Crystal structures, morphological, optical, adsorption, kinetic and photocatalytic degradation studies of activated carbon loaded BiOBr nanoplates prepared by solvothermal method

Inorganic Chemistry Communications 104 (2019) 134–144

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Short communication

Crystal structures, morphological, optical, adsorption, kinetic and photocatalytic degradation studies of activated carbon loaded BiOBr nanoplates prepared by solvothermal method

T

K.S. Bhavsara, P.K. Labhanea, R.B. Dhakeb, , G.H. Sonawanec, ⁎

⁎

a

MGSM's, Arts, Science, and Commerce College, Chopda, Dist. Jalgaon, M.S., India Department of Chemistry, D.D.N Bhole College, Bhusawal, M.S., India c Kisan Arts, Commerce and Science College, Parola, Dist. Jalgaon, M.S., India b

GRAPHICAL ABSTRACT

Plausible mechanism for the photocatalytic degradation of RhB on AC-BiOBr nanocomposite.

ARTICLE INFO

ABSTRACT

Keywords: BiOBr nanoplates Activated carbon Rhodamine B Nanocomposite Photocatalytic study Adsorption Kinetic study

Herein, the activated carbon (AC) loaded BiOBr nanoplates (AC-BiOBr) was synthesized by solvothermal method. The characterizations were done by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM) and x-ray diffraction (XRD) to perceive the morphology, composition, inner structure, and phase of prepared samples respectively. FESEM images revealed that BiOBr is composed of variable sizes of microspheres, constructed by many interlaced nanoplates. Thermogravimetric analysis (TGA) confirms the stability and purity of samples. The as-prepared activated loaded composites were evaluated via the removal of Rhodamine B (RhB) dye under visible light. The ACBiOBr nanocomposites exhibited superior adsorption capacity, consequently improved photocatalytic efficiency. The better degradation efficiency of the AC-BiOBr nanocomposite is attributed to higher the adsorption capacity of dye on its surface, and enhanced charge separation through adsorbed O2 in AC. Kinetic parameters like pseudo-first-order and pseudo-second-order were determined. The adsorption of RhB dye on AC-BiOBr follows the second-order kinetic model. Langmuir adsorption isotherm is the most fitted

⁎

Corresponding authors. E-mail addresses: [email protected] (R.B. Dhake), [email protected] (G.H. Sonawane).

https://doi.org/10.1016/j.inoche.2019.04.002 Received 8 March 2019; Received in revised form 1 April 2019; Accepted 3 April 2019 Available online 06 April 2019 1387-7003/ © 2019 Published by Elsevier B.V.

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isotherm for adsorption of RhB dye on AC-BiOBr. The composite surface provides the more adsorption surface sites for photodegradation, in addition, could restrict the toxic intermediates to release in air or solution phase.

1. Introduction

separation of photogenerated electrons and hole pairs, which sequentially enhance the photocatalytic performances. [39–43]. Zhang et al. [44] reported the superiority of BiOBr as a visible light photocatalyst over commonly used TiO2. Chen et al. [45] prepared the porous BiOBr by microwave assisted–ionic liquid method and reported that the porous structure of BiOBr has enhanced photocatalytic efficiency by lowering the recombination of the photogenerated electron-hole pair. Additionally, the photocatalytic activity of the BiOBr can be modified by immobilizing it on activated carbon [46–47]. Ao et al. [48] reported that the BiOBr-activated charcoal composite has better degradation efficiency for methylene blue under both UV and visible light. However, still there is scope to explored the physicochemical and the photocatalytic properties of the carbon loaded BiOBr composite for the degradation of stable dye like RhB. Herein, we prepared the activated carbon-BiOBr composite with different content of activated carbon (AC) by solvothermal method and reported the effect of immobilization of AC on structural, morphological, thermal, adsorption and photocatalytic behaviour of BiOBr nanoplates.

Recently, effective and low-cost heterogeneous photocatalysis has been explored for the destruction of various organic pollutants present in the aquatic environment [1–4]. TiO2, ZnO, CdS, ZrO2, SnO2, Cu2O, and CeO2 are the most commonly used semiconductor in heterogeneous photocatalysis [5–15]. The semiconductor photocatalytic oxidations facilitate simple operation, no secondary pollution and complete degradation of organic pollutants, accordingly, emerging green technology processes for environmental remediation [16–17]. However, these semiconductors have limited their photoactivity to UV light only. Further the separation and recovery of powder photocatalyst is another major issue [18–19]. Recently, many researchers have attempted different techniques to improve the photocatalytic activity of these semiconductors [20–23]. Immobilization could be the most suitable approach to enhance the photocatalytic property of the semiconductors [24–27]. It facilitates the separation, recovery, larger surface area, strong light-absorbing power, optimum porosity, strong interfacial electronic effects and reusability of photocatalysts [28–30]. Carbonbased binary composites have a broad absorption spectrum and can expand their visible light absorption abilities and thereby, improve the photocatalytic performance [31–33]. Martins et al. [34] reported the degradation of tetracycline by TiO2-activated carbon composite under UV light. The immobilization of TiO2 on activated carbon increases the BET surface area and lowers the crystal size of the catalyst, thereby improve the photocatalytic efficiency. Besides, it also provides more surface of the catalyst, large and wide dimension pores for the adsorption of organic pollutant [35–36]. Recently, BiOBr has gained an enormous attraction as an alternative photocatalyst for the degradation of organic pollutants under UV and visible light [37–38]. The BiOBr structure is made of a layer of [Bi2O2]2+ and the distinctive layer structure can facilitate the effective

2. Materials and methods 2.1. Materials For the synthesis of activated carbon loaded BiOBr nanocomposite, analytical grade Bismuth nitrate Bi(NO3)26H2O, potassium bromide (KBr), activated carbon (AC) ethylene glycol (EG) and Rhodamine B (RhB) has been procured from the Merck India. All the chemicals were used as they were received from the supplier without further purification. Dye solution was prepared in double distilled water.

Fig. 1. Flowchart for the synthesis of activated carbon loaded BiOBr nanocomposites. 135

Inorganic Chemistry Communications 104 (2019) 134–144

310

220

211

212

200

104

110

102

nanocomposite. The measurements were carried out on NOVA 1000 e surface area and porosity analyzer (Quanta chrome®). Thermogravimetric analyses (TGA) were performed in a thermo-balance model SDT Q600 V20.9 Build 20; samples were heated at a rate of 20 °C/min. The UV–Vis diffuse reflectance spectra (DRS) of samples were recorded by a spectrophotometer (Hitachi, U-3900).

A C 80-B iO B r 112

001

101

K.S. Bhavsar, et al.

Intensity (a.u.)

A C 60-B iO B r

2.4. Adsorption experiment

A C 40-B iO B r

The adsorption study was done by mixing the RhB dye solution with AC-BiOBr nanocomposite. The mixed solution was stirred in dark for 90 min to ensure adsorption-desorption equilibrium between the RhB dye and AC-BiOBr nanocomposite. The change in concentration of RhB dye solution with time was noticed by recording the absorption values on UV–Vis double beam Shimadzu UV-1800 spectrophotometer at λ max = 554 nm. The amount of RhB dye adsorbed on AC-BiOBr nanocomposite is calculated by Eq. (1),

A C 20-B iO B r

B iO B r

qt = (C0 AC

10

20

30

40

50

60

70

(1)

Ct ) V / W

where qt (mg/ g) is the adsorption capacity at time t; C0 (mg/ L) is the initial RhB dye concentration and Ct (mg/ L) is the RhB dye concentration at time t; V (L) is the initial volume of RhB dye solution and W (g) is the amount of nanocomposite.

80

Fig. 2. XRD pattern of AC, BiOBr and AC-BiOBr nanocomposites.

2.5. Photocatalytic study

2.2. Synthesis of activated carbon loaded BiOBr nanoplates

Photocatalytic activities of as-prepared samples were evaluated under visible light using 250 W Xenon lamp placed horizontally in a batch reactor. The reactor was surrounded by a cooling jacket circulated with 0.72 M NaNO2 solution to eliminate UV light radiation (cutoff < 385 nm) [49]. 50 mL RhB (25 mg/L) solution with 15 mg catalyst were stirred at dark for 90 min to attain the adsorption-desorption equilibrium. Then the solutions exposed to visible radiation. The 5 mL aliquot was collected from the mixed solution at regular time intervals and subjected to centrifugation. The supernatants were used to record the absorption spectra on Shimadzu UV-1800 spectrophotometer. The degradation of RhB dye was monitored by UV–visible absorption spectroscopy with distilled water as the reference medium. All photocatalytic evaluation was done at room temperature. The percentage removal of RhB from aqueous solution was determined using the following Eq. (2),

The BiOBr and AC-BiOBr nanocomposites were synthesized via a simple solvothermal method [48]. In a typical synthesis of 20% AC loaded BiOBr (AC20-BiOBr), 1.96 g Bi (NO3)3.5H2O and 0.240 g AC were dispersed in 40 mL ethylene glycol (EG) by the ultrasonic probe (Dakshin Ultrasonic, Mumbai) at a frequency of 20 kHz. To this dispersed solution, KBr (0.47 g dissolved in a 40 mL EG) was added dropwise with continuous stirring. Later, the suspension was poured into a 100 mL Teflon-lined autoclave and heated at 160 °C for 16 h in hot air oven. The solid product was collected by filtration, washed with distilled water and dried at 80 °C for 24 h to get AC20-BiOBr. Other composites 40% AC loaded BiOBr (AC40-BiOBr), 60% AC loaded BiOBr (AC60-BiOBr) and 80% AC loaded BiOBr (AC80-BiOBr) samples were prepared by varying the content of AC. For comparison, BiOBr catalyst was also prepared without the addition of AC. The flowchart for the synthesis of activated carbon loaded BiOBr are shown in Fig. 1.

%removal =

C0

Ct C0

× 100

(2)

where C0 and Ct are the initial and final concentration of the RhB dye in aqueous solution, respectively.

2.3. Characterization The XRD pattern of as-prepared samples was recorded on Brucker D8 advance powder diffractometer (Cu Kα radiations λ = 1.5416 Å) in the range of 20° to 80° at room temperature. The morphology and purity of samples were investigated with Field Emission Scanning Electron Microscope (FESEM) and High-resolution Transmission electron microscopy (HR-TEM). The nanoporous structure, total surface area, average pore diameter, and pore volumes were determined by recording N2 adsorption-desorption isotherm of the as-prepared

3. Results and discussion 3.1. XRD analysis The crystalline phases of AC, BiOBr and AC-BiOBr samples were recorded by X-ray diffraction (Fig. 2). In the XRD patterns of BiOBr and AC-BiOBr samples, the dominating peaks at 2θ ≈ 10.900°, 21.927°,

Table 1 Physical Parameters of BiOBr and AC-BiOBr nanocomposites. Samples

BiOBr AC20BiOBr AC40BiOBr AC60BiOBr AC80BiOBr

Lattice parameters (Å)3 A

C

3.9132 3.9129 3.9177 3.918 3.9151

8.1475 8.1484 8.0893 8.1032 8.1082

Volume cell (Å)3

X-ray density

c/a ratio

Atomic packing factor

Crystallite size (D) (nm)

W-H Grain Size (nm)

Strain (ε)

124.7639 124.7600 124.1575 124.3897 124.2860

4.2552 4.2553 4.2760 4.2680 4.2716

2.0821 2.0825 2.0648 2.0682 2.0710

0.5010 0.5011 0.4966 0.4975 0.4982

13.0788 13.0011 17.1818 17.3237 12.8648

17.3375 13.6515 18.5180 18.5427 11.3224

0.00267 0.00079 0.00087 0.00081 −0.00062

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25.156°, 31.732°, 32.318°, 39.324°, 46.268°, 50.681°, 53.341°, 57.235°, 67.682°, 70.937°, and 76.637° corresponds to lattice planes (001), (0 0 2), (1 0 1), (1 0 2), (1 1 2), (2 0 0), (1 0 4), (2 1 1), (2 1 2), (2 2 0), (2 1 4) and (3 1 0) respectively, revealed that the prepared samples are in good crystalline nature and agree with the JCPDS data (09-0393). Evidently, confirms the tetragonal structure of BiOBr and AC-BiOBr. Further, the XRD pattern of plane activated carbon shows the major peak at 2θ ≈ 20° to 30°, confirms its typical amorphous structure. Moreover, the small hump appears at 2θ ≈ 40° to 50° in the XRD pattern of AC indicates α axis of graphite structure [50]. The immobilization of BiOBr on AC does not alter the tetragonal nature of BiOBr, as there is no significant change in the value of c/a ratio (Table 1). Further, the loading of activated carbon lowers the crystallite size and alter the lattice parameters values of BiOBr nanoparticles [51]. The average crystallite size (D) was calculated by using Debye Scherrer's formula [52–53], Eq. (3),

0.030 BiOBr Y=0.008+0.00267x

0.025

β cosθ

0.020 0.015 0.010 0.005 0.000 0.0

0.5

1.0

1.5

2.0

2.5

4 sinθ

D=

0.020

βcosθ

0.012

(4)

V = a2c 0.008

The peak broadening and crystalline size can be qualitatively related to the local environment and the strain produced in the lattice of BiOBr. The peak broadening with lattice strains can be understood by taking various peaks appeared in the XRD pattern. The strain produced in the lattice of samples was estimated by Stokes and Wilson [54–55] formula Eq. (5),

0.004

0.000 0.0

0.5

1.0

1.5

2.0

2.5

cos

4 sinθ

0.020

0.016

0.012

0.008

0.004

0.000 0.0

0.5

1.0

1.5

2.0

=

k + 4 sin D

(5)

where β is FWHM in radians, D is the crystallite size in nm, λ wavelength of the X-ray and ε is the strain. The intercept and the slope of the WeH plot of βcosθ versus 4sinθ (Fig. 3) give the average crystallize (D) size and strain (ε) respectively. Fig. 3 shows the WeH plot of βcosθ versus 4sinθ of BiOBr and ACBiOBr nanocomposites. The WeH plots are predicted parallel to the 4sinθ axis in the absence of strain while it should be a non-zero slope in the presence of strain produced in the lattice [54]. The lattice strains calculated from WeH plots are tabulated in Table 2. It is observed that loading of activated carbon on BiOBr leads to change in crystallite size and lattice strain, owing to a decrease in lattice parameters, the volume of the unit cell. The immobilizations of activated carbon beyond 60% produce a negative strain on the unit cell of BiOBr. This negative strain may be due to lattice shrinkage [56].

AC80-BiOBr Y=0.01225-0.000618x

βcosθ

(3)

where λ is the wavelength of the X-ray radiation (λ = 1.5416 Å), K is the shape factor (0.9), β is the FWHM in radians; θ is the Bragg's angle in degree. The volume of the unit cell was calculated from the values of lattice constants by the formula given in Eq. (4),

AC40-BiOBr Y=0.00749+0.000874x

0.016

K cos

2.5 3.2. SEM and EDX analysis

4 sinθ Fig. 3. The WeH plot of β cosθ versus 4 sinθ for BiOBr and AC-BiOBr nanocomposites.

The FESEM images of BiOBr and AC-BiOBr nanocomposites prepared by solvothermal route are shown in Fig. 4. FESEM images revealed that BiOBr is composed of variable sizes of microspheres,

Table 2 Pore size distribution of BiOBr and AC-BiOBr nanocomposites. Sample

Pore volume (cc/ g)

Micro pore Volume cc/ g

Meso pore Volume cc/ g

Micro pore area m2/ g

External surface Area m2/g

Surface area m2/g

Pore Diameter Dv (d)

BiOBr AC20BiOBr AC40BiOBr AC60BiOBr AC80BiOBr

0.041 0.052 0.136 0.226 0.330

– 0.016 0.066 0.128 0.157

0.041 0.036 0.070 0.098 0.173

– 26.814 107.108 223.398 265.685

– 57.838 174.130 293.916 472.113

18.379 84.653 281.238 517.313 737.798

3.671 1.295 1.301 1.326 1.316

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Fig. 4. SEM images of BiOBr and AC-BiOBr nanocomposites.

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Fig. 5. TEM images of BiOBr nanocomposites.

Fig. 6. EDX spectra of BiOBr and AC-BiOBr nanocomposites.

constructed by many interlaced nanoplates. The thickness of interlaced nanoplates in BiOBr is in the range of 24 to 49 nm. However, the loading of AC on BiOBr sinks the thickness of interlaced nanoplates and in the range of 12 nm to 33 nm. The elemental compositions of BiOBr and AC-BiOBr composites are analyzed by energy dispersive X-ray spectroscopy (EDX) analysis measurement shown in Fig. 5. The spectra indicate the presence of Bi, Br, O, and C as the major elements. EDX

spectra further approve the purity of the samples as no traces of other foreign elements were noticed in the spectra. Further, the interior structure of BiOBr was elucidated by TEM (Fig. 6a & b). The TEM images revealed that the BiOBr samples are small in nanoscale with distinct grain boundaries and strongly deposited on the activated carbon. Moreover, the bright and distinct spots in selected area electron diffraction (SAED) pattern (Fig. 6c) validate the highly crystalline 139

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K.S. Bhavsar, et al. 1.0

1.16 1.12

Absorbance

1.08

200

BiOBr (443) 0.6

20% AC-BiOBr (479) 40% AC-BiOBr (495) 60% AC-BiOBr (509)

BiOBr 40 % AC-BIOBr 60 % AC-BiOBr

220

0.8

180

0.4

160

1.04

0.2

1.00

0.0 300

140 350

400

450

500

120

550

0.96

100

0.92

80

0.88

60 40

0.84

20 0.80 300

350

400

450

500

0

550

0.0

Wavelength (nm)

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/Po

Fig. 7. Diffuse reflectance spectra of BiOBr and AC-BiOBr nanocomposites. 0.05

100 0.04

40 % AC-BiOBr BiOBr

Weight loss (%)

90 0.03

80 0.02

70

BiOBr BiOBr-AC20 BiOBr-AC40 AC

60

0.01

0.00

-0.01

50 100

200

300

400

500

600 Pore Diameter (nm)

Temperature °C

Fig. 9. a: N2 adsorption and desorption isotherms of the BiOBr and AC-BiOBr nanocomposites. b: Pore size distribution of the BiOBr and AC40BiOBr nanocomposites.

Fig. 8. TGA curve of BiOBr and AC-BiOBr nanocomposites.

nature of the BiOBr. The High-resolution TEM (Fig. 6d) image of BiOBr shows that the lattice fringe spacing 0.17 nm, 0.27 nm, and 0.40 nm corresponding to the (211), (110), and (002) crystal plane of tetragonal BiOBr (JCPDS 09-0393).

to the removal of moisture content. Further, weight loss in the composite above 120 °C may be attributed to the exclusion of functional groups having mostly oxygen and decomposition of carbon skeleton [59].

3.3. Optical study of as-prepared nanocomposites

3.5. Textural properties of BiOBr and AC-BiOBr nanocomposites

The UV–vis diffuse reflectance spectra of the BiOBr and AC-BiOBr composites at room temperature are shown in Fig. 7. The increase in carbon content of AC shifts the absorption maxima of BiOBr towards higher wavelength. The absorption edges were considered to calculate the band gap energies (Eg) of the samples and equation used is, λg = 1239/Eg, where λg is the band gap wavelength [57]. The band gap energies are 2.79, 2.58, 2.50, and 2.43 eV for BiOBr, AC20-BiOBr, AC40-BiOBr, and AC60-BiOBr respectively. The decrease in band gap energies with the increase in carbon content could be due to including of overlapping absorption spectrum of AC and BiOBr [58].

Photocatalytic efficiency of the catalyst moreover depends on its adsorbing power. The adsorption and desorption isotherm of N2 at 77 K for BiOBr and AC-BiOBr are clearly of type IV with a distinct hysteresis loop observed in the range of 0.45–0.95 p/po. (Fig. 9a). The pore size distribution calculated according to the BJH method, the corresponding porosity distribution of the BiOBr and AC-BiOBr composites was shown in Fig. 9b. Porous structure parameters for the composite were calculated and tabulated in Table 2. The loading of AC on BiOBr leads to increase in active surface area for adsorption. The total pore volume for AC80-BiOBr is 0.330 cm3/g and mesoporous volume is 0.173 cm3/g. According to IUPAC classifications, pores can be divided into three types, i.e. micropores (d < 2 nm), mesopores (2 < d < 50 nm) and macropores (d > 50 nm) [60]. In our results, BiOBr is mainly mesoporous while an AC-BiOBr composite shows presence of multimodal micro and mesopores. Such micro and mesoporous structural design having the larger surface area could play a significant role in the adsorption process and the catalyst reaction.

3.4. Thermogravimetric analysis Thermogravimetric analysis (TGA) was done in air by varying temperature from 25 °C to 600 °C at a rate of 20 °C/min and shown in Fig. 8. For pure BiOBr, there is no substantial loss up to 600 °C, indicates stability and purity of samples. However, for AC-BiOBr, initial weight loss in the temperature range 100 °C to 120 °C would mainly due 140

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concentration increased from 50 mg/L to 70 mg/L. The optimum contact time was found to be 90 min, which was used for all further adsorption studies.

120

100

3.6.2. Adsorption isotherms 3.6.2.1. Langmuir isotherm model. The linear form of Langmuir isotherm [61–62], is represented as Eq. (6),

qt

80

60

Ce 1 C = + e qe ab a

50 mg/L 60 mg/L 70 mg/L

40

where Ce equilibrium concentration of dye is, qe is an amount of dye adsorbed per unit mass of adsorbent at equilibrium (mg/g), ‘a’ is monolayer coverage capacity (mg/g), ‘b’ is Langmuir isotherm constant (L mg−1). Langmuir isotherm describes homogeneous monolayer adsorption as it based on the assumption that the once the adsorption of dye takes place at a specific site where no further adsorption of dye occurs. The favorable, unfavorable, linear or irreversible adsorption nature can be described by the dimensionless or separation factor values RL. The relation 1 > RL > 0 indicates the adsorption is favorable, RL > 1 indicates adsorption is unfavorable, while RL = 1 directs the adsorption is linear [63]. Fig. 11 depicts the results of Langmuir adsorption isotherms for BiOBr and AC-BiOBr. The present study reports, RL value lies between 0 and 1 specifies a favorable adsorption process (Table 3).

20

0 0

25

50

75

100

Time (min) Fig. 10. Amount of RhB dye adsorbed qt (mg/g) with time for different initial dye concentration; adsorbent dose 0.6 g/L of AC-BiOBr nanocomposites.

0.18 0.16 0.14

3.6.2.2. Freundlich isotherm model. Freundlich isotherm [61–62] describes the multilayer adsorption mechanism with the heterogeneous surface where the adsorption capability is mainly related to the concentration of dye at equilibrium and is given by Eq. (7):

0.12 0.10

ce/qe

(6)

0.08

log qe = log KF +

0.06

0.02 0.00 2

4

6

8

10

12

14

16

18

(7)

where KF is Freundlich isotherm constant ((mg/ g) (L/ g)1/n) related to adsorption capacity, Ce is equilibrium concentration of dye, 1/n is a measure of adsorption density. The linear nature of Freundlich isotherm is shown by the plot of log qe versus log Ce. The slope values show the adsorption nature. If slope (1/n) is < 1, it indicates chemisorptions; > 1 indicates co-operative adsorption while closer to 0 indicates heterogeneity [62]. In the present study, the slope value 0.126 to 0.259 indicates the adsorption of RhB dye over AC-BiOBr is chemisorptions (Table 3). Langmuir isotherm has higher regression coefficient (r2) values while Freundlich isotherm has lower regression coefficient (r2) values. This could be due to the homogeneous distribution of active sites on the activated carbon surface, as the Langmuir equation assumes the surface of catalyst is homogeneous. Accordingly, the Langmuir adsorption isotherm is the most appropriate isotherm for adsorption of RhB dye on AC-BiOBr.

0.04

0

1 log Ce n

20

Ce Fig. 11. Langmuir plot for adsorption of RhB dye by activated carbon loaded BiOBr nanocomposites.

3.6. Adsorption study 3.6.1. Effect of dye concentration on adsorption Fig. 10 shows initial dye concentration versus the adsorption rate of RhB dye on AC40-BiOBr. It was observed that with the increase in dye concentration, the rate of adsorption increases with time. Initially, up to 25 min, the rate of adsorption is faster and this could be due to the availability of more adsorption sites on the composite. However, after 25 min, it increases gradually and finally attains the equilibrium. This could be due difficulties to occupy remaining active sites. The amount of dye adsorbed qt increases from 78.49 to 98.54 mg/g as initial dye

3.6.3. Adsorption kinetics The adsorption kinetics is an important parameter as it describes the efficiency of adsorption and mechanism of the adsorption process. The kinetics of adsorption of RhB dye on AC-BiOBr was studied with the help of pseudo-first-order, pseudo-second-order models.

Table 3 Freundlich and Langmuir isotherm constants for adsorption of Rh B on AC-BiOBr nanocomposites for different dye concentration and adsorbent dose of 0.30–0.40 g/L at contact time 120 min. RhB Dye Concentration mg/L

50 70 90

Freundlich coefficient

Langmuir coefficient

KF (L/g)

n

1/n

r2

a (mg/g)

b (g/L)

RL

r2

52.97 66.07 48.75

3.861 7.937 5.814

0.259 0.126 0.172

0.961 0.746 0.701

125 90.9 100

0.00013 0.00004 0.00031

0.9936 0.9969 0.9729

0.999 0.999 0.996

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Table 4 Comparison of adsorption rate constants, calculated and experimental qe values for different initial dye concentrations and adsorbent dose for different kinetic models. Adsorbent (g/L) AC-BiOBr

RhB Dye concentration (mg/L)

0.50 0.60 0.70

Adsorbent (g/L) AC-BiOBr

0.50

Pseudo-first-order qe (exp) (mg/g)

K1 (min−1)

qe (cal) (mg/g)

r2

50 60 70 50 60 70 50 60 70

90.50 107.8 112.3 79.00 91.00 106.0 71.01 81.82 92.83

0.039 0.028 0.016 0.048 0.048 0.027 0.101 0.075 0.039

40.83 33.57 16.63 35.48 36.39 36.72 19.05 18.66 34.67

0.979 0.954 0.966 0.935 0.979 0.865 0.953 0.932 0.865

RhB dye concentration (mg/L)

Pseudo-Second-order

50 60 70 50 60 70 50 60 70

0.60 0.70

1.2

K2x10−3 (g/mg min)

qe (cal) (mg/g)

h

r2

1.50 1.70 0.30 2.50 2.50 1.40 7.30 16.0 2.30

100 111.11 111.11 83.33 100 111.11 71.43 83.33 100

14.92 20.41 3.215 17.54 25.00 17.85 55.55 111.1 18.86

0.998 0.997 0.999 0.999 0.999 0.999 0.998 0.999 0.999

BiOBr AC20BiOBr

50ppm 60 ppm 70 ppm

1.0

1.0

AC40BiOBr AC60BiOBr AC80BiOBr

0.8

0.6

Ct/C0

t / qt

0.8

0.6

0.4

0.4

0.2

0.2 20

40

60

80

100

0.0

Time (min) Fig. 12. Second-order-kinetics plots for the removal of RhB at different initial dye concentrations; adsorbent dose 0.6 g/L of AC-BiOBr nanocomposites.

qt ) = log qe

K1 t 2.303

0

20

40 60 Time(min)

80

100

120

Fig. 13. Analogy of the photocatalytic degradation of RhB dye using AC-BiOBr nanocomposites.

3.6.3.1. The pseudo-first-order model. Lagergren [64] described the linear form of pseudo-first-order and is given by the equation (Eq. 8):

log(qe

Dark

t 1 t = + and h = K2 qe2 qt qe K2 qe2

(8)

(9) −1

−1

where K2 is rate constant for second-order adsorption (g mg min ), h is the initial rate (mg/ gmin). K2 and qe are determined from the slope and intercept of the plot of t qt vs. t (Fig. 12). The calculated qe values are in good agreement with the experimental qe values. Further, the corelation coefficient (r2) values are very close to 1, consequently indicates the adsorption of dye on AC-BiOBr follows the second-order kinetic model.

where qe and qt are the amount of dye adsorbed (mg/g) on AC-BiOBr at equilibrium and at time t, respectively. K1 is the rate constant of pseudo-first-order adsorption (min−1). The rate constant values were estimated by plotting log(qe − qt) versus t. The calculated K1 and correlation coefficient r2 values are tabulated in Table 4. The calculated qe values from first-order-kinetics plots were too small as compared to experimental qe values indicates the non-applicability of the pseudo-first-order model for the prediction of adsorption kinetics of RhB dye on AC-BiOBr.

3.7. Photocatalytic study Photocatalytic performance of BiOBr and AC-BiOBr composites were studied for the removal of RhB dye. After ensuring adsorptiondesorption equilibrium in dark between catalyst and RhB dye solution,

3.6.3.2. The pseudo-second-order model. The pseudo-second-order kinetic model is expressed as [65] follows (Eq. 9): 142

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nanocomposites are excellent catalyst for photodegradation of RhB dye under visible light. The excellent photocatalytic efficiency of AC-BiOBr nanocomposite is mainly due to increased adsorption along with the decrease in recombination of photogenerated electron-hole pair in ACBiOBr nanocomposite. Besides, AC restricts toxic intermediates formed during photodegradation to release in air or solution. In summary, the AC supported photocatalysts can be a green approach to the degradation of environmental pollutants. References [1] Kunal Mondal, Ashutosh Sharma, Recent advances in the synthesis and application of photocatalytic metal–metal oxide core–shell nanoparticles for environmental remediation and their recycling process, RSC Adv. 6 (2016) 83589–83612. [2] Alex Ibhadon, Paul Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (2013) 189–218. 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Fig. 14. Plausible mechanism for the photocatalytic degradation of RhB on ACBiOBr nanocomposite.

it was subjected to photocatalytic degradation under visible light by assuming time t = 0. The analogy of BiOBr and AC-BiOBr nanocomposites for the photocatalytic degradation of RhB dye is shown in Fig. 13. The degradation rate of RhB solution in 120 min of visible irradiation is maximum for AC80-BiOBr and is 95.1%, which is greater than those of BiOBr (33.5%), AC20-BiOBr (49.1%), AC40-BiOBr (67.8%), and AC60-BiOBr (85.6%). This suggests that the removal of RhB dye mainly depends on the adsorption of the samples. Subsequently, for photodegradation, the adsorptions play an important role, the higher the adsorption efficiency of dye on the sample, the higher will be the degradation. Further, Nethaji S et al. [46] reported that the electron transfer between a catalyst and excited dye molecules is influenced by adsorption of dye on the catalyst surface. Besides, the intermediate organic substances formed during photodegradation are adsorbed on the composite catalyst surface and finally get oxidized to CO2 and H2O. Accordingly, the composite surface not only provides the more adsorption surface for photodegradation but also restrict the toxic intermediates to release in air or solution phase. Furthermore, AC improved the charge separation by the reaction between the adsorbed oxygen (O2 present in AC) and the photogenerated electron, consequently, improve in photocatalytic efficiency owing to delay in electron–hole pair recombination [66]. Plausible degradation of RhB solution is shown in Fig. 14. 4. Conclusions In this report, we have summarized the structural, morphological, thermal, adsorption and photocatalytic behavior of pure BiOBr and ACBiOBr nanocomposites fabricated via a solvothermal method. SEM and TEM images confirm the mesoporous BiOBr nanoplates homogeneously dispersed on AC surface. Adsorption parameters for the Langmuir and Freundlich were determined and the equilibrium data were best described by the Langmuir isotherm model. The adsorption kinetics RhB dye on AC-BiOBr was found to follow the pseudo-second-order kinetic model. The photocatalytic results demonstrate that AC-BiOBr 143

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