A New Application of a Mesoporous Hybrid of Tungsten Oxide and

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A New Application of Mesoporous Hybrid of Tungsten Oxide and Carbon as an Adsorbent for Elimination of Sr2+ and Co2+ from Aquatic Environment Yanan Wang, Huiyan Huang, Shengxia Duan, Xia Liu, Ju Sun, Tasawar Hayat, Ahmed Alsaedi, and Jiaxing Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03818 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Sustainable Chemistry & Engineering

1

A New Application of Mesoporous Hybrid of Tungsten

2

Oxide and Carbon as an Adsorbent for Elimination of

3

Sr2+ and Co2+ from Aquatic Environment

4

Yanan Wanga, Huiyan Huangb, Shengxia Duana,b, Xia Liua, Ju Suna, Tasawar Hayatd,e,

5

Ahmed Alsaedid, Jiaxing Lia,c,d*

6

a

7

Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, P.R.

8

China.

9

b

CAS Key Laboratory of Photovoltaic and energy conservation materials, Institute of

School of Chemical and Environmental Engineering, Wuyi University, Jiangmen

10

529020, China.

11

c

12

Institutions, P.R. China.

13

d

14

21589, Saudi Arabia.

15

e

16

*Corresponding author. Tel.: +86-551-65596617; Email: [email protected] (J. Li)

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

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

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

17 18 19 20 21 22 23

KEYWORDS: Tungsten oxide and carbon, Sr2+, Co2+, adsorption. 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

ABSTRACT: The mesoporous tungsten oxide and carbon (WOx/C) nanowire network

2

was successfully synthesized using an environmental–friendly solvothermal method.

3

The as–prepared composite was applied as an adsorbent for metal ions’ elimination

4

from wastewater via batch experiments. The adsorption results suggested that the

5

adsorption interactions between WOx/C and Sr2+ and Co2+ were pH–dependent while

6

the effect of ionic strength toward sorption capacity depended on the solution acidity

7

and adsorbates. The sorption process followed the Langmuir model with the

8

maximum sorption capacity of Sr2+ and Co2+ onto WOx/C to be 175.0 and 326.0 mg/g

9

at 308 K, respectively. Thermodynamic parameters namely △S0, △H0 and △G0

10

indicated an endothermic, but spontaneously adsorption process. All the exhibited

11

results demonstrated that the as–synthesized WOx/C nanowire network has the

12

potential to be an effective adsorbent for heavy metal ions’ remediation from aqueous

13

solutions.

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2


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1

INTRODUCTION

2

With the increasing demand for energy and the fast consumption of fossil fuels,

3

nuclear energy has been considered as a promising alternative to alleviate the resource

4

shortage due to its effectiveness and cleanliness. However, environmental hazardous

5

nuclides can be inevitably produced and released into the environment in processing

6

of nuclear fuel and treatment of spent fuels, posing threat to the public’s health.

7

Among the hazardous nuclides, strontium (Sr) and cobalt (Co), both in stable and

8

radioactive forms, are two nonnegligible toxic elements existing not only in nuclear

9

waste repositories but also in petrochemical, mineral mining and smelting

10

wastewaters (1). Moreover, the development in advanced technology also increased

11

the possibility of Sr and Co emission to the environment. For example, 90Sr, a pure β

12

radiator usually derived from fission process, have been applied to thickness gauge (2)

13

and radiotherapy (3). However,

14

human being due to its long half–life and chemical properties. Specifically, Sr2+ can

15

be easily assimilated in human body through alimentary thereby depositing in the

16

liver, lungs and kidneys because of its high solubility. Once taken into human body,

17

Sr2+ can easily replace the calcium (Ca) in bones and other human organs or tissues

18

due to the similarity with Ca, causing anemia, leukemia along with other chronic

19

diseases (4, 5). Moreover, the application of Co is even more extensive, ranging from

20

Co–containing alloy materials of various properties (6-8) to dyeing (9), electroplating

21

(10) and so forth. The discharge of Co2+ by these afore mentioned industries can cause

22

severe problems to human health, such as asthma–like allergies, heart disease, thyroid

90

Sr is one of the most dangerous radionuclides for

3



1

damage and bleeding, considering its non–biodegradable properties in natural

2

conditions and accumulation propensity in living organisms. Therefore, it is crucial to

3

restrict the concentration of Sr2+ and Co2+ to acceptable levels in the environment.

4

Recently, numerous techniques have been developed for elimination of Sr2+ and

5

Co2+ from aqueous solutions, including precipitation, adsorption and ion–exchange.

6

Among these methods, adsorption has been proven to be the most effective one with

7

the advantages of simplicity, high efficiency, low cost, etc. Researchers have

8

performed extensive works to investigate appropriate adsorbents for removal of

9

hazardous nuclides from aqueous solutions, including carbon materials (11-14),

10

titanate materials (15), clay composites (16, 17), chitosan-based materials (18, 19) etc.

11

However, these adsorbents either have poor adsorption property or may produce

12

secondary pollution during the synthetic and/or application process. For instance,

13

graphene nanomaterials have presented desirable performance in the adsorption of

14

Sr2+ and Co2+ due to the large surface area, abundant functional groups and excellent

15

dispersity in aqueous solution (20-22). However, recent researches (23-26) revealed

16

that graphene nanomaterials released to the environment can be accumulated by

17

organisms which can subsequently interfere into their biophysical activities like

18

metabolism, leading to the unpredictable effect in the long run. Specifically, metal

19

oxides have attracted great attentions of many researchers to dangerous heavy metal

20

ions’ treatment with the advantages of low cost, simple synthetic process and

21

desirable adsorption performance.

22

Recent years have witnessed great progress in water purification techniques by 4


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1

applying metal oxides, such as iron oxides, aluminum oxides, zinc oxides, and

2

manganese oxides (27-32), etc. For example, Lukashev et al. (33) reported a

3

maximum sorption capacity of 0.43 mmol/g Co(II) onto mechanically activated

4

γ–Fe2O3 (34). A selective extraction method presented by Dong et al. (35) revealed

5

the importance of Mn and Fe oxides for Pb and Cd adsorption. In addition to

6

inorganic heavy metal ions, the laterally grown ZnO nanorods on reduced graphene

7

oxide hydrogel contributed to excellent methylene blue (MB) elimination efficiency

8

because of improved photocatalytic activity (36).

9

Although some achievements have been made by these metal oxides in the

10

adsorption of heavy metal ions from aqueous solutions, limitations such as somewhat

11

lower adsorption capacity, slow adsorption kinetics and acid nonresistance restrict

12

their practical application in water purification. Hence, it is imperative to explore new

13

metal oxides with both remarkable adsorption performance and acid resistance.

14

Tungsten oxides WO3-x (x=0–1) are promising candidates for heavy metal ions’

15

adsorption due to their attractive properties such as uniform size and shape, resistance

16

to acid, thermal stability and low cytotoxicity (37-41). Therefore, tungsten oxides,

17

especially for W18O49, are favored by researchers from photoelectric to semiconductor

18

fields. W18O49 was reported to be applied for heavy metal ions’ adsorption.

19

Additionally, W18O49, as an interesting non–stoichiometric tungsten oxide, possesses

20

abundant oxygen vacancies on the surface, which might provide many active sites for

21

metal ion complexation. Furthermore, most metal oxides cannot compete with other

22

materials in adsorption capacity in many circumstances, thereby limiting their 5



1

application in water purification. To further improve its adsorption capacity,

2

incorporating organic functional groups with favorable affinity to the heavy metals

3

ions and organic dyes into the WOx, namely inorganic-organic hybrid WOx-based

4

composites, may be an effective approach. These organic functional groups including

5

thiol, amino, carboxylic and sulfur groups possess stronger complexation or chelating

6

ability towards heavy metal ions, and have been used to functionalize the inorganic

7

adsorbents to improve their adsorption performance. Hu et al. (42) synthesized

8

Zn2GeO4-ethylenediamine inorganic-organic hybrid nanowires using ethylenediamine

9

as solvent, which showed superb removal capacity for Pb2+. Carbonaceous materials

10

via hydrothermal carbonization of glucose, with abundant hydrophilic functional

11

groups, commonly possessed stronger chelating ability towards heavy metal ions, and

12

have been used to functionalize the inorganic adsorbents to improve their adsorption

13

performance. Therefore, we envision that a combination of tungsten oxide 1D

14

framework with carbonaceous materials would fabricate a superior adsorbent with

15

unique heterogeneous surface, strong affinity for heavy metal ions and organic dyes in

16

a one-pot and scalable approach.

17

In this work, inorganic–organic carbon encapsulated W18O49 hybrid network

18

(WOx/C) was successfully fabricated via a simple one–pot solvothermal method,

19

which was adopted as adsorbent to remove heavy metal ions of Sr2+ and Co2+ from

20

aqueous solution. Influencing factors, such as sorption temperature, pH and ionic

21

strength (I) were investigated. The results suggested that WOx/C hybrid network can

22

be potentially applied as an adsorbent for water purification. 6


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1

EXPERIMENTAL SECTION

2

Materials. Adsorbate (Sr2+ and Co2+) stock solutions were prepared by

3

dissolving their nitrate salts (99.9%, Sigma–Aldrich) in 0.01 M HNO3 solution and

4

then diluted to desired concentrations. All reagents in this experiment were

5

commercially ordered and used without further purification. Deionized water was

6

adopted for all experimental work.

7

Preparation of WOx/C inorganic–organic nanowire network. A simple

8

solvothermal method was adopted to produce WOx/C using WCl6 as a precursor and

9

triethylene glycol as solvent. Specifically, 0.8 g WCl6 was dissolved into 40 mL

10

triethylene glycol under vigorous stirring to form a precursor solution. 0.6 g glucose

11

(used as carbon source) was added into the precursor solution and stirred for 1 h. The

12

mixed solution was transferred into a 100 mL teflon–lined stainless steel autoclave

13

and kept at 200 ºC for 6h. After naturally cooling to room temperature, the product

14

was collected via centrifugation and washed respectively with deionized water and

15

ethanol several times to remove the inorganic and organic impurities. Finally, after

16

freeze–drying overnight the obtained samples are collected as WOx/C and WOx,

17

respectively.

18

Characterizations. The surface morphologies of pristine WOx/C coupled with

19

WOx/C adsorbed Sr2+ and Co2+ were characterized through scanning electron

20

microscopy (SEM, S–4800, field emission) equipped with a GENESIS4000 energy

21

dispersive

22

(Nicolet–8700 FT–IR spectrophotometer) with KBr pellets was applied to verify the

X–ray

spectroscope.

Fourier

transform

7


infrared

spectroscopy


1

surface functional groups in the 4000–400 cm-1 region. X–ray powder diffraction

2

(XRD), equipped with a Cu–Kα radiation source (λ=1.5406Å) scanning from 20–80°

3

with a step size of 0.02°, was also adopted to identify WOx/C samples before and after

4

adsorption. Besides, X–ray photoelectron spectroscopy (XPS) was carried out on

5

Thermo Escalab 250, conducting at 150 W with Al Kα radiation. Specific surface area

6

was analyzed by N2 adsorption–desorption isotherm recorded from a Micromeritics

7

ASAP 2010 surface area analyzer. Zeta potential was measured by Zetasizer Nano ZS

8

(Malvern, England).

9

Batch sorption experiment. The sorption experiments of Sr2+ and Co2+ onto

10

WOx/C were performed in polyethylene tubes by batch techniques. In the process, the

11

solution pH was adjusted by adding negligible amount of 0.01–1M HNO3 and/or

12

NaOH solutions. The sorption isotherms were conducted at 288, 308 and 328 K under

13

the pH of 5.5 and 4.5 respectively for Sr2+ and Co2+ to make sure that no precipitation

14

existed within the concentration of 2–27 mg/L for Sr2+ and 4–80 mg/L for Co2+. The

15

suspension of WOx/C and NaNO3 was pre–equilibrated for 24 h, and then stock

16

solution of Sr2+ and Co2+ was gradually injected into this prepared suspension. After

17

being adjusted to the desired pH, the suspensions were immediately transferred to a

18

shaker, constantly shaking for 24 h to reach complete reaction equilibrium. After that,

19

the suspensions were centrifuged to separate the solid and liquid phase. The Sr2+

20

concentration of the supernatant was determined by inductively coupled

21

plasma–atomic emission spectroscopy (ICP–AES, iCAP6000, Thermo Fisher

22

Scientific) while Co2+ concentration was measured by atomic absorption spectroscopy 8


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1 2 3

(AAS–6300C, Shimadzu, Japan). The sorption capacity of metal ions (sorption percentage (%) and Qe (mg/g)) can be calculated according to the equations below:

4

(1)

5

(2)

6

Where C0 (mg/L) is the initial concentration and Ce (mg/L) is the equilibrium

7

concentration after adsorption. m (g) and V (L) are the mass of WOx/C adsorbent and

8

the total volume of the suspension, respectively. The reported experimental data were

9

the average value of duplicate data within 5% relative errors.

10

RESULTS AND DISCUSSION

11

Characterization

12

XRD pattern was applied to characterize the crystal structure of the synthesized

13

WOx/C material (Fig. 1A). Two peaks located at 2θ 23.46°and 48.03° in pristine

14

WOx/C correspond to (0 1 0) and (0 2 0) from the monoclinic phase of W18O49 with

15

cell diameters of a = 18.23073 Å, b = 3.79557 Å, c = 14.11241 Å, and β = 116.1579°

16

(JCPDS Card no. 71–2450), revealing the main component of WOx. The existence of

17

carbon can be proved in FT–IR spectra (Fig. 1B), where carbon related functional

18

groups were clearly observed in 2800–2900 cm-1 (characteristic –CH stretching),

19

1710 and 1400 cm-1 for C=O, 1620 cm-1 for C=C and 980 for C–O bond (43). The

20

abundant carbonaceous functional groups were ascribed to the possible incomplete

21

carbonization of glucose at the hydrothermal temperature at 200 ºC on the surface of

22

tungsten oxide (44), indicating the preparation of WOx/C. To investigate the surface 9



1

charged species of WOx/C as a function of solution pH, the zeta potential analysis was

2

conducted by determining the species’ activities in solution and the result is given in

3

Fig. 1C. The negatively charged surface of WOx/C is probably due to the

4

deprotonation of its surface functional groups such as -OH, -COOH, C=O, etc.,

5

providing easier accessibility for positively charged contaminants. Fig. 1D shows the

6

N2 adsorption–desorption isotherm of the mesoporous WOx/C. Based on the BET data,

7

the specific surface area of mesoporous WOx/C nanowire network is calculated to be

8

92.5 m2/g, a large surface area for the immobilization of contaminants from aqueous

9

solutions.

10 11

Fig. 1 XRD patterns (A), FTIR spectra (B), zeta potential (C) and BET analysis (D) of

12

the synthesized WOx/C.

13

XPS technique was applied to obtain a further insight into the composition and

14

surface functional groups of WOx/C. As shown in Fig. 2A, the presence of strong C 1s

15

peak revealed that the surface of tungsten oxides was successfully functionalized by

16

carbonaceous species during the synthetic process. The peaks at 491.88 and 428.49 10


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1

eV were assigned to W 4p core level while peaks at 259.81 and 246.60 eV

2

corresponded to W 4d core level. The double peak appearing at 35.60 and 37.70 eV

3

was assigned to W 4f7/2 and W 4f5/2 sublevels because of the spin–orbit splitting

4

(45). The deconvolution of W 4f, O 1s and C 1s XPS spectra were presented in Fig.

5

2B-D, where the W 4f XPS spectra of WOx/C could be perfectly fitted by three

6

oxidation states. The peaks appearing at 37.7 and 35.65 eV were associated with W6+

7

(46). The peaks observed at binding energy of 36.5 and 34.86 eV corresponded to W5+

8

(47). The rather short W–W distance of 0.26 nm and the oxygen vacancy occurring at

9

the shear planes provided places for W5+ to be formed. Moreover, the peak found at

10

34.2 eV indicated the presence of W4+ oxidation state, which might be derived from

11

the dismutation of W5+ (48). The different oxidation states of W6+, W5+ and W4+ were

12

the typical nonstoichiometric characteristics of W18O49 nanomaterials (47, 49). All the

13

results indicated a successful preparation of WOx/C. Besides, O 1s XPS spectra (Fig.

14

2C) exhibited two peaks: 530.41 and 532.22 eV, in accordance with the W–O–W

15

bonds and the O–atoms in the vicinity of an O–vacancy in W18O49 nanowire network

16

(50, 51) or the residual water and/or C–O bond derived from the residual adsorbed

17

molecules (52), respectively. The C 1s spectra were deconvoluted to three peaks,

18

corresponding to C=O&O–C=O, C–O and C=C, respectively (Fig. 2D). The XPS

19

results are totally conformed to the FT-IR analysis, revealing the generation of

20

abundant carbonaceous species and various oxygen containing functional groups on

21

the surface of WOx.

11



1 2

Fig. 2 XPS spectra of low–resolution (A), high–resolution W 4f (B), O 1s (C) and C

3

1s (D) of WOx/C.

4 5

Fig. 3 TEM image of WOx (A) and WOx/C (B).

6

The morphologies of WOx/C were detected through TEM to corroborate its

7

nanowires morphology (Fig. 3). The TEM image clearly showed that a large quantity

8

of loose twisted nanowires with length up to several micrometers contained in the

9

WOx/C network exhibited remarkable flexibility compare with pristine WOx, which

10

was aggregated and assembled from the radial nanowires. These loose structures 12


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1

indicated that they are more conducive to the adsorption of pollutants. The diameter

2

of the single WOx/C nanowires is ~1-2 nm.

3

The micro structure for the as–synthesized WOx/C is captured through SEM (Fig.

4

4A–B). Fig. 4A-B exhibits the morphology of WOx/C nanowire network, which

5

consists of nanowires with diameter of several to dozens of nanometers and several

6

hundred nanometers in length, adhering to and intertwining with each other to form

7

porous structures, which provides large surface area and easy access for outside

8

substances to its inner space. As a comparison, the surface morphologies of WOx/C

9

after adsorption are also presented (Fig. 4C–F). As depicted in Fig. 4C-D, in spite of

10

the existence of extra aggregates in the WOx/C, the basic porous inner structures can

11

still be easily distinguished. This result suggested that Sr2+ ions entered into the pores

12

of WOx/C and caused aggregation while the number of Sr2+ ions was not enough to

13

fill up the WOx/C. Although several bumps were observed in Co2+–adsorbed WOx/C

14

(Fig. 4E), which seems to differ from the original one (Fig. 4A) largely, the inner

15

porous structure almost remains intact as a matter of fact (as shown in Fig. 4F). This

16

maybe results from the interaction of Co2+ ions mainly with the outer surface of

17

WOx/C. According to the SEM images before and after adsorption, the adsorption

18

mechanism between Sr2+ and WOx/C seems to be inner–sphere complexation while

19

the Co2+ ions tend to be immobilized by WOx/C via outer–sphere chelation or ion

20

exchange, which is in accordance with the adsorption results below.

13



1 2

Fig. 4 SEM images of pristine (A), Sr2+–adsorbed (C) and Co2+–adsorbed (E) WOx/C;

3

(B), (D) and (F) correspond to their SEM images in higher magnification,

4

respectively.

5 6

ADSORPTION PERFORMANCE

7

Kinetic study. The adsorption kinetics is very important because it can reflect

8

the rate–controlling step of the adsorption process. The adsorption kinetics were

9

studied at the concentrations of 0.1 g/L WOx/C, 21 mg/L Sr2+ and 80 mg/L Co2+ (Fig.

10

5). The reaction equilibrium can be reached within 10 and 120 min for Sr2+ and Co2+,

11

respectively.

12

To obtain insights into the adsorption behavior of WOx/C toward the two heavy

13

metal ions, three kinetic models, namely pseudo–first–order, pseudo–second–order

14

model and intra–particle diffusion models (53, 54), were utilized to study the

15

adsorption process. The equations of three models were given as below: 14


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1

(3)

2

(4)

3

(5)

4

Where qt (mg/g) and qe (mg/g) represent the adsorption amount at reaction time t

5

and after equilibrium, k1 (min–1) and k2 (g·mg–1·min–1) are denoted as the kinetic

6

adsorption rate constant of the first and second-pseudo kinetic two models,

7

respectively. kint (mg·g–1·min–0.5) is the intra diffusion rate constant and C is a

8

constant related to thickness and interlayer. The parameters and fitting coefficients are

9

listed in Table 1. The adsorption process can be fitted by pseudo–second–order model

10

both for both metal ions with the correlation coefficient of 0.997 and 0.981,

11

respectively, demonstrating a possible chemisorption process as the rate–limiting step

12

(55). The intraparticle diffusion is not the only rate controlling step in the adsorption

13

reaction according to the nonzero value of C (13) and multi-linearity in Fig. 5C-D,

14

suggesting that the adsorption process onto WOx/C consists of several steps.

15

Specifically, the heavy metal ions in the solution first transfer to the external surface

16

of WOx/C, which can be reflected from the initial linear portion in Fig. 5C-D. The

17

second linear part stands for the gradual adsorption stage, in which the heavy metal

18

ions diffuse through the pores of WOx/C. For the adsorption of Co2+ ions, there is a

19

final equilibrium stage in which the intra–particle diffusion begins to slow down due

20

to the sharply decreased ions concentration in aqueous solution and the reduction of

21

available adsorption sites on WOx/C. The slope of the linear portion can be

22

interpreted as the indication of the rate of the adsorption process that the adsorption 15



Page 16 of 43

1

rate for Sr2+ is remarkable in the first 10 min and then decreases afterwards. In

2

contrast, the adsorption rate for Co2+ is much slower. The ever-decreasing adsorption

3

rate with time can be explained as that the ions diffused into the inner part of WOx/C

4

can block the pores so that the free path of heavy metal ions is narrowed.

5 6

Fig. 5 Adsorption kinetics of Sr2+ (A) and Co2+ (B) on WOx/C and fitting to three

7

kinetic equations: solid line for pseudo–first–order model, dash line for

8

pseudo–second–order model and dotted line for intra–particle diffusion model; (C)

9

and (D) are the linear plots of intra–particle diffusion model for Sr2+ and Co2+,

10

m/V=0.1 g/L, I = 0.01 M NaNO3, pHSr=5.5, pHCo=4.5, T=288 K.

11

Table 1 Parameters for the three kinetic models. Model

Equation (linearization)

Adsorbate 2+

Sr

Co

Pseudo-seco

2+

2+

Sr nd-order

R2

k1=0.407 (min–1) qe=87.780 (mg·g–1)

Pseudo-first -order

Parameters

0.996

k1=0.152 (min–1) qe=105.291 (mg·g–1)

0.946

k2=0.0238 (g·mg–1·min–1) qe=88.602 (mg·g–1)

16


0.997

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Co

2+

2+

Sr Intra-particl ediffusion

2+

Co

k2=0.0023 (g·mg–1·min–1) qe=111.100 (mg·g–1)

0.981

kint=0.349 (mg·g–1·min–0.5) C=83.966 (mg·g–1)

0.937

kint=1.938 (mg·g–1· h–0.5) C=81.848 (mg·g–1)

0.814

1

Effect of solution acidity and ionic strength. The influence of solution pH

2

values on the sorption performance towards Sr2+ and Co2+ on WOx/C is exhibited in

3

Fig. 6. The results of the samples without adsorbent at varied solution pH were also

4

given in Fig. 6 to estimate the contribution of precipitation process, from which we

5

can learn that the precipitation process can be ignored for Sr2+ adsorption throughout

6

the experimental pH while for Co2+ the effect of precipitation is unnoticeable when

7

solution pH < 7. What’s more, the sorption capacity increased with increasing pH

8

values in the range of 2.0–6.0, which could be explained by the electrostatic attraction

9

between positively charged metal species (Fig. 6C–D) and increasing negatively

10

charged WOx/C surface (Fig. 1C). However, when the solution pH continued to

11

escalate, the sorption behavior of the two metal ions showed apparent differences (Fig.

12

6A–B): for Sr2+, the adsorption capacity remained the maximum value at pH 6.0–9.0;

13

while WOx/C exhibited an increasing sorption tendency throughout the experimental

14

pH toward Co2+ and the increment in the pH range of 7.0–8.0 was noticeable. A

15

possible explanation for this phenomenon is that the binding sites for Sr2+ ions on the

16

WOx/C have reached saturation at pH 6.0 and further increment in solution pH could

17

not lead to larger adsorption capacity. In addition, Fig. 6A also exhibits that ionic 17



1

strength has negligible impact on Sr2+ sorption onto WOx/C in the pH range from

2

2.0–6.0 and the ionic strength independency demonstrated the inner–sphere chelation

3

(56), which confirms the previous SEM analysis result. When solution pH exceeded

4

6.0, although very little, the restricting effect of high ionic strength on Sr2+ adsorption

5

can still be observed, suggesting an ion exchange adsorption mechanism in basic

6

solution. The sorption capacity of Co2+ was positively related to the solution pH in the

7

whole range and sorption capacity in ionic strength of 0.001 M NaNO3 was larger

8

than that of both 0.01 M and0.1 M NaNO3 (Fig. 6B). The consistently increasing

9

trend resulted from the positively charged Co2+ ions (Fig. 6D) and gradually increased

10

negatively charged WOx/C surface with the increase of pH. While the ionic strength

11

dependency revealed the ionic exchange sorption mechanism. The ever–decreasing

12

discrepancy under various ionic strength condition in high pH might be related to the

13

occurring of Co(OH)2(aq) (Fig. 6D). As a result, solution pH of 5.5 and 4.5 was

14

determined for the following thermodynamic study of Sr2+ and Co2+ adsorption onto

15

WOx/C, respectively, to ensure that no precipitation occurs during the adsorption

16

process. 100

100

A

Adsorption (%)

Adsorption (%)

80 I=0.1 M NaNO3 I=0.01 M NaNO3

60

I=0.001 M NaNO3 40

no adsorbent

20 0 1.5

B

80

I=0.1 M NaNO3 I=0.01 M NaNO3

60

I=0.001 M NaNO3 no adsorbent

40 20 0

3.0

4.5

pH 6.0

7.5

9.0

2

C

4

6

pH

8

10

12

D

1.0 2+

Co

2+

Percentage (%)

Sr

0.97

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 43

0.96

0.95

0.8 Co(OH)2(aq)

0.6 0.4

+

CoOH 0.2

0.94

17

2

4

6

pH

8

10

12

0.0

2.8

4.2

18


5.6

pH

7.0

8.4

9.8

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Fig. 6 Effect of pH and ionic strength on the sorption of A: Sr2+ and B: Co2+ onto

2

WOx/C, in which orange spheres represent the percentage of metal ions remaining in

3

the solution; the species distribution of C: Sr2+ and D: Co2+ as a function of pH in

4

aqueous solution, C0= 21 and 80 mg/L for Sr2+ and Co2+, respectively, m/V = 0.1 g/L,

5

T = 288 K.

6

Sorption isotherms and thermodynamics. To investigate the thermodynamic

7

properties of metal ions sorption onto WOx/C, the sorption isotherms of Sr2+and Co2+

8

were measured at three different temperatures (Fig. 7A–B). Considering the

9

precipitation possibility of metal ions in basic solutions, the isotherms were obtained

10

at low pH condition, 5.5 for Sr2+ and 4.5 for Co2+. The Langmuir and Freundlich

11

models are denoted as follows:

12

(6)

13

(7)

14

Where Ce(mg·L-1) is the concentration of adsorbate in equilibrium; Qe (mg·g-1) is

15

the amount of adsorbate immobilized by WOx/C, and Cs max represents the maximum

16

sorption capacity of WOx/C toward an adsorbate, corresponding to a complete

17

adsorbates coverage on WOx/C’s surface at high concentration of an adsorbate; b

18

(L·g-1) refers to the Langmuir constant. While KF and n in the Freundlich model are

19

denoted as Freundlich constants with respect to the sorption capacity and sorption

20

intensity, respectively. As depicted in Fig. 7A–B, Langmuir model exhibited a better

21

fitting results compared to the Freundlich model for both adsorbates’ sorption. Table 2

22

listed the relative parameters of both Langmuir and Freundlich models. The saturated 19



Page 20 of 43

1

sorption capacity of WOx/C derived from the Langmuir model were 175 and 326

2

mg·g-1 for Sr2+ and Co2+ at T = 308 K, superior to many other reported adsorbents

3

(Table 3).

4

Table 2 Parameters for the Langmuir and Freundlich models. Langmuir model Adsorbate

2+

Sr

Co

5

2+

Freundlich model

Csmax(mg·g-1)

b(L·mg-1)

R2

n

R2

288

114.0

0.076

0.904

10.8

0.632

0.845

308

175.0

0.059

0.871

12.2

0.703

0.814

328

269.7

0.042

0.907

12.4

0.791

0.875

288

269.8

0.009

0.964

4.1

0.759

0.976

308

326.0

0.008

0.996

4.3

0.783

0.991

328

374.9

0.009

0.988

5.0

0.812

0.984

T(K)

KF(mg1-n·Ln·g-1)

Table 3 Sorption capacity of Co(II) and Sr(II) by some adsorbents. Adsorbent

pH

Adsorbate

T (K)

Cs max (mg·g-1)

Ozonized GO

6.8

Co(II)

303

372

GO

6.8

Co(II)

303

198

WOx/C

4.5

Co(II)

308

326

This study

Zero-valent iron nanoparticles

>6.5

Co(II)

-

172

(57)

nZVI-graphene composite

5.7

Co(II)

303

134

(58)

γ-Fe2O3 nanotubes

6.0

Co(II)

298

60.6

(59)

20


Ref

(11)

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphene oxide-magnetite hybrid

7.0

Co(II)

318

23

(60)

Fe3O4

6.9

Co(II)

295

17.4

(61)

WOx/C

5.5

Sr(II)

308

175

This study

Thiacalixarene-functionalized GO

7.0

Sr(II)

298

101

(62)

Titanate nanotubes

8.0

Sr(II)

-

91.7

(63)

Magnetic zeolite composite

-

Sr(II)

298

83.7

(64)

APTES-Mt

8.5

Sr(II)

301

65.6

(65)

Graphene oxide-magnetite hybrid

7.0

Sr(II)

318

18

(60)

1

Thermodynamic data. Further analysis about how temperature affected

2

adsorbate sorption efficiency onto WOx/C was carried out as well. Fig. 7C–D

3

presented the plots of lnK0 versus 1/T and their linear fitting lines, based on which the

4

thermodynamic parameters namely △G0, △S0 and △H0 are calculated according to

5

equations below:

6

(8)

7

(9)

8

Where K0 and R (8.314 J mol-1·K-1) represent the sorption equilibrium constant

9

and the ideal gas constant; T is the temperature in kelvin. According to eq. (9), △S0

10

and △H0 can be calculated by multiplying R and –R with the intercept and slope in Fig.

11

7C–D, respectively. As shown in Table 4, the negative△G0 values indicate

12

spontaneous sorption processes of Sr2+ and Co2+. Increasing sorption temperature

13

gave even more negative △G0, suggesting a sorption process favored by higher

14

temperature, which is in accordance with the fact that higher reaction temperature 21



1

always came along with larger sorption capacity. The positive value of △H0 (34.9 and

2

6.5kJ·mol-1 for Sr2+ and Co2+, respectively) revealed endothermic sorption process.

3

The most widely accepted explanation for this phenomenon was that the energy

4

absorbed for the dehydration of ionic adsorbates from their aqueous complex

5

exceeded that of releasing from their attaching process to the solid surface of WOx/C

6

(66). The positive △S0 (34.9 and 29.4 for Sr2+ and Co2+, respectively) indicated

7

structural changes during the immobilization process. Based on the calculated data, it

8

can be concluded that the disorder of the solid–solution system increased with the

9

order of Co2+ and Sr2+.

10

Additionally, the recyclability of WOx/C was investigated by five successive

11

adsorption-desorption experiments, using 0.1 M HNO3 solution as the desorption

12

agent. As depicted in Fig. 8, a slight decline in adsorption percentage was observed

13

for both the Sr2+ and Co2+ after five cycles, from ~38% to ~34% for Sr2+ and from

14

~15% to ~12% for Co2+, respectively, which could be the result of the mass loss of the

15

adsorbent during adsorption/desorption process. The regeneration results also

16

suggested that the prepared WOx/C possessed desirable recyclability and

17

recoverability when used as an adsorbent for heavy metal ions’ elimination from

18

aquatic environment.

22


Page 22 of 43

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Fig. 7 Sorption isotherms of A: Sr2+ (pH 5.5) and B: Co2+ (pH 4.5) at various

3

temperatures as well as their fitting results: solid lines for Langmuir models and

4

dotted lines for Freundlich models; and plots of lnK0 versus 1/T for Sr2+ (C) and Co2+

5

(D) coupled with their linear fittings, m/V = 0.1 g/L, I = 0.01 M NaNO3.

6

Table 4 Thermodynamic parameters for heavy metal ions’ sorption on WOx/C. Adsorbate

2+

Sr

2+

Co

T(K)

△G0(kJ·mol-1)

288

–5.7(2)

308

–5.7(6)

328

–5.8

288

–2.0

308

–2.4

328

–3.2

△S0(J·mol-1·K-1)

△H0(kJ·mol-1)

34.9

5.0

29.4

6.5

7

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Adsorption percentage(%)


42

Sr Co

35 28 21 14 7 0

1

2

3

4

5

Cycle number

1 2

Fig. 8 Adsorption recyclability of Sr2+ and Co2+ on WOx/C over five cycles, pH 5.5

3

for Sr2+ and 4.5 for Co2+, m/V = 0.1 g/L, T = 288 K.

4 5

POSSIBLE ADSORPTION MECHANISM

6

A comparison adsorption experiment was conducted to make it clear that if the

7

high adsorption capacity towards Sr2+ and Co2+ was attributed to the WOx or the

8

introduced carbonaceous species and oxygen containing functional groups in

9

functionalization process. Herein, reacting conditions using single WOx and WOx/C

10

sample as adsorbent were kept the same for the Sr2+ and Co2+ adsorption. As depicted

11

in Fig. 9A-B, the sorption isotherms of the two samples exhibited similar trend except

12

that WOx/C had a higher adsorption capacity than WOx for both Sr2+ and Co2+,

13

implying that carbon functionalization on WOx was conducive to metal ions’

14

immobilization. But which part of WOx/C is responsible for the metal ions’ binding

15

remains unclear. Therefore, characterization of WOx/C after metal ions loading was

16

collected and analyzed.

17

From Fig. 9C, it can be clearly seen that no predominant new peaks but the (0 1

18

0) and (0 2 0) peak shifting to a slightly lower diffraction angle were observed in Sr2+ 24


Page 24 of 43

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

and Co2+–adsorbed WOx/C compared to the pristine WOx/C (Fig. 1A), from which we

2

could conclude that the adsorption of Sr2+ and Co2+ barely affected its lattice structure

3

apart from the slightly expanded interlayer spacing because of outer heavy metal ions’

4

intrusion. The FT–IR spectra of the raw WOx/C and metal ions–loaded WOx/C were

5

measured to detect the surface functional groups changes before and after adsorption,

6

as exhibited in Fig. 9D. The characteristic band at 3423 cm-1 indicated the vibration of

7

O–H of water arising from the hydrothermal carbonization process. The absorption

8

band in the region 580–940 cm-1 was ascribed to the stretching vibration of short

9

W=O bonds, while small bands around 819 and 725 cm-1 were attributed to the

10

O–W–O stretching modes (67). Moreover, the peaks at 655 and 582 cm-1 were related

11

to the W–O–W stretching vibration (68). Meanwhile, the peak at 1623 cm-1 is

12

associated with the W–OH, also known as the oxygen vacancies. The disappearance

13

of the small peak at ~980 cm-1, which was assigned to the C–O vibration, was

14

observed for both Sr2+ and Co2+–loaded samples compared to the pristine WOx/C (Fig.

15

1B). This is probably because of the binding reaction between C–O and heavy metal

16

ions, suggesting that the enhanced adsorption capacity of WOx/C is mainly arising

17

from the introduced carbonaceous functional groups. Considering the tiny changes

18

can be overlapped by the strong peaks in FT–IR spectra, XPS analysis was applied to

19

detect changes in bond respective.

25



1 2

Fig. 9 Adsorption isotherms of Sr (A) and Co (B) at 328 K by WOx and WOx/C; XRD

3

patterns (C) and FTIR spectra (D) of heavy metal ion–loaded WOx/C.

4

In the full XPS spectra, characteristic peak of heavy metal ions was observed at

5

133.94 and 781.92 eV respectively for Sr2+ and Co2+ after sorption (Fig. 10A),

6

confirming the strong sorption ability of WOx/C. After adsorption, only two sharp

7

peaks could be observed in the deconvolution XPS spectra of W 4f (Fig. 10B-C),

8

suggesting partial conversion of W5+ to W4+ and W6+ in comparison to pristine WOx/C

9

(Fig. 2B). The detailed information about each peak, including the binding energy,

10

corresponding oxidation state and functional groups as well as their percentage of

11

WOx/C before and after adsorption were listed in Table 5 and 6. Slightly shifting of

12

binding energy could be observed for both metal ion–loaded samples, similar to

13

previously reported results (69). The fitted O 1s results of heavy metal ion–loaded

14

WOx/C (Fig. 10D-E) showed a conformed trend toward higher binding energy (0.24

15

eV for Sr and 0.34 eV for Co) relative to pure WOx/C (as shown in Table 5),

16

indicating a change of the local bonding environments (70). The decreased percentage 26


Page 26 of 43

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

of O vacancies after heavy metal ions’ adsorption (as shown in Table 6) suggests that

2

the adsorbed metal ions may take up the O vacancies through the formation of

3

W–O–Sr and W–O–Co bonds. The deconvoluted C 1s spectra were exhibited in Fig.

4

10F-G. The C 1s spectra consist of three peaks corresponding to C=O&O–C=O, C–O

5

and C=C, respectively. Contrary to the O 1s spectra, a slightly shift to lower binding

6

energy of WOx/C after adsorption was observed, especially for the peak referring to

7

C–O bond with 0.27 and 0.51 eV decrease after Sr2+ and Co2+ immobilization,

8

respectively (Table 5). A remarkable decline of C–O functional groups in C 1s spectra

9

after adsorption, from 43% to 26% for Sr2+ and 32% for Co2+, is presented in Table 6,

10

implying binding bonds of Sr2+–OC and Co2+–OC for Sr2+ and Co2+ with WOx/C,

11

respectively. To summarize, the high adsorption capacity of WOx/C toward Sr2+ and

12

Co2+ is the synergistic result of O vacancies and C–O functional groups.

13 14

Fig. 10 XPS spectra of Sr2+, Co2+ loaded WOx/C (A) and corresponding

15

deconvolution of W 4f, O1s and C 1s spectra ( B, D and F for Sr2+ loaded WOx/C and 27



Page 28 of 43

1

C, E and G for Co2+ loaded WOx/C).

2

Table 5 The binding energy and assignment of W 4f and O 1s XPS spectral bands for

3

pure and metal ion–loaded WOx/C Element

WOx/C (eV)

Heavy metal ion–loaded WOx/C (eV) 2+

Sr W 4f

O 1s

C 1s

Co

Assignments

2+

37.7

37.8

37.8

W6+

35.7

35.7

36.6

W6+

36.5

36.8

36.4

W5+

34.9

35.6

35.4

W5+

34.2

34.8

35.0

W4+

532.2

532.6

532.6

O vacancy

530.4

530.6

530.8

W–O–W

286.6

286.6

286.5

C=O&O–C=O

285.8

285.5

285.2

C–O

284.6

284.6

284.6

C=C

4 5

Table 6 Fraction of oxidation states and functional groups in high–resolution W 4f, O

6

1s and C 1s spectra pure, and Sr2+, Co2+ loaded WOx/C Percentage (%)

WOx/C

WOx/C +Sr

WOx/C +Co

W6+

84

87

88

W5+

14

9

8

W4+

2

5

4

28


Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O vacancy

49

32

28

W–O–W

51

68

72

C=O&O–C=O

9

21

21

C–O

43

25

32

C=C

48

54

47

1 2

CONCLUSION

3

WOx/C nanowire network was successfully fabricated and their sorption capacity

4

and efficiency toward Sr2+and Co2+ were examined. The immobilization of Sr2+ was

5

processed via inner–sphere complexes while ionic exchange or outer–sphere

6

complexation was applied for Co2+sorption. Spectroscopic analysis revealed that the

7

C–O functional groups and O vacancies provided by WOx/C are active sorption sites

8

for adsorbate sorption from aqueous solutions. The calculated maximum sorption

9

capacity was 175.0 and 326.0mg/g for Sr2+ (pH 5.5) and Co2+ (pH 4.5) at 308 K,

10

respectively. This study provided a potential application of WOx/C as an adsorbent for

11

metal ions’ removal from aqueous solutions, considering its easy fabrication, excellent

12

sorption efficiency, and easy separation process.

13

29



1

AUTHOR INFORMATION

2

Corresponding Author

3

*Tel/Fax: +86-551-65596617. Email: [email protected].

4

Notes

5

The authors declare no competing financial interest.

6

There is no supporting information.

7

ACKNOWLEDGMENT

8

Financial supports from National Natural Science Foundation of China (21677146) and

9

the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority

10

Academic Program Development of Jiangsu Higher Education Institutions are acknowledged.

11 12

30


Page 30 of 43

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1

Table of Contents Graphic and Synopsis

2 3

Mesoporous WOx/C nanowaires with high sorption capacity of Sr2+ and Co2+ were

4

successfully synthesized using an environmental–friendly method.

5

43


A New Application of a Mesoporous Hybrid of Tungsten Oxide and

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