Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely synthesized mesoporous iron–zirconium bimetal oxide

Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely synthesized mesoporous iron–zirconium bimetal oxide

Accepted Manuscript Title: Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely synthesized mesoporous iron-zirconium bimetal...

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Accepted Manuscript Title: Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely synthesized mesoporous iron-zirconium bimetal oxide Author: Yi Wang Dongfang Liu Jianbo Lu Jian Huang PII: DOI: Reference:

S0927-7757(15)00088-6 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.01.060 COLSUA 19707

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

31-8-2014 24-1-2015 27-1-2015

Please cite this article as: Y. Wang, D. Liu, J. Lu, J. Huang, Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely synthesized mesoporous ironzirconium bimetal oxide, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2015), http://dx.doi.org/10.1016/j.colsurfa.2015.01.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.



Enhanced adsorption of hexavalent chromium from aqueous solutions on facilely



synthesized mesoporous iron-zirconium bimetal oxide Yi Wanga, Dongfang Liua*, Jianbo Lub*, Jian Huanga

3  4 

a

Key Laboratory of Environmental Remediation and Pollution Control/Ministry of

Education Key Laboratory of Pollution Processes and Environmental Criteria, Nankai



University, Tianjin, 300071, China.



School of Environmental and Municipal Engineering, Tianjin Chengjian University,

Tianjin 300384, China

cr



b

a

* Corresponding author: Tel.: +8613752092530, Fax: 86-22-23501117

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ip t



Email address: [email protected] (Dongfang Liu)

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Postal address: Dongfang Liu, Professor

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Environmental Engineering Program

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College of Environmental science and Engineering, Nankai University, 94 Weijin

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Road, Tianjin, China, 300071 b

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an

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* Corresponding author: Tel.: +8613752670427

Email address: [email protected] (Jianbo Lu)

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Postal address: Jianbo Lu, Associate Professor

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Environmental and Municipal Engineering

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School of Environmental and Municipal Engineering, Tianjin Chengjian University,

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26 Jinjing road, Tianjin, China, 300384

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Abstract: Mesoporous iron-zirconium bimetal oxide (MIZO) which was templated

23 

by cetyltrimethylammonium bromide (CTAB) was facilely synthesized through

24 

co-precipitation for the first time. MIZO was applied in the adsorption of hexavalent

25 

chromium [Cr(VI)] in aqueous solutions, with comparison to ordinary iron-zirconium

26 

bimetal oxide (IZO). The properties of MIZO and IZO were characterized by N2

27 

adsorption-desorption isotherms, X-ray diffraction (XRD), scanning electron

28 

microscope (SEM), Zetasizer analyzer and X-ray Photoelectron Spectroscopy (XPS).

29 

In general, MIZO showed better performance than IZO within the Cr(VI) adsorption.

Page 1 of 27

The maximum adsorption capacity of MIZO was 59.88 mg/g, which was obviously

31 

greater than that of IZO (37.04 mg/g). Besides, MIZO had a wider pH tolerance range.

32 

When pH increased from 2 to 8, the Cr(VI) removal rate of MIZO decreased from

33 

94% to 81% and that of IZO decreased from 88% to 43%. Further, MIZO had higher

34 

Cr(VI) removal than IZO when coexisting with common anions (chloride, sulfate, etc.)

35 

and regenerated by NaOH. The characterization of MIZO and IZO indicated that the

36 

enhanced adsorption of Cr(VI) on MIZO was closely related to its uniform

37 

mesoporous structure characteristics andhigh zeta potential. Based on the experiment

38 

results, MIZO has been proven to be an excellent adsorbent for Cr(VI).

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Key words: hexavalent chromium; mesoporous structure; iron-zirconium bimetal

40 

oxide; enhanced adsorption

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1. Introduction

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Many wastewaters from industry, such as electroplating, metal cleaning and

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leather tanning, contain high concentration of Cr(VI). Cr(VI) is highly toxic to most

45 

living organisms because of its carcinogenic and mutagenic properties. The typical

46 

Cr(VI) concentration in wastewater varies from 5 to 220 mg/L [1]; it is much higher

47 

than the maximum allowed Cr(VI) concentration in drinking water (0.05 mg/L) and

48 

inland surface water (0.1 mg/L), respectively [2]. Cr(VI) species may be in the form

49 

of hydrogen chromate (HCrO4-,), dichromate (Cr2O72-), or chromate (CrO42-) in

50 

solutions. The repulsive electrostatic interactions between these Cr(VI) anion species

51 

and negatively charged soil particles enable Cr(VI) to transfer freely in aqueous

52 

environments [3]. Therefore, it is definitely necessary to remove Cr(VI) from

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Cr(VI)-containing wastewater before discharging.

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Cr(VI) removal methods include electrocoagulation, chemical precipitation,

55 

membrane separation, ion exchange and adsorption. To our best knowledge,

56 

adsorption is one of the most promising methods due to its high efficiency, low

57 

operation cost, and flexibility of design [4]. The inorganic and organic adsorbents are

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two available adsorbents; the inorganic adsorbents have been proved to be a common

Page 2 of 27

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water treatment material, including activated carbon [5, 6] and metal oxides [7]. In

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comparison with the activated carbon, metal oxides are more promising perspectives

61 

because of the following aspects: i) metal oxides are very common and has good

62 

selectivity to heavy metal ions; ii) metal oxides can be easily prepared into granule

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particles or be in situ formed to realize engineering application [8]. Iron oxide is a low cost, eco-friendly and widely investigated adsorbent for

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anionic pollutants removal [9]. Study revealed that iron (hydr)oxides can effectively

66 

adsorb Cr(VI) and immobilize Cr(VI) [10]. Zirconium oxide has strong affinity to

67 

Cr(VI) and high resistance to attacks from acids, alkalis, oxidants, and reductants [7].

68 

It has been of great interest to the academia and industrial communities to

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incorporating some metals (e.g. aluminium, manganese and cerium) into iron oxides

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to prepare bimetal oxides which may show more excellent physicochemical properties

71 

and higher adsorption capacity than their simple metal oxide [11]. Therefore, an

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iron-zirconium bimetal oxide originating from the combination of iron oxide and

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zirconium oxide is anticipated to show a great potential for Cr(VI) removal.

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Apart from the well-documented influence factors like pH, time, initial

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concentration of pollutant, previous studies also demonstrated that the pore structure

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and size of adsorbents played a significant role in adsorption performance [2].

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Mesoporous material with appropriate size distribution and desired pore structure is

78 

aimed to generate high density of adsorptive sites. The metal oxides with mesoporous

79 

structures have large pore volume and specific surface area, and adjustable pore

80 

diameters [12], which is desirable for adsorption. Some mesoporous materials,

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including mesoporous silica [13], mesoporous carbon [14] and mesoporous metal

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oxide [15], have been tested in Cr(VI) removal , but the results were not satisfactory.

83 

Either the adsorption capacity of adsorbents was low (<23 mg/g) [15], or adsorbent

84 

synthesis procedure needed high temperature (673-873 K) [13], which increased

85 

synthesis cost. Therefore, it is in strong demand to develop a new route for the

86 

synthesis of mesoporous material with high Cr(VI) adsorption capacity. Given the

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great potential of iron-zirconium bimetal oxide for Cr(VI) removal, mesoporous

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iron-zirconium bimetal oxide becomes the first option in the current research.

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In this study, a facile co-precipitation method was established to prepare an

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innovative mesoporous iron-zirconium bimetal oxide (MIZO) and ordinary

91 

iron-zirconium bimetal oxide (IZO). The main objectives of this research are to (i)

92 

characterize the prepared MIZO and IZO; (ii) compare and evaluate their Cr(VI)

93 

adsorption performance; and finally; (iii) explain the adsorption mechanism.

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2. Materials and methods

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2.1 Materials

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Reagents used in this research were analytical grade and purchased from

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Sinopharm Chemical Regent Co., Ltd (Beijing Chemical Company). The stock

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solution of 1000 mg/L Cr(VI) was prepared by dissolving 2.8290 g potassium

99 

dichromate (K2Cr2O7) in 1 L deionized water. Other Cr(VI) solutions were obtained

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by diluting the 1000 mg/L Cr(VI) stock solution with deionized water.

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2.2 Synthesis procedure of MIZO and IZO

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MIZO templated by a cationic surfactant CTAB was synthesized through the

103 

co-precipitation of ferric trichloride (FeCl3) and zirconium oxychloride (ZrOCl2)

104 

solutions. These two solutions were prepared by dissolving FeCl3·6H2O (0.06 mol)

105 

and ZrOCl2·8H2O (0.012 mol) in 100 mL and 20 mL ethanol respectively at room

106 

temperature (298 ± 1 K). Then the two solutions were mixed together by magnetic

107 

with stirring for 30 min. CTAB solution, prepared by dissolving CTAB (0.036 mol) in

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90 mL deionized water, was added to the above iron and zirconium mixture with

109 

intensely stirring to form homogeneous yellow gel. Aqueous ammonia (mass ratio

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25%) was added into the yellow gel till pH up to 9.5±0.02. After this, the yellow gel

111 

was strongly stirred for 60 min, and then placed in a thermostatic oven maintained at

112 

353 K for 24 h. After air-cooled to room temperature, the yellow precipitate was

113 

washed firstly with deionized water for three times and then with ethanol for three

114 

times. Finally, the precipitate was dried in an oven at 376 K for 24 h.

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IZO was synthesized with same procedure, only without adding CTAB solution into the iron and zirconium mixture.

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2.3 Characterization of MIZO and IZO The specific surface area, pore volume, and pore size distribution of MIZO and

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IZO were determined by N2 adsorption/desorption isotherm measurement at 77.3 K

120 

using a model of NOVA 6000 automated gas sorption system (Quantachrome, USA).

121 

Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) model and

122 

pore size distribution was calculated by Barrett-Joyner-Halenda (BJH) model. The

123 

XRD patterns of the adsorbents were determined by RIGAKU UltimaIV-185 X-ray

124 

diffractometer (Cu Ka radiation, λ=1.5406 A) over a range of 0-90° operated at 40 mA

125 

and 40 kV, with a scan rate of 4°/min and step size of 0.02°. The surface morphology

126 

was determined using a scanning electron microscope (SEM, S-3500N, Hitachi Ltd.,

127 

Japan). The zeta potential of the adsorbents before and after adsorption was measured

128 

by a Zetasizer analyzer (Nano ZS, Malvern Co., UK) at room temperature. The

129 

adsorbents before and after adsorption was also analyzed using an X-ray

130 

photoelectron spectrometer (XPS, Kratos AXIS ULTRA DLD, Japan) with an Al-Kα

131 

X-ray source. The XPS results were corrected by C1s at a binding enegery of 284.6

132 

eV. The CasaXPS software was used to fit the XPS spectra peaks. The atomic

133 

percentages of different elements and O species were obtained in terms of the relative

134 

ratio of area.

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2.4 Sorption experiments

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Batch sorption experiments include sorption isotherm, sorption kinetics, the

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effect of pH and coexistent anions on Cr(VI) adsorption onto MIZO and IZO. All

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sorption experiments were conducted in well capped 50 mL centrifuge tubes. 40 mL

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Cr(VI) solution with required concentration and 40 mg MIZO or IZO were added into

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a tube and then the tube was shaken in a thermostatic shaker (ZHWY-2102C,

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Shanghai Zhicheng Analytical Instrument Manufacting Co., Ltd) with 210 rpm at 303

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K for 24 h. After shaking, the adsorbent was separated by 0.45μm cellulose acetate

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membrane. The residual Cr(VI) concentration in the filtrate was determined

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spectrophotometrically at 540 nm using a UV-vis spectrophotometer (UV-1601,

145 

Shimadzu), by following the 1, 5- diphenycarbazide method (APHA, 1998). The pH

146 

value of the experimental solutions was adjusted by dilute HCl and NaOH solutions.

Page 5 of 27

147  148 

In order to verify the result of this study, all the experiments were repeated twice. The initial Cr(VI) concentration, initial pH of the Cr(VI) solution, and adsorbent dosage were fixed at 50 mg/L, 5±0.02, and 1 g/L, respectively, unless otherwise stated.

150 

Sorption isotherms were studied by varying initial Cr(VI) concentration from 5 mg/L

151 

to 100 mg/L, and the data were fitted using both Langmuir and Freundlich isotherm

152 

model. Kinetic studies were carried out by taking samples at predetermined time

153 

intervals. Lagergren pseudo-first-order and pseudo-second-order models were applied

154 

to analyze the data. Effect of pH on Cr(VI) removal was tested under initial pH of the

155 

Cr(VI) solutions ranging from 1.0 to 10.0. The effect of common coexistent anions in

156 

wastewater on Cr(VI) adsorption was determined by adding chloride (Cl-), nitrate

157 

(NO3-), acetate (CH3COO-), sulfate (SO42-), phosphate (H2PO4-), and carbonate (CO32-)

158 

into Cr(VI) solutions separately. The concentration of the coexistent anions ranged

159 

from 1 to 100 mmol/L.

160 

2.5 Desorption experiments

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To study the regeneration and reusability of the adsorbents, four consecutive

162 

sorption/regeneration cycles were carried out. In the tests, 40 mg adsorbent after the

163 

adsorption of Cr(VI) was put into 20 mL 0.05 mol/L NaOH solution, and the mixture

164 

was then shaken in the thermostatic shaker at 210 rpm at 303 K for 4 h for desorption.

165 

After each cycle of sorption/desorption, the regenerated adsorbent was washed with

166 

deionized water until neutral pH was researched, and then the adsorbent was filtered

167 

and freeze dried for reuse in the next cycle.

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2.6 Calculation of Cr(VI) removal rate and adsorption capacity

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The Cr(VI) percentage removal (removal rate) was determined using Eq. (1):

% removal =

C0 − Ce × 100                                                                                             (1) C0

171 

where, C0 (mg/L) and Ce (mg/L) are the initial Cr(VI) concentration and the

172 

equilibrium Cr(VI) concentration, respectively.

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Cr(VI) adsorption capacity of MIZO or IZO was calculated from the following mass balance relation:

Page 6 of 27

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qe =

(C0 − Ce ) ×V m

(2)

where, qe (mg/g) is the equilibrium amount of Cr(VI) adsorbed per unite mass of

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adsorbent. V (L) and m (g) are the test solution volume and mass of adsorbent,

178 

respectively.

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3. Results and discussion

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3.1 Characterization of the adsorbents

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The N2 adsorption/desorption isotherms and pore distributions of MIZO and IZO

182 

are shown in Fig. 1. The specific surface area and porosity are shown in Table 1.

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According to IUPAC classification, the adsorption/desorption isotherms of MIZO

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were type IV with H1 hysteresis loops at P/P0 range from 0.5 to 0.9, which is typical

185 

for mesoporous materials [12, 16]. No similar hysteresis loops were found in IZO.

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The pore size distribution of MIZO was much narrower and steeper than that of IZO.

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This means that the pore distribution of MIZO was more uniform. Also, MIZO had

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larger pore volume and average pore size than IZO.

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Fig. 2 shows the XRD patterns of MIZO and IZO. For IZO, no distinct principle

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peaks for any phase of Fe2O3 or ZrO2 were observed, indicating iron oxides and

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zirconium oxides are in amorphous phase in IZO. For MIZO, peaks at 33.14°, 35.84°,

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40.52°, 54.25°, and 64.15° in agreement with the phase of α-Fe2O3 (JCPDS

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No.33-0664), and peaks at 28.38° and 34.17° represented ZrO2 (JCPDS No.78-0047)

194 

were found. These peaks of Fe2O3 and ZrO2 observed in MIZO were of weak intensity.

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Thus, both the crystallinity of MIZO and IZO were poor.

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The morphology of MIZO and IZO were observed through SEM and shown in

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Fig. 3. MIZO was composed of relatively uniform and homogeneous spherical

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particles (Fig. 3a), while IZO was composed of dispersed irregular nanoparticles with

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various sizes (Fig. 3b). Hence, the addition of CTAB promoted MIZO to form

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particles with uniform sizes.

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The zero charge (pHzpc) values of MIZO and IZO were determined by plotting

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the zeta potential values of the particulates suspension solutions (1 g/L) versus pH

203 

(Fig. 4). As pH increased, the zeta potential of MIZO and IZO decreased from

Page 7 of 27

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positive value to negative value. The pHzpc of MIZO is about 10.2 and that of IZO is

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7.5. This result indicates that the addition of CTAB to prepare iron-zirconium bimetal

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oxide would raise the pHzpc value of the product.

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3.2 Effect of initial Cr(VI) concentration and adsorption isotherms The Cr(VI) removal rate of MIZO and IZO for different initial Cr(VI)

209 

concentrations and adsorption isotherms are shown in Fig. 5. For both MIZO and IZO,

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the Cr(VI) removal decreased when initial Cr(VI) concentration increased (Fig. 5a).

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MIZO possessed higher removal rate than that of IZO in the entire concentration

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range (5-100 mg/L). In low initial Cr(VI) concentration (< 20 mg/L), both MIZO and

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IZO were excellent for Cr(VI) adsorption, with removal rate above 96%. After initial

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Cr(VI) concentration exceeding 25 mg/L, Cr(VI) adsorbed by IZO met a significant

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drop, while Cr(VI) adsorbed by MIZO can still maintain above 93% at initial Cr(VI)

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concentration below 50 mg/L. Obviously, MIZO had a better adsorption performance

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in a wider initial Cr(VI) concentration range than IZO.

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To evaluate the maximum adsorption capacity of the adsorbents and calculate

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adsorption parameters, Langmuir and Freundlich isotherm models were used for data

220 

analysis. The Langmuir adsorption isotherm describes the homogeneous surface,

221 

assuming that adsorption at one site does not affect adsorption at an adjacent site. The

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linear equation of Langmuir model was represented as follows [17]:

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Ce Ce 1 + = qe Qm bQm

(3)

The Freundlich isotherm is assuming a heterogeneous surface. This model is

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summation of adsorption on all surface sites with different bonds. The equation is

226 

given in logarithm form as follows [18]:

227 

ln qe =

1 Ce + ln K F n

(4)

228 

where, qe (mg/g) is the equilibrium adsorption capacity of the adsorbents and Ce

229 

(mg/L) is the equilibrium concentration of the adsorbate; Qm (mg/g) is the monolayer

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adsorption capacity and b is a constant related to the free adsorption energy; KF and

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1/n are Freundlich constants corresponding to adsorption capacity and adsorption

Page 8 of 27

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intensity, respectively. The isotherm parameters were calculated from the linear fitting of the above

234 

equations (Eq.(3) and Eq.(4)) and the results are presented in Table 2. By comparing

235 

the regression coefficients (R2) of the two models, it was obvious that Langmuir

236 

model can better describe the adsorption behavior of Cr(VI) onto both MIZO and IZO

237 

than Freundlich model, indicating that the adsorbed layer was monolayer coverage.

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From Langmuir adsorption isotherms, the calculated maximum adsorption of MIZO

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for Cr(VI) was 59.88 mg/g, which was evidently greater than that of IZO (37.04

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mg/g). Enhancement of adsorption capacity of MIZO was possibly due to its uniform

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pore size distribution, which can enhance Cr(VI) accessibility to the active sites [19]

242 

and the highly positive charged surface properties made it have strong affinity to

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anions. This result also shows that the key factor influencing on the adsorption

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capacity is a uniform pore size but not the surface area of the adsorbent. Because both

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the BET surface areas values of MIZO and IZO are between 75 and 78 m2/g. Previous

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researches also reported a similar result that the Qm did not directly correlate with the

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specific areas of the materials [20].

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A dimensionless separation factor RL (Eq. (5)) [21] was used to test whether the

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adsorption of Cr(VI) onto the adsorbents was favorable or not, expressed as follows: RL=1/(1+bC0)

(5)

251 

where, C0 (mg/L) is the initial solute concentration and b (L/mg) is the Langmuir

252 

adsorption equilibrium constant. The value of RL indicates the type of the isotherm to

253 

be either unfavorable (RL>1), linear (RL=1), favorable (0< RL<1) or irreversible (RL=0).

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The values of RL with initial Cr(VI) concentration of 50 mg/L for MIZO and IZO

255 

were 0.015 and 0.021, respectively. And the values of RL for the current experimental

256 

conditions were all in the range of 0-1, which confirmed the favorable uptake of

257 

Cr(VI) onto the MIZO and IZO.

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To further estimate the performance of MIZO and IZO for Cr(VI) removal, their

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Qm values were compared to other adsorbents reported in previous researches in Table

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3. Nevertheless, the experimental conditions such as pH, initial Cr(VI) concentration

261 

and operating temperature, were different from one research to another, in general,

Page 9 of 27

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MIZO is efficient for Cr(VI) removal with Qm 59.88 mg/g and may be developed as a

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potential adsorbent for decontamination of Cr(VI) solutions.

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3.3 Effect of contact time and adsorption kinetics  Fig. 6 shows the Cr(VI) adsorption capacities of MIZO and IZO for Cr(VI) as a

266 

function of time. Both the adsorption of Cr(VI) onto MIZO and IZO were very fast.

267 

The amount of the adsorbed Cr(VI) on MIZO was greater than that of IZO at a given

268 

time during the adsorption process. The adsorption rate of Cr(VI) on these two

269 

adsorbents was slightly different. In the first 30 min, about 97% and 99% of the Cr(VI)

270 

equilibrium adsorption capacity were adsorbed onto MIZO and IZO, respectively.

271 

After that, the adsorbed Cr(VI) amount onto MIZO showed a slight increase and that

272 

of IZO remained almost constant. The equilibrium time for both MIZO and IZO were

273 

less than 1 hr, much faster than those of peat (about 6 hr) and activated carbon (about

274 

10-70 hr) [22]. The rapid approach to equilibrium suggests that most of the adsorption

275 

sites of the adsorbents existed in the exterior of the adsorbents and were easily

276 

accessible by the Cr(VI) species.

cr

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adsorption

kinetics

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data

were

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analyzed

by

using

Lagergren

pseudo-first-order [23] (Eq. (6)) and pseudo-second-order [24] (Eq. (7)) kinetic

279 

models, expressed as follows:

280 

281 

282  283 

Ac ce pt e

278 

ln(qe − qt ) = ln qe − K1t

(6)

t 1 t = + 2 qt K 2 q e qe

(7)

The initial adsorption rate h (mg/g·min) can be defined as: h = K 2 qe 2

(min-1)

284 

where,

285 

pseudo-second-order adsorption rate constant, respectively; qe (mg/g) and qt (mg/g)

286 

are the adsorption capacity at equilibrium and time t, respectively. The kinetic

287 

parameters estimated by the two models are presented in Table 4. It is evident from

288 

the estimated regression correlation (R2) that for both MIZO and IZO, the data were

289 

described better by pseudo-second-order model than pseudo-first-order, implying the

K1

and

K2

(g/mg·min)

(8) are

the

pseudo-first-order

and

Page 10 of 27

rate-limiting step may be a chemical sorption involving valency forces through

291 

sharing or exchanging of electrons between adsorbent and adsorbate. The K2 value

292 

estimated for MIZO (0.02 g/mg/min) was less than that for IZO (0.03 g/mg/min),

293 

suggesting that adsorption rate of Cr(VI) by MIZO was in some extent slower that of

294 

IZO.

295 

3.4 Effect of pH

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The effect of pH on Cr(VI) removal is shown in Fig. 7. Overall, acidic

297 

environment was favorable for Cr (VI) adsorption. pH had more significant effect on

298 

the adsorption of Cr(VI) onto IZO than onto MIZO. The highest Cr(VI) removal rate

299 

by adsorption of MIZO and IZO occurred at pH about 3 and 2, respectively. After this

300 

point, the removal rate of Cr(VI) adsorbed by MIZO was still more than 87% as the

301 

pH increased to 6, slightly reduced to 81% as pH increased to 8, and then reduced

302 

significantly when the initial pH increased from 8.0 to 10.0. However, the Cr(VI)

303 

removal by IZO experienced a direct decrease after pH 2.

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The above pH dependency is largely related to the Cr(VI) speciation and the

305 

properties of the adsorbent. The following are important equilibrium reactions

306 

affected by solution pH for Cr(VI) species [2]:

308  309  310 

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d

304 

⎯⎯ → H + + CrO4 2− , pKa=5.9 HCrO4 − ←⎯ ⎯

(8)

⎯⎯ → H + + HCrO4 − , pKa=4.1 H 2CrO4 ←⎯ ⎯

(9)

⎯⎯ → 2 HCrO4 − , pKa=2.2 Cr2O7 2 − + H 2O ←⎯ ⎯

(10)

The surface charge of IZO was positive below pHzpc 7.5 (Fig. 4), which was

311 

caused by the protonation of hydroxyl groups on the surface of IZO. The relatively

312 

higher removal of Cr(VI) in the pH range of 2-6 was mainly due to the electrostatic

313 

attraction between positive surface and Cr(VI) anions. The reason for the removal

314 

decrease in this pH range was that the protonation was weakened as pH increased.

315 

Alkaline pH range resulted sharply decrease in Cr(VI) removal by IZO, because (i)

316 

strong electrostatic repulsion between the negatively charge sites on the surface and

317 

Cr(VI) anions when the solution pH is higher than pHzpc of IZO and (ii) competition

318 

adsorption between OH- and CrO42- for sorption sites [12]. In contrast, the surface

Page 11 of 27

charge of MIZO was positive in the whole experimental pH range (1-10) because its

320 

pHzpc is higher than 11 (Fig. 4). This characteristic of MIZO contributed to its high

321 

Cr(VI) removal in a wider pH range. The decrease in Cr(VI) removal rate of MIZO at

322 

pH above 8 might be attributed to the competition adsorption between OH- and Cr(VI)

323 

for sorption sites. The significant decline of Cr(VI) removal by MIZO and IZO under

324 

strongly acidic conditions (pH<2) adsorption may be caused by the dissolution of the

325 

adsorbents at too low solution pHs. Besides, when the concentration of H+ ions is too

326 

high (i.e., low pH), CrO42- ions are probably transferred to Cr2O72-. Because the

327 

dimension of Cr2O72- ion is about twice as large as a CrO42-, making it difficult to

328 

enter the pores on the adsorbents [16].

us

cr

ip t

319 

Most adsorbents reported in previous researches were effective for Cr(VI)

330 

removal at pH less than 6.0. However, at neutral to alkaline pH, the Cr(VI) removal

331 

capacity is drastically reduced by these adsorbents [25, 26]. Nevertheless, many

332 

wastewaters exist at near neutral to slightly alkaline pH. Therefore, an adsorbent with

333 

high Cr(VI) removal at neutral to alkaline pH would be very effective in such cases.

334 

Although the removal rates of MIZO for Cr(VI) at near neutral to slightly alkaline pH

335 

(removal rate 81-87% when pH is 6.0-8.0) are less than that at acidic pHs (removal

336 

rate>90%), MIZO is an acceptable adsorbent at near neutral to slightly alkaline

337 

conditions.

338 

3.5 Effect of coexistent anions

M

d

Ac ce pt e

339 

an

329 

As it is known, various anions may coexist with Cr(VI) in wastewaters and

340 

compete with Cr(VI) for adsorption sites. Fig. 8 shows the effect of six common

341 

anions (Cl-, NO3-, CH3COO-, SO42-, H2PO4-, CO32-) on Cr(VI) adsorbed by MIZO and

342 

IZO. Each anion was investigated at a constant concentration range (1-100 mmol/L).

343 

For each coexistent anion, the adsorption capacity decreased as the concentration of

344 

the anion increased. Compared with IZO, MIZO had higher or same adsorption

345 

capacity when coexisting with the coexistent anions except for CO32- with a

346 

concentration range of 10-100 mmol/L. The effect of these coexistent anions on

347 

MIZO

348 

CO32->H2PO4->SO42->CH3COO->NO3-≈Cl-.

for

Cr(VI)

adsorption And

followed this

order

the of

order: IZO

was:

Page 12 of 27

H2PO4->CO32->SO42->CH3COO->NO3-≈Cl-. The different effect order of MIZO and

350 

IZO may due to their different chemical composition. Further analysis revealed that

351 

Cl- and NO3- exhibited no noticeable effect on the removal of Cr(VI). SO42-, H2PO4-

352 

and CO32- markedly inhibited the Cr(VI) adsorption. For example, the presence of

353 

SO42-, H2PO4- and CO32- with concentration of 100 mmol/L would decrease the Cr(VI)

354 

adsorbed by MIZO from 46 to 12, 6 and 2 mg/g, respectively, and that of IZO was

355 

from 32 to 12, 5 and 5 mg/g, respectively. The great decrease of the coexisting with

356 

H2PO4- can be explained by their similar tetrahedral structure that both chromium and

357 

phosphate have [27]. The decrease caused by CO32- might due to the formation of

358 

chromyl-carbonate complexes which would prevent the complexing between Cr(VI)

359 

and adsorption sites of the metal oxides. The middle effect of CH3COO- is may be

360 

caused by the organic and weakly acidic properties of CHCOOH. Because, as a kind

361 

of organic, it will complex with the adsorbents and thus inhibit Cr(VI) adsorption. On

362 

the other hand, as a kind of acids, it will ionize hydrogen ions to reduce the pH of the

363 

solution and improve the adsorption of Cr(VI).

364 

3.6 Desorption and reuse

d

M

an

us

cr

ip t

349 

Considering the practical applicability, the regeneration and reuse of the

366 

adsorbents are also important. In this study, the adsorption capacities of MIZO and

367 

IZO for Cr(VI) in the successive sorption/regeneration cycles were shown in Fig. 9.

368 

After the first regeneration, the adsorption capacity of MIZO for Cr(VI) decreased

369 

from 47.07 to 44.32 mg/g, and that of IZO decreased from 30.88 to 28.31 mg/g.

370 

Slightly decreases were observed in the following three cycles for both adsorbents.

371 

The adsorption capacity of Cr(VI) on the regenerated MIZO was 38.06 mg/g in the

372 

fourth cycle and the adsorbent still remained more than 80% of its original Cr(VI)

373 

adsorption capacity, and these values of IZO were 23.66 and 76%,respectively. These

374 

indicate that the Cr-loaded adsorbents can be effectively desorbed and regenerated via

375 

NaOH treatment.

376 

3.7 Adsorption mechanism

Ac ce pt e

365 

377 

To elucidate the interactions at the interface of solid-liquid, zeta potential

378 

variations of MIZO at different pH values before and after the adsorption of Cr(VI)

Page 13 of 27

were tested, as shown in Fig. 4. The pHzpc of MIZO was about 10.2This indicated that

380 

the adsorbent surface was positively charged at pH below 10.2 and electrostatic

381 

attraction existed between Cr(VI) anions and adsorbent surface. After Cr(VI) sorption,

382 

the pHpzc of adsorbent was decreased to 8.2. Clearly, the presence of Cr(VI) anions

383 

made the surface of MIZO more negatively charged. It has been reported that the

384 

formation of outer-sphere surface complexes cannot shift the pHpzc because no

385 

specific chemical reactions took place between the adsorbate and the surface of

386 

adsorbent [28]. The results of zeta potentials measurements indicate that Cr(VI)

387 

formed negatively charged inner-sphere complexes on MIZO. Therefore, specific

388 

adsorption rather than a purely electrostatic force existed at the aqueous Cr(VI)/MIZO

389 

interface.

us

cr

ip t

379 

To find out the probable interaction process between MIZO and Cr(VI), and the

391 

active adsorption sites of MIZO, the XPS spectra of MIZO before and after Cr(VI)

392 

adsorption were analyzed. The atomic percentages of different elements on the

393 

surface of original and Cr-loaded MIZO were summarized in Table 5. After Cr(VI)

394 

adsorption, the Cl percentage on MIZO decreased from 1.7% to 0.3% while the Cr

395 

percentage on Cr-loaded MIZO surface increased from 0% to 5.3%. The deceased Cl

396 

is replaced by Cr. Although the replacement of Cl might play some role in Cr

397 

adsorption, it was pointed out that the release of Cl at higher Cr(VI) concentrations

398 

was relatively low. Furthermore, the Cl percentage was only 1.7% on the surface of

399 

original MIZO. When the exchangeable Cl on its surface was completely replaced by

400 

Cr, it was far less than the observed removal Cr(VI). Therefore, it was presumed that

401 

Cl exchange was not the dominant mechanism for Cr(VI) adsorption.

M

d

Ac ce pt e

402 

an

390 

Fig.11 illustrated the XPS of Cr 2p, Fe 2p, Zr 3d and O 1s of MIZO before and

403 

after Cr(VI) adsorption. The high resolution Cr 2p spectrum of Cr-loaded MIZO (Fig.

404 

11a) showed that both Cr 2p3/2 and Cr 2p1/2 were appeared at 573~578 eV and

405 

583~589 eV, respectively [29]. Binding energy 576.89 and 574.75 eV were obtained

406 

after XPS-peak-differentation-imitating analysis of Cr 2P3/2, which represented

407 

Cr(VI) and Cr(III) [30], respectively. Hence, Cr(VI) was reduced to Cr(III) on MIZO

408 

surface during the adsorption. The Cr(III) accounted for 21.75% of Cr(VI) based on

Page 14 of 27

409 

the peak area. After Cr(VI) adsorption, the peaks for Fe 2p (Fig. 11b) and Zr 3d (Fig. 11b)

411 

moved to lower energy, probably suggesting that hydroxyl groups bonded to metal

412 

(M-OH) were involved in Cr(VI) adsorption and the M-O groups were with low

413 

binding energies formed. Furthermore, according to the binding energy of different

414 

oxygen species on MIZO, the O 1s XPS spectra were divided into two peaks, which

415 

were separately assigned to M-OH (hydroxyl boned to metal) and M-O (oxygen

416 

bonded to metal). As for the M-OH, its percentage decreased from 81.5 % to 0% after

417 

Cr(VI) adsorption whereas the percentage of M-O obviously increased from 18.5% to

418 

99.9%. The decreased of M-OH ratio further confirmed that hydroxyl groups on the

419 

adsorbent surface were involved in Cr(VI) adsorption, and it’s the main adsorption

420 

mechanisms.

an

us

cr

ip t

410 

According to the zeta potential andXPS analyses, a conclusion can be drawn that

422 

electrostatic attraction, ion exchange and complexation were simultaneously

423 

responsible for Cr(VI) adsorption onto MIZO, and the formation of inner-sphere

424 

complexes through hydroxy replacement played a dominant role.

425 

4. Conclusions

d

Ac ce pt e

426 

M

421 

In this study, MIZO and IZO have been synthesized and used as adsorbents for

427 

the removal of Cr(VI) in solutions. It was found that the Cr(VI) adsorption isotherm

428 

of MIZO and IZO can be excellently fitted on Langmuir model. The maximum

429 

adsorption capacity of Cr(VI) onto MIZO and IZO were calculated as 59.88 and 37.04

430 

mg/g, respectively. Both the adsorption rate of MIZO and IZO for Cr(VI) were rapid

431 

and the adsorption kinetics fitted Lagergren pseudo-second order. MIZO can adapt to

432 

a broader pH range (2-8) than IZO (2-4). Coexistent SO42-, H2PO4- and CO32- anions

433 

competed aggressively with Cr(VI) anions for adsorption sites, and MIZO had higher

434 

Cr(VI) removal rate for most testing coexistent anions. The enhancement of Cr(VI)

435 

adsorption onto MIZO than IZO may be attributed to the uniform mesoporous

436 

structure characteristics and higher pHzpc value of MIZO. In addition, MIZO could be

437 

effectively regenerated by NaOH solutions. Moreover, in contrast to the traditional

438 

methods to prepare mesoporous metal oxide, our synthesis procedure only involves a

Page 15 of 27

439 

simple adding CTAB as template by co-precipitation, without the need for extreme

440 

high reaction temperature and/or strict synthesis protocols. The excellent adsorption

441 

efficiency for Cr(VI) and facile synthesis procedure make MIZO a promising

442 

adsorbent for decontamination of Cr(VI) pollution in water systems.

443 

Acknowledgments This study was funded by the National Natural Science Foundation of China (No.

ip t

444 

51108298), the Natural Science Foundation of Tianjin (No. 12JCYBJC14800).

446 

References

447  448  449  450  451  452  453  454  455  456  457  458  459  460  461  462  463  464  465  466  467  468  469  470  471  472  473  474  475  476  477  478 

[1] P.K. Ghosh, Hexavalent chromium [Cr(VI)] removal by acid modified waste activated carbons, J.

cr

445 

us

Hazard. Mater. 171 (2009) 116-122.

[2] X.-H. Wang, F.-F. Liu, L. Lu, S. Yang, Y. Zhao, L.-B. Sun, S.-G. Wang, Individual and competitive adsorption of Cr (VI) and phosphate onto synthetic Fe–Al hydroxides, Colloids Surf., A:

an

Physicochemical and Engineering Aspects. 423 (2013) 42-49.

[3] Y. Li, B. Gao, T. Wu, D. Sun, X. Li, B. Wang, F. Lu, Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide, Water Res. 43 (2009) 3067-3075. [4] C.E. Barrera-Díaz, V. Lugo-Lugo, B. Bilyeu, A review of chemical, electrochemical and biological

M

methods for aqueous Cr (VI) reduction, J. Hazard. Mater. 223 (2012) 1-12. [5] Z. Hu, L. Lei, Y. Li, Y. Ni, Chromium adsorption on high-performance activated carbons from aqueous solution, Sep. Purif. Technol. 31 (2003) 13-18.

d

[6] T. Karthikeyan, S. Rajgopal, L.R. Miranda, Chromium (VI) adsorption from aqueous solution by Hevea Brasilinesis sawdust activated carbon, J. Hazard. Mater. 124 (2005) 192-199.

Ac ce pt e

[7] L.A. Rodrigues, L.J. Maschio, R.E. da Silva, M.L.C.P. da Silva, Adsorption of Cr (VI) from aqueous solution by hydrous zirconium oxide, J. Hazard. Mater.173 (2010) 630-636. [8] K. Wu, H. Wang, R. Liu, X. Zhao, H. Liu, J. Qu, Arsenic removal from a high-arsenic wastewater using in situ formed Fe–Mn binary oxide combined with coagulation by poly-aluminum chloride, J. Hazard. Mater. 185 (2011) 990-995.

[9] L. Zeng, X. Li, J. Liu, Adsorptive removal of phosphate from aqueous solutions using iron oxide tailings, Water Res. 38 (2004) 1318-1326.

[10] X. Ke, L. Chunguang, L. Juntan, P. Weigong, Study on chromium (VI) removal from aqueous solution using Fe-Mn bimetal oxide, Electric Technology and Civil Engineering (ICETCE), 2011 International Conference on, IEEE, Lushan, 2011, pp. 1569-1572. [11] Y. Zhang, M. Yang, X.-M. Dou, H. He, D.-S. Wang, Arsenate Adsorption on an Fe−Ce Bimetal Oxide Adsorbent:  Role of Surface Properties, Environ. Sci. Technol. 39 (2005) 7246-7253. [12] B. Chen, Z. Zhu, Y. Guo, Y. Qiu, J. Zhao, Facile synthesis of mesoporous Ce–Fe bimetal oxide and its enhanced adsorption of arsenate from aqueous solutions, J. Colloid Interface Sci. 398 (2013) 142-151. [13] J. Li, X. Miao, Y. Hao, J. Zhao, X. Sun, L. Wang, Synthesis, amino-functionalization of mesoporous silica and its adsorption of Cr (VI), . Colloid Interface Sci. 318 (2008) 309-314. [14] Y. Guo, J. Qi, S. Yang, K. Yu, Z. Wang, H. Xu, Adsorption of Cr (VI) on micro-and mesoporous rice husk-based active carbon, Mater. Chem. Phys. 78 (2003) 132-137.

Page 16 of 27

479  480  481  482  483  484 

[15] P. Wang, I. Lo, Synthesis of mesoporous magnetic γ-Fe2O3 and its application to Cr (VI) removal

485  486  487  488  489  490  491  492  493  494  495  496  497  498  499  500  501  502  503  504  505  506  507  508 

[18] L. Wei, G. Yang, R. Wang, W. Ma, Selective adsorption and separation of chromium (VI) on the magnetic iron–nickel oxide from waste nickel liquid. J. Hazard. Mater. 164 (2009), 1159-1163. [19] W. Xu, J. Wang, L. Wang, G. Sheng, J. Liu, H. Yu, X.-J. Huang, Enhanced arsenic removal from water by hierarchically porous CeO2–ZrO2 nanospheres: Role of surface- and structure-dependent properties, J. Hazard. Mater. 260 (2013) 498-507. [20] Y. Kim, C. Kim, I. Choi, S. Rengaraj, J. Yi, Arsenic removal using mesoporous alumina prepared via a templating method, Environ. Sci. Technol. 38 (2004) 924-931. [21] S. Yadav, V. Srivastava, S. Banerjee, C.-H. Weng, Y.C. Sharma, Adsorption characteristics of modified sand for the removal of hexavalent chromium ions from aqueous solutions: Kinetic, thermodynamic and equilibrium studies, Catena. 100 (2013) 120-127. [22] P. Yuan, M. Fan, D. Yang, H. He, D. Liu, A. Yuan, J. Zhu, T. Chen, Montmorillonite-supported magnetite nanoparticles for the removal of hexavalent chromium [Cr(VI)] from aqueous solutions, J. Hazard. Mater. 166 (2009) 821-829. [23] Y. Ho, G. McKay, The sorption of lead (II) ions on peat, Water Res. 33 (1999) 578-584. [24] Y.-S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451-465. [25] S. Deng, R. Bai, Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms, Water Res. 38 (2004) 2424-2432. [26] C.-H. Weng, Y. Sharma, S.-H. Chu, Adsorption of Cr (VI) from aqueous solutions by spent activated clay, J. Hazard. Mater. 155 (2008) 65-75. [27] C. Su, R.W. Puls, Arsenate and arsenite removal by zerovalent iron: kinetics, redox transformation, and implications for in situ groundwater remediation, Environ. Sci. Technol. 35 (2001) 1487-1492. [28] G. Zhang, H. Liu, R. Liu, J. Qu, Removal of phosphate from water by a Fe–Mn binary oxide adsorbent, J. Colloid Interface Sci. 335 (2009) 168-174.

509  510  511  512 

[29] D. Park, Y.-S. Yun, J. M. Park, XAS and XPS studies o chromium-binding groups of biomaterial during Cr(VI) biosorption, J. Colloid Interface Sci. 317 (2008) 54-61. [30] M. C. Biesinger, C. Brown, J. R. Mycroft, R. D. Davidson, N. S. Mclntyre, X-ray photoeletron spectroscopy studies of chromium compounds, Surf. Interface Anal. 36 (2004) 1550-1563. 

[16] S. Asuha, X.G. Zhou, S. Zhao, Adsorption of methyl orange and Cr(VI) on mesoporous TiO2 prepared by hydrothermal method, J. Hazard. Mater. 181 (2010) 204-210. [17] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem.

d

M

an

us

cr

ip t

Soc. 40 (1918) 1361-1403.

Ac ce pt e

513 

from contaminated water, Water Res. 43 (2009) 3727-3734.

Page 17 of 27

513  514  515  516  517  518  519 

Table legends

520 

Table 5 Atomic percentages from XPS analysis of MIZO before and after Cr(VI) adsorption

Table 1 BET surface area, average pore diameters (BJH), and pore volume of MIZO and IZO Table 2 Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption onto MIZO and IZO (adsorbent dosage=1g/L, pH=5.0±0.02, T=303 K) Table 3 Comparison of adsorption capacities for Cr(VI) onto MIZO and other reported adsorbents. Table 4 Largergren pseudo-first-order and pseudo-second-order parameters for Cr(VI) adsorption onto MIZO and IZO (initial Cr(VI)=50 mg/L, adsorbent dosage=1g/L, pH=5.0±0.02, T=303 K)

Ac ce pt e

d

M

an

us

cr

ip t

521 

Page 18 of 27

521  Fig. 1. N2 adsorption/desorption isotherms of (a) MIZO and (b) IZO. Inset the corresponding pore size distribution of MIZO and IZO. Fig. 2. XRD patterns of MIZO and IZO. Fig. 3. SEM images of the adsorbents: (a) MIZO and (b) IZO Fig. 4. Zeta potentials of MIZO, IZO and Cr-loaded MIZO as a function of pH Fig. 5. (a) Cr(VI) removal rate of MIZO and IZO as a function of different initial Cr(VI) concentrations.

ip t

(b) Adsorption isotherms of Cr(VI) onto MIZO and IZO (pH=5±0.02, adsorbent dosage =1g/L, T=303 K) (initial Cr(VI)= 50 mg/L, pH=5±0.02, adsorbent dosage =1g/L, T=303 K) Fig. 7. Effect of initial pH on Cr(VI) adsorption onto MIZO and IZO

us

(initial Cr(VI)= 50 mg/L, adsorbent dosage =1g/L, T=303 K)

cr

Fig. 6. Adsorption kinetics of Cr(VI) onto MIZO and IZO.

Fig. 8. Effect of coexistent anions on Cr(VI) adsorption onto the adsorbents: (a) MIZO and (b) IZO.

an

(initial Cr(VI)= 50 mg/L, adsorpbent dosage=1 g/L, initial pH= 5±0.02, T=303 K)

M

Fig. 9. Cr(VI) adsorption capacity of MIZO and IZO in four successive adsorption-desorption cycles. Fig. 10. XPS of (a) Cr 2p, (b) Fe 2p, (c) Zr 3d and (d) O 1s on MIZO and Cr-loaded MIZO  

d

537  538  539  540  541  542 

Figures Captions

Ac ce pt e

522  523  524  525  526  527  528  529  530  531  532  533  534  535  536 

Page 19 of 27

542 

Table 1 BET surface area, average pore diameters (BJH), and pore volume of MIZO and IZO Sample

Average pore size (nm)

BET area (m2/g)

Pore volume (cm3/g)

MIZO IZO

6.50 3.76

75.843 77.836

0.165 0.146

543  Table 2 Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption onto MIZO and IZO (adsorbent dosage=1 g/L, pH=5.0±0.02, T=303 K)

MIZO IZO

Langmuir isotherm

Freundlich isotherm 2

Qm (mg/g)

b (L/mg)

R

KF

59.88 37.04

1.34 0.92

0.9992 0.9946

21.68 14.57

546 

R2

2.89 3.81

0.8915 0.7989

Capacity (mg/g)

reference

FS-100

3.0

69.3

[5]

SHT

3.0

F-400

2.0

Synthesized activated carbons

Rubber wood saw dust

2.0

Terminalia arjuna nuts Hazelnut shell Fe-Al Fe-Zr

69.1

[5]

48.5

[31]

44.1

[6]

1.0

28.4

[32]

2.0

17.7

[33]

4.5

47.4

[2]

5.0

37.0

Current research

Ac ce pt e

Bimetal (hydr)oxide

Mesoporous adsorbents

an

pH

d

Commercial activated carbons

us

Table 3 Comparison of adsorption capacities for Cr(VI) onto MIZO and other reported adsorbents Adsorbent type

548  549 

n

M

547 

ip t

Sample

cr

544  545 

Fe-Ni

5.0

30.0

[18]

Fe-Mn

6.0

8.0

[10]

Fe-Zr

5.0

59.9

Current research

TiO2

6.0

33.9

[16]

Active carbon

5.0

28.2

[14]

r-Fe2O3

2.5

11.6

[15]

Page 20 of 27

Table 4 Largergren pseudo-first order and pseudo-second order parameters for Cr(VI) adsorption onto MIZO and IZO (initial Cr(VI)=50 mg/L, adsorbent dosage=1 g/L, pH=5.0±0.02, T=303 K) Pseudo-first order K1 min-1

qe mg/g

R2

K2 g/mg/min

qe mg/g

h mg/g/min

R2

MIZO IZO

0.812 0.449

45.47 30.30

0.8356 0.8086

0.02 0.03

47.10 31.35

44.37 29.28

0.9996 0.9999

ip t

Sample

Table 5 Atomic percentages from XPS analysis of MIZO before and after Cr(VI) adsorption Atomic percentages %

Fe

Zr

O

Cl

Cr

MIZO Cr-loaded MIZO

16.4 15.5

4.4 3.9

77.5 75.0

1.7 0.3

5.3

us

552  553 

Pseudo-second order

cr

549  550  551 

554 

Ac ce pt e

d

M

an

555 

Page 21 of 27

556  0.16

14

0.015 0.010 0.005 0.000

6

0

10

20

30

40

50

60

Pore diameter

4

Adsorption Desorption

2

(a)

0.12

12

dV(d) (cm 3 /g/min)

8

0.020

Quantity Adsorbed (STP, cm 3/g)

10

dV (d) (cm 3 /g/m in)

Quantity Adsorbed (STP, cm 3/g)

12

0.025

0

10 8

0.10 0.08 0.06 0.04 0.02 0.00

6

0

10

20

30

40

50

60

Pore diameter (nm)

4

Adsorption Desorption

2

(b)

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

Relative pressure, P/P

0.6

0.8

1.0

us

cr

Relative pressure, P/P0 559  0 adsorption/desorption 559  isotherms of Fig. 1. N2 (a) MIZO and (b) IZO. Inset the corresponding pore size distribution of MIZO and IZO.  

an

IZO

Intensity

569  570  571 

0.14

0.030

ip t

0.035

14

558 

M

MIZO

20

30

40

50

60

70

80

d

10

2-Theta (degree)

90

Fig. 2. XRD patterns of MIZO and IZO.     (a)                 

584  585 

Fig. 3 SEM images of the adsorbents: (a) MIZO and (b) IZO  

Ac ce pt e

572  573  574  575  576  577  578  579  580  581  582  583 

 

(b) 

Page 22 of 27

40

MIZO Cr-MIZO

30

IZO

10

10.2

8.8

0 2

4

6

-10

7.5 8 pH

10

12

ip t

Zeta potential (mV)

20

-20

-40

60

90

50

70 60

MIZO

IZO

50 40 30

590  591  592  593 

20

40

60 C0 (mg/L)

20

80

MIZO Langmuir

10

IZO Freundlich

(b) 0

100

0

10

20

Ce (mg/L)

Ac ce pt e

0

30

d

(a)

589 

40

M

Removal rate (%)

80

an

100

us

  Fig. 4. Zeta potentials of MIZO, IZO and Cr-loaded MIZO as a function of pH  

Adsorption capacity (mg/g)

586  587  588 

cr

-30

30

40

50

 

Fig. 5. (a) Cr(VI) removal rate of MIZO and IZO as a function of different Cr(VI) initial concentrations. (b) Adsorption isotherms of Cr(VI) onto MIZO and IZO. (pH=5±0.02, adsorbent dosage =1 g/L, T=303 K)   50 45

qt (mg/g)

40 35 30 25

MIZO IZO First-Pseudo-Order Second-Pseudo-Order

20 15 0

594  595  596  597 

5

10

15

20

25 30 Time (min)

200

400

600

  Fig. 6. Adsorption kinetics of Cr(VI) onto MIZO and IZO. (initial Cr(VI)=50 mg/L, pH=5±0.02, adsorbent dosage =1 g/L, T=303 K)  

Page 23 of 27

100

60

40

MIZO

IZO

ip t

Removal rate (%)

80

20

1

2

3

4

5

6

7

8

9

0 mM

1mM

10 mM

us

  Fig. 7. Effect of initial pH on Cr(VI) adsorption onto MIZO and IZO (initial Cr(VI)= 50 mg/L, adsorbent dosage =1 g/L, T=303 K)   50 50 100 mM

0 mM

Adsorption capacity (mg/g)

40

20

10

0

10 mM

100 mM

(b)

30

M

30

1 mM

40

NO3-

Cl-

CH3COO-

20

10

d

Adsorption capacity (mg/g)

(a)

SO42- H2PO4-

0

CO32-

Cl-

NO3-

CH3COO- SO42-

H2PO4-

CO32-

Fig. 8. Effect of coexistent anions on Cr(VI) adsorption onto the adsorbents: (a) MIZO and (b) IZO. (initial Cr(VI)=50 mg/L, adsorpbent dosage=1 g/L, initial pH=5±0.02, T=303 K)

Ac ce pt e

604  605  606 

10

Initial pH

an

598  599  600  602 

cr

0

50

Adsorption capacity (mg/g)

MIZO

40

30

20

10

0

607  608  609  610 

IZO

fresh

second third Recycle number

fourth

  Fig. 9. Cr(VI) adsorption capacity of MIZO and IZO in four successive adsorption-desorption cycles  

Page 24 of 27

Cr2p3/2

Intensity

(a) Cr 2p

Cr2p1/2

580

575

613 

cr

585

570

Binding Energy (eV)

  (c) Zr 3d

730

725

715

710

Intensity

Intensity

M 705

Ac ce pt e

Binding Energy

(d) O 1s

542

615  616  617  618 

720

d

614 

735

540

538

529.76 eV M-OH(81.5%)

536

534

532

Binding Energy (eV)

530

188

526

186

184

182

180

178

176

Binding Energy (eV)

(d) O 1s

527.42 eV M-O(18.5%)

528

Cr-loaded

 

528.98 eV M-O (100.0%) Cr-loaded

Intensity

740

721.00 eV

Cr-loaded

708.94 eV

Intensity

721.36 eV

708.96 eV

(b) Fe 2p

182.24 eV

590

Cr(III)

182.18 eV

595

ip t

Cr(VI)

Cr-loaded

us

 

179.87 eV

612 

179.82 eV

 

an

611 

542

540

538

536

534

532

530

528

Binding Energy (eV)

526

 

Fig. 10. XPS of (a) Cr 2p; (b) Fe 2p; (c) Zr 3d and (d) O 1s on MIZO and Cr-loaded MIZO  

Page 25 of 27

OH OH OH OH OH OH OH

O

O

O Cr OH

618 

O

O

 

Graphical_Abstract 

cr

619 

O

ip t

OH

O Cr

Ac ce pt e

d

M

an

us

620 

Page 26 of 27

620 

Highlights 

621 

 

We established a facile co-precipitation method to prepare an innovative

623 

mesoporous iron-zirconium bimetal oxide (MIZO). The adsorption properties of

624 

MIZO for Cr(VI) was studied. We found MIZO has much better Cr(VI) removal than

625 

ordinary iron-zirconium bimetal oxide and most previous reported adsorbents, in

626 

terms of adsorption capacity, the effect of pH and coexistent anions, etc. Our synthesis

627 

procedure will promote the synthesized adsorbent to form uniform pore distribution

628 

with large pore size, high zeta potential and thus enhance the adsorption.

629  630 

   

Ac ce pt e

d

M

an

us

cr

ip t

622 

Page 27 of 27