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Adsorption and photodegradation efficiency of TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite nanophotocatalysts for the removal of cyanide Parisa Eskandari, Mehrdad Farhadian, Ali Reza Solaimany Nazar, and Byong-Hun Jeon Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05073 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Adsorption and photodegradation efficiency of TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite nanophotocatalysts for the removal of cyanide

Parisa Eskandari a, Mehrdad Farhadian a, Ali Reza Solaimany Nazar a, Byong-Hun Jeon b

aDepartment

of Chemical Engineering, Faculty Engineering, University of Isfahan, Isfahan, Iran.

bDepartment

of Earth Resources and Environmental Engineering, Hanyang University, Seoul,

South Korea.

 Corresponding author. Tel.: +98 313 793 4532; fax: +98 313 793 4031. E-mail address: [email protected] (M. Farhadian). 1 ACS Paragon Plus Environment

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Abstract The synthesized TiO2/Fe2O3 nanostructures supported on powder activated carbon (PAC) and zeolite at different mole ratios of Fe3+/TiO2 were characterized by XRD, XRF, FESEM, EDX, TEM, FTIR, BET and, PL analyses and their cyanide photodegradation mechanism was thoroughly discussed. The results confirmed not only TiO2/Fe2O3/PAC had higher photocatalytic and adsorption capability but also better structural stability and reusability for cyanide removal than TiO2/Fe2O3/zeolite. The first order kinetics model indicated that the photodegradation rate using TiO2/Fe2O3/PAC was 1.3 times higher than that of TiO2/Fe2O3/zeolite. The response surface methodology (RSM) assessment showed that pH, irradiation time and initial cyanide concentration using UV/H2O2/TiO2/Fe2O3/zeolite system had more effects on the degradation respectively; whereas the effectiveness of UV/H2O2/TiO2/Fe2O3/PAC process was highly influenced by initial cyanide concentration than the other two parameters. High R2 and well-fitted residual plots approved the accuracy of the models in predicting the cyanide degradation efficiency using both the photocatalysts.

Keywords: Cyanide; TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC photocatalysts; Kinetic; Photodegradation, and adsorption comparison; Response surface methodology (RSM)

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Introduction The global cyanide production of 2-3 million tons/year 1 and its widespread application in various industries has led to the contamination of water bodies with high concentration of cyanide2 3, 4.

This poisonous compound poses serious threats to the environment

5-7.Therefore,

cyanide

concentration in water and wastewater should decrease up to the permissible standard level before releasing to the environment. The conventional methods are ineffective in degrading cyanide from wastewater and generate toxic byproducts, or they are only applicable for low concentration of cyanide removal. In contrast, cost-effective and environmentally sustainable heterogeneous photocatalytic degradation technologies have been noticeably used to purify the cyanide contaminated water and industrial wastewaters

7-9.

The photocatalytic oxidation is characterized

by photoactivated reactions where photons interact with catalyst and generate free reactive radicals leading to the degradation of cyanide 7, 10. The degradation efficiency is dependent on the type of catalyst used for the reaction in this method. Photocatalytic degradation of cyanide using TiO2 has been identified as one of the most effective technologies due to its excellent photochemical properties, inertness, low water solubility, and stability11 12, 13. However, the reaction catalyzed by only TiO2 leads to the fast recombination of charge carriers and a low quantum yield which should be enhanced by modifying the TiO2 with some transitional metals such as Fe2O3 due to the similar radius of Fe3+ and Ti4+ and also the role of Fe2O3 as a coupling semiconductor enhancing photocatalytic activity. Therefore, Fe2O3 could extend the optical absorbtion edge to visible light region for TiO2

14, 15.

In spite of improving the degradation rate by doping with Fe2O3, the

shortcomings of TiO2/Fe2O3 powder include poor recovery and reusability

16.

Activated carbon

and zeolite have been suggested as excellent support materials for nanostructure loading because of their large surface area, porous structure, and high chemical and thermal stability7, 17, 18. 3 ACS Paragon Plus Environment

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Currently, the study on the enhancement of photocatalytic activity performance of different photocatalysts for cyanide degradation has gained much attention19-21. However, it is noted that most of the previous literatures have focused on optimizing single parameter leading to two major disadvantages: (1) more experimental runs would make the study time-consuming22 and (2) it fails to determine the correct optimum conditions due to ignoring the interaction between the variable parameters and their combined influences on the degradation 23. Such drawbacks can be eliminated using response surface methodology7, 22. To the best of our knowledge, no studies have been performed so far to compare the structural, photocatalytic and adsorption behavior of TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite for cyanide removal from water. Therefore, the goal of this study was to synthesize TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite nanocatalysts. Subsequently, the TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite were compared for the photocatalytic activities in the presence of UV light and H2O2 and choosing the best photocatalyst for cyanide degradation has been focused. Furthermore, the mechanism of TiO2/Fe2O3 nanoparticles for cyanide photodegradation was discussed in details. Moreover, a series of experimental runs was designed using Box-Behnken design model to determine the interaction of operational parameters and their optimized conditions for maximum cyanide degradation. Kinetics of the photocatalytic degradation of cyanide in the presence of both the nanocatalysts were also studied. Both the nanocatalysts were compared for their reusability and for performance of the different elements of advanced oxidation processes (AOPs). Experimental 2.1. Preparation of TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite composite photocatalysts The catalysts were synthesized by a chemical co-precipitation method at different molar ratios of Fe3+/TiO2 while the most effective molar ratio of Fe3+/TiO2 was 0.06 based on the



screening experiments. Therefore, all of the experiments were carried out at 0.06 Fe3+/TiO2 molar ratio. The procedure of both catalysts syntheses at 0.06 molar ratio of Fe3+/TiO2 follows: The 0.47 g of powder activated carbon (Merck, purity 99%) and natural zeolite (0.47 g) were separately suspended into the deionized water, and mixed at the 70 °C using stirrer-heater. The 0.69 mL of titanium tetrachloride (TiCl4, Merck, purity 99%) and 0.061 g of Iron (III) chloride (FeCl3, Merck, purity 99%) were added into the suspensions at the Fe2O3/TiO2: PAC (zeolite) ratio of 1. The pH of the solutions (TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite) were adjusted to 2.5 using NaCl (Merck, purity 99%) and HCl (Merck, purity 99%) and continuously stirred at 70°C for 4 h. The solutions were cooled to the room temperature (25 °C) followed by filtration and washing with deionized water to completely remove the Cl- and Na+. Drying of the samples was carried out inside an electric oven at 80°C for 2h. The synthesized photocatalyst were calcined at 400 °C for 2 h inside a muffle furnace to obtain the TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite composite photocatalysts. 2.2. Morphology and chemical component test The crystalline phase and structure of the synthesized TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite catalysts were investigated using powder X-ray diffraction (XRD) (BrukerD8 Advance, Germany). The chemical composition and structure of the chemical compounds were identified using the X-ray Fluorescence (XRF) (Bruker S4-Pioneer, Germany). The morphology and size distribution of nanoparticles were investigated using field-emission scanning electron microscope (FESEM) (Nanosem 450, USA) equipped with Energy Dispersive X-ray (EDX) was used to recognize chemical composition of samples . In order to further investigate the structural properties of synthesized nanoparticles and transmission electron microscopy (TEM) measurement was operated (Philips EM 208S, Netherland) at an acceleration voltage of 100 kV.


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Fourier transform infrared (FTIR), (Jasco, 6300, Japan) spectroscopy was carried out to further confirm the chemical composition of compounds. Brunauer-Emmett-Teller (BET) surface area and pore volume were determined by N2 adsorption at 77 K (Belsorp mini, Japan). To study the recombination situation of electron-hole pairs, photoluminescence (Varian Cary Eclipse fluorescence spectrophotometer) emission spectra were carried out by the supporting of a fluorescence spectrometer with a wavelength of 280 nm acting as the excitation wavelength. 2.3. Photocatalytic degradation experiment A cylindrical batch reactor (400 mL) was designed in which an 8W UV lamp (254 nm wavelength, SNXIN Co. China) was devised (Figure 1). Water entered the reactor jacket and the reactor temperature was adjusted at 25 °C. A magnetic stirrer was used to continuously mix the powder suspension (TiO2/Fe2O3/PAC or TiO2/Fe2O3/zeolite). Synthetic cyanide contaminated water having different sodium cyanide (Merck, purity 99%) concentrations were prepared and pH was adjusted according to the experiment design. Besides, 1.4 g/L of the catalyst and 400 mg/L of H2O2 (37% W/W Merck) were dispersed into the contaminated water, and placed under the ultraviolate at constant stirring condition for 60-180 min. After completion of the reaction, the photocatalyst was filtered and separated from the cyanide solution by centrifugation at 8000 rpm for 10 min. The cyanide concentration was measured according to APHA 4500-CN-D standard method. The photocatalytic cyanide degradation efficiency was measured using eq 1. 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦(%) =

𝐶0 ― 𝐶𝑒 𝐶0

(1)

× 100

where, C0 and Ce (mg/L) are the initial and final cyanide concentrations respectively24. 2.4. Response surface methodology Response surface methodology was adopted to simultaneously describe the effect of independent numeral factors and their interaction on the response based on experimental data as 6 ACS Paragon Plus Environment


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wells as the optimization of such variables to obtain the best response. Since the response could be affected by various numerous variables, it is practically impossible to identify and control such large number of variables. Therefore, choosing the variables which have major effects on the responses is highly essential. In this study among five independent variables including initial cyanide concentration, pH, catalyst concentration, hydrogen peroxide concentration and irradiation time, three most effective factors, initial cyanide concentration, pH, and irradiation time were considered as variables based on the screening experiments. The catalyst concentration (TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite) and hydrogen peroxide concentration were kept constant at 1.4 g/L and 400 mg/L respectively for all the runs. The levels of each variable were divided into three levels, low (-1), middle (0) and high (+1) (Table 1). The Box-Behnken design model was used to design the experiment, and the cyanide degradation efficiency was chosen as the response. The number of experiments is determined according to eq 2. (2)

𝑁 = 2𝑘(𝑘 ― 1) +𝑐𝑝

where, k and cp are the number of factors and the number of the central point respectively 25. Results and Discussion 3.1. Crystal structures and characterization analyses The XRD patterns of the zeolite, PAC, TiO2/Fe2O3/zeolite, and TiO2/Fe2O3/PAC are shown in Figure 2 (a-d). The XRD patterns of the natural zeolite at 2θ values of 11.41°, 13.12° and 26.51° corresponded to JCPDS. The XRD of PAC reflection revealed the broad peaks at 2θ values of 31.0° and 51.0° (Figure 2c). The patterns of synthesized catalysts (TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC) confirmed the presence of different phases of TiO2 (Figure 2 (b and d)). For instance, the major peaks at 25.3˚ and 27.4˚ indicated the anatase and rutile phases, respectively (Figure 2b). The lack of Fe3+ patterns in Figure 2(b and d) can be attributed to the similar radius


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values of Fe3+ (0.64 Å)) and Ti4+ (0.68 Å), and the low concentration of Fe-doping26. This clearly indicates the incorporation of Fe doping into the crystal structure of TiO2 without disturbing its structure. The presence of Fe2O3 was verified by the XRF, EDX and FTIR analyses. The XRF analysis of zeolite and two catalysts revealed the weight percentage of different compounds including Fe2O3 (Table 2). Moreover, TiO2 was the most abundant as compared to the other compounds. The similar SiO2/Al2O3 ratios of the TiO2/Fe2O3/zeolite and zeolite confirmed the deposition of oxides on Si (Al)–O bands in the TiO2/Fe2O3/zeolite nanoparticle. The FESEM images of zeolite, TiO2/Fe2O3/zeolite, PAC and TiO2/Fe2O3/PAC (Figure 3) showed the porous structures with variable shape. Such changes guaranteed the uniform loading of TiO2/Fe2O3 on the surfaces of the zeolite and PAC. Moreover, the size distribution of TiO2/Fe2O3/zeolite showed the sizes of the most nanoparticles are in the range of 20-60 nm (Figure 4a), and more than 80% of the TiO2/Fe2O3/PAC nanoparticles are ≤ 50 nm (Figure 4b). The EDX spectrum of TiO2/Fe2O3/zeolite (Figure 5g) and TiO2/Fe2O3/PAC (Figure 6e) verified the presence of Fe, Ti, and O (related to nanoparticles) as well as C, Si, and Al (related to PAC and zeolite). Further, EDX mapping of TiO2/Fe2O3/zeolite (Figure 5a-e)

and TiO2/Fe2O3/PAC (Figure 6a-d)

illustrated extensive

dispersion of Fe, Ti, C, O, Si and Al elements confirming that Fe2O3 and TiO2 were well distributed on the surface of PAC and zeolite. The TEM images of both synthesized nanostructures (Figure 7) showed the Fe2O3 particles were uniformly attached to the TiO2 surface. The sizes of particles varied and were up to 100 nm which were consistent with the FE-SEM results. The FTIR analysis comparison of the support materials and synthesized nanocomposites indicated only slight differences such as broadening of some bands and spectral shifts in the synthesized nanocomposites (Figure 8). These changes are presumably due to the interaction between the nanooxides and zeolite or PAC. The results of Figure 8a illustrated that the peak at 1033 cm−1 was



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associated to the stretching vibration band of Si(Al)–O in the zeolite27. The region within 14001710 cm-1 was related to the skeleton vibration of C-C groups (Figure 8b). The peaks at 560 cm-1 and 1100 cm-1 of Figure 8(a, b)were assigned to the Fe-O and Ti-O functional group respectively, while the strong peak of 3500 cm-1 was related to (O-H) of TiO2 and Fe2O3 particles 28, 29. The BET results (Table 3) confirmed that the surface area and pore volume of TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC were lower than those of zeolite and PAC. It can be owing to the penetration of TiO2 and Fe2O3 nanoparticles into the pores of zeolite and PAC. Further, the difference between the surface area and pore volume of TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC could be attributed to the difference in the surface structural features of zeolite (microporous material with ordered channel structure) 30, and activated carbon (hydrophobic surface) 31 and their solubility in aqueous solution. 3.2. Model fitting and statistical analysis The regression analysis between the experimental results and model based values proved that the quadratic model, was statistically significant for the estimation of cyanide degradation efficiency (Table 4 and 5). The regression equations for both catalysts are formulated in eqs 3 and 4. 𝑌Ti𝑂2/𝐹𝑒2𝑂3/zeolite(%) = 86.00 ― 1.34 ∗ 𝑋1 +1.48 ∗ 𝑋2 ―1.93 ∗ 𝑋3 ―1.00 ∗ 𝑋2 ∗ 𝑋3 +1.93𝑋32 (3) 𝑌Ti𝑂2/𝐹𝑒2𝑂3/PAC(%) = 89.41 ― 2.05 ∗ 𝑋1 +1.51 ∗ 𝑋2 ―2.34 ∗ 𝑋3 ―1.00 ∗ 𝑋2 ∗ 𝑋3 +0.64𝑋22 +2.45𝑋23 (4) The analysis of variance (ANOVA) results (Table 4 and 5) showed that the reduced quadratic models were highly significant with the p -values < 0.0001 for both catalysts, and F9 ACS Paragon Plus Environment

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values of 106.83 and 609.05 for TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC respectively. The lack of fit values, for the regression of Eq. (3) and for Eq. (4) were 0.798 and 0.565, respectively, implying that the lack of fit of both equations are not significant relative to the pure error. The independent variables of pH, irradiation time, initial cyanide concentration, square pH and the interaction between the irradiation time and pH, all are the significant model terms in eqs (3 and 4) (p pHzps=6.3 )(eqs 5 and 6)23, the repulsion of the negatively charged cyanide from the negatively charged surface of the nanocatalysts results in the reduction of the cyanide photodegradation Besides, the saddle shape of the curve (Figure 13 (a and b)) proved the existence of the interaction between the irradiation time and pH. 𝑝𝐻 < 𝑝𝐻𝑍𝑃𝑆:𝑇𝑖𝐼𝑉 ―𝑂𝐻 + 𝐻 + →𝑇𝑖𝐼𝑉 ― 𝑂𝐻2+

(5)

𝑝𝐻 > 𝑝𝐻𝑍𝑃𝑆:𝑇𝑖𝐼𝑉 ―𝑂𝐻 + 𝑂𝐻 ― →𝑇𝑖𝐼𝑉 ― 𝑂 ― + 𝐻2𝑂

(6)

3.4. Assessment of the optimal conditions, validation experiments and photocatalytic reaction mechanism The

optimized

conditions

for

the

cyanide

photodegradation

(89%)

using

TiO2/Fe2O3/zeolite were 200 mg/L of initial cyanide concentration, pH=10 and irradiation time=160 min (Table 6). Moreover, 97% cyanide photodegradation efficiency at the presence of TiO2/Fe2O3/PAC was achieved at its optimal conditions of [CN]- of 300 mg/L, pH of 10 and irradiation time of 170 min. However, under optimal conditions, the maximum predicted removal values in the presence of TiO2/Fe2O3/zeolite and TiO2/Fe2O3/PAC were 92.42% and 97.37%, respectively with satisfactory desirability of 1.00 (Figure 14 (a and b)). This indicated a good agreement of the models with the experimental results.



The assessment of adsorption comparison approved that the adsorption efficiency of PAC (40%) and TiO2/Fe2O3/PAC (49%) were much more than those of zeolite and TiO2/Fe2O3/zeolite (2% and 7%, respectively) (Table 6). This could be due to the difference in the surface structural features such as surface area due to the distinct interaction of supports with the active phase. The higher surface area of TiO2/Fe2O3/PAC (254.64 m2/g) compared to that of TiO2/Fe2O3/zeolite (112.69 m2/g) could justify the reason for better adsorption performance of TiO2/Fe2O3/PAC than TiO2/Fe2O3/zeolite. Another reason for this could be contributed to the surface functional group. The FTIR analysis demonstrated the existence of C-C group on the surface of the TiO2/Fe2O3/PAC which was the probable adsorption site and had the potential to adsorb more cyanide as compared with the TiO2/Fe2O3/zeolite that lacked C-C group. Moreover, the results of experiments indicated that the AOP process not only had the noticeable contribution on the cyanide removal in comparison with adsorption process, which only transfers cyanide to another phase without degrading it, but also it decomposed cyanide to harmless materials. Photocatalytic process as a kind of AOP process photodegrades cyanide using electron-hole pairs resulting from photocatalytic oxidation reaction after light irradiation and at the presence of H2O2 33. Figure 15 shows the mechanism of photocatalytic activity of TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite for cyanide degradation. The photocatalytic mechanism is based on the absorption of photon hv with the energy equal to or greater than band gap of semiconductors which leads to the generation of an electron-hole pair on the surface of nanocatalyst (eq (7))34, 35. When catalysts (TiO2/Fe2O3/PAC and TiO2/Fe2O3/zeolite) are irradiated with UV light, the electrons from TiO2 and Fe2O3 could be promoted from the valance band (VB) to the conduction band (CB) and a positive hole could be formed in the VB35-37. While some excited-state electrons can recombine with holes in the VB, some others can be transferred to the surface of the photocatalyst and react with Fe3+ (eq (8))37, 38.


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Besides, Fe3+ can act not only as an electron trap (eq (8)) but also as a hole trap (eq (9)) to enhance photocatalytic efficiency by reducing the recombination rate of h+/e- pairs33, 38, 39. This is due to the energy levels of Fe3+/Fe4+ and Fe3+/Fe2+ which are above the VB edge of TiO2 and below the CB edge of TiO2 respectively38, 40. However, Fe2+ and Fe4+ generated (eqs (8 and 9)) are more unstable as compared with Fe3+ which leads to the transfer of trapped charges from Fe2+ and Fe4+ to Fe3+ in the photocatalyst surface 33, 38, 40. Fe2+ can be oxided to Fe3+ ions by transferring electrons to absorbed O2 on the surface of TiO2 (eq (10)) or surface of Ti4+ ions (eq (11))38-40.Moreover, adsorbed O2 can be reduced to °O2― (eqs (10 and 12))38, 40 and the unstable °O2― can be transferred to OH°quickly in water(eq (13)) playing a significant role in further degrading cyanide. Fe4+ ions can be reduced by releasing electrons while surface hydroxyl group turns into hydroxyl radical (eq (14))37,

38, 40.

Therefore, Fe3+ can have positive effects on the reduction of the hole-electron

recombination rate and the enhancement of the photocatalytic activity. However, the mechanism of photoactivity of TiO2/Fe2O3 could be different when the concentration of Fe3+ ions is too high as Fe3+ can act as the recombination centers for photo-generated electrons and holes (eqs (15-17)) which triggers the reduction of photocatalytic activity37-40. Meanwhile, the presence of H2O2 can also be a key factor improving the degradation efficiency as H2O2 can act as a conduction band electron acceptor41 decreasing the recombination rate and contribute to the generation of hydroxyl radicals by direct UV light photolysis (eq (18)), capturing electrons (eq (19)) or reacting with °O2― (Eq.(20))41, 42. Finally, generated hydroxyl radicals degrade cyanide to cyanate (eq (21)) and then cyanate can be oxidized to produce bicarbonate and nitrogen gas, nitrite or nitrate (eqs (22-24)) 4345.

The formation of nitrite or nitrate depends on the amount of excess H2O2 present during the

reaction44, 45. Further, the hydrolysis of cyanate to ammonium and bicarbonate ions (eq (25)) is more favorable for acidic condition (pH

PAC and

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