Synthesis of Fe3O4 flower-like hierarchical nanostructures with high adsorption performance toward dye molecules

Accepted Manuscript Title: Synthesis of Fe3 O4 flower-like hierarchical nanostructures with high adsorption performance toward dye molecules Author: Maryam Khosravi Saeid Azizian PII: DOI: Reference:

S0927-7757(15)30064-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.06.049 COLSUA 20007

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

4-2-2015 18-6-2015 26-6-2015

Please cite this article as: Maryam Khosravi, Saeid Azizian, Synthesis of Fe3O4 flower-like hierarchical nanostructures with high adsorption performance toward dye molecules, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.06.049 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.

Synthesis of Fe3O4 flower-like hierarchical nanostructures with high adsorption performance toward dye molecules

*

Maryam Khosravi, Saeid Azizian

Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

Highlights

-

Flower-like iron oxide nanostructure (FL-IONs) prepared from different precursor. Different surfactant and reductive agent can affect on the morphology of FL-IONs. The FL-IONs showed excellent ability to remove dyes from water within few minutes.

Abstract

*

CORRESPONDING AUTHOR: E-mail address: [email protected] and [email protected]

Tel: +988138282807 Fax: +988138380709

1

3D assembly of iron oxide nanosheets with flower-like structures were synthesized via decomposition of different iron alkoxide precursors by heat treatment. The products were characterized by X-ray diffraction, scanning electronic microscopy and N2 adsorption/desorption isotherm, methods. The SEM results show that both samples have flower like structure but one of then is composed of smooth nanosheets but the other one composed of rough nanosheets. BET surface area and pore size distribution of samples were obtained using N2 adsorption data. The obtained nanostructured flower-like iron oxide act as highly effective sorbent for fast removal of organic dyes pollutants, such as Reactive Orange (RO), Reactive Yellow (RY) and Bismarck Brown (BB), from simulated sewage system. The adsorption equilibrium and kinetic data were modeled with classical and recently developed models.

Keywords: Nanostructures, Oxides, Adsorption, Magnetic Materials 1. Introduction

The environmental and health problems related with polluted water are increasing due to increase of population and industries [1]. Sewage from tanneries, chemical plants, battery manufacturing plants and other industries contains a lot of toxic materials, such as heavy metal ions and organic compounds. Textile effluent contains organic dyes; wastewater from hospital contains drug remainders, bacteria and viruses; they will form cluster in the organism and cannot be degraded by the organism easily and finally cause different illnesses in vivo. Dyes and pigments are widely applied in our daily life and in industries including textiles, leather, pharmaceutical and cosmetic. Every year large amounts of dyes releases into the effluent

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during the dyeing process, which has caused seriously environmental and health problems. So, removal of dye pollutants from wastewater has attracted significant attention in recent years [2]. Different methods have been developed to remove dye pollutants from water such as co precipitation [3-5], biological treatment [6], flocculation, catalytic degradation, biodegradation [7,8] and adsorption [9]. Among the above mentioned methods, adsorption is regarded as one of the most promising approaches, because of easy operation, high efficiency, insensitive to toxic substances and economic process. This method has been used to remove different dyes pollutants from aqueous solutions [10,11]. Different adsorbents including zeolites [12], activated carbon [13] and polymeric materials [14] were commonly used for removal of organic dyes. The mentioned adsorbents have low adsorption capacities or low rate of adsorption. Recently, nanoadsorbents and nanotechnology have garnered worldwide attention for their application in environmental remediation and pollution control, because nanostructured materials offer large surface area and having higher adsorption capacity and or high rate of adsorption [15,16]. For example, iron oxide nanomaterials are used for the removal of toxic heavy metal ions [17], dyes [18-20] and organic pollutants from waste water because of the lost cost, abundant availability and environmentally benign nature [21]. Different iron oxide particles with 0D, 1D, 2D and 3D nanostructures have been successfully synthesized [22-24]. Among various morphologies of iron oxide nanomaterials, three-dimensional (3D) flower-like nanostructures composed of hierarchically assembled subunits, have several advantages for adsorption, such as high surface area, easy mass transformation and easy separation [25].

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Zhong and Co-workers reported controllable synthesis of iron oxide with flower structures for the removal of As(V) and Cr(VI) from water [25]. Cao et al. synthesized flower-like α-Fe2O3 nanostructures for removal of heavy metal ions [26]. Recently, we have shown that the synthesis conditions affect on the structure of nanostructured flower-like iron oxides and therefore on their removal performance for adsorption of dyes from water [20]. In this paper, nanostructured flower-like iron oxide with different morphologies has been synthesized by decomposition of the iron alkoxide precursors that are prepared by heating up the solution of different materials. Excellent adsorption ability of the prepared adsorbents for anionic and cationic dyes such as Reactive Orange, Reactive Yellow and Bismarck Brown in wastewater treatment was found.

2. Experimental

2.1. Materials

FeCl3.6H2O (≥99%), urea (≥99.8%), ethylene glycol (≥99.5%), sodium acetate trihydrat (≥99.5%), polyvinylpyrrolidone (PVP) were purchased from Merck Co. and pluronic F127 (MW=12600) from Sigma-Aldrich. Table S1 shows the structures of the used dyes including Reactive Orange 13, Reactive Yellow 15 (Alvan Sabet Co. Iran) and Bismarck Brown (Merck Co.).

2.2. Synthesis of iron oxide

4

Flower-like iron oxide with different morphologies have been prepared through a two-step process including reflux at 195 oC and calcination of the obtained iron alkoxide precursor. The obtained products were named as FL-IONsa and FL-IONsb.

FL-IONsa preparation

The starting solution was prepared by mixing 0.75 gr of FeCl3.6H2O, 0.41 gr of NaAc.3H2O and 0.5 gr of PVP in 25 ml of ethylene glycol in a round flask. The solution was stirred with a magnetic stirrer and heated to refluxing temperature (ca. 195 oC). After refluxing for 0.5 h the stirring was stopped and the mixture was cooled to room temperature. The precipitate was collected by centrifugation and washed with absolute ethanol four times. The obtained precipitate was calcined at 500 ᵒC under N2 atmosphere for 3 h and then cooled to room temperature under N2 atmosphere. The obtained product named FL-IONsa which is a black powder and the yield of synthesized FL-IONsa is %47.

FL-IONsb preparation

In a typical experiment, 1.2 gr of FeCl3.6H2O, 5.4 gr of urea and 3.48 gr of F127 were mixed with 180 ml of ethylene glycol under magnetic stirring vigorously for 0.5 h until a uniform suspension was formed. The solution was subsequently heated and refluxed at 195 oC, the precipitates were collected by centrifugation washed with ethanol four times, and finally dried in the air at room temperature.

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The dried powder of the precursor was calcined at 500 oC under N2 atmosphere for 3 h and then cooling down the system until the room temperature under N2 atmosphere. The obtained black powder named FL-IONsb and the yield of synthesized FL-IONsb is %44..

2.3. Characterization

The morphology and the size of the synthesized samples were characterized by scanning electron microscopy (SEM) (XL30-philips). The crystal structure of the samples was determined by Xray diffraction (XRD)(ADP 2000 ITALSTRUCTURE italia). The N2 adsorption/desorption experiments were carried out using Quantachrome Novawin2.

2.4. Adsorption equilibrium and kinetic experiments

Adsorption experiments were performed for FL-IONsa and FL-IONsb from both equilibrium and kinetic approaches. The initial pH values for RO, RY and BB solutions were 7.2, 7.1 and 6.9, respectively. In the equilibrium adsorption experiments, 5 ml of dye aqueous solutions (RO, RY and BB) with different concentrations (6-120 mg/l) were added in to 5 mg of adsorbents. Then, the samples were placed in a water batch shaker and shaken at 150 rpm for 24 h. After that, the adsorbent was separated from the solution by external magnet and the equilibrium concentration of RO, RY and BB in the bulk (Ce) was determined at 498 nm, 415 nm and 459 nm, respectively using a UV/Vis spectrophotometer (PG Iustrumet LTD model T80).

6

For calculation of the amount of adsorbed dyes per unit mass of adsorbents (mg/g) at equilibrium (qe) the following equation was used: qe 

(C0  Ce ) V W

(1)

where C0 and Ce are the initial and equilibrium dye concentrations, respectively; W is the adsorbent mass (g) and V is the solution volume (L). The kinetic adsorption experiments were performed at three different initial concentrations of RO (20, 40, 80 mg/l), RY (20, 50, 80 mg/l) and BB (20, 35, 80 mg/l). In each concentration a series of 5ml of RO, RY and BB solution were added in to 5 mg of adsorbents. The samples were placed in a shaker (150 rpm, 25 oC) and the dye concentration at different time intervals (Ct) was determined with UV/Vis spectrophotometer. The amount of dye adsorbed per unit mass of adsorbent at different times, q t (mg/g) was calculated by: qt 

(C0  Ct ) V W

(2)

2.5. Effect of pH

The influence of pH on the RO, RY and BB removal efficiency was studied at different pH (4≤ pH ≤ 11). In this study, a series of 5 ml of dye solution with concentration 40 mg/l but different pH were agitated with 5 mg of FL-IONsa and FL-IONsb at 150 rpm and 25 oC for 4 h. Then, the solutions were analyzed with UV/Vis spectrophotometer. Removal percentage was calculated for each sample by the following equation: % Re 

( A0  A)  100 A

(3)

where A0 and A are the initial and equilibrium absorbance of the samples.

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2.6. Utilization of used adsorbent

In order to investigate the regeneration cycle of the as-prepared FL-IONs, the following experiments were performed. First, the fresh adsorbents were added to a new dye solution (20 mg/l) and the removal percentage was obtained after 5 min. Second, the used adsorbent was washed with distilled water several times and used as adsorbent for another new dye solution. Third, the used adsorbent was washed with NaOH solution (0.01M) for 45 min and then used for dye removal from new solution. Then their removal efficiencies were compared.

3. Results and discussions

3.1. Structural and morphological characteristics

The chemical composition of the products was analyzed using powder XRD. As shown in Figs. (1a-1b) all of the identified peaks could be assigned to the Fe3O4. The XRD pattern of FL-IONsa (Fig. 1a) shows seven characteristic peaks at 2θ= 30.1 o, 35.8o, 37.1 o, 43.1o, 53.8 o, 57.3o and 63o that can be indexed to (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of Fe3O4 [25]. Fig. 1b shows the XRD pattern of the FL-IONsb. The diffraction peaks locating at 2θ= 18.2 o, 30.1o, 35.8 o, 37.1o, 43.1 o, 53.8o, 57.3o and 63 o can be indexed to (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes, respectively. Morphologies of the calcined products were illuminated by scanning electron microscopy (SEM). SEM image in Fig. 2a shows that the sample is composed of numerous flower-like Fe3O4

8

nanostructure, with diameters of about 2 µm. A typical flower-like nanostructure is shown in the magnified SEM image in Fig. 2b which demonstrate that each flower-like nanostructure composed of smooth nanosheets with a thickness of about 80 nm. Fig. 3 represents SEM images of FL-IONsb. At low magnification (Fig. 3a) the particles look hierarchical microspheres. Higher magnification (Fig. 3b) shows that each microsphere has a flower-like structure with irregular rough nanosheets where their thickness is about 100 nm. The difference between FL-IONsa and FL-IONsb is just using different precursors. Comparison of Figs. 2 and 3, shows that the different precursors can affect on the structure of product. For precursors consist of PVP (surfactant) and sodium acetate trihydrat (precipitation agent) the petals of the prepared Fe3O4 flowers are smooth nanosheets but when the precursors consist of F127 (surfactant) and urea (precipitation agent) the petals are small, non-smooth and irregular. Therefore, from the SEM images it can be concluded that the different precursors can affect on the morphology of the prepared iron oxide. So, by using different precursors but the same procedure as illustrated in Scheme 1, we could easily obtain Fe3O4 with different morphologies. The

structural

properties

of

the

prepared

FL-IONs

were

determined

using

N2

adsorption/desorption isotherm (Fig. 4). The results are summarized in Table 1, and the pore size distribution plots are presented in Fig. S1.

3. 2. Adsorption kinetic study

As mentioned earlier, recently we have shown that iron oxide nanosphere and flower-like iron oxide nanostructures are good adsorbents for dye removal [18-20].

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In the present work the synthesis of novel 3D flower-like iron oxide from different precursors was reported and we are going to investigate their capability as adsorbent to remove various dye compounds from water. The high rate of adsorption is one of the most important characters of each adsorbent. Figs. 5 and S2 demonstrate the kinetic data of RO, RY and BB sorption by FL-IONb and FL-IONsa at different initial concentrations, respectively. These data show that, an increase in the rate of adsorption and also in the equilibrium amounts of ad-species (qe) by increase of initial concentration. It is clear that dye removal has been performed within few minutes which denote very fast adsorption of dye by FL-IONsb and FL-IONsa. The results of these experiments (Figs. 5 and S2) show that the rate of adsorption of RO, RY and BB by both adsorbents are fast but the adsorption rate by FL-IONsb is slightly faster than the adsorption by FL-IONsa. This high rate of adsorption is very important from practical point of view. For modeling of adsorption kinetics of RO, RY and BB by both adsorbents, different adsorption kinetic models were applied. Pseudo-first order rate equation (PFO) or so-called Lagergern equation which is based on adsorption capacity is one of the most widely kinetic equations [27]. Based on this model the rate of adsorption is proportional to the available vacant sites of adsorption. The integrated form of this model can be given as follows: qt  qe (1  e

 k1t

(4)

)

where k1(s−1) is the pseudo-first order rate constant, qe and qt (mg/g) are the amount of solute adsorbed at equilibrium and time t, respectively. The k1 parameter is the time scale factor the value which decides how fast equilibrium in the system can be approached.

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The pseudo-second order (PSO) equation which the rate of adsorption is proportional to the square of available vacant sites [27]. The integrated form of this equation is: qt 

k 2 qe 2 t 1  k 2 qe t

(5)

where k2 (g/mg s) is the pseudo-second order rate constant. PSO equation was first empirically presented [28] and then was derived theoretically by Azizian [27]. He showed that this equation is applicable when the change of adsorbate concentration is noticeable. According to Azizian ,s derivation, k2 is a complicated function of C0 as well as adsorption and desorption rate constants. In order to investigate the sorption of pollutants from aqueous solution Elovich rate equation has been widely used. The integrated form of Elovich equation is expressed as follows: 1 qt  ( ) ln(1  abt ) b

(6)

where a and b are equation constants. Mainly, Elovich equation is applied to describe chemical adsorption onto heterogeneous surfaces [29]. The mixed 1,2-order equation (MOE), that is the combination of first- and second-order equation, is expressed by [30] qt  q e

1  exp(k1t ) 1  F2 exp( k1t )

(7)

where F2 (F2 < 1) is determining the share of second-order term in the order rate equation. F2 

k 2 qe k1  k 2 q e

,

(8)

F1  F2  1

This model can describe the adsorption onto heterogeneous surface [30]. Most recently, Marczewski solved the Langmuir kinetic equation analytically and derived an equation which is mathematically similar to Eq. (7), but valid for homogeneous surfaces [31]. The modified pseudo-n-order (MPnO), can be expressed as [32]:

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qt  qe (1  e  nk t )1 / n

(9)

where k′=kqen−1 and n is the order of rate equation. For n=1 this equation converts to the PFO equation. This equation is also valid for heterogeneous surface. The fractal-like model utilized for adsorption on heterogeneous surfaces at solid/solution systems [33,34], the fractal-like pseudo-first order (FL-PFO) and fractal-like pseudo-second order (FLPSO) models indicate that there are different paths for adsorption on the active sites or mainly there are different active sites for adsorption during the time course of adsorption [33,34]. FLPFO and FL-PSO adsorption rate equations are derived as follows: 

(10)

qt  qe [1  e  kt ] 2

qt 

kqe t  1  kq e t 

(11)

where k is the rate constant and  is constant. The result of fitting are listed in (Tables S2 to S7). Based on the obtained correlation coefficient values, r2, the FL-PSO and FL-PFO models describes the adsorption of the dyes by FL-IONsa better than the other kinetic models. This means that, the rate coefficient of adsorption in these systems is dependent to the time. The physical meaning of this time dependency is changing of reaction path with time [33,34] due to surface heterogeneity of adsorbent [34]. For FL-IONsb the Elovich model describe the experimental kinetic data very well. This means that, the FL-IONsb provide heterogeneous surface for adsorption of dyes. The results of fitting by the FL-PFO, FL-PSO and Elovich equations are shown by solid lines in Figs 5 and S2. The present study shows that all systems approach to the equilibrium within 2 min which denotes very fast adsorption of the dyes by FL-IONsa and Fl-IONsb. This rate of adsorption is higher than previously reported data for dye adsorption by iron oxides [18-20].

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3. 3. Adsorption equilibrium studies

Figs. 6 and S3 show the adsorption isotherms of RO, RY and BB by FL-IONsb and FL-IONsa. It is clear that in all systems, the qe value increases with concentration and then approaches to the saturation value. The equilibrium adsorption data were analyzed with six isotherm equations (Tables S8 to S13). The Langmuir isotherm includes several assumptions which are surface homogenicty and absence of interactions between adsorbed molecules on the surface [35]. The Langmuir equation is given as follows: qe 

q m K L Ce 1  K L Ce

(12)

where q m (mg/g) and KL (L/mg) are known as Langmuir constants and referred to maximum sorption capacity and affinity of sorption, respectively. The Freundlich isotherm is an empirical equation, utilizing for heterogeneous surfaces [35]. The Freundlich isotherm can be written as: q e  K F C e1 / n

(13)

where K F (Lmg(1-(1/n))/g) is Freundlich constant and n indicating surface heterogeneity. Redlich–Peterson (R-P) is one of the three-parameter equations: qe 

K R Ce

(14)

1   R Ce 

where KR, ˛R, are the Redlich–Peterson constants.  is the parameter which is between 0 and 1 Redlich and Peterson incorporated the features of the Langmuir and Freundlich isotherms into a single equation There are two limiting behaviors: Langmuir form for  =1 and Henry’s law form for  =0 [35]. 13

The Toth isotherm model is another equation developed to improve Langmuir isotherm fittings. It is useful in describing adsorption onto heterogeneous surface [35]: qe 

q m bT C e

(15)

[1  (bT C e )1/ nT ] nT

Where bT (mg/g), n T (L/mg) are the Toth isotherm constants. n T indicating heterogeneity of surface. The Langmuir–Freundlich isotherm which is used for heterogeneous surfaces is a combination of Langmuir and Freundlich isotherms [36,37]: qe 

q m KCe1 / n

(16)

1  KCe1 / n

where K is the adsorption constant. The extended Langmuir equation is modified the Langmuir model and is better than or comparable to the Langmuir, Langmuir–Freundlich and Toth models in terms of accuracy in fitting experimental adsorption equilibrium data. [38]: qe 

qm K L Ce

(17)

1  K L Ce  a K L Ce

where K L, ˛a are the extended Langmuir constants. The highest correlation coefficient found for Langmuir-Freundlich model. This means a heterogeneous adsorption in all studied systems, which is in agreement whit the results from kinetic studies. The predicted results of the L-F isotherm are shown in Figs. 6 and S3. The obtained adsorption capacities for RO, RY, BB on the FL-IONsa are 88.0, 57.9 and 88.4 mg/g respectively, and the adsorption capacities of the prepared FL-IONsb for RO, RY, BB are 89.0, 57.0, 87.6 mg/g, respectively. The obtained maximum adsorption capacities, q m, for FL-IONsa and FL-IONsb are higher or comparable with different iron oxide nanostructures [18-20] (Table

14

S14). This indicates that FL-IONsa and FL-IONsb are suitable adsorbent for dye removal from aqueous solution.

3. 4. Effect of pH

pH is an important variable in the removal efficiency by adsorption method. So we have investigated the effect of pH on the adsorption capabilities of the prepared iron oxides. Effect of solution pH on the removal percentage of RO, RY and BB by FL-IONsa and FL-IONsb are shown in Fig. S4. For both adsorbents, the removal percentage of BB increases in alkaline solutions. By increasing of the pH value, the number of negative charge sites on the surface of adsorbents increases which favors for the adsorption of cationic dye (BB) due to electrostatic attraction between the negative surface of adsorbent and cationic dye. In the case of anionic dye (RO and RY), the removal percentages decreases with the increase of pH value. The pH of point zero charge (pHpzc) of adsorbent is an important characteristic, and for the prepared iron oxides it was determined according to the previous report [39]. pHpzc has determined by plotting final pH (pHf), versus initial pH, (pHi) of solution (Fig. S5). According to this plot the pH

pzc

for FL-

IONsa and FL-IONsb are found to be as 4.1 and 4.2, respectively. At solution pH higher than pHpzc the surface of the adsorbent has negative charge. Therefore the removal efficiency for both RO and RY decreases by increasing pH because of repulsion force between anionic dyes and negative surface.

3. 5. Effect of temperature

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Temperature is an important controlling factor in the real applications of adsorbents for dye removal, since most of the textile dye effluents are produced at relatively high temperatures. In order to determine the effect of temperature on the removal of RO, RY and BB by FL-IONsa and FL-IONsb, adsorption experiments were tested at four temperatures (25 oC, 35 oC, 45 oC and 55 o

C). The results show that the removal percentage of all studied dyes on to FL-IONsa and FL-

IONsb increase with temperature (Fig. S6) indicating an endothermic adsorption.

3.6. Performance of used adsorbent

Figs. 7 and S7 show the performance of fresh adsorbent and the used adsorbent treated with water or NaOH solution, for removal of RO, RY and BB form aqueous solution. As this figures show the adsorbent can be regenerated by NaOH solution moderately.

4. Conclusion

In this paper, novel 3D flower-like iron oxide nanostructures were prepared using different material but with the same procedure. The type of surfactant and reductive agent can affect on the structure of the prepared iron oxide. The obtained products show excellent adsorption performance for organic dyes (RO, RY and BB) in wastewater treatment. The kinetic studies 16

reveal that the rate of adsorption onto FL-IONs b is rapid and mainly take place within 2 min. The equilibrium and kinetic data showed that both FL-IONsa and FL-IONsb provide heterogeneous surface for adsorption of dyes (RO, RY and BB). The optimum pH for removal RO and RY have been obtained in acidic pH but for BB the optimum pH is in alkaline solution.

Acknowledgment The authors acknowledge the financial support of Bu-Ali Sina University.

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Figure captions Fig. 1. XRD patterns of the prepared iron oxides: (a) FL-IONsa, (b) FL-IONsb.

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Fig. 2. SEM images of FL-IONsa with different magnifications.

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Fig. 3. SEM images of FL-IONsb with different magnifications.

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Fig. 4. Nitrogen adsorption-desorption isotherms of (a) FL-IONsa (b) FL-IONsb.

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Fig. 5. Kinetic data for adsorption of (a) RO,(b) RY and (c) BB by FL-IONsb at different initial concentrations. The solid lines represents the predicted values by Elovich model.

Fig. 6. Adsorption isotherm of (a) RO, (b) RY and (c) BB by FL-IONsb. The solid lines represent the predicted values by Langmuir-Freundlich isotherm.

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Fig. 7. Dye removal percentage by the fresh adsorbent and the used adsorbent (FL-IONsb) treated with water or NaOH solution.

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Scheme 1. Schematic illustration of preparation of different nanostructured flower-like iron oxide.

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Table 1. Obtained structural properties of the FL-IONs, based on N2 adsorption isotherm. BET

Total pore

Average pore

surface area

volume

diameter

(m2/g)

(cm3/g)

(nm)

FL-IONsa

82

0.334

16.3

FL-IONsb

74

0.218

11.8

Sample

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