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Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review Yidong Zou, Xiangxue Wang, Ayub Khan, Pengyi Wang, YunHai Liu, Ahmed Alsaedi, Tasawar Hayat, and Xiangke Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 22, 2016

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Page 1 of 48

Environmental Science & Technology

1

Environmental Remediation and Application of Nanoscale Zero-Valent Iron and

2

Its Composites for the Removal of Heavy Metal Ions: A Review

3

Yidong Zou1,2, Xiangxue Wang1,3, Ayub Khan1, Pengyi Wang1, Yunhai Liu2, Ahmed

4

Alsaedi4, Tasawar Hayat4,5, Xiangke Wang1,3,4*

5

1. School of Environment and Chemical Engineering, North China Electric Power

6

University, Beijing 102206, P. R. China

7

2. School of Chemistry, Biological and Materials Sciences, East China Institute of

8

Technology, Nanchang, 330013, P. R. China

9

3. Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher

10

Education Institutions P.R. China

11

4. NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah

12

21589, Saudi Arabia

13

5. Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

14 15

*:

Corresponding

author.

Email:

[email protected]

16

+86-10-61772890; Fax: +86-10-61772890.

17

1

ACS Paragon Plus Environment

(X.

Wang),

Tel:


18

TOC

19 20

2


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Page 3 of 48


21

ABSTRACT: The presence of heavy metals in the industrial effluents has recently

22

been a challenging issue for human health. Efficient removal of heavy metal ions

23

from environment is one of the most important issues from biological and

24

environmental point of view, and many studies have been devoted to investigate the

25

environmental behavior of nanoscale zero-valent iron (NZVI) for the removal of toxic

26

heavy metal ions, present both in the surface and underground wastewater. The aim of

27

this review is to show the excellent removal capacity and environmental remediation

28

of NZVI-based materials for various heavy metal ions. A new look on NZVI-based

29

materials (e.g. modified or matrix-supported NZVI materials) and possible interaction

30

mechanism (e.g. adsorption, reduction and oxidation) and the latest environmental

31

application. The effects of various environmental conditions (e.g. pH, temperature,

32

coexisting oxy-anions and cations) and potential problems for the removal of heavy

33

metal ions on NZVI-based materials with the DFT theoretical calculations and

34

EXAFS technology are discussed. Research shows that NZVI-based materials have

35

satisfactory removal capacities for heavy metal ions and play an important role in the

36

environmental pollution cleanup. Possible improvement of NZVI-based materials and

37

potential areas for future applications in environment remediation are also proposed.

38

3



39

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1. INTRODUCTION

40

Rapid growth of industrialization, results in the high demand of metals for their

41

applications in various products, such as storage batteries pigments, automobile fuels,

42

photographic films, various explosives, coating materials, aeronautics and steel

43

industries

44

environmental pollution and attracted considerable attention due to their potential

45

hazards to human safety and the environmental stability

46

traditional organic pollutants, heavy metal ions, such as Cr(VI), Pb(II), Fe(III), Cu(II),

47

Zn(II), Hg(II) and Ni(II), are difficult to degrade into cleaning products 5, which

48

accumulated in living organisms and most of those are known to be highly toxic or

49

carcinogenic

50

cellular processes, kidney, brain functions and liver via its progressive accumulation

51

and multiple-toxicity

52

safety, effectively separate and enrichment undesirable heavy metal ions from

53

environment is still a very significant but challenging project for environmental

54

pollution control.

1-5

. Apart from their goodness, heavy metal ions are also responsible for

6, 7

. Compared with

3, 8-10

. For example, lead poisoning can cause severe injuries to basic

11-14

. Thus, in order to maintain ecological stability and public

55

During the past few decades, a variety of conventional and modern methods

56

including chemical precipitation 15-17, electrochemical treatment 16, electro dialysis 16,

57

evaporative recovery

58

oxidation/reduction

59

membrane

60

contaminated water. However, most of these processes suffer from various drawbacks,

61

for example, chemical precipitation is comparably simple and reliable but requires

62

high installation cost for its large tanks to obtain the effective precipitation. Of all the

63

known methods, adsorption technique has been regarded as a simple and effective

64

tool for the enrichment of heavy metal ions from wastewater owing to its wide

65

adaptability, environment-friendly and low cost

66

remediation, adsorption method has been extensively adopted to remove pollutants

67

(e.g., organic and inorganic pollutants) from aqueous solutions 40, 41.

68

20, 35

18

, solvent extraction

23-25

, reverse osmosis

19

, ultra filtration

26

, filtration

20

, ion-exchange

27

, adsorption

28-34

21, 22

,

and

technologies have been proposed to separate heavy metal ions from

36-39

. Especially in environmental

To the best of our knowledge, many researches have been focused on carbon 4


Page 5 of 48


5, 6

, clay minerals

42, 43

nanotubes

70

metal oxides (NMOs)

71

carbon (OMC)

72

metal ions from environment. Nanoscale zero valent iron (NZVI), an environmentally

73

benign material 40, 41, has been used successfully to treat various metal ions in aqueous

74

solutions (e.g., Pb(II), Ba(II), Zn(II), As(III), Cr(VI), As(V), Cu(II), Cd(II) and Co(II))

75

51-53

76

sites 51, 52, 54, 55. Besides, NZVI has also been applied for the stabilization of biosolids

77

56, 57

78

61-65

79

the treatment of nuclear waste 54, atrazine 68 and herbicides 69. NZVI has showed great

80

potentials to reductive transformation of heavy metal ions

81

the sufficient mobility, excellent reactive longevity and low toxicity of NZVI

82

Unfortunately, tiny particle size and powder state restricts the direct application of

83

NZVI, and the intrinsic characteristics of NZVI to react with surrounded media or

84

agglomerate during preparation process decreases the reactivity of the nanoparticles

85

and also results in poor mobility and successful transport of NZVI to the

86

contaminated area for the continuous in-situ remediation

87

agglomeration of iron particles in fixed bed column or any other dynamic flow system

88

results in high-pressure drop, thus restricting the direct use of NZVI for field scale

89

application. To address this issue, NZVI-based materials, including surface modified

90

NZVI (SM-NZVI), porous material supported NZVI (e.g., resin

91

nanoscaled magnesium hydroxide (Mg(OH)2)

92

multiwalled carbon nanotubes

93

kaolinite

80

94

bentonite

84, 85

95

pollutants from environment. Although NZVI-based materials as an efficient

96

adsorbent for separation and enrichment of heavy metal ions from aqueous solutions

97

has been investigated extensively, the review about the mechanism and application of

98

NZVI-based materials in environmental pollution cleanup is still scarce.

28

, layered double hydroxides (LDHs)

43-45

69

9, 45

, activated carbon 46, carbon film

and graphene oxides

49, 50

, nanosized

47, 48

, ordered mesoporous

as efficient adsorbents to separate heavy

, via its controllable particle size, high reactivity and abundant reactive surface

, the removal of tetracycline

58

, and the decolouration of dyes

, zeolite

81

, clay

79

59, 60

, antibiotic metronidazole

, nitrate pollution

55, 66, 67

, as a membrane anti-fouling agent and for

70, 71

, which is attributed to 41

.

72, 73

. Moreover, the

67

76

, silica

, activated carbon

77, 78

74, 75

,

and

) and inorganic clay mineral supported NZVI (e.g.,

65

, montmorillonite

82

, rectorite

83

, palygorskite and

) have been successfully synthesized as efficient adsorbents to separate

5



Page 6 of 48

99

This review presents a brief view on several typical pristine NZVI, surface

100

modified NZVI, porous material supported NZVI and inorganic clay mineral

101

supported NZVI, including their synthesis, applications and adsorption behavior of

102

heavy metals (e.g., Pb(II), Cd(II), Ni(II), Cr(VI) and Cu(II) etc) from water system

103

under varying experimental conditions, also the underlying mechanism responsible

104

for the environmental behavior, as well as their reusability and selectivity during

105

environmental in-situ remediation. Furthermore, the various kinds of surface modified

106

NZVI and substrates supported NZVI were briefly introduced according to their

107

various physicochemical properties and adsorption mechanism. In addition, similar

108

adsorbents were compared and summarized for their adsorptive performance on heavy

109

metal ions from natural environment.

110

2. SYNTHESIS OF NZVI AND ITS COMPOSITES FOR HEAVY METAL

111

REMOVAL

112

2.1. Pristine NZVI. In the past two decades, NZVI has attracted a great attention

113

as a promising reactant for reductive removal of various environmental contaminants

114

86, 87

115

pesticides

69, 92, 95, 96

116

pollutants

23, 51

117

produced by the reduction of dissolved iron using sodium borohydride solutions or by

118

milling or grinding processes 100.

(e.g., chlorinated solvents , organic dyes

78, 88-92

, trichloroethylene

71, 93, 94

, organochlorine

55, 66, 67, 84, 97, 98

, decabromodiphenyl 99 and inorganic

) from agricultural and industrial wastewaters, which can be readily

119

The traditional method for pristine NZVI synthesized by the liquid phase

120

reducing method, and the detail process was showed in Figure S1 (See Supporting

121

Information)

122

methods was introduced to produce NZVI by electroplating iron particles and to

123

remove the nanoscale iron particles into the solution instantaneously

124

particle was plated on the cathode by putting ferric chloride in solution to reduce the

125

ferric ion to iron particle. Compared with the common methods, the new technology

126

releases the strong reducing agent or effective oxidants

127

sodium and boron in the solvents. For the application in environmental pollution

128

management and environmental in-situ remediation, the synthesis of pristine NZVI

101

. A novel technology combining electrochemical and ultrasonic

103

6


102

. The iron

and reduces the use of

Page 7 of 48


129

should be inexpensive, convenience, efficient in large-scale and simple. Therefore, not

130

all these methods are practical for environmental applications. To date, only a few

131

studies have investigated the synthesis of pristine NZVI for environmental

132

remediation applications. Boparai et al.

133

addition of NaBH4 aqueous solutions to FeCl3 solution with continuous stirring to

134

prepare pristine NZVI, and applied it to remove Cd2+ ions from aqueous solutions. In

135

a similar manner, Ramos et al.

136

sodium borohydride reduction of ferric iron and it showed that NZVI was an effective

137

adsorbent for the removal of arsenic from wastewaters. In general, pristine NZVI

138

could apply to remove and recycle many kinds of heavy metals and environmental

139

in-situ remediation through reduction and oxidation processes.

23

52

used the ‘bottom-up’ method of drop wise

reported that the pristine NZVI was prepared from

140

2.2. Surface Modified NZVI. Pristine NZVI tends to aggregate rapidly due to

141

the weak Van der Waals forces, high surface energy and intrinsic magnetic

142

interactions 53, 95, making it difficult for NZVI to interact with target contaminants and

143

difficult to disperse in water for the heavy metal ions management, which limits their

144

practical applications. To improve its high dispersion and prevent aggregation of

145

NZVI, different kinds of surface modified NZVI have been synthesized by adding

146

functional groups on the surfaces with chemical modification (covalent or

147

non-covalent functionalization)

148

polymer or surfactant

149

technology or metal-doped methods (e.g., Pd, Pt, Ni, Ag and Cu)

150

physically more stable and chemically more reactive NZVI could be obtained, and the

151

removal ability of pollutants was also enhanced.

78, 104

or attaching a stabilizer such as a soluble

64, 105, 106

. Through various surface chemical modification 40, 105-107

, the

152

Many researchers have focused on coating iron nanoparticles with stabilizer via

153

steric hindrance and electrostatic repulsion. Various polyelectrolyte coatings have

154

been studied with varying success 108-110, including butyl methacrylate, caboxymethyl

155

cellulose, 4-styrenesulphonate, polyacrylic acid

156

cellulose

157

acetate-co-itaconic

158

These surface stabilizers can be coated onto the surface of pristine NZVI to provide

105, 106

, polystyrene sulphonate,

98

, polymethylmethacrylate, polyaspartate, polyvinyl alcohol-co-vinyl 109

, chitosan

111

, triblock copolymers

75

7


and xanthan gum

104, 112

.


Page 8 of 48

159

electrostatic repulsion, and steric or electrosteric stabilization, which could decrease

160

the aggregation and increase the mobility of NZVI for environmental heavy metal

161

ions in-situ remediation.

162

All reported modifiers, including various polymer electrolytes and surfactants,

163

such as chitosan, xanthan gum, cetyltrimethyl ammonium bromide (CTAB), and

164

carboxy methylated cellulose (CMC), has been applied to modify the pristine NZVI to

165

improve its dispersion and stability and to enhance its removal ability of some various

166

heavy metal ions from natural environment

167

3-aminopropyltriethoxysilane (APS) as an effective microemulsion to synthesize

168

amino-functionalized NZVI, and this material could rapidly separate Pb(II) from

169

water system and easily recycled by an external magnetic field, which might be

170

suitable for heavy metal remediation and had potential industrial applications. Su et al.

171

113

172

NZVI served as efficient adsorbents to remove Cd(II) from aqueous solutions, and the

173

synthetic process and reaction process were shown in Figure S2. In general, metal

174

doped NZVI could reduce the aggregation and increase the reactivity of pristine NZVI.

175

Furthermore, Zhao et al. 109 and Dong et al. 114 adopted different stabilizers (polyvinyl

176

alcohol (PVA), polyacrylic acid (PAA), Tween-20 and starch) to modify the pristine

177

NZVI due to their low cost and environmental compatibility, and this kind of

178

stabilizer can not only control the particle size during the Fe0 formation process, but

179

also prevent the agglomeration of the NZVI nanoparticles in liquid phase. Thus, the

180

size and reactivity of NZVI are significantly improved, which has been applied to

181

remove heavy metal ions (e.g., Cu(II), Sb(III) and Sb(V)) from wastewater, and the

182

results showed high efficiency and reaction rate for the remediation of heavy metal

183

ions.

108-110, 113, 114

. Liu et al.

110

used

successfully demonstrated the Au doped NZVI, Cu doped NZVI and Ag doped

184

Introducing modifiers or stabilizers to pristine NZVI has also improved the

185

surface oxygen-containing functional groups, such as epoxide, hydroxyl and carboxyl,

186

and these oxygen-containing functional groups could form strong complexes with

187

heavy metal ions, and allow the various surface modified NZVI to act as adsorbents

188

for heavy metal ion preconcentration and elimination, which could create the 8


Page 9 of 48


189

beneficial conditions to environmental in-situ remediation and cleanup.

190

2.3. Porous Material Supported NZVI. Recently, there is great interest in the

191

synthesis of porous material supported NZVI because of their high surface area, gap

192

structure and unique property. They can also provide stable sites for pristine NZVI

193

nanoparticles loading to prevent their oxidation and aggregation. Researchers

194

immobilized NZVI on porous solid support material, such as PVDF membrane, resin

195

67

196

green-tea

197

multiwalled carbon nanotubes

198

reduced graphene oxide 118 to restrain the aggregation of NZVI and applied the porous

199

material supported NZVI particles to remove pollutants from aqueous solutions. In

200

these porous material supported NZVI, the supported materials on NZVI surface

201

prevent the aggregation of the nanoparticles to some extent, which thereby increases

202

the surface area and stable sites for the removal of pollutants from environment.

, polystyrene resins

64

, silica, nanoscale magnesium hydroxide

116

, activated carbon

77, 78

, mesoporous carbon

115

, carbon

117

, carboxymethyl cellulose

79

76

,

, cellulose acetate

48

, mesoporous silica

99

93

,

and

203

Among these composites, graphene family nanomaterials (GFNs) supported

204

NZVI have attracted the most research interest due to the superior physicochemical

205

properties of GFNs

206

adsorption capacity for Cr(VI) ions (up to 162 mg·g-1) than the bare NZVI (148

207

mg·g-1). In 2016, Li et al.

208

synthesis of graphene-oxide nanosheets supported NZVI (NZVI/GNS), which had an

209

effective adsorption ability for in-situ remediation of Cr(VI)-polluted water, and the

210

adsorption capacity was achieved to 21.72 mg·g-1. Interestingly, the reduced graphene

211

oxide supported NZVI (NZVI/rGOs) was also developed by a hydrogen/argon plasma

212

reduction method to increase the reactivity and stability of NZVI, and the

213

microstructure characteristics of NZVI/rGOs have been reported by our groups in

214

Figure 1

215

removal capacities for Cr(VI) (187.16 mg·g-1) and Pb(II) (396.37 mg·g-1)

216

same time, the as-prepared NZVI/rGOs had been reported to remove U(VI) ions

217

under anoxic conditions, and the adsorption tendency of U(VI) on NZVI/rGOs was

218

similar to that on bare NZVI, achieving an equilibrium capacity of 4174 mg·g-1

119, 120

. According to Jabeen’ reported

119

120

, G-NZVI had a higher

reported a chemical reduction technology for the

121

. The NZVI/rGOs showed excellent water treatment performance with

9


118

. At the

122

.


123

Page 10 of 48

219

Wang et al.

also used NZVI/rGOs to remove As(III) and As(V) from wastewater,

220

and the adsorption capacities of As(III) and As(V) calculated from the Langmuir

221

model were 35.83 mg·g-1 and 29.04 mg·g-1, respectively. The porous material

222

supported NZVI can effectively prevent the agglomeration of NZVI nanoparticles by

223

the electrostatic repulsion or steric hindrance, and it can also increase the reactive

224

active sites and improve the processing efficiency of NZVI, thus improving the

225

interaction activity with various heavy metal ions.

226 227

Figure 1. SEM images of NZVI particles (A) and NZVI/rGOs (1:4) (B); TEM images of the

228

NZVI particles (C) and NZVI/rGOs (1:4) (D); HRTEM of NZVI/rGOs (1:4) (E) and the

229

corresponding EDS pattern (F). Reproduced with permission from [121], Copyright 2016,

230

Elsevier.

231

2.4. Inorganic Clay Mineral Supported NZVI. To improve the stability and

232

increase the reaction active sites of NZVI, inorganic clay mineral supported NZVI

233

materials have been developed rapidly and given great opportunity to environmental

234

in-situ remediation. Common inorganic clay mineral and its composites such as

235

kaolinite

236

organobentonite

237

bentonite

80

124

, multifunctional kaolinite

84, 85, 97

68, 91, 96

, rectorite

, zeolite

81

, montmorillonite

83

, calcium polysulfide

82

, clay

65

,

116

, palygorskite and

have been used to restrain the aggregation of NZVI and applied to 10


Page 11 of 48


238

remove various pollutants from natural environment. Clay is an abundant natural

239

resource and is also a suitable supporting material due to its potential applicability in

240

pollutants' adsorption and various composites. Compared with porous material

241

supported NZVI, inorganic mineral can not only decrease the aggregation and

242

improve the dispersion as well as stabilization, but also interact with various natural

243

pollutants and prevent the migration and accumulation of pollutants.

244

In pollutant removal, matrix material supported NZVI, especially the inorganic

245

clay mineral supported materials, have attracted the most research interest and been

246

used as efficient adsorbents to remove various heavy metal ions from wastewater.

247

According to the works of Shahwan’ group

248

(NZVI-kaol) was prepared and applied to remove Cu2+ and Co2+ ions from wastewater,

249

and it showed that the sizes could be adjusted between 10 and 80 nm. In order to

250

explore the various adsorption mechanism, the montmorillonite-supported NZVI

251

(Mt-NZVI) was synthesized by borohydride reduction method

252

showed high adsorption capability towards inorganic arsenic in aqueous solutions. It

253

demonstrated that Mt-NZVI upon reaction with water and oxygen formed a number of

254

iron corrosion product and provided new adsorption sites for As(III) and As(V), and

255

then increased the adsorption capacity for arsenic from aqueous solutions.

13,

80

, kaolinite-supported NZVI

82

, and Mt-NZVI

256

Various kinds of NZVI-based materials have shown great adsorption capacity for

257

heavy metal ions from environment, and the main adsorption parameters was

258

summarized in Table 1. The specific surface area of NZVI is an important role to

259

influence the adsorption capacity for various heavy metal ions, and in general, the

260

adsorption capacity of NZVI-based materials can depend on basic physicochemical

261

properties of adsorbents and experimental conditions, such as pH, ionic strength,

262

temperature, concentration of adsorbents and adsorbates. According to the results of

263

many researches (Table 1), the adsorption capacity and physicochemical property

264

improved obviously with various complex methods and stabilizers, which might due

265

to the stability, surface active sites and oxygen-containing groups. It indicated that

266

various NZVI-based materials could be used through complex with various stabilizers

267

in practically environmental heavy metal ions’ remediation. 11



Page 12 of 48

Table 1. Adsorption Capacities and Mainly Parameters of Various Heavy Metal Ions Interaction with NZVI-Based Materials

268

Adsorbents

Raw materials

Target metals

BET (m2·g-1)

Optimum pH

Initial C[tareget] (mg·L-1)

Adsorption capacity (mg·g-1)

Kinetic model

Bare NZVI

FeCl3·6H2O + NaBH4

As(V)

33.5

7.0

11.1

38.2

n.a.

Bare NZVI

FeCl3·6H2O + NaBH4

As(III)

24.4

7.0

1.0

3.5

pseudo-first-order

Bare NZVI

FeCl3·6H2O + NaBH4

As(V)

25

7.0

1.0

1.0

pseudo-first-order

n.a.

127

Bare NZVI

FeCl3·4H2O + NaBH4

Ba(II)

n.a.

7.0

13.7

1.1

pseudo-second-order

D-R isotherm

128

Bare NZVI

FeCl2·4H2O + NaBH4

Cu(II)

n.a.

6.5

50

250

n.a.

n.a.

13

Bare NZVI

FeCl2·4H2O + NaBH4

Co(II)

14.2

8.0

800

172

n.a.

n.a.

129

Bare NZVI

FeCl3 + NaBH4

Cd(II)

n.a.

7.0

112

769.2

pseudo-second-order

Langmuir isotherm

52

Bare NZVI

Nano iron

Cr(VI)

n.a.

5.0

15.6

47.2

n.a.

n.a.

130

Amino-functionalize d NZVI (APS-NZVI)

FeCl3·6H2O + CTAB

Pb(II)

n.a.

7.0

100

111

n.a.

NZVI-Fe3O4

FeCl3·6H2O + CTAB FeSO4·7H2O

Cr(VI)

n.a.

8.0

20

29.43

pseudo-second-order

Sineguelas waste-modified (S-NaOH-NZVI)

FeCl2·4H2O + S-NaOH

Pb(II)

35.56

6.5

1000

266

pseudo-second-order

12


Isotherm model Langmuir isotherm Freundlich isotherm

Freundlich isotherm Langmuir isotherm Freundlich isotherm

Refs

125 126

110 131

12

Page 13 of 48


Coated NZVI

FeSO4·7H2O + KBH4

Pb(II)

n.a.

5.0

100

100

n.a.

n.a.

132

Au doped NZVI

FeCl3·6H2O + NaBH4

Cd(II)

n.a.

8.5

40

188

n.a.

n.a.

113

PVP modified NZVI

FeSO4·7H2O + NaBH4

Sb(III)/Sb(V)

11.3

7.0

5.0

6.99/1.65

pseudo-second-order

Langmuir isotherm

109

FeSO4·7H2O + NaBH4

Cr(VI)

n.a.

5.5

20

66.7

pseudo-first-order

n.a.

133

FeCl3·6H2O + NaBH4

As(III)

69.4

6.5

2.0

18.2

n.a.

Langmuir isotherm

134

FeSO4·7H2O + NaBH4

Cr(VI)

n.a.

5.5

10

33

n.a.

n.a.

117

G-NZVI

FeCl3·6H2O + NaBH4

Cr(VI)

170

4.25

n.a.

162

pseudo-second-order

Langmuir isotherm

120

MWCNT-NZVI

FeSO4·7H2O + NaBH4

Cr(VI)

n.a.

7.0

20

200

pseudo-first-order

n.a.

135

MWCNT-reinforced nanofibrous mats-supported NZVI

FeCl3·6H2O + NaBH4

Cu(II)

n.a.

5.5

50

107.8

pseudo-second-order

Langmuir isotherm

136

G-NZVI

FeCl3·6H2O + NaBH4

Pb(II)

n.a.

5.0

250

585.5

pseudo-second-order

Freundlich isotherm

137

Magnetic Fe3O4/graphene -supported NZVI

FeCl3·6H2O + FeSO4·7H2O

Cr(VI)

n.a.

8.0

100

66.2

pseudo-first-order

Langmuir isotherm

138

NZVI/rGO

FeSO4·7H2O + NaBH4

As(III)/

n.a.

7.0

7.0

35.83/29.04

pseudo-second-order

Langmuir

123

Ultrasonic modified NZVI (US-NZVI) Activated carbon supported-NZVI (NZVI/AC) Carboxymethyl cellulose supported

13



Page 14 of 48

As(V)

isotherm

NZVI/rGO

FeCl3·6H2O + NaBH4

Cd(II)

117.97

5.0

150

425.72

pseudo-second-order

n.a.

121

NZVI/rGO

FeCl3·6H2O + NaBH4

Cr(VI)/ Pb(II)

117.97

5.0

100

187.16/396. 37

n.a.

Langmuir isotherm

118

Mg(OH)2-supported NZVI

FeSO4·7H2O + NaBH4

Pb(II)

40.2

6.86

1000

1986.6

n.a.

n.a.

76

Langmuir/ Freundlich isotherm Langmuir isotherm

Cellulose-supported NZVI

FeCl3 + NaBH4

Cr(VI)

9.55

3.0

10

562.8

pseudo-second-order

G-NZVI

FeCl3·6H2O + NaBH4

Cr(VI)

n.a.

7.0

20

21.72

pseudo-first-order

NZVI-kaol

FeCl2·4H2O + NaBH4

Cu(II)/ Co(II)

6.7

6.0

39/18

49/65

n.a.

n.a.

80

Bentonite-supported NZVI

Fe2O3 + NaBH4

Cr(VI)

39.94

6.0

73

7.3

pseudo-first-order

n.a.

140

NZVI-kaol

FeCl3·6H2O + NaBH4

Pb(II)

26.11

5.1

500

48

n.a.

n.a.

141

Zeolite-supported NZVI

Fe(NO3)3·9H2O + NaBH4

Pb(II)

29.1

5.5

200

105.5

pseudo-second-order

NZVI-kaol

FeCl3·6H2O + NaBH4

Cu(II)/ Ni(II)

40.76

7.0

50

33.74/32.25

pseudo-second-order

Mt-NZVI

FeCl3·6H2O + NaBH4

Zn(II)/ Pd(II)

n.a.

5.0

50

30.2/29.0

pseudo-second-order

Sepiolite-supported NZVI

FeCl3·6H2O + NaBH4

Cr(VI) / Pd(II)

141.42

6.0

50

43.86/44.05

pseudo-first-order

269

* n.a.: not application. 14


Langmuir isotherm Langmuir isotherm D–R isotherm Langmuir isotherm

139

119

142 143 144 145

Page 15 of 48


270

According to literature survey, most inorganic clay mineral supported NZVIs are

271

used for the separation of heavy metal ions 76, 80-82, 143, 146, 147. For instance, Uzum et al.

272

80

273

decreased the aggregation of iron nanoparticles and enhanced the activity of bare

274

NZVI to remove Cu2+ and Co2+ ions from aqueous solutions. This research indicated

275

that NZVI-kaol exhibited excellent removal abilities for Cu2+ and Co2+ ions from

276

wastewaters, and the various interaction mechanism could attributed to adsorption and

277

redox reactions, respectively. Cu2+ was mainly bonded with a redox reaction, and

278

transfer into Cu2O and Cu0, however, Co2+ was mainly bonded with an adsorption

279

process by various oxyhydrogen groups. Compared with the bare NZVIs, the clay

280

mineral supported NZVIs have lower aggregation property, higher efficiency, higher

281

sorption capacity and more stability in the removal of heavy metal ions from aqueous

282

solutions.

283

3. THE ROLE OF NZVI-BASED MATERIALS IN HIGHLY EFFICIENT

284

HEAVY METAL REMOVAL

reported a borohydride reduction method to synthesize NZVI-kaol, which

285

3.1. Interaction Mechanism between NZVI-Based Materials and Heavy

286

Metals. NZVI-based materials have been widely investigated for the treatment of

287

heavy metal ions and demonstrated tremendous potential and prosperous application

288

for the aggregation and in-situ remediation of heavy metal ions. However, the

289

interaction mechanism of contaminants with NZVI-based materials is still under

290

debate. Mechanistic study of heavy metal ions' adsorption by NZVI-based materials is

291

paramount in the explanation of the reaction process, which is beneficial for the

292

optimization of the adsorption conditions and desorption/regeneration conditions. The

293

mechanistic studies have been carried out with either the assistance of the DFT

294

theoretical calculation models, EXAFS technology or comprehensive experimental

295

observations on adsorption characteristics

296

mechanism of heavy metal ions on NZVI-based materials is summarized in Table 2.

297

In the various adsorption systems, heavy metal ions express various physiochemical

298

properties, and interaction with NZVI-based materials through physical & chemical

299

reactions, including adsorption, redox, aggregation, ion exchange, hydroxylation as

60, 148-150

. The possible interaction

15



Page 16 of 48

300

well as subsequent precipitation. While the main interaction mechanism could be

301

regarded as adsorption, reduction and oxidation process.

302

3.1.1 Adsorption Mechanism. Adsorption technology has been considered as a

303

typical reaction process for the removal of heavy metal ions due to the abundant

304

oxygen-containing functional groups on the surface of various adsorbents, and

305

NZVI-based materials has been confirmed with large amount of active sites or

306

functional groups

307

could attribute to adsorption reaction. Moreover, iron hydroxides or oxides could be

308

formed on the surface of NZVI-based materials, which could improve their adsorption

309

capacities in a Fe0-H2O system in the natural environment 60. Furthermore, adsorption

310

process has been considered to the main interaction mechanism and has been used to

311

explain various removal processes of heavy metal ions with NZVI-based materials.

312

Kanel et al.

313

Laser light scattering analysis demonstrated the formation of NZVI-As(III)

314

inner-sphere surface complexes. Li et al.

315

adsorption capacity to Cd(II), and the possible interaction mechanism is shown in

316

Figure 2(A), which indicates that the interaction is attributed to adsorption rather than

317

redox process. Han et al.

318

zero-valent aluminum (ZVAl) as reactive medium in PRBs to treat heavy metal

319

wastewater containing Cr(VI), Cu(II), Cd(II), Ni(II) and Zn(II) ions, and the possible

320

interaction mechanism is shown in Figure 2(B). The removal process was divided into

321

four interaction processes, namely reduction process, adsorption process, hydroxide

322

precipitates and electron transfer, however, the removal process was mainly

323

dominated by adsorption process. In compared to Lv et al.

324

the reported adsorption process of heavy metal ions on NZVI-based materials (the

325

XPS spectrum in Figure S3), this interaction mechanism could be complex. However,

326

in real applications, the reaction system could include many kinds of coexistent heavy

327

metal ions, and the complex interaction process should be considerable.

126

121, 123

. Herein, the possible mechanism of NZVI-based materials

applied pristine NZVI to remove As(III) from groundwater, and the

149

121

found that the NZVI/rGO showed great

reported that the mixture of acid-washed ZVI and

16


138

and Wang et al.

123

on

Page 17 of 48


328 329

Figure 2. (A) Possible schematic mechanisms and reaction process of Cd(II) removal by

330

NZVI/rGOs Reproduced with permission from [121], Copyright 2015, Elsevier, (B) Possible

331

mechanisms of the heavy metal ion removal by ZVI/ZVAl in PRBs (reduction process,

332

adsorption process, hydroxide precipitates and electron transfer). Reproduced with permission

333

from [149], Copyright 2016, Elsevier.

334

Recently, the adsorption mechanism studied by batch experiments, advanced

335

technologies in characterization and theoretical calculation models, which has

336

proposed more valuable reference basis for the application of NZVI-based materials

337

in environment remediation. The removal and remediation of heavy metal ions was

338

conducted in aqueous solutions, and the main solvent molecule, i.e., water, played an

339

important role in the adsorption process. Hence a further research could be discussed

340

between water and NZVI-based materials, and then the interaction process of heavy

341

metal ions could be understood. The interaction mechanism of heavy metal ions on

342

NZVI-based materials can also be theoretically modeled, and the main mechanism of

343

water with iron surface by (NZVI)/H2O system in an oxygen environment was

344

investigated with density functional theory (DFT) calculations

345

demonstrated DFT study of the adsorption process in water system at the (100) and

346

(111) surfaces of iron, which could interact with heavy metal ions, and the results of

347

DFT calculation was shown in Figure 3. It indicated that the adsorbed water molecule

348

could transfer into H + OH (H-Fe-OH) species with an activation barrier of 15.7

349

kcal/mol for the (100) surface, and the positive activation barrier was beneficial to the

350

adsorption process of heavy metal ions on NZVI-based materials.

17


151, 152

. Lazar et al.

151


351 352

Figure 3. Dissociation of water molecule on the Fe(100) surface calculated PW91 density

353

functional (black curve, dots) and HSE06 hybrid functional (red curve, dashed lines). The

354

arrows show the heights of energy barriers along the reaction path. Reproduced with

355

permission from [151], Copyright 2012, American Chemical Society.

356

3.1.2 Reduction Mechanism. Generally, for multivalent heavy metal ions, the

357

interaction is mainly attributed to the reduction process, and the reducing action of Fe0

358

in natural environment is multistep processes, where Fe0 acts as an electron donor to

359

many heavy metal ions

360

heavy metal ions and NZVI-based materials, influenced the valence and

361

physicochemical properties of heavy metal ions in aqueous solutions. The reduction

362

of NZVI-based materials for the heavy metals includes two distinct mechanism: 1) the

363

reduction of heavy metals by Fe0 directly

364

metals on the core-shell structure of NZVI and then gradually reduction of adsorbed

365

heavy metals by Fe2+ derived from NZVI

366

intermediate valence could be reduced to the lowest valence. However, for few

367

intermediate valent heavy metals, adsorption process and reduction by Fe2+ is also

368

possible.

66, 88

. Reduction reaction as an important interaction between

76

; 2) the primarily adsorption of heavy

85

. For most heavy metals, which has no

369

At the same time, for some multivalent elements, reduction process could

370

consist in most heavy metal ions' interaction with matrix in the natural environment,

371

and the biotoxicity of the initial valence heavy metal ions reduced and formed more

372

stable valence with the reduction process. Liu et al. 76 reported that a novel composite

373

(Mg(OH)2) supported NZVI (NZVI@Mg(OH)2) showed exceptional removal 18


Page 18 of 48

Page 19 of 48


374

capacity because of synergistic effect, which included at least three paths as shown in

375

Figure S4, and NZVI showed possible reduction process of Pb(II) to Pb0, and about

376

47% Pb0 was formed by reduction reaction. And the main reactions, including

377

reduction, adsorption and precipitation, are listed as Eqs. (1)-(5). Moreover, the

378

reduction mechanism has been confirmed through many experiments and

379

characterization. Zhou et al.

380

restored into Sb(0) by beta zeolite supported NZVI, which could be affirmed with

381

XPS analysis, and the reduction process controlled the reaction process and showed

382

great challenges to other stable metal elements. While for unstable metal elements,

383

reduction process could be useful and benefit to the in-situ remediation of heavy

384

metal ions from the natural environment. With the further research of the reduction

385

process about heavy metal ions on NZVI-based materials, EXAFS technology has

386

also been applied to characterize the products. Li et al.

387

charged pillared bentonite supported NZVI (NZVI/Al-bent) could improve the

388

reductive transformation and the removal of Se(VI) into less soluble Se(II) from

389

wastewater by Fe2+ and NZVI. And the EXAFS results (Figure S5) show that the shell

390

of Se-O bond is not obtained for the samples of Se(VI) reacted with NZVI/Al-bent,

391

indicating that the absence of adsorbed Se(VI) on the solid, and Se(VI) is completely

392

reduced with the reduction process.

150

investigated that more than 80% Sb(III) could be

85

reported that the positively

393

Mg(OH)2(s) → Mg2+ +2OH-

(1)

394

Pb2+ + 2OH- → Pb(OH)2(s)

(2)

395

Pb2+ + Fe0(s) → Fe2+ + Pb0(s)

(3)

396

Fe2+ + 2H2O → Fe(OH)2(s) + 2H+

(4)

397

Fe0(s) +2H2O → Fe2+ + H2(g) +2OH-

(5)

398

3.1.3 Oxidation Mechanism. In some special environmental remediation

399

systems, in contrast to the reduction process, oxidation process is also considered as a

400

possible react mechanism

401

controlled with the Fenton reaction in the presence of oxygen, and then some strong

402

oxidants are produced [as followed Eqs. (6)-(8)]

403

oxidization of heavy metal ions in aqueous solutions, but in usual reaction system of

69, 103

, such as the Fe0-H2O mixture system, and it has been

90

, which can improved the

19



Page 20 of 48

404

natural environment, the oxidation process could be rare in the removal of heavy

405

metal ions on NZVI-based materials.

406

Fe0 + O2 + 2H+ → H2O2 + Fe2+

(6)

407

Fe2+ + H2O2 → Fe3+ +·OH + OH-

(7)

408

2Fe3+ + Fe0 → 3Fe2+

(8)

409

Bhowmick et al.

82

prepared novel Mt-NZVI by the liquid phase reducing

410

method, and applied the Mt-NZVI for the elimination of both As(III) and As(V) from

411

aqueous solutions. During the reaction process, Fe(II) and Fe(III) could be formed by

412

Fe0 and solution (Eqs. (9)-(10)), and it can further produce various derivatives of iron.

413

As(III) could be oxidized partially to As(V), moreover, the oxidation process of Fe0

414

can also form various oxidizing medium in water system such as H2O2, HO0, O20-

415

[Eqs. (11)-(13)]

416

for various targets through forming various derivatives of iron, and it was beneficial

417

for the separation of heavy metal ions.

27, 82

. It indicated that the Fe0 could provide great oxidizing capacity

418

2Fe0 + O2 + 2H2O → 2Fe2+ + 4OH-

(9)

419

4Fe2+ + 2H2O + O2 → 4Fe3+ +4HO-

(10)

420

Fe2+ + O2 → Fe3+ + O20-

(11)

421

Fe2+ + O20- + 2H+ → Fe3+ + OH0 + HO-

(12)

422

Fe2+ + O20- + 2H+ → Fe3+ + H2O2

(13)

423

3.1.4 Other Special Interaction Mechanism. Generally, in mixing reaction

424

systems, various interactions could dominate the removal process due to various

425

environmental conditions. Occasionally, the main interaction mechanism is controlled

426

by magnetic interaction forces, Van der Waals forces, electrostatic interactions and

427

specific surface bonding. Wang et al.

428

aqueous solutions, and they found that the removal of As(III) was attributed to surface

429

complexation at pH < 9.1 and electrostatic interactions at pH > 9.1. NZVI-based

430

materials could also form amorphous precipitate with heavy metal in aqueous

431

solutions, Li et al 118 reported that NZVI/rGO exhibited excellent removal capacity for

432

Cr(III) with the forming precipitation of (CrxFe1-x)(OH)3 and CrxFe1-xOOH. In most

433

reaction process, synergistic process dominated the remediation process, for example,

123

used NZVI/rGOs to remove As(III) from

20


Page 21 of 48


76

434

Liu et al

used NZVI@Mg(OH)2 to remove Pb(II) from mixture solutions, and the

435

possible mechanism was shown in Table 2. It indicated that the interaction

436

mechanisms between adsorbents and target metals were attributed to adsorption,

437

reduction, ion exchange reaction, hydroxylation and even subsequent precipitation,

438

which is benefical for the aggregation of pollutants.

439

In fact, during the practical environmental application, adsorption process,

440

reduction process, oxidation process even other special interaction could be existed

441

among various materials. However, the main interaction could be attributed to one or

442

two kinds of interactions due to the environmental medium. Thus, the true interaction

443

mechanism between heavy metal ions and NZVI-based materials could be understood

444

via to real reaction conditions.

445

Table2. The pathways and mechanism of various heavy metal ion interaction with

446

NZVI-based materials Target

Adsorbents

Interaction mechanism or reaction

Ref

metals (Synergistic process) adsorption, reduction, ion exchange reaction, NZVI@Mg(OH)2

Pb(II)

76 hydroxylation and subsequent precipitation. Precipitation and reduction sorption. (1) 2HCrO4- + 3Fe0 + 14H+ → 3Fe2+ + 2Cr3+ + 8H2O;

NZVI/rGO

Cr(III)

(2) HCrO4- + 3Fe0 + 3H+ → 3Fe2+ + Cr3+ + 2H2O;

118

(3) (1-x)Fe3+ + xCr3+ + 3H2O → (CrxFe1-x)(OH)3↓+ 3H+; (4) (1-x)Fe3+ + xCr3+ + 2H2O → CrxFe1-xOOH↓+ 3H+ NZVI/rGO

Cd(II)

Adsorption (electrostatic interactions and specific surface bonding).

121

Bare NZVI

As(III)

Surface adsorption or formation of arsenic-iron co-precipitates.

153

Cu(II)

Reduction and the formation of Fe(III)-heavy metals co-precipitate.

102

Chitosan beads-supported NZVI Van der Waals forces, magnetic interactions forces, and/or surface Mt-NZVI

Zn(II)

144 complexation. 21



Page 22 of 48

Redox, adsorption, precipitation, and co-precipitation. (1) Fe0 + 2H2O + 1/2O2 → Fe2+ + 4OH- (basic solution); NZVI-kaol

(2) Fe0 + 2H2O → Fe2+ + H2 +2OH- (acid solution);

Ni(II)

143

(3) Ni2+ + Kaolinite → Ni2+ － Kaolinite (adsorption); (4) Ni2+ + FexOyHz → Ni2+ － FexOyHz (adsorption) Reduction and electron transport Bentonite-supported

(Fe0 oxidized to ferrous ions through anoxic corrosion or reductive

Se(VI)

85

NZVI dissolution of iron by Se(VI) under anaerobic conditions.) Beta

Sb(III) adsorbed on the surface of NZVI-zeolite, and reduced into Sb(0)

zeolite-supported

Sb(III)

immediately accompanied by the oxidation of NZVI. Fe(0) and iron oxides

NZVI

kept evolution, and the degree of oxidation was promoted.

447 448

3.2. Effect of Environmental Conditions. In general, the adsorption behavior of

449

heavy metal ions on NZVI-based materials can be described by batch experiments in

450

various conditions, including solution pH, contact time, dosage of adsorbent,

451

temperature, coexisting oxy-anions and cations. Due to the importance of heavy metal

452

in environment, evaluation of the migration, transfer, accumulation and environmental

453

effect of heavy metal ions in wastewater is important in environmental pollution

454

in-situ remediation.

455

Among these parameters, pH values could affect the surface properties of

456

materials and species of the molecules or irons. The pH values of the aquatic

457

environment generally ranges from 5.0 to 9.0, and the adsorption of heavy metal ions,

458

such as Pb(II), Cu(II), Cr(VI) and Ni(II) by NZVI-based materials may conduct in the

459

pH range of 5.0-9.0 141, 146, which can ensure the consistency of natural conditions. In

460

the lower pH or higher pH values, the adsorption capacity of heavy metal ions on

461

NZVI-based materials tends to be decreased sharply as confirmed in 82, 120, 147 and the

462

pH-dependent behavior can be explained by ionization process between adsorbates

463

and adsorbents, which lead the repulsion effect and decrease the net target adsorption.

464

At lower pH values, the iron corrosion increased and produced abundant hydrogen, 22


150

Page 23 of 48


465

which is beneficial for hydrogenation reaction. At higher pH values, passive film of

466

iron hydroxide could be formed with the iron corrosion in solution, which exhibited

467

further adsorption process

468

oxygen-containing groups (e.g., oxide or oxyhydroxide) become deprotonated and the

469

surface becomes negatively charged as pH increases, which is favorable for adsorbing

470

more heavy metal ions via electrostatic interactions. It assumes that the adsorption

471

properties of NZVI are influenced strongly by solution pH.

154,

155

. A plausible explanation is that more

472

The interactive effects of redox and adsorption process for heavy metal ions on

473

NZVI-based materials could be described by kinetic models, and the common models

474

including pseudo-first-order kinetics and pseudo-second-order kinetics. For example,

475

bare NZVI could remove about 40% of Cr(VI) in 60 min from aqueous solutions, and

476

the NZVI/GNS showed a higher adsorption capacity for Cr(VI) (70% within 60 min),

477

and the maximum adsorption capacity was calculated to be 20 mg·g-1 (C[initial] = 25

478

mg·L-1)

479

NZVI/GNS could also be fitted by the pseudo-first-order kinetics model, and the

480

activity toward Cr(VI) removal by the NZVI/GNS was about 2.8 times as high as that

481

of the bare NZVI, which indicated that the adsorption process was mainly attributed

482

to physical interaction process because of the high sorption capacity of GNS. On the

483

contrary, the adsorption process of Cr(VI) ions on Mt-NZVI was well fitted by the

484

pseudo-second-order kinetics, and 95% Cr(VI) could be removed by the composites

485

within 30 min, and the maximum adsorption capacity was calculated to be 145 mg·g-1

486

(C[initial] = 20 mg·L-1), which showed that the process was attributed to chemical

487

interaction involving valence forces

488

aqueous solutions over the NZVI/GNS and Mt-NZVI is shown in Figure 4 119, 156, and

489

the removal of Cd2+ by NZVI/rGOs was also fitted better with pseudo-second-order

490

kinetics, which were shown in Figure S6

491

velocity showed that strong chemisorption, redox interaction or strong surface

492

complexation contributed to the coagulation of heavy metal ions onto the surface of

493

NZVI-based materials which was important for the application of the NZVI-based

494

materials to remove pollutants from natural environment.

119

. Interestingly, the Cr(VI) removal processes using bare NZVI and

131

. The kinetics removal behavior of Cr(VI) in

121

. At the same time, the fast adsorption

23



A

B

495 496

Figure 4. (A) Comparison of various Fe materials with the same amount of Fe0 = 0.016 g in

497

Cr removal. Reproduced with permission from [156], Copyright 2013, Elsevier; (B) Removal

498

of Cr(VI) in aqueous solutions over the NZVI/GNS with different GNS amounts; dash lines is

499

the corresponding pseudo-first-order kinetics fitting curves. Reproduced with permission

500

from [119], Copyright 2016, Elsevier.

501

Generally, temperature can change the removal rate of molecules and the energy

502

of the reaction system. The common adsorption isotherms employed to represent the

503

sorption equilibrium of heavy metal ions on NZVI-based materials include Freundlich,

504

Langmuir, Temkin and Dubinin-Radushkevich (D-R). At the same time, solution

505

temperature studies can express the important thermodynamic parameters values, such

506

as Gibb’s free energy (△G0), entropy (△S0) and enthalpy (△H0). Attempt to

507

understand the thermodynamic behavior of heavy metal ions on NZVI-based

508

materials have generally been inconclusive. Adsorption isotherm describes the

509

interaction process between adsorbates and adsorbents, which is essential in

510

optimizing the application of adsorbents. The As(V) adsorption on bare NZVI follows

511

Freundlich as well as Langmuir isotherms 126, and the adsorption behavior was tended

512

to a spontaneous exothermic adsorption reaction. The adsorption isotherms of Cr(VI)

513

on NZVI-Fe3O4 nanocomposites proved the fitness of the Langmuir and Freundlich

514

models

515

tendency of the heterogeneous surface of NZVI-Fe3O4 nanocomposites, and the

516

adsorption process was confirmed by the spontaneous character of the negative △G0

517

value. Figure 5A shows the adsorption isotherms of Cd(II) on NZVI and its composites

518

121

131

, and the better imitative effect of Freundlich model showed higher

, and it found that the adsorption process was endothermic and spontaneous, which 24


Page 24 of 48

Page 25 of 48


519

is beneficial for the practical application of environmental remediation. With the same

520

mechanism, the adsorption isotherms of Sb(III) by zeolite-supported NZVI were

521

investigated at various temperatures (Figure 5B) 150. In general, the temperature has a

522

positive effect on the adsorption capacity of heavy metal ions on NZVI-based

523

materials, which is beneficial for environmental in-situ remediation. A

B

524 525

Figure 5. (A) adsorption isotherms of Cd(II) removal by the rGOs, NZVI and NZVI/rGOs

526

(1:4). Reproduced with permission from [121], Copyright 2015, Elsevier; (B) adsorption

527

isotherms of Sb(III) on zeolite-supported NZVI. Reproduced with permission from [150],

528

Copyright 2015, Springer Science.

529

Dosage of adsorbent (NZVI-based materials) is an also significant factor to

530

affect the removal efficiency of pollutants, because using an optimum dosage of

531

adsorbent for removal process in the natural environment is useful for its

532

cost-effective application. According to Arshadi et al. 12, a dose of 0.15 g containing

533

NZVI decorated sineguelas waste (S-NaOH-NZVI) can be used to remove Pb(II)

534

from aqueous solutions and reached the maximum percentage removal (89%) for

535

Pb(II) at initial concentration of 10 mg·g-1. However, in order to obtain higher

536

percentage removal (93%) for Pb(II) at the same initial concentration, a dose of 1.50 g

537

was required. In another study, sepiolite-supported NZVI (S-NZVI) was used to

538

remove Cr(VI) and Pb(II) ions from groundwater 145, and the removal percentages of

539

Cr(VI) and Pb(II) were 98.8 % and 97.31%, respectively, which were obtained within

540

10 min when the dose of adsorbent was 3.2 g·L-1. Compared with the solid content of

541

0.05 g·L-1, the removal efficiency of target improved 1.5 ~ 2 times, which indicated

542

that the optimum dosage could reduce the cost of water environment manage. 25



543

Page 26 of 48

There are many publications on the effect of coexisting oxy-anions and cations 82, 134, 135

544

on heavy metal ions' adsorption/reduction by NZVI-based materials

545

the adsorption process, the coexisting oxy-anions could lead chemical competition

546

reaction

547

As(III) adsorption by Mt-NZVI was found to reduce in the order of PO43- > HCO3- >

548

SO42- > NO3-, which might be attributed to that the anions enhanced the ionic strength

549

and competed with targets, simultaneously

550

strength exceeds the driving force of competitive adsorption, the overall effect is

551

positive, otherwise negative effect. The results showed that HCO3-, SO42- and NO3-

552

had insignificant effect on the adsorption for both the species of As(V) and As(III) on

553

Mt-NZVI

554

adsorption percentage increased from 11.0% to 28.4% for As(V) and 13.0% to 35.5%

555

for As(III), which were via the inner-sphere complexes formed by PO43- and competed

556

for the adsorption sites on the adsorbent surface. It could be explained that the higher

557

valent anions have greater interfering effect than other monovalent anions in

558

As(V)/As(III) adsorption by Mt-NZVI. Similarly, various cations, such as Ca2+, Mg2+,

559

Zn2+, Fe2+, Cu2+ and Cd2+, have an important influence to the adsorption process. Zhu

560

et al.

561

and Fe2+ suppressed the adsorption process while Ca2+ and Mg2+ had a positive effect

562

to the reaction process. However, compared with the effect of solution pH, coexisting

563

oxy-anions and cations were found to be insignificant to the removal process.

134

. During

82, 119, 157

. For example, the effect of coexisting oxy-anions on As(V) and

135

. When the promotional effect of ionic

. Interestingly, with increase in C[PO43-] from 2 to 5 mg·L-1, the

82

reported that NZVI/AC showed great adsorption capacity to As(V)/As(III),

564

3.3. Comparative Adsorption Capacity. To the best of our knowledge,

565

NZVI-based materials have shown various adsorption capacities for heavy metal ions,

566

which was attributed to the surface characteristics and oxygen-containing functional

567

groups. For example, the removal of Pb(II) ions from wastewater by various

568

NZVI-based materials has been studied by Liu et al.

569

were calculated to be 775.4, 1718.4 and 1986.6 mg·g-1 for Mg(OH)2, bare NZVI

570

nanoparticles and NZVI@Mg(OH)2, respectively. Interestingly, although the high

571

adsorption capacity was obtained by Mg(OH)2 or bare NZVI, the removal

572

performance has obviously improved with NZVI@Mg(OH)2 composite. Compared

76

, and the sorption capacities

26


Page 27 of 48


573

with the NZVI-based materials, such as NZVI-zeolite composite for Pb(II) (806.0

574

mg·g-1)

575

supported NZVI for Pb(II) (225.0 mg·g-1) 12 and NZVI-kaol for Pb(II) (440.5 mg·g-1)

576

147

577

which is regardless of the varied experimental conditions. In order to better

578

understand the adsorption behavior and removal performance of heavy metal ions,

579

various adsorption thermodynamic parameters and sorption capacity with Pb(II) ions

580

as an example adsorbed on various materials are tabulated in Table S1, including

581

carbon-based materials

582

materials

583

capacity, and various physicochemical property can control the adsorption process 172,

584

173

585

in-situ remediation.

586

4. ENVIRONMENTAL SIGNIFICANCE AND FUTURE APPLICATION

81

, NZVI-graphene composite for Pb(II) (585.5 mg·g-1)

137

, sineguelas waste

, the NZVI@Mg(OH)2 composite possesses the highest removal capacity for Pb(II),

7, 134, 158

, graphene-based materials

159-161

and other new

8, 11, 162-171

, which indicate that various materials have high adsorption

. The NZVI-based materials show great application potentials in environmental

587

With the rapid development and extensive applications of heavy metals and its

588

composites, its release into the environment is inevitable and poisonous. Therefore,

589

the rapid adsorption of heavy metal ions appears to be particularly important and

590

urgent due to its high activity and toxicity in the environment. Generally, since the

591

adsorption behavior strongly influences the toxicity, accumulation, adhesion and

592

migration of various heavy metal ions in the environment, especially in clay minerals,

593

NZVI as an important environmentally benign reducing agent, has shown great

594

potentials to pollutant removal and environmental in-situ remediation. It is necessary

595

and significant to investigate the adsorption behavior and reduction mechanism of

596

heavy metal ions on NZVI-based materials from aqueous solutions, and it is

597

beneficial for us to understand the migration rules and interaction mechanism of

598

heavy metal ions in environmental medium, especially in clay minerals, groundwater

599

and wastewater.

600

To date, many researchers have confirmed that the adsorption processes of heavy

601

metal ions on NZVI-based materials from natural environment are attributed to

602

chemical reaction and monolayer molecule sorption by batch adsorption experiments 27



603

or theoretical study. Batch experimental results have also proved that the optimum pH

604

values of adsorption process on most NZVI-based materials is 4.0 to 7.0, which is

605

conformed to the aquatic environmental range (pH = 5.0 ~ 9.0), and most of NZVI

606

composites with excellent properties show great adsorption capacity and removal

607

performance. These findings provide crucial insight regarding the fate and adsorption

608

of heavy metal ions on NZVI-based materials in the natural environment and would

609

partly allow us to assess its environmental impact. At the same time, NZVI-based

610

materials are expected to be an efficient adsorbent and reductant for heavy metal ions

611

and used for in-situ environmental remediation, which could provide us a simple

612

method for the efficient elimination of environmental pollutants from aqueous

613

solutions.

614

In summary, NZVI-based materials can be a promising material to remove heavy

615

metal ions efficiently from aqueous solutions by using a simple and rapid chemical

616

reaction and adsorption process. Although universal acceptance of NZVI-based

617

material as a remediation material may well occur, a further understanding of

618

environment behavior, remediation mechanism and influence factor of heavy metal

619

ions adsorbed on NZVI-based materials has also not been demonstrated. Future

620

research should seek to establish a sound body of evidence upon the interaction

621

mechanism of NZVI-based materials in natural environment, which could confirm

622

that NZVI-based materials applied in environment in-situ remediation and reduce the

623

effect of heavy metal ions pollutants or other common pollutants.

624

ASSOCIATED CONTENT

625

AUTHOR INFORMATION

626

Corresponding Authors: *Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail:

627

[email protected] or [email protected] (W. Wang).

628

Notes

629

The authors declare no competing financial interest.

28


Page 28 of 48

Page 29 of 48


630

Supporting Information

631

Preparation of pristine nanoscale zero-valent iron (NZVI), Figures S1 to S6 and Table

632

S1. The Supporting Information is available free of charge via the Internet at

633

http://pubs.acs.org.

634 635

ACKNOWLEDGMENTS

636

Financial supports from NSFC (21225730, 91326202, 21577032, 21377132,

637

21307135, 41273134), the Fundamental Research Funds for the Central Universities

638

(JB2015001), the Project of East China Institute of Technology Graduate Student

639

Innovation Fund (YC2015-S273), the Jiangsu Provincial Key Laboratory of Radiation

640

Medicine and Protection and the Priority Academic Program Development of Jiangsu

641

Higher Education Institutions are acknowledged.

642 643

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1177

List of Proper Noun (Full name and Abbreviated Name) in this review paper Full Name

Abbreviated Name

Nanoscale zero valent iron

NZVI

Layered double hydroxides

LDHs

Nanosized metal oxides

NMOs

Ordered mesoporous carbon

OMC

Surface modified NZVI

SM-NZVI

Cetyltrimethyl ammonium bromide

CTAB

Carboxy methylated cellulose

CMC

3-aminopropyltriethoxysilane

APS

Polyvinyl alcohol

APA

Polyacrylic acid

PAA

Graphene family nanomaterials

GFNs

Graphene supported NZVI

G-NZVI

Graphene-oxide nanosheets supported NZVI

NZVI/GNS

Reduced graphene oxide supported NZVI

NZVI/rGOs

Kaolinite-supported zero-valent iron nanoparticles

NZVI-kaol

Montmorillonite-supported NZVI

Mt-NZVI

Amino-functionalized NZVI

APS-NZVI

Sineguelas waste-modified NZVI

S-NaOH-NZVI

Ultrasonic modified NZVI

US-NZVI

Carboxymethyl cellulose supported NZVI

CMC-NZVI

Activated carbon supported-NZVI

NZVI/AC

(Mg(OH)2) supported NZVI

NZVI@Mg(OH)2

Pillared bentonite supported NZVI

NZVI/Al-bent

Zero-valent aluminum

ZVAl

1178

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