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Efficient adsorption and photocatalytic degradation of Rhodamine B dye over Bi2O3-bentonite nanocomposites: A kinetic study
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JIEC 2741 1–8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7
Efficient adsorption and photocatalytic degradation of Rhodamine B dye over Bi2O3-bentonite nanocomposites: A kinetic study Q1 Sandip
P. Patil a, Bhaskar Bethi b, G.H. Sonawane c,*, V.S. Shrivastava a, Shirish Sonawane b
a
Nano-Chemistry Research Laboratory, G. T. Patil College, Nandurbar-425 412 (M.S.) India Chemical Engineering Department, NIT, Warangal-506 004 (Telangana) India c Deptartment of Chemistry, Kisan Arts, Commerce and Science College, Parola-425 111 (M.S.) India b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 April 2015 Received in revised form 16 October 2015 Accepted 1 December 2015 Available online xxx
Bi2O3-bentonite nanocomposites successfully synthesized by intercalation method are used for photocatalytic degradation of Rhodamine B (Rh B) under visible light irradiation. Bi2O3-bentonite shows enhanced photocatalytic efficiency than pure Bi2O3 due to intercalation with bentonite. Removal of Rh B is achieved upto 98.5% using 3 gL1 photocatalyst at pH 3. It is found that increase in light absorption and decrease in electron-hole recombination enhances photocatalytic efficiency. Photocatalytic degradation of Rh B by Bi2O3-bentonite proceeds via advanced oxidative process. The plausible mechanism of photocatalytic degradation Rh B reported by LC-MS shows generation of different degradation products including benzoic acid and benzonium ion. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Bi2O3-bentonite Rhodamine B Visible light photodegradation Advanced oxidative process
8 9
Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Ever increasing discharge of large excess of dyes by dye manufacturing and textile industries leads to severe environmental problems [1–5]. Many of the dyes are hazardous to humans as well as aquatic life [6,7]. Rh B is one of them which is extensively used for dyeing and their discharge in water bodies impart adverse effects on human and animal health [8,9]. Weathering of organic dyes in wastewater produces toxic metabolites [10]. Many researchers have tried various methods like oxidation, adsorption, degradation and catalytic reduction for removal of various dyes from wastewater [11–14]. Recent studies show that AOP is one of the effective techniques for the removal of dyes from wastewater [15–17]. AOPs are considered to be destructive and low or nonwaste generating technologies for the treatment of polluted water [18]. AOPs completely mineralize contaminants and their intermediates. Thus post treatment is not required as secondary wastes are not generated. Recently various researchers focus on degradation of toxic organic compounds present in wastewater by photocatalysis using various semiconductor metal oxides [19]. Some of the semiconductor metal oxides like TiO2 and ZnO have higher band gap thus
* Corresponding author. Tel.: +912597222441; fax: +912597223688. E-mail address: [email protected] (G.H. Sonawane).
for the excitation of electron from valence band to conduction band require high energy UV radiations. Therefore lot of studies have been devoted either to modify energy band gap of these semiconductor metal oxides or to find alternative to them in order to utilise solar energy. These methods involves a) doping with other elements, b) fabrication of hetero-junction structure by combining semiconductor with metals or other semiconductors to enhance solar light sensitivity [20]. There are several attempts has been made to use nanocomposites as a photocatalyst for the dye degradation. Earlier studies indicate that clay supported semiconductor nanocomposites are found to be promising method for wastewater treatment [21]. It has enhanced photocatalytic activity as it provides larger surface area, basal space and cation exchange capacity. Bentonite is abundant and inexpensive clay. It is effectively combined with different semiconductors to form composites due to its adsorptiondesorption property. Meshram et al. [15] reported removal of phenol, Riaz et al. [22] reported removal of orange G and Hamane et al. [23] reported removal of Pb+2, using different bentonite based composites. Bi2O3 has lower band gap 2.58-2.85 eV [24] sensitive to visible light irradiation has also found to exhibit good photocatalytic performances for removal of different dyes [25–29]. The combination of Bi2O3 with bentonite clay is a promising method to enhance the photocatalytic efficiency of Bi2O3. However, Bi2O3-bentonite nanoclay composite is unique catalyst which
http://dx.doi.org/10.1016/j.jiec.2015.12.002 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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shows the activity in the visible light. Bentonite nanoclay has the more cation exchange capacity for the adsorption. The novelty involved in this work is the synthesis of heterogeneous visible light activated photocatalyst and its performance in the dye degradation and its reusability for 3 times. Bi2O3-bentonite nanocomposite was shown a good adsorption and photocatalytic property in visible light for the degradation of dye. It has indicates that, the scale-up for the treatment of textile waste water in bulk volume is possible using solar light and this photocatalyst. Present study involves incorporation of Bi2O3 semiconductor with bentonite clay to obtained Bi2O3-bentonite nanocomposite. The resulting nanocomposites were characterised by SEM, EDS and XRD. Then nanocomposites are applied for photocatalytic and adsorptive removal of Rh B under visible light irradiation. Effect of key operating parameters like initial dye concentration, pH, contact time and catalyst dose for the degradation of Rh B were investigated systematically. Then photodegradation intermediates were identified by LC-MS analysis and degradation pathway of Rh B was proposed.
BiONO3 [24,31,32], then 5wt% of modified nanoclay is added into it. Resulting mixture is sonicated for 30 min to obtain uniform suspension. Then dil. NaOH is added to this solution with continuous stirring for 30 min. Then Bi2O3-bentonite nanocomposites are obtained by centrifugation and successive dispersions in alcohol. Finally nanocomposites are dried under vacuum and calcinated at 400 8C for 3-4 h.
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Characterization of nanocomposite
108
The SEM images were taken using the Hitachi S-4800 (Japan) FESEM. EDS analysis was performed by using Bruker X Flash 5030. The XRD pattern of the samples were measured on a Bruker D 8 Advance X-ray diffractometer (Germany) using monochromatized Cu Ka (l = 0.15418 nm) radiation under 40 kV and 40 mA and scanning over the range of 10o 2u 60o.
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Adsorption study
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74
Experimental
75
Materials and methods
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All the chemicals used in the present study are of A.R. grade. Natural bentonite was procured from Ganatra mines (Jaipur, India). Bismuth nitrate, Cetyl trimethyl ammonium bromide (CTAB), Rh B are procured from S.D.Fine Chemicals, India and are used as supplied without further purification.
The adsorption of Rh B on Bi2O3-bentonite is studied in dark for 60 min. The mixtures are shaken to achieve equilibrium between them. Then changes in Rh B concentration are determined by Systronics-2203 UV-Visible double beam spectrophotometer at lmax = 546 nm. The amount of Rh B adsorbed on Bi2O3-bentonite is calculated by eq.1.
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81
Preparation of Rh B solution
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Stock solution (500mgL1) is prepared by distilled water. The experimental solutions are prepared by diluting stock solution with distilled water. The molecular structure of Rh B is shown in Fig. 1.
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Synthesis of Bi2O3-bentonite nanocomposite
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The bentonite was ground, sieved and washed with deionised water for several times. Differential sedimentation technique was used to remove silica and iron oxides. Then mixture was stirred for 1 h and kept undisturbed for overnight. After filtration, the solid mass was exposed to slow evaporation to obtain dry purified bentonite. Intercalation of bentonite was carried out as reported by Sonawane et al. [30] by using CTAB. 3.64 g CTAB (on the basis of CEC of bentonite) was dissolved in 75 mL distilled water having 3.9 103 gmol HCl. This mixture was stirred under sonication at 80 8C for 30 min to get clear solution. 3.25 g purified bentonite (CEC 72meq/100 g) was added to above solution and sonicated for another 1.5 h. Then solid obtained by filtration was dispersed in hot water for several times to obtain chloride free, modified nanoclay. 2 g Bi(NO3)3.5H2O is dissolved in 20 mL nitric acid (1.5 M) to avoid hydrolyzation of Bi3+ ions by preventing precipitation of
H 3C H 3C
CH 3 N
O
N
COOH
Fig. 1. The structure of Rh B.
+
Cl
qt ¼
ðC o C t ÞV W
(1)
where qt (mg g1) is the adsorption capacity at time t; Co (mg L1) is the initial Rh B dye concentration; Ct (mg L1) is the Rh B concentration at time t; V (L) is the initial volume of dye solution and W (g) is the amount of nanocomposite.
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Photocatalytic experiment
127
The photocatalytic performance of Bi2O3-bentonite nanocomposite is studied through photocatalytic degradation of Rh B under visible light irradiation. The photocatalytic degradation of Rh B is carried out in photocatalytic reactor, having 500 W halogen lamp. Cooling water jacket is used to maintain temperature inside the reactor. Different nanocomposite doses are added to the 50 mL dye solution and then placed in a photocatalytic reactor. After certain time intervals, adequate amount of sample is withdrawn and centrifuged to remove the nanocomposite. Then changes in dye concentration are determined by Systronics-2203 UV-Visible double beam spectrophotometer. The percentage removal of Rh B is calculated by eq. (2).
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Removal percentage ¼
C o C t 100 Co
(2)
Results and discussion
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SEM and EDS analysis
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Elemental analysis of material surface layer is obtained by electron dispersive X-ray spectroscopy (EDS). Fig. 2a shows that Bi2O3-bentonite contains C K(3.46%), O K(55.87%),Bi K(20.7%), Na K(2.72%), Si K(11.08%), Al K (3.3%), Ca K(2.03%) and Mg K(0.83%). Absence of Bi peaks in EDS of bentonite while presence of peaks of Bi and O in EDS of Bi2O3-bentonite micrograph proves existence of Bi2O3 in the nanocomposite. Fig. 2b shows absence of crystalline particles on bentonite clay. The Bi2O3 material has nanorod shape [24]. From Fig. 2c it was clearly observed that, Bi2O3 nanorods are well dispersed over
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-
CH 3
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Fig. 2. The EDS spectra of (a) Bi2O3-bentonite; The SEM images of (b) bentonite and (c) Bi2O3-bentonite.
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bentonite clay which reflects good combination between them and also proves existence of Bi2O3-bentonite nanocomposite.
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XRD analysis
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X-ray powder diffraction (XRD) is a useful tool to characterise the phase structure of the materials. The X-ray diffraction pattern of Bi2O3, bentonite and Bi2O3-bentonite are
shown in Fig. 3. XRD patterns of bentonite clay contain major peaks at 2u of 19.64o, 24.56o, 25.76o and 34.1o. The diffraction pattern of Bi2O3 shows major peaks at 2u of 21.28o, 28.64o, 33.47o and 34.41o. The XRD analysis indicates that, all the diffraction peaks of Bi2O3-bentonite are similar to those of Bi2O3. The broadening of peaks probably occurs due to bridging of Bi2O3 to the neighbouring silicate units between the layers of bentonite clay.
Fig. 3. XRD patterns of the bentonite, Bi2O3 and Bi2O3-bentonite.
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(a)
14
25
12 10 8
t/qt
qt (mg g-1)
20 15
6
10
4 2
5
0 0
0 20
40
60
80
Time (min)
(b)
80
qt (mg g-1)
70 60 50 40 30 20 10 0 20
40
40
60
80
100
Time (min) Fig. 5. Second order kinetics plots for the removal of Rh B at different initial dye concentrations; Bi2O3-bentonite dose 3 gL1, pH 3.
90
0
20
100
60
80
100
Time (min) Fig. 4. a. Effect of contact time on adsorption of Rh B with catalyst dose 1 gL1; pH 3 and dye concentration 20 mgL1. b. Amount of dye adsorbed qt (mg g1) with time for different initial dye concentration; pH 3, Bi2O3-bentonite dose 1gL1.
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Adsorption of Rh B on bentonite, Bi2O3 and Bi2O3-bentonite
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Effect of contact time The effect of contact time on adsorption of Rh B by bentonite, Bi2O3 and Bi2O3-bentonite is shown in Fig. 4a. This shows that adsorption of Rh B is higher over Bi2O3-bentonite as compare to bentonite and Bi2O3. Fig. 4b shows that initially amount of Rh B adsorbed increases with increase in initial dye concentration and contact time. The rate of adsorption is faster for first 20 min then it becomes slower to attain equilibrium. The faster adsorption in first 20 min is due to availability of more number of sites for adsorption. Later on repulsion between solute and bulk phase causes difficulties to occupy remaining sites.
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Adsorption kinetics Mechanism of adsorption is given by adsorption kinetics. The adsorption kinetics of Rh B was studied with pseudo-second-order model. The pseudo-second-order model is given as [33],where K2 is pseudo-second-order rate constant (g mg1 min1), h is initial rate (mg g1 min1). The linear plot of t/qt vs t with correlation coefficient r2 0.998 shows good agreement with experimental qe values (Fig. 5). This indicates that adsorption belongs to pseudosecond-order kinetics.
189
Adsorption isotherm
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Freundlich isotherm describes non-ideal, reversible and multilayer adsorption with heterogeneous system. Linear nature of plot of log qe vs log Ce (Fig. 6) shows the applicability of Freundlich adsorption isotherm. The slope (1/n) value of plot indicates nature adsorption. The slope (1/n) less than one indicates chemisorption; more than one indicates co-operative adsorption while closer to
zero indicates heterogeneity. The greater 1/n values (Table 1) indicates adsorption of Rh B on Bi2O3-bentonite is co-operative. Langmuir assumes that monolayer homogeneous adsorption occurs only at finite number of identical and equivalent sites without interaction between adsorbed molecules. Langmuir isotherm is described by dimensionless factor RL. The RL values indicate nature of adsorption. RL > 1 indicates unfavourable adsorption; RL = 1 indicates linear adsorption; 0< RL < 1 indicates favourable adsorption and RL = 0 indicates irreversible adsorption. The present study with RL values between 0 and 1 indicates favourable adsorption process (Table 1). Freundlich isotherm has higher r2 values, 1/n and KF values while Langmuir isotherm has lower r2 values. Thus, Freundlich adsorption isotherm is the most appropriate isotherm for adsorption of Rh B on Bi2O3-bentonite.
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Photocatalytic study
211
Photocatalytic activity of bentonite, Bi2O3 and Bi2O3-bentonite is studied for the removal of Rh B. The dye solutions along with catalyst are placed in dark to carry out adsorption for 60 min, then photocatalytic degradation preceded by considering t = 0 under visible light irradiation. Fig. 7 shows that A/Ao decreases rapidly when degradation was carried out with Bi2O3-bentonite than bentonite and Bi2O3 alone. Where Ao is the initial absorbance and A is absorbance at time t. The percentage removal of Rh B by bentonite, Bi2O3 and Bi2O3-bentonite after 60 min adsorption are 57.2%, 8.7% and 72.3% respectively. These values indicate that Bi2O3-bentonite has higher adsorption efficiency as compare to bentonite and Bi2O3. After visible light irradiation for 80 min, removal of Rh B by bentonite achieved upto 62%, by Bi2O3 upto 58.4% and by
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2.5 2
log q
0
1.5 1 0.5 0 0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
log Ce Fig. 6. Freundlich plot for adsorption of Rh B by Bi2O3-bentonite.
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Table 1 Freundlich and Langmuir isotherm constants for adsorption of Rh B on Bi2O3-bentonite for different dye concentration and photocatalyst dose of 0.4 to 2.8 gL1 at pH 3, contact time 60 min. Dye concentration mgL1
Freundlich coefficient
20 40 60 80
n
1/n
r2
a (mgg1)
b (gL1)
RL
r2
12.62 9.70 11.07 1.13
0.4968 0.5917 0.6689 0.3425
2.013 1.690 1.495 2.920
0.944 0.915 0.933 0.983
31.25 47.62 111.11 166.67
0.7273 0.4375 0.1956 0.0822
0.0643 0.0540 0.0785 0.0132
0.892 0.999 0.979 0.964
Bi2O3-bentonite upto 95.8% respectively. In the removal of Rh B by Bi2O3 and Bi2O3-bentonite, photocatalytic degradation is the dominant process than adsorption. To investigate the optimum amount of photocatalyst required for the removal of Rh B, amount of photocatalyst was varied. The percentage removal of Rh B by Bi2O3-bentonite at different photocatalyst doses 0.4 to 2.8 gL1 for 20 mgL1 dye concentration was studied under same experimental conditions. It was observed that, as Bi2O3-bentonite dose increases from 0.4 to 2.8gL1, the percentage removal also increases from 90.9 to 97.6% under visible light irradiation. It was concluded from Table 2 that, Rh B removal by Bi2O3bentonite was an efficient method by comparison of various removal efficiencies, operational times and initial dye concentrations with similar studies in the literature. To confirm the stability and efficiency of high performance of Bi2O3-bentonite nanocomposite as well as cost effectiveness of the process, the reusability of Bi2O3-bentonite nanocomposite in the photocatalytic degradation of Rh B under visible light irradiation
1.0
was investigated. To study its reusability, the powdered nanocomposite was allowed to settle by gravity after completion of each photocatalytic experiment. The recovered nanocomposite was reused for 3 times under same experimental conditions. Fig. 8 shows removal Rh B by Bi2O3-bentonite after 1st run achieved upto 98.5% after 80 min. After 4th run it decreases down to 91.7%. The catalytic activity was found to decrease slightly after 4th run. This decrease may be attributed to loss of reused catalyst during sampling each time and irreversible changes of the surface of the photocatalyst by dye molecules [37]. Fig. 8 shows that Bi2O3bentonite has excellent stability and does not suffer from photocorrosion during photodegradation. This reflects significant efficiency and practical applicability of Bi2O3-bentonite nanocomposite.
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Effect of pH on Rh B degradation
259
Photocatalytic degradation of dyes is significantly affected by pH of the dye solution [38,39]. The effect of pH is studied from pH 1 to 9 at 20mgL1 initial dye concentration for 1gL1 photocatalyst dose. It is observed that percent removal at pH 1 is 87.9%, then it increases upto 95.8% for pH 3 then it decreases down to 57.8% for pH 9. Several other authors also reported, the optimum pH for the removal of Rh B has been found to be about 3 [40].
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Degradation mechanism and identification of metabolites
267
Degradation of Rh B can be explained on the basis of AOPs promoted by heterogeneous photocatalysis. This process involves generation of e and h+ which again generates O2 and OH radicals respectively. These radicals are highly reactive and act as degrading agents. Moreover, photo-generated hydrogen atom from water is responsible for reductive degradation. Since Rh B is cationic dye and poor e donor, the initial step during its photodegradation is adsorption of dye on clay surface then e--hole pair get generated by absorption of light by Bi2O3 nanorods intercalated between bentonite clay structure. e and h+
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0.8
Bi2O3-bentonite Bentonite Bi2O3
0.6
A/Ao 0.4
0.2
0.0 0
20
40
60
80
Time (min)
120
Fig. 7. Photocatalytic degradation of Rh B (20mgL1) with Bentonite, Bi2O3 and Bi2O3-bentonite.
Table 2 Rh B removal efficiencies (%) of various methods. Nanomaterial BiVO4/reduced graphene oxide BiOBr/montmorillonite BiOBr-Graphene oxide BiOBr-Graphene Vis-Fe0-H2O2-citrate-O2 Bi2O3-bentonite
Removal efficiency (%)
Operation time (min)
Sources
98.5 98.9 95 87.4 62 98.5
600 120 45 135 120 80
[4] [8] [34] [35] [36] This study
100 Percent removal
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Langmuir coefficient
KF (Lg1)
80 1st run
60
2nd run
40
3rd run 4th run
20 0 1st run
2nd run
3rd run
4th run
Cycles Fig. 8. Reusability performance of Bi2O3-bentonite nanocomposite.
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Percentage Removal
6
100 90 80 70 60 50 40 30 20 10 0
degradation pathway of Rh B is shown in Fig. 11a. Fig. 11a indicates the N-deethylation, ring opening of Rh B to form different degradation products including benzoic acid and benzonium ion which are relatively non-toxic compare to Rh B. The plausible mechanism for catalytic oxidation of Rh B on Bi2O3-bentonite nanocomposite photocatalyst is shown below. ½DyeðRh BÞ þ H2 O þ Bi2 O3 bentonite
adsorption
!
hv
Aqueous soluon
0.1 M i-PrOH
Fig. 9. Photocatlytic degradation of Rh B with addition of 0.1 M i-PrOH.
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again generate degrading agents, O2 and OH radicals. These are responsible for degradation of Rh B. Isopropanol is commonly used as diagnostic tool for OH mediated mechanisms [41–45]. The present study involves isopropanol as a scavenger. As direct oxidation of smaller alcohols by photogenerated holes is negligible, it is neglected. The results in Fig. 9 show that photodegradation of Rh B decreases down to 66.3% with the addition of 0.1 M i-PrOH as compare to reaction in aqueous medium. The inhibition of photodegradation of Rh B suggests that OH are predominantly active species in the photocatalytic degradation of Rh B. In order to understand the degradation pathway and intermediates that are formed during photocatalytic degradation of Rh B, LC-MS analysis was carried out. The mass spectra of Rh B before and after degradation are shown in Fig. 10. The probable
Rh BBi2 O3 bentonite
Bi2 O3 bentonite þ H2 O!e þ h
þ
þ H2 O
h ! OH
(4) 299 298 (5)
(6)
O2
e !O2
(7)
301 300
303 302
305 304
yields
Rh Bþ OH þ O2 ! CO2 þ H2 O þ Benzolc acid þ Benzonium ion
(8)
Mechanistic pathway of photodegradation of Rh B dye using Bi2O3–bentonite nanocomposite is shown in Fig. 11b. Initially clay is intercalated with higher d spacing. Catalyst Bi2O3 is attached onto surface of the clay platelets. With the irradiation of the visible light, there is generation of electron and hole which are attacking onto the Rh B dye molecule. Though the complete mineralization is not possible but the formation of the side products benzoic acid
Fig. 10. Mass spectra of Rh B solution (a) before and (b) after photodegradation.
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(a) O
+
H 3C
H 2N
Rh B
H 3C
m/z 104
H 2N
7
N
CH 3
O
N
+
NH2
HO
CH 3
m/z 114
NH
COOH COOH
m/z 196 H 2N
m/z 122
m/z 443
O
CH 3
CH 3
m/z 274
H 2N
H 3C
NH
O
NH
O
N
+
CH 3
COOH
COOH
m/z 415 m/z 362
(b)
Fig. 11. a The possible reaction intermediates after photocatalytic degradation under visible light irradiation. b Mechanistic pathway of photodegradation of Rh B dye using Bi2O3–Bentonite nanocomposite.
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and benzonium ion takes place which are less toxic than the Rh B dye.
317
Conclusion
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Bi2O3-bentonite nanocomposites have been successfully synthesized by sonochemical method and its photocatalytic performances under visible light are investigated. The results indicate that the Bi2O3-bentonite nanocomposite exhibits enhanced photocatalytic efficiency in Rh B removal than Bi2O3 and bentonite clay. Increase in photocatalytic efficiency is due to increased adsorption and decrease in recombination of photo e--hole pair in the Bi2O3 with bentonite. The semiconductor based photocatalyst, the Bi2O3 incorporated with bentonite clay can be a promising photocatalyst used effectively for the removal of dyes from wastewater. Dye removal is favoured at pH 3. Adsorption kinetics is found to follow pseudo-second-order kinetics. Whereas adsorption isotherm found to follow Freundlich isotherm. The
dimensionless factor RL values show the favourable adsorption process. The scavenging study by addition i-PrOH shows that, OH are the active species responsible for the photodegradation Rh B. LC-MS analysis shows the non generation of secondary waste after degradation of Rh B by Bi2O3-bentonite. Furthermore, catalyst reusability study reveals that reusable Bi2O3-bentonite nanocomposite has high stability, high mineralization efficiency. Thus Bi2O3-bentonite nanocomposite could be potential material for wastewater treatment.
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Acknowledgements
340
The authors have gratefully acknowledged to Central Instrumentation Centre, University Institute of Chemical Technology, NMU, Jalgaon for SEM and XRD analysis. Authors are also thankful to Principal, Kisan College, Parola and Principal, G. T. Patil College, Nandurbar for providing necessary laboratory facilities.
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JIEC 2741 1–8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7
Efficient adsorption and photocatalytic degradation of Rhodamine B dye over Bi2O3-bentonite nanocomposites: A kinetic study Q1 Sandip
P. Patil a, Bhaskar Bethi b, G.H. Sonawane c,*, V.S. Shrivastava a, Shirish Sonawane b
a
Nano-Chemistry Research Laboratory, G. T. Patil College, Nandurbar-425 412 (M.S.) India Chemical Engineering Department, NIT, Warangal-506 004 (Telangana) India c Deptartment of Chemistry, Kisan Arts, Commerce and Science College, Parola-425 111 (M.S.) India b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 April 2015 Received in revised form 16 October 2015 Accepted 1 December 2015 Available online xxx
Bi2O3-bentonite nanocomposites successfully synthesized by intercalation method are used for photocatalytic degradation of Rhodamine B (Rh B) under visible light irradiation. Bi2O3-bentonite shows enhanced photocatalytic efficiency than pure Bi2O3 due to intercalation with bentonite. Removal of Rh B is achieved upto 98.5% using 3 gL1 photocatalyst at pH 3. It is found that increase in light absorption and decrease in electron-hole recombination enhances photocatalytic efficiency. Photocatalytic degradation of Rh B by Bi2O3-bentonite proceeds via advanced oxidative process. The plausible mechanism of photocatalytic degradation Rh B reported by LC-MS shows generation of different degradation products including benzoic acid and benzonium ion. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Bi2O3-bentonite Rhodamine B Visible light photodegradation Advanced oxidative process
8 9
Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Ever increasing discharge of large excess of dyes by dye manufacturing and textile industries leads to severe environmental problems [1–5]. Many of the dyes are hazardous to humans as well as aquatic life [6,7]. Rh B is one of them which is extensively used for dyeing and their discharge in water bodies impart adverse effects on human and animal health [8,9]. Weathering of organic dyes in wastewater produces toxic metabolites [10]. Many researchers have tried various methods like oxidation, adsorption, degradation and catalytic reduction for removal of various dyes from wastewater [11–14]. Recent studies show that AOP is one of the effective techniques for the removal of dyes from wastewater [15–17]. AOPs are considered to be destructive and low or nonwaste generating technologies for the treatment of polluted water [18]. AOPs completely mineralize contaminants and their intermediates. Thus post treatment is not required as secondary wastes are not generated. Recently various researchers focus on degradation of toxic organic compounds present in wastewater by photocatalysis using various semiconductor metal oxides [19]. Some of the semiconductor metal oxides like TiO2 and ZnO have higher band gap thus
* Corresponding author. Tel.: +912597222441; fax: +912597223688. E-mail address: [email protected] (G.H. Sonawane).
for the excitation of electron from valence band to conduction band require high energy UV radiations. Therefore lot of studies have been devoted either to modify energy band gap of these semiconductor metal oxides or to find alternative to them in order to utilise solar energy. These methods involves a) doping with other elements, b) fabrication of hetero-junction structure by combining semiconductor with metals or other semiconductors to enhance solar light sensitivity [20]. There are several attempts has been made to use nanocomposites as a photocatalyst for the dye degradation. Earlier studies indicate that clay supported semiconductor nanocomposites are found to be promising method for wastewater treatment [21]. It has enhanced photocatalytic activity as it provides larger surface area, basal space and cation exchange capacity. Bentonite is abundant and inexpensive clay. It is effectively combined with different semiconductors to form composites due to its adsorptiondesorption property. Meshram et al. [15] reported removal of phenol, Riaz et al. [22] reported removal of orange G and Hamane et al. [23] reported removal of Pb+2, using different bentonite based composites. Bi2O3 has lower band gap 2.58-2.85 eV [24] sensitive to visible light irradiation has also found to exhibit good photocatalytic performances for removal of different dyes [25–29]. The combination of Bi2O3 with bentonite clay is a promising method to enhance the photocatalytic efficiency of Bi2O3. However, Bi2O3-bentonite nanoclay composite is unique catalyst which
http://dx.doi.org/10.1016/j.jiec.2015.12.002 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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shows the activity in the visible light. Bentonite nanoclay has the more cation exchange capacity for the adsorption. The novelty involved in this work is the synthesis of heterogeneous visible light activated photocatalyst and its performance in the dye degradation and its reusability for 3 times. Bi2O3-bentonite nanocomposite was shown a good adsorption and photocatalytic property in visible light for the degradation of dye. It has indicates that, the scale-up for the treatment of textile waste water in bulk volume is possible using solar light and this photocatalyst. Present study involves incorporation of Bi2O3 semiconductor with bentonite clay to obtained Bi2O3-bentonite nanocomposite. The resulting nanocomposites were characterised by SEM, EDS and XRD. Then nanocomposites are applied for photocatalytic and adsorptive removal of Rh B under visible light irradiation. Effect of key operating parameters like initial dye concentration, pH, contact time and catalyst dose for the degradation of Rh B were investigated systematically. Then photodegradation intermediates were identified by LC-MS analysis and degradation pathway of Rh B was proposed.
BiONO3 [24,31,32], then 5wt% of modified nanoclay is added into it. Resulting mixture is sonicated for 30 min to obtain uniform suspension. Then dil. NaOH is added to this solution with continuous stirring for 30 min. Then Bi2O3-bentonite nanocomposites are obtained by centrifugation and successive dispersions in alcohol. Finally nanocomposites are dried under vacuum and calcinated at 400 8C for 3-4 h.
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Characterization of nanocomposite
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The SEM images were taken using the Hitachi S-4800 (Japan) FESEM. EDS analysis was performed by using Bruker X Flash 5030. The XRD pattern of the samples were measured on a Bruker D 8 Advance X-ray diffractometer (Germany) using monochromatized Cu Ka (l = 0.15418 nm) radiation under 40 kV and 40 mA and scanning over the range of 10o 2u 60o.
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Adsorption study
115
74
Experimental
75
Materials and methods
76 77 78 79 80
All the chemicals used in the present study are of A.R. grade. Natural bentonite was procured from Ganatra mines (Jaipur, India). Bismuth nitrate, Cetyl trimethyl ammonium bromide (CTAB), Rh B are procured from S.D.Fine Chemicals, India and are used as supplied without further purification.
The adsorption of Rh B on Bi2O3-bentonite is studied in dark for 60 min. The mixtures are shaken to achieve equilibrium between them. Then changes in Rh B concentration are determined by Systronics-2203 UV-Visible double beam spectrophotometer at lmax = 546 nm. The amount of Rh B adsorbed on Bi2O3-bentonite is calculated by eq.1.
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Preparation of Rh B solution
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Stock solution (500mgL1) is prepared by distilled water. The experimental solutions are prepared by diluting stock solution with distilled water. The molecular structure of Rh B is shown in Fig. 1.
85
Synthesis of Bi2O3-bentonite nanocomposite
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The bentonite was ground, sieved and washed with deionised water for several times. Differential sedimentation technique was used to remove silica and iron oxides. Then mixture was stirred for 1 h and kept undisturbed for overnight. After filtration, the solid mass was exposed to slow evaporation to obtain dry purified bentonite. Intercalation of bentonite was carried out as reported by Sonawane et al. [30] by using CTAB. 3.64 g CTAB (on the basis of CEC of bentonite) was dissolved in 75 mL distilled water having 3.9 103 gmol HCl. This mixture was stirred under sonication at 80 8C for 30 min to get clear solution. 3.25 g purified bentonite (CEC 72meq/100 g) was added to above solution and sonicated for another 1.5 h. Then solid obtained by filtration was dispersed in hot water for several times to obtain chloride free, modified nanoclay. 2 g Bi(NO3)3.5H2O is dissolved in 20 mL nitric acid (1.5 M) to avoid hydrolyzation of Bi3+ ions by preventing precipitation of
H 3C H 3C
CH 3 N
O
N
COOH
Fig. 1. The structure of Rh B.
+
Cl
qt ¼
ðC o C t ÞV W
(1)
where qt (mg g1) is the adsorption capacity at time t; Co (mg L1) is the initial Rh B dye concentration; Ct (mg L1) is the Rh B concentration at time t; V (L) is the initial volume of dye solution and W (g) is the amount of nanocomposite.
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Photocatalytic experiment
127
The photocatalytic performance of Bi2O3-bentonite nanocomposite is studied through photocatalytic degradation of Rh B under visible light irradiation. The photocatalytic degradation of Rh B is carried out in photocatalytic reactor, having 500 W halogen lamp. Cooling water jacket is used to maintain temperature inside the reactor. Different nanocomposite doses are added to the 50 mL dye solution and then placed in a photocatalytic reactor. After certain time intervals, adequate amount of sample is withdrawn and centrifuged to remove the nanocomposite. Then changes in dye concentration are determined by Systronics-2203 UV-Visible double beam spectrophotometer. The percentage removal of Rh B is calculated by eq. (2).
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Removal percentage ¼
C o C t 100 Co
(2)
Results and discussion
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SEM and EDS analysis
143
Elemental analysis of material surface layer is obtained by electron dispersive X-ray spectroscopy (EDS). Fig. 2a shows that Bi2O3-bentonite contains C K(3.46%), O K(55.87%),Bi K(20.7%), Na K(2.72%), Si K(11.08%), Al K (3.3%), Ca K(2.03%) and Mg K(0.83%). Absence of Bi peaks in EDS of bentonite while presence of peaks of Bi and O in EDS of Bi2O3-bentonite micrograph proves existence of Bi2O3 in the nanocomposite. Fig. 2b shows absence of crystalline particles on bentonite clay. The Bi2O3 material has nanorod shape [24]. From Fig. 2c it was clearly observed that, Bi2O3 nanorods are well dispersed over
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-
CH 3
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Fig. 2. The EDS spectra of (a) Bi2O3-bentonite; The SEM images of (b) bentonite and (c) Bi2O3-bentonite.
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bentonite clay which reflects good combination between them and also proves existence of Bi2O3-bentonite nanocomposite.
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XRD analysis
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X-ray powder diffraction (XRD) is a useful tool to characterise the phase structure of the materials. The X-ray diffraction pattern of Bi2O3, bentonite and Bi2O3-bentonite are
shown in Fig. 3. XRD patterns of bentonite clay contain major peaks at 2u of 19.64o, 24.56o, 25.76o and 34.1o. The diffraction pattern of Bi2O3 shows major peaks at 2u of 21.28o, 28.64o, 33.47o and 34.41o. The XRD analysis indicates that, all the diffraction peaks of Bi2O3-bentonite are similar to those of Bi2O3. The broadening of peaks probably occurs due to bridging of Bi2O3 to the neighbouring silicate units between the layers of bentonite clay.
Fig. 3. XRD patterns of the bentonite, Bi2O3 and Bi2O3-bentonite.
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(a)
14
25
12 10 8
t/qt
qt (mg g-1)
20 15
6
10
4 2
5
0 0
0 20
40
60
80
Time (min)
(b)
80
qt (mg g-1)
70 60 50 40 30 20 10 0 20
40
40
60
80
100
Time (min) Fig. 5. Second order kinetics plots for the removal of Rh B at different initial dye concentrations; Bi2O3-bentonite dose 3 gL1, pH 3.
90
0
20
100
60
80
100
Time (min) Fig. 4. a. Effect of contact time on adsorption of Rh B with catalyst dose 1 gL1; pH 3 and dye concentration 20 mgL1. b. Amount of dye adsorbed qt (mg g1) with time for different initial dye concentration; pH 3, Bi2O3-bentonite dose 1gL1.
168
Adsorption of Rh B on bentonite, Bi2O3 and Bi2O3-bentonite
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Effect of contact time The effect of contact time on adsorption of Rh B by bentonite, Bi2O3 and Bi2O3-bentonite is shown in Fig. 4a. This shows that adsorption of Rh B is higher over Bi2O3-bentonite as compare to bentonite and Bi2O3. Fig. 4b shows that initially amount of Rh B adsorbed increases with increase in initial dye concentration and contact time. The rate of adsorption is faster for first 20 min then it becomes slower to attain equilibrium. The faster adsorption in first 20 min is due to availability of more number of sites for adsorption. Later on repulsion between solute and bulk phase causes difficulties to occupy remaining sites.
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Adsorption kinetics Mechanism of adsorption is given by adsorption kinetics. The adsorption kinetics of Rh B was studied with pseudo-second-order model. The pseudo-second-order model is given as [33],where K2 is pseudo-second-order rate constant (g mg1 min1), h is initial rate (mg g1 min1). The linear plot of t/qt vs t with correlation coefficient r2 0.998 shows good agreement with experimental qe values (Fig. 5). This indicates that adsorption belongs to pseudosecond-order kinetics.
189
Adsorption isotherm
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Freundlich isotherm describes non-ideal, reversible and multilayer adsorption with heterogeneous system. Linear nature of plot of log qe vs log Ce (Fig. 6) shows the applicability of Freundlich adsorption isotherm. The slope (1/n) value of plot indicates nature adsorption. The slope (1/n) less than one indicates chemisorption; more than one indicates co-operative adsorption while closer to
zero indicates heterogeneity. The greater 1/n values (Table 1) indicates adsorption of Rh B on Bi2O3-bentonite is co-operative. Langmuir assumes that monolayer homogeneous adsorption occurs only at finite number of identical and equivalent sites without interaction between adsorbed molecules. Langmuir isotherm is described by dimensionless factor RL. The RL values indicate nature of adsorption. RL > 1 indicates unfavourable adsorption; RL = 1 indicates linear adsorption; 0< RL < 1 indicates favourable adsorption and RL = 0 indicates irreversible adsorption. The present study with RL values between 0 and 1 indicates favourable adsorption process (Table 1). Freundlich isotherm has higher r2 values, 1/n and KF values while Langmuir isotherm has lower r2 values. Thus, Freundlich adsorption isotherm is the most appropriate isotherm for adsorption of Rh B on Bi2O3-bentonite.
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Photocatalytic study
211
Photocatalytic activity of bentonite, Bi2O3 and Bi2O3-bentonite is studied for the removal of Rh B. The dye solutions along with catalyst are placed in dark to carry out adsorption for 60 min, then photocatalytic degradation preceded by considering t = 0 under visible light irradiation. Fig. 7 shows that A/Ao decreases rapidly when degradation was carried out with Bi2O3-bentonite than bentonite and Bi2O3 alone. Where Ao is the initial absorbance and A is absorbance at time t. The percentage removal of Rh B by bentonite, Bi2O3 and Bi2O3-bentonite after 60 min adsorption are 57.2%, 8.7% and 72.3% respectively. These values indicate that Bi2O3-bentonite has higher adsorption efficiency as compare to bentonite and Bi2O3. After visible light irradiation for 80 min, removal of Rh B by bentonite achieved upto 62%, by Bi2O3 upto 58.4% and by
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2.5 2
log q
0
1.5 1 0.5 0 0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
log Ce Fig. 6. Freundlich plot for adsorption of Rh B by Bi2O3-bentonite.
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Table 1 Freundlich and Langmuir isotherm constants for adsorption of Rh B on Bi2O3-bentonite for different dye concentration and photocatalyst dose of 0.4 to 2.8 gL1 at pH 3, contact time 60 min. Dye concentration mgL1
Freundlich coefficient
20 40 60 80
n
1/n
r2
a (mgg1)
b (gL1)
RL
r2
12.62 9.70 11.07 1.13
0.4968 0.5917 0.6689 0.3425
2.013 1.690 1.495 2.920
0.944 0.915 0.933 0.983
31.25 47.62 111.11 166.67
0.7273 0.4375 0.1956 0.0822
0.0643 0.0540 0.0785 0.0132
0.892 0.999 0.979 0.964
Bi2O3-bentonite upto 95.8% respectively. In the removal of Rh B by Bi2O3 and Bi2O3-bentonite, photocatalytic degradation is the dominant process than adsorption. To investigate the optimum amount of photocatalyst required for the removal of Rh B, amount of photocatalyst was varied. The percentage removal of Rh B by Bi2O3-bentonite at different photocatalyst doses 0.4 to 2.8 gL1 for 20 mgL1 dye concentration was studied under same experimental conditions. It was observed that, as Bi2O3-bentonite dose increases from 0.4 to 2.8gL1, the percentage removal also increases from 90.9 to 97.6% under visible light irradiation. It was concluded from Table 2 that, Rh B removal by Bi2O3bentonite was an efficient method by comparison of various removal efficiencies, operational times and initial dye concentrations with similar studies in the literature. To confirm the stability and efficiency of high performance of Bi2O3-bentonite nanocomposite as well as cost effectiveness of the process, the reusability of Bi2O3-bentonite nanocomposite in the photocatalytic degradation of Rh B under visible light irradiation
1.0
was investigated. To study its reusability, the powdered nanocomposite was allowed to settle by gravity after completion of each photocatalytic experiment. The recovered nanocomposite was reused for 3 times under same experimental conditions. Fig. 8 shows removal Rh B by Bi2O3-bentonite after 1st run achieved upto 98.5% after 80 min. After 4th run it decreases down to 91.7%. The catalytic activity was found to decrease slightly after 4th run. This decrease may be attributed to loss of reused catalyst during sampling each time and irreversible changes of the surface of the photocatalyst by dye molecules [37]. Fig. 8 shows that Bi2O3bentonite has excellent stability and does not suffer from photocorrosion during photodegradation. This reflects significant efficiency and practical applicability of Bi2O3-bentonite nanocomposite.
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Effect of pH on Rh B degradation
259
Photocatalytic degradation of dyes is significantly affected by pH of the dye solution [38,39]. The effect of pH is studied from pH 1 to 9 at 20mgL1 initial dye concentration for 1gL1 photocatalyst dose. It is observed that percent removal at pH 1 is 87.9%, then it increases upto 95.8% for pH 3 then it decreases down to 57.8% for pH 9. Several other authors also reported, the optimum pH for the removal of Rh B has been found to be about 3 [40].
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Degradation mechanism and identification of metabolites
267
Degradation of Rh B can be explained on the basis of AOPs promoted by heterogeneous photocatalysis. This process involves generation of e and h+ which again generates O2 and OH radicals respectively. These radicals are highly reactive and act as degrading agents. Moreover, photo-generated hydrogen atom from water is responsible for reductive degradation. Since Rh B is cationic dye and poor e donor, the initial step during its photodegradation is adsorption of dye on clay surface then e--hole pair get generated by absorption of light by Bi2O3 nanorods intercalated between bentonite clay structure. e and h+
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0.8
Bi2O3-bentonite Bentonite Bi2O3
0.6
A/Ao 0.4
0.2
0.0 0
20
40
60
80
Time (min)
120
Fig. 7. Photocatalytic degradation of Rh B (20mgL1) with Bentonite, Bi2O3 and Bi2O3-bentonite.
Table 2 Rh B removal efficiencies (%) of various methods. Nanomaterial BiVO4/reduced graphene oxide BiOBr/montmorillonite BiOBr-Graphene oxide BiOBr-Graphene Vis-Fe0-H2O2-citrate-O2 Bi2O3-bentonite
Removal efficiency (%)
Operation time (min)
Sources
98.5 98.9 95 87.4 62 98.5
600 120 45 135 120 80
[4] [8] [34] [35] [36] This study
100 Percent removal
226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244
Langmuir coefficient
KF (Lg1)
80 1st run
60
2nd run
40
3rd run 4th run
20 0 1st run
2nd run
3rd run
4th run
Cycles Fig. 8. Reusability performance of Bi2O3-bentonite nanocomposite.
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Percentage Removal
6
100 90 80 70 60 50 40 30 20 10 0
degradation pathway of Rh B is shown in Fig. 11a. Fig. 11a indicates the N-deethylation, ring opening of Rh B to form different degradation products including benzoic acid and benzonium ion which are relatively non-toxic compare to Rh B. The plausible mechanism for catalytic oxidation of Rh B on Bi2O3-bentonite nanocomposite photocatalyst is shown below. ½DyeðRh BÞ þ H2 O þ Bi2 O3 bentonite
adsorption
!
hv
Aqueous soluon
0.1 M i-PrOH
Fig. 9. Photocatlytic degradation of Rh B with addition of 0.1 M i-PrOH.
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292 293 294 295 296 297
again generate degrading agents, O2 and OH radicals. These are responsible for degradation of Rh B. Isopropanol is commonly used as diagnostic tool for OH mediated mechanisms [41–45]. The present study involves isopropanol as a scavenger. As direct oxidation of smaller alcohols by photogenerated holes is negligible, it is neglected. The results in Fig. 9 show that photodegradation of Rh B decreases down to 66.3% with the addition of 0.1 M i-PrOH as compare to reaction in aqueous medium. The inhibition of photodegradation of Rh B suggests that OH are predominantly active species in the photocatalytic degradation of Rh B. In order to understand the degradation pathway and intermediates that are formed during photocatalytic degradation of Rh B, LC-MS analysis was carried out. The mass spectra of Rh B before and after degradation are shown in Fig. 10. The probable
Rh BBi2 O3 bentonite
Bi2 O3 bentonite þ H2 O!e þ h
þ
þ H2 O
h ! OH
(4) 299 298 (5)
(6)
O2
e !O2
(7)
301 300
303 302
305 304
yields
Rh Bþ OH þ O2 ! CO2 þ H2 O þ Benzolc acid þ Benzonium ion
(8)
Mechanistic pathway of photodegradation of Rh B dye using Bi2O3–bentonite nanocomposite is shown in Fig. 11b. Initially clay is intercalated with higher d spacing. Catalyst Bi2O3 is attached onto surface of the clay platelets. With the irradiation of the visible light, there is generation of electron and hole which are attacking onto the Rh B dye molecule. Though the complete mineralization is not possible but the formation of the side products benzoic acid
Fig. 10. Mass spectra of Rh B solution (a) before and (b) after photodegradation.
Please cite this article in press as: S.P. Patil, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.12.002
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G Model
JIEC 2741 1–8 S.P. Patil et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
(a) O
+
H 3C
H 2N
Rh B
H 3C
m/z 104
H 2N
7
N
CH 3
O
N
+
NH2
HO
CH 3
m/z 114
NH
COOH COOH
m/z 196 H 2N
m/z 122
m/z 443
O
CH 3
CH 3
m/z 274
H 2N
H 3C
NH
O
NH
O
N
+
CH 3
COOH
COOH
m/z 415 m/z 362
(b)
Fig. 11. a The possible reaction intermediates after photocatalytic degradation under visible light irradiation. b Mechanistic pathway of photodegradation of Rh B dye using Bi2O3–Bentonite nanocomposite.
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and benzonium ion takes place which are less toxic than the Rh B dye.
317
Conclusion
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Bi2O3-bentonite nanocomposites have been successfully synthesized by sonochemical method and its photocatalytic performances under visible light are investigated. The results indicate that the Bi2O3-bentonite nanocomposite exhibits enhanced photocatalytic efficiency in Rh B removal than Bi2O3 and bentonite clay. Increase in photocatalytic efficiency is due to increased adsorption and decrease in recombination of photo e--hole pair in the Bi2O3 with bentonite. The semiconductor based photocatalyst, the Bi2O3 incorporated with bentonite clay can be a promising photocatalyst used effectively for the removal of dyes from wastewater. Dye removal is favoured at pH 3. Adsorption kinetics is found to follow pseudo-second-order kinetics. Whereas adsorption isotherm found to follow Freundlich isotherm. The
dimensionless factor RL values show the favourable adsorption process. The scavenging study by addition i-PrOH shows that, OH are the active species responsible for the photodegradation Rh B. LC-MS analysis shows the non generation of secondary waste after degradation of Rh B by Bi2O3-bentonite. Furthermore, catalyst reusability study reveals that reusable Bi2O3-bentonite nanocomposite has high stability, high mineralization efficiency. Thus Bi2O3-bentonite nanocomposite could be potential material for wastewater treatment.
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Acknowledgements
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The authors have gratefully acknowledged to Central Instrumentation Centre, University Institute of Chemical Technology, NMU, Jalgaon for SEM and XRD analysis. Authors are also thankful to Principal, Kisan College, Parola and Principal, G. T. Patil College, Nandurbar for providing necessary laboratory facilities.
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Please cite this article in press as: S.P. Patil, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.12.002
G Model
JIEC 2741 1–8 8
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