A review on nanofibers membrane with amino-based ionic liquid for heavy metal removal

Journal Pre-proof A review on nanofibers membrane with amino-based ionic liquid for heavy metal removal Choi Yee Foong, M.D.H. Wirzal, M.A. Bustam PII:

S0167-7322(19)33938-8

DOI:

https://doi.org/10.1016/j.molliq.2019.111793

Reference:

MOLLIQ 111793

To appear in:

Journal of Molecular Liquids

Received Date: 1 August 2019 Revised Date:

17 September 2019

Accepted Date: 21 September 2019

Please cite this article as: C.Y. Foong, M.D.H. Wirzal, M.A. Bustam, A review on nanofibers membrane with amino-based ionic liquid for heavy metal removal, Journal of Molecular Liquids (2019), doi: https:// doi.org/10.1016/j.molliq.2019.111793. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

1 2

A Review on Nanofibers Membrane with Amino-based Ionic Liquid for Heavy Metal Removal

3

Choi Yee Foong a, M.D.H. Wirzal a,b* and M. A. Bustama,b

4

a

5

Perak, Malaysia

6 7

Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar,

b

Center of Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS, 32610 Bandar

Seri Iskandar, Perak, Malaysia

8

ABSTRACT

9 10 11

Due to rapid growth of human population and urbanization, concerns over

12

water contamination become a critical issue for many regions especially in developing

13

countries, thus an efficient and cost effective method for water purification especially

14

heavy metal removal in urgent need. Toxic heavy metals hold severe threats to

15

human health and agriculture because they are non-degradable and be likely to amass

16

in the environment. Electrospun membrane fabricated through electrospinning

17

techniques consists of dual functionality; efficient microfiltration capability with

18

excellent surface functionality as a good design for separation membranes that can

19

effectively remove heavy metals in contaminated water sources. On the other hand,

20

Ionic liquids (IL) shown good performance as an extractant in the separation of heavy

21

metal ions. With advancement of technology, more simple and efficient membrane

22

system are required to ensure the efficiency and reliability of filtration system to

23

remove the heavy metal ions. In this research study reports an overview of

24

investigation of electrospun membrane and amino-based ionic liquids for heavy metal

25

removal from waste water applications.

26 27

Keywords: Electrospinning, Nanofibers, Membrane, Amino-based Ionic Liquids,

28

Heavy Metal Removal

29 30

*Correspondence authors Email addresses: [email protected] (M.D.H. Wirzal)

i

31

1.0

Introduction

32 33

Water pollution is a serious concern in the twenty-first century caused by

34

population explosion, industrialization and urbanization and World Health

35

Organization (WHO) reported that by year 2050, there are about one billion people

36

lack access to safe drinking water and water shortage may affect up to four billion

37

people [1, 2]. Accessing to fresh water supply are depleting gradually nowadays,

38

microbial and hazardous chemical pollutants need to be removed in order to make

39

water drinkable, thus research are more focusing on the development of a more

40

efficient and cost effective method for water purification.

41

On the other hand, heavy metal pollution is an international issue growing

42

with the development of modern industry as toxic heavy metal wastes are discharge

43

directly or indirectly to the environment. This toxic waste such as chromium (Cr),

44

mercury (Hg), arsenic (As) and copper (Cu) have bad effect to human health as well

45

as agriculture as the non-degradability of it tends to amass in the environment. In

46

order to reduce this growing problem, cost-effective and efficient technologies to

47

pre-concentrate and remove the heavy metals are investigated [3]. Conventional

48

methods for the heavy metal ions removal such as ion exchange, chemical

49

oxidation/reduction, reverse osmosis, ultrafiltration and others are effective but few

50

factors that limit their usage including harsh operation conditions, less efficiency and

51

possibility of secondary contaminants [4].

52

Biosorption methods using agricultural wastes and industrial byproducts for

53

the heavy metal removal is relatively efficient and cost saving method, however,

54

high chemical oxygen demand, biological oxygen demand and high total organic

55

carbon due to soluble organic compound released from the reaction used up the

56

oxygen in the water thus intimidate the aquatic life [5]. In addition, nanosized metal

57

oxides was being introduced due to its high capacity and selectivity towards the

58

heavy metals, unfortunately, it tends to aggregate in nature thus reduce and somehow

59

loss its high capability and selectivity.

60

Electrospinning is a smart technology in fabricating electrospun to form

61

nanofibers and nanostructured materials with large surface area- to- volume ratio

62

(10-40m2/g) and high porosity (>80%) from various polymer. Apart from that, a

2

63

variety of surface groups on the electrospun allow them to further functionalization

64

and functional nanoparticle incorporation for efficient heavy metal removal [6].

65

Adsorption and size exclusion are the two ways of using polymeric

66

electrospun membrane in water filtration, different type of impurities can be

67

removed from the effluent depends on the membrane pore sizes. Pore size of the

68

microfiltration (MF) membrane range of 0.1 - 5µm could be used to reject particles

69

such as protozoa and bacteria. Viruses, emulsified oils, colloids and protein can be

70

removed by ultrafiltration (UF) membrane with pore size range 0.01 – 0.1µm.

71

Particles size 0.001 – 0.01µm could be removed by nanofiltration (NF) membrane

72

and reverse osmosis (RO) membrane is being used to reject particles size from 0.001

73

– 0.0001µm [7].

74

Volatile organic solvents are selected to prepare the polymer solution and it

75

will evaporate during the jet’s course allowing the electrospun nanofibers formation,

76

this would cause the high level of harmful volatile compounds released to the

77

environment as well as the solvents are unable for effective recovery and recycle

78

used [8]. Ionic liquids (IL) could be used to replace the volatile solvents since these

79

non-volatile solvents will remain in the fibers as they can be removed by an array of

80

strategy which allow them to be reused and recovery effectively [9]. The IL removal

81

can be done easily through chemical separation by using other organic solvents or

82

water extraction or distilled off the desired products. The main advantage of using IL

83

for electrospinning techniques is nanoscale and microscopic fibrous material with

84

high surface area can be obtained. It also can be an invaluable tool for the dissolution

85

of biopolymers which insoluble is most organic solvents.

86

Furthermore, the nature of functionality on the nanofiber surface is also the

87

determinant of the membrane capability of toxic metal removal. As the heavy metal

88

ions removal using electrospun membrane is basically based on the interactions

89

between the functional sites on the nanofiber surface and the heavy metal ions, the

90

interaction can be physical (affinity or electrostatic interactions) or chemical

91

(chelating or coordination complex formation). Therefore, by incorporating suitable

92

surface functional group to the nanofibers membrane would increase the efficiency

93

of the heavy metal removal too [10].

94

Moreover, in order to enhance the safety and environmental friendliness of

95

the conventional extraction separation technique which using organic phase and

96

aqueous phase for the metal ions removal, IL can be used as ideal substitutes due to 3

97

their stability, non-volatility and adjustable miscibility and polarity [11]. The metal

98

ion association and extraction process strongly depends on the alkyl chain length and

99

hence determine the hydrophobic and hydrophilic properties of IL [12] while

100

hydrophobic IL are preferred to perform the extraction process efficiently [13].

101

However, the IL’s poor biodegradability or the toxicity of ILs’ degradation

102

products which limits their used in various applications, thus bio-based IL is being

103

explore as the green character of the IL can be improved by synthesis it using

104

renewable compounds [14-16].

105

In the present study, a suitable bio-based ionic liquid impregnated to the solid

106

support which is polymer electrospun membrane through immersion to form a

107

hybrid membrane for heavy metal ion removal purpose. Many scientific articles have

108

been published adequately documenting both electrospun and bio-based ionic liquids

109

are beneficial for heavy metal removal application. The heavy metal ion extraction

110

using bio-based IL under aqueous phase required less harsh thermal and chemical

111

condition as its functional groups could be used to chelate and extract metal ion

112

compared to the classic reaction media [17]. Moreover, the high specific surface area

113

(10-1000m2g-1), high porosities (up to 80%), and easily tuneable surface

114

functionality of the electrospun membrane as the solid support IL is a promising

115

material for the adsorption of organic impurities and toxic metal ions from aqueous

116

solution [18].

117

In the review article, properties of ionic liquids, application of bio-based

118

ionic liquids, electrospinning techniques as well as the use of electrospun fibrous

119

membrane for heavy metal removal have been critically summarized.

120 121

2.0

Ionic Liquids

122 123

Ionic Liquids (ILs) is a liquid encompassed exclusively of ions. In addition,

124

by using the boiling point of water as reference point, ILs exist in the liquid state that

125

below 100°C which called ‘room-temperature ionic liquid’.

126

The unique properties of the ILs enable it to be used in various application.

127

The ILs’ particular properties and attribute to specific application are summarize

128

next section.

129 4

130 131 132

2.1

Properties of Ionic Liquids

133 134

The ILs’ properties are difficult to be report general tendencies, previously

135

researchers concluded a general set of claims for ILs that is non-volatile, non-

136

flammable, intrinsically ‘green”, highly electrochemical stable, highly thermally

137

sable as well as comprising simple ionic species [19]. However, due to intense

138

investigation have been done on the physical properties of IL and more type of IL

139

are being explored, the results show that none of these generic properties is

140

ubiquitous and these all are depending on the quantification of ILs’ impurities which

141

would affect its thermos-physical properties. Furthermore, the discrepancies of the

142

ILs’ physic-chemical properties occurred may due to the different experimental

143

techniques used and thus results on the data uncertainty estimation [20].

144

2.1.1 Viscosity

145 146

ILs’ viscosity is relatively high when compared to other conventional

147

solvents, variety of ILs reported that, their viscosity is in the range of 66 to 1100cP

148

at 20-25°C. The strong intermolecular interaction is the results of the van der Waals

149

forces, hydrogen bonding as well as Coulombic forces. However, this would affect

150

the active transport properties such as diffusion in the engineering point of view and

151

become an issue in practical catalytic application [21]. The viscosity can be reduced

152

by mixing the ILs with molecular solvents, however, the physic-chemical properties

153

of the IL would be affected as well; the vapor pressure of ILs will be increase,

154

flammability and lower electrochemical stability would be established. Temperature

155

is also another key point to change the ILs’ viscosity and this effect can be used for

156

ease of handling in the lab or in larger scale commercial reactor design.

157 158

2.1.2 Vapor Pressure

159 160

Vapor pressure of a substance is related to its volatility, as a substance with

161

high vapor pressure, the tendency of it to vaporize which from liquid state transfer to 5

162

gas state higher. IL has negligible vapor pressure; however this is arguably as what

163

does it mean by ‘negligible’, thus more research or studies should be conducted to

164

investigate and understand the gas-phase structure of the IL and how this affect the

165

vapor pressure of different ILs.

166 167

2.1.3

Melting Point

168 169

Low melting point of the ILs is the other often-reported property, this is all

170

related to the relationship between the cation/anion structure and the lower melting

171

point (Tm) of the ILs.

172

First, the size of the ions plays an important roles, for example both ions are

173

molecular ions will be larger than simple ions such as K+ and Cl- , larger separation

174

exist between their minimum energy position thus decreasing electrostatic

175

interaction, as a result of larger ions, decrease the Tm.

176

Next, the number and strength of interactions between the ions as 1) Larger and

177

more widely separated ions will decrease the lattice energies. Tm will be reduced as

178

larger ion size decrease the Coulombic force (refer to Eq. 1).

179


 =

180

  



(1)

181 182

Ke = Coulombs constant

183

q1 and q2= charge on ion 1 and ion 2

184

r = distance between the charges

185 186

Decrease localized charge density as the lattice energies reduced. Charge

187

delocalization and shielding. Nitrate ion, uneven charge distribution as the negative

188

charge distributed partially around the oxygen, thus the interaction weakens and the

189

melting point reduced. Furthermore, the positive or negative ions are being shielded

190

physically caused the close contact between the charged centers restricted thus the

191

lattice energy reduced. Reduce hydrogen bonding. Decline of ion symmetry to

192

disturb crystallization.

6

193

Ion structures that prevent the crystallization – inhibit the close contacts of

194

the ions forming, thus reduce Tm. Total electrostatic energy can be calculated using

195

Eq.2.

196

 =

197

  ℇ 

(2)

198 199

dmin = distance to the nearest counter-ion

200

q= charge on ion

201

ℇo= permittivity

202

M = Madelung constant [22]

203 204

The Madelung constant, M is determined by the arrangement of ions in the

205

crystal structure. The crystal structure with lower symmetry distribution, lower M

206

value, the stability of the crystal structure decreases and thus resulting the lower Tm.

207

Thirdly, packing efficiency. Less efficiency packing with more free volume

208

will contribute to a lower Tm, for example, asymmetry ion will result in a lower

209

lattice energy and thus lower Tm. furthermore, the conformational flexibility of the

210

ions, as the conformation degrees of freedom provide a high flexibility and

211

contribute a lower Tm as a result of an increase in the entropy of melting and steric

212

hindrance to the formation of a close- packed crystal structure. This property may

213

become uncertain due to the presence of impurities or it may undergo supercooling

214

effect.

215 216

2.1.4 Thermal Properties

217 218

Generally, thermal gravimetric analysis (TGA) techniques is being used to

219

identify the thermal properties of IL by heating it at a controlled rate and the

220

decrease in ILs’ mass is studied. However, because the experiment is conducted

221

relatively fast, it is hard to find out the ‘onset’ point of the IL [23]. It is very crucial

222

to identify the ‘onset’ point of the ILs especially for the usage of IL in batteries or

223

synthesis application, as these ILS exposed to the temperature in excess of these

224

‘onset temperature’, then extreme temperature and pressure rises spontaneously and

225

cause explosions. 7

226

Thus, the seldom-used technique, accelerating rate calorimetry (ARC) can be

227

used in order to provides important information regarding the safe use of ILs. In this

228

technique, small temperature steps are being utilized. At each steps, instrument

229

detect whether an exothermic process is occurring as identify the ILs’ ability to be

230

stabilize at that temperature.

231

2.1.5

Electrochemical Properties

232 233

The good electrochemical stability and high ionic conductivity of ILs allow

234

them to be extensive use in batteries, supercapacitors, fuel cells, solar cells,

235

electrowinning and other related applications. The structures of the cation and anion

236

in the terms of thermodynamics refers to oxidative and reductive stability of IL. IL

237

can be exposed to most positive and negative potential without an electron-transfer

238

reaction occurring which leads to reduction of the cation or oxidation of the anion.

239

However, in actual applications, these kinetics reaction of the ILs would be affected

240

by other factors such as electrode surface morphology and nature, temperature,

241

viscosity, conductivity and so on.

242 243

2.1.6 Surface Tension

244 245

The ILs’ surface tension has relatively minor number of researches discussed.

246

According to W. Martino et al [24], when compared to organic solvents, ILs have

247

relatively moderate surface tensions.

248 249

2.1.7

Non-flammability and Non-volatile Properties

250 251

ILs have been considered as non-volatile and thus non-flammable at ambient

252

and higher temperature which enable it to be centered on their possible to be an

253

‘green’ alternative to volatile organic solvents. However, it is not totally safe when

254

working with or near a heat or ignition source even though many ILs have negligible

255

vapor pressure, due to the certain decomposition products formed during the ILs’

256

thermal decomposition may sensitive to combustion. Thus, safety precaution should

257

be maintained when handling the ILs near a heat source [25]. 8

258 259 260 261 262

2.1.8 Biocompatibility

263 264

Biocompatibility is the main issue in the ‘green chemistry’ contexts, however,

265

this should not only concern about how the ILs intrinsically react to the

266

environmental or human/animal which expose to the materials, the possibility of

267

workplace or environmental impact of exposure to the IL also need to be take into

268

account.

269

environment in one form or another when using the IL in large volumes, and it

270

would immediately impact on the local ecosystem. Thus, ILs’ ultimate

271

biodegradation and toxicity need to be carefully considered as low toxicity does not

272

necessarily mean complete biocompatibility.

It is inevitable that small amounts of the IL will escape into the

273

Toxicity can be expressed into chemical toxicity and osmosis toxicity. The

274

chemical toxicity can be tested by exposure of the cell lines or small organisms to

275

the IL solution and determine the LD50 which is the lethal dose for 50% of the

276

population. The experiment is carried out by gradually increase the IL concentration

277

to the aqueous solution and basically the alkyl chain length of the cation become

278

more, the toxicity will be increased. This due to the ability of alkyl group to dissolve

279

into the membrane bilayer and thus disrupt the normal membrane function of cells

280

[26].

281

A hypertonic condition to the normal mammalian cell is refer to the aqueous

282

concentration which greater than 0.1 molar, M or any external medium that has

283

water activity lower than the internal cell. Most ILs have high molar concentration

284

which are 4 – 5 molar which contribute the its strong and immediate dehydrating

285

effect on a cell as water rushes across the cell membrane toward the lower water

286

activity environment, thus rapid dehydration occurs and will cause the cell death in

287

animal cells. Due to this reason, most cellular organisms are unable to survive in

288

pure ILs or in aqueous mixtures of the ILS where the water activity lower than the

289

internal solution of the organism.

290 9

291 292 293 294 295

2.2

Application of Ionic Liquids

296 297

The unique properties of the ILs enable it to be used in wide range of

298

application. How the ILs’ characteristic translate to the application, Table 1 provide

299

some examples to explain the particular characteristic contribute to certain

300

application.

301

Table 1: Properties of the ILs contribute to specific applications. Applications

Description

Pharmaceutical

Solid state active pharmaceutical ingredients’ chemical and

Application

physical properties could be affected by the polymorphism effects. The compounds show the different crystallographic arrangements and different hydrogen bonds self-arrangement once polymorphism effect occurred [27, 28], this would cause the contaminated drugs with low water solubility and are predisposed to poor and variable oral bioavailability [29]. The low melting point of the ILs enable the active pharmaceutical ingredients

always

polymorphism

in

effects

liquid

phase thus prevent

occurred

[30].

In

the

addition,

administration of drug to the patients would be easier when it is available in liquid state. Biotechnological Enzymes and proteins, DNA and RNA can be dissolve and Applications

stabilize in ILs. When the thermal energy and motion of the water molecules increases, the hydrogen bonding between the water molecules and between the water molecules and the protein coil weaken, this caused the protein structure uncoil. However, the hydrogen bond interaction in the IL solutions is higher, thus it can prevent the uncoiling of protein, thus in other words, protein

10

show remarkable stability in IL solution. In addition, research showed that protein degradation process can be slowed down when storage in the IL medium as the condition – the environment usually rich in organisms and proteases which are catalytic toward the breaking down of larger proteins are avoided. The hypertonic properties of the IL aqueous solution are not favorable for the cellular organisms survive. In addition, the active site of the Protease has been disrupted by IL medium, thus it is less active for protein denaturation [31].

Material

The solvency properties of ILs are very broad due to the its

Processing/

cation-anion pair function, it’s not only concerned to the

Extraction

potential of these ‘green solvents’ in order to replace the classical organic solvents. In addition, IL can readily dissolve biopolymers that are not easily soluble in most traditional solvents enable it to be used for biomass material processing. Used IL for the liquid-liquid extraction due to their immiscibility with common solvents. When two immiscible liquids which do not mix with each other is potentially to be used in separation techniques called liquid-liquid extraction or solvent extraction. In this system, a liquid solvent is used to remove a liquid component from a liquid mixture. The equilibrium constant, partition coefficient for this process can be determined in Eq 3.  ()

K =  () 

(3)

Where a(A) is the activity of the compound, 1 and 2 represent the two liquid phases which are not miscible with each other. When K is ~1, the compound will be found in both phases, while when the K > 10 or < 0.1, this indicate that the compound is predominantly in one or the other phase. The distribution coefficient is a related measure that takes account

11

of all forms of A as a function of pH.

Biomass

Cellulose, hemicellulose, keratin, lignin and chitin are the

Dissolution

complex mixture of materials which also known as biomass, these are valuable sustainable resources. However, it is difficult to process in order to obtain it due to the intrinsic nature and properties.

For example, cellulose, a harsh

condition is required to process it which available in various forms to produce, fibers for fabrics, films for packaging and so forth, this is due to the naturally designed intrinsic insolubility of cellulose. In nature, it is rare to obtain pure cellulose, usually lignocellulose which compose of cellulose, hemicellulose and lignin is presence. Lignin, a high molecular weight, rigid, aromatic-ether polymer that provides strength to woodly materials should be removed from the mixture as presence of it strongly interferes with the action of the cellulose enzymes, thus its removal is preferable before the second stage of the process. According to Tan et al, IL xylenesulfonate anion used to selectively dissolve the lignin component from the mixture which can reduce the harsh condition applied [32]. Energetic

A class of material with high amount of stored chemical energy

Material

such as shock, heating or applying friction that can be released are referred to energetic materials. Hypergolic ILs, a type of IL is being designed to ignite hen suitable oxidizer is being contacted and it is the replacement for hydrazine which is highly toxic and difficult to handle for use in propellants [33].

Electrochemical

The ion conductivity unique properties of IL enable it to be

Device

used as electrolyte in electrochemistry. Although its ion 12

Applications

conductivity is not typically as high as the existing electrolytes such as aqueous salt solutions (used in metal-air batteries) and organic carbonate-liquid based electrolytes (used in most common lithium-ion batteries), however its’ thermal stability enable it to gain much interest for the IL-based electrolytes for devices. The solvent-based electrolytes are intrinsically volatile; this will creates drying out problems when temperature elevated. In addition, under adverse condition, organic solvent electrolyte may flammable and potentially explosive. Thus, the thermal management of the market battery is very crucial to limit the internal temperature rise as significant heat generated during the rapid charging and discharging. However, this would add the cost and weight of the device. More thermally stable of the IL than organic electrolytes enable it to provide a basis of safer and more efficient device [34].

302 303

3.0

Bio-based Ionic Liquids

304 305

The IL’s poor biodegradability or the toxicity of ILs’ degradation products

306

which limits their used in various applications, thus bio-based IL is being explore as

307

the green character of the IL can be improved by synthesis it using renewable

308

compounds [14-16]. In addition, although best performing ILs are derived from non-

309

renewable sources such as petroleum or natural gas, but this would result in large

310

scale deployment, thus ILs derived from inexpensive and renewable agents are

311

highly desirable [35]. Natural resources such as amino acids and amino alcohols

312

from protein; sugar from cellulose, chitin, starch and other polysaccharides; aromatic

313

aldehydes from lignin as well as a diverse group of other compounds such as fatty

314

acids from vegetable or algae-derived oils can be used as IL precursors. Many

315

research has been conducted to convert the bio-polymer monomer such as amino

316

acids, sugars, aromatic aldehydes and acids to IL anions or cations.

317

13

318

3.1

Selection Criteria of Bio-based IL

319 320

Bio-based ILs are more useable in the field of bio-polymer processing, used

321

as reaction media and metal ion separation as their structure, thermal stability and

322

other characteristics are better compared to conventional ILs. However, any IL is

323

being implemented to the industrial process, few important criteria should be

324

properly considered.

325 326

3.1.1

Thermal Stability

327 328

It should be stable under the industrial process condition for a long period of

329

time. Thermal stability test is required for the selection of the suitable bio-based ILs

330

for the process implementation, while the testing condition should mimic the real

331

condition thus the setting for the test should use the relevant temperatures for

332

relevant duration and by working under the same atmosphere and pressure as the

333

actual process. Developing an accelerated aging protocols are required and should be

334

uniform.

335 336 337

3.1.2

Chemical Stability

338

The limited stability of the protonated tertiary nitrogen in simple protic

339

amino acid cation ILs which lower the its suitability to be used in many chemical

340

environments. Furthermore, the degradation of the original IL anion would be

341

happening through anion exchange and then the volatilization of protonated acid

342

occurred, thus this limits the ILs being implemented to certain process [36].

343 344 345

3.1.3

Efficiency of Bio-based IL synthesis

346

There are few synthesis bio-based IL pathways required protecting agents

347

and use large quantities of catalysts during the process, however, these will cause the

348

produced ILs has reduced atom efficiency and green character. But this can be

349

lightened as bio-based IL can be reused for a considerable number of times. 14

350 351 352 353 354 355 356 357 358 359

3.2

Amino Acid Based Ionic Liquids

360

A wide range of biomolecules can be incorporated into ILs; however amino

361

acid has special positions amongst biomolecules as it can be converted into both

362

anions and cations as well as its side chain consists of variety of functional groups

363

which enable it to incorporate chirality and a wide range of properties into the IL

364

easily. Furthermore, amino acids are cheap and abundant compared to others chiral

365

pool. Due to these, amino acid IL play a major role in the context of green and

366

sustainable chemistry [37]. Figure 1 shows the synthesis of amino acid derived Ionic

367

Liquids. In a single Amino acid molecule contains both an amino group and a

368

carboxylic acid residue with various side groups and a chiral carbon atom, thus it can

369

be an excellent candidate to act as a platform for functional ILs (Figure 2).

370 371

Figure 1: Synthesis of amino acid derived Ionic Liquids.

372 373

15

374

Figure 2: Design of amino acid for Ionic Liquids.

375 376 377 378

3.2.1

Amino Acids as Anions

379

Ohno and co-worker were first prepared the IL using amino acid (AA) as

380

anion, 1-ethyl-3-methylimidazolium cation [emim] which is a common cation used

381

for the IL preparations was selected. The cation was exposed to the anion exchange

382

resin (Figure 3) to prepare the [emim][OH] aqueous solution, then neutralized the

383

hydroxides using a slight excess of an equimolar amino acid aqueous solutions.

384 385

Figure 3: Ionic Liquids derived from amino acid through neutralization method [38].

386 387

20 natural amino acids were used to prepare room temperature IL using the

388

same methods. From the experiments, all the resulting amino acid ionic liquids were

389

transparent and nearly colorless liquids at room temperature (Figure 4). The amino

390

acid IL had different values of glass transition temperature (Tg) as well as ionic

391

conductivity reflecting their side chain structures. This may due to the introduction

392

of a functional group such as a hydrogen bond donor or acceptor lower the ionic

393

conductivity through intra/intermolecular interactions.

16

394 395

Figure 4: Resulted amino acid ionic liquids. Upper side from left to right:

396

[emim][Leu], [emim][Lys], [emim][Met], [emim][Phe], [emim][Phe], [emim][Pro],

397

[emim][Ser], [emim][Thr], [emim][Trp], [emim][Tyr] and [emim][Val]. Lower side

398

from left to right: [emim][Ala], [emim][Arg], [emim][Asn], [emim][Asp],

399

[emim][Cys], [emim][Gln], [emim][Glu], [emim][Gly], [emim][His], and [emim][Ile]

400

[39].

401

However, the thermal stability of these [emim][AA] was less satisfactory as

402

these amino acid derived IL start to decompose at approximately 200°C. Then,

403

phosphonium cation (P4444) were used to prepare ILs and alanine (Ala) as typical

404

amino acid [38]. The resulted ILs generally had low viscosity and high thermal

405

stability.

406

On the other hand, some fully biodegradable, non-toxic and potentially

407

renewable IL were produced by combining choline cation [Cho] with amino acid

408

anions. Five [Cho][AA] were prepared by Moriel and co-worker by using renewable

409

and non-toxic natural products which were choline hydroxide as cation and amino

410

acids as anions through a simple and straightforward procedures (Figure 5). In

411

addition, the synthesis is a green route in which the by-product of the process was

412

water.

413 414

Figure 5: [Cho][AA] ionic liquids synthesis pathway.

415 17

416

The resulted [Cho][AA] were being used as catalyst for the Knoevenagel

417

condensation between benzaldehyde and three different active methylene. Its showed

418

good conversions and high selectivity running at room temperature due to the

419

aminoacetate part of the amino acid ionic liquids as the promoter of the condensation

420

reactions [40]. Most of the [Cho][AA] ILs were being proven that it has low toxicity

421

with good biodegradability using enzymes and bacteria. The extra carboxyl or amide

422

groups on the amino acid side chain presence enable it more susceptible for

423

microbial breakdown [41].

424 425 426 427 428 429 430

3.2.2

Amino Acids as Cations The major amino acid-derived IL cations synthesis pathways (Figure 6) are 1)

431

use of modified amino acid as positive charge carriers through protonation with

432

strong acid and 2) through quaternization of N-containing molecules using amino

433

acid derived alkylating agents. However, in many cases, pre-treatment required to

434

form the conjugated N-heterocycle before the N-alkylation to stabilize the introduced

435

positive charge.

436

437 18

Figure 6: Cation amino acid derived Ionic Liquids [13].

438 439 440

Simple cation amino acid IL is formed through a simple protonation reaction

441

by mixing the correct molar ratio of amino acid and relevant strong acid in water,

442

then followed by evaporation of water in air and finally vacuum as showed in Figure

443

7. This is a typical atom-economic reaction without any poisonous by-product. High

444

melting points of some of the resulted IL. Esterification is an efficient way to

445

minimize the hydrogen bonding, the results showed that the melting points of the IL

446

(-18 to 75°C) reduced significantly as the number of hydrogen bonding decreased.

447

However, there are little influence on the quaternary nitrogen cations and the ILs

448

have relatively low decomposition temperatures (150-230 °C) [42].

449 450

Figure 7: Schematic of simple protonation reaction for cation amino acid derived IL.

451 452

Another example of the formation of IL surfactants through amino acid

453

protonation was conducted by Trivedi et al. Thionyl chloride added slowly to

454

isopropyl or isobutyl alcohol at 0°C for esterification purpose, then amino acids were

455

added to the reaction mixtures. To obtained the pure crystals of amino acid ester

456

hydrochloride,

457

recrystallization with methanol/hexane after titurated with hexane at 0°C. Next,

458

using hot water to dissolve the equimolar amounts of the AAECls and surfactants

459

(Figure 8). Thermal decomposition temperatures of the resulted amino acid derived

460

IL surfactants from 221°C to 280°C and it has better surface activity than

461

conventional surfactants [43].

crude

amino

acid

ester

19

hydrochlorides

(AAECls)

were

462

Figure 8: Schematic for the amino acid derived IL surfactants.

463 464 465 466 467

3.3

Application of Bio-based ILs

468 469

3.3.1

Bio-polymer Processing Application

470

The de-polymerization ability is the determinant of industrial process

471

efficiency for certain bio-polymers, for example, the easy de-polymerization

472

properties of starch and protein which enable them to be process effectively. On the

473

other hand, the de-polymerization of lignocellulose is difficult as it is almost

474

insoluble in conventional solvents as the strong hydrogen bonding between the

475

polysaccharide chains which contribute to its high degree of crystallinity and

476

insolubility. Thus, high temperatures, harsh chemical condition or enzymes are

477

required in order to convert lignocellulose to a value-added chemical, usually

478

pretreatment steps such as mechanical treatment, steam explosion or high

479

temperature treatment in dilute acid or bases are included to lower the cellulose’s

480

crystallinity [35].

481

By dissolving the crystalline biopolymer in IL to make the cellulose chain

482

more accessible to the de-polymerization agents is the another option for the

483

lignocellulose pretreatment as the chloride ion, a IL anion is the most beneficial

484

choice to break the inter-chain bonds due to their high hydrogen bond-accepting

485

characteristics [44]. The IL cation are beneficial too for the cellulose dissolution but

486

the effect is less noticeable than the anion as it has shorter alkyl chains. [BMIM][CL],

20

487

[EMIM][CH3COO], and [BMPy][CL] are the ILs which can be used for the cellulose

488

dissolution [45, 46].

489

The lignocellulose constituents, lignin and hemicellulose which remain

490

insoluble after cellulose dissolution can be removed easily. This step is crucial as

491

lignin will interrupt the cellulose de-polymerization process. After that, re-

492

precipitation of cellulose conducted by adding water, then evaporation process with

493

high temperature condition carried out to remove the water, IL should have

494

regenerated during this process as some residue IL in the cellulose can potentially

495

interfere with further chemical processing, thus it is important to select the suitable

496

IL for the lignocellulose pre-treatment as it should thermally stable for the prolonged

497

times at the regeneration temperature [44].

498

Intensive research has been conducted by using conventional ILs for the

499

cellulose, lignin and other bio-polymers dissolution, however, few studies were done

500

on the use of bio-based ILs for this application [47]. A research has been conducted

501

by using the ILs which generated from the lignocellulosic feedstock was being used

502

for the pretreatment of the same lignocellulose material [35]. Sugars and residual

503

lignin and hemicellulose were produced through the enzymatic hydolysis process

504

from the biomass which obtained from the first pretreatment, after that these

505

elements was further process through pyrolysis, biological treatment or catalyzed

506

oxidation. The product from the de-polymerization, aromatic aldehydes were used

507

for the fabrication of bio-based ILs which can be utilized for the initial process stage.

508

These ILs can dissolve cellulose effectively and removed lignin under moderate

509

conditions which need 3 hours at 160°C and it is better compared to the conventional

510

IL [EMIM][Cl]. In addition, the production cost of the lignin-derived bio-IL was

511

cheaper than the conventional IL.

512

Bio-based IL can be used as nanoparticle catalyst stabilization during the

513

biopolymer breakdown process as nanoparticles in conventional ILs are being used

514

to process lignocellulose as well as derived products. For example, during the

515

cellulose de-polymerization process, the IL [BMIM][Cl] was being used to stable the

516

heterogeneous Platinum (Pt) and Rhodium (Rh) catalyst and a homogeneous

517

Ruthenium (Ru) catalyst in H2 at 150°C, this enable better yield of sorbitol and

518

glucose (51%) and allow the cellulose converted fully [48]. Besides that, single step

519

cellulose conversion can be done by using boric acid functionalized IL in order to

520

stabilize the Ru (0) nanoparticle catalysts together with [BMIM][Cl] and yield 94% 21

521

of sorbitol under temperature 80°C [49]. An environmental friendly system can be

522

established by using bio-based IL potentially replace both IL constituents of this

523

reaction medium.

524

Processing of bio-polymer lignin also implement this IL-nanoparticle system

525

too, for example, used the Pd nanoparticles that produced in situ from H2PdCl4 in

526

[BMIM][MeSO4] together with N-pentyl-4-methylpyridinium iron bis(discarbollide)

527

as a co-catalyst during the lignin conversion process under the controlled condition

528

at 120°C for 18 hours, 72% of lignin convert successfully to aromatic aldehyde and

529

this catalyst could be reused at least 3 times without significant loss of activity. The

530

bio-based IL are recommended to replace the [BMIM] cation that being used in the

531

catalyst system [50].

532 533 534 535 536 537

3.3.2

Used as Reaction Media and Organocatalysts

538

reactions by taking the advantages of the chiral moieties retention of the starting

539

materials such as amino acid. In addition, recover of the chiral catalyst from the

540

substrate, reagent and the product mixture easily by solubility difference is the main

541

point that beneficial of using the ionic-liquid-supported chiral catalyst in various

542

processes [51]. However, processes such as distillation or extraction are required to

543

separate the products yield, process aids and another form of IL regeneration after

544

the lignocellulose treatment and the processes usually are thermal treatment or

545

harsher condition during the reaction itself, thus the thermal stability of the selected

546

ILs remain the key determinant.

Bio-based ILs have been used as solvents or organocatalysts in many

547

Functional ionic liquids can be designed by adding the functional groups to

548

the side chain of ionic liquids; the chiral unit bond covalently to the IL moiety to

549

develop the chiral ionic liquids (CILs). Chiral unit of the CIL act as catalytic site and

550

the IL moiety as chiral-induction group during the asymmetric catalysis process. For

551

instant, functionalized CIL, proline-based bio-IL was being used as catalyze in the

552

asymmetric Michael process for adding the cyclohexanone to trans-β-nitrostyrene

553

under room temperature and co-catalyst (15 mol% of IL and 5 mol% f trifluoroacetic

554

acid (TFA)) without additional solvents were being used. The reaction gave 22

555

quantitative yields and very high enantiomeric excess (ee) which was 97% within 20

556

hours. The biphasic characteristic of IL enable it maintained the functionalized CILs

557

and it can be reused during the process through the diethyl ether precipitation, the

558

recycled CIL showed the same activity during the next cycle with marginally

559

reduced selectivity, there are observed loss of activity of the recycled CIL during the

560

third and fourth cycle, but it still can achieve excellent yields and ee values [52].

561

In addition, fructose-derived IL family were produced [53] through the

562

conversion of the fructose to hydroxymethyleneimidazole 1 and double alkylation of

563

it. This bio-based IL was being used in the Heck reaction to allow the coupling

564

process between the aromatic and vinylic system; bromobenzene and n-butyl crylate

565

in phosphonium salts respectively in the presence of palladium catalyze and

566

triethylaminate at temperature 100°C. Good yield of trans-cinamic n-butyl ether and

567

stable solution were established while using the complexes dichlorobis palladium (II)

568

and palladium (II) acetate were used as catalyst. It can be reused after the reaction

569

without loss of activity for at least two run [54].

570 571

3.3.3

Metal Ion Separations

572

Bio-based IL’s functional groups can be used to chelate and extract the metal

573

ion, this process can be done under aqueous phase thus less harsh thermal and

574

chemical conditions are required compared to the classic reaction media. This

575

enhance the opportunity to apply wider range of bio-based ILs. Hydrophobic ILs is

576

preferred to enable the extraction process perform efficiently as it can be phase

577

separate from the water spontaneously, however, most of the bio-based IL are

578

hydrophilic in nature thus hydrophobicity need to be introduced. Previously,

579

hydrophobicity can be done by adding the fluorinated anions such as Tf2N

580

conventional ILs, this approach can be used for the bio-based cation too, however,

581

bio-based alkylating agents such as fatty acid potentially replace these expensive and

582

sometimes toxic fluorinated anions [49].

–

to the

583

Basically, the most common metal ion extraction using bio-based ILs involve

584

three steps (Figure 9), which are 1) complexation of the metal ions by the functional

585

groups of IL cation or anion, 2) phase separation by mixing the aqueous metal ion

586

solution and hydrophobic IL under strong agitation, sometimes heating is needed.

587

Then, acid is being used for stripping the metal ions from IL phase. 23

588

Figure 9: Process of bio-based IL metal ion extraction [13].

589 590 591

Betaine, side product of beetroots production was being used to synthesis

592

task-specific ionic liquid for Schadium (III) recover and betaine salts synthesis

593

process

594

bis(trifluoromethylsulfonyl)imide in water to form [Hbet][Tf2N] (Figure 10), the

595

ionic liquid compound can be removed from the easily from the aqueous solution as

596

it is hydrophobic at room temperature [55]. However, efficiency of the extraction

597

using the pure [Hbet][Tf2N] is poor, thus extractant, zwitterionic betaine was added

598

to the system. The rare-earth extraction efficiency of the [Hbet][Tf2N] increased to

599

more than 90% by adding the excess zwitterionic betaine to the aqueous phase [56].

done

by

dissolving

the

betaine

hydrochloride

and

lithium

600

601 602

Figure 10: Chemical Structures of a) [Hbet][Tf2N] and b) zwitterionic betaine

603

[56].

604 605

Schadium (III) extraction from the red mud leachates can be done by mixing

606

the pre-saturated [Hbet][Tf2N] with feed solution while small amount of HCl (1 M)

607

or HNO3 (1M) to adjust the pH. Then, to obtain the homogenous mixture and

608

chemical equilibrium conditions of the extraction mixture, it was heated to 70°C and

609

shaken at 700rpm in a thermoshaker for 10 minutes, then induce settling of the

610

mixture phases by permitted to cool to room temperature for one hour. Full phase

611

separation was induced with a clear and flat interphase mixture was observed, the

612

metal concentration in the aqueous phase was determined. Approximately 95% of Sc

613

(III) can be extracted in the pH range 1.5-3.5 as the [Hbet][Tf2N] metal ion

614

extraction is more likely occur via the proton exchange mechanism on the carboxylic

24

615

acid function. The maximum loading of the Sc (III) ion reached when the absence of

616

COOH absorption bands as there are no free carboxylic acid function in the ionic

617

liquid phase. The ionic liquid phase was separated through the precipitation stripping

618

by adding solid oxalic acid.

619

The reused ionic liquid phase showed the same Sc (III) extraction efficiency

620

as it was washed two times with distilled water in the 1:1 phase ratio as the residue

621

acid during the stripping step was removed.

622

The affinity of [Hbet][Tf2N] toward other metals ion such as iron, aluminum,

623

titanium, calcium, sodium and silicon as well as few minor constituents which were

624

present in red mud leachates was identified too. Researcher focus on the scandium

625

extraction more as it represents ≥ 90% of the economic value of the minor elements

626

present in red mud [57]. The extraction of different metal ions from the feed

627

solutions was conducted at various acidity. However, [Hbet][Tf2N] showed better

628

affinity towards Sc (III) (extraction percentage, %E > 90%) compared with other

629

rare-earth metal ion (%E between 4% to 12%) at the initial pH 3. Low affinity of

630

[Hbet][Tf2N] towards major elements present in the red mud was showed (%E less

631

than 5%), except for Fe (III) and the stronger extractability of the Fe (III) and Sc (III)

632

established due to the smaller radius ion of these metal ions, thus higher charge

633

density which leads to more stable complexes formed with the carboxylate groups.

634

This enable more efficiency extraction for Fe (III) and Sc (III) in the ionic liquid

635

phase [58].

636

Dries Parmentier et al synthesized fatty acid based task-specific ionic liquids

637

(TSIL) and measured their salt extraction capabilities. Four highly hydrophobic

638

TSIL,

639

methyltrioctylammonium linoleate and tetraoctylammonium oleate were produced

640

under the same condition through the two step synthesis; 1) Same molar amount of

641

sodium hydroxide added to the fatty acid already dissolved in ethanol by stirring it

642

overnight

643

tetraalkylammonium chloride which dissolved in toluene and water mixed with the

644

sodium fatty acid to enable the metathesis reaction to occur in a period of 3 hours

645

under room temperature. Then, purification of the organic phase was carried out to

646

obtained the viscous liquid.

tetraoctylammonium

under

room

linoleate,

temperature

to

methyl-trioctyl-ammonium

synthesis

sodium

fatty

oleate,

acid

2)

647

The both alkali metal and period IV transition metal salt extraction efficiency

648

using the four TSIL synthesized was evaluated by adding the metal solution to the IL 25

649

and the mixture was stirred with a vortex mixer for 2 hours then transfer to

650

centrifuge at 3750 rpm for 10 minutes. The extraction was conducted under

651

controlled laboratory conditions at 20°C. The results showed that it has no or

652

negligible extraction efficiency towards alkali metal salts, in contrast, it has good

653

extraction efficiency (greater than 99%) for period IV transition metal salts, this may

654

due to some ion exchange occurs which reduce the extraction efficiency for the

655

alkali metal salts [59].

656 657 658 659 660 661 662 663 664 665

4.0

Electrospinning

666 667 668

4.1

Basic Concept of Electrospinning

669

electrospinning process which is a useful method and valid for frequent organic and

670

inorganic systems [60]. The nanofibers formed using this method have high porous

671

network structure, large surface area to volume ratio and the dimensions of the

672

nanofibers can be personalized and enhanced simply during manufacturing [61].

673

Both natural and synthetic polymer can be used to harvest nanofibers in tube, wire,

674

or particulate form [61, 62]. To make higher volumes of nanofibers, many methods

675

can be used [60].

Nanomaterials with firmly controlled size distribution can be invented using

676

The high volume production of light weight, highly functional, nanoscale,

677

mesh-like structures is allowed by electrospinning technique [63]. The setup of

678

electrospinning is as follow:

679

26

680

Figure 11: Electrospinning Setup [64].

681 682

Three key constituents to achieve the electrospinning process which are a

683

high voltage supplier, a capillary tube with needle (small diameter) as well as a metal

684

collector. High voltage in the electrospinning method is to generate an electrically

685

charged jet of polymer solution. Thus, the solution jet is vanishes or solidifies before

686

getting to the collection and is composed as an interconnected mesh of small fibers

687

[65, 66]. To prevent excessive fusion between the fibers as membrane permeability

688

would be reduced by over-crosslinking, solvent vapour exposure time must control

689

[67].

690

There are two unlike polarity electrode are used during the electrospinning

691

process. One electrode is placed into the spinning solution/melt and the other

692

attached to the collector while usually, the collector is being grounded. The polymer

693

solution detained by its surface tension and when charge is induced on the surface of

694

the liquid, the contraction of the surface charges to the counter electrode which is

695

directly opposite to the surface tension [68].

696

When the electrical field force increases, the hemi-spherical surface of the

697

polymer solution at the tip of syringe’s needle extends to form a pointed shape

698

known as Taylor cone [69]. Further increasing the electric field will increase the

699

repulsive electrode to overcome the surface tension of liquid and the charged jet of

700

the polymer solution is ejected from the tip of Taylor cone. Elongation process

701

undergoes by the discharged polymer solution jet and this will allow the jet to

702

become very long and thin. The solvent will evaporate at the same time, leaving

703

behind a charged polymer fiber [70].

27

704

Very thin and fine fibers with diameters between less than 3nm to over

705

1micrometer are effectively fabricated using electrospinning method. As the

706

processing circumstances involved are simple and straight forwards, therefore most

707

of the polymers were liquefied in solvents before electrospinning. When the solute

708

polymer dissolved totally in a correct quantity of solvent, it will become a fluid form

709

called polymer solution. Then, it can introduce into a syringe with needle [70].

710 711 712

4.2

Multilayer Electrospinning and Mixing Electrospinning

713

Lately, there are two progressive electrospinning practices are announced.

714

There are multilayer electrospinning and mixing electrospinning in order to harvest

715

complex scaffolds having unlike polymers. Each polymer is constructing to

716

procedure a single layer and is consecutively collected on the same grounded

717

collector for the multilayer electrospinning. A multilayered non-woven nanofibrous

718

mesh is formed and a hierarchically ordered structure collected of different polymer

719

meshes can be obtained. For the mixing electrospinning, a mixed fiber mesh is

720

obtained when two different polymer solutions are instantaneously invented from

721

two different needles under different processing condition. The polymer fibers are

722

varied on the same collector. In order to classify specific polymer fiber on the

723

identical collector, each polymer solution can dissolve with a fluorescence dye and

724

the layered structure is pictured by confocal laser scanning micrographs. Figure 12

725

shows the Schematic diagram of (a) multilayer electrospinning and (b) mixing

726

electrospinning [71].

727

728 729

Figure 12: Multilayer Electrospinning and Mixing Electrospinning [71]. 28

730

The determinations of using multilayer electrospinning are it is a simple way

731

to fabricate nanofiber membrane and the thickness of the scaffold and nanostructure

732

fiber can be controlled easily. Therefore, in the future, scaffold has the potential for

733

industrialize [72]. In order to strengthen the electrospun mat and avoiding

734

delamination of the electrospun multilayer, hot pressed technique can be adopted [73,

735

74].

736 737 738

4.3

Electrospun Removal Mechanism of Heavy Metals

739

Heavy metal ions and compounds from contaminated metal effluent removed

740

by the electrospun fibrous membrane is due to the interaction between the heavy

741

metal ions and the functional sites on the nanofibers’ surface, for instance physical

742

affinity and electrostatic interaction, or chemical chelating and complexation, thus

743

the electrospun membrane’s capability to remove heavy metal are determined by the

744

functional site which inherent or anchored on the membrane surface [10].

745

Electrospun membrane is perfect microfiltration medium as its micro or nano-porous

746

structure, thus in combination of strong interaction and effective microfiltration

747

properties of the electrospun membrane enable it has high efficiency towards the

748

heavy metal removal.

749

Basically, toxic metal removal involves static adsorption and dynamic

750

adsorption process. The efficiency of the static adsorption process can be measured

751

based on the sorption capacity and functional surface’s adsorption rate while for the

752

dynamic adsorption or depth filtration involved the pressure drop, permeation flux as

753

well as membrane’s working life. Heavy metal removal using electrospun membrane

754

are more applied to static mode.

755 756 757

4.3.1

Adsorption Isotherm

758

A curve that expresses the variation in the amount of gas adsorbed by the

759

absorbent with pressure at constant temperature. Three adsorption isotherm are

760

commonly used to discuss the wastewater treatment application which are Langmuir

761

[75], Freundlich [76], and Redlich-Peterson isotherms [77]. By applying these

29

762

adsorption isotherm, modelling of heavy metal adsorption on the electrospun fibrous

763

membrane is realized.

764

First, Langmuir equation describe a relationship between the number of

765

active sires of the surface undergoing adsorption and pressure. Figure 13 shows the

766

dynamic equilibrium exists between adsorbed gaseous molecules and the free

767

gaseous molecules.

768 769

Figure 13: Dynamic Equilibrium exists between adsorbed molecules and the free molecules.

770

Where A(g) is un-adsorbed molecule, B(s) is unoccupied electrospun fibrous

771

membrane surface and AB is absorbed molecule. The Langmuir isotherm equation

772

can be showed as below equation (Eq. 4). 

773



=





+







×" $ #

(4)



% = residual metal concentration

774 775

& = amount of metal adsorbed onto the

776

adsorbent at equilibrium

777

' = Langmuir constant of the system

778 779

By plotting the graph 





versus



#

, a straight line obtained with slope



and

. The maximum adsorption capacity (qm) of the heavy metal

780

interception point

781

molecules on the membrane can be identified.





782

There are some limitations of the Langmuir Equation, as the molecules do

783

not interact with each other and it assumes that the adsorption is monolayer, however

784

this only valid under low pressure condition, the molecules will attract more and

785

more molecules toward each other under high pressure condition thus the assumption

786

breaks down.

787

Next, Freundlich Isotherm which is an empirical relationship between the

788

amount of molecules adsorbed by a unit mass of solid adsorbent and pressure at a

789

specific temperature are established by German scientist Freundlich. During the

790

actual adsorption process, there may have interactions between the adsorbed 30

791

molecules, thus when a molecule absorbed on one specific site, this would make

792

other molecules to be adsorbed on the nearby sites, Freundlich isotherm equation (Eq.

793

5) provide better fitting result on this circumstances. 1 n

794

log &+ = log '
+ " $ log %+ (5)

795

'. and n are Fruendlich constants of the system which indicate the

796

adsorption capacity and adsorption intensity of the adsorbents. By plotting the graph,

797

log qe against log ce, then kF and n can be determined. The experimental isotherms

798

always seem to approach saturation at high pressure and thus Freundlich isotherm is

799

not valid at high pressure. However, the Redlich-Peterson equation (Eq. 6) is more suitable to be used

800 801

for the diffusion of the adsorbed metal ions into the porous adsorbent. 01

ln / & + − 13 = g ln %+ + In B (6)

802

+

803

& = adsorbing amount on unite adsorbent

804

g = exponent between 0 and 1

805

% = equilibrium concentration of heavy metal ions

806

A and B = Redlich-Peterson isotherm constant of the system

807 808 809

4.3.2

Kinetic Adsorption

810

Two kinetic models which are pseudo-first order rate equation and pseudo-

811

second-order rate equation can be used to determine the adsorption rate of heavy

812

metal ions on the surface of electrospun membranes [78].

813

Linear form of the pseudo-first order rate equation are describe in Eq. 7.

814 815

' <

1 678 9&+ − &: ; = log &+ − 2.303 (7)

816 817

k1 = adsorption rate constant (h-1)

818

qe = amounts of heavy metal ions absorbed at equilibrium (mg/g)

819

qt = amounts of heavy metal ions absorbed at time t (mg/g)

820 31

821

A straight line can be obtained by plotting the log (qe – qt) versus t if the

822

model is applicable, where the gradient of the graph is (-k1/2.303) and the

823

interception point is log qe. The value of the qe and k1 can be determined.

824

The pseudo-second order rate equation in the linear form shows in Eq. 8.

825

A

826



=



 





+ t " $ (8)  

827

k2 = adsorption rate constant (g/mg/min)

828

qe = amounts of heavy metal ions absorbed at equilibrium (mg/g)

829

qt = amounts of heavy metal ions absorbed at time t (mg/g)

830 831

Plot t/qt versus t, linear graph shows if the model is applicable, with the

832

interception point 1/k2qe2 and gradient 1/qe, while the rate constant k2 and

833

equilibrium concentration qe can be calculated.

834 835 836 837 838 839 840 841

4.4

Electrospun Fibrous Membrane Surface Functionality

842

The heavy metal ions’ adsorption efficiency by electrospun membrane

843

affected significantly by the membrane surface morphology and surface functionality.

844

Electrospun surface morphology is refer to the surface-to-volume ratio properties of

845

the membrane and this is the critical factor which can determine the adsorption

846

capacity while it can be adjusted through the phase separation process during

847

electrospinning or post treatment of the membrane. The phase separation process

848

was accomplished by monitoring the solvent during the electrospinning as the

849

solvent rich regions at the beginning of the process will transform to pores. Thus

850

solvents with higher volatility and vapor pressure contributes to higher tendency for

851

pore formation [79]. The high surface-to-volume ratio enhance the adsorption

32

852

efficiency for all mechanism such as ion exchange, charge interactions, chemical

853

chelating, complex formation and physical adsorption.

854

Another key factor that control the interaction between the metal ions and

855

electrospun membrane is surface functionality of the membrane and it could be

856

altered through the surface chemical modification methods. The modification by

857

anchoring appropriate functional sites to the nanofibers membrane would depend on

858

the adsorption mechanism involved. Different functional groups such as carboxylate

859

groups, sulfonate groups, amino groups, and special ligands could be introduced to

860

the nanofiber surface depends on the metal ions’ nature.

861 862 863

4.4.1

Carboxylate Group (COOH)

864

Chelation process occurred between the carboxylates and metal ions. For

865

examples, carboxylate groups could be introduced to the cellulose acetate

866

microfibers membrane by grafting poly (methacrylic acid) (PMAA) through Ce(VI)

867

initiated radical polymerization, the chain can provide adsorptive –COOH groups on

868

the microfibers. Carboxylate groups converted from carboxylic acid by changing the

869

pH value, while the experimental results showed that the Cu(II), Cd(II) and Hg(II)

870

removed by the modified cellulose acetate membrane through the chelating

871

interactions between metal ions and carboxylates, however, the contaminated water

872

at pH value 3.4, no Cu(II) adsorption were observed due to the possibility of

873

carboxylate converted back to carboxylic acid under this condition. The membranes’

874

adsorption capacity increased when pH value of the contaminated water increased

875

and the modified membrane could be reused through the desorption mechanism;

876

immersing the used membrane in saturated ethylene dinitrilo tetra-acetic acid (EDTA)

877

solution with concentration about 200 mg/L for the Hg(II) desorption [80].

878 879 880

4.4.2

Amino Groups

881

A variety of metal ions could bind with amine groups. Study was conducted

882

by Pimolpun Kampalanonwat et al to evaluate the adsorption capacity of aminated

883

Polyacrylonitrile (PAN) towards four types of metal ions, Cu(II), Ag(II), Fe(II) and

884

Pb(II). The modified PAN membranes were achieved by grafting the amidino 33

885

diethylenediamine chelating groups on the PAN surface through heterogeneous

886

reaction with diethylenetriamine (DETA) and AlCl3.6H2O as catalyst. Initial pH and

887

metal ion solution’s initial concentration were determinant of the amounts of the

888

metal ions adsorbed onto the APAN membranes, the results showed that the

889

membrane adsorption reached equilibrium at about 10 hours for Cu(II) ions and

890

about 5 hours for Ag(II), Fe(II) and Pb(II) ions. Langmuir isotherm was fitted for

891

this experiments and the metal ions’ maximal adsorption capacities were 156.6,

892

155.5, 116.5, and 60.6 mg/g, respectively. Desorption of the membrane could be

893

done with hydrochloric acid (HCI) aqueous solution for the APAN membrane reused

894

purpose [81].

895

The amidoxime groups or N-amino amide could be a strong chelating

896

chelating sites to a wide range of heavy metal ions. An amidoxime PAN nanofibers

897

were fabricated through the reaction with hydroxylamine hydrochloride as the matrix

898

for metal ions chelation. The results showed that the modified PAN nanofibers

899

which fabricated through electrospinning techniques had almost two times higher

900

saturated adsorption capacity to Cu(II), Cd(II) was 219 mg/g and 510.4 mg/g

901

compared to the conventional amidoxime PAN membrane. In the 1 mol/ℓ nitric acid

902

solution, the desorption rate of Cu (II) and Cd(II) from the metal ion chelated

903

amidoxime PAN nanofibers membrane was 87% and 92% respectively under 60

904

minutes [82].

905

Post treatment is another method in order to amino functionalized the

906

electrospun. For instance, Polyacrylonitrile/polyaniline (PAN/PANI) core/shell

907

nanofibers fabricated

908

polymerization of aniline as a selective adsorbent for Cr(VI) ions, while by tuning

909

the polymerization temperature, different electrospun morphology were obtained.

910

Excellent adsorption capability was exhibited by the core/shell mats and the

911

saturated capacity of the nanofibers membrane is 24.96, 37.24 and 52.00mg/g for

912

105, 156, and 207mg/L initial Cr(VI) solution respectively. Pseudo second order

913

kinetics model was followed and the adsorption was best fit using the Langmuir

914

isotherm. In addition, results showed that the adsorption capacity increase with

915

temperature. The Cr(VI) reduced to non-toxic Cr(III) by using the PAN/PANI core

916

shell nanofibers mat and it can be regenerated and reused after NaOH treatment [83].

via electrospinning technique followed by in situ

917

34

918 919

4.4.3

Sulfonate Groups

920

Composite polystyrene (PS) and styrene-isoprene-styrene block copolymer

921

(SIS) submicron ion-exchange fibers (IEF) fabricated through electrospinning

922

method and further sulfonated with sulfuric acid in order to produce strong acidic

923

cation IEF. Through this process many sulfonated acid groups introduced onto the

924

benzene wreath of styrene. The remarkable characteristics of the PS/SIS IEF were

925

high porosity (85%) and high specific surface (760 m2/g) contributed to higher value

926

of ion-exchange capacity (4.78 mmol/g) and greater adsorption rate for Cu (II)

927

(305.9mg/g) due to more IEF functional groups expose to the metal ions [84].

928 929 930

4.5

Electrospinning and Ionic Liquids

931

As mentioned earlier, ILs are being explore and used as a replacement for

932

volatile organic solvent system as well as the able of it to be reused and recovery

933

enable it to be explore extensively for electrospinning techniques. Furthermore,

934

nanoscale and microscopy nanofibers membrane with high surface area can be

935

obtained by using IL as solvents for the electrospinning techniques. It also can be an

936

invaluable tool for the dissolution of biopolymers which insoluble is most organic

937

solvents. For example, cellulose, a natural abundance, renewability and widespread

938

use of cellulose fibers in various commercial applications enable it to be focused

939

more, however cellulose is insoluble in organic solvents and unable to melt, cause

940

the electrospinning of the cellulose using the cellulose derivatives such as cellulose

941

acetate, having higher solubility in organic solvents [85]. Unfortunately, fibrous

942

fabricated from the cellulose derivatives are tends to degrade more easily than the

943

native cellulose, thus thermal stability of the fiber decreased. Using RTIL

944

[BMIM][Cl], cellulose nanometer-to-micrometer diameter fiber fabricated due to

945

RTIL act as hydrogen bond acceptors that disrupt the hydrogen bonds extensive

946

network which presents in the crystalline polymer [86].

947

Incorporation of nanoparticles into the electrospun bio-polymer fibers

948

through coaxial electrospinning with nanoparticles core and bio-polymer sheath done

949

successfully as RTIL can enhance the inorganic nanoparticles’ dispersion due to

35

950

their polar, electrolyte nature seems to reduce the tendency of nanoparticles to

951

aggregate.

952

Natural silk fibers have excellent mechanical properties including high

953

strength, toughness, elasticity as well as failure resistance, in addition, their

954

renewable source enable it to have excellent biocompatibility, biodegradability, and

955

oxygen and water permeability. Unfortunately, extensive hydrogen bonding exist in

956

the fibroin cores’ crystalline regions of silk affect them hydrophobic in nature thus

957

make their dissolution challenges. Bombyx mori (B.mori) silk dissolved by

958

Imidazolium-based RTILs which contain chloride and glycine anions; [BMIM][Cl]

959

used as solvent for silk and silk containing polymers [87]. Young’s modulus of the

960

B.mori silk was being enhanced significantly (up to 460%) compared to pristine silk

961

electrospun fibers by adding SWNTs as nanocomposites while research have been

962

proven that silk nanocomposite fibers, silk fibroin and silk fibroin/ wool keratose

963

fibers capable of binding heavy metal ions due to the presence of carboxylic,

964

sulfonate and amine groups. Thus, the reinforced silk-containing electrospun

965

membrane from RTIL with dispersed nanofiller exhibit enhanced mechanical

966

strength and high affinity towards toxic heavy metals can ideally suited for the

967

filtration applications [88].

968

All the non-volatile solvents need to be removed from the fibers being

969

formed as incomplete solvent removal can cause polymer plasticization and results

970

in the fiber fusion partially or fully. In addition, retention of electrical charge on the

971

cellulose

972

chloride/dimethylacetamide (LiCl/DMAc) cause the fibers arrange vertically on the

973

collector and tends to collapse and agglomerate once the electrical field is removed

974

[89]. These are the general problems by using RTIL as alternate solvent system for

975

electrospinning techniques, to retain electrospun morphology during the fiber

976

collection, additional processes are required.

electrospun

using

the

non-volatile

solvent

system,

lithium

977 978

For instance, a grounded ethanol coagulation bath used for the cellulose

979

fibers collection and the removal of ionic liquid, [BMIM][Cl] and to confirm the

980

RTIL removed completely, elemental analysis was conducted. Reports proven that

981

water used as coagulant for the electrospinning of cellulose using the same ionic

982

liquid solvent system [90], but ethanol can remove [BMIM][Cl] from cellulose fibers

983

more effective than water. However, ethanol is highly flammable and can ignite 36

984

easily thus mixture of water/ethanol can reduce the fire hazard as well as enhance the

985

RTIL kinetic removal from the fibers. The cellulose fiber newly produces were

986

piling up on the bath solution surface faster than they were sinking thus stationary

987

coagulation baths method unable to remove the non-volatile solvents from

988

electrospun fibers effectively, in order to avoid fibers build up and agglomeration on

989

the coagulant baths, surfactants can be added to the coagulations or recirculation of

990

coagulant [91].

991

Besides that, other methods for fiber collection and remove the non-volatile

992

from the electrospun are developed such as disc or drum rotating collector used

993

enable the intermittent immersion of fibers in a coagulation bath during rotation

994

(Figure 14) or the fibers collected immersed into the coagulants. Actually small

995

degree of fiber interconnectivity results from some fiber fusion may be advantageous

996

too as it can enhance the overall mechanical stability of non-woven mats without

997

decrease the membrane surface area and porosity or increase fiber diameter

998

significantly [92].

999 1000

Figure 14: Modified Electrospinning Setup with temperature control and coagulation

1001

[93].

1002 1003

4.5.1

RTIL Solution Properties affect the Fiber Morphology

1004

A complex relationship between solution properties and spinning parameters

1005

impact the fiber thinning, while the mathematical models of the electrospinning

1006

process proposed that solution conductivity plays a key role in fiber morphology as

1007

highly conductivity solvents will form the ultrathin and uniform fibers due to the

1008

predominance of fiber jet non-axis symmetric mode. In contrast, at high charge

1009

densities, fiber jet whipping instabilities over the axis-symmetric instability modes

1010

caused the droplet and bead formations. 37

1011

RTIL-based polymer solution is more favorable for the thin and uniform

1012

fibers formation due to its high conductivity properties as the solvents are composed

1013

solely of ions. Unfortunately, high viscosities and surface tensions typically between

1014

the water and organic solvents of the RTIL contribute the droplets or beaded fibers

1015

formation because the surface tension and viscosities of the polymer solution are

1016

high enough to susceptible for axis-symmetric instability modes.

1017

A method is proposed to eliminate this limitation is increase the working

1018

temperature of electrospinning technique since the viscosity of RTILs tends to be a

1019

strong function of temperature; viscosity and surface tension of the spinning solution

1020

decrease as the its temperature increase. The temperature-controlled electrospinning

1021

achieved by generate an enclosure with constant temperature surrounding the syringe.

1022

Vogel-Fulcher-Tammann (VFT) equation used to determine the solution viscosity at

1023

the electrospinning temperature [94]. For instance, stable jet formation observed and

1024

smooth and continuous fibers for the 3, 4 and 5wt% polyacrylonitrile (PAN) from

1025

[BMIM][Br] when the polymer solution temperature increased from 70°C to 85°C,

1026

as the temperature increased contribute to viscosity reduced and moderate decrease

1027

in surface tension and increase in conductivity of the polymer solutions. No fiber

1028

formation for the solution concentration less than 3wt% as the viscosity is

1029

insufficient chain entanglement density [90].

1030

Another approach to reduce the RTIL/polymer solution’s viscosity and

1031

surface tension is incorporate co-solvent. For example, co-solvent dimethyl sulfoxide

1032

(DMSO) was added to the [AMIM][Cl]/cellulose solution and noticed that the

1033

viscosity and surface tension of the solution decreased as the co-solvent volume

1034

increased; even at high co-solvent concentrations, no precipitation formed. A balance

1035

between viscosity, surface tension and conductivity which maintained with ratio of

1036

the polymer concentration and co-solvents contribute to smooth cellulose fiber

1037

formation which suppress the axis-symmetric instabilities of jet fiber [91].

1038 1039 1040 1041

5.0

Previous Applications of Nanofiber Membranes

1042

Table 2.2 shows previous studies that have been conducted on the fabrication

1043

of electrospun membrane with ionic liquids for various applications. From the 38

1044

studies, it can be concluded that mostly IL are being used as alternate solvents for the

1045

electrospun fabrication and results showed that the morphology of the membrane

1046

was being enhanced. Moreover, it can be seen that there is no fabrication of

1047

electrospun membrane with bio-based IL entrapped in the membrane for heavy metal

1048

removal application yet.

1049

39

Materials

Ionic Liquids

Polyvinyl alcohol (PVA) and Chitosan

1-ethyl-3Electrospin methylimidazolium chloride ning (EMIMCl) and 1-butyl-3methylimidazolium bromide (BMIMBr)

Immersion

Silk

1-ethyl-3Electrospin methylimidazolium chloride ning (EMICl), 1-butyl-3methylimidazolium chloride (BMICl), 1-butyl-2,3dimethylimidazolium chloride, (DMBICl), 1,2dimethyl-3hexadecylimidazolium bromide (DMHdIBr), and 1ethyl-3-methylimidazolium glycine (EMIGly) Aliquat 336 Electrospin

Mixing polymer solution

Cellulose

Membrane Fabrication Method

Ionic Liquid Incorporation Method

Results

-

in

-

-

Mixing

in

-

40

Application

References

Table 2: Studies on the fabrication of electrospun membrane with ionic liquids for various applications.

Ionic liquids entrapped in the electrospun polymer membrane. Conductivity enhanced from 6 ×10-6 S/cm to 0.10 S/cm. Thermoelectric (TE) activity exist by demonstrating Seebeck coefficient up to 17.92µV/K. EMIGly is the most effective solvent for dissolving silk and preparing silknanocomposite solutions. Thermal stability of the B.mori Silk enhanced by adding carbon nanotubes to it.

Energy storage/conv ersion devices

[95]

Fiber diameter decrease from 1.60µm to 0.3µm

Metal

-

[87]

ion

[96]

triacetate (CTA)

ning

polymer solution

-

Cellulose

1-ethyl-3Electrospin methylimidazolium acetate ning [C2mim][OAC]

Chitin thermally dissolved in IL

-

-

Cellulose and Chitosan

Lignocell ulosic Biomass

Poly(viny lidene fluoride) (PVDF)

1-ethyl-3Electrospin methylimidazolium acetate ning [EmIm][Ac]

1-ethyl-3Electrospin methyllimozoliuem acetate ning [C2mim][OAC]

1-ethy1-2methylimidazolium bis(trifluoromethanesulfony l)amide [C2mim][NTf2]

Electrospin ning

Used to dissolve the nonderivatized cellulose and chitosan, then removed by ethanol. Dissolved the treated and untreated hemps.

-

Incorporated into PVDF

-

-

-

-

-

-

41

by adding 4wt% Aliquat 336. Nanofiber structure become dense as the Aliquat concentration increased. Cadmium (II) extraction rate increased with the Aliquat 336 content increased. Using pure ionic liquid [C2mim][CH3CO2] as solvent, cellulose nanofibers fabricated and the average diameters within 470±110nm. Thermal stabilities of electrospun fibers have higher thermal stability compared to cellulose casting film and slightly lower (~10K) than that raw cellulose. Highly viscose and non-volatile IL allows the fibers to have mostly micron-sized diameters. Chitosan-cellulose composite microfibrils create extensive surface area which allowing large amount water holding capacity. It has antibacterial effects against E. coli, affording 65% and 36% bacterial resistance at pH 6 and 7. The lignin content lower than 4.5% produced the most stable jet and showed the best spinnability. Lower lignin content solution produced finer and uniform micrometer-sized fiberwebs. Electrospinning was stable and smooth and beadles’ fiber ontained with dual fiber distribution for the IL concentration ≤10wt%. Average fiber diameter decrease as the highest

removal

-

Wound dressing application.

[97]

[98]

-

[99]

Biomedical Applications

[100 ]

Poly(viny lidene fluoride) (PVDF)

1-butyl-3methylimidazolium hexaflurophosphate [BMIM][PF6]

Electrospin ning

Mixing solvents.

in

-

-

-

Electrosp 1-ethyl-3-methimidazolium un acetate [EMIM][Ac] Polyvinyl idene fluoridecohexafluor opropyle ne (PVDF/H FP) and cellulose

Electrospin ning

Synthetic 1-ethyl-3Electrospin wood methylimidazolium acetate ning (cellulose [EmIm][Ac]

IL used for cellulose regeneration and PVDFHFP membrane coated with cellulose in IL.

-

Solvents for synthetic wood and Poly(3-

-

-

-

42

(10wt%) ionic liquids present in the polymer solution. All PVDF with 5 and 10wt% Ionic Liquids were not cytotoxic to C2C12 cells. The mean fiber diameter increase and rough fiber surface obtained by incorporated of the IL the PVDF/IL composite nanofibers. With increasing the IL incorporation concentration, irregular beads along the fibers disappeared. Better stretchability and higher electrical conductivity due to the high beta content of the PVDF/IL electrospun. The cellulose matrix fills inside the PVDF-HFP membrane pores (0.3µm) and reduce the its porosity (35%). Resulted membrane has higher mechanical strength as the elastic modulus increased from 17MPa to 54MPa as well as the tensile strength improve from 5.5MPa to 8.6MPa. The hydrophobic PVDF-HFP transform to superhydrophilic as the regenerated cellulose has highly amorphous structures which readily uptake water and it was successfully applied for selective separation of water from oil with efficiency up to 99.98%. Micron-sized fibers fabricated successfully. The synthetic wood/ Poly(3-octylthiophene) fiber possesses good luminescent characteristic

Micro- and nanoscale electronic device applications.

[101 ]

Oil/Water Separation

[102 ]

Biotechnolo gical applications

[103 ]

/xylan/lig nin) and Poly(3octylthio phene) as additive

octylthiophene )

as attribute the surface plasmon (SP) resonance in the light-emintting Poly(3-octylthiophene).

43

for optical sensors during the healing process.

6.0

Conclusion

This article reports the ionic liquids and electrospinning techniques which has been extensively explored by researcher. The unique properties of electrospun are the most influential factors which contribute for the heavy metal removal applications. On the other hand, the green character of bio-based ionic liquids as well as its stability, non-volatility and adjustable miscibility and polarity are beneficial for the heavy metal removal too. Experimental trials are needed to ensure the effectiveness of electrospun membrane with bio-based ionic liquids incorporation towards the waste water heavy metal removal.

44

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Typically provide comprehensive overview related to electrospinning and amino-acid IL. Reviewing their multifunctional properties, fabrication method and various application. Explore possible electrospinning and amino-acid IL combination for heavy metal removal.

Conflict of interest

The authors declare that they have no conflict of interest.