Hierarchical flower-like nickel(II) oxide microspheres with high adsorption capacity of Congo red in water

Journal of Colloid and Interface Science 504 (2017) 688–696

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Regular Article

Hierarchical flower-like nickel(II) oxide microspheres with high adsorption capacity of Congo red in water Yingqiu Zheng a, Bicheng Zhu a, Hua Chen a, Wei You a, Chuanjia Jiang a,⇑, Jiaguo Yu a,b,⇑ a b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Monodispersed hierarchical flower-

like NiO microspheres were synthesized.
NiO microspheres with hierarchical pore structure and high specific surface area.
NiO microspheres exhibited high Congo red adsorption capacity of 534.8 mg g–1.
Adsorption of Congo red onto NiO mainly due to strong electrostatic attraction.

a r t i c l e

i n f o

Article history: Received 1 April 2017 Revised 3 June 2017 Accepted 5 June 2017

Keywords: Hierarchical microsphere Nickel(II) oxide Congo red Anionic dye Adsorption

a b s t r a c t Monodispersed hierarchical flower-like nickel(II) oxide (NiO) microspheres were fabricated by a facile solvothermal reaction with the assistance of ethanolamine and a subsequent calcination process. The as-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption isotherms, zeta potential measurement and Fourier transform infrared spectroscopy. Flower-like nickel(II) hydroxide microspheres with uniform diameters of approximate 6.3 lm were obtained after the solvothermal reaction. After heat treatment at 350 °C, the crystal phase transformed to NiO, but the hierarchical porous structure was maintained. The as-prepared microspheres exhibited outstanding performance for the adsorption of Congo red (CR), an anionic organic dye, from aqueous solution at circumneutral pH. The pseudo-second-order model can make a good description of the adsorption kinetics, while Langmuir model could well express the adsorption isotherms, with calculated maximum CR adsorption capacity of 534.8 and 384.6 mg g–1, respectively, for NiO and Ni(OH)2. The adsorption mechanism of CR onto the as-synthesized samples can be mainly attributed to electrostatic interaction between the positively charged sample surface and the anionic CR molecules. The as-prepared NiO microspheres are a promising adsorbent for CR removal in water treatment. Ó 2017 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding authors at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China (J. Yu). E-mail addresses: [email protected] (C. Jiang), [email protected] (J. Yu). http://dx.doi.org/10.1016/j.jcis.2017.06.014 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.

Water is vital for life, and water quality is closely related to human health. However, surface water has been seriously polluted in recent decades with the development of modern manufactory

Y. Zheng et al. / Journal of Colloid and Interface Science 504 (2017) 688–696

and energy industries [1–4]. Synthetic dyes have wide applications in industries such as textile, papermaking, printing and leather tanning [5–7]. Their discharges into the aquatic environment bring risk to human health since many organic dyes are toxic and even carcinogenic [8]. Therefore, it is of great importance to remove the dye pollutants from water. To eliminate organic dyes from wastewater, various techniques have already been developed, e.g. adsorption [9–11], coagulation [12,13], aerobic or anaerobic treatment [14,15], photodegradation [16–18], chemical oxidation [19], and membrane separation [20]. Compared to other methods, adsorption has remarkable advantages such as simple design and operation, low cost, and high efficiency, thus making it one of the most effective dye removal techniques. Various materials have been applied for the removal of dyes, e.g., activated carbons [21], zeolites [22], fly ash [8], polymer [23], and clay minerals [24]. However, most of these conventional adsorbents still have drawbacks, such as slow adsorption kinetics and low adsorption capacity, as well as low selectivity. Hence, it is indispensable to exploit new adsorbents for removing organic dyes from the wastewater. For this purpose, porous metal oxides have drawn considerable attention based on the advantage of high specific surface area, fast diffusivities as well as excellent adsorption capacity [25]. Nickel(II) oxide (NiO) is an important and promising transitionmetal oxide which possesses the advantages of high chemical and thermal stability, environmental compatibility and low cost. It has been widely used in many fields, such as lithium-ion batteries [26], gas sensor [27], supercapacitors [28], and catalysis [29]. Moreover, with an isoelectric point (IEP) of 10.3 [30,31], NiO particles have positive surface charge at circumneutral pH (pH = 6–8), making it suitable to adsorb anionic dyes from aqueous solution. Therefore, besides the above applications, NiO also exhibits excellent performance of adsorption for anionic dye, such as Congo red (CR) and Methyl orange (MO). However, the adsorption ability of these NiO materials for organic dyes are relatively low, and it is imperative to design highly efficient NiO-based adsorbents for organic dye removal from water. In this work, we synthesized nearly monodispersed flower-like NiO microspheres by a solvothermal method with the aid of ethanolamine, and investigated their performance for CR adsorption in water. The use of ethanolamine in material synthesis reactions could reduce the size of nanocrystals, improve the dispersion properties [32,33], and modify the morphology of the synthesis products. Materials with various morphologies have been successfully prepared with the assistance of ethanolamine, such as flower-like ZnO [34], hierarchical porous b-Co(OH)2 [35] and nanosheetassembled hierarchical hollow a-Fe2O3 microspheres [36]. Herein, flower-like NiO microspheres were successfully synthesized, and showed excellent adsorption capacity for CR, indicating great potential for application in water treatment.

689

of ethanol and DI water (2:1, v/v). Subsequently, 8 mL of ethanolamine is mixed into this solution. To form a homogeneous solution, the mixture is stirred continually for 30 min at 25 °C. After stirring, the solution is sealed in a Teflon-line autoclave (100 mL) and then heated to 160 °C and kept for 12 h. After cooling down, the green products are collected by centrifuging and rinsed thoroughly using DI water and ethanol. The abluent precipitates are dried for 8 h at 50 °C by a vacuum oven to collect Ni(OH)2 microspheres. Finally, the NiO microspheres are obtained after calcining the Ni(OH)2 microspheres for 2 h at 350 °C with a 2 °C min–1 heating rate. The commercial NiO used for comparison is denoted as NiO-C.

2.2. Characterization X-ray diffraction (XRD) is conducted on a X-ray diffractometer irradiation (D/Max-RB, Rigaku, Japan) using Cu Ka (k = 0.15418 nm). Morphology and microstructure are investigated by a field-emission scanning electron microscope (JSM-7500F, JEOL, Japan) and a transmission electron microscope (JEM-2100F, JEOL). The nitrogen adsorption analyses are operated on a nitrogen adsorption apparatus (ASAP 2020, Micromeritics, USA), the pore size distributions are obtained by the Barrett–Joyner–Halenda (BJH) method [37]. FT-IR spectra are collected on a FT-IR spectrometer (Affinity-1, Shimadzu, Japan).

2.3. Adsorption experiments The adsorption isotherms are obtained using the batch method with a thermostatic shaker without adjusting pH. The process is as follows: A certain amount of the adsorbent (10 mg) is mixed into a group of conical flasks (250 mL) with 100 mL of dilute CR solutions (20–100 mg L–1) inside. After being sealed, the solutions are incubated in the shaker to reach equilibrium at 30 °C. Subsequently, the suspensions are centrifuged and the CR concentrations left in the supernatant solution are measured with a UV–vis spectrophotometer (UVmini 1240, Shimadzu, Japan). The equilibrium CR adsorption capacity is evaluated by Eq. (1) in Table 1. For adsorption kinetic experiments, with a fixed initial concentration of 50 mg L–1 for CR solutions, the samples are prepared in a similar way to the isotherm experiments. The solutions are sampled and centrifuged at different time intervals and the equilibrium CR concentrations in solution at equilibrium are measured using an identical way to adsorption isotherms study. The CR adsorption uptake at time t is measured through Eq. (2) listed in Table 1.

3. Results and discussion 2. Experimental

3.1. Crystal phase

2.1. Preparation of samples

XRD patterns for the three samples (NiO, Ni(OH)2 and the commercial NiO-C) are shown in Fig. 1. All the observed peaks in curve (a) correspond to the hexagonal phase b-Ni(OH)2 (JCPDS No.14– 0117) [38]. Curve (b) shows five peaks located at 37.2°, 43.3°, 62.8°, 75.4°and 79.4°, which are indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattice planes of NiO (JCPDS No.47– 1049) with face-centered-cubic (fcc) structure [39]. No other peaks were observed, which indicates pure crystalline NiO can be gained by calcination of the precursor. Compared with curve (b), curve (c) shows the same diffraction patterns but with much sharper diffraction peaks, suggesting the grain size of commercial NiO is much larger.

Nickel chloride (NiCl26H2O), urea (CO(NH2)2), nickel oxide (NiO), ethanolamine (HOCH2CH2NH2) and ethanol are obtained from the Shanghai Chemical Industrial Company. The chemicals mentioned above are of analytical grade and used as received without refining. Deionized (DI) water is also employed during the whole fabrication and experimental process. The monodispersed hierarchical Ni(OH)2 microspheres are prepared by the solvothermal reaction and NiO microspheres are obtained by a subsequent calcination process. Typically, 0.95 g of NiCl26H2O and 0.48 g of urea are added to a 60 mL mixed solvent

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Table 1 Equations used in this work. Equation number

Equation

Parameters and meaning

1

e ÞV qe ¼ ðC 0 C W

C0 (mg L–1): initial CR concentration in solution Ce (mg L–1): CR concentration in solution at equilibrium qe (mg g–1): the amount of CR adsorbed at equilibrium Ct (mg g–1): CR concentration in solution at time t qt (mg g–1): the amount of CR adsorption at time t V (L): volume of the solution W (g): mass of adsorbent used qe (mg g–1) and qt (mg g–1): the same as above k1 (min–1): rate constant of pseudo-first-order models k2 (min g mg–1): rate constant of pseudo-second-order models qm (mg g–1): maximum adsorption capacity corresponding to a complete monolayer coverage qe (mg g–1) and Ce (mg L–1): the same as above KL (L mg–1): the Langmuir adsorption constant KF: the Freundlich constant n: the adsorption intensity factor DG0 (kJ mol–1): the Gibbs free energy change R: ideal gas constant, 8.314 J mol–1 K–1 T (K): temperature KC: the distribution coefficient DH0 (kJ mol–1): the enthalpy change DS0 (J mol–1 K–1): the entropy change

2

qt ¼

ðC 0 C t ÞV W

3

k1 t logðqe  qt Þ ¼ log qe  2:303

4

t qt

¼ k 1q2 þ q1 t

5

Ce qe

¼ K L1q þ qC e

6

ln qe ¼ ln K F þ 1n ln C e

7

DG0 ¼ RT ln K C

8

DG0 ¼ DH0  T DS0

2 e

m

e

m

assembled from numerous curved nanosheets, the average thickness of which is approximately 10–20 nm. After calcination at 350 °C for 2 h, no significant morphology change was observed and the flower-like morphology was still well maintained (Fig. 2c and d). Fig. 2e and f display the TEM images of the NiO sample, which reveal the nanosheets are crumpled and there are plenty of pores (ca.2–4 nm) in the nanosheets. 3.3. Specific surface area The specific surface area and porosity properties of the synthesized samples were analyzed through N2 adsorption–desorption measurement. The N2 adsorption–desorption isotherms of the samples are shown in Fig. 3, while the inset figure presents the BJH pore size distribution. The shape of the two isotherms can be classified as type IV isotherm [37], which indicates there are mesopores in the samples. The two hysteresis loops are of type H3, implying the presence of narrow slit-like pores formed via the stacking of 2D nanosheets within the microspheres [40]. A major peak centered around 50 nm appeared in the curve of pore size distribution for Ni(OH)2 sample, which is ascribed to the mesopores formed by aggregation of the nanosheets, while another peak around 2 nm can be inferred, which correspond to micropores and small mesopores in the Ni(OH)2 nanosheets. The NiO sample also exhibited two peaks, centered at about 3 nm and 50 nm, respectively. The peak centered at 50 nm confirms that the microstructure of the microspheres was maintained after calcination. The peak centered at 3 nm is in accordance with the TEM characterization results (Fig. 2f), and indicates that these small pores in the nanosheets grew slightly larger after calcination. The specific surface area (SBET), average pore diameter (dp), and total pore volume (Vp) of the three samples (NiO, Ni(OH)2, and NiOC) are displayed in Table 2. Among the three samples, the NiO sample has much higher SBET (107 m2 g–1) than that of Ni(OH)2 (57 m2 g–1) and NiO-C (2 m2 g–1). With larger SBET, the NiO sample is expected to provide more active adsorption sites, therefore giving rise to higher adsorption capacity. 3.4. FT-IR spectra

Fig. 1. XRD patterns of the Ni(OH)2, NiO and NiO-C samples.

3.2. Morphology FE-SEM and TEM were utilized to investigate the microstructure as well as the morphology of the monodispersed NiO and Ni(OH)2 microspheres (Fig. 2). Fig. 2a and b demonstrate the FE-SEM images for Ni(OH)2 microspheres, which are highly monodisperse, with an average diameter of 6.3 lm (Fig. 2a). Moreover, closer observation demonstrate that the microspheres exhibit wellordered and highly uniform hierarchical structure and highly porous texture. The hierarchical flower-like microspheres were

To investigate the interactions between the sample and CR molecule in the adsorption process, the FT-IR spectra of NiO prior to and after CR adsorption were analyzed (Fig. 4). The IR band at around 3433 cm–1 correspond to the hydroxyl groups (AOH) stretching vibration, while the adsorption around 1384 cm–1 belongs to the AOH groups vibration of deformation mode in adsorbed water [41]. Furthermore, the weaker adsorption at 2374 cm–1 originates from the stretching mode vibration of CO2 [42]. Furthermore, the band around 1634 cm–1 can be associated with the bending mode of the AOH [43], while the sharp peak at 1610 cm–1 is due to the C@C stretching vibration of the aromatic ring in CR molecule [44]. It is noticed that after adsorbing CR, the adsorption band at around 1634 cm1 disappeared, which is most likely overlapped by the stretching mode of C@C. In the low wavenumber region, the peak at about 559 cm1 originates from the stretching mode of NiAO [42,45]. Moreover, three new bands appeared after CR adsorption at 1043, 1176 and 1220 cm–1, which are associated with the stretching vibration of sulfonate group (ASO 3 ) of CR [46], suggesting CR molecules were adsorbed onto NiO microspheres. 3.5. XPS analysis The surface elemental composition and chemical status of the NiO sample were investigated by X-ray photoelectron spectroscopy (XPS). Fig. 5a displays the XPS survey spectrum of the

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Fig. 2. FE-SEM images of Ni(OH)2 microspheres (a and b), NiO microspheres (c and d), and TEM images of NiO microspheres (e and f).

Table 2 Parameters of the porous structure of the NiO, Ni(OH)2, and NiO-C samples.

Fig. 3. Nitrogen adsorption–desorption isotherms and corresponding pore size distributions (inset) of the Ni(OH)2 and NiO samples.

Samples

Specific surface area (m2 g–1)

Average pore diameter (nm)

Total pore volume (cm3 g–1)

NiO Ni(OH)2 NiO-C

107 57 2

6.9 8.4 8.7

0.18 0.12 0.004

spectrum of Ni 2p and reveals four easily discernible regions: the region of Ni 2p3/2 contains two peaks centered at ca. 853.1 and 855.0 eV, a broad satellite peak of Ni 2p3/2 at ca. 861 eV, two other peaks for Ni 2p1/2 at ca. 870.5 and 872.4 eV and the satellite peaks of Ni 2p1/2 centered at 879 eV, respectively [47]. The highresolution O 1s region (Fig. 5c) of the sample reveals two peaks centered at 528.8 and 530.6 eV, which can be attributed to the O 1s orbital of lattice oxygen for NiO and hydroxyl groups on the surface of NiO, respectively [48]. 3.6. Zeta potential analysis and adsorption mechanism

NiO sample, and Ni, O, and C elements are observed. The XPS peak for the C 1s core level at the binding energy of 284.8 eV is due to the adventitious hydrocarbon. Fig. 5b shows the high-resolution

Zeta potential analysis was utilized to investigate the surface charge of the sample particles in aqueous solution. As shown in

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Fig. 4. FT-IR spectra of CR and NiO sample prior to and after adsorption of CR.

Fig. 6, the IEP of the NiO, Ni(OH)2 and NiO-C samples were 9.6, 7.9, and 6.7, respectively. At the pH of the suspensions (ca. pH 7), the zeta potential of NiO is more positive than that of Ni(OH)2 and the commercial NiO-C. In light of the above results, the adsorption mechanism of CR onto NiO surface is suggested to be electrostatic adsorption and illustrated in Fig. 7, where R represents the CR molecule except for ASO–3 groups. When dispersed in aqueous suspension, there are abundant hydroxyl groups (AOH) on NiO surface. The surface AOH can protonate and deprotonate, which are expressed by the following equations:

Ni  OH þ Hþ ! Ni  OHþ2 Ni  OH þ OH ! Ni  O þ H2 O The extents of these reactions depend on the solution pH relative to the IEP, when the pH of the solution is below the IEP, the surface hydroxyl groups are prone to protonate, which makes the surface to have a net positive charge. Consequently, the negatively charged ASO–3 group of CR would be attracted to the positively charged surface of the NiO particles through electrostatic interaction [49,50]. 3.7. Adsorption kinetics The kinetic curves for Congo Red adsorption of the three samples are exhibited in Fig. 8. The three curves all reveal that the initial CR uptake rate was quite fast during the first 15 min and after that became slower gradually against time, with adsorption equilibria reached within 2 h. Under the same experimental conditions,

Fig. 6. Variation in zeta potentials versus pH of the suspensions of the NiO, Ni(OH)2, and NiO-C samples.

the as-synthesized NiO exhibited much higher adsorption capacity than the commercial NiO and Ni(OH)2, because of the much larger specific surface area as well as more positive surface charge. For further investigation on the mechanisms of CR adsorbed onto the samples, the data were fitted to two kinetics models (that is, pseudo-first-order and pseudo-second-order) and the linear form of both models are displayed in Table 1 [44,51]. The fitted lines are graphically displayed in Fig. 9, and the corresponding parameters (k1 and k2) obtained by linear regression are listed in Table 3. Fig. 9a displays that some experimental data deviate from the fitted straight lines for the pseudo-first-order, while almost all the experimental data are distributed on the fitted straight lines in Fig. 9b. In addition, the calculated equilibrium adsorption quantity approaches to the experimental data of the samples for the pseudo-second-order model. With higher correlation coefficient (R2) and smaller differences between the calculated and experimental equilibrium data, the pseudo-second-order model provides a better fit, indicating it is more suitable for the adsorption process. 3.8. Adsorption isotherms The adsorption isotherms can be depicted graphically by plotting the adsorption quantity against the liquid-phase concentration at equilibrium. Fig. 10 shows the relationship between the adsorption quantity and concentration of CR in solution at equilibrium for the Ni(OH)2 and NiO samples. The equilibrium adsorption quantity increases as the equilibrium concentrations increase and then reach adsorption saturation. The equilibrium adsorption data were obtained at 30 °C and the most widely-used two models,

Fig. 5. XPS survey spectrum (a), high-resolution XPS spectra of the Ni 2p (b) and O 1s (c) of the NiO sample.

Y. Zheng et al. / Journal of Colloid and Interface Science 504 (2017) 688–696

693

Fig. 7. Schematic illustration of the CR adsorption process onto NiO.

Fig. 8. Kinetic curves for CR adsorption on NiO, Ni(OH)2, and NiO-C. (Initial CR concentration = 50 mg L–1, adsorbent dose = 100 mg L–1, T = 30 °C, n = 3).

Langmuir isotherm and Freundlich isotherm, were utilized to investigate the adsorption isotherms. The Langmuir isotherm

model supposes that the adsorption surface is homogeneous on which the sites for adsorption is identical for all adsorbate molecules; in addition, the adsorbate molecules adsorbed on the samples do not interacted with each other. Furthermore, it assumes that an active site can only be taken up by one adsorbate molecule, therefore, the thickness of the adsorption layer is single molecule. On the other hand, the Freundlich adsorption model describes the nonideal adsorption process and refers to multilayer adsorption; it deems the adsorption surface to be heterogeneous, and the adsorption sites are nonidentical to occupy due to the differences in adsorption energy. The linear forms of the two isotherms are expressed by Eqs. (5) and (6) in Table 1 [52–54]. The corresponding isotherm parameters obtained by linear regression are displayed in Table 4. The fitted straight lines (Fig. 11) graphically reveal that linearized Langmuir isotherms exhibited a better fit for the equilibrium adsorption data than Freundlich adsorption isotherms. With higher R2 (correlation coefficient), the Langmuir adsorption model has better applicability in describing the CR adsorption equilibrium process, suggesting the samples have a saturated adsorption capacity.

Fig. 9. The linear fitted kinetic curves for CR adsorption on NiO and Ni(OH)2: pseudo-first-order (a) and pseudo-second-order kinetics (b).

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Table 3 Adsorption kinetics parameters for NiO, Ni(OH)2 and NiO-C. Adsorbents

qe,exp (mg g–1)

Pseudo-first-order model –1

qe,cal (mg g ) Ni(OH)2 NiO NiO-C

286.1 443.2 49.1

Pseudo-second-order model –1

k1 (min ) –3

28.7  10 73.3  10–3 36.1  10–3

171.4 289.9 30.5

R

2

0.954 0.953 0.968

Fig. 10. Equilibrium isotherms of Congo red adsorption for the NiO and Ni(OH)2 samples. (Initial CR concentration = 20–100 mg L–1, adsorbent dose = 100 mg L–1, T = 30 °C, n = 3).

For the Langmuir isotherm, the constant separation factor or the equilibrium parameter, RL, can be calculated by the following equation:

RL ¼

SD (%)

1 1 þ K LC0

As a dimensionless constant, the parameter RL indicates whether the type of the adsorption is favorable (0 < RL < 1) or unfavorable (RL > 1), the linear case (RL = 1) or irreversible (RL = 0). The calculated values of RL are in the range of 0.012–0.127, further confirming that the adsorption of CR is quite favorable.

23.9 26.1 19.4

qe,cal (mg g–1) 293.2 454.5 51.6

k2 (g mg–1min–1) –3

0.73  10 0.57  10–3 2.28  10–3

R2

SD (%)

0.999 0.999 0.996

4.68 2.27 7.03

Based on the fitting results of Langmuir model (Table 4), the maximal CR adsorption capacity of NiO is 534.8 mg g–1, much higher than the saturated adsorption capacity of Ni(OH)2 (384.6 mg g–1) and the commercial NiO (68.6 mg g–1) under the same circumstance. The differences in adsorption capacity may derive from two reasons: one is the increase of specific surface area, which gives rise to more active sites for adsorption, and the other is the much more positive surface charge, which is conducive to a stronger electrostatic interaction between CR molecules and the adsorbents. Furthermore, the maximal CR adsorption capacity of the flower-like NiO was also superior to other metal (hydr)oxides reported in the literature (Table 5). It is known that large specific surface area would provide more active sites, thus typically leading to a higher adsorption capacity. However, as compared with other adsorbents, the flower-like NiO microspheres do not possess the largest specific surface areas, while exhibiting the most excellent adsorption capacity. Apart from the specific surface area, the high positive surface charge is crucial for the adsorption process, which can facilitate the CR molecules to approach to the surface of the adsorbents and result in a stronger electrostatic interaction. Moreover, the hierarchical porous structure is also vital for the adsorption, due to promoted diffusion of CR molecules through the interconnected porosity at different length scales. Based on the analysis, it is concluded that the unique hierarchical porous structure and high positive zeta potential of NiO contribute to its excellent adsorption performance. 3.9. Adsorption thermodynamics The thermodynamics analysis can be utilized to evaluate the inherent energy change during the adsorption and the influence

Table 4 Parameters of Langmuir isotherm and Freundlich isotherm for CR adsorption on Ni(OH)2, NiO and NiO-C. Samples

Ni(OH)2 NiO NiO-C

Langmuir isotherm

Freundlich isotherm

R2

KL (L mg–1)

qmax (mg g–1)

R2

KF (mg g–1)(L mg–1)1/n

n

0.994 0.998 0.964

0.14 0.84 0.069

384.6 534.8 68.6

0.907 0.935 0.725

9.06 11.81 1.19

6.35 3.40 10.9

Fig. 11. Corresponding linearly fitted isotherms: Langmuir isotherm (a) and Freundlich isotherm (b).

Y. Zheng et al. / Journal of Colloid and Interface Science 504 (2017) 688–696 Table 5 The maximum adsorption capacity for Congo red by different adsorbents for comparison. Samples

Adsorption capacity (mg g–1)

Reference

Monodispersed hierarchical NiO microspheres NiO nanosheets Hierarchical porous NiO architectures Hierarchical hollow NiO–SiO2 microspheres Hierarchically porous NiO–Al2O3 nanocomposite Hierarchical NiO nanospheres Hierarchical ZnO microsphere Ni0.6Fe2.4O4 nanoparticles CoFe2O4 nanoparticles Sea urchin-like a-FeOOH hollow spheres Calcined MgAl-CO2– 3 -LDH Hierarchical porous MgBO2(OH) microspheres HAM@c-AlOOH/Fe(OH)3 with hierarchical structure

535

This work

168 224 204

[55] [56] [57]

357

[42]

440 334 73 242 275

[58] [59] [60] [61] [62]

143 228

[63] [64]

253

[65]

695

as-synthesized NiO microspheres can be considered as a promising adsorbent for CR removal in water treatment.

Acknowledgements

Table 6 Parameters of adsorption thermodynamics for CR adsobed on NiO microspheres. Temperature (K)

DG0 (kJ mol–1)

DH0 (kJ mol–1)

DS0 (J mol–1 K–1)

303 308 313 318 323

5.178 5.722 6.762 7.560 9.076

53.11

191.6

of temperature on adsorption for further investigation. The parameters of thermodynamics can be obtained by Eqs. (7) and (8) (Table 1) [66,67]. KC can be calculated by KC = Ca/Ce, where Ca and Ce are the CR concentration adsorbed on the adsorbent and in solution at equilibrium. The calculated DG0, DH0 and DS0 are displayed in Table 6. The value of DG0 read from the table is negative, which indicates the process for CR adsorbed on NiO sample is a spontaneous reaction, while the positive DH0 and DS0 suggest the adsorption reaction is an endothermic one and the randomness increases after the adsorption process [66,68].

4. Conclusions In summary, monodispersed hierarchical NiO and Ni(OH)2 microspheres were obtained by a simple solvothermal reaction with addition of ethanolamine. The NiO microspheres with hierarchical pore structure and relatively large diameter (about 6.3 lm) exhibit both high CR adsorption capacity and easy separation, which are highly desirable characteristics of adsorbents for removal of aqueous pollutants. The pseudo-second-order model can well express the adsorption kinetics data, while the Langmuir model fit the adsorption isotherm well. The maximum adsorption quantity of NiO and Ni(OH)2 microspheres respectively approach to 534.8 and 384.6 mg g–1, which are competitive or even superior to other recently reported metal (hydr)oxides adsorbents. The main adsorption mechanism is the electrostatic interaction between the synthesized microspherical NiO and CR molecules, while the hierarchical porous structure with relative large specific surface area and high positive zeta potential for the surface of NiO particles contribute to the excellent adsorption performance of the NiO microspheres. With a relatively high adsorption capacity, the

The research was partially supported by National Natural Science Foundation of China (21433007, 21573170, 51372190 and 5132010500), National Basic Research Program of China (2013CB632402), Fundamental Research Funds for the Central Universities (2015-III-034), Natural Science Foundation of Hubei Province of China (No. 2015CFA001) and Self-determined and Innovative Research Funds of SKLWUT (2017-ZD-4).

References [1] L. Ruhl, A. Vengosh, G.S. Dwyer, H. Hsu-Kim, G. Schwartz, A. Romanski, S.D. Smith, The impact of coal combustion residue effluent on water resources: a North Carolina example, Environ. Sci. Technol. 46 (2012) 12226–12233. [2] A. Holbach, Y. Bi, Y. Yuan, L. Wang, B. Zheng, S. Norra, Environmental water body characteristics in a major tributary backwater of the unique and strongly seasonal Three Gorges Reservoir, China, Environ. Sci.: Processes Impacts 17 (2015) 1641–1653. [3] S.E. Diringer, B.J. Feingold, E.J. Ortiz, J.A. Gallis, J.M. Araujo-Flores, A. Berky, W. K. Pan, H. Hsu-Kim, River transport of mercury from artisanal and small-scale gold mining and risks for dietary mercury exposure in Madre de Dios, Peru, Environ. Sci.: Processes Impacts 17 (2015) 478–487. [4] A.D. Tappin, S. Comber, P.J. Worsfold, Orthophosphate-P in the nutrient impacted River Taw and its catchment (SW England) between 1990 and 2013, Environ. Sci.: Processes Impacts 18 (2016) 690–705. [5] V. Garg, Basic dye (methylene blue) removal from simulated wastewater by adsorption using Indian Rosewood sawdust: a timber industry waste, Dyes Pigm. 63 (2004) 243–250. [6] X. Liu, W. Gong, J. Luo, C. Zou, Y. Yang, S. Yang, Selective adsorption of cationic dyes from aqueous solution by polyoxometalate-based metal–organic framework composite, Appl. Surf. Sci. 362 (2016) 517–524. [7] L.N. Jin, X.Y. Qian, J.G. Wang, H. Aslan, M. Dong, MIL-68 (In) nano-rods for the removal of Congo red dye from aqueous solutionOriginal Research Article, J. Colloid Interface Sci. 453 (2015) 270–275. [8] S. Wang, Y. Boyjoo, A. Choueib, Z.H. Zhu, Removal of dyes from aqueous solution using fly ash and red mud, Water Res. 39 (2005) 129–138. [9] Z. Yan, B. Zhu, J. Yu, Z. Xu, Effect of calcination on adsorption performance of Mg–Al layered double hydroxide prepared by a water-in-oil microemulsion method, RSC Adv. 6 (2016) 50128–50137. [10] H.H. Peng, J. Chen, D.Y. Jiang, X.L. Guo, H. Chen, Y.X. Zhang, Merging of memory effect and anion intercalation: MnOx-decorated MgAl-LDO as a highperformance nano-adsorbent for the removal of methyl orange, Dalton. Trans. 45 (2016) 10530–10538. [11] H. Wang, H. Gao, M. Chen, X. Xu, X. Wang, C. Pan, J. Gao, Microwave-assisted synthesis of reduced graphene oxide/titania nanocomposites as an adsorbent for methylene blue adsorption, Appl. Surf. Sci. 360 (2016) 840–848. [12] M. Chafi, B. Gourich, A.H. Essadki, C. Vial, A. Fabregat, Comparison of electrocoagulation using iron and aluminium electrodes with chemical coagulation for the removal of a highly soluble acid dye, Desalination 281 (2011) 285–292. [13] B. Shi, G. Li, D. Wang, C. Feng, H. Tang, Removal of direct dyes by coagulation: the performance of preformed polymeric aluminum species, J. Hazard. Mater. 143 (2007) 567–574. [14] D. Cui, Y.Q. Guo, H.Y. Cheng, B. Liang, F.Y. Kong, H.S. Lee, A.J. Wang, Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor, J. Hazard. Mater. 239 (2012) 257–264. [15] F.P. van der Zee, S. Villaverde, Combined anaerobic-aerobic treatment of azo dyes—a short review of bioreactor studies, Water Res. 39 (2005) 1425–1440. [16] D. Xu, B. Cheng, S. Cao, J. Yu, Enhanced photocatalytic activity and stability of Z-scheme Ag2CrO4-GO composite photocatalysts for organic pollutant degradation, Appl. Catal. B: Environ. 164 (2015) 380–388. [17] F.F. Dias, A.A.S. Oliveira, A.P. Arcanjo, F.C.C. Moura, J.G.A. Pacheco, Residuebased iron catalyst for the degradation of textile dye via heterogeneous photoFenton, Appl. Catal. B: Environ. 186 (2016) 136–142. [18] X.P. Qiu, J.S. Yu, H.M. Xu, W.X. Chen, W. Hu, H.Y. Bai, G.L. Chen, Interfacial effect of the nanostructured Ag2S/Co3O4 and its catalytic mechanism for the dye photodegradation under visible light, Appl. Surf. Sci. 362 (2016) 498–505. [19] M.E. Osugi, K. Rajeshwar, E.R.A. Ferraz, D.P. de Oliveira, Â.R. Araújo, M.V.B. Zanoni, Comparison of oxidation efficiency of disperse dyes by chemical and photoelectrocatalytic chlorination and removal of mutagenic activity, Electrochim. Acta 54 (2009) 2086–2093. [20] J.W. Lee, S.P. Choi, R. Thiruvenkatachari, W.G. Shim, H. Moon, Submerged microfiltration membrane coupled with alum coagulation/powdered activated carbon adsorption for complete decolorization of reactive dyes, Water Res. 40 (2006) 435–444.

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Y. Zheng et al. / Journal of Colloid and Interface Science 504 (2017) 688–696

[21] Y. Aldegs, M. Elbarghouthi, A. Elsheikh, G. Walker, Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon, Dyes Pigm. 77 (2008) 16–23. [22] C.K. Lee, S.S. Liu, L.C. Juang, C.C. Wang, K.S. Lin, M.D. Lyu, Application of MCM41 for dyes removal from wastewater, J. Hazard. Mater. 147 (2007) 997–1005. [23] Y. Qin, L. Wang, C. Zhao, D. Chen, Y. Ma, W. Yang, Ammonium-functionalized hollow polymer particles as a pH-responsive adsorbent for selective removal of acid dye, ACS Appl. Mater. Interfaces 8 (2016) 16690–16698. [24] V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint, Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials, Chem. Eng. J. 148 (2009) 354–364. [25] M.A. Behnajady, S. Bimeghdar, Synthesis of mesoporous NiO nanoparticles and their application in the adsorption of Cr(VI), Chem. Eng. J. 239 (2014) 105–113. [26] H.S. Jadhav, G.M. Thorat, J. Mun, J.G. Seo, Self-assembled hierarchical 3D–NiO microspheres with ultra-thin porous nanoflakes for lithium-ion batteries, J. Power Sources 302 (2016) 13–21. [27] K. Tian, X.X. Wang, H.Y. Li, R. Nadimicherla, X. Guo, Lotus pollen derived 3dimensional hierarchically porous NiO microspheres for NO2 gas sensing, Sens. Actuators B 227 (2016) 554–560. [28] G. Cheng, W. Yang, C. Dong, T. Kou, Q. Bai, H. Wang, Z. Zhang, Ultrathin mesoporous NiO nanosheet-anchored 3D nickel foam as an advanced electrode for supercapacitors, J. Mater. Chem. A 3 (2015) 17469–17478. [29] Y. Ye, Y. Zhao, L. Ni, K. Jiang, G. Tong, Y. Zhao, B. Teng, Facile synthesis of unique NiO nanostructures for efficiently catalytic conversion of CH4 at low temperature, Appl. Surf. Sci. 362 (2016) 20–27. [30] G.A. Parks, The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems, Chem. Rev. 65 (1964) 177–198. [31] J.A. Lewis, Colloidal Processing of Ceramics, J. Am. Ceram. Soc. 83 (2000) 2341– 2359. [32] A.K. Diallo, M. Gaceur, S.B. Dkhil, Y. Didane, O. Margeat, J. Ackermann, C. Videlot-Ackermann, Impact of surfactants covering ZnO nanoparticles on solution-processed field-effect transistors: From dispersion state to solid state, Colloids Surf. A 500 (2016) 214–221. [33] Z. Chen, X. Qin, T. Zhou, X. Wu, S. Shao, M. Xie, Z. Cui, Ethanolamine-assisted synthesis of size-controlled indium tin oxide nanoinks for low temperature solution deposited transparent conductive films, J. Mater. Chem. C 3 (2015) 11464–11470. [34] X. Wang, Q. Zhang, Q. Wan, G. Dai, C. Zhou, B. Zou, Controllable ZnO Architectures by Ethanolamine-Assisted Hydrothermal Reaction for Enhanced Photocatalytic Activity, J. Phys. Chem. C 115 (2011) 2769–2775. [35] C. Mondal, M. Ganguly, P.K. Manna, S.M. Yusuf, T. Pal, Fabrication of porous bCo(OH)2 architecture at room temperature: a high performance supercapacitor, Langmuir 29 (2013) 9179–9187. [36] T.W. Sun, Y.J. Zhu, C. Qi, G.J. Ding, F. Chen, J. Wu, a-Fe2O3 nanosheet-assembled hierarchical hollow mesoporous microspheres: Microwave-assisted solvothermal synthesis and application in photocatalysis, J. Colloid Interface Sci. 463 (2016) 107–117. [37] K.S.W. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure. Appl. Chem. 57 (1985) 603–619. [38] X. Hu, S. Liu, C. Li, J. Huang, J. Luv, P. Xu, J. Liu, X.Z. You, Facile and environmentally friendly synthesis of ultrathin nickel hydroxide nanosheets with excellent supercapacitor performances, Nanoscale 8 (2016) 11797– 11802. [39] P. Lv, H. Zhao, Z. Zeng, C. Gao, X. Liu, T. Zhang, Self-assembled threedimensional hierarchical NiO nano/microspheres as high-performance anode material for lithium ion batteries, Appl. Surf. Sci. 329 (2015) 301–305. [40] R. Chen, J. Yu, W. Xiao, Hierarchically porous MnO2 microspheres with enhanced adsorption performance, J. Mater. Chem. A 1 (2013) 11682–11690. [41] W. Yang, Z. Gao, J. Wang, J. Ma, M. Zhang, L. Liu, Solvothermal one-step synthesis of Ni-Al layered double hydroxide/carbon nanotube/reduced graphene oxide sheet ternary nanocomposite with ultrahigh capacitance for supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 5443–5454. [42] C. Lei, X. Zhu, Y. Le, B. Zhu, J. Yu, W. Ho, Hierarchically porous NiO–Al2O3 nanocomposite with enhanced Congo red adsorption in water, RSC Adv. 6 (2016) 10272–10279. [43] L. Qi, B. Cheng, W. Ho, G. Liu, J. Yu, Hierarchical Pt/NiO Hollow Microspheres with Enhanced Catalytic Performance, ChemNanoMat 1 (2015) 58–67. [44] Z. Li, B. Yang, S. Zhang, B. Wang, B. Xue, A novel approach to hierarchical sphere-like ZnAl-layered double hydroxides and their enhanced adsorption capability, J. Mater. Chem. A 2 (2014) 10202–10210. [45] G. Duan, W. Cai, Y. Luo, F. Sun, A hierarchically structured Ni(OH)2 monolayer hollow-sphere array and its tunable optical properties over a large region, Adv. Funct. Mater. 17 (2007) 644–650.

[46] I.M. Ahmed, M.S. Gasser, Adsorption study of anionic reactive dye from aqueous solution to Mg–Fe–CO3 layered double hydroxide (LDH), Appl. Surf. Sci. 259 (2012) 650–656. [47] J. Wu, W.J. Yin, W.W. Liu, P. Guo, G. Liu, X. Liu, D. Geng, W.M. Lau, H. Liu, L.M. Liu, High performance NiO nanosheets anchored on three-dimensional nitrogen-doped carbon nanotubes as a binder-free anode for lithium ion batteries, J. Mater. Chem. A 4 (2016) 10940–10947. [48] S.B. Ivan, I. Popescu, I. Fechete, F. Garin, V.I. Parvulescu, I.C. Marcu, The effect of phosphorus on the catalytic performance of nickel oxide in ethane oxidative dehydrogenation, Catal. Sci. Technol. 6 (2016) 6953–6964. [49] M. Liu, J. Xu, B. Cheng, W. Ho, J. Yu, Synthesis and adsorption performance of Mg(OH)2 hexagonal nanosheet–graphene oxide composites, Appl. Surf. Sci. 332 (2015) 121–129. [50] B. Zhu, P. Xia, W. Ho, J. Yu, Isoelectric point and adsorption activity of porous gC3N4, Appl. Surf. Sci. 344 (2015) 188–195. [51] F. Chen, M. Hong, W. You, C. Li, Y. Yu, Simultaneous efficient adsorption of Pb2+ and MnO 4 ions by MCM-41 functionalized with amine and nitrilotriacetic acid anhydride, Appl. Surf. Sci. 357 (2015) 856–865. [52] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [53] H.M.F. Freundlich, Uber die adsorption in losungen, J. Phys. Chem. 57 (1906) 385–470. [54] B. Cheng, Y. Le, W. Cai, J. Yu, Synthesis of hierarchical Ni(OH)2 and NiO nanosheets and their adsorption kinetics and isotherms to Congo red in water, J. Hazard. Mater. 185 (2011) 889–897. [55] J. Zhao, Y. Tan, K. Su, J. Zhao, C. Yang, L. Sang, H. Lu, J. Chen, A facile homogeneous precipitation synthesis of NiO nanosheets and their applications in water treatment, Appl. Surf. Sci. 337 (2015) 111–117. [56] L. Ai, Y. Zeng, Hierarchical porous NiO architectures as highly recyclable adsorbents for effective removal of organic dye from aqueous solution, Chem. Eng. J. 215 (2013) 269–278. [57] C. Lei, X. Zhu, B. Zhu, J. Yu, W. Ho, Hierarchical NiO-SiO2 composite hollow microspheres with enhanced adsorption affinity towards Congo red in water, J. Colloid Interface Sci. 466 (2016) 238–246. [58] T. Zhu, J.S. Chen, X.W. Lou, Highly efficient removal of organic dyes from waste water using hierarchical NiO spheres with high surface area, J. Phys. Chem. C 116 (2012) 6873–6878. [59] C. Lei, M. Pi, C. Jiang, B. Cheng, J. Yu, Synthesis of hierarchical porous zinc oxide (ZnO) microspheres with highly efficient adsorption of Congo red, J. Colloid Interface Sci. 490 (2017) 242–251. [60] S. Zeng, S. Duan, R. Tang, L. Li, C. Liu, D. Sun, Magnetically separable Ni0.6Fe2.4O4 nanoparticles as an effective adsorbent for dye removal: Synthesis and study on the kinetic and thermodynamic behaviors for dye adsorption, Chem. Eng. J. 258 (2014) 218–228. [61] X. Wu, W. Wang, F. Li, S. Khaimanov, N. Tsidaeva, M. Lahoubi, PEG-assisted hydrothermal synthesis of CoFe2O4 nanoparticles with enhanced selective adsorption properties for different dyes, Appl. Surf. Sci. 389 (2016) 1003–1011. [62] B. Wang, H. Wu, L. Yu, R. Xu, T.T. Lim, X.W. Lou, Template-free formation of uniform urchin-like a-FeOOH hollow spheres with superior capability for water treatment, Adv. Mater. 24 (2012) 1111–1116. [63] B. Li, Y. Zhang, X. Zhou, Z. Liu, Q. Liu, X. Li, Different dye removal mechanisms between monodispersed and uniform hexagonal thin plate-like MgAl–CO32– LDH and its calcined product in efficient removal of Congo red from water, J. Alloys Compd. 673 (2016) 265–271. [64] Z. Zhang, W. Zhu, R. Wang, L. Zhang, L. Zhu, Q. Zhang, Ionothermal confined self-organization for hierarchical porous magnesium borate superstructures as highly efficient adsorbents for dye removal, J. Mater. Chem. A 2 (2014) 19167– 19179. [65] Y. Zhang, Y. Ye, X. Zhou, Z. Liu, G. Zhu, D. Li, X. Li, Monodispersed hollow aluminosilica microsphere@hierarchical c-AlOOH deposited with or without Fe(OH)3 nanoparticles for efficient adsorption of organic pollutants, J. Mater. Chem. A 4 (2016) 838–846. [66] S. Chowdhury, R. Mishra, P. Saha, P. Kushwaha, Adsorption thermodynamics, kinetics and isosteric heat of adsorption of malachite green onto chemically modified rice husk, Desalination 265 (2011) 159–168. [67] J. Zhou, Y. Cheng, J. Yu, G. Liu, Hierarchically porous calcined lithium/ aluminum layered double hydroxides: Facile synthesis and enhanced adsorption towards fluoride in water, J. Mater. Chem. 21 (2011) 19353–19361. [68] J. Fu, Z. Chen, M. Wang, S. Liu, J. Zhang, J. Zhang, R. Han, Q. Xu, Adsorption of methylene blue by a high-efficiency adsorbent (polydopamine microspheres): kinetics, isotherm, thermodynamics and mechanism analysis, Chem. Eng. J. 259 (2015) 53–61.