1 Characterization of composite nanofibers

194

The morphology of composite nanofibers is shown in Fig. 1. As shown in Fig. 1 (a) and (d), PAN

195

nanofibers without beads were randomly deposited to form a nonwoven mat. Smooth surfaces and

196

round cross sections of PAN nanofibers were clearly observed. PAN nanofibers diameters were

197

uniform and ranged from 140 to 200 nm. Compared with PAN nanofibers, PAN/PANI nanofibers had

198

similar morphology, smooth surfaces and uniform diameters without beads (Fig. 1 (b) and (e)).

199

Furthermore, as can be seen from the FTIR spectra of PAN nanofiber, PANI powder and PAN/PANI

200

composite nanofiber (Fig.S2), PANI is present in the composite and there was no chemical reaction

201

between PANI and PAN. These results illustrate that high quantities of PAN nanofibers and

202

PAN/PANI nanofibers can be successfully manufactured by electrospinning.

203

Fig. 1 (c) and (f) show the PAN/PANI/AgNWs composite nanofiber morphology. The diameters of

204

PAN/PANI/AgNWs composite nanofibers were not as uniform as those of PAN nanofibers and

205

PAN/PANI nanofibers, which might have been caused by AgNWs traversing the composite

206

nanofibers (Fig. 2 (a)). However, there were no obvious beads on the PAN/PANI/AgNWs composite

207

nanofibers. These results indicate that a relatively high quantity of PAN/PANI/AgNWs composite

208

nanofibers can be fabricated by co-electrospinning.



209 210 211

Fig. 1. FESEM images of (a, d) PAN, (b, e) PAN/PANI, (c, f) PAN/PANI/AgNWs nanofibers at different magnifications.

212

TEM was used to identify the existence and distribution of AgNWs in the composite nanofibers.

213

Fig. 2 (a) and (b) show representative TEM images of PAN/PANI/AgNWs composite nanofibers,

214

which reveal the successful embedment of AgNWs with 50-nm-diameters in composite nanofibers.

215

Fig. 2 (b) shows that some of the incorporated AgNWs were dispersed inside the nanofibers and

216

completely aligned along the nanofibers’ axes without any aggregation, causing nanofiber electrical

217

conductivity improvement. From Fig. 2 (a), it can be confirmed that some of the incorporated

218

AgNWs crossed the fiber and were partly exposed to the external of the nanofibers instead of being

219

parallel to the fiber. The distribution of AgNWs concurrently enhanced the electrical conductivity

220

and intrinsic antibacterial performance of the composite nanofibers. Moreover, nanofibers could limit

221

the excess of released silver and provide composite nanofibers with persistent antimicrobial activity.

222

AgNWs embedded in nanofibers were further verified by EDS analysis, and the loading amount

223

was estimated as 5.22 wt.%, as shown in Fig. 2 (c). According to EDS mapping images (Fig. S4) of

224

the PAN/PANI/AgNWs, AgNWs were uniformly distributed in the composite nanofibers.


Page 12 of 26

Page 13 of 26


225 226

Fig. 2. (a) (b) TEM images of PAN/PANI/AgNWs at different magnifications. (c) EDS analysis of

227

PAN/PANI/AgNWs.

228

3.2 Disinfection efficacy of nanocomposite membranes

229

3.2.1 Filtration and static bactericidal performance of nanocomposite membrane

230

The bacterial filtration removal efficiencies of nanocomposite membranes are shown in Fig. 3. No

231

E. coli and S. aureus colonies formed on the re-incubated agar culture medium of effluents with

232

initial microbial concentrations of both 105 and 107 CFU/mL, which indicated that all bacteria were

233

removed by a sieving mechanism. This result is consistent with the results of 13.8 nm pore size

234

nanofibrous membrane filtration. In our previous study 19, AgNW-CC only removed 2.2 log E. coli at

235

most by filtration. The results illustrate that the electrospun nanofibrous membrane is of great

236

significance because it can act as an effective filter for the complete removal of E. coli and S. aureus.

237 238

Fig. 3. Removal efficiency of E. coli and S. aureus at different concentrations in filtration tests: (a) E. coli, (b)



S. aureus.

239 240

E. coli and S. aureus were completely removed from the influent by the sieving mechanism.

241

However, sieved bacteria can be released and cause membrane biofouling during continuous

242

filtration. Therefore, it is necessary to evaluate the extent of bacterial inactivation sieved on the

243

nanocomposite membrane. The inactivation efficiencies of sieved bacteria on different

244

nanocomposite membranes are presented in Fig. 4; those of E. coli and S. aureus on PAN-CC

245

membranes were measured as 1.9 log and 1.4 log, respectively. For the PAN/PANI-CC membrane,

246

inactivation efficiencies of E. coli and S. aureus were measured as 1.8 log and 1.6 log, respectively.

247

These phenomena were caused by nonculturable bacteria. The inactivation efficiencies of E. coli and

248

S. aureus for the PAN/PANI/AgNWs-CC membrane were slightly higher than those of the PAN-CC

249

and PAN/PANI-CC membranes, which suggests that the released silver from AgNWs can contribute

250

to inactivation efficiency.

251 252

Fig. 4 Inactivation efficiencies of sieved bacteria on different nanocomposite membranes in filtration tests (107

253

CFU/mL of bacterial concentration of influent)

254

The static bactericidal effects of the three types of nanocomposite membranes are shown in Fig. 5.

255

Fig. 5 provides typical photographs of E. coli and S. aureus colonies on three types of nanocomposite

256

membranes starting with different bacterial concentrations. For PAN-CC and PAN/PANI-CC,


Page 14 of 26

Page 15 of 26


257

continuous bacterial colonies were prevalent on the agar culture medium at both bacterial

258

concentrations, implying that E. coli and S. aureus could survive on the surface of PAN-CC and

259

PAN/PANI-CC. As shown in Fig. 5 left, no bacteria colonies were seen on agar medium at either

260

bacterial concentration, indicating that PAN/PANI/AgNWs-CC has significant antimicrobial activity

261

against E. coli and S. aureus. The results above indicate that PAN-CC and PAN/PANI-CC exhibited

262

no antimicrobial effect but that AgNWs played a key role in the antibacterial performance of the

263

composites. It can be concluded that the silver released from AgNWs is the primary cause of this

264

result. The silver released included Ag+ and AgNWs, which was generated from the oxidation of

265

AgNWs

266

oxidative damage, and DNA damage in pathogens

267

excellent intrinsic antibacterial activity. The results shown in Fig. 5 also indicate that sieved bacteria

268

can be completely inactivated within 8 h from the silver released.

20

and detachment from composite nanofibers. They can cause cell membrane damage, 21, 22

; thus, PAN/PANI/AgNWs-CC possesses

269



270 271

Fig. 5 Typical photographs of re-cultivated (a) E. coli colonies and (b) S. aureus colonies on agar culture

272

plates, with seeded bacterial concentrations on composite nanofiber membrane of 107 CFU/mL and 105

273

CFU/mL.

274

3.2.2 Electrochemical disinfection performance of nanocomposite membrane

275

Electrochemical bacterial inactivation efficiencies using different membranes under different

276

voltages and flow rates are presented in Fig. 6. The effects of voltage are shown in Fig. 6 (a) and (b).

277

E. coli and S. aureus inactivation efficiencies were both greatly enhanced with increasing voltage,

278

which indicated that a stronger electric field results in more efficient disinfection. In PAN-CC

279

experiments, the maximum inactivation efficiencies of E. coli and S. aureus were only 3.6 log and

280

3.3 log, respectively. For PAN/PANI-CC, which had higher electrical conductivity, the maximum

281

inactivation efficiencies of E. coli and S. aureus were improved to 4.3 log and 4.5 log, respectively.

282

According to the Nyquist plots of different nanofiber membranes (Fig. S5), compared with PAN-CC


Page 16 of 26

Page 17 of 26


283

membrane, PAN/PANI-CC membrane might exhibit a faster electron transfer rate and better

284

electrochemical performance under the relatively high applied voltages, resulting in higher

285

inactivation efficiencies of sieved bacteria of PAN/PANI-CC membrane. However, when the applied

286

voltage was below 3 V, the inactivation efficiencies of sieved bacteria of PAN/PANI-CC membrane

287

had no significant increase to PAN-CC membrane, for the improvement of conductivity of doping

288

PANI

289

PAN/PANI/AgNWs-CC

290

PAN/PANI/AgNWs-CC, at 6 V applied voltage, E. coli and S. aureus were both completely

291

inactivated (7 log), which illustrated that AgNWs on nanofibers had an important germicidal effect.

292

The nanocomposites without AgNWs exhibited lower disinfection performance.

on

the

composite

material

exhibited

might much

not better

tremendous

enough.

disinfection

In

comparison,

performance.

For

293

The disinfection performance of nanocomposite membranes was also investigated at various flow

294

rates, and the results are shown in Fig. 6 (c) and (d). With increasing flow rates, the E. coli and S.

295

aureus inactivation efficiencies both decreased, which indicates that longer contact time can enhance

296

disinfection efficiency.



297 298

Fig. 6. Inactivation efficiencies of sieved E. coli and S. aureus in electrochemical disinfection tests: (a), (c)

299

effect of voltage (1 mL/min flow rate ); (b), (d) effect of flow rate (6 V).

300

According to Liu’s report, silver nanowire-carbon nanotube coated polyurethane sponge only

301

inactivated 0.2 log of E. coli without an electric field but achieved over 6 log in an electric field 14.

302

AgNW/CNT cotton inactivated 77% of E. coli at + 20 V, whereas at 0 V, it inactivated approximately

303

20% 16. Our experimental results are consistent with the above studies. Fig. 6 (a) and (b) show that

304

increasing voltage played an effective role in enhancing the disinfection efficiency. Increasing

305

voltage can likewise improve energy consumption. Flow rate was also an important factor:

306

decreasing the flow rate provided better disinfection. However, the filtration efficiency and energy

307

consumption were also influenced by changing flow rate. Thus, comprehensive considerations must

308

be taken into account for both energy consumption and disinfection performance in practical


Page 18 of 26

Page 19 of 26

309


applications.

310

Increasing voltage and decreasing flow rate are conducive to enhancing the disinfection

311

performance of nanocomposites. However, considering efficiency and economy, a 3 V applied

312

voltage and 1 mL/min flow rate, which inactivated more than 99.999% bacteria under this condition,

313

is recommended.

314

Additionally, as shown in the left of Fig. 7, in the 5-recycles test, the inactivation efficiencies of

315

E.coli of each recycle were 5, 5.2, 4.7, 4.9 and 4.6 log under the applied voltage of 3 V, respectively.

316

As shown in the right of Fig. 7, when E. coli was electrolyzed at 3 V, the inactivation efficiencies of

317

E.coli was decreased to 4.1 log after 10 h continuous operation. Similarly, the electrochemical

318

inactivation efficiencies of E. coli were both slightly decreased after 5-recycles and 10 h continuous

319

operation at 6 V. These results revealed that PAN/PANI/AgNWs-CC has a relatively good

320

disinfection reusability and stability under a relatively low flow rate.

321

According to the changes of the enfluent flow rate in 10 h continuous operating test with different

322

E.coli influent concentration (Fig. S6), it could be inferred that PAN/PANI/AgNWs-CC has a

323

relatively good flux under a relatively low bacterial concentration influent, but its flux would be

324

obviously decreased when the bacterial concentration and flow rate of influent were high enough.



325 326

Fig. 7. Inactivation efficiencies of sieved E. coli in 5-recycles and 10 h continuous operating electrochemical

327

disinfection tests at 3, 6 V (1 mL/min flow rate ).

328

3.2.3 Electrochemical disinfection mechanism of nanocomposite membrane

329

With applied voltages of 0 to 6 V, the effluent silver concentrations were all below 15 ppb, which

330

is far below the national drinking water standard of 100 ppb. This result shows that silver release can

331

be controlled by embedding AgNWs on nanofibers and that the electrochemical filtration system is

332

safe for drinking water disinfection. Although the contact time between silver and bacteria was very

333

short, these results also indicate that the intrinsic antimicrobial activity of silver (metal 23, 24 and Ag+

334

ions 20, 25) could not account for the dramatic disinfection efficiency enhancement in an electric field.

335

The excellent disinfection capability against E. coli and S. aureus in this study revealed that the

336

electric field plays an important role in electrochemical disinfection. FESEM was performed to

337

evaluate the voltage effects on cell morphology of model bacteria E. coli on PAN/PANI/AgNWs-CC

338

membrane. Typical FESEM images of E. coli in contact with PAN/PANI/AgNWs-CC membranes

339

after electrolysis at 0, 3, and 6 V are presented in Fig. 8. FESEM images (Fig. 8 (a) − (d)) clearly


Page 20 of 26

Page 21 of 26


340

show morphological changes after electrolysis at different applied voltages. As shown in Fig. 8 (a),

341

when no voltage was applied, cells were still intact and plump. When E. coli was electrolyzed on

342

PAN/PANI/AgNWs-CC at 3 V, the cells were shriveled and dehydrated but maintained the

343

macroscopic cell membrane structures (Fig. 8 (b)). When the applied voltage was increased to 6 V,

344

small pores were observed on bacteria surfaces, and the cell membrane integrity was destroyed (Fig.

345

8 (c) and (d)), which resulted in the release of cellular contents and the ultimate death of the

346

microorganism.

347

In electrochemical disinfection, a high-intensity electric field of brief duration can generate 14, 16, 26

348

electroporated pores on the bacteria surfaces

. This was a pre-existing microbicidal

349

electroporation mechanism. Electroporation causes damage to bacteria membranes, including the

350

loss of semipermeability and occurrence of pores, which can lead to ion leakage and escape of

351

metabolites. Severe electroporation-generated pores are irreparable and give rise to cell death. Dye

352

staining experiments further confirmed that electroporation involves cellular membrane damage and

353

eventual cell death 14, 26.

354

In this study, as a result of the nanocomposites’ special one-dimensional nanostructure, an

355

extremely high electric field can be induced on the sharp nano-tips of AgNWs. Once the bacterial

356

cells were attached to AgNWs, which exposed the external nanofibers (as shown in Fig. 2 (a)), the

357

transmembrane potential induced by the high-intensity electric field would exceed the critical value.

358

This process would cause tiny pores on the cell membranes. Electroporation could occur repeatedly

359

and cause more pores on the membranes, leading to eventual cell death.



360 361

Fig. 8. FESEM images of E. coli on PAN/PANI/AgNWs-CC filter after electrolysis in 10 mM NaCl at applied

362

voltages of (a) 0 V, (b) 3 V, and (c), (d) 6 V.

363

3.3 Potential environmental applications of PAN/PANI/AgNWs-CC nanofiber membrane

364

filtration/electrochemical system

365

The novel electrochemical PAN/PANI/AgNWs-CC membrane presented here can be applied as a

366

drinking water purification technology for pathogen removal and inactivation. In this study, we

367

combined physical membrane filtration with electrochemical disinfection, which provides

368

advantages over using one process alone. Our electrochemical filtration system can be competitive

369

with other POU technologies based on the following advantages:

370

(1) Low cost of energetic requirements. For instance, at 3 V, the average current was 10 mA, and the

371

average power was 30 mW; thus, the electricity usage only requires 0.125 KW·h to treat 1 m3 of


Page 22 of 26

Page 23 of 26


372

water (4167 h at 4 mL/min).

373

(2) Efficient bacterial removal and inactivation. Without applied voltage, PAN/PANI/AgNWs-CC

374

membrane reduced the number of culturable bacteria in the effluent to 0, and the sieved bacteria

375

could be completely inactivated in 8 h upon silver release. At the applied voltage of 6 V, the system

376

could completely inactivate the sieved bacteria within a few seconds. According to Liu’s report,

377

silver nanowire-carbon nanotube coated polyurethane sponge inactivated over 6 log of E. coli with

378

applied voltage above 10 V 14. AgNW/CNT cotton inactivated 77% of E. coli at + 20 V and at a flow

379

rate of 1 L/h with several seconds 16. rGO-Ag-CF inactivated 100% E. coli at 1.5 V with 5 min 17.

380

Compared with these existed reports, the electrochemical disinfection performance (inactivated 7-log

381

at 6 V with several seconds) of our composite material can be very competitive.

382

(3) Short disinfection contact time. Our experiment results showed that a 7 log E. coli and S.aureus

383

inactivation were achieved within only few seconds.

384

(4) The silver released was less than 15 ppb, which is far below the national drinking water standard

385

of 100 ppb.

386

There have also reported some nanocomposite materials applied in the bacteria filtration and 16

387

inactivation, such as AgNW/CNT cotton

, silver nanowire-carbon nanotube coated polyurethane

388

sponge 14, multiwalled carbon nanotube microfilter 13, etc. However, as far as we know, there is no

389

report about the PAN/PANI/AgNWs-CC composite nanofiber filtration membrane so far, which has

390

physical interception, conductive and antimicrobial performance and can be prepared by a facile and

391

rapid co-electrospinning technology. Nowadays, with its rapid development, electrospinning

392

technology makes it possible to prepare larger membrane which has a greater scope of application

393

and a better prospect. Importantly,

compared with other silver nanocomposite materials, the



394

PAN/PANI/AgNWs-CC membrane in this work can restrict the excess of the released silver, thus

395

endowing the composite nanofibers with persistent antimicrobial activity. Hence our study gives a

396

new viewpoint of POU drinking water disinfection.

397

However, the expensive silver nanowire has limited the application of the membrane

398

filtration/electrochemical system. The synthesis of other nanomaterial (such as cheaper CuO wire)

399

may address this challenge. Moreover, the actual performance of PAN/PANI/AgNWs-CC in field

400

applications is unknown. It is likely that raw water quality could affect the performance and lifespan

401

of PAN/PANI/AgNWs-CC, such as ion species and dissolved oxygen. Additionally, the disinfection

402

of viruses and other microbial pathogens by this system should be investigated before putting into

403

practical application. In brief, although there are many human health problems that still need to be

404

solved urgently, the superb disinfection performance and the simple and rapid synthesis method

405

offered by PAN/PANI/AgNWs-CC make it a great potential option to provide safe drinking water in

406

less developed or rural areas.

407

Supporting Information

408

Detail information were presented in this part, including further characterization of the PAN/PANI

409

nanofiber (FESEM, FTIR spectra) and AgNWs (FESEM, TEM, XRD), EDS mapping images of

410

PAN/PANI/AgNWs, Nyquist plots of different nanofiber membranes and the tabulation of previous

411

reports about silver nanocomposite material for water disinfection.

412

Acknowledgments

413

The authors are grateful for the financial support provided by the National Natural Science Fund

414

of China (No. U1401235). The authors thank Professor Weijia Zhou, Jian Sun, Yaping Zhang,

415

Junfeng Chen, Zhuyu Niu, Cao Yang, Yuancai Lv for valuable comments on this manuscript.


Page 24 of 26

Page 25 of 26


416

Reference

417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458

1.

Elimelech, M., The global challenge for adequate and safe water. Journal of Water Supply Research and

Technology-Aqua 2006, 55, (1), 3-10. 2.

Wardlaw, T.; Salama, P.; Brocklehurst, C.; Chopra, M.; Mason, E., Diarrhoea: why children are still dying and what

can be done. Lancet 2010, 375, (9718), 870-872. 3.

WHO and UNICEF: Progress on drinking water and sanitation. Hydrologie Und Wasserbewirtschaftung 2014, 58, (4),

244-245. 4.

Albert, J.; Luoto, J.; Levine, D., End-User Preferences for and Performance of Competing POU Water Treatment

Technologies among the Rural Poor of Kenya. Environmental Science & Technology 2010, 44, (12), 4426-4432. 5.

Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q., One-dimensional

nanostructures: Synthesis, characterization, and applications. Advanced Materials 2003, 15, (5), 353-389. 6.

Agarwal, S.; Greiner, A.; Wendorff, J. H., Electrospinning of Manmade and Biopolymer Nanofibers-Progress in

Techniques, Materials, and Applications. Advanced Functional Materials 2009, 19, (18), 2863-2879. 7.

Fridrikh, S. V.; Yu, J. H.; Brenner, M. P.; Rutledge, G. C., Controlling the fiber diameter during electrospinning.

Physical Review Letters 2003, 90, (14): 144502. 8.

Liu, H. Q.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G., Polymeric nanowire chemical sensor. Nano Letters 2004,

4, (4), 671-675. 9.

Li, Q. L.; Mahendra, S.; Lyon, D. Y.; Brunet, L.; Liga, M. V.; Li, D.; Alvarez, P. J. J., Antimicrobial nanomaterials for

water disinfection and microbial control: Potential applications and implications. Water Research 2008, 42, (18), 4591-4602. 10. Rai, M.; Yadav, A.; Gade, A., Silver nanoparticles as a new generation of antimicrobials. Biotechnology Advances 2009, 27, (1), 76-83. 11. Drees, K. P.; Abbaszadegan, M.; Maier, R. M., Comparative electrochemical inactivation of bacteria and bacteriophage. Water Research 2003, 37, (10), 2291-2300. 12. Jeong, J.; Kim, J. Y.; Cho, M.; Choi, W.; Yoon, J., Inactivation of Escherichia coli in the electrochemical disinfection process using a Pt anode. Chemosphere 2007, 67, (4), 652-659. 13. Vecitis, C. D.; Schnoor, M. H.; Rahaman, M. S.; Schiffman, J. D.; Elimelech, M., Electrochemical Multiwalled Carbon Nanotube Filter for Viral and Bacterial Removal and Inactivation. Environmental Science & Technology 2011, 45, (8), 3672-3679. 14. Liu, C.; Xie, X.; Zhao, W. T.; Liu, N.; Maraccini, P. A.; Sassoubre, L. M.; Boehm, A. B.; Cui, Y., Conducting Nanosponge Electroporation for Affordable and High-Efficiency Disinfection of Bacteria and Viruses in Water. Nano Letters 2013, 13, (9), 4288-4293. 15. Radjenovic, J.; Farre, M. J.; Mu, Y.; Gernjak, W.; Keller, J., Reductive electrochemical remediation of emerging and regulated disinfection byproducts. Water Research 2012, 46, (6), 1705-1714. 16. Schoen, D. T.; Schoen, A. P.; Hu, L. B.; Kim, H. S.; Heilshorn, S. C.; Cui, Y., High Speed Water Sterilization Using One-Dimensional Nanostructures. Nano Letters 2010, 10, (9), 3628-3632. 17. Kumar, S.; Ghosh, S.; Munichandraiah, N.; Vasan, H. N., 1.5 V battery driven reduced graphene oxide-silver nanostructure coated carbon foam (rGO-Ag-CF) for the purification of drinking water. Nanotechnology 2013, 24, (23): 235101. 18. Martins, C. R.; de Freitas, P. S.; De Paoli, M. A., Physical and conductive properties of the blend of polyaniline/dodecylbenzenesulphonic acid with PSS. Polymer Bulletin 2003, 49, (5), 379-386. 19. Hong, X. S.; Wen, J. J.; Xiong, X. H.; Hu, Y. Y., Silver nanowire-carbon fiber cloth nanocomposites synthesized by UV curing adhesive for electrochemical point-of-use water disinfection. Chemosphere 2016, 154, 537-545.



459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474

20. Liu, J. Y.; Hurt, R. H., Ion Release Kinetics and Particle Persistence in Aqueous Nano-Silver Colloids. Environmental Science & Technology 2010, 44, (6), 2169-2175. 21. Chernousova, S.; Epple, M., Silver as Antibacterial Agent: Ion, Nanoparticle, and Metal. Angewandte Chemie-International Edition 2013, 52, (6), 1636-1653. 22. Suresh, A. K.; Pelletier, D. A.; Doktycz, M. J., Relating nanomaterial properties and microbial toxicity. Nanoscale 2013, 5, (2), 463-474. 23. Scanlan, L. D.; Reed, R. B.; Loguinov, A. V.; Antczak, P.; Tagmount, A.; Aloni, S.; Nowinski, D. T.; Luong, P.; Tran, C.; Karunaratne, N.; Pham, D.; Lin, X. X.; Falciani, F.; Higgins, C. P.; Ranville, J. F.; Vulpe, C. D.; Gilbert, B., Silver Nanowire Exposure Results in Internalization and Toxicity to Daphnia magna. Acs Nano 2013, 7, (12), 10681-10694. 24. Kwak, J. I.; An, Y. J., A review of the ecotoxicological effects of nanowires. International Journal of Environmental Science and Technology 2015, 12, (3), 1163-1172. 25. Newton, K. M.; Puppala, H. L.; Kitchens, C. L.; Colvin, V. L.; Klaine, S. J., Silver nanoparticle toxicity to Daphnia magna is a function of dissolved silver concentration. Environmental Toxicology and Chemistry 2013, 32, (10), 2356-2364. 26. Liu, C.; Xie, X.; Zhao, W. T.; Yao, J.; Kong, D. S.; Boehm, A. B.; Cui, Y., Static Electricity Powered Copper Oxide Nanowire Microbicidal Electroporation for Water Disinfection. Nano Letters 2014, 14, (10), 5603-5608.

475


Page 26 of 26

Filtration and Electrochemical Disinfection ... - ACS Publications

Recommend Documents