Zirconium-loaded Mesostructured Silica Nanoparticles Adsorbent for

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Applied Chemistry

Zirconium-loaded Mesostructured Silica Nanoparticles Adsorbent for Removal of Hexavalent Chromium from Aqueous Solution Sugeng Triwahyono, N. Salamun, Aishah Abdul Jalil, S.M. Izan, H.D. Setiabudi, and D. Prasetyoko Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02167 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Industrial & Engineering Chemistry Research

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Zirconium-loaded Mesostructured Silica Nanoparticles Adsorbent for

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Removal of Hexavalent Chromium from Aqueous Solution

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S. Triwahyonoa, N. Salamuna, A.A. Jalilb,c*, S.M. Izana, H.D. Setiabudid and D.

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Prasetyokoe

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a

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81310 Johor Bahru, Johor, Malaysia

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b

Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, UTM

Department of Chemical Engineering, Faculty of Chemical Engineering, Universiti

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Teknologi Malaysia, UTM 81310 Johor Bahru, Johor, Malaysia

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c

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UTM 81310 Johor Bahru, Johor, Malaysia

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d

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Lebuhraya Tun Razak, 26300 Gambang, Pahang, Malaysia

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e

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Mathematics and Natural Sciences, Institut Teknologi Sepuluh Nopember, Surabaya,

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60111 Indonesia

Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia,

Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang,

Laboratory of Material Chemistry and Energy, Department of Chemistry, Faculty of

18 19 20 21 22

*Corresponding author:

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Aishah Abdul Jalil (PhD)

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Tel: +60-7-5535581

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Email: [email protected]

Fax: +60-7-5536165

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

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ABSTRACT

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Zirconium-loaded mesostructured silica nanoparticles (Zr/MSN) was developed for

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the removal of hexavalent chromium (Cr(VI)) from aqueous solutions. XRD, TEM,

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and BET results revealed that both MSN and Zr/MSN showed a characteristic highly

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ordered hexagonal pore structure with a high specific area of 821-1219 m2 g-1. FTIR

7

analysis showed that the presence of Zr diminished the absorbance band that was

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assigned to silanol groups of the structural defect sites, while XPS analysis of the

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binding energy of Si-O-Zr indicated interactions between the silanol groups and

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zirconium. The interaction of zirconium and silanol groups from the structural defect

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sites generated bidentate zirconium, which acted as an active site for the adsorption.

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The experimental results showed that the modification of MSN with zirconium

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significantly enhanced the adsorption capacity for Cr(VI). The equilibrium isotherm

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data showed that the adsorption process was best described by the Langmuir isotherm

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with 104 mg g-1 maximum adsorption capacity. While the kinetics of Cr(VI)

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adsorption followed a pseudo-second-order kinetic model. The thermodynamic

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properties confirmed that the adsorption of Cr(VI) onto Zr/MSN was spontaneous and

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endothermic in nature, with activation energy of 24 kJ mol-1 showing that the

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adsorption was a chemisorption process.

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Keywords: mesostructured silica nanoparticles, zirconium, chromium, pseudo-second-

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order kinetic model, endothermic


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1 2 3

1.

Introduction Chromium is one of the heavy metals present in effluents produced from the

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aerospace, electroplating, leather, mining, dyeing, fertilizer, and photography

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industries [1]. In the aqueous phase, chromium mostly exists as trivalent chromium

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(Cr(III)) and hexavalent chromium (Cr(VI)). Most of the hexavalent compounds are

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considered to be more hazardous due to their carcinogenic properties, which can cause

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severe health problems in humans. Therefore, it is essential to reduce the Cr(VI)

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content of wastewater to below the tolerance limit before it is discharged into the

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environment [2].

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Several research groups have developed technologies to reduce the Cr(VI)

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content from aqueous media, such as adsorption, electrochemical precipitation, ion

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exchange, and reverse osmosis techniques [2-5]. Among these technologies,

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adsorption is considered as an easy and efficient technique to remove Cr(VI), owing to

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its relatively low cost and simplicity of design and operation. The conventional

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adsorbents for adsorption of organic and inorganic metal compounds have been

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studied widely, which include activated carbon, zeolite, natural clay, and biomaterials

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[6-9]. However, the inherent drawbacks associated with most of these adsorbents leads

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to relatively low adsorption capacity and selectivity, mechanical and thermal

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instability, or the need for a longer equilibrium time, eventhough the adsorbents may

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be economical to some extent in practice. Consequently, it is desirable to develop an

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adsorbent with a better Cr(VI) removal ability.

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Recent studies have shown that mesoporous powder materials, such as MCM-

24

41, SBA-15, FSM, and HMS, possess a large surface area and a meso-scale pore size,

25

promise a good adsorption capability for removal of toxic compounds and/or heavy

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metals from aqueous solutions [10]. Nevertheless, these materials possess a neutral 3 ACS Paragon Plus Environment


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framework that limits their application as a catalyst, support, and adsorbent. Recently,

2

efforts have been made to modify the mesoporous molecular sieve in order to increase

3

the active sites for catalysis or adsorption and to enhance their practicability. Several

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studies have been reported on the modification of mesoporous silica with organic

5

compounds, such as amino groups and organosulfur for the removal of Cr(VI) [11-13].

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Sepehrian and co-workers, however, prepared mesoporous silica incorporated with

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several inorganic metals such as Zr(IV), La(III), Al(III), and Ce(IV) to study the

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adsorption behavior for certain heavy metals [14]. In addition, Zhang et al. reported

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the introduction of Ti into the framework of pure MCM-41 by direct synthesis and a

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post-synthetic impregnation method [2]. They concluded that inorganic metals, such as

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Zr(IV), Al(III), and Fe(III), have affinity toward inorganic anions, including fluoride,

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phosphate, arsenic, and chromium. Our research group studied MCM41-type mesostructured silica nanoparticles (MSN)

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for removal of Cr(VI). Recently, MSN have drawn a great deal of research attention,

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especially regarding adsorption, due to the excellence properties of MSN, such as high

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surface area, small particles, presence of intraparticle and interparticle pores, and easily

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modified silica framework. In this study, MSN was modified with zirconium ion to enhance

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the interaction between the adsorbent and Cr(VI). The Cr(VI) adsorption was carried out in a

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batch system. In addition to the adsorption and its kinetics study, the interaction of Zr and

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Cr(VI) ions with MSN was observed by FTIR, XPS, and solid-state NMR studies.

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2.

Experimental

2.1

Adsorbent preparations and Characterization

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The MSN adsorbent was prepared by sol-gel and co-condensation technique

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conferring to the method reported in the literature [15]. Then, the MSN was

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impregnated with Zr(OH)4 as the Zr source and NH4OH solution as the solvent

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followed by drying at 383 K and calcination in air at 823 K to give Zr/MSN. Several

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different amount of Zr were prepared and the samples were labelled as Zr1/MSN,

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Zr5/MSN and Zr10/MSN for 1, 5 and 10 wt% of zirconium loading, respectively. The

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weight percentage of zirconium loading onto MSN was confirmed by a MPAES

8

analysis, and the results are shown in Table S1. The weight percentage of Zr loaded in

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Zr1/MSN, Zr5/MSN and Zr10/MSN are approximately 1, 5 and 10 wt%, respectively.

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The X-ray diffraction (XRD, Bruker AXS D8 with λ= 1.5418Å (Cu Kα

11

radiation) at 40 mA and 40 kV) was used to determine the patterns of the adsorbents.

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Surface area and pore volume were measured with a SA 3100 Beckman Coulter. Prior

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to measurement, the sample was outgassed at 573 K for 1 h. The infrared spectra of

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the adsorbents in the range of 900-4000 cm-1 was observed by Agilent Cary 640 FTIR

15

Spectrometer [16]. FESEM-EDX (JEOL JSM-6701F) and TEM (JEOL JEM-2100F)

16

were used to determine the morphological properties as well as the elemental

17

composition of the adsorbents. Prior to the analysis, the sample was coated with

18

platinum. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a

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Kratos Ultra spectrometer with MgKα radiation source (10 mA, 15 kV) to determine

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the chemical oxidation state of the zirconium on MSN. The binding energies were

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referenced to the C (1s) binding energy at 284.5 ± 0.1 eV as internal standard. While,

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the formation of unpaired electron which corresponds to the structure and environment

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of the surface species was observed with a JEOL JES-FA100 ESR spectrometer. A

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Bruker Advance 400 MHz 9.4 T (79.4 MHz) spectrometer with 7.5 kHz spinning rate

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of, 60 s pulses intervals was used to record 29Si MAS NMR. In this analysis, external



1

standard reference used was tetramethylsilane (TMS) in order to obtain the chemical

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shift of the solid materials.

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2.2

Adsorption study

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The Cr(VI) stock solution was prepared by dissolving chromium trioxide

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(CrO3) in NH4OH solution at 353 K overnight ((NH4)2CrO4). The experiment was

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conducted by batch adsorption system with 50 mg L-1 Cr(VI) initial concentration

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under stirring condition till equilibrium. 0.1 M NaOH or 0.1 M HCl was used to adjust

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the pH of the working solutions. The experiment of adsorption isotherm was carried

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out at 303 K with 5 to 150 mg L-1 Cr(VI) initial concentration, while the kinetic and

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thermodynamic studies have been done at 303-323 K with 10 mg L-1 Cr(VI) initial

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concentration. The chromium concentration was analyzed with a Perkin Elmer model

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A Analyst 400 atomic absorption spectrophotometer. In order to evaluate the reusability of Zr/MSN, NaOH treated Cr(VI)-Zr/MSN

15 16

was used for re-adsorption of Cr(VI) in a batch experiments. The Cr(VI)-Zr/MSN was

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treated with 0.01 M NaOH in order to remove the Cr(VI) content, followed by

18

washing with distilled water and drying overnight at 383 K. The treatment was

19

repeated till no-Cr(VI) leaching was observed from the Zr/MSN. Then the resulting

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Zr/MSN was used in Cr(VI) adsorption at 298 K.

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3.

Results and Discussion

3.1

Characterization of MSN and Zr/MSN

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The low-angle XRD pattern of MSN, Zr1/MSN, Zr5/MSN, and Zr10/MSN

2

samples (Fig. 1A) exhibited three distinct diffraction peaks at 2θ = 2.4º (100), 4.0º

3

(110), and 4.36º (200). This pattern is typical of a hierarchical mesostructured material

4

with a p6mm 2D hexagonal structure [17,18]. However, the intensity of the diffraction

5

peak at (100) decreased with increasing Zr loading, showing decreases in the degree of

6

orderliness and uniformity of the mesoporous structure. The Zr may interact with

7

silanol groups related to the structural defect in the silicate framework to form

8

deformed-tetrahedral coordination inside the framework, resulting in the low ordering

9

of the structure [19-21]. Pal et al. also reported the decrease of XRD peak due to Zn-

10

metal loading on mesoporous silica. They reported that the introduction of Zn below

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Zn/Si = 1/25 did not change the XRD peak of mesoporous silica. However, the

12

introduction of Zn at and above 2.5/25 (Zn/Si ratio) lowered the XRD peaks which

13

may be decreasing the long-range ordering of the mesophases [22].

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Fig. 1 (A) XRD patterns of (a) MSN (b) Zr1/MSN (c) Zr5/MSN and (d) Zr10/MSN at 2θ=1.5-9

16

º. (B) XRD patterns of (a) ZrO2 and (b) wide angle for spectrum (d) in (A).

17



1

The wide-angle XRD patterns of Zr10/MSN and crystalline ZrO2 are presented

2

in Fig. 1B. ZrO2 phase characteristic peak was not observed, implying that the Zr ions

3

may be highly dispersed on the surface or pore wall or interacted/bonded with silica

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framework. A similar results was reported by Jiang et al. on Zr/MCM-41 in which the

5

peak for bulk ZrO2 phase was not detected in the Zr/MCM-41 sample [19-23]. They

6

concluded that the Zr ions has bonded with both outer and inner channel of the sample,

7

and well-dispersed in the amorphous wall of MCM-41. In contrast, Dong et al.

8

reported that the peak corresponding to the metastable tetragonal zirconia was

9

observed when a high concentration of Zr on MCM-41 (ratio of Si/Zr > 15) was

10 11

calcined at ≥ 1373 K [24]. Nitrogen physisorption and its corresponding pore size distribution of MSN and

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Zr10/MSN are displayed in Fig. S1. Both adsorbents exhibited sorption isotherms type

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IV and type H1 hysteresis loops, confirming a distinctive feature of a uniform

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cylindrical mesoporous materials. In addition, both adsorbents exhibited two steps of

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capillary condensation at P/Po= 0.3 and P/Po=0.9 indicating presence of intraparticle

16

pores within the sample and interparticle pores. However, for Zr10/MSN, the size of

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the hysteresis loop was larger than MSN. This may be due to the larger capillary

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condensation after introduction of Zr onto the MSN [25]. The surface area slightly

19

decreased with increasing Zr loading due to the increasing average pore diameter of

20

MSN from 2.32 to 3.06 nm (Table 1). Enlargement of the average pore diameter may

21

be caused by the introduction of a different size of metal, i.e., the radius of the Zr4+ ion

22

and Si4+ ion are 0.084 nm and 0.026 nm, respectively. The changes of the textural

23

properties of MSN due to the introduction of Zr are similar to those of Zr or Ti loaded

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onto MCM-41, where the addition of metal ions lowered the surface area and

25

increased the pore size and volume of the MCM-41 [16,19,21]. They claimed that the


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changes were due to the partial collapsing of the ordering of mesostructure material. In

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addition, they noted that the metal ions had interacted with the silica framework, and

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thus altered the pore size of the MCM-41.

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Table 1. Textural properties of MSN, Zr1/MSN, Zr5/MSN and Zr10/MSN Sample

d100 (nm)

aο (nm)

S (m2 g-1)

Dp (nm)

Vp (cm3 g-1)

t (nm)

MSN Zr1/MSN Zr5/MSN Zr10/MSN

3.61 3.73 3.85 3.83

4.17 4.31 4.45 4.42

1219 1067 989 821

2.32 2.77 2.86 3.06

0.537 0.961 1.126 1.339

2.10 1.68 1.45 1.11

d100, d-value (100) reflections; aο, unit cell parameter= 2d100/31/2; S, BET surface area obtained from N2 adsorption; Dp, average pore diameter obtained from BJH method; Vp, pore volume; t, porewall thickness=aο-Dp

The morphological properties of both MSN and Zr10/MSN were studied with

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FESEM-EDX and HRTEM images (Fig. 2). Based on the FESEM and TEM images,

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the average particle size of MSN and Zr/MSN was in the range 50–100 nm. The

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HRTEM and fast Fourier transform pattern (inset) images confirmed the mesoporous

15

structure with a hexagonal arrangement. The results obtained resembled the XRD

16

results, in which both MSN and Zr/MSN showed a highly ordered hierarchichal

17

hexagonal pore structure. Though, the Zr/MSN possessed broaden and weak peak at

18

2θ = 2.4º (100), showing a lower in the ordering of mesostructure, which has been

19

evidenced with the results of TEM. The EDX data exhibited the presence of Pt, C, Si

20

and O in the MSN and Zr was also confirmed in the Zr/MSN. The presence of C and

21

Pt could be anticipated from the sample preparation using carbon tape and the Pt

22

sputtering instrument, which is a standard procedure for FESEM observations. Similar

23

report was observed by Salas et al. [21] and Zhao et al. [23] where the mesostructure

24

ordering in MCM-41 was decreased due to introduction of high amount of Zr. This is



Page 10 of 34

1

due to the presence of zirconium ion which partially destructed channels in the

2

hierarchical hexagonal array.

A

B

Element

Mass, %

Si

25.91

O

30.56

C

26.63

Pt

16.90

Total

100.00

Element

Mass, %

Si

20.10

O

29.97

Zr

8.17

C

28.50

Pt

13.26

Total

100.00

3 4

Fig. 2 TEM, FESEM and EDX analyses of (A) MSN and (B) Zr10/MSN

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Fig. 3 shows the XPS Zr3d and O1s spectra of Zr10/MSN. The binding energies for

7

Zr3d5/2 and Zr3d3/2 were 182.8 and 184.9 eV, respectively, thus indicating the presence of

8

Zr4+. The value for Zr3d5/2 was higher compared to Zr(IV) oxide (ZrO2, 182.2 eV) and

9

closer to that of zirconium silicate (ZrSiO4, 183.3 eV) [19, 26, 27]. Thus, it is possible

10

that the Zr in the MSN may be in the form of Zr4+ connected to four O atoms, (Zr(O)4)4-.

11

In fact, the O1s XPS spectrum of the Zr/MSN sample consisted of three components, thus

12

indicating three different oxygen environments. The binding energy at 532.9 eV and 534

13

eV indicated the presence of Si-O-H and Si-O-Si bonds, respectively. Whereas, binding

14

energy at 530.9 eV was related to the Si-O-Zr bond, showing a relatively similar binding

15

energy of ZrO2 at 530.2 eV, hence strong indication of the presence of O atoms bridging

16

Zr and Si. Similar results were concluded by Jones et al. [26] and Rodriguez-Castellon et

17

al. [28] in which the binding energy of Si-O-Zr bond was slightly higher than that of


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ZrO2 (>0.7 eV), and therefore argues against the presence of zirconium as segregated

2

oxide phase.

Fig. 3 XPS spectra of Zr10/MSN (A) Zr3d and (B) O1S

3 4 5

The fingerprint FTIR spectra for all samples were recorded in the spectral range

6

1300-700 cm-1 as shown in Fig. 4A. The absorption bands at 1072 and 960 cm−1 can be

7

assigned to the extension and flexural vibrations of Si-O-Si bonds, and vibration of Si-

8

OH groups and/or overlapping of Si-OH with Si-O-Zr bond vibrations, respectively. The

9

introduction of Zr onto MSN decreased and shifted the absorbance band at 1072 cm-1 to

10

1068 cm-1 due to a deteriorating silica framework and formation of Si-O-Zr. The

11

absorbance band at 960 cm-1 was intensified slightly due to the Si-O-Zr bond vibrations,

12

which are formed from the interaction between Zr and Si-O-Si or Si-O-H [29]. Similar

13

results were observed when Mn or Zr-Mn were loaded onto MCM-41, in which the

14

peaks at 1100 cm-1 on MCM-41 shifted to 1086 and 1080 cm-1 due to the increase of the

15

mean Si-O distance in the walls that is caused by the substitution of the small Si4+ by the

16

larger Mn2+ and Zr4+ [18]. Zhang et al. reported the intensification of the peak at 960 cm-1

17

due to the introduction of Ti onto MCM-41, and suggested that the intensification is due

18

to the overlapping of the peaks attributed to the original Si-OH and new Si-O-Ti bond

19

vibrations [2]. 11 ACS Paragon Plus Environment


1

2 3

Fig. 4 (A) FTIR spectra of (a) MSN, (b) Zr1/MSN (c) Zr5/MSN and (d) Zr10/MSN and (B&C)

4

FTIR spectra after vacuo heating at 373 K for (a) MSN, (b) Zr10/MSN (c) Cr-MSN and (d)

5

Cr-Zr10/MSN

6 7

Fig. 4B and 4C show the FTIR spectra for both MSN and Zr10/MSN before and

8

after adsorption of Cr(VI). The altering of the spectra indicated the interaction between

9

the adsorbent and Cr(VI) ions. Fig. 4B shows a sharp peak at 3740 cm-1, corresponding

10

to the stretching vibration of SiOH groups correlated to the external surface terminating

11

the silica framework. While, structural defects of SiOH groups were observed at 3720–

12

3590 cm-1. Nevertheless, the intensity of the peaks were slightly reduced after

13

introduction of zirconium ions onto MSN without altering the peak at 3740 cm-1. A

14

major decrease of the peaks was observed at 3720–3590 cm-1 for Cr(VI) adsorbed MSN

15

and Zr10/MSN which indicated that the hydroxyl groups have interacted with Zr and Cr

16

ions. The lattice stretching absorbance bands of MSN and Zr10/MSN were observed at

17

peaks ranging from 1300 to 2100 cm-1 (Fig. 4C). The vibration bands at 1750 cm-1 may

18

be corresponded to the presence of surface defects hydroxyl groups due to removal of


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1

lattice atoms or an incomplete condensation. The introduction of Zr ions reduced the

2

intensity of the peak at 1750 cm-1, confirming that the Zr ions interacted with the surface

3

defects. Moreover, the adsorption of Cr(VI) led to form new peaks at 1425-1430, 1530

4

and 1610 cm-1 showing that the altering in the lattice evidenced the interactions between

5

Cr(VI) and adsorbents. FTIR and XPS results were unswerving as they showed the

6

presence of Si-O-Zr bonding that indicated the interactions between hydroxyl groups

7

from Si with Zr. FTIR data showed the absence of structural defect sites due to the

8

interaction with Zr, whereas XPS data clarified the presence of the Si-O-Zr structure

9

from the measured binding energy.

10

Fig. 5A shows the 29Si MAS-NMR spectra of the MSN and Zr10/MSN before and

11

after adsorption of Cr(VI). Both adsorbents exhibited broad signals between 0 and -200

12

ppm, which can be resolved into two major peaks with chemical shifts at ca. -100 and -

13

111 ppm. The signal at -110 ppm result from Q3 and signal at -111 from Q4 silicon

14

nuclei, where Qx attributed to a silicon nucleus with x siloxane linkages, i.e., Q3 to

15

silanol Si–(O–Si)3 and Q4 to silanol Si–(O–Si)4 in the framework. However, after Cr(VI)

16

adsorption, Q2 (-92 ppm) was observed for both Cr-MSN and Cr-Zr10/MSN indicating

17

the presence of disilanol Si–(O–Si)2(–O–X)2 in the framework. Additionally, the peak

18

width and shape of the spectra depend on the phase composition of the sample [30]. It is

19

noteworthy that Zr/MSN exhibits a larger population of Q2 and Q3 coordinations

20

showing more Si ions on Q4 coordination being substituted by Zr ions in the framework.

21

The oxidation state of zirconium incorporated onto MSN and chromium adsorbed

22

onto MSN and Zr10/MSN after Cr(VI) adsorption was studied by electron spectra resonance

23

(ESR) spectroscopy. ESR is a physical method for observing resonance adsorption of

24

microwave power by unpaired electron spin states in a magnetic field. For the MSN sample,

25

evacuation above 573 K generated a signal at g = 2.04, which originated from a Si-O• radical,



1

i.e., a non-bridging oxygen hole center (NBOHC). The Zr in Zr10/MSN with electronic state

2

Zr4+ should be ESR silent. However, a slight oxygen deficiency can lead to the formation of a

3 4

Fig. 5 (A) 29Si MAS-NMR and (B&C) ESR spectra MSN, Cr-MSN, Zr10/MSN and Cr-

5

Zr10/MSN

6 7

paramagnetic Zr3+ state. Fig. 5B and 5C illustrate the ESR spectra of MSN and Zr10/MSN

8

before and after Cr(VI) adsorption. The Zr10/MSN and Cr-Zr10/MSN exhibited peaks for g║ =

9

1.96 and g┴ = 1.98, which are very close to those reported for Zr3+ [31]. However, the

10

decrease in the ESR relative intensity of Cr-Zr10/MSN can be explained as a decrease in the

11

number of unpaired electrons, and the decreased numbers of electrons were considered as

12

those electrons that have participated in the bonding action [32]. After adsorption of Cr(VI),

13

only peak at g = 1.98 was related to the Cr(V)/Cr(VI) signal, which revealed that there was

14

no reduction of Cr(VI) to Cr(III). As reported from previous studies, the resonance signal at a

15

magnetic-field position corresponding to a g-value of 1.97 confirms the reduction of Cr(VI)

16

to Cr(III) [33]. Thus, it was suggested that total chromium bound on adsorbent was

17

predominantly in the Cr(VI) form with negligible or insignificant amounts of Cr(III) [34].

18 19 14 ACS Paragon Plus Environment

Page 14 of 34

Page 15 of 34 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 3

3.2

Adsorption studies

4 5

3.2.1 Effect of modification on MSN

6 7

Fig. 6A illustrates the adsorption capacity of Cr(VI) on bare MSN and Zr/MSN at

8

different Zr loadings. The results indicated that the adsorption capacity of Cr(VI) was

9

proportionally improved after introduction of zirconium ions onto the MSN. The

10

maximum adsorption capacity was 1.2, 5.1, 8.4, and 9.8 mg g-1 for MSN, Zr1/MSN,

11

Zr5/MSN, and Zr10/MSN, respectively. These changes may be caused by the

12

modification of the MSN framework with heteroelement (Zr), which apparently

13

increased the active sites and subsequently increased the adsorption capacity of the

14

adsorbent. Bare mesostructured silica nanoparticles show a limited application because

15

the deficiency of active sites in the framework. It can be noted that an increase in the Zr

16

ion loading, the adsorption rate should also increase owing to a higher number of

17

hydroxyl groups generated on the heteroelement zirconium and MSN framework. Hence,

18

there will be more active sites which are able to attract Cr(VI) ions on the surface of

19

Zr/MSN.

20 21 22 23



1 2

Fig. 6 (A) Effect of zirconium loading on adsorption of Cr(VI) onto Zr10/MSN, (B)

3

desorption of Cr(VI) using two different condition and (C) re-adsorption of Cr(VI) using

4

Zr10/MSN after desorption process for regeneration study

5 6

From the characterization results, the adsorption mechanism of Cr(VI) onto MSN

7

and Zr/MSN is proposed in Scheme 1. FTIR spectra showed possible interactions of the

8

silanol groups with the metal ions with evidenced by the disappearance of defect sites.

9

Furthermore, the XPS spectrum of Zr3d elucidated the oxidation state of the Zr4+ ions,

10

which indicates that Zr in the MSN may be in the form of Zr4+ connected to four O

11

atoms, i.e., (Zr(O)4)4-. The O1s XPS spectrum illustrated the presence of a Si-O-Zr bond.

12

Therefore, it can be concluded that the available structural defect sites interacted with Zr

13

ions to form the structure as presented in Scheme 1. Modification with Zr ions increased

14

the active sites on the surface, which led to an enhanced adsorption capacity of the MSN.

15


Page 16 of 34

Page 17 of 34 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

Scheme 1 Proposed adsorption mechanism of Cr(VI) onto MSN and Zr/MSN

3 4

Repeated availability is an important parameter for an advanced adsorbent. Such

5

adsorbents not only possess higher adsorption capacity and capability, but also have to

6

show better desorption properties, which will significantly reduce the overall adsorbent

7

cost. Based on the results of desorption experiments (Fig. 6B), the Cr(VI) desorption

8

capacity increased with the increase of pH. At pH 14, almost 78% of the Cr(VI) ions

9

were liberated into the liquid. This result indicates that the Cr(VI) adsorption onto

10

Zr10/MSN is not completely reversible. The bonded Cr(VI) was only partially removed at

11

pH below 7, while they were sharply desorbed in more alkaline solution owing to the

12

tendency to form soluble sodium chromate. Similar phenomena were observed for other

13

adsorbents for Cr(VI) recovery, such as wheat bran, magnetically modified multi-wall

14

nanotubes, and activated carbon and magnetic iron nanoparticles doped on ordered

15

mesoporous carbon [35-36]. To evaluate the adsorption potential of regenerated

16

Zr10/MSN, four cycles of adsorption–desorption were carried out while maintaining the

17

conditions of experiment. As shown in Fig. 6C, the percentage of adsorption of Cr(VI)

18

was found to be 82, 75, 70 and 69%, respectively, for first, second, third and fourth cycle

19

of batch adsorption. After four cycles of reusability study, the adsorption percentage of

20

Zr10/MSN decreased by 15%. It is noteworthy that our results are up to par with other

21

inorganic adsorbents for adsorption of heavy metals. Salleh et al., reported the adsorption

22

capacity of Pd2+ on Ni/Al2O3-E exhibit a loss of 18 % after five cycles of adsorption

23

[37]. On the other hand, adsorption of Pb2+ and MnO4- on NH2/MCM-41/NTAA showed

24

a decrease of 8 and 10%, respectively after five cycles [38]. This result suggests that



Page 18 of 34

1

Zr10/MSN has practical potential to be used as an adsorbent for Cr(VI) removal and

2

comparable with other studies.

3

A comparison of the Cr(VI) adsorption capacities of various types of mesoporous

4

silica adsorbents is given in Table 2. In this study, Zr10/MSN gave a high maximum

5

adsorption capacity (104 mg g-1) and surface area (821 m2 g-1) compared to the other

6

adsorbents listed. Introduction of zirconium ions to the MSN, the mesoporous structure

7

remained constant, and the Cr(VI) adsorption capacities increased caused by the

8

increasing active sites on the framework and heteroelement zirconium. Previously, Salah

9

et al. reported that functionalization of mesoporous silica nanoparticles with amino

10

groups enhanced the adsorption of Cr(VI) [39]. In their study, USG-41 provided

11

sufficient pore area for functionalization and coordination of the dichromate dianion,

12

thus providing higher adsorption capacity. Lower adsorption capacities were obtained

13

(39 and 48 mg g-1) using the support materials MCM-41 and SBA-15. Chen et al. studied

14 15

Table 2. Comparison of Zr10/MSN with MCM41-type adsorbents for Cr(VI) adsorption

16 17 18 19

Adsorbent

Surface Area (m2 g-1) 821 813 850 585

Zr10/MSN AP*-MCM41 AP*-SBA15 Fe3+-Magnetic MCM-41 SBA-15 Modified with 2342 mercaptopyridine AP*: 3-aminopropyltrimethoxy-silane

qmax (mg g-1) 104 39 48 67-99

This study [39] [39] [40]

95

[12]

Reference

20 21

the removal of Cr(VI) using Fe3+-magMCM-41 [40]. The incorporation of magnetic

22

Fe3O4 nano-particles into the MCM-41 matrix produced a high adsorption capacity (67–

23

99 mg g-1), due to high magnetization and surface area. In general, modification of

24

mesoporous materials with organic or inorganic groups as adsorbent will provide

25

excellent properties for elimination of Cr(VI) or any other metal ion.

26


Page 19 of 34 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

3.2.2 Effect of pH solution and contact time

2 3

Cr(VI) adsorption capacity of Zr10/MSN in response to the initial pH and contact

4

time are shown in Fig. 7A. The pH of solution primarily influences the degree of

5

ionization, the adsorbent surface charge, and the speciation of adsorbate species during

6

adsorption [41]. The pH-dependent of Cr(VI) adsorption on Zr10/MSN is concomitant to

7

the presence of Cr(VI) species in aqueous solution. At pH value of 2-12, Cr(VI) exist

8

dominantly as HCrO4- (bichromate) and CrO42- (chromate) anions in water solution.

9

HCrO4- was the predominant species at pH 2-4, whereas CrO42- was the predominant

10

species in the pH range 9-12. The molar percentage distributions of HCrO4- and CrO42-

11

were 95% and 5%, 80% and 20%, and 2% and 98% for pH 5, 6, and 8, respectively. This

12

means that the Cr(VI) was adsorbed on the Zr10/MSN as either the HCrO4- or the CrO42-

13

species [42]. The positive charge of the adsorbents surface rose with lowering pH, and so

14

more Cr(VI) ions were adsorbed at lower pH. At pH 2, the Cr(VI) was solely adsorbed as

15

HCrO4-; however, at pH 5 the Cr(VI) was adsorbed as HCrO4- and CrO42-. The maximum

16

adsorption capacity observed at pH 5 which was probably caused by the CrO42- species

17

being more selectively adsorbed on the Zr10/MSN than the HCrO4- species. However,

18

apparently due to the competitive adsorption of hydroxyl ions, the adsorption capacity

19

decreased with a further increase of pH [43].



1 2

Fig. 7 Effect of (A) pH and contact time and (B) adsorbent dosage and initial Cr(VI)

3

concentration on the adsorption of Cr(VI) onto Zr10/MSN

4 5

The time to achieve steady-state equilibrium is depended to the nature of the

6

adsorption system. From the results, the adsorption rate increased rapidly during the

7

initial stage of the adsorption and then decreased to some extent. The slower adsorption

8

rate at the end was apparently due to the saturation of active sites with Cr(VI) ion. After

9

an equilibrium time of 1.5 h, the adsorption remained constant. As for other adsorbent

10

reported previously, amino-functionalized mesoporous silica attained its equilibrium at

11

240 min to adsorb Cr(VI), which shows that different adsorbents show different rates of

12

adsorbing Cr(VI) [13].

13 14

3.2.3 Effects of adsorbent dosage and initial concentration of Cr(VI)

15 16

The effect of adsorbent dosage and initial concentration of Cr(VI) on the

17

adsorption capacity of Cr(VI) onto Zr10/MSN are shown in Fig. 7B. By varying the

18

Zr10/MSN dosage (1–5 g L-1), the adsorption of Cr(VI) on Zr10/MSN decreased with 20 ACS Paragon Plus Environment

Page 20 of 34

Page 21 of 34 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

increasing adsorbent dosage due to the higher unsaturated adsorption sites during the

2

adsorption process, and thus the adsorption process occurs ineffectively due to

3

overlapping of adsorption sites [44]. The effect of initial concentration of Cr(VI) ions onto Zr10/MSN were determined

4 5

by varying the initial Cr(VI) ions concentration from 5 to 150 mg L-1. When the initial

6

Cr(VI) concentration increased, an increment in the adsorption capacity was observed

7

from 4.8 to 58.2 mg g-1. This is due to an increase in the driving force of the

8

concentration pressure gradient resulting in higher probability of collision between

9

Cr(VI) ions and the active sites [45]. At certain point, the adsorption capacity remained

10

constant because of the saturation of active sites with adsorbate species.

11 12

3.3

Isotherm studies

13 14

Isotherm results provide information on the capacity of the adsorbent and the

15

parameters of the adsorption system. In this study, two types of isotherm (Langmuir and

16

Freundlich) models have been used to clarify the adsorption equilibrium between Cr(VI)

17

ions and adsorbent. The isotherm constants for both models were determined by a linear

18

regression method (Table 3).

19

The Langmuir model assumes that a monomolecular layer is formed when

20

adsorption takes place without any interaction between the adsorbed molecules [46]. The

21

essential characteristics of the Langmuir isotherm can be expressed in terms of a

22

dimensionless constant separation factor RL, which is given as:

23 24

RL = 1 KL + Cο

(1)

25



Page 22 of 34

1

where KL (L mg-1) is the Langmuir equilibrium constant, and Cο (mg L-1) is the highest

2

initial concentration of adsorbate. The RL value estimates the degree of suitability of

3

adsorbents toward metal ions, which gives an indication of the possibility of the

4

adsorption process proceeding. It is identified that RL > 1 unfavorable; RL = 1 linear; 0
∆S°, indicates that

7

the adsorption was dominated by enthalpic rather than the entropic changes [51].

8 9 10

The activation energy (Ea) is defined as the minimum kinetic energy required by the adsorbate ions to react with the active sites available on the surface of the biosorbent. Arrhenius equation is used to obtain the value of Ea:

11 12

ln k 2 = ln A − Ea

(6)

RT

13

14

where the Ea value was calcuated from the slope of ln k2 vs. 1/T, and was 24 kJ mol-1 for

15

Zr10/MSN (Fig. S4B; Table 5). Generally, activation energies which are less than 4.2 kJ

16

mol-1) are distinguish as physical adsorption, while activation energies ranging from 8.4

17

to 83.7 kJ mol-1 as chemical adsorption. Based on the Ea value, the Cr(VI) adsorption

18

onto Zr10/MSN proceeded through a chemical adsorption process.

19 20

Table 5. Thermodynamic parameters for the Cr(VI) adsorption using

21

Zr10/MSN at different temperatures

22 T (K)

23 24

303 313 323

qe -1 (mg g ) 9.8 10.8 11.1

ο

∆G -1 (kJ mol ) -3.9 -6.1 -7.3

ο

ο

∆H -1 (kJ mol )

∆S -1 (J (mol·K) )

Ea -1 (kJ mol )

47.4

169.9

24

25 25 ACS Paragon Plus Environment


1

4.

Conclusion

2 3

This study illustrated the utilization of zirconium-loaded mesostructured silica

4

nanoparticles (Zr/MSN) for efficient adsorption of Cr(VI). The MSN retained its

5

characteristic mesoporous structure with hexagonal arrangement following the

6

incorporation of zirconium. The decrease in the band assigned to the OH group of

7

structural defect sites from FTIR spectra indicates an interaction with zirconium and

8

chromium. This result was consistent with XPS analysis in which the binding energy for

9

Si-O-Zr was also observed. The Zr/MSN with the highest zirconium loading gave the

10

highest Cr(VI) adsorption capacity. The modification of the siliceous framework with Zr

11

apparently increases the number of active sites in the material. In addition, Zr10/MSN

12

showed good reusability and could be effectively regenerated by an alkali solution. The

13

equilibrium data were well described by the Langmuir isotherm model with a maximum

14

adsorption capacity of 104 mg g-1. The kinetics of adsorption followed a pseudo-second-

15

order model and the calculated activation energy was found to be 24 kJ mol-1 indicating

16

that chemisorption contribute to the control of the rate of adsorption. The thermodynamic

17

parameters reveal that the Cr(VI) adsorption onto Zr10/MSN was endothermic and

18

spontaneous in nature.

19 20 21

Acknowledgements

22 23

The authors acknowledge the research grant provided by the Universiti Teknologi

24

Malaysia under the Research University Grant No. 12J13 and 10H30.

25


Page 26 of 34

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1 2 3 4

5

Table of Contents Graphic

•

Incorporation of Zr on MSN retained the high ordered mesoporous structure. Zr

6

interacted with structural defect sites of MSN to form more active sites. Maximum

7

adsorption capacity of Cr(VI) on Zr10/MSN was 104 mg g-1.

8


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Zirconium-loaded Mesostructured Silica Nanoparticles Adsorbent for

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