Efficient reduction of bromate in water by nano-iron hydroxide impregnated granular activated carbon (Fe-GAC)

Accepted Manuscript Efficient reduction of bromate in water by nano-iron hydroxide impregnated granular activated carbon (Fe-GAC) Jian-hong Xu, Nai-yun Gao, Dong-ye Zhao, Wei-xian Zhang, Qin-kun Xu, Aihong Xiao PII: DOI: Reference:

S1385-8947(15)00437-4 http://dx.doi.org/10.1016/j.cej.2015.03.110 CEJ 13461

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 January 2015 22 March 2015 23 March 2015

Please cite this article as: J-h. Xu, N-y. Gao, D-y. Zhao, W-x. Zhang, Q-k. Xu, A-h. Xiao, Efficient reduction of bromate in water by nano-iron hydroxide impregnated granular activated carbon (Fe-GAC), Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.03.110

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1

Efficient reduction of bromate in water by nano-iron hydroxide

2

impregnated granular activated carbon (Fe-GAC)

3

Jian-hong XU1 Nai-yun GAO1,

4

Ai-hong XIAO1

5

1

6

200092, P.R.China.

7

2

8

Engineering Center, Auburn University, Auburn, AL 36849, USA

9

3

∗

Dong-ye ZhAO2 Wei-xian ZHANG1 Qin-kun XU3

State Key Laboratory of Pollution Control and Resource Rescue, Tongji University, Shanghai

Environmental Engineering Program, Department of Civil Engineering, 238 Harbert

Southwest University of Science and Technology, Mianyang, 621000, PR China

10 11

[Abstract]: Nano-iron hydroxide impregnated granular activated carbon (Fe-GAC) was

12

prepared and tested for reduction of bromate in water. SEM, BET and FTIR characterization of

13

Fe-GAC revealed that nanoscale iron hydroxide (FeOOH) particles with SO4 2- attached were

14

evenly distributed on the surface of GAC. Fe-GAC can effectively remove bromate in water

15

through firstly adsorption then reduction. The nano-iron hydroxides on GAC enhanced bromate

16

removal rate and the equilibrium bromate adsorption capacity. More FeOOH of Fe-GAC

17

favored the reduction of bromate. Bromate reduction by Fe-GAC was the main mechanism for

18

bromate removal. Fe-GAC performed well through a broad pH range ( 2-10 ) with the optimal

19

pH 6-8 for bromate reduction. According to XPS analysis, Fe(III) of Fe-GAC was reduced to

20

Fe(II) during the bromate reduction to bromide. Fe(III)/Fe(II) acted as a catalyst and accelerated

21

the bromate reduction rate by carbon of GAC. The four inorganic anions (SO42-, Cl- , CO33- or

22

PO4 3-) all exhibited inhibiting effects on bromate removal by Fe-GAC with the following order:

23

PO4 3->CO33- > Cl- > SO42-. In all, Fe-GAC is a promising material for efficient reduction of

24

bromate in water.

25 26 27

Key words: bromate, nano-iron hydroxide, reduction, Fe(III)/Fe(II), catalyst

28 ∗

Corresponding author phone: 86-21-65982691 Fax:86-21-65986313 e-mail: [email protected]

29

1. Introduction

30

Drinking water source is often contaminated by oxyanions such as nitrate

31

(NO3-), bromate (BrO3-), perchlorate (ClO4- ) and arsenic (AsO33- or AsO43- ), which

32

can pose potential health risks. Bromate is commonly produced during the

33

ozonation of water containing bromide (Br-) [1, 2]. Several studies have confirmed

34

that bromate is a potential human carcinogen for causing renal cell tumors in rats

35

and male mice [3, 4]. The International Agency for Research on Cancer (IARC) has

36

classified bromate as a group B-2 carcinogen. To mitigate human exposure, World

37

Health Organization has recommended a maximum contaminant level (MCL) of

38

0.078 µmol/L (10 µg of BrO3-/L) for bromate in drinking water [5]. Therefore, the

39

stiffening of regulations generates strong demands to control bromate (BrO3-)

40

contamination.

41

Materials containing metal (hydr)oxides have been the most common

42

adsorbents for bromate such as amorphous aluminum hydroxide [6], nano

43

crystalline akaganeite (β-FeOOH) [7], granular ferric hydroxide [8], calcined

44

Mg-Al layered double hydroxides [9]. Among the materials, nano-iron (hydr)oxide

45

particles can offer high adsorption capacities for bromate removal. However, direct

46

addition of these nano-iron (hydr)oxide particles into water is not feasible in

47

engineering practice, because these materials from treated water ought to be

48

removed subsequent to the adsorption. Granular activated carbon (GAC) may be an

49

excellent supporting media for the nano-iron (hydr)oxide particles, for granular

50

activated carbon (GAC) has a high chemical stability, mechanical robustness, large

51

specific surface area and commercial availability. Granular activated carbon

52

impregnated with Fe(II) salt or Fe(III) salt (Fe-GAC) contained iron (hydr)oxide

53

particles which effectively removed perchlorate reported by our group [10], at the

54

same time, Fe-GAC can remove many other pollutants such as arsenate, selenite or

55

trichloroethylene [11-14], for which the mechanism was through only adsorption.

56

However, there has been no report of nano-iron hydroxide impregnated granular

57

activated carbon (Fe-GAC) for bromate removal.

58

Adsorption has attained much attention due to its low cost and high efficiency,

59

but reductive removal of bromate can be more desirable. Recently, reduction of

60

bromate was achieved by using GAC [15-17], carbon nanofibre supported catalysts

61

[18], nanoscale zero valent iron (NZVI) [19, 20], Fe(II) in Fe-Al-LDH [21, 22], the

62

effect of Fe(III) as catalysts [23], bioreactors [24] and electrochemical reduction

63

[25]. In particular, the catalytic effect of Fe(III) and GAC as the electron donor

64

played the important role in the bromate reduction process. Nano-iron hydroxide

65

impregnated granular activated carbon (Fe-GAC) contained nano-iron hydroxide,

66

GAC and Fe(III). However, there has been no report on the effectiveness of

67

Fe-GAC for bromate reduction. furthermore, there has been no report on the

68

mechanism for bromate removal by Fe-GAC.

69

In this study, Fe-GAC is as an adsorbent/reductive agent for bromate removal.

70

The specific objectives were to: 1) Prepare and characterize Fe-GAC by field

71

emission scanning electron microscope (FESEM), Fourier Transform Infrared

72

Spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) surface area. 2) Evaluate

73

the sorption kinetics, isotherms and pH effect on bromate removal. and 3) Elucidate

74

the underlying mechanisms of bromate removal by Fe-GAC through products and

75

X-ray photoelectron spectroscopy (XPS) analyses.

76

2. Experimental section

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2.1 Chemical reagents

78

Ferrous sulfate (FeSO4·7H2O), sodium bromate (NaBrO3, Sigma), sodium

79

hydroxide (NaOH), hydrochloric acid (HCl) and coal based GAC with grain sizes

80

of 105-148 µm were purchased from Sinopharm Chemical Reagent (Shanghai,

81

China). All the chemical reagents were of analytical grade.

82

2.2 Synthesis of Fe-GAC

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Nanoscale iron hydroxide impregnated granular activated carbon (Fe-GAC) was

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prepared as follows. At first, GAC was washed under the assistance of ultrasound

85

irradiation. The hydrolysis of Fe(II) was initiated by mixing known amounts (2, 4,

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and 8 g) of FeSO4·7H2O and 5 g of GAC in 50 mL of deionized (DI) water in a 250

87

mL glass flask under strong magnetic stirring at 100-120oC. After 24 h hydrolysis,

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the solids were washed with DI water for three times to remove impurities and

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dried at about 120°C for 4-6 h until its mass became constant. The dried Fe-GAC

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was stored in open bags prior to use. To determine the iron content of the Fe-GAC

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prepared with different FeSO4·7H2O masses (2, 4 and 8 g), 0.1 g of Fe-GAC was

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sampled and added to 100 mL of 1:1 HCl solution in a flask installed in a rotary

93

shaker. Upon vigorous shaking (200 rpm) for 4 h, the iron concentration in water

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was determined by a Spectro Genesis Inductively Coupled Plasma-Optical

95

Emission Spectroscopy (ICP-OES), which reflected the content of iron-impregnated

96

on Fe-GAC. Results showed that the aqueous iron concentrations of Fe-GAC

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prepared with 2, 4 and 8 g of FeSO4·7H2O were 5.83, 9.2 and 12.3 mg/L,

98

corresponding to 0.6 wt.%, 0.9 wt.% and 1.2 wt.% relative to the GAC mass,

99

respectively. Accordingly, the Fe-GAC materials were denoted as Fe(0.6)-GAC,

100

Fe(0.9)-GAC and Fe(1.2)-GAC, respectively.

101

2.3 Characterization

102

The surface morphology of the adsorbents was determined by Field emission

103

scanning electron microscope (FESEM) (JSM-6701F, Japan). FTIR analysis was

104

carried out to identify the function groups. To this end, the adsorbent materials were

105

palletized with KBr. FTIR spectra were recorded in the range of 4000-400 cm-1

106

with a Nicol et 5700 spectrometer. X-ray photoelectron spectroscopy (XPS)

107

analysis was performed with a RBD upgraded PHI-5000C ESCA system (Perkin

108

Elmer, USA) with Mg Kɑ radiation (hν=1253.6 eV). XPS spectrum was obtained at

109

several angles such as 90o and 35o relative to the sample surface plane. Binding

110

energies were calibrated by using the containment carbon (C1s=284.6 eV).

111

2.4 Experiments

112

The kinetics tests were carried out in 250 mL conical flasks containing 200 mL

113

of 0.1 mmol/L BrO3- solution at about pH 5±0.5. The effects of different

114

concentrations of bromate were tested in 250 mL conical flasks containing 100 mL

115

of 0.05-0.3 mmol/L BrO3- solution at pH 5±0.5. The tests were initiated by addition

116

0.1 g of Fe-GAC or GAC to each of the reactors. The reactors were placed on a

117

rotary shaker operated at 200 rpm to ensure a complete mixing state. For kinetic

118

tests, 5 mL of each sample was collected at each designated sampling time. The

119

samples were filtered through 0.45 µm membrane filters and the filtrated were then

120

analyzed for bromate remaining or bromide production in the aqueous phase. To

121

test

122

tests were allowed to proceed for 72 h, then 5 mL of each was sampled, filtered and

123

analyzed for bromate or bromide. To test the effect of solution pH, the initial pH

124

was pre-adjusted to a desired level from 1.5 to 12 with 0.1 M HCl or 0.1 M NaOH.

125

After the reaction proceeded for 24 h, 5 or 25 mL of each sample was collected and

126

analyzed for bromate and iron concentrations. To test the effect of different

127

inorganic anions, 0, 0.02 or 0.1 g NaCl, Na2SO4, Na2CO3 or Na3PO4 was added to

128

the solution containing 0.1 g Fe-GAC and 100 mL 0.1 mmol/L bromate solution,

129

respectively, then 0, 100 or 500 mg/L NaCl, Na2SO4, Na2CO3 or Na3PO4 was

130

formed. After the reaction proceeded for 24 h, 5 mL of each sample was collected

131

and analyzed for bromate.

132

2.5 Analytical methods

the effects of different concentrations of bromate, the reduction/adsorption

133

Bromate and bromide were analyzed using a Dionex ion chromatograph

134

(DX-120) equipped with a suppressed conductivity detector, using an AS20 column,

135

an AG20 guard column, and a 250 µL sample loop. A degassed 20 mmol/L KOH

136

solution was used as the eluent and the suppressor current was set at 75 mA for

137

BrO3- or Br- analysis. The eluent flow rate was set at 1.0 mL/min. The detection

138

limit was 0.22 µg/L for BrO3-. The iron concentrations in water were determined by

139

a Spectro Genesis Inductively Coupled Plasma-Optical Emission Spectroscopy

140

(ICP-OES).

141

2.6 Models for kinetics

142 143 144

The amount of bromate adsorbed onto GAC composites is calculated by Eqs. (1) or (2): (1)

qt = (C0 − Ct ) ∗V / M

(2)

qt = (Ct − C0 ) ∗V / M

145 146

where qt is the amount of bromate adsorbed or bromide production per unit mass of

147

the GAC composites (mmol/g) at a given time t; C0 and Ct are the bromate or

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bromide concentrations in the bulk solution at time 0 and t, respectively; V is

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solution volume and M is the mass of the GAC composites (g).

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The bromate removal kinetic data are correlated with the adsorption kinetics

151

models:

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log( qe − qt ) = log qe −

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1 1 t = + t (Pseudo-second order kinetics model) 2 qt k2 qe qe

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where q e and qt are the amounts of bromate adsorbed onto the GAC composites at

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chemical equilibrium and at time

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are the adsorption rate constants for the two models, respectively.

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3. Results and discussion

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3.1 Material characterization

k1 t (Pseudo-first order kinetics model) 2.303

(3) (4)

t, respectively; and, k1 (h-1) and k2 (h g/mmol)

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3.1.1 SEM analysis

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SEM is used to obtain the surface morphology. Fig. 1(a)-(d) shows the SEM

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images at 40000× magnification. Fig. 1(a) revealed the smooth surface of virgin

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GAC. Figs. 1b, 1c, and 1d show the SEM images of Fe-GAC prepared at various

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concentrations of ferrous sulfate (FeSO4·7H2O), where a thick and uniform layer

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composed of rod-shape particles covered almost the entire GAC surface, indicating

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the successful immobilization of iron hydroxide particles on the surface of GAC.

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Those prepared at elevated iron concentrations resulted in a thicker layer of iron on

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the GAC surface. It is noteworthy that the iron hydroxide particles were in the

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nanoscale, with a mean size of about 100 nm.

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3.1.2 FTIR analysis

170

The FTIR spectra of Fe-GAC prepared at various concentrations of ferrous

171

sulfate (FeSO4·7H2O) have been obtained from pellets containing the same

172

amounts of adsorbents and KBr. The FTIR spectra of Fe(0.6)-GAC, Fe(0.9)-GAC,

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Fe(1.2)-GAC and GAC are illustrated in Fig. 2. Based on a previous study [26], the

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asymmetric (vasCOO, 1547-1560 cm-1) and symmetric (vsCOO, 1395-1409 cm-1)

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vibrations correspond to aqueous carboxylates. The two major bands observed at

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1558 and 1398 cm-1 are thus ascribed to carboxylates existing on Fe-GAC. For the

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metal carboxylates, the COasym generally smaller than the COsym indicated that the

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bidentate bridging (type IV) groups were formed [27]. As seen in Fig. 2, a strong

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band at 1558 cm-1 and a weak band at 1398 cm-1 ( ᵞCOasym and ᵞCOsym,

180

respectively) indicated the bidentate bridging (type IV) groups were the complex

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structure of the iron hydroxide with GAC. Tresintsi et al. [28] have reported that a

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broad peak observed about at 1120 cm-1 approximated the tetrahedral symmetry of

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free SO42-. In Fig. 2, the band at between 1100 and 1193 cm-1 was then assigned to

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SO42- in Fe-GAC, which was attached through outer sphere complexation [29]. For

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GAC, there was little band observed between 1100 and 1193 cm-1, indicating that

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there was no SO42- or little SO42- on GAC. Furthermore, as seen in Fig.2, the band

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of SO42- at between 1100 and 1193 cm-1 was stronger for Fe-GAC with more iron

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content than that with less iron content. The findings have shown that Fe(1.2)-GAC

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contained more SO42- than the other materials. The band at 3137 cm-1 was assigned

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to OH in Fe-GAC. Therefore, the predominant forms of iron hydroxide particles in

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Fe-GAC are FeOOH with SO42- being attached.

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3.1.3 BET analysis

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BET characterization results of the GAC, Fe(0.6)-GAC, Fe(0.9)-GAC and Fe

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(1.2)-GAC are shown in Table 1. The BET surface areas of Fe-GAC decreased

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from 918 (GAC) to 488-632 m2/g (Fe-GAC). At the same time, the BET surface

196

areas of Fe-GAC decreased from 632 to 488 m2/g with the iron content increasing

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from 0.6 to 1.2%. Correspondingly, their total pore volumes declined from 0.442

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cm3 /g (GAC) to 0.315 cm3 /g (Fe (0.6)-GAC) or 0.304 cm3 /g (Fe(II) (0.9)-GAC) or

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0.251 cm3 /g (Fe (1.2)-GAC). These findings suggested that nanoscale iron

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hydroxide deposited on the GAC surface and partially covered the cavities.

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Furthermore, the adsorption average pore diameter of Fe(0.6)-GAC (2.01 nm),

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Fe(0.9)-GAC (2.03 nm) or Fe(1.2)-GAC (2.12 nm) was larger than that of plain

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GAC (1.97 nm), at the same time, the adsorption average pore diameter of Fe-GAC

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showed an increasing trend with higher iron concentration, maybe because more

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iron hydroxide filled or covered the more micro pore to enhance the average pore

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sizes of GAC.

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3.1.4 XPS analysis

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Fig. 3 shows the results of XPS analysis of Fe(1.2)-GAC or Fe(1.2)-GAC after

209

reaction with 50 mg/L BrO3-. As seen in Fig. 3a, the elements on the materials

210

surface include iron, oxygen, sulfur and carbon. Sulfur on the GAC surface was

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primarily derived from sulfate. The iron content on the surface of Fe(1.2)-GAC

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after reaction with 50 mg/L BrO3- was 1.1% according to XPS characterization,

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lower than that (2%) of Fe(1.2)-GAC, which is likely due to partial loss of iron

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from Fe-GAC in water during the reaction with bromate or due to a different part

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on GAC.

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The change of the iron performance before or after reaction with bromate was

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unknown. To confirm the Fe 2p binding energy region or the valance of iron formed

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on GAC, the iron of Fe(1.2)-GAC and Fe(1.2)-GAC after reaction with bromate

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were characterized by XPS. Fig. 3b illustrates the Fe 2p binding energy region, the

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peak of iron shifted to lower binding energy (ev) from 711.7 to 710 ev. The

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positions of Fe 2p3 of Fe(1.2)-GAC peaked at 711.7 ev, indicating that Fe3+ was the

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predominant surface species in Fe-GAC. The findings have shown that the Fe3+ was

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formed from oxidation of ferrous ions by dissolved oxygen. The positions of Fe 2p3

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of Fe(1.2)-GAC after reaction peaked at 710 ev, indicating that Fe2+ was the

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predominant surface species on the surface of Fe(1.2)-GAC after reaction with

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bromate. The findings have shown that the Fe3+ was reduced to Fe2+ by the reaction

227

of bromate with Fe(1.2)-GAC, More details on the reaction mechanisms are

228

discussed in Section 3.6.

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3.2

Kinetics

230

The initial solution pH level of Fe(1.2)-GAC or GAC (0.1 g GAC composites

231

in 100 mL solution) without pH control is about pH 5±0.5. Fig.4 gives the resultant

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bromide products concentrations and the bromate concentrations at the different

233

times during the bromate removal kinetic tests by GAC or Fe(1.2)-GAC. Evidently,

234

the bromate concentrations decreased with the time nearly proportionally with the

235

bromide production in both cases. The observation indicates that the bromate was

236

reduced to bromide by GAC or Fe(1.2)-GAC. For Fe(1.2)-GAC, the bromate

237

concentration decreased from 0.1 mmol/L to zero after 50 h, i.e., 100% removal of

238

bromate was achieved by Fe(1.2)-GAC, with 80% of bromate being reduced to

239

bromide after 72 h. For GAC, the bromate concentration decreased slowly from 0.1

240

mmol/L to 0.034 mmol/L after 72 h, i.e., 66% of bromate was removed by GAC, of

241

which, 80% of bromate was reduced to bromide. The findings have shown that the

242

bromate reduction was the main mechanism for bromate removal by GAC or

243

Fe(1.2)-GAC.

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Table 2 presents the corresponding bromate removal kinetic parameters. While

245

both of the kinetic models were able to adequately interpret the experimental

246

kinetic data for bromate removal for both GAC and Fe(1.2)-GAC, the pseudo

247

second-order models (R2>0.99) performed slightly better than pseudo first-order

248

models (R2 >0.98).). The equilibrium bromate adsorption (qe) (0.10 mmol/g) of

249

Fe(1.2)-GAC was 1.5 times higher than that of GAC (0.068 mmol/g) according to

250

pseudo second-order models, indicating that the immobilized nano-iron hydroxide

251

particles on GAC enhanced the equilibrium bromate removal capacity. In the

252

pseudo second-order adsorption models, the rate constant of Fe(1.2)-GAC (3.02

253

h· g/mmol) was 1.2 times greater than that of GAC (2.43 h·g/mmol), indicating that

254

the immobilized nano-iron hydroxide particles on GAC enhanced bromate removal

255

rate. Furthermore, as seen in Fig.4, for Fe(1.2)-GAC, the concentration of bromate

256

was decreased rapidly and only a small amount of bromide was produced during

257

0-10 h, while a great amount of bromide was produced rapidly during 10-40 h and

258

the concentration of bromate was decreased slowly after 10 h. For GAC, the

259

concentration of bromate was decreased more slowly than that of Fe(1.2)-GAC,

260

only a small amount of bromide was produced during 0-10 h, while a great amount

261

of bromide was produced rapidly during 10-72 h. Moreover, the GAC even

262

produced more bromide

263

longer contact time [15] was favorable for bromate reduction by GAC. Based on

264

the observations, it seemed plausible to conceive a two-step process for bromate

265

removal by GAC or Fe(1.2)-GAC. First, bromate was adsorbed onto GAC or

266

Fe(1.2)-GAC by ion exchange between sulfate and bromate or electrostatic

267

attraction, and then reduced to bromide by GAC or Fe(1.2)-GAC on the sorbent

268

surface. The SO42- attached on Fe-GAC effected the bromate absorption by ion

than Fe(1.2)-GAC during 36-72 h, indicating that a

269

exchange between sulfate and bromate. The overall removal of bromate is due to

270

concurrent adsorption and reduction of bromate to the innocuous bromide by GAC

271

or Fe(1.2)-GAC.

272

3.3 Effect of different concentrations of bromate

273

The effect of different concentrations of bromate for bromate removal by GAC

274

or Fe-GAC in 72 h has shown in Fig. 5, the vertical axis represented the equilibrium

275

adsorption capacity (qe) of Fe-GAC for bromate removal in the different

276

concentrations of bromate (0.05-0.30 mmol/L). As seen in Fig. 5, the equilibrium

277

adsorption capacity (qe) of Fe-GAC or GAC for bromate removal was increased

278

with the increase of bromate concentrations, at the same time, the equilibrium

279

removal capacity (qe) of Fe-GAC or GAC followed the order: Fe (1.2)-GAC > Fe

280

(0.9)-GAC > Fe(0.6)-GAC >GAC. For the case of Fe(1.2)-GAC, the final

281

concentration of bromate in the solution phase was undetectable for the different

282

concentrations of bromate (0.05-0.30 mmol/L), and bromate removal was linearly

283

proportional to the concentrations of bromate, the maximum equilibrium adsorption

284

capacity (qe) of Fe (1.2)-GAC has got to 0.3 mmol/g. For the case of Fe(0.9)-GAC

285

or Fe(0.6)-GAC, all of bromate at the low concentrations of bromate (0.05-0.15

286

mmol/l) was adsorbed, the maximum equilibrium adsorption capacity (qe) of

287

Fe(0.9)-GAC or Fe(0.6)-GAC has got to 0.24 or 0.22 mmol/g. The findings have

288

shown that the equilibrium adsorption capacity (qe) of Fe-GAC for bromate

289

removal in the different concentrations of bromate (0.05-0.3 mmol/l) was increased

290

with the increase of iron content. For GAC, the maximum equilibrium removal

291

capacity (qe) was 0.16 mmol/g, lower than that of Fe-GAC, indicating that the

292

nano-iron hydroxide particles on GAC played the important role in bromate

293

removal.

294

As seen Table 1, there were different BET surface area for the different iron in

295

GAC, the lower iron content has the higher BET surface area. The higher BET

296

surface area was favored the adsorption [30]. However, the equilibrium removal

297

capacity (qe) of Fe-GAC for bromate removal in the different concentrations of

298

bromate (0.05-0.3 mmol/l) was increased with the increase of iron content. The

299

findings have shown that the bromate removal capacity of Fe-GAC was mainly

300

relative to the amount of the nano-iron hydroxide particles on GAC, which played

301

the important role in bromate removal.

302

The bromide products during the bromate removal by GAC or Fe(1.2)-GAC in

303

the different concentrations of bromate (0.05-0.30 mmol/L) was found, and Fig. 6

304

gave the final bromide production amount per unit mass of Fe-GAC (g) at different

305

initial bromate concentrations. As seen in Fig. 6, bromide concentrations increased

306

with the increase of initial bromate concentrations, and the amount of bromide

307

products by Fe-GAC or GAC followed the order: Fe (1.2)-GAC > Fe (0.9)-GAC >

308

Fe(0.6)-GAC >GAC. Fe(1.2)-GAC with the highest iron-hydroxide content

309

produced the most bromide among the four types of materials, which is consistent

310

with the amount of bromate adsorbed/removed by these materials.

311

The total bromate removal versus bromate reduction for Fe(1.2)-GAC was

312

quantified at various bromate equilibrium concentrations (Fig. 7). It is evident from

313

that more than 80% of bromate was removed through the chemical reduction at

314

lower bromate concentrations (<0.1 mmol/L). At elevated bromate concentrations,

315

it remained that more than 70% of bromate was removed through the reduction

316

mechanism, leaving a small fraction (<30%) being adsorbed. The findings have

317

shown that bromate reduction by Fe(1.2)-GAC was the main mechanism for

318

bromate removal. The portion of bromate removal due to reduction decreased with

319

increasing concentration of bromate. The same method was adopted to quantify the

320

total bromate removal versus bromate reduction for GAC, more than 50% of

321

bromate was removed through the reduction mechanism. The findings revealed

322

GAC itself can reduce bromate, the loading of nano-iron hydroxide greatly

323

enhanced the reductive removal of bromate.

324

3.4 Effect of initial pH

325

According to the kinetic discussion in the Section 3.2, complete bromate

326

removal by Fe(1.2)-GAC was observed in the bromate concentration of 0.1 mmol/L

327

at equilibration time of 52 h. For a better research of the pH effects, 24 h was

328

selected as the reaction time. The effects of initial solution pH on Fe(1.2)-GAC

329

removal of bromate are shown in Fig. 8. As seen from Fig. 8, the highly effective

330

removal of bromate was observed in the broad pH range (2-8) with the maximum

331

removal (0.077 mmol/g) occurring in the pH range 6-8, which was due to the dual

332

removal mechanisms of the adsorption and reduction. The findings were consisted

333

with an earlier study using activated carbon for bromate reduction [19]. At the same

334

time, water can provide hydrogen ion as medium for bromate reduction by GAC or

335

Fe2+ in neutral and acidic environment [21], then bromate reductive removal was

336

observed in the broad pH range (2-8). The zeta potential of Fe-GAC was decreased

337

with the the increase of pH reported by our group [10]. The surface potential of

338

Fe-GAC was more positive at low pH, and the surface turned more negative with

339

the elevated pH. However, bromate reductive highly removal was observed in the

340

broad pH range (2-8), indicating that the zeta potential of Fe-GAC has little effect

341

on the bromate removal. When the pH value increased from pH 8 to 10.35, the

342

bromate reductive removal decreased significantly from 0.075 to 0.052 mmol/g,

343

maybe because Fe3+ combined with OH- to form Fe (OH)3 on the surface of GAC

344

when pH>8, and prevented the electron transferring from GAC. Then the bromate

345

reductive removal decreased rapidly from pH 8 to 10.35.

346

The final pH was measured after the tests. When the initial pH value was in

347

the range of 2-6, the final pH went up, indicating that hydrogen ions were

348

consumed and hydroxyl ions were produced. When the initial pH value was in the

349

range of 8-11, the final pH went down, maybe because carbon was reduced by

350

bromate to produce carbon dioxide to make the pH value down. The iron

351

concentration of Fe-GAC at pH 2.07 or 10.37 after the tests is 1.2 or 0.2 mg/L,

352

while the iron concentration of Fe-GAC at pH value (3-9) is zero, indicating that

353

Fe-GAC is very stable and no iron is loss at pH 3-9. At the same time, the iron

354

released from Fe-GAC at pH 2.07 or 10.37 resulted in the decrease of bromate

355

removal capacity.

356

3.5

Effect of inorganic anions

357

The effects of different inorganic anions (0, 100 or 500 mg/L SO42-, Cl- , CO32-,

358

PO43-) on Fe(1.2)-GAC for initial bromate concentration of 0.1 mmol/L removal at

359

pH 5±0.5 are illustrated in Fig.9. As seen in Fig.9, the four anions all exhibited

360

inhibiting effects in the bromate removal to different degrees with the following

361

order: PO43->CO32- > Cl- > SO42-. The percentage of the bromate removal by

362

Fe(1.2)-GAC in the absence of these inorganic anions is 81%, PO43- and CO33- in

363

the presence of 500 mg/L decreased the bromate removal from 81% to 8% and to

364

12% , maybe because PO43- and CO32- as tetrahedral anions will form inner-sphere

365

complexes with iron hydroxide to precipitate at the surfaces of GAC [31, 32], and

366

prevented the electron transferring from GAC, then the bromate removal rate

367

decreased rapidly from 81% to 8% and to 12%. Anions such as SO42- or Cl- often

368

are weakly bound with surface sites of iron hydroxides forming outer-sphere

369

surface complexes [33]. The effect of SO42- or Cl- on bromate removal by

370

Fe(1.2)-GAC in the presence of 500 mg/L was as ionic strength effect, then the

371

bromate removal rate decreased slowly from 81% to 68% and to 62%. Furthermore,

372

as seen in Fig.9, the percentage of the bromate removal decreased with the increase

373

of concentrations of SO42-, Cl- , CO32-, or PO43-, indicating that the higher

374

concentrations of SO42-, Cl- , CO32-, or PO43- inhibited the bromate removal by

375

Fe(1.2)-GAC.

376

3.6 Mechanism analysis

377

Wang et al. [16] and Siddiqui et al. [17] have reported that GAC removed the

378

bromate through concurrent adsorption and reduction to the innocuous bromide, the

379

bromate reduction by GAC involved electron transferring from the carbon surface

380

to BrO3- ions with bromine (HOBr/Br-) as intermediate. The reaction of bromate

381

removal by GAC was as following eqs (5) or (6):

382

C + BrO3− → 2BrO − + CO2

(5)

383

C + 2 BrO − → 2 Br − + CO2

(6)

384

Xie et al. [23] has studied that the effect of Fe(III) on the bromate reduction by

385

humic substances in aqueous solution, where Fe(III) is reduced to Fe(II) and

386

thereby accelerated the bromate reduction rate by humic substances. Zhong et al.

387

[21] have reported that Fe(II) in Fe-Al LDH (SO4 type) adsorbed bromate firstly,

388

then reduced it to bromide and transferred Fe2+ to FeOOH. The reaction of bromate

389

removal by Fe(II) in Fe-Al LDH (SO4 type) was as following eqs (7) or (8).

390

6Fe 2+ + 6H + + BrO3 → 6Fe3+ + 3H 2O + Br -

391

6Fe 2+ + 3H 2O + BrO3- → 6Fe3+ + 6OH - + Br -

-

(7)

(8)

392

The Fe(III) or Fe(II) loading on carbon materials in the catalytic wet peroxide

393

oxidation played an important role for the model organic compounds removal [34,

394

35]. In this study, nano-iron hydroxides on GAC enhanced bromate removal rate

395

and the bromate reductive removal capacity. To explain the mechanism of the

396

bromate reduction by Fe-GAC and the role of nano-iron hydroxides, the Fe 2p

397

binding energy region or the valance of iron formed on GAC was confirmed by the

398

XPS spectra of Fe-GAC before or after reaction with the bromate at concentration

399

of 50 mg/L (Fig. 3). As seen in Fig.3, the binding energy region positions of Fe 2p3

400

before reaction with the bromate peaked at 711.7 ev, indicating that Fe(III) was

401

the predominant surface species, for Fe(II) could be oxidized to Fe(III) by dissolved

402

oxygen in water and from air. The binding energy region positions of Fe 2p3 after

403

reaction with the bromate peaked at 710 ev, indicating that Fe(II) was the

404

predominant surface species, maybe for the Fe(III) was reduced to Fe(II) after

405

reaction with the bromate. The bromate reduction by Fe(1.2)-GAC involved

406

electron transferring from the carbon surface to BrO3- ions, Fe(III) is reduced to

407

Fe(II) that accelerated the bromate reduction rate by carbon of Fe(1.2)-GAC. The

408

process is as follows: carbon transfers electrons to Fe(III) to form Fe(II), and the

409

regenerated Fe(II) donates the electrons to bromate, resulting in the bromate

410

reduction. The addition of Fe(III) accelerates the bromate reduction rate by carbon.

411

The Fe(III)/Fe(II) couple acts as a catalyst for bromate reduction by GAC. The

412

scheme is illustrated in Fig.10. The reaction of electron transferring from the carbon

413

surface to Fe(III) to form Fe(II) is as following eq (9):

414

4Fe3+ + 2H 2O + C → 4Fe2+ + CO2 + 4 H +

(9)

415

The reaction time for bromate removal by Fe(II) (Fe–Al LDHs) required 2 h

416

[22] shorter than the time for bromate removal by GAC [19]. The regenerated Fe(II)

417

from Fe(III) reacted with carbon of GAC transferred the electrons to bromate and

418

accelerated the bromate reduction rate by carbon, the more Fe(III)/Fe(II) couple as

419

a catalyst resulted in more bromate reduction by GAC. Hence, the immobilized

420

nano-iron hydroxides on GAC enhanced bromate removal rate from 2.43 (GAC) to

421

3.02 h·g/mmol and the bromate reductive removal capacity from 0.068 (GAC) to

422

0.10 mmol/g. The bromate reduction was the main mechanism for bromate removal

423

by Fe-GAC.

424

4. Conclusions

425

The major research findings are recapped as follows:

426

(1) Fe-GAC can effectively reduce bromate in water through coupled adsorption

427

and reduction where reduction of bromate to bromide is the key mechanism for

428

bromate removal.

429

(2) The equilibrium bromate removal capacity and the bromate removal rate

430

followed the order for the different concentrations of bromate (0.05-0.30 mmol/L):

431

Fe(1.2)-GAC > Fe(0.9)-GAC > Fe (0.6)-GAC>GAC. While GAC alone can reduce

432

part of bromate, the iron hydroxide loading greatly enhances the bromate removal

433

rate and the bromate reductive removal capacity.

434

(3) Fe(1.2)-GAC performed well through a broad pH range ( 2-10 ) with the

435

optimal pH 6-8 for bromate reduction. The four anions all exhibited inhibiting

436

effects in the bromate removal to different degrees with the following order:

437

PO43->CO32- > Cl- > SO42-.

438

(4) Fe(III)/Fe(II) couple acts as a catalyst for bromate reduction by the carbon of

439

GAC, the addition of Fe(III) could accelerate the bromate reduction rate.

440

The nano-iron hydroxides loaded on GAC may serve as an effective and

441

promising material for efficient reduction of bromate in water.

442

Acknowledgments

443

This work was financially supported by the National Natural Science Foundation of China

444

(No. 51208364, 51178321), National Major Project of Science & Technology Ministry of China

445

(No.2012ZX07403-001).

446

References

447

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448

bromate during ozonation of waters containing bromide, J. Am. Water Works Assoc. 85 (1993)

449

73-81.

450

[2] W. R. Hagg, J. Holgne, Ozonation of bromide-containing waters: kinetics of formation of

451

hypobromous acid and bromate, Environ. Sci.Technol. 17( 1983)261-267.

452

[3] J. Fawell, M. Walker, Approaches to regulatory values for carcinogenswith particular

453

reference to bromate, Toxicology. 221 (2006) 149-156.

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50 (2011) 9280-9285.

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carbon (Fe(II)-GAC) as a sorbent for perchlorate in water, Chem.Eng.J. 222 (2013) 520-526.

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[11] M. Jang, W. F. Chen, F. S. Cannon, Preloading hydrous ferric oxide into granular activated

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carbon for arsenic removal, Environ. Sci. Technol. 42 (2008) 3369-3374.

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adsorbents for arsenic removal, Environ. Sci. Technol. 39 (2005) 3833-3843.

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[13] N. Zhang, L.S. Lin, D.C. Gang, Adsorptive selenite removal from water using iron-coated

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GAC adsorbents, Water Res. 42 (2008) 3809-3816.

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[14] A.M. Cooper, D. Kiril, D. Hristovski, P. Westerhoff, P. Sylvester, The effect of carbon

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nanoparticle-impregnated granulated activated carbons, J. Hazard. Mater. 183 (2010) 381-388.

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using powdered activated carbon, Journal of Environmental Sciences. 22 (2010) 1846-1853.

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Water Res. 30 (1996) 1651-1660.

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[18] P. Yaseneva, C.F. Marti, E. Palomares, X.L. Fan, T. Morgan, P.S. Perez, M.Ronning, F.

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Huang, T. Yuranova, L.K. Minsker, S. Derrouiche, A.A. Lapkin, Efficient reduction of bromates

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(hydr)oxide

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[19] X.Q. Wu, Q. Yang, D.C. Xu, Y. Zhong, K. Luo, X.M. Li, H.B. Chen, G.M. Zeng,

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Simultaneous adsorption/reduction of bromate by nanoscale zerovalent iron supported on

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modified activated carbon, Ind. Eng. Chem. Res. 52 (2013) 12574-12581.

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method for the reduction of bromate, J.Hazard.Mater. 250 (2013) 345-353.

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bromate reduction: synthesis and reactivity, J. Colloid. Interf. Sci. 354 (2011) 798-803.

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537 538 539

540 541 542 543 544 545 546 547 548 549 550 551

(a)

(b)

552

553 554

(c)

(d)

555 556 557 558

Fig.1. SEM images of GAC(a), Fe(0.6)-GAC(b), Fe(0.9)-GAC(c) and Fe(1.2)-GAC(d).

110 100

4

1 2 3 4

80

Fe(0.6)-GAC Fe(0.9)-GAC Fe(1.2)-GAC GAC

2

70

3

60

1

Trancemittance (%)

90

50

8 9 3 1

7 3 1 3

30 3500

3000

0 5 1 1

8 5 5 1

40

2500

2000

1500

1000

500

-1

Wavenumber (cm ) 559 560 561

Fig.2. FTIR spectra of Fe(0.6)-GAC (1), Fe(0.9)-GAC (2), Fe(1.2)-GAC (3) and GAC (4).

m u r t c e p s l l a r e v O

1 2

8000

Fe(1.2)-GAC Fe(1.2)-GAC-after- reaction

a

s 1 a N

1

6000

g M L L K O s 1 C

s 1 O

3 p 2 e F

4000

g M L L K e F 3 p 2 s

2

3 p 3 e F

2000

0 1200

1000

800

600

400

200

0

Binding energy 562

1 2

54000

Fe(1.2)-GAC Fe(1.2)-GAC-after-reaction

b

56000

52000 50000 48000

7 . 1 1 7

46000

0 1 7

44000

1

2

42000 40000 38000 36000 34000 740

730

720

710

700

690

Binding energy(ev) 563 564 565

Fig.3. XPS analysis of Fe(1.2)-GAC and Fe(1.2)-GAC-after-reaction: (a) overall XPS spectra; and (b) individual XPS Fe 2p binding energy region.

0.08

0.08 0.06 0.06 0.04 0.04

-

Fe(1.2)-GAC-BrO 3 -

GAC-Br GAC-BrO3

0.02

0.02 -

Bromide concentration(mmol/L)

Bromate concentration (mmol/L)

0.10

Fe(1.2)-GAC-Br 0.00

0.00 0

10

20

30

40

50

60

70

80

90

100

110

time(h)

566 567 568 569 570 571 572 573

Fig.4. BrO3- remaining concentrations and Br- production concentrations with the increase of time. (Initial BrO3-=0.1 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC; agitation rate: 200 rpm; pH =5±0.5 ).

0.32 0.30 0.28

Bromate removal q(mmol/g)

0.26 0.24 0.22 0.20

GAC Fe(0.6)-GAC Fe(0.9)-GAC Fe(1.2)-GAC

0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Initial bromate concentrations(mmol/L) 574 575 576 577 578

Fig.5. Bromate removal by GAC, Fe(0.6)-GAC, Fe(0.9)-GAC or Fe(1.2)-GAC in different concentrations of bromate. (Initial BrO3-=0.05-0.30 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC; agitation rate: 200 rpm; pH =5±0.5; equilibrium time = 72 h).

Bromide production (mmol/g)

0.25

0.20

0.15

GAC Fe(0.6)-GAC Fe(0.9)-GAC Fe(1.2)-GAC

0.10

0.05

0.00 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Initial bromate concentration (mmol/L) 579 580 581

Fig.6. Bromide products by GAC, Fe(0.6)-GAC, Fe(0.9)-GAC or Fe(1.2)-GAC in different bromate concentrations. (Initial BrO3-=0.05-0.30 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC;

582 583 584 585

agitation rate:200 rpm; pH =5±0.5 ; time =72 h)

586

Bromate removal or reduction qe(mmol/g)

0.30

0.8

ratio of removal to reduction

0.25

0.6

bromate reduction bromate removal

0.20

0.15

0.4

0.10 0.2 0.05

0.00 0.00

0.0 0.05

0.10

0.15

0.20

0.25

0.30

) L / l o m m ( 0 C n o i t a r t n e c n o c e t a m o r B

587 588 589 590 591

Fig.7. Bromate removal or reduction qe (mmol/g) in different bromate concentrations. (Initial BrO3-=0.05-0.30 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC; agitation rate:200 rpm; pH =5±0.5; time = 72 h)

592 593 594

0.08

0.06 0.05 0.04 0.03

-

BrO3 removal qe (mmol/g)

0.07

0.02 0.01 0.00 0

2

4

6

pH

595

8

10

12

596 597 598

Fig.8. Effects of initial pH on bromate reduction by Fe(1.2)-GAC in the different initial pH. (Initial BrO3-=0.1 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC; agitation rate: 200 rpm, time = 24 h ).

1.2

0 mg/L 100 mg/L 500 mg/L

1.1 1.0

Bromate removal

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

SO4

599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615

2-

-

Cl

3-

CO3

PO4

3-

Fig.9. Effect of different inorganic anions on bromate removal by Fe(1.2)-GAC at pH 5±0.5. (Initial BrO3-=0.05-0.30 mmol/L; Fe-GAC: 1 g/L; temperature: 25oC; agitation rate:200 rpm; pH =5±0.5; time = 24 h)

Carbon (reduced)

Carbon (oxidized)

Fe(II) Br-

Fe(III)

BrO3-

616 617 618 619

Fig.10. Electron-transfer process from the electron donor (carbon) to the electron acceptor (bromate) via Fe(III).

620

Table 1

621

(1.2)-GAC.

BET characterization results of the GAC, Fe(0.6)-GAC, Fe(0.9)-GAC and Fe

Surface area (m2· g-1)

622 623 624

1

Pore volume (cm3·g-1)

Pore size (nm)

NO.

Names

BET

Micro1

BJH2

Total3

Micro

BJH

Ave4

BJH

1

GAC

918

906

289

0.442

0.417

0.177

1.97

2.36

2

Fe(0.6)-GAC

632

584

240

0.315

0.262

0.145

2.01

2.60

3

Fe(0.9)-GAC

581

570

232

0.304

0.271

0.142

2.03

2.63

4

Fe(1.2)-GAC

488

302

212

0.251

0.160

0.132

2.12

2.69

Micro: micropore. 2BJH: cumulative pores between 1.7 and 300 nm from BJH adsorption branch. 3Single

point adsorption total pore volume at P/P0>0.99 (corresponding to less than 200 nm pores). 4Adsorptio average pore width (4 V/A by BET).

625 626 627

Table 2

Kinetics parameters for BrO3- removal by Fe(1.2)-GAC or GAC. Pseudo-first order

Materials

k1 (h-1) qe

628 629 630 631 632 633 634 635

Pseudo-second order R2

k2 (h·g/mmol)

qe

R2

Fe(1.2)-GAC

0.068

0.061

0.99

3.02

0.10

0.99

GAC

0.036

0.044

0.98

2.43

0.068

0.99

) 0 1 7 ( ) I I ( e F

3

) 0 1 7 ( ) I I ( e F

n o i t c a e r r e t f a C A G ) 2 . 1 ( e F

C A G ) 2 . 1 ( e F

640

O r B

) 7 . 1 1 7 ( ) I I I ( e F

639

) 7 . 1 1 7 ( ) I I I ( e F ) d e z ni oi d b rx ao C(

638

r B

) d e nc od u b re ar C(

637

Graphical abstract 636

641

Highlights

642 643

(1) Fe-GAC can effectively remove bromate through firstly adsorption then

644

reduction.

645

(2) Bromate reduction by Fe-GAC was the main mechanism for bromate removal.

646

(3) Nano-iron hydroxides on GAC enhanced bromate removal rate and the

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equilibrium reductive bromate removal capacity.

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(4) More FeOOH of Fe-GAC favored the removal of bromate.

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(5) Fe(III)/Fe(II) acted as a catalyst and accelerated bromate reduction by GAC.

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