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carbon nanotubes composite catalyst for degradation of chlorpyrifos
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
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Effect of Zr+4 doping on characteristics and sonocatalytic activity of TiO2/ carbon nanotubes composite catalyst for degradation of chlorpyrifos
T
Asaad F. Hassana,b,∗, Hassan Elhadidya,c,d,∗ CEITEC – Central European Institute of Technology, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ 61662, Brno, Czech Republic b Chemistry Department, Faculty of Science, Damanhour University, Damanhour, Egypt c Faculty of Science, Physics Department, Mansoura University, Mansoura, 35516, Egypt d Faculty of Mathematics and Physics, Institute of Physics, Charles University, Ke Karlovu 5, Prague, CZ 121 16, Czech Republic a
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonocatalysis Nanotitania Carbon nanotube Composite Chlorpyrifos
Nanotitania/carbon nanotubes composites were prepared using the sol-gel method with different nanotitania: carbon nanotubes ratios. The composite sample was modified with zirconium cation (IV). The prepared composites were characterized with different techniques using thermogravimetric analysis (TGA), nitrogen adsorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). Sonocatalytic degradation efficiency of the prepared sonocatalysts was tested for sonocatalytic degradation of chlorpyrifos at different application conditions such as the effect of chlorpyrifos concentration, temperature, catalyst dosage, ultrasonic power and the presence of radiation light. Zirconium modified nanotitania/carbon nanotubes composite (ZCT10) shows the maximum sonocatalytic activity (91.5% after 60 min) and obeyed pseudo-first order kinetic models. The optimum degradation efficiency was also confirmed at 2.0 g/L as catalyst dosage and increase with both of application temperature and ultrasonic power. The presence of radiation light can also enhance the activity of sonocatalysts. Catalyst reusability experiments showed that catalysts are well reusable for degradation of chlorpyrifos by the sonocatalytic process.
1. Introduction Chlorpyrifos is one of the most widely used organophosphate insecticides [O, O-diethyl O-3, 5, 6-trichloro-2-pyridinyl phosphorothioate] due to its broad spectrum of activity [1]. It has been used in a variety of crops, including corn, soybeans, nuts, tree, alfalfa, citrus, wheat, peanuts and vegetables [2]. Degradation of chlorpyrifos in soil was found to be with half-lifetime about 10–120 days [3]. Hydrolysis of chlorpyrifos is accompanied by the formation of a compound known as 3, 5, 6-trichloro-2-pyridinol with adverse effects on the degradation of the parent compound and soil microbial activity [4]. Chlorpyrifos is responsible for the catalytic inhibition of acetylcholinesterase, which leads to in vivo accumulation of acetylcholine, resulting in the destruction of the nervous system and cell death [5]. Chlorpyrifos has been detected in air, rain, marine, streams, sediments, rivers, freshwater, fog and groundwater [6]. Degradation methods for organic pollutants such as oxidation are preferred because they can degrade organic pollutants rather than accumulating them on a certain medium
∗
creating another pollutant. Advanced oxidation processes such as heterogeneous Fenton catalysis, photocatalysis, and sonocatalysis are very important procedures in mineralization and decomposition of hazardous organic pollutants in water. Heterogeneous sonocatalysis is one of the most recent and promising advanced oxidation processes. Application of ultrasound irradiation to the organic aqueous solution is accompanied by the creation of acoustic cavitation, namely the formation, growth and implosive collapse of bubbles in solution. That bubbles collapse generates a localized hot spot with a temperature of about few thousand Kelvin and about one thousand atmospheric pressure [7] where nonvolatile organic pollutant molecules can hardly enter the bubbles for pyrolysis degradation. Also under the last conditions, decomposition of organic pollutants may be related to either direct pyrolysis and/or production of reactive radicals such as oxygen (•O), hydrogen (•H) and hydroxyl (•OH). The following chain reaction equations (Eq. (1 ─ 3) explain the production of active species [8,9]: H2O+ Ultrasound radiation → •H+•OH
Corresponding authors. Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ 61662, Brno, Czech Republic. E-mail addresses: [email protected] (A.F. Hassan), [email protected] (H. Elhadidy).
https://doi.org/10.1016/j.jpcs.2019.01.018 Received 5 October 2018; Received in revised form 10 December 2018; Accepted 14 January 2019 Available online 16 January 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
(1)
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
A.F. Hassan, H. Elhadidy
O2+ Ultrasound radiation →2•O •
O+H2O → 2HO
•
and the formed precipitate was stirred for 4 h and dried at 90 °C. The dried precipitate was heated at 550 °C for 3 h and stored after cooling as CT2, CT4, CT10, CT20 and CT50, respectively. ZCT10 was prepared by a method reported by Yean and Tsai [17,22] where an appropriate amount of Zr (NO3)4 was added to a mixture of CT10 powder and NaOH where the molar ratio of Zr: Ti was 0.005. The produced powder was heated at 300 °C for 3 h to get ZCT10.
(2) (3)
Addition of semiconductive catalysts (sonocatalysts) with the ultrasound radiation to enhance the performance of sonochemical degradation of organics due to additional nucleation sites for development of cavity microbubbles and increasing the generation of new reactive radicals and raise the rate of pyrolytic degradation. ZnO [10], CdS [11] and TiO2 [12] have been reported as efficient semiconductor sonocatalysts. Titanium dioxide is the most acceptable catalyst based on its chemical and biological inertness, non-toxicity and higher stability. The use of pure TiO2 as a sonocatalyst is limited mainly by the difficult separation of TiO2 particles from the solution after treatment and the recombination of the generated active holes and electrons, crystal growth and phase transformation. Many efforts have been made to avoid the last drawbacks in TiO2 applications such as doping with another inorganic material such as CdS [11], SiO2 [13], YAlO3 [14], Fe2O3 [15] and CNTs [16]. Doping of TiO2 sonocatalysts in suitable transition metals such as Fe+3 [17], La+3 [18], Cr+3 [19] and Cu+2/Zn+2 [20] improved its catalytic activity by decreasing band gap and increasing charge carriers lifetime. To the best of our knowledge, there were no studies in pertinent literature about the application of the sonocatalytic process for chlorpyrifos removal on the surface of Zr+4@ nanotitania/ carbon nanotube composites. In this report, titanium dioxide was combined with different ratios with multiwalled carbon nanotube and doped in zirconium cation solution to improve its catalytic efficiency. The prepared catalysts were characterized by TGA, nitrogen adsorption, XRD, TEM, EDX, FTIR, and DRS techniques. Sonocatalytic degradation of chlorpyrifos was studied using the prepared catalysts at different application conditions.
2.4. Characterization of sonocatalysts TGA for TiO2, CNTs, CT10 and ZCT10 were performed using thermoanalyzer apparatus (Shimadzu D-50, Japan) at a nitrogen flow rate of 50 mL/min and a heating rate of 10 °C/min up to 700 °C. Textural characterization for TiO2, CNTs, CT10, CT50, and ZCT10 was determined via nitrogen adsorption at ─196 °C using NOVA2000 gas sorption analyser (Quantachrome Corporation, USA) and systematic errors in the measurements were ∓ 0.1% of the measured values. The crystalline characteristics of TiO2, CNTs, CT10 and ZCT10 were determined using powder X-ray diffractometer (Shimadzu XD-1, Japan). Transmission electron microscopy (TEM) was conducted on a JEOLJEM-2100 (Tokyo, Japan) at an acceleration voltage of 200 kV to examine the size and shape of CNTs and sonocatalysts (TiO2, CT10, and ZCT10). EDX (energy dispersive X-ray spectra) was performed to determine the elemental information of TiO2, CNTs, CT10 and ZCT10. DRS (diffuse reflectance spectroscopy) analyses for TiO2, CT10, and ZCT10 were applied using a UV-vis scanning spectrophotometer (PerkinElmer Lambda 35) in the range 200–800 nm. Fourier transform infrared spectroscopy was performed for TiO2, CNTs, CT10 and ZCT10 in the range between 400 and 4000 cm−1 using a Mattson 5000 FTIR spectrometer. 2.5. Catalytic activity
2. Materials and methods
Sonocatalytic activity of catalysts was measured via degradation of chlorpyrifos aqueous solution using a controllable ultrasonic apparatus (Soniprep150, UK). Many experiments were carried out to indicate the effect of ultrasonic radiation time (5─ 90 min), catalyst dosage (0.5─ 2.5 g/L), power (50─ 200 W), temperature (20─ 40 °C) and the effect of light radiation using 400 W, UV/visible Hg lamp, Westinghouse, UAE. Chlorpyrifos concentration change was followed by HPLC (Alignet 1200 series). The optimum HPLC condition was achieved with water: acetonitrile (80: 20) as mobile phase with a flow rate of 1.5 mL/min, Zorbax Eclipse XDB-C18 column at 40 °C. The systematic errors in the concentration measurements by HPLC were ∓ 0.25 %. The measurements were repeated 3 times, and the average concentration values were used. The errors bars from both systematic and experimental errors were calculated and illustrated in different figures.
2.1. Materials Carbon nanotubes (CNTs), titanium (IV) isopropoxide, Zr (NO3)4 and cetyltrimethylammonium bromide (CTAB) were purchased from Alpha Aeser. Chlorpyrifos was purchased from Sigma-Aldrich. All reagents were used without further purification. 2.2. Oxidation of CNTs Oxidized carbon nanotubes were prepared by the method reported by Datsyuk et al. [21] where 0.5 g of CNTs was dispersed in 40 mL of NH4OH (25%) and H2O2 (30 wt %) in ratio1:1 in 150 mL round bottom flask fitted with a condenser. The last mixture was heated to 80 °C for 6 h. The previous solid was washed up till neutral washing solution and dried at 50 °C.
2.6. Catalyst reusability
2.3. Preparation of sonocatalysts
CT10 and ZCT10 were selected to test catalysts reusability after four sonocatalytic cycles where, 30 mg/L as chlorpyrifos initial concentration, 0.15 g of nanocatalyst, 100 mL as a volume of solution at 25 °C, and after a reaction time of 60 min were used as experimental conditions. After each cycle, the catalyst was washed with distilled water and dried at 80 °C for successive reuse.
TiO2 was prepared by dissolving 0.5 g of CTAB in 25 mL absolute ethanol and gentle stirring. Additions of 3.65 mL of titanium isopropoxide drop by drop to the last CTAB solution and the reaction mixture was stirred for two hours. Then 10 mL of distilled water added drop by drop to the solution till the formation of a white precipitate. The formed suspension solution was stirred for four hours and dried at 90 °C. The produced white precipitate annealed to the muffle at 550 °C for 3 h (TiO2). TiO2/CNTs catalysts (CT) were prepared by using a method as described by Lei Zhu [11] with slight modifications. Dissolving 0.5 g CTAB in 25 mL absolute ethanol and mixed with a certain weight of CNTs (20, 40, 100, 200 or 500 mg) and stirring for 2 h. Addition of 3.65 mL of titanium isopropoxide to the last solution and stirring for another 2 h followed by addition of 10 mL distilled water drop by drop
3. Results and discussion 3.1. Characterization of catalysts Fig. 1 A shows thermal degradation of TiO2, CNTs, CT10 and ZCT10. CNTs exhibited about 2% weight loss up to 120 °C due to evaporation of adsorbed water. The second stage of thermal degradation observed in the range from 120─ 350 °C is related to decarboxylation of the carboxylic groups [21]. At 500 °C weight loss reached to about 5.7% which 181
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Fig. 1. TGA curves (A), nitrogen adsorption isotherms at −196 °C and inserted PSD (B), linear BET plots (C) and XRD patterns (D) for TiO2, CNTs, and selected sonocatalysts.
effect of oxidation process in the removal of some impurities and creation of new CNTs cavities. Surface area for CT50 > CT10 (about 14.2%) indicating that surface area increase with the increase in CNTs contents. The decrease in CNTs surface area with the formation of TiO2/ CNTs composite may be related to the penetration of TiO2 particles in pores of CNTs. Total pore volume was found to be related directly to surface area where, VT for CNTs > CT50 > ZCT10 > CT10 > TiO2 (0.605, 0.389, 0.310, 0.279 and 0.055 cm3/g, respectively) which can be related to the porous nature of CNTs. Nonlocal density functional theory (NLDFT) and the data from nitrogen adsorption isotherms at 77 K [26] were used to calculate the pore size distribution from the isotherms (see the insets in Fig. 1B). Pore size distribution (PSD) plots were located at 2.42, 7.72, 5.90, 7.08 and 6.18 nm for TiO2, CNTs, CT10, CT50, and ZCT10 materials, respectively indicating the mesoporous nature of all the solid materials. Wide range XRD patterns for TiO2, CNTs, CT10 and ZCT10 are presented in Fig. 1D. Synthesized TiO2 exhibited its characteristic peaks at 2θ of 25.3, 40, 48 and 63° which were related to the characteristic (111), (004), (200) and (204) as planes of anatase TiO2, respectively (JCPDS 21-1272) [27]. CNTs exhibited characteristic peaks at 2θ = 25.8 and 42.8° which are related to (002) and (100) of the hexagonal graphite [28]. XRD pattern of CT10 and ZCT10 is approximately the same as TiO2 indicating that CNTs without any effect on the crystal form of TiO2 as confirmed in previous works [11,16]. Fig. 2 shows the morphology of oxidized CNTs arrays as layer-bylayer with hollow structure and the EDX analysis for oxidized CNTs indicates a higher purity of the sample with 97.4% of carbon content
is attributed to the decomposition of hydroxyl function groups on the surface of CNTs [23] while the last stage in degradation starting from 500 to 700 °C is attributed to thermal oxidation of disordered carbon atoms [24]. Nanotitania TGA curve shows a lower moisture content at T < 120 °C compared with CNTs due to the absence of polar surface function groups but exhibited weight loss in the range 150─ 450 °C (9.3% weight loss) which is related to the decomposition of organic compounds occluded in TiO2 gel [25]. CT10 and ZCT10 show a trend in thermal stability between TiO2 and CNTs except for the higher moisture content in case of ZCT10 (5% at 120 °C). It can be related to the incorporation of zirconium atoms (Lewis acidic sites) in the matrix of ZCT10 which are responsible for raising moisture surface adsorption. Nitrogen adsorption isotherms, pore size distribution (PSD) and Linear BET plot are presented in Fig. 1B and C for CNTs, TiO2, CT10, CT50, and ZCT10. All curves are classified as type II according to IUPAC. Surface area, total pore volume, and pore diameter are listed in Table 1. BET surface area of TiO2 and CNTs are 70.2 and 313.3 m2/g, respectively. The higher surface area for CNTs can be related to the
Table 1 Textural characterization of TiO2, CNTs, CT10, CT50 and ZCT10 derived from nitrogen adsorption isotherms. Textural parameters
TiO2
CNTs
CT10
CT50
ZCT10
SBET(m2/g) VT(cm3/g) Pore diameter(nm)
70.2 0.055 2.42
313.3 0.605 7.72
189.2 0.279 5.90
216.2 0.389 7.08
207.4 0.310 6.18
182
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Fig. 2. TEM images (a, b, c, d) and EDX elemental microanalysis (e, f, g, h) for CNTs, TiO2, CT10, and ZCT10, respectively.
which may be related to the increase in surface electric charge of the composite [10,30]. The introduction of zirconium ion also reduces the band energy gap of ZCT10 and the expected catalyst efficiency will increase due to the higher electrical charge of the incorporated Zr+4. Chemical function groups on oxidized CNTs, TiO2, CT10, and ZCT10 were examined using FTIR. Fig. 3B shows that CNTs after oxidation have various types of C=O function groups. The peaks located at 895, 1190 and 1720 cm−1 were attributed to C‒H, C‒O, and C=O stretching vibration, respectively [16]. The spectrum of TiO2 showed a strong absorption band at 480 cm−1 due to the stretching vibration of O‒Ti‒O while the bands at 1630 and 3400 cm−1 are related to the bending vibration of surface hydroxyl groups from adsorbed water molecules [17,31]. For CT10 and ZCT10 there is an observable decrease in the intensity of absorption peaks of CNTs at 1190 and 1720 cm−1which indicate the effect of the TiO2 particle.
and 2.2% of oxygen content due to oxidation process while the others are residual impurities due to preparation and oxidation procedures. TEM image showed that TiO2 has a granular solid shape with about 20 nm particle sizes and its EDX analysis confirms higher purity. TEM images and EDX analysis for CT10 and ZCT10 solid catalysts confirms the attachment of TiO2 nanoparticles to the surface of CNTs and there is no observable difference between the previous two samples except the presence of zirconium metal cation (0.4%) in case of ZCT10 sample. Fig. 3A displays the diffuse reflectance spectra of TiO2, CT10, and ZCT10. Equation (4) defines the threshold wavelength required for excitation of a semiconductor [29].
λ (nm) =
1240 Ebg (ev )
(4)
Applying the previous equation indicated that energy band gap (Ebg) for TiO2, CT10, and ZCT10 are 3.32, 3.01 and 2.90 eV, respectively. It is also observed that the incorporation of CNTs in CT10 nanocomposite raised its catalytic efficiency due to the reduction in band energy gaps
Fig. 3. DRS (A) of TiO2, CT10, and ZCT10 and FTIR (B) of CNTs, TiO2, CT10 and ZCT10. 183
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Fig. 4. Degradation efficiency of chlorpyrifos with and without sonocatalysts (A) and PFO kinetic plots (B) at 25 °C.
Table 2 where, regression coefficient (R2) around 0.9760 indicating the good applicability of PFO models and the apparent rate constant (Kapp) ranged between 0.00765 and 0.03185 min−1 and increased with the increase in the efficacy of chlorpyrifos degradation in the same order of catalyst degradation efficiencies where Kapp for ZCT10 > Kapp for TiO2 by about four times. The introductions of CNTs to TiO2 raise the catalytic activity of composite till some extents. In particular, the maximum sonocatalytic degradation of chlorpyrifos was confirmed in the presence of CT10. The presence of CNTs enhances the free movement of the separated electrons through the tubes and reduces the possibilities of recombination [16]. At higher CNTs contents in the sonocatalysts, the degradation of chlorpyrifos is decreased which may be related to the decrease in TiO2 contents and the strong adsorption effect of CNTs function groups towards chlorpyrifos rather than degradation in the generated hot spots involved in sonocatalysis degradation. Modification of CT10 with Zr+4 (ZCT10) enhances its degradation efficiency based on the ability of Zr+4 to promote the separation of the electron-hole pairs in dispersed particles of TiO2 by acting as an electron trap (Zr+4 +e− → Zr+3) and hole trap (Zr+4 +h+ → Zr+4) [16,18].
3.2. Sonocatalytic degradation of chlorpyrifos Before starting the sonocatalysis examination, mixtures of chlorpyrifos solution and catalysts were stirred for 30 min to control equilibrium adsorption. Fig. 4A shows the adsorption and sonocatalytic degradation of chlorpyrifos in absence and presence of ultrasonic radiation, respectively using TiO2, CT2, CT4, CT10, CT20, CT50, and ZCT10 at 25 °C. The adsorption of chlorpyrifos during 30 min was found to be very small with optimum removal value about 3% in case of TiO2 and increase with the increase in CNTs content till reach 17% in case of CT50 and ZCT10. In the absence of any catalyst, the degradation (sonolysis) was found to be 29% after 60 min. In the presence of sonocatalysts the degradation increases in the order of ZCT10 > CT10 > CT20 > CT50 > CT4 > CT2 > TiO2 where the degradation efficiency reaches 91.5% in the case of ZCT10 after 60 min compared with 43% after the same time in the case of TiO2 and 75% in the presence of CT10. Sonocatalytic degradation of chlorpyrifos by sonocatalysts obeyed pseudo-first order kinetic models with respect to chlorpyrifos concentration (eq. (5))
Log(C0 /Ct ) =
1 K app t 2.303
3.3. Effect of operational conditions on sonocatalytic degradation of chlorpyrifos
(5)
Where, Co and Ct are the initial and time concentration of chlorpyrifos at time t, respectively. Fig. 4 B predicts the relation between time (min) against log(Co/Ct) at 25 °C. PFO parameter collected in
3.3.1. Effect of catalyst dosage Different dosages of ZCT10 (0.5–2.5 g/L) were performed to obtain
Table 2 PFO kinetic parameters for sonocatalytic degradation of chlorpyrifos in the presence of sonocatalysts at 25 °C & PFO kinetic model and Arrhenius parameters for chlorpyrifos degradation in the presence of TiO2, CT10 and ZCT10 at 20, 30 and 40 °C. Kinetic parameters
TiO2
CT2
CT4
CT10
CT20
CT50
ZCT10
R2 Kapp (min-1)
0.9649 0.00765
0.9780 0.01039
0.9734 0.01101
0.9726 0.01781
0.9739 0.01439
0.9845 0.01269
0.9786 0.03185
TiO2
2
R Kapp(min-1)
CT10
ZCT10
20 °C
30 °C
40 °C
20 °C
30 °C
40 °C
20 °C
30 °C
40 °C
0.9425 0.00706
0.9762 0.00828
0.9527 0.00918
0.9753 0.00158
0.9745 0.02468
0.9863 0.02532
0.9730 0.03483
0.9489 0.03911
0.9385 0.04886
Arrhenius parameters
R2 Ea(kJ/mol)
TiO2
CT10
ZCT10
0.9919 12.1
0.9243 11.8
0.9998 8.6
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Fig. 5. Sonocatalytic degradation of chlorpyrifos at different catalyst dosages (A) in the presence of ZCT10, sonocatalytic degradation of chlorpyrifos (B), PFO kinetic plots (C) and linear plot of Arrhenius model (D) using TiO2, CT10 and ZCT10 at 20, 30, and 40 °C.
the optimum sonocatalysts dosage (Fig. 5A). From Fig. 5A we observed that as the catalyst dosages increase the degradation of chlorpyrifos also increase where the degradation efficiency after 45 min about 73% and 94% using 0.5 and 2.0 g/L, respectively. The previous result can be related to the production of more reactive radicals and more cavitation microbubbles [32] and the increase in the local temperature at cavity collapse because of the presence of more nucleation sites for the cavitation bubbles which enhance the pyrolysis of water molecules to form hydroxyl radicals [33,34]. Catalyst dosage 2.5 g/L gave the same efficiency as 2.0 g/L dosage which may be related to aggregation of sonocatalysts particles, higher adsorption factor and the blocking of transmission of ultrasound waves near the catalyst [17,35]. From the effect of catalyst dosage, we choose 2.0 g/L as the optimum value for applications.
enhancement of the reaction rate between radicals and chlorpyrifos [37]. The increase in the sonocatalysis application temperature may decrease the cavitation energy and facilitate the formation of cavitation bubbles and raising the rate of free radicals production (•OH and •OOH) [38]. Upon analysis of PFO parameters at different application temperatures (Table 2): (i) regression coefficient, R2, ranged between 0.9385 and 0.9863 for all the investigated sonocatalysts at all the application temperatures indicating the good applicability of PFO model. (ii) For all the investigated sonocatalysts as temperature increases, the rate constant for degradation also increases. Applying Arrhenius equation (eq. (6), Fig. 5D) we can calculate the apparent activation energy, Ea (J/mol):
3.3.2. Effect of application temperature Fig. 5B and C, D illustrate the effect of temperature on the catalytic degradation of chlorpyrifos using TiO2, CT10, and ZCT10 at 20, 30 and 40 °C. It is observed that as temperature increases the catalytic efficiency slightly increases in all the investigated catalysts (Fig. 5B). In particular, after 60 min degradation efficiencies were found to be 91.5, 95.4 and 98.7% at 20, 30 and 40 °C, respectively. Fig. 5C shows the application of the PFO kinetic model at different temperatures and the calculated parameters are collected in Table 2. The increase in sonocatalytic degradation of chlorpyrifos with temperature may be related to many factors, such as, the increase in water vapor pressure which causes a less violent collapse of bubbles [36]. Also, the increase in the mass transfer of radicals increased which is accompanied by
Here, R (8.314 J/mol K) is the gas constant, A is a pre-exponential factor (min−1). Values in Table 2 indicate a good applicability of Arrhenius model (as supported by higher regression coefficients, 0.9243–0.9998). The calculated activation energy values for sonocatalytic degradation of chlorpyrifos in the presence of TiO2, CT10 and ZCT10 were 12.1, 11.8 and 8.6 kJ/mol, respectively. The lower Ea value in case of ZCT10 is accompanied by higher reaction rate due to the lower energy required to break down bonds in chlorpyrifos molecules [39].
LnK app = −
Ea + lnA RT
(6)
3.3.3. Effect of power on degradation efficiency It is known that most of the used power in sonocatalysis is transferred into heat as the main factor in the production of •OH and •H from 185
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
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Fig. 6. Effect of power on sonocatalytic degradation of chlorpyrifos (A) and sonocatalytic degradation of chlorpyrifos in the presence and absence of radiation light (B) using ZCT10.
water. The effect of applied power of ultrasonic generator on the degradation of chlorpyrifos was investigated at 50, 100, 150 and 200 W in the presence of ZCT10 as a sonocatalysts (Fig. 6A). The degradation efficiency increased with the power of ultrasonic generator, which explained by (i) disorder of solution increase with power is accompanied with the increase in the rate of mass transfer of chlorpyrifos molecules, radicals and degradation byproducts between solution bulk and catalyst surface [12,40]. (ii) The increase in the number of cavitation bubbles leading to more free radicals (•OH) [41]. (iii) Enhancement of active sites on the surface of the catalyst due to cleaning action of higher applied power [42]. (iv) The increase in acoustic energy with power leads to an increase in pressure, temperature and the collapse time [41,43,44]. 3.3.4. Effect of light radiation on sonocatalytic degradation efficiency The combination of light radiation and ultrasonic radiation was investigated using ZCT10. Fig. 6 B shows the effect of light radiation on chlorpyrifos sonocatalytic degradation which predicts the increase in the degradation rate of organic pollutant with a light beside ultrasonic radiations. After 90 min the efficiency of degradation in the presence and absence of light was 100 and 95%, respectively. Several reasons stand behind the beneficial effect of sonocatalysis and photocatalysis coupling such as (i) the increase in •OH production [10] (ii) production of conduction band electrons and valence band energy when catalyst irradiated with radiation light with energy equals or more than the energy band gap of catalyst [45].
Fig. 7. Catalyst reusability studies for sonocatalytic degradation of chlorpyrifos in the presence of CT10 and ZCT10 at 25 °C.
sonocatalysts was found to increase with increasing the catalyst dosage till 2.0 g/L. As application temperature and ultrasonic power increase the efficiency of degradation also increases. The presence of radiation light raises the efficiency of degradation which reaches 100% after 90 min (in the case of ZCT10) in the presence of radiation light compared with 95% in the absence. Catalyst reusability data showed that the prepared solid materials are well reusable nanocatalyst for sonocatalytic degradation of chlorpyrifos even after many application cycles.
3.4. Catalyst reusability Catalysts reusability was investigated after four cycles of sonocatalysis. Reusability results are shown in Fig. 7. Catalyst efficiency slightly decreases with catalyst reusing where after four application cycles the decrease in CT10 and ZCT10 efficiency was found to be 0.6 and 0.4%, respectively as reported by many authors [46–48]. The slight decrease in catalytic activity may be related to the slight decrease in surface area due to the coagulation of nanoparticles [49].
Prime novelty statement In this manuscript, we prepared Zr+4@ Nanotitania/carbon nanotube composites using sol-gel method. The prepared samples were characterized with different tools. Degradation of chlorpyrifos was studied using sonocatalytic mechanism under different application conditions. To the best of our knowledge, application of sonocatalytic process for chlorpyrifos removal on the surface of Zr+4@ Nanotitania/ carbon nanotube composites was not studied up to now.
4. Conclusion The present study showed that modification of nanotitania/carbon nanotubes composite with Zr+4 reduces its energy band gap and promotes the separation of the electron-hole pairs in dispersed particles of TiO2 followed by raising its sonocatalytic efficiency to the extent of 91.5% degradation after 60 min. The degradation efficiency of
Acknowledgment This research was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the Project CEITEC 2020 186
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
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(Project No. LQ1601) and by the Academy of Sciences of the Czech Republic (Institutional Project No. RVO:68081723).
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of Zr+4 doping on characteristics and sonocatalytic activity of TiO2/ carbon nanotubes composite catalyst for degradation of chlorpyrifos
T
Asaad F. Hassana,b,∗, Hassan Elhadidya,c,d,∗ CEITEC – Central European Institute of Technology, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ 61662, Brno, Czech Republic b Chemistry Department, Faculty of Science, Damanhour University, Damanhour, Egypt c Faculty of Science, Physics Department, Mansoura University, Mansoura, 35516, Egypt d Faculty of Mathematics and Physics, Institute of Physics, Charles University, Ke Karlovu 5, Prague, CZ 121 16, Czech Republic a
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonocatalysis Nanotitania Carbon nanotube Composite Chlorpyrifos
Nanotitania/carbon nanotubes composites were prepared using the sol-gel method with different nanotitania: carbon nanotubes ratios. The composite sample was modified with zirconium cation (IV). The prepared composites were characterized with different techniques using thermogravimetric analysis (TGA), nitrogen adsorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). Sonocatalytic degradation efficiency of the prepared sonocatalysts was tested for sonocatalytic degradation of chlorpyrifos at different application conditions such as the effect of chlorpyrifos concentration, temperature, catalyst dosage, ultrasonic power and the presence of radiation light. Zirconium modified nanotitania/carbon nanotubes composite (ZCT10) shows the maximum sonocatalytic activity (91.5% after 60 min) and obeyed pseudo-first order kinetic models. The optimum degradation efficiency was also confirmed at 2.0 g/L as catalyst dosage and increase with both of application temperature and ultrasonic power. The presence of radiation light can also enhance the activity of sonocatalysts. Catalyst reusability experiments showed that catalysts are well reusable for degradation of chlorpyrifos by the sonocatalytic process.
1. Introduction Chlorpyrifos is one of the most widely used organophosphate insecticides [O, O-diethyl O-3, 5, 6-trichloro-2-pyridinyl phosphorothioate] due to its broad spectrum of activity [1]. It has been used in a variety of crops, including corn, soybeans, nuts, tree, alfalfa, citrus, wheat, peanuts and vegetables [2]. Degradation of chlorpyrifos in soil was found to be with half-lifetime about 10–120 days [3]. Hydrolysis of chlorpyrifos is accompanied by the formation of a compound known as 3, 5, 6-trichloro-2-pyridinol with adverse effects on the degradation of the parent compound and soil microbial activity [4]. Chlorpyrifos is responsible for the catalytic inhibition of acetylcholinesterase, which leads to in vivo accumulation of acetylcholine, resulting in the destruction of the nervous system and cell death [5]. Chlorpyrifos has been detected in air, rain, marine, streams, sediments, rivers, freshwater, fog and groundwater [6]. Degradation methods for organic pollutants such as oxidation are preferred because they can degrade organic pollutants rather than accumulating them on a certain medium
∗
creating another pollutant. Advanced oxidation processes such as heterogeneous Fenton catalysis, photocatalysis, and sonocatalysis are very important procedures in mineralization and decomposition of hazardous organic pollutants in water. Heterogeneous sonocatalysis is one of the most recent and promising advanced oxidation processes. Application of ultrasound irradiation to the organic aqueous solution is accompanied by the creation of acoustic cavitation, namely the formation, growth and implosive collapse of bubbles in solution. That bubbles collapse generates a localized hot spot with a temperature of about few thousand Kelvin and about one thousand atmospheric pressure [7] where nonvolatile organic pollutant molecules can hardly enter the bubbles for pyrolysis degradation. Also under the last conditions, decomposition of organic pollutants may be related to either direct pyrolysis and/or production of reactive radicals such as oxygen (•O), hydrogen (•H) and hydroxyl (•OH). The following chain reaction equations (Eq. (1 ─ 3) explain the production of active species [8,9]: H2O+ Ultrasound radiation → •H+•OH
Corresponding authors. Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ 61662, Brno, Czech Republic. E-mail addresses: [email protected] (A.F. Hassan), [email protected] (H. Elhadidy).
https://doi.org/10.1016/j.jpcs.2019.01.018 Received 5 October 2018; Received in revised form 10 December 2018; Accepted 14 January 2019 Available online 16 January 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
(1)
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
A.F. Hassan, H. Elhadidy
O2+ Ultrasound radiation →2•O •
O+H2O → 2HO
•
and the formed precipitate was stirred for 4 h and dried at 90 °C. The dried precipitate was heated at 550 °C for 3 h and stored after cooling as CT2, CT4, CT10, CT20 and CT50, respectively. ZCT10 was prepared by a method reported by Yean and Tsai [17,22] where an appropriate amount of Zr (NO3)4 was added to a mixture of CT10 powder and NaOH where the molar ratio of Zr: Ti was 0.005. The produced powder was heated at 300 °C for 3 h to get ZCT10.
(2) (3)
Addition of semiconductive catalysts (sonocatalysts) with the ultrasound radiation to enhance the performance of sonochemical degradation of organics due to additional nucleation sites for development of cavity microbubbles and increasing the generation of new reactive radicals and raise the rate of pyrolytic degradation. ZnO [10], CdS [11] and TiO2 [12] have been reported as efficient semiconductor sonocatalysts. Titanium dioxide is the most acceptable catalyst based on its chemical and biological inertness, non-toxicity and higher stability. The use of pure TiO2 as a sonocatalyst is limited mainly by the difficult separation of TiO2 particles from the solution after treatment and the recombination of the generated active holes and electrons, crystal growth and phase transformation. Many efforts have been made to avoid the last drawbacks in TiO2 applications such as doping with another inorganic material such as CdS [11], SiO2 [13], YAlO3 [14], Fe2O3 [15] and CNTs [16]. Doping of TiO2 sonocatalysts in suitable transition metals such as Fe+3 [17], La+3 [18], Cr+3 [19] and Cu+2/Zn+2 [20] improved its catalytic activity by decreasing band gap and increasing charge carriers lifetime. To the best of our knowledge, there were no studies in pertinent literature about the application of the sonocatalytic process for chlorpyrifos removal on the surface of Zr+4@ nanotitania/ carbon nanotube composites. In this report, titanium dioxide was combined with different ratios with multiwalled carbon nanotube and doped in zirconium cation solution to improve its catalytic efficiency. The prepared catalysts were characterized by TGA, nitrogen adsorption, XRD, TEM, EDX, FTIR, and DRS techniques. Sonocatalytic degradation of chlorpyrifos was studied using the prepared catalysts at different application conditions.
2.4. Characterization of sonocatalysts TGA for TiO2, CNTs, CT10 and ZCT10 were performed using thermoanalyzer apparatus (Shimadzu D-50, Japan) at a nitrogen flow rate of 50 mL/min and a heating rate of 10 °C/min up to 700 °C. Textural characterization for TiO2, CNTs, CT10, CT50, and ZCT10 was determined via nitrogen adsorption at ─196 °C using NOVA2000 gas sorption analyser (Quantachrome Corporation, USA) and systematic errors in the measurements were ∓ 0.1% of the measured values. The crystalline characteristics of TiO2, CNTs, CT10 and ZCT10 were determined using powder X-ray diffractometer (Shimadzu XD-1, Japan). Transmission electron microscopy (TEM) was conducted on a JEOLJEM-2100 (Tokyo, Japan) at an acceleration voltage of 200 kV to examine the size and shape of CNTs and sonocatalysts (TiO2, CT10, and ZCT10). EDX (energy dispersive X-ray spectra) was performed to determine the elemental information of TiO2, CNTs, CT10 and ZCT10. DRS (diffuse reflectance spectroscopy) analyses for TiO2, CT10, and ZCT10 were applied using a UV-vis scanning spectrophotometer (PerkinElmer Lambda 35) in the range 200–800 nm. Fourier transform infrared spectroscopy was performed for TiO2, CNTs, CT10 and ZCT10 in the range between 400 and 4000 cm−1 using a Mattson 5000 FTIR spectrometer. 2.5. Catalytic activity
2. Materials and methods
Sonocatalytic activity of catalysts was measured via degradation of chlorpyrifos aqueous solution using a controllable ultrasonic apparatus (Soniprep150, UK). Many experiments were carried out to indicate the effect of ultrasonic radiation time (5─ 90 min), catalyst dosage (0.5─ 2.5 g/L), power (50─ 200 W), temperature (20─ 40 °C) and the effect of light radiation using 400 W, UV/visible Hg lamp, Westinghouse, UAE. Chlorpyrifos concentration change was followed by HPLC (Alignet 1200 series). The optimum HPLC condition was achieved with water: acetonitrile (80: 20) as mobile phase with a flow rate of 1.5 mL/min, Zorbax Eclipse XDB-C18 column at 40 °C. The systematic errors in the concentration measurements by HPLC were ∓ 0.25 %. The measurements were repeated 3 times, and the average concentration values were used. The errors bars from both systematic and experimental errors were calculated and illustrated in different figures.
2.1. Materials Carbon nanotubes (CNTs), titanium (IV) isopropoxide, Zr (NO3)4 and cetyltrimethylammonium bromide (CTAB) were purchased from Alpha Aeser. Chlorpyrifos was purchased from Sigma-Aldrich. All reagents were used without further purification. 2.2. Oxidation of CNTs Oxidized carbon nanotubes were prepared by the method reported by Datsyuk et al. [21] where 0.5 g of CNTs was dispersed in 40 mL of NH4OH (25%) and H2O2 (30 wt %) in ratio1:1 in 150 mL round bottom flask fitted with a condenser. The last mixture was heated to 80 °C for 6 h. The previous solid was washed up till neutral washing solution and dried at 50 °C.
2.6. Catalyst reusability
2.3. Preparation of sonocatalysts
CT10 and ZCT10 were selected to test catalysts reusability after four sonocatalytic cycles where, 30 mg/L as chlorpyrifos initial concentration, 0.15 g of nanocatalyst, 100 mL as a volume of solution at 25 °C, and after a reaction time of 60 min were used as experimental conditions. After each cycle, the catalyst was washed with distilled water and dried at 80 °C for successive reuse.
TiO2 was prepared by dissolving 0.5 g of CTAB in 25 mL absolute ethanol and gentle stirring. Additions of 3.65 mL of titanium isopropoxide drop by drop to the last CTAB solution and the reaction mixture was stirred for two hours. Then 10 mL of distilled water added drop by drop to the solution till the formation of a white precipitate. The formed suspension solution was stirred for four hours and dried at 90 °C. The produced white precipitate annealed to the muffle at 550 °C for 3 h (TiO2). TiO2/CNTs catalysts (CT) were prepared by using a method as described by Lei Zhu [11] with slight modifications. Dissolving 0.5 g CTAB in 25 mL absolute ethanol and mixed with a certain weight of CNTs (20, 40, 100, 200 or 500 mg) and stirring for 2 h. Addition of 3.65 mL of titanium isopropoxide to the last solution and stirring for another 2 h followed by addition of 10 mL distilled water drop by drop
3. Results and discussion 3.1. Characterization of catalysts Fig. 1 A shows thermal degradation of TiO2, CNTs, CT10 and ZCT10. CNTs exhibited about 2% weight loss up to 120 °C due to evaporation of adsorbed water. The second stage of thermal degradation observed in the range from 120─ 350 °C is related to decarboxylation of the carboxylic groups [21]. At 500 °C weight loss reached to about 5.7% which 181
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
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Fig. 1. TGA curves (A), nitrogen adsorption isotherms at −196 °C and inserted PSD (B), linear BET plots (C) and XRD patterns (D) for TiO2, CNTs, and selected sonocatalysts.
effect of oxidation process in the removal of some impurities and creation of new CNTs cavities. Surface area for CT50 > CT10 (about 14.2%) indicating that surface area increase with the increase in CNTs contents. The decrease in CNTs surface area with the formation of TiO2/ CNTs composite may be related to the penetration of TiO2 particles in pores of CNTs. Total pore volume was found to be related directly to surface area where, VT for CNTs > CT50 > ZCT10 > CT10 > TiO2 (0.605, 0.389, 0.310, 0.279 and 0.055 cm3/g, respectively) which can be related to the porous nature of CNTs. Nonlocal density functional theory (NLDFT) and the data from nitrogen adsorption isotherms at 77 K [26] were used to calculate the pore size distribution from the isotherms (see the insets in Fig. 1B). Pore size distribution (PSD) plots were located at 2.42, 7.72, 5.90, 7.08 and 6.18 nm for TiO2, CNTs, CT10, CT50, and ZCT10 materials, respectively indicating the mesoporous nature of all the solid materials. Wide range XRD patterns for TiO2, CNTs, CT10 and ZCT10 are presented in Fig. 1D. Synthesized TiO2 exhibited its characteristic peaks at 2θ of 25.3, 40, 48 and 63° which were related to the characteristic (111), (004), (200) and (204) as planes of anatase TiO2, respectively (JCPDS 21-1272) [27]. CNTs exhibited characteristic peaks at 2θ = 25.8 and 42.8° which are related to (002) and (100) of the hexagonal graphite [28]. XRD pattern of CT10 and ZCT10 is approximately the same as TiO2 indicating that CNTs without any effect on the crystal form of TiO2 as confirmed in previous works [11,16]. Fig. 2 shows the morphology of oxidized CNTs arrays as layer-bylayer with hollow structure and the EDX analysis for oxidized CNTs indicates a higher purity of the sample with 97.4% of carbon content
is attributed to the decomposition of hydroxyl function groups on the surface of CNTs [23] while the last stage in degradation starting from 500 to 700 °C is attributed to thermal oxidation of disordered carbon atoms [24]. Nanotitania TGA curve shows a lower moisture content at T < 120 °C compared with CNTs due to the absence of polar surface function groups but exhibited weight loss in the range 150─ 450 °C (9.3% weight loss) which is related to the decomposition of organic compounds occluded in TiO2 gel [25]. CT10 and ZCT10 show a trend in thermal stability between TiO2 and CNTs except for the higher moisture content in case of ZCT10 (5% at 120 °C). It can be related to the incorporation of zirconium atoms (Lewis acidic sites) in the matrix of ZCT10 which are responsible for raising moisture surface adsorption. Nitrogen adsorption isotherms, pore size distribution (PSD) and Linear BET plot are presented in Fig. 1B and C for CNTs, TiO2, CT10, CT50, and ZCT10. All curves are classified as type II according to IUPAC. Surface area, total pore volume, and pore diameter are listed in Table 1. BET surface area of TiO2 and CNTs are 70.2 and 313.3 m2/g, respectively. The higher surface area for CNTs can be related to the
Table 1 Textural characterization of TiO2, CNTs, CT10, CT50 and ZCT10 derived from nitrogen adsorption isotherms. Textural parameters
TiO2
CNTs
CT10
CT50
ZCT10
SBET(m2/g) VT(cm3/g) Pore diameter(nm)
70.2 0.055 2.42
313.3 0.605 7.72
189.2 0.279 5.90
216.2 0.389 7.08
207.4 0.310 6.18
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Fig. 2. TEM images (a, b, c, d) and EDX elemental microanalysis (e, f, g, h) for CNTs, TiO2, CT10, and ZCT10, respectively.
which may be related to the increase in surface electric charge of the composite [10,30]. The introduction of zirconium ion also reduces the band energy gap of ZCT10 and the expected catalyst efficiency will increase due to the higher electrical charge of the incorporated Zr+4. Chemical function groups on oxidized CNTs, TiO2, CT10, and ZCT10 were examined using FTIR. Fig. 3B shows that CNTs after oxidation have various types of C=O function groups. The peaks located at 895, 1190 and 1720 cm−1 were attributed to C‒H, C‒O, and C=O stretching vibration, respectively [16]. The spectrum of TiO2 showed a strong absorption band at 480 cm−1 due to the stretching vibration of O‒Ti‒O while the bands at 1630 and 3400 cm−1 are related to the bending vibration of surface hydroxyl groups from adsorbed water molecules [17,31]. For CT10 and ZCT10 there is an observable decrease in the intensity of absorption peaks of CNTs at 1190 and 1720 cm−1which indicate the effect of the TiO2 particle.
and 2.2% of oxygen content due to oxidation process while the others are residual impurities due to preparation and oxidation procedures. TEM image showed that TiO2 has a granular solid shape with about 20 nm particle sizes and its EDX analysis confirms higher purity. TEM images and EDX analysis for CT10 and ZCT10 solid catalysts confirms the attachment of TiO2 nanoparticles to the surface of CNTs and there is no observable difference between the previous two samples except the presence of zirconium metal cation (0.4%) in case of ZCT10 sample. Fig. 3A displays the diffuse reflectance spectra of TiO2, CT10, and ZCT10. Equation (4) defines the threshold wavelength required for excitation of a semiconductor [29].
λ (nm) =
1240 Ebg (ev )
(4)
Applying the previous equation indicated that energy band gap (Ebg) for TiO2, CT10, and ZCT10 are 3.32, 3.01 and 2.90 eV, respectively. It is also observed that the incorporation of CNTs in CT10 nanocomposite raised its catalytic efficiency due to the reduction in band energy gaps
Fig. 3. DRS (A) of TiO2, CT10, and ZCT10 and FTIR (B) of CNTs, TiO2, CT10 and ZCT10. 183
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Fig. 4. Degradation efficiency of chlorpyrifos with and without sonocatalysts (A) and PFO kinetic plots (B) at 25 °C.
Table 2 where, regression coefficient (R2) around 0.9760 indicating the good applicability of PFO models and the apparent rate constant (Kapp) ranged between 0.00765 and 0.03185 min−1 and increased with the increase in the efficacy of chlorpyrifos degradation in the same order of catalyst degradation efficiencies where Kapp for ZCT10 > Kapp for TiO2 by about four times. The introductions of CNTs to TiO2 raise the catalytic activity of composite till some extents. In particular, the maximum sonocatalytic degradation of chlorpyrifos was confirmed in the presence of CT10. The presence of CNTs enhances the free movement of the separated electrons through the tubes and reduces the possibilities of recombination [16]. At higher CNTs contents in the sonocatalysts, the degradation of chlorpyrifos is decreased which may be related to the decrease in TiO2 contents and the strong adsorption effect of CNTs function groups towards chlorpyrifos rather than degradation in the generated hot spots involved in sonocatalysis degradation. Modification of CT10 with Zr+4 (ZCT10) enhances its degradation efficiency based on the ability of Zr+4 to promote the separation of the electron-hole pairs in dispersed particles of TiO2 by acting as an electron trap (Zr+4 +e− → Zr+3) and hole trap (Zr+4 +h+ → Zr+4) [16,18].
3.2. Sonocatalytic degradation of chlorpyrifos Before starting the sonocatalysis examination, mixtures of chlorpyrifos solution and catalysts were stirred for 30 min to control equilibrium adsorption. Fig. 4A shows the adsorption and sonocatalytic degradation of chlorpyrifos in absence and presence of ultrasonic radiation, respectively using TiO2, CT2, CT4, CT10, CT20, CT50, and ZCT10 at 25 °C. The adsorption of chlorpyrifos during 30 min was found to be very small with optimum removal value about 3% in case of TiO2 and increase with the increase in CNTs content till reach 17% in case of CT50 and ZCT10. In the absence of any catalyst, the degradation (sonolysis) was found to be 29% after 60 min. In the presence of sonocatalysts the degradation increases in the order of ZCT10 > CT10 > CT20 > CT50 > CT4 > CT2 > TiO2 where the degradation efficiency reaches 91.5% in the case of ZCT10 after 60 min compared with 43% after the same time in the case of TiO2 and 75% in the presence of CT10. Sonocatalytic degradation of chlorpyrifos by sonocatalysts obeyed pseudo-first order kinetic models with respect to chlorpyrifos concentration (eq. (5))
Log(C0 /Ct ) =
1 K app t 2.303
3.3. Effect of operational conditions on sonocatalytic degradation of chlorpyrifos
(5)
Where, Co and Ct are the initial and time concentration of chlorpyrifos at time t, respectively. Fig. 4 B predicts the relation between time (min) against log(Co/Ct) at 25 °C. PFO parameter collected in
3.3.1. Effect of catalyst dosage Different dosages of ZCT10 (0.5–2.5 g/L) were performed to obtain
Table 2 PFO kinetic parameters for sonocatalytic degradation of chlorpyrifos in the presence of sonocatalysts at 25 °C & PFO kinetic model and Arrhenius parameters for chlorpyrifos degradation in the presence of TiO2, CT10 and ZCT10 at 20, 30 and 40 °C. Kinetic parameters
TiO2
CT2
CT4
CT10
CT20
CT50
ZCT10
R2 Kapp (min-1)
0.9649 0.00765
0.9780 0.01039
0.9734 0.01101
0.9726 0.01781
0.9739 0.01439
0.9845 0.01269
0.9786 0.03185
TiO2
2
R Kapp(min-1)
CT10
ZCT10
20 °C
30 °C
40 °C
20 °C
30 °C
40 °C
20 °C
30 °C
40 °C
0.9425 0.00706
0.9762 0.00828
0.9527 0.00918
0.9753 0.00158
0.9745 0.02468
0.9863 0.02532
0.9730 0.03483
0.9489 0.03911
0.9385 0.04886
Arrhenius parameters
R2 Ea(kJ/mol)
TiO2
CT10
ZCT10
0.9919 12.1
0.9243 11.8
0.9998 8.6
184
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Fig. 5. Sonocatalytic degradation of chlorpyrifos at different catalyst dosages (A) in the presence of ZCT10, sonocatalytic degradation of chlorpyrifos (B), PFO kinetic plots (C) and linear plot of Arrhenius model (D) using TiO2, CT10 and ZCT10 at 20, 30, and 40 °C.
the optimum sonocatalysts dosage (Fig. 5A). From Fig. 5A we observed that as the catalyst dosages increase the degradation of chlorpyrifos also increase where the degradation efficiency after 45 min about 73% and 94% using 0.5 and 2.0 g/L, respectively. The previous result can be related to the production of more reactive radicals and more cavitation microbubbles [32] and the increase in the local temperature at cavity collapse because of the presence of more nucleation sites for the cavitation bubbles which enhance the pyrolysis of water molecules to form hydroxyl radicals [33,34]. Catalyst dosage 2.5 g/L gave the same efficiency as 2.0 g/L dosage which may be related to aggregation of sonocatalysts particles, higher adsorption factor and the blocking of transmission of ultrasound waves near the catalyst [17,35]. From the effect of catalyst dosage, we choose 2.0 g/L as the optimum value for applications.
enhancement of the reaction rate between radicals and chlorpyrifos [37]. The increase in the sonocatalysis application temperature may decrease the cavitation energy and facilitate the formation of cavitation bubbles and raising the rate of free radicals production (•OH and •OOH) [38]. Upon analysis of PFO parameters at different application temperatures (Table 2): (i) regression coefficient, R2, ranged between 0.9385 and 0.9863 for all the investigated sonocatalysts at all the application temperatures indicating the good applicability of PFO model. (ii) For all the investigated sonocatalysts as temperature increases, the rate constant for degradation also increases. Applying Arrhenius equation (eq. (6), Fig. 5D) we can calculate the apparent activation energy, Ea (J/mol):
3.3.2. Effect of application temperature Fig. 5B and C, D illustrate the effect of temperature on the catalytic degradation of chlorpyrifos using TiO2, CT10, and ZCT10 at 20, 30 and 40 °C. It is observed that as temperature increases the catalytic efficiency slightly increases in all the investigated catalysts (Fig. 5B). In particular, after 60 min degradation efficiencies were found to be 91.5, 95.4 and 98.7% at 20, 30 and 40 °C, respectively. Fig. 5C shows the application of the PFO kinetic model at different temperatures and the calculated parameters are collected in Table 2. The increase in sonocatalytic degradation of chlorpyrifos with temperature may be related to many factors, such as, the increase in water vapor pressure which causes a less violent collapse of bubbles [36]. Also, the increase in the mass transfer of radicals increased which is accompanied by
Here, R (8.314 J/mol K) is the gas constant, A is a pre-exponential factor (min−1). Values in Table 2 indicate a good applicability of Arrhenius model (as supported by higher regression coefficients, 0.9243–0.9998). The calculated activation energy values for sonocatalytic degradation of chlorpyrifos in the presence of TiO2, CT10 and ZCT10 were 12.1, 11.8 and 8.6 kJ/mol, respectively. The lower Ea value in case of ZCT10 is accompanied by higher reaction rate due to the lower energy required to break down bonds in chlorpyrifos molecules [39].
LnK app = −
Ea + lnA RT
(6)
3.3.3. Effect of power on degradation efficiency It is known that most of the used power in sonocatalysis is transferred into heat as the main factor in the production of •OH and •H from 185
Journal of Physics and Chemistry of Solids 129 (2019) 180–187
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Fig. 6. Effect of power on sonocatalytic degradation of chlorpyrifos (A) and sonocatalytic degradation of chlorpyrifos in the presence and absence of radiation light (B) using ZCT10.
water. The effect of applied power of ultrasonic generator on the degradation of chlorpyrifos was investigated at 50, 100, 150 and 200 W in the presence of ZCT10 as a sonocatalysts (Fig. 6A). The degradation efficiency increased with the power of ultrasonic generator, which explained by (i) disorder of solution increase with power is accompanied with the increase in the rate of mass transfer of chlorpyrifos molecules, radicals and degradation byproducts between solution bulk and catalyst surface [12,40]. (ii) The increase in the number of cavitation bubbles leading to more free radicals (•OH) [41]. (iii) Enhancement of active sites on the surface of the catalyst due to cleaning action of higher applied power [42]. (iv) The increase in acoustic energy with power leads to an increase in pressure, temperature and the collapse time [41,43,44]. 3.3.4. Effect of light radiation on sonocatalytic degradation efficiency The combination of light radiation and ultrasonic radiation was investigated using ZCT10. Fig. 6 B shows the effect of light radiation on chlorpyrifos sonocatalytic degradation which predicts the increase in the degradation rate of organic pollutant with a light beside ultrasonic radiations. After 90 min the efficiency of degradation in the presence and absence of light was 100 and 95%, respectively. Several reasons stand behind the beneficial effect of sonocatalysis and photocatalysis coupling such as (i) the increase in •OH production [10] (ii) production of conduction band electrons and valence band energy when catalyst irradiated with radiation light with energy equals or more than the energy band gap of catalyst [45].
Fig. 7. Catalyst reusability studies for sonocatalytic degradation of chlorpyrifos in the presence of CT10 and ZCT10 at 25 °C.
sonocatalysts was found to increase with increasing the catalyst dosage till 2.0 g/L. As application temperature and ultrasonic power increase the efficiency of degradation also increases. The presence of radiation light raises the efficiency of degradation which reaches 100% after 90 min (in the case of ZCT10) in the presence of radiation light compared with 95% in the absence. Catalyst reusability data showed that the prepared solid materials are well reusable nanocatalyst for sonocatalytic degradation of chlorpyrifos even after many application cycles.
3.4. Catalyst reusability Catalysts reusability was investigated after four cycles of sonocatalysis. Reusability results are shown in Fig. 7. Catalyst efficiency slightly decreases with catalyst reusing where after four application cycles the decrease in CT10 and ZCT10 efficiency was found to be 0.6 and 0.4%, respectively as reported by many authors [46–48]. The slight decrease in catalytic activity may be related to the slight decrease in surface area due to the coagulation of nanoparticles [49].
Prime novelty statement In this manuscript, we prepared Zr+4@ Nanotitania/carbon nanotube composites using sol-gel method. The prepared samples were characterized with different tools. Degradation of chlorpyrifos was studied using sonocatalytic mechanism under different application conditions. To the best of our knowledge, application of sonocatalytic process for chlorpyrifos removal on the surface of Zr+4@ Nanotitania/ carbon nanotube composites was not studied up to now.
4. Conclusion The present study showed that modification of nanotitania/carbon nanotubes composite with Zr+4 reduces its energy band gap and promotes the separation of the electron-hole pairs in dispersed particles of TiO2 followed by raising its sonocatalytic efficiency to the extent of 91.5% degradation after 60 min. The degradation efficiency of
Acknowledgment This research was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the Project CEITEC 2020 186
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(Project No. LQ1601) and by the Academy of Sciences of the Czech Republic (Institutional Project No. RVO:68081723).
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