Kinetics and Mechanism of Enhanced Photocatalytic Activity under

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Kinetic and mechanism of enhanced photocatalytic activity under visible light using synthesized PrxCd1-xSe nanoparticles Alireza Khataee, Amirreza Khataee, Mehrangiz Fathinia, Younes Hanifehpour, and Sang Woo Joo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie402352g • Publication Date (Web): 20 Aug 2013 Downloaded from http://pubs.acs.org on September 1, 2013

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Industrial & Engineering Chemistry Research

Abstract Praseodymium (Pr)-doped CdSe nanoparticles with different Pr contents were successfully synthesized by an easy hydrothermal method. The prepared samples were characterized by Xray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis demonstrated that the particles were excellently crystallized and corresponds to cubic CdSe phase. SEM images indicated that the size of the particles were in the range of 50–100 nm. Photocatalytic efficiency of pure and Pr-doped CdSe samples was evaluated by monitoring the decolorization of malachite green (MG) in aqueous solution under visible light irradiation. The color removal efficiency of 47.16 % for CdSe and 94.32 % for Pr0.06Cd0.94Se was achieved at 120 min. It was found that the presence of the Cl−, SO4−2 and HCO3− ions obstructed the photocatalytic degradation of MG, but the presence of S2O82− accelerated it. Also, the effect of operational parameters at optimum conditions was modeled regarding the nonlinear regression analysis.

Keywords: Photocatalysis; CdSe nanoparticles; Praseodymium; Decolorization; Nonlinear regression.

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1. Introduction In most of industrial factories, including the textile, cosmetic, leather, pharmaceutical, and food industries, large amounts of dyes are annually produced and used in many different sections.1 The discharge of these huge amounts of pollutants including synthetic dyes to the environment leads to chronic pollution of water sources because of their bioaccumulation in aquatic and wildlife.2 Also, various problems associated to their carcinogenicity and toxicity to aquatic life could be immerged. Malachite green is an example of most commonly used dye for the dyeing of cotton, silk, paper and leather between all other dyes of its category. Moreover, it is used as a pesticide in aquaculture industry due to its efficiency in treatment of parasitic, fungal and bacterial infections in fish and fish eggs.3 Despite the wide application ranges which were reported for MG, its hazardous and carcinogenic effects limit its effective application. In this context, MG can cause allergy, dermatitis, and cancer in humans.4 Additionally, it decreases food intake capacity, growth and fertility rates, causes damage to heart, eyes, liver and kidney thereby compromising aquatic life.3 Therefore, the development of new techniques for managing the effluents containing such dye is crucial due to its harmful impacts on receiving waters. Among various physical, chemical and biological techniques, heterogeneous photocatalysis via the use of semiconductor photocatalysts has been considered as a cost-effective alternative method for polluted water treatment. The advantages of photocatalytic technique over the traditional ones are the rapid oxidation rate, high efficiency, no formation of polycyclic products and oxidation of pollutants even in the low levels.2 Different semiconductors have been used to exhibit photocatalytic behavior. TiO2 and ZnO are the most commonly utilized semiconductors for organic pollutant degradation.5, 6 However, because of the wide band gap of TiO2 and ZnO (band gap around 3.2 eV and 3.37 eV for TiO2 and ZnO, respectively), their practical application is confined due to the need of an ultraviolet 3 ACS Paragon Plus Environment

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excitation source, which is only a small part (3−5%) of solar light.6-8 Therefore, to overcome this problem, visible-light-driven photocatalysts were developed for photocatalysis application under solar energy.9-12 Among these materials, small band gap semiconductors such as CdSe, the II– VI semiconducting materials and the member of cadmium chalcogenide family, have aroused extensive attention.13 Due to its low band gap and the rapid generation of electron-hole pairs (charge carriers) by photoexcitation, CdSe has been regarded as an efficient semiconductor under visible light irradiation.14, 15 However, still some issues are remained with the visible light activated semiconductors such as CdSe which prevent their practical application: instability upon light illumination leads to photocorrosion or photodissolution on a photocatalyst surface during photocatalytic reaction; low adsorption capacity and separation efficiency of electron-hole pairs resulted in low photocatalytic activity.16, 17 Generally, the adsorption capacity of CdSe particles could be improved by producing CdSe nanomaterials containing enhanced specific surface area or depositing CdSe onto the materials with high surface area. By preparing a hybrid or composite semiconductor materials containing CdSe and metal ion doping of CdSe, the fast recombination of electron-hole pairs could be efficiently inhibited and nanoparticles of photocatalysts are obtained. Regarding this context, CdSe-Pt nanorods,18 hybrid CdSe-Au nanostructures,19 carbon nanomaterials-CdSe composites20 have been successfully prepared and reported for degradation of various organic compounds. However, according to the literature reports,21-24 metal ion doping of semiconductors has considerably increased the photocatalytic efficiency of these materials. Yang et al.17 reported that metal ion doping of semiconductors not only could affect their photophysical behavior, but also expedites the photochemical reaction. Moreover, metal ion may change surface characteristics by the generation of a Schottky barrier through the metal in contact with the semiconductor surface, which acts as an electron trap. There are also many rare earth



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metals such as Pr3+,25 Sm3+,23 Nd3+,23 Ce3+,26 Y3+ 27 and Eu3+ 28 that have been utilized to promote the separation of photogenerated electron and hole for improving photocatalytic property. Nevertheless, there have been no reports regarding the improvement of CdSe photocatalytic property under visible light irradiation via doping of transition rare earth metals such as Pr, a lanthanide metal. On the other hand, for the fabrication of CdSe samples, various methods such as the sol– gel and electrochemical were wildly utilized.10,

29, 30

Most of these synthesis processes

generally require highly sophisticated equipment to produce high purity samples. In the case of sol-gel technique, such high calcination temperatures will increase the nanoparticle size and decrease the specific surface area.21 In order to produce highly photoactive nanomaterials, a practical technique with a low temperature procedure should be applied to synthesis samples with high surface area. The hydrothermal technique is a one-pot chemical method, which facilitates the control of particle size, particle morphology and phase composition by adjusting experimental parameters of the solution.31, 32 So, in this work we present a new method to prepare cubic CdSe nanoparticles using cadmium acetate as a cationic source and sodium selenite as an anionic source in the presence of ethylenediaminetetraacetic acid (EDTA), as template agent to avoid further aggregation of synthesized nanoparticles, using a hydrothermal method. Pr was specifically selected here due to its efficient function in improvement of TiO2 photocatalytic properties under visible light in the previously reported papers.25,

33

It was reported that doping of Pr3+ to TiO2

effectively enhanced its photocatalytic activity under visible light through increasing the separation efficiency of interfacial charges by Pr3+ and the red shift of the optical absorption edge of TiO2 by Pr3+ doping.25 The physical properties of the synthesized samples were characterized by XRD and SEM techniques. The prepared CdSe and PrxCd1-xSe nanoparticles were used as a photocatalyst for decolorization of MG as a model pollutant (See Table 1).


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Table 1. Characteristics of Malachite Green.

Color index

Chemical structure

name

C.I. Basic Green 4

Molecular

λmax

formula

(nm)

C23H25ClN2

619

Color index number

42000

Mw (g/mol)

364.911

Moreover, the performance of synthesized samples in terms of decolorization efficiency and kinetic rate constant were studied and compared. To the best of our knowledge, there is no previous literature report concerning the use of CdSe and PrxCd1-xSe nanoparticles for the removal of MG. Other objectives of this work are to investigate the effect of inorganic ions on the decolorization efficiency of MG. Also, we have modeled the kinetic of the process using nonlinear regression analysis.

2. Experimental details 2.1. Materials All chemicals used in this study were of analytical grade and were used without further purification.

Cadmium

acetate

dihydrate

(Cd(CH3COO)2.2H2O

98%),

hydrazine

monohydrate (N2H4.H2O 80%), sodium selenite (Na2SeO3 99%) were purchased from Loba Chemie Co. (India). NaOH, NaCl, NaHCO3 and K2S2O8 were purchased from Merck.Praseodymium (III) nitrate hexahydrate (Pr(NO3)3.6H2O 99.99%), was purchased from Aldrich. EDTA (C10H14N2O8Na2.2H2O) was purchased from Rankem Co. (India). MG and ethanol (90%) was purchased from ShimiBoyakhsaz Co. (Iran) and Iran Daru Co. (Iran), respectively. 6 ACS Paragon Plus Environment


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2.2. Synthesis of CdSe and Pr-doped CdSe samples CdSe and Pr-doped CdSe nanoparticles with variable Pr mole fractions (0-10 % mol) were prepared by hydrothermal method using N2H4.H2O as the reducing agent. In a typical synthesis, 1 mmol Na2SeO3 powder and 1 mmol NaOH and appropriate molar ratios of Pr(NO3)3.6H2O and Cd(CH3COO)2.2H2O were first dissolved in 70 mL distilled water. Under middle speed stirring, 1 mmol EDTA was dissolved in 20 mL distilled water and added to the above solution. After being stirred uniformly, hydrazine was added drop wise to the above solution. The resulting solution was moved into a 150 mL teflon-lined stainless-steel autoclave, placed in an oven at 180 ◦C for 24 h, and then the autoclave was allowed to cool to room temperature naturally. As-synthesized CdSe and PrxCd1-xSe nanoparticles were collected and washed with distilled water and ethanol several times in order to remove residual impurities and then dried at 60 °C for 5 h. The final black powders were obtained as a result.

2.3. Characterization instruments To determine the crystal phase composition of the prepared CdSe and Pr-doped CdSe samples, XRD characterization was carried out at room temperature using a D8 Advance, Bruker, Germany diffractometer with monochromatic high-intensity CuKa radiation (l=1.5406 A˚), the accelerating voltage of 40 kV and the emission current of 30 mA. The Debye–Scherrer formula was employed to calculate the average crystalline size of the catalysts.34 SEM (S-4200, Hitachi, Japan) was used to observe the surface state and morphology of the prepared nanoparticles using an electron microscope.


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2.4. Photocatalytic activity and experimental procedures The photocatalytic activity of undoped and Pr-doped CdSe nanoparticles was evaluated by the decolorization of MG in an aqueous solution under visible light. In a typical process, 0.1 g of the photocatalyst powder was added into 100 mL MG solution with an initial concentration of 5 mg/L. The suspension of photocatalyst and MG was magnetically stirred in a quartz photoreactor in the dark for 15 min to establish an adsorption/desorption equilibrium of the dye. Then, the solution was irradiated by a 6W fluorescent visible lamp (GK-140, China) as the light source for a set irradiation time. Visible light irradiation of the reactor was performed for 15, 30, 45, 60, 90 and 120 min. Samples were withdrawn regularly from the reactor, and dispersed powders were removed in a centrifuge. The color removal was evaluated by determining its absorbance at λmax= 619 nm by using UV–Vis spectrophotometer, Lightwave S2000 (England). The decolorization efficiency (DE (%)) was expressed as the percentage ratio of decolorized dye concentration to that of the initial one. In the experiment requiring inhibitors, different amounts of probe molecules (NaCl, Na2SO4, NaHCO3 and K2S2O8) were added to the MG solution before the introduction of the PrxCd1-xSe as the catalyst. The continuance of the procedure was identical for the photocatalytic activity evaluation as mentioned above.

3. Results and discussion 3.1. Characterization of the synthesized samples The XRD patterns of the CdSe and Pr0.06Cd0.94Se nanoparticles are depicted in Figure 1a and 1b, respectively. The XRD diffraction peaks at 2θ of 25.4°, 42°, and 49.6° can be classified to the (100), (002), (101), (112) and (211) plane reflections. It can be associated with cubic crystal structure CdSe according to the standard powder diffraction data (JCPDS 65-2891 for CdSe, cubic).35,

36

Figure 1b shows that, the same typical peaks for CdSe are 8

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detected for Pr0.06Cd0.94Se sample indicating that there is no change in the crystal structure upon Pr doping. No peak for impurities was detected, confirming that the applied hydrothermal method in this study was successful in synthesizing the samples. Moreover, the sharp diffraction peaks in the XRD spectra of the synthesized samples express that the synthesized products are high crystalline. Calculating from the Debye–Scherrer formula,34 the average size of the crystals was about 15 nm. In order to further clarify the size and shape of the nanoparticles, SEM image was taken at different magnifications.

Figure 1. Powder X-ray diffraction pattern of (a): CdSe and (b): Pr0.06Cd0.94Se samples.

Figures 2 and 3 show the SEM microphotographs of the CdSe and Pr doped CdSe samples, respectively. In Figure 2, uniform and spherical nanoparticles of about 50-100 nm in 9 ACS Paragon Plus Environment

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diameter with a little agglomeration can be seen. In Figure 3, very uniform and sphericalshaped Pr doped CdSe nanoparticles can be observed. The diameter of these particles is around 50-100 nm. These figures confirm that doping of Pr3+ into the structure of CdSe does not change the morphology of CdSe nanoparticles.

(a)

(b) Figure 2. (a) and (b): SEM images of CdSe nanoparticles synthesized at 150 ºC and 24 h at different magnifications.



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(a)

(b) Figure 3. (a) and (b): SEM images of Pr0.06Cd0.94Se nanoparticles synthesized at 150 ºC and 24 h at different magnifications.

3.2. Effect of operating conditions on the photocatalysis of MG 3.2.1. Effect of Pr3+ content of PrxCd1-xSenanoparticles To explore the optimum conditions of photocatalytic activity, the degradation process of MG was studied in the presence of PrxCd1-xSe with different mole fraction (x = 0.00, 0.02, 0.06, 11 ACS Paragon Plus Environment

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0.08 and 0.10) under visible light irradiation. Figure 4 shows the decolorization efficiency of MG over different Pr-doped CdSe photocatalysts during 120 min of reaction. From Figure 4a, it can be clearly seen that the samples doped with appropriate amount of Pr ion had much higher photocatalytic activity compared with pure CdSe, especially the sample with 6% molar ratio of Pr, which exhibited the best photocatalytic activity. The reason of high photocatalytic activity of Pr0.06Cd0.94Se can be explained by the following mechanism. Generally, rare earth metal ions such as Pr3+, can perform either as a mediator of interfacial charge transfer or as a recombination center in the crystalline structure of CdSe.28, 37 So depending on the dopant concentration, the doped CdSe activity or efficiency is changeable. At low mole fractions of dopant, Pr3+ ions can capture photoinduced electrons retarding electron/hole recombination rate and thereafter enhancing the interfacial charge transfer for MG degradation.38 However, when the mole fractions of dopant is higher than the optimal value, the recombination rate may increase since the distance between trapping sites in a crystal structure of CdSe is decreased which lead to decrease in photocatalytic activity.21 So, an optimum content of Pr3+ is essential to separate photoinduced electron/hole pairs and increase the life time of charge carries. To understand the reaction kinetic of MG decolorization in this study, the following pseudofirst-order kinetic equation is used to fit experimental data:

ln(

C0 ) = k app t C

(1)

where kapparent (kapp) is the pseudo-first-order rate constant (min-1), C0 is the initial concentration at time 0 min, and C is the concentration at the reaction time of t min.



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(a)

(b)

(c) Figure 4. (a): The effect of Pr3+ dopant content on the decolorization of 5 mg/L MG (catalyst loading 1.0 g/L); (b): Apparent pseudo first-order reaction kinetic and (c): Variation of apparent kinetic constant for MG decolorization during the photocatalytic reaction at different Pr3+ contents. 13 ACS Paragon Plus Environment

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The fitting curves of ln(C0/C) verses time during 0–120 min at different Pr3+ mole fractions were plotted and presented in Figure 4b. Figure 4b indicates that photodecolorization of MG over PrxCd1-xSesamples follows pseudo-first-order kinetic. Figure 4c shows the kinetic constant during the photocatalytic reaction at different Pr3+ mole fractions by the pseudo first-order model. As can be seen, the kinetic constant of photocatalytic reaction increases with increasing the Pr3+ content up to 6 % and then decreases. The possible reason for this behavior was explained above.

3.2.2. Enhanced photocatalytic decolorization mechanism of MG on Pr0.06Cd0.94Se In order to properly illustrate the photocatalytic activity of the pure CdSe and the Pr0.06Cd0.94Se samples and investigate the possible reaction mechanism, the changes in the UV-Vis absorption spectra of MG during the photocatalytic process at different irradiation times are shown in Figure 5. The decreasing concentration of MG during the photocatalytic reaction is used to evaluate the activity of the photocatalysts. Two steps are included in the photocatalytic removal of a dye: the adsorption of dye molecules, and the degradation. Figure 5a and b represent both the adsorption and degradation effects for CdSe and Pr0.06Cd0.94Se samples under visible light irradiation, respectively. At the identical conditions, the Pr0.06Cd0.94Se exhibits significantly higher photocatalytic activity than that of pure CdSe. From Figure 5a, it can be clearly seen that when pure CdSe used as a photocatalyst, MG concentration reaches to a constant amount (47.16 % color removal efficiency) after 120 min of reaction and the degradation is stopped. There are several probable reasons for the low activity of pure CdSe including photocorrosion and photodissolution occurred on the surface of pure CdSe in the photocatalytic reaction.17 The other major issue is that, when the band-gap energy of a semiconductor is low, the life time of the produced electrons and holes in its surface is low.3 So, in the case of CdSe the presumed reason is that, after a 60 min of irradiation and producing charge carries on the surface of CdSe, the rate of recombination of charge carriers is dominated to the rate of other



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reactions in the solution, which lead to the decrease in its photocatalytic activity. However, in Figure 5b, it is obvious that after 15 min of adsorption step and 120 min of irradiation, the concentration of the MG solution gradually reduces with increasing irradiation time.

(a)

(b)

Figure 5. Adsorption and degradation of MG under visible light irradiation using (a): pure CdSe and (b): Pr0.06Cd0.94Se nanoparticles

Therefore, these results indicate that Pr3+ dopant really could play an important role in the improvement of photocatalytic activity of CdSe. This behavior can be attributed to the fact that, 15 ACS Paragon Plus Environment

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the number of the produced electrons and holes and their life time are dependent on the kind, amount of dopant and size of the semiconductor particles.16,

38

Herein, the Pr3+ dopant

operates as an electron scavenger on the surface of CdSe, suppressing the recombination of electron-hole pairs and enhancing their life time; so, the photocatalytic activity of photocatalyst is increased. Incorporating our achieved results into the other literature reports,5,

33, 39

the probable

mechanism for the enhanced photocatalysis of Pr0.06Cd0.94Se was proposed as follow. Under the visible light irradiation, the electrons (e−) are excited from the valence band to the conduction band of CdSe, and the holes (h+) are produced. Pr3+ dopant in CdSe can effectively scavenge the e− and inhibit their recombination with h+ due to the existence of partially filled 4f-orbital, 4f2. The produced Pr2+ ions, with three electrons in 4f orbital, are very unstable, so that the e− can be easily descavenged and passed to the adsorbed O2 molecules, promoting the O2•− and •OH

formation subsequently. Simultaneously, the

adsorbed photogenerated holes oxidize water molecules or surface bound hydroxide species to generate •OH species. The degradation mechanism for the Pr0.06Cd0.94Se was shown in Eqs 2-11.



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Pr0.06 Cd 0.9 4Se + visible light → Pr0.06 Cd 0.9 4S (e − ads + h + ads )

( 2)

Pr 3+ + e − ads → Pr 2+ (electron scavenging step)

(3)

O 2 ads + e − ads → O 2 O2

•− ads

•−

+ Pr 3+ → Pr 2+ (electron scavenging step)

Pr 2+ + O 2 ads → Pr 3+ + O 2 O2 •

•− ads

( 4)

ads

•− ads

(electron transferring step)

+ H + ads →• OOH ads

(5) ( 6) (7 )

OOH ads + H + ads + e − ads → H 2 O 2 ads

(8)

H 2 O 2 ads + e − ads →• OH ads + OH − ads

(9)

h + ads + H 2O ads →• OH ads + H + ads

(10)

h + ads + OH − ads →• OH ads

(11)

Thus, the scavenging and transferring of the charge carriers in CdSe surface can be attributed to Pr3+ dopant in its crystal structure which further improves the photocatalytic activity of the catalyst.

3.2.3. Effect of photocatalyst concentration and reusability The initial rate of photocatalytic decolorization process is dependent on the photocatalyst concentration.21, 29, 36 In order to compare the effect of different dosage of Pr0.06Cd0.94Se (0.05–1.5 g/L) on the decolorization efficiency of MG, a series of experiments were done with 5 mg/L initial concentration of MG. The results are presented in Figure 6a. Figure 6b indicates that photocatalytic decolorization of MG with different amounts of Pr0.06Cd0.94Se samples follows pseudo-first-order kinetic. Also, Figure 6c shows the kinetic constant during the photocatalytic reaction using different amounts of Pr0.06Cd0.94Se by the pseudo first-order model. As can be seen in Figure 6c, the kapp significantly increases with an increase in the amount of photocatalyst from 0.05 to 1.0 g/L; it reaches to the maximum value (0.0223 min-1) with the content of 1.0 g/L. By further increase of photocatalyst amount, the kapp is decreased slightly. These results indicate


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that the photocatalyst dosage of 1.0 g/L would be the optimum amount. The assumed reason is that, when the concentration of photocatalyst is increased, the available photocatalyst active sites, the number of photons adsorbed and the number of dye molecules adsorbed are also increased, so, the decolorization efficiency is increased in the presence of 1.0 g/L photocatalyst.16,

21

However, by further increase of the photocatalyst dosage beyond the optimum amount, the kapp is diminished because of increasing the turbidity of the suspension. So, the penetration depth of the photons reduced and less photocatalyst nanoparticles could be excited.40 This leads to a decrease in the color removal efficiency.

(a)

(b) 18 ACS Paragon Plus Environment


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(c)

Figure 6. (a): The effect of photocatalyst loading on the decolorization of 5 mg/L MG by the Pr0.06Cd0.94Se; (b): Apparent pseudo first-order reaction kinetics and (c): Variation of apparent kinetic constant for MG decolorization during the photocatalytic reaction at different photocatalyst concentration.

The reusability is one of the most important factors for a photocatalyst. Figure 7 shows the reusability tests of Pr0.06Cd0.94Se photocatalyst in the decolorization of MG, during 10 cycle experiments under optimum conditions as: 5 mg/L of MG, 1.0 g/L of Pr0.06Cd0.94Se photocatalyst and irradiation time of 120 min. After each decolorization experiment, the photocatalyst was washed with distilled water and then dried at 60 °C for 5 h and then used in new experiment. As can be seen in Figure 7, Pr0.06Cd0.94Se exhibited excellent chemical stability without any considerable decomposition or photocorrosion during the 10 cycles of photocatalytic reaction which is an important advantageous for practical applications.


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Figure 7. Reusability behavior of Pr0.06Cd0.94Se in photocatalytic decolorization of 5 mg/L of MG in the presence of 1.0 g/L of Pr0.06Cd0.94Se and irradiation time of 120 min.

3.2.4. Effect of initial MG concentration Photocatalytic decolorization of MG was investigated at different concentrations of dye solutions varying from 5 to 15 mg/L, in the presence of 1.0 g/L Pr0.06Cd0.94Se under UV irradiation. Figure 8a shows the decolorization efficiency of different concentrations of MG dye solution in the certain irradiation time intervals. Figure 8b shows that photocatalytic decolorization of MG with its various initial concentrations follows pseudo-first-order kinetic. Also, Figure 8c presents the kinetic constant during the photocatalytic reaction using different initial concentrations of MG. It can be observed from Figure 8c that the kapp declines with increasing the initial concentration of the dye. This behavior can be related to the several factors. By increasing the dye concentration, more and more molecules of the dye get adsorbed on the surface of the photocatalyst. Therefore, requirement of the reactive oxygen species (such as •OH and •O2−) in order to degrade the dye molecules, increases. However, the formation of •OH and •O2− on the photocatalyst surface remains constant for a given light intensity, catalyst dosage and 20 ACS Paragon Plus Environment


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duration of irradiation. Hence, the available hydroxyl radicals were not enough to oxidize the high concentration of MG and more generated intermediates from its degradation.2 Accordingly, the degradation efficiency of the dye decreases as the concentration increases. Moreover, the increasing of substrate concentration can lead to the production of intermediates, which may adsorb on the surface of the catalyst. Slow diffusion of the produced intermediates from the catalyst surface can result in the deactivation of active sites of the photocatalyst and consequently, a reduction in the degradation efficiency. Another reason for reducing the color removal at high substrate concentrations may be due to the absorption of light photon by the dye itself that leads to a lesser availability of photons for hydroxyl radical production.1, 41

(a)


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(b)

(c)

Figure 8. (a): The effect of MG concentration on the decolorization of 5 mg/L MG(Pr0.06Cd0.94Se loading 1.0 g/L); (b): Apparent pseudo first-order reaction kinetics; and (c): Variation of apparent kinetic constant for MG decolorization during the photocatalytic reaction at different MG concentration.

3.2.5. Effect of inorganic ions on the photocatalytic activity The effect of different inorganic ions including NaCl, Na2SO4, NaHCO3 and K2S2O8were tested on the kapp of photocatalytic reaction. The decolorization efficiency of MG in the presence of inorganic ions such as Cl−, SO42−, HCO3− is shown in Figure 9a. In the absence of inorganic 22 ACS Paragon Plus Environment


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ions, the adsorption efficiency during 15 min of reaction under dark was 35.59 % and the decolorization efficiency of MG during 120 min of photocatalysis was 94.32 %. Figure 9a shows that, when the NaCl, Na2SO4 and NaHCO3are added to the reaction solution separately, with the concentration ratio of 1:1 to MG, both the adsorption and decolorization efficiency of MG decrease considerably. This behavior can be related to the adsorption properties of these ions. According to Figure 9a, the adsorption of Cl−, SO42− and HCO3− ions on the Pr0.06Cd0.94Se surface is stronger than that of MG. So, the surface of the Pr0.06Cd0.94Se is dominated by these ions which lead to a decrease in adsorption efficiency of MG. This phenomenon demonstrates that the preferential adsorption of pollutant molecules on the surface of the catalyst is a critical step in photocatalytic reaction.42 Moreover, Figure 9 shows that the addition of the three mentioned inorganic anions decreases the photocatalytic decolorization efficiency of MG. The inhibition effect of these ions is due to the h+ and •OH scavenging properties of these adsorbed ions as shown in Eqs. 12–17. The results are in good agreement with the published papers which reported a strong inhibiting effect of inorganic ions.42, 43

Cl − ads + h + ads → Cl • ads Cl • ads + Cl − ads → Cl 2 SO 4

2−

SO 4

2−

•− •−

+ h + ads → SO 4

ads

+ • OH ads → SO 4

− ads

2− ads

(13)

ads

ads

HCO 3 CO 3

(12)

(14)

ads •− ads

+ • OH ads → CO 3

+ • OH ads → CO 3

+ OH − ads

(15)

+ H 2O

(16)

+ OH − ads

(17)

•− ads

•− ads

Moreover, the above reactions demonstrate that the CdSe surface active sites are blocked by the produced radical anions which are not easily oxidizable, and subsequently the photocatalytic activity decreases. Consequently, it can be deduced that MG is first adsorbed on the surface of the catalyst and then is oxidized under the effect of holes and hydroxyl radicals.


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(a)

(b)

(c)

Figure 9. (a):The effect of addition of Cl−, SO42− and HCO3−ions on the decolorization of 5 mg/L MG (Pr0.06Cd0.94Se loading 1.0 g/L); (b): Apparent pseudo first-order reaction kinetics; and (c): Variation of apparent kinetic constant in the presence of different inorganic ions with the concentration ratio of 1:1 to MG.



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The effect of the addition of K2S2O8 on the photocatalytic activity is shown in Figure 10. With the addition of K2S2O8, the photocatalytic activity is significantly increased. When the ratio of K2S2O8 to MG was 6:1, the decolorization efficiency of MG is 98.25 % after being irradiated for 60 min. The persulfate ions (S2O82−) are strong electron acceptor and they can capture the photogenerated electrons rapidly to produce hydroxyl radicals through Eqs. 18 and 19.42

S2 O8 SO 4

2− ads

•− ads

+ e − ads → SO 4

2−

+ H 2O ads → SO 4

ads

+ SO 4

2− ads

•−

(18)

ads

+ • OH ads + H +

(19)

According to the number of previous studies,42,

44

the persulfate ion is a more efficient

electron acceptor than molecular oxygen. So the addition of K2S2O8 decreased the recombination of electrons and holes, thus greatly enhanced the decolorization efficiency. This results reconfirmed the critical rule of •OH radicals in the degradation of MG using Pr0.06Cd0.94Se nanocatalyst under visible light irradiation.

(a)


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(b)

(c)

Figure 10. (a):The effect of addition of S2O82− ions, with different the concentration ratios to MG, on the decolorization of 5 mg/L MG(Pr0.06Cd0.94Se loading 1.0 g/L); (b): Apparent pseudo first-order reaction kinetics; and (c): Variation of apparent kinetic constant in the presence of different concentrations of S2O82− ions.

3.2.6. Kinetic modeling of the photocatalytic process The aim of this section is to develop a kinetic model to estimate the pseudo-first order rate constant (kapp) of MG degradation by photocatalytic process using Pr0.06Cd0.94Se nanocatalyst under visible light irradiation. Non-linear regression analysis was used to find the relation between kapp and operational parameters including initial concentration of MG, the presence of different inorganic inhibitors with the concentration ratio of 1:1 to MG and initial 26 ACS Paragon Plus Environment


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concentration of K2S2O8 in the optimum concentration of catalyst dosage of 0.1 g/L. According to pseudo-first order kinetic assumption, when straight lines with high correlation coefficient values (R2> 0.90) were obtained from the plots of ln(C0/C) versus process time, the proposed kinetic was confirmed.45 Moreover, the non-linear relation of operational variables and kapp can be also modeled with empirical power-law type separately as Eq. 20.46 kapp= α (operational variable)β

(20)

Where α and β are the model parameters. So, the non-linear relation of operational variables including initial MG concentration, the presence of different inorganic inhibitors with the concentration ratio of 1:1 to MG and initial concentration of K2S2O8 and kapp were modeled with empirical power-law separately. The model parameters (α and β) were calculated for each operational variable with non-linear regression analysis of experimental data, and the obtained results are shown in Figures 8c, 9c and 10c. The kapp declines by increasing of initial concentration of MG (Figure 8c) and also it is reduced in the presence of various inorganic inhibitors (Figure 9c). On the other hand, the kapp increases by increasing the initial concentration of K2S2O8 (Figure 10c). As it was discussed in previous section, degradation of MG follows pseudo-first order kinetic, and it is deduced that the kapp is a function of operational variables as follows:

k app = k '

[K 2S2O 8 ]a0 [MG ]0b [Inhibitor ]c

(21)

Non-linear analysis regression was employed to estimate a, b, and c. Then, with known values of kapp, [K2S2O8]0, [MG]0 and [Inhibitor],which were applied in experimental conditions, k' can be computed for each run. The average values of k' was attained as 0.0458 (mg/L min). By substituting the estimated values into Eq. (21), the Eq. 22 was obtained for prediction of kapp at various experimental conditions:


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[K 2S2 O8 ]00.704 k app = 0.0458 [MG ]00.740 [Inhibitor ]0.795

(22)

A comparison between experimental and calculated kapp, which was obtained from the mathematical model (Eq. 22), was exhibited in Figure 11. It can be perceived that the calculated results from the kinetic model are in good agreement with experimental data (R2= 0.999).

Figure 11. A comparison between experimental and calculated apparent reaction rate constants obtained from expression 22, the new kinetic model.

4. Conclusions In this research, a simple and efficient hydrothermal method was applied for the synthesis of CdSe and Pr doped CdSe nanoparticles as a new visible light activated photocatalyst. The results of XRD and SEM analyses confirmed that the doping of Pr3+ ions into CdSe structure neither change the crystal structure nor the morphology of CdSe nanoparticles. Synthesized CdSe and Pr doped CdSe nanoparticles were used as the photocatalyst for removal of MG under visible light irradiation. The decolorization efficiency was 47.16 % and 94.32 % for CdSe and Pr0.06Cd0.94Se, respectively at 120 min of photocatalytic process. The mole fraction of 0.06 of Pr in nanoparticles showed the highest photocatalytic activity. It was shown that the presence of the 28 ACS Paragon Plus Environment


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inorganic ions such as Cl−, SO42− and HCO3− reduced the decolorization efficiency of MG, but the presence of S2O82− accelerated the reaction. Furthermore, non-linear regression analysis was applied to develop a kinetic model for estimating the kapp of photocatalytic decolorization. The experimental results for kapp were in good agreement with theoretically calculated data (R2= 0.999) and confirmed the significance of the mathematical model.

Acknowledgments The authors thank the University of Tabriz, Iran for all the supports provided. This work was funded by the Grant 2011-0014246 of the National Research Foundation of South Korea.

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Graphical abstract 169x141mm (300 x 300 DPI)

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Kinetics and Mechanism of Enhanced Photocatalytic Activity under

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