A facile homogeneous precipitation synthesis of NiO nanosheets and their applications in water treatment

Accepted Manuscript Title: A facile homogeneous precipitation synthesis of NiO nanosheets and their applications in water treatment Author: Junfeng Zhao Yang Tan Kang Su Junjie Zhao Cheng Yang Lingling Sang Hongbin Lu JianHua Chen PII: DOI: Reference:

S0169-4332(15)00375-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.02.071 APSUSC 29750

To appear in:

APSUSC

Received date: Revised date: Accepted date:

28-10-2014 9-2-2015 9-2-2015

Please cite this article as: J. Zhao, Y. Tan, K. Su, J. Zhao, C. Yang, L. Sang, H. Lu, J.H. Chen, A facile homogeneous precipitation synthesis of NiO nanosheets and their applications in water treatment, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.02.071 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.

Highlights: NiO nanosheets were synthesized via a facile homogeneous precipitation method.

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The NiO nansheets have a large surface area.

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This preparation method was low-cost, simple equipments, easy preparation, short reaction time and better repeatability.

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The product also showed a favourable ability to remove Cr(VI) and

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Congo Red (CR) in water treatment.

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A facile homogeneous precipitation synthesis of NiO nanosheets

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and their applications in water treatment

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Junfeng Zhaoa,b*, Yang Tana, Kang Sua, Junjie Zhaoa, Cheng Yanga, Lingling Sanga, Hongbin Luc, JianHua Chena,b

School of Chemistry and Materials Engineering, Changshu Institute of Technology,

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a

Changshu, P. R. China.

Jiangsu Laboratory of Advanced Functional Materials, Changshu Institute of

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b

National Laboratory of Solid State Microstructures and College of Engineering and

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c

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Technology, Changshu, P. R. China

Applied Sciences, Nanjing University, Nanjing, P. R. China

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

Corresponding author Tel.: +86-512-52251842; fax. : +86-512-52251842. E-mail address: [email protected] (J. F. Zhao)

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Abstract NiO nanosheets were successfully synthesized by a facile homogeneous precipitation method with the assistance of ethanol amine. The sample was

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characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), and nitrogen dsorption-desorption

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techniques. The results demonstrated that the as-prepared product was cubic NiO

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nanosheets with a large surface area of 170.1 m2 g-1. Further, the as-prepared product was used to investigate its potential application for wastewater treatment. The

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maximum adsorption capacity for Cr (VI) and Congo red (CR) on NiO nanosheets has

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been determined using the Langmuir equation and found to reach up to 48.98 mg g-1 and 167.73 mg g-1, respectively. It could be concluded that NiO nanosheets with

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special surface features had the potential as adsorbents for wastewater treatment.

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Keywords: NiO nanosheets; Homogeneous precipitation; Adsorption; Cr(VI); Congo red

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1. Introduction Over the past few years, the environmental pollution especially industrial wastewater has become a serious environmental problem for the global society.

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Inorganic heavy metal ions and organic dyes are often present in industrial or urban

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waste waters, which can bring many detrimental effects on environment and human

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health [1, 2]. Until now, many effective methods, such as chemical precipitation [3, 4], ion exchange [5, 6], adsorption [7, 8] for the removal of heavy metal ions, and

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coagulation [9], chemical oxidation [10, 11], adsorption [12, 13], electrochemical processes [14] for the remove of organic dyes were developed. Among all these

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available methods, adsorption onto the surface of the solid materials is generally considered to be one of the most versatile, effective and economic methods for the

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water treatment due to its advantage of low-cost, clean and green virtues[15]. Especially, the development of nano science and technology has pioneered a new path

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on the studies of water treatment. Nanostructured materials demonstrate potential advantages in water treatment application due to their large surface area, high available surface adsorption site density, surface defects, fast diffusivities and well-defined morphology [16-24]. As a well-know important p-type wide-bandgap oxide semiconductor, NiO-based

nanomaterials have been extensive investigated because of its promising applications in catalysis [25], lithium ion batteries [26, 27], electrochemical supercapacitors [28], and so on. Recent reports have also shown that NiO nanostructures hold great promise in removal of organic dyes and inorganic heavy metal ions [29-35]. Most of these

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reported NiO adsorbents generally synthesized via surfactant-assisted solvothermal and electronchemical method. For large scale production, these processes are cost-expensive, time consuming, poor repeatability and not environmentally friendly.

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production of NiO with the comparable adsorption capacities.

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Hence, it is important to develop a facile synthesis route that allows the low cost

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Herein, we report a facile synthesis of NiO nanosheets by the homogeneous precipitation method and their implementation as adsorbents in removing Cr (VI) and

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Congo red (CR). This present approach has significant advantages such as low-cost, simple equipments, easy preparation, short reaction time and better repeatability.

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More importantly, the as-prepared NiO nanosheets exhibit an excellent ability in wastewater treatment.

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2.1 Preparation

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2. Experimental

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All reagents were of analytical grade and used without further purification. The

synthesis route is summarized as follows: Ni2+

ETA

complexing

ETA-Ni2+

hydrolysis reflux

Ni(OH)2

calcination

NiO

In a typical synthesis, 0.005 mol of NiCl2·6H2O and 20 mL of ethanol amine

(ETA) were added to a conical flask, and continuously stirred at 80 oC to form a clear

colloid solution. Then, 200 mL of distilled water was added into the colloid solution. The clear mixture turned to a green suspension after reflux at 180 oC for 2 h. The resulting suspension was then centrifuged and washed by absolute ethanol and deionized water for several times. The washed suspension was dried at 80 oC for 24 h

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and then calcined at 300 oC for 3 h. 2.2 Characterization X-ray powder diffraction (XRD) pattern of the sample was obtained with a

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Rigaku Dmax-2200PC X-ray diffractometer in the diffraction angle range 2θ = 20-90º

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using Cu-K radiation. The field emission scanning electron microscopy morphology

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(FE-SEM) and the energy dispersive X-ray spectrum (EDX) were conducted on Zeiss Sigma field emission scanning electron microscope. Atomic force microscope (AFM)

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images were obtained using AFM (Bruker innova-sys). Transmission electron microscopy (TEM) analysis was performed by means of JEM-2000. The surface areas

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of the powders are measured by Brunauer-Emmett-Teller method (AS AP 2020).

2.3 Cr (VI) adsorption

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spectrophotometer.

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UV-vis adsorption spectra were collected by a Shimadzu UV-3600 UV-visible

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K2Cr2O7 was chosen as the sources of Cr(VI). Simulated wastewater with

different Cr(VI) concentrations (10, 15, 20, 30, 40, 60, 80, 100, 150 mg L-1) were

prepared by dilution of the stock K2Cr2O7 standard solution with deionized water. Typically, the adsorption studies of Cr(VI) ions onto NiO powders were investigated in a model in a glass flask under magnetic stirring with a constant speed. In each experiment, 0.2 g of adsorbent was added to 100 mL of simulated wastewater at room temperature (25 oC) and without the further pH adjustment. After the completion of preset time intervals, about 4.5 mL of solution was taken out and centrifuged for separating the solution from the adsorbent. The residual Cr(VI) concentration in the

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solution was then determined by a UV-vis instrument (λmax = 540 nm). 2.4 CR adsorption The dye adsorption studies were performed in a standard solution of CR dye. In

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the adsorption kinetics experiment, 0.08 g of the as-prepared NiO powders was mixed

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with 50 mL of aqueous solution of CR with various concentrations (50, 100, 200, 300,

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400, 500 mg L-1). At different time intervals, 4.5 mL of the solution was taken out and centrifuged. The CR concentrations in the remaining solutions were analyzed by

3. Results and discussion

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3.1 Characterization of the NiO powders

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UV-vis spectroscopy at λmax = 498 nm.

The X-ray diffraction (XRD) was used to characterize the purity and crystallinity

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of the as-prepared samples. As shown in Fig. 1, the main peaks at 37.26o, 43.32o,

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62.84o, 75.18o, and 79.23o correspond to the (111), (200), (220), (311) and (222)

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planes of NiO (JCPDS 78-0643), indicating the formation of NiO cubic phase with high purity.

The morphology and structure of the as-prepared sample were determined by

SEM, AFM and TEM. SEM and AFM morphologies of the samples are shown in Fig. 2a, 2b and 2f. It can be observed that NiO presents a sheets-like morphology, with thickness of about 20 nm. Fig. 2c and 2d show the TEM images of NiO, which further identified their sheets-like morphology. In order to further prove the synthesis of NiO, EDX is performed to reveal that these nonosheets structure is composed purely of NiO. The typical EDX pattern of NiO crystal is shown in Fig. 2e.

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To further determine the specific surface area and pore size distribution of the as-prepared NiO nanosheets, nitrogen adsorption-desorption measurements were performed. Fig. 3 display the nitrogen adsorption-desorption isotherm and the

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corresponding Barrett-Joyner-Halenda (BJH) pore size distribution curve. Acoording

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to the IUPAC classification, the isotherm is ascribed to a type IV with a type H3 hysteresis loop, indicating the existence of mesopores (pore diameter 2-50 nm) in the

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as-prepared NiO nanosheets. The BET surface area of the sample is 170.1 m2 g-1 and

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the average pore diameter of the sample is approximately 24.6 nm, determined by BJH method.

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3.2 Removal of Cr(VI)

Cr(VI) is considered as a highly toxic pollutant. In this study, the adsorption

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performance of the as-prepared NiO nanosheets was first investigated by adsorptive removal of Cr(VI) from simulated wastewater. The adsorbed amount was calculated

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by Eqs. (1).

qt 

(C0  Ct )V W

(1)

Where C0 is the initial concentration of Cr(VI) (mg L-1), qt (mg g-1) is the amount

adsorbed per gram of adsorbent at time t (min), Ct is the concentration of Cr(VI) at time t of adsorption (mg L-1), V is the initial volume (L) the Cr(VI) solution, and W is the weight of the adsorbent (g).

The effect of the contact time was analyzed using the plot of qt vs. t (Fig. 4) for different initial Cr(VI) concentrations, keeping the adsorbent mass (0.2 g) constant. Obviously, the adsorption rates within the first 5 min were surprisingly fast under all

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the concentrations. The adsorption process almost finished within 60 min, and no significant change was observed from 60 to 180 min, demonstrating the high efficiency of the NiO nanosheets for the removal of Cr(VI) in water. This

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phenomenon is not only attributed to the high concentration gradient at the initial

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stage, but also associated with the advantageous structure and the adequate vacant

adsorption sites on the surface of the adsorbent. It is noteworthy that the adsorbent

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achieved a maximum Cr (VI) adsorption capacity of 28 mg g-1 after only 10 min of

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adsorption at the initial concentration of 80 mg L-1. Obviously, the features of fast adsorption rate and excellent adsorption capacity could allow the NiO nanosheets to

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find a potential application for rapid treatment of high concentration Cr(VI) wastewater.

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The adsorption kinetics was obtained by fitting the experimental data with a

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pseudo-second-order kinetic model, and the equation is given below: t 1 t   2 qt k2 qe qe

(2)

Where qe and qt (mg g-1) are the amounts of adsorbed at equilibrium and at any

time t (min), respectively. k2 (g mg-1 min-1) is the pseudo-second-order rate constant. The kinetic parameters and the correlation coefficients (R2) can be

determined by linear regression. As shown in Fig. 5 and Table 1, the pseudo-second-order model fits quite well with experimental date at all the initial concentrations with high correlation coefficients, and the values of qe ,cal are very

close to the experimentally observed values of qe ,exp . These results indicate that the adsorption system studied belongs to the second-order kinetic model.

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Adsorption capacity at different aqueous equilibrium concentration can be illustrated by the adsorption isotherm. To investigate the interaction between Cr(VI) and NiO nanosheets, two well-know models, Langmuir and Freundlich isotherm,

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were selected to analyze the equilibrium adsorption date. The isotherm models can be

qm K L Ce 1  K L Ce

qe  K F C

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qe 

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expressed with the following equations:

1 n e

(3)

(4)

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Where Ce is equilibrium (residual) concentration of solute (mg L-1), qe is the

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amount of adsorbed at equilibrium (mg g-1), qm is the maximum adsorption capacity and K L is the Langmuir adsorption model constant (L mg-1). For the Freundlich

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equation, K F is the adsorption model constant (L g-1) and n is Freundlich adsorption

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model exponent. The related parameters obtained from the two isotherm models are

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listed in Table 2. Fig. 6 shows that the adsorption of Cr(VI) onto the NiO nanosheets can be better described by the Langmuir model , with a correlation coefficient R2 value of 0.9619, than to that of Freundlich model, with a correlation coefficient of 0.9252, which indicates that the adsorption process conforms better to the Langmuir model. The maximum adsorption capacity ( qm ) calculated from the non-linear

simulation equation was 48.98 mg g-1, which were a little higher than the experiment data. 3.2 Removal of Congo red (CR) To further investigate the advantage of the NiO nanosheets in water treatment, the adsorption performance for CR was also studied in our work. Fig. 7 shows the

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time profile of CR adsorption at different initial concentration with 0.08 g of the NiO nanosheets. For all the concentrations, the adsorption is very fast during the first 5 min, and the equilibrium is achieved within 180 min. The maximum CR adsorption

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capacity of NiO nanosheets was found to be 115.32 mg g-1 at the initial concentrations

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of 200 mg L-1. Simultaneously, the removal efficiencies were found to be 85.8, 94.7

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and 92.3% at the initial CR concentrations of 50, 100 and 200 mg L-1, respectively. This reveals the high efficiency of the NiO nanosheets for the removal of CR in

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aqueous solutions.

Fig. 8a and Fig. 8b show the adsorption spectra of CR solution (100 and 200 mg

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L-1 respectively) in the present of the NiO nanosheets for various durations in the adsorption process. The adsorption peaks intensity corresponding to the CR molecule

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at 498 nm decrease gradually as the adsorption time increase and almost completely disappear after about 240 min.

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To understand and characteristics of the adsorption process, the kinetics and

isotherm were also determined and modeled in detail. The pseudo-second-order kinetics of CR onto the NiO nanosheets is shown in Fig. 9. It can be deduced that the adsorption process fits quite well with the pseudo-second-order model. Moreover, as shown in Table 3, the correlation coefficient (R2 > 0.999) further suggests that the

adsorption followed the pseudo-second-order model perfectly. Fig. 10 shows the adsorption isotherms of CR onto NiO nanosheets. The equilibrium adsorption data were also analyzed by using Langmuir and Freundlich isotherm models. The related parameters obtained from non-linear regression by both models are summarized in

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Table 4. The experimental data fit better to the Langmuir isotherm with a correlation coefficient value of 0.9604 than to the Freundlich isotherm with correlation coefficient of 0.9186, indicating the monolayer adsorption of CR on the NiO

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nanosheets.

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3.3 Comparison of adsorption properties

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Table 5 compares the maximum adsorption capacity of NiO nanosheets with other hierarchical structured materials previously used for removal of Cr(VI) or CR in

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water. Our results indicated that the NiO nanosheets possessed much higher adsorption capacity to Cr(VI) than mesoporous NiO [30]. This value is even higher

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than that of previously reported hierarchical structured adsorbents, such as, manganese oxide nanofibers, 14.6 mg g-1 [36], ceria microspheres, 6.76 mg g-1 [37],

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and lepidocrocite (γ-FeOOH) nanoflakes, 17.5 mg g-1 [38]. Simultaneously, the adsorption capacity of CR onto NiO nanosheets is comparable with many other

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previously reported adsorbents. For example, the

qm value of hierarchical NiO

nanosheets and hierarchical NiO architectures for CR adsorption are 151.7 and 223.8 mg g-1[29, 31]. These observations clearly emphasize that the NiO nanosheets presented in this study is a potential adsorbent material for heavy metal ion and organic dyes contaminants.

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4. Conclusion In summary, NiO nanosheets have been prepared by a facile, environmentally friendly and low-cost procedure homogeneous precipitation method with the

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assistance of ethanol amine. The as-prepared NiO nanosheets exhibited predominant

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adsorption ability to heavy metal ion pollutants and high adsorption ability to organic

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dyes, which may be attributed to large specific surface areas and the exist of mesoporous structure. The maximum adsorption capacity of NiO nanosheets for

kinetics

and

isotherm

of

adsorption

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Cr(VI) and CR is 48.98 mg g-1 and 167.73 mg g-1, respectively. Furthermore, the process

were

found

to

obey

the

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pseudo-second-order kinetics and Langmuir isotherm models, respectively. These studies provide valuable information for design the NiO with large specific surface

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areas, which may have potential application in wastewater treatment.

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Reference [1] M. M. Pendergast, E. M. V. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci. 4 (2011) 1946-1971.

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[2] L. H. Ai, H. T. Yue, J. Jiang, Sacrificial template-directed synthesis of mesoporous

cr

manganese oxide architectures with superior performance for organic dye adsorption,

us

Nanoscale, 4 (2012) 5401-5408.

[3] M. M. Matlock, B. S. Howerton, D. A. Atwood, Chemical precipitation of heavy

an

metals from acid mine drainage, Water Res. 36 (2002) 4757-4764.

[4] M. Kiel, D. Dobslaw, K. H. Engesser, Comparison of biological and chemical

M

treatment processes as cost-effective methods for elimination of benzoate in saline wastewaters, Water Res. 66 (2014) 1-11.

te

d

[5] I. Dobrevsky, M. D. Todorova, T. Panayotova, Electroplating rinse waste water treatment by ion exchange, Desalination 108 (1997) 277-280.

Ac ce p

[6] B. An, Q. Q. Liang, D. Y. Zhao, Removal of arsenic(V) from spent ion exchange brine using a new class of starch-bridges magnetite nanoparticles, Water Res. 45 (2011) 1961-1972.

[7] M. Arulkumar, K. Thirumalai, P. Sathishkumar, T. Palvannan, Rapid removal of chromium from aqueous solution using novel prawn shell activated carbon, Chem. Eng. J. 185-196 (2012) 178-186. [8] A. B. Albadarin, C. Mangwandi, A. H. Al-Muhtaseb, G. M. Walker, S. J. Allen, M. N. M. Ahmad, Kinetic and thermodynamics of chromium ions adsorption onto low-cost dolomite adsorbent, Chem. Eng. J. 179 (2012) 193-202.

Page 14 of 32

[9]

A.

K.

Verma,

R.

R.

Dash,

P.

Bhunia,

A

review

on

chemical

coagulation/flocculation technologies for removal of colour from textile wastewaters, J. Environ. Manage. 93 (2012) 154-168.

ip t

[10] V.V. Tarasov, G. S. Barancova, N. K. Zaitsev, D. X. Zhang, Photochemical

cr

kinetics of organic dye oxidation in water, Process Saf. Environ. 81 (2003) 243-249.

us

[11] Y. Zhang, D. L. Li, Y. Chen, X. H. Wang, S. T. Wang, Catalytic wet air oxidation of dye pollutants by polyoxomolybdate nanotubes under room condition, Appl Cataly

an

B: Environ. 86 (2009) 182-189.

[12] S. B. Wang, H. T. Li, Structure directed reversible adsorption of organic dye on

M

mesoporous silica in aqueous solution, Micropor. Mesopor. Mater. 97 (2006) 21-26. [13] A. Gil, F. C. C. Assis, S. Albeniz, S. A. Korili, Removal of dyes from

te

d

wastewaters by adsorption on pillared clays, Chem. Eng. J. 168 (2011) 1032-1040. [14] X. P. Zhu, J. R. Ni, J. J. Wei, X. Xing, H. N. Li, Destination of organic pollutants

Ac ce p

during electrichemical oxidation of biologically-pretreated dye wastewater using boron-doped dimond anode, J. Hazard. Mater. 189 (2011) 127-133. [15] S. H. Ran, Y. G. Zhu, H. T. Huang, B. Liang, J. Xu, B. Liu, J. Zhang, Z. Xie, Z. R. Wang, J. H. Ye, D. Chen, G. Z. Shen, Phase-controlled synthesis of 3D flower-like Ni(OH)2 architectures and their applications in water treatment, Cryst. Eng. Comm. 14

(2012) 3063-3068. [16] H. Li, G. F. Wang, F. Zhang, Y. Cai, Y. D. Wang, I. Djerdj, Surfactant-assisted synthesis of CeO2 nanoparticles and their application in wastewater treatment, RSC Adv. 2 (2012) 12413-12423.

Page 15 of 32

[17] X. H. Lu, D. Z. Zheng, J. Y. Gan, Z. Q. Liu, C. L. Liang, P. Liu, Y. X. Tong, Porous CeO2 nanowires/nanowire arrays: electrochemical synthesis and application in water treatment, J. Mater. Chem. 20 (2010) 7118-7122.

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[18] X. Q. Gao, F. Xiao, C. Yang, J. D. Wang, X. T. Su, Hydrothermal fabrication of

cr

W18O49 nanowire networks with superior performance for water treatment, J. Mater.

us

Chem. A, 1 (2013) 5831-5834.

[19] T. Hao, C. Yang, X. H. Rao, J. D. Wang, C. G. Niu, X. T. Su, Facile additive

an

synthesis of iron oxide nanoparticles for efficient adsorptive removal of Congo red and Cr (VI), Appl. Surf. Sci. 292 (2014) 174-180.

M

[20] T. Zhang, X. W. Zhang, J. W. Ng, H. Y. Yang, J. C. Liu, D. D. Sun, Fabrication of magnetic cryptomelane-typr manganese oxide nanowires for water treatment, Chem.

te

d

Commun. 47 (2011) 1890-1892.

[21] I. Urban, N. M. Ratcliffe, J. R. Duffield, G. R. Elder, D. Patton, Functionalized

Ac ce p

paramagnetic nanoparticles for waste water treatment, Chem. Comm. 46 (2010) 4583-4585.

[22] E. Álvarez-Ayuso, A. García-Sánchez, X. Querol, Adsorption of Cr(VI) from synthetic solutions and electroplating wastewaters on amorphous aluminium oxide, J. Hazard. Mater. 142 (2007) 191-198. [23] L. A. Rodrigues, L. J. Maschio, R. E. Silva, M. L.C. P. Silva, Adsorption of Cr(VI) from aqueous solution by hydrous zirconium oxide, J. Hazard. Mater. 173 (2010) 630-636. [24] A. M. Asiri, S. B. Khan, K. A. Alamry, H. M. Marwani, M. M. Rahman, Growth

Page 16 of 32

of Mn3O4 on cellulose matrix: nanohybird as a solid phase adsorbent for trivalent chromium, Appl. Surf. Sci. 270 (2013) 539-544. [25] Q. H. Zhang, C. J. Cao, T. Xu, M. Sun, J. Z. Zhang, Y. Wang, H. L. Wan,

ip t

NiO-polyoxometalate nanocomposites as efficient catalysts for the oxidative

cr

dehydrogenation of propane and isobutene, Chem. Commun. (2009) 2376-2378.

us

[26] B. Varghese, M. V. Reddy, Y. W. Zhu, S. L. Chang, C. H. Teo, G. V. S. Rao, B. V. R. Chowdari, T. S. W. Andrew, C. T. Lim, C. H. Sow, Fabrication of NiO nanowall

an

electrodes for high performance lithium ion battery, Chem. Mater. 20 (2008) 3360-3367.

M

[27] V. Aravindan, P. S. Kumar, J. Sundaramurthy, W. C. Ling, S. Ramakrishna, S. Madhavi, Electrospun NiO nanofibers as high performance anode material for Li-ion

te

d

batteries, J. Power. Source, 227 (2013) 284-290. [28] S. J. Ding, T. Zhu, J. S. Chen, Z. Y. Wang, C. L. Yuan, X. W. Lou, Controlled

Ac ce p

synthesis of hierarchical NiO nanosheet hollow spheres with enhanced supercapacitive performance, J. Mater. Chem. 21 (2011) 6602-6606. [29] L. H. Ai, Y. Zeng, Hierarchical NiO architectures as highly recyclable adsorbents for effective removal of organic dye from aqueous solution, Chem. Eng. J. 215-216 (2013) 269-278.

[30] M. A. Behnajady, S. Bimeghdar, Synthesis of mesoporous NiO nanoparticles and their application in the adsorption of Cr(VI), Chem. Eng. J. 239 (2014) 105-103. [31] B. Cheng, Y. Le, W. Q. Cai, J. G. Yu, Synthesis of hierarchical Ni(OH)2 and NiO nanosheets and their adsorption kinetics and isotherms to congo red in water, J.

Page 17 of 32

Hazard. Mater. 185 (2011) 889-897. [32] P. P. Zhang, X. M. Ma, Y. M. Guo, Q. Q. Cheng, L. Yang, Size-controlled synthesis of hierarchical NiO hollow microspheres and the adsorption for congo red in

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water, Chem. Eng. J. 189-190 (2012) 188-195.

cr

[33] H. J. Wang, C. F. Wang, Y. Y. Sun, Y. Cao, Film-form dye adsorbents of NiO:

us

synthesis, biological activity and application on dye sorption, Chem. Eng. J. 209 (2012) 442-450.

an

[34] X. W. Li, S. L. Xiong, J. F. Li, J. Bai, Y. T. Qian, Mesoporous NiO ultrathin nanowire networks topotactically transformed from -Ni(OH)2 hierarchical

M

microspheres and their superior electrochemical capacitance properties and excellent capability for water treatment, J. Mater. Chem. 22 (2012) 14276-14283.

te

d

[35] L. X. Song, Z. K. Yang, Y. Teng, J. Xia, P. Du, Nickel oxide nanoflowers: formation, structure, magnetic property and adsorptive performance towards organic

Ac ce p

dyes and heavy metal ions, J. Mater. Chem. A, 1 (2013) 8731-8736. [36] A. H. Qusti, Removal of chromium(VI) from aqueous using manganese oxide nanofibers, J. Ind. Eng. Chem. 20 (2014) 3394-3399. [37] H. Y. Xiao, Z. H. Ai, L. Z. Zhang, Nanaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres as novel adsorbents for wastewater treatment, J. Phys. Chem. C 113 (2009) 16625-16630. [38] Y. Jia, T. Luo, X. Y. Yu, Z. Jin, B. Sun, J. H. Liu, X. J. Huang, Facile one-pot synthesis of lepidocrocite (γ-FeOOH) nanoflakes for water treatment, New J. Chem. 37 (2013) 2551-2556. [39] T. Zhu, J. S. Chen, X. W. Lou, Highly efficient removal of organic dyes from

Page 18 of 32

wastewater using hierarchical NiO spheres with high surface area, J. Phys. Chem. C 116 (2012)6873-6878. [40] Z. Song, L. F. Chen, J. C. Hu, R. Richards, NiO(111) nanosheets as efficient and

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recyclable adsorbents for dye pollutant removal from wastewater, Nanotechnology 20

Ac ce p

te

d

M

an

us

cr

(2009) 275707.

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Figure legends Fig.1 XRD patterns of NiO powders Fig.2 SEM, TEM and AFM images of the as-synthesized product, (a) low magnification SEM image, (b) high magnification SEM image, (c) low magnification

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TEM image, (d) high magnification SEM image and SAED pattern of the as-synthesized product, (e) AFM images, (d) EDX pattern of NiO nanosheets.

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Fig.3 Nitrogen adsorption-desorption isotherms and pore-size distribution (inset) of the NiO nanosheets.

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Fig.4 Adsorption capacity of the NiO nanosheets for Cr(VI) removal as a function of the contact time.

an

Fig.5 The pseudo-second-order kinetics plots of Cr(VI) adsorption on the NiO nanosheets.

M

Fig.6 Adsorption isotherm curves of Cr(VI) adsorption on the NiO nanosheets. Fig.7 Adsorption capacity of the NiO nanosheets for CR removal as a function of the

d

contact time.

Fig.8 UV-vis adsorption spectra change of CR after being treated by the NiO

te

nanosheets, (a) 100 mg L-1, (b) 200 mg L-1.

Ac ce p

Fig.9 The pseudo-second-order kinetics plots of CR adsorption on the NiO nanosheets.

Fig.10 Adsorption isotherm curves of CR adsorption on the NiO nanosheets.

Page 20 of 32

Table 1 Kinetic parameters for the adsorption of Cr(VI) onto NiO nanosheets. qe,exp (mg g-1)

20 40 60 80

9.37 14.37 23.77 29.32

Pseudo-second-order -1

-1

k2 (g mg min )

qe,cal (mg g-1)

R2

0.1282 0.0534 0.0493 0.071

9.56 14.45 23.73 29.36

0.99996 0.99994 0.9999 0.99999

Table 2 Isotherm parameters for the adsorption of Cr(VI) onto NiO nanosheets. -1

2

qm (mg g )

KL(L mg )

R

KF

48.98

0.01323

0.96194

1.95122

cr

Freundlich -1

n

R2

0.5717

0.92527

us

Langmuir

ip t

C0 (mg L-1)

Table 3 Kinetic parameters for the adsorption of CR onto NiO nanosheets qe,exp (mg g-1)

50 100 200

26.82 59.23 115.4

Pseudo-second-order -1

-1

k2 (g mg min )

qe,cal (mg g-1)

R2

26.86 59.84 118.06

0.9999 0.9998 0.9963

an

C0 (mg L-1)

M

0.0212 0.0048 0.0007

Table 4 Isotherm parameters for the adsorption of CR onto NiO nanosheets. Langmuir

Freundlich

167.73

0.0064

R2

KF

n

R2

0.96035

8.1557

0.4489

0.9186

d

KL(L mg-1)

Ac ce p

te

qm (mg g-1)

Page 21 of 32

Table 5 Comparison of the adsorption capacities of Cr(VI) and CR onto various adsorbents.

Cr(VI)

17.5

132.8

Cr(VI) CR

78.1 151.7

15.5 201

CR

440

CR

223.8

CR CR

36.1 525

170.1 71.09 94.1 65 254

30 240 30 60 120 60

References

This work [30] [36] [37] [23]

ip t

48.98 167.73 4.73 14.6 6.76 66

Time (min)

cr

Cr(VI) CR Cr(VI) Cr(VI) Cr(VI) Cr(VI)

150

[38]

300

[22] [31]

90

[39]

58.3

120

[29]

80 44.9

300 180

[40] [35]

us

g-1)

Equilibrium

222

Ac ce p

te

d

Mesoporous NiO Manganese oxide nanofibers Ceria microspheres Hydrous zirconium oxide Lepidocrocite (γ-FeOOH) nanoflakes Amorphous aluminium oxide Hierarchical NiO nanosheets Hierarchical NiO nanospheres Hierarchical NiO architectures NiO (111) nanosheets NiO nanoflowers

qm (mg

an

NiO nanosheets

SBET (m2 g-1)

Pollution type

M

Adsorbents

Page 22 of 32

Ac

ce

pt

ed

M

an

us

cr

i

Figure 1

Page 23 of 32

Ac ce p

te

d

M

an

us

cr

ip t

Figure 2

Page 24 of 32

Ac

ce

pt

ed

M

an

us

cr

i

Figure 3

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 4

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 5

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 6

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 7

Page 29 of 32

Ac

ce

pt

ed

M

an

us

cr

i

Figure 8

Page 30 of 32

Ac

ce

pt

ed

M

an

us

cr

i

Figure 9

Page 31 of 32

Ac

ce

pt

ed

M

an

us

cr

i

Figure 10

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