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Synthesis, characterization and catalytic wet air oxidation property of mesoporous Ce1−xFexO2 mixed oxides
Materials Chemistry and Physics 155 (2015) 223e231
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Synthesis, characterization and catalytic wet air oxidation property of mesoporous Ce1xFexO2 mixed oxides Anushree, Satish Kumar*, Chhaya Sharma Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano-scaled Ce1xFexO2 mixed oxides were synthesized and characterized. The catalytic activity was assessed for CWO of paper industry wastewater. Surface area, pore volume and crystallite size affected the catalytic activity. Ce0.4Fe0.6O2 exhibited highest efficiency in catalytic wet oxidation. Biodegradability index increased appreciably after treatment.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 21 January 2015 Accepted 21 February 2015 Available online 26 February 2015
Nano-structured Ce1xFexO2 mixed oxides with different Fe loading have been synthesized via coprecipitation route and were evaluated as heterogeneous catalysts for catalytic wet air oxidation of wastewater. The synthesized catalysts were characterized by XRD, FT-IR, TGA, N2 adsorption/desorption, FE-SEM, TEM and EDS. The influence of initial pH, reaction temperature, reaction time and catalyst dose has been evaluated for the removal of COD and color from paper industry wastewater. The catalytic performance of Ce1xFexO2 mixed oxides was found to be superior to those of individual oxides of Ce and Fe. The molar ratio of Ce/Fe markedly influenced the activity of mixed oxides. Ce0.4Fe0.6O2 mixed oxide exhibited maximum COD (74%) and color (82%) removal under optimum experimental conditions. The biodegradability of wastewater increased from an initial level of 0.27e0.47 after treatment. The highest catalytic activity of Ce0.4Fe0.6O2 mixed oxide was strongly influenced by its high surface area, high pore volume, small particle size and high dispersion of Fe2O3 into ceria lattice. © 2015 Elsevier B.V. All rights reserved.
Keywords: Mesoporous Ce1xFexO2 mixed oxide Nano-catalyst Wastewater
1. Introduction The paper industry generates considerable amount of wastewater which can adversely affect the receiving aquatic environment through slime production, scum formation, toxicity and esthetical issues, as it has low BOD (biochemical oxygen demand),
* Corresponding author. E-mail address: [email protected] (S. Kumar). http://dx.doi.org/10.1016/j.matchemphys.2015.02.034 0254-0584/© 2015 Elsevier B.V. All rights reserved.
high COD (chemical oxygen demand), high color and low biodegradability (ratio of BOD5/COD) values [1]. This is due to the presence of more than 200e300 organic compounds and approximately 700 organic and inorganic compounds, i.e. chlorinated compounds, suspended solids, fatty acids, tannins, stillbenes, resin acids, lignin and its derivatives, sulfur and sulfur compounds etc. [2,3]. Various methods including precipitation, flocculation, adsorption and reverse osmosis have been widely studied to remove these compounds, but they require post-treatments to dispose the separated contaminants. The biological processes are
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most commonly used for full-scale secondary treatment of paper industry wastewater but their efficiency is very low (45e60% COD removal), as some of the pollutants are not biodegradable [4,5]. Therefore, other technologically advanced method is required in order to decrease the non-biodegradable components prior to biological treatment [6,7]. Among various advance oxidation processes (AOP's), the catalytic wet air oxidation (CWAO) is an extensively used chemical technique [8e10]. Rodríguez et al. [11] studied Cu/CNF catalyst for liquid phase oxidation of textile effluent at 160 C, 8.7 bar pressure and 6.6 g/L catalysts loading. Hosseini et al. [12] reported the Ru/Ir oxide coated Ti monolith for wet oxidation of pharmaceutical wastewater at 230 C, 50 bar pressure for 5 h. Although the removal efficiencies were good, but harsh operating conditions were the major problem. In the view of above examples, the application of CWAO process for wastewater treatment at milder operating conditions is of great interest. It has been reported that wet air oxidation (WAO) occur through the chain autocatalytic-oxidation process with the help of dissolved oxygen [13,14]. Therefore, increased amount of dissolved oxygen greatly influences the WAO process. CeO2 has enormous applications in heterogeneous catalysis due to its redox properties (Ce4þ/ € pe et al. [16] Ce3þ) and high oxygen storage capacity (OSC) [15]. Tcho suggested that increased activity of CeO2 nano-crystals is due to the existence of a more reactive type of oxygen. Later on Guzman et al. [17] confirmed these species to be superoxides (O 2 ), which could be formed when an electron trapped at reduced ceria surface (Ce3þ site), is transferred to an adsorbed O2 molecule. The reaction can be written as follows: Ce(III) þ O2(g) / Ce(IV) þ O 2 (a) Xu et al. [18] also detected that the oxygen storage capacity (OSC) for small nano-crystals was high for fully covered (supercharged) crystals with attached O2 molecules in the form of superoxide ions. Usually pure CeO2 is not used because of its rapid sintering at higher temperatures [19,20]. Therefore, the study of mixed CeO2 systems is of great interest. Cation dopants with oxidation states lower than þ4 leads to formation of oxygen vacancies. Size and concentration of oxygen vacancies depends on the size and oxidation state of dopant, respectively [21,22]. Various ceria-based systems have been investigated and it was observed that doped ions influence the catalytic properties by forming an active phase at the interface of catalyst [23]. Particular attention was given to CeeFe mixed oxides, because ceria and iron oxides both are of great importance in catalysis [24]. Fe2O3 interact with CeO2 and densify it by increasing oxide ion vacancies, which provide higher oxygen storage capacity [25,26]. Ironecerium mixed oxide was used successfully as a photocatalyst for the degradation of phenol, methylene blue, and congo red [27]. It has been reported as an effective catalyst for total oxidation of CO and CH4 [28]. CO, NOx, and C3H8 were removed from air using Pd/FexCe1xOy as an efficient three way catalyst [29]. Magnetic, nano-scaled Fe/Ce composite was found to be an efficient fenton like heterogeneous catalyst for degradation of 4chlorophenol [30]. Nano-crystalline CeeFe mixed oxides were reported to be a successful catalyst for CO oxidation [31]. The present work aimed to investigate the role of Fe2O3 in modifying the surface and catalytic properties of CeO2 at nanoscale. Various characterization techniques were employed in order to study the physicochemical properties of synthesized catalysts. These catalysts were studied for wet oxidation of paper industry wastewater at milder operating conditions. The influence of various reaction variables have been investigated followed by studies for reusability of the catalyst.
2. Experimental 2.1. Material and methods All the chemicals used for catalyst preparation were of analytical grade. Combined wastewater after primary clarifier was procured from a nearby paper industry. The pH of wastewater was adjusted with 1 M NaOH or 1 M H2SO4 solutions. The wastewater was characterized before and after treatment in accordance with standard methods [32]. COD was determined by closed reflux titrimetric method and color was measured at 465 nm using a UV-VIS double beam spectrophotometer (Analytic Jena Model SPEKOL 2000). BOD5 was determined by measuring dissolved oxygen before and after incubation at 20 C for 5 days. 2.2. Catalyst synthesis A series of Ce1xFexO2 (x ¼ 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) mixed oxides, as well as pure CeO2 and Fe2O3 were prepared by a facile coprecipitation method. The metal nitrate solutions i.e. Ce(NO3)3.6H2O and Fe(NO3)3.9(H2O) were stirred sufficiently to obtain a homogeneous solution. A solution of NaOH (0.5 M) was added drop wise to the mixture. When, the pH value was increased to ~10, the resulting solution was maintained at 70 C with continuous stirring for 2 h. The obtained slurry was washed with distilled water, followed by drying at 110 C and calcination at 400 C in air for 4 h. 2.3. Catalyst characterization Powder X-Ray diffraction (PXRD) for phase identification was carried out in the scanning angle (2q) range from 20 to 80 using diffractometer (Bruker AXS Diffractometer D8) with Cu Ka radiation (l ¼ 0.15418 nm) at a scan speed of 2 /min. Average crystallite size of the catalysts were calculated using the scherrer equation and the lattice parameter were estimated by means of standard indexation method. Fourier transform infrared spectroscopy (FT-IR) was employed as an additional probe to confirm the presence of inorganic species. FT-IR spectra were collected on a spectrophotometer (Perkin Elmer FT-IR C91158, resolution 4 cm1) using KBr pellet method in transmission mode from 800 to 400 cm1. Sample was first mixed with KBr at a ratio of 1/9, followed by pressing of the mixture into a transparent pellet. N2 adsorptionedesorption measurements were recorded (Quantachrome ASiQwin™) with previous vacuum-degasification of sample at 120 C for 12 h. The specific surface area of catalysts was calculated using the BET method, pore size distribution was analyzed by DFT method, and the pore volume was assessed from the adsorbed amount of nitrogen. The weight loss measurement was carried out thermo gravimetrically (EXSTAR TG/DTA 6300) from 25 to 800 C at heating rate of 10 C min1 in dry air. The microstructure of prepared catalysts was analyzed by field emission scanning electron microscopy (FE-SEM, Quanta 200F, 20 kV) and transmission electron microscopy (TEM, Tecnai G2 STWIN, 200 kV). The samples were prepared by gold coating method for FE-SEM analysis. Samples for TEM analysis were prepared by ultrasonication in ethanol followed by evaporating the drop of resultant suspension onto a carbon-coated copper grid. Elemental analysis of samples was carried out with energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, 51 XMX 1005). 2.4. Catalytic wet air oxidation The experiments were carried out in a three necked glass reactor operating at a constant temperature under atmospheric pressure. The experimental set-up used for CWAO of wastewater is
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Fig. 1. Experimental set-up for catalytic wet oxidation.
shown in Fig. 1. The wastewater was adjusted to desired pH and transferred to the glass reactor. After catalyst addition the aqueous suspension was stirred with excess oxygen. After completion of reaction, mixture was allowed to settle and supernatant solution was collected for further analysis.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD analysis XRD patterns of Ce1xFexO2 mixed oxides along with pure CeO2 and Fe2O3 are presented in Fig. 2(a). XRD patterns illustrate the effect of incorporation of Fe3þ into ceria matrix with varied composition. The diffraction pattern for cubic CeO2 match well with the characteristic peaks at 2q values of 28.5 , 33 , 47.4 , 56.3 , corresponding to (111), (200), (220), (311) crystal planes, respectively (JCPDS 81-0792). In pure Fe2O3 all reflections were characteristic of tetragonal hematite structure with peaks at 2q values of 33.1, 35.6 , 49.4 , 54 , 62.4 , 64 , corresponding to (104), (110), (024), (116), (214), (300) crystal planes, respectively (JCPDS 860550). In mixed oxides no peaks corresponding to Fe2O3 phase were found in samples upto 60 mol% Fe content. A weak diffraction peak at 2q value of 35.4 corresponding to (110) crystal plane of Fe2O3
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was obtained in Ce0.2Fe0.8O2 mixed oxide. It showed the successful incorporation of Fe into ceria lattice upto Ce0.4Fe0.6O2, while in case of Ce0.2Fe0.8O2 precipitation of Fe2O3 takes place as a separate phase. No diffraction peaks from impurities were observed confirming the successful synthesis of mixed oxides. In Ce0.8Fe0.2O2, characteristic ceria peak (111) shifted to higher angles (28.5e28.7 ) and the shifting increased with increasing Fe content. This shifting is shown in the low angle region from 25 to 35 (Fig. 2(b)). Similar findings have also been reported by Sirichaipraserta et al. [33]. The average crystallite size and lattice parameter of CeO2 and Fe2O3 were calculated from (111) and (110) diffraction peaks, respectively (Table 1). The peak width increased with increasing Fe content, which lead to decrease in crystallite size. The lattice parameter of CeO2 (5.416 Å) decreased linearly with increasing Fe content, indicating the lattice constraint due to smaller ionic radius of Feþ3 (0.64 Å) than that of Ceþ4 (1.01 Å) and Ceþ3 (1.11 Å). This lattice constraint induces the generation of oxygen vacancies [34,35]. 3.1.2. N2 adsorptionedesorption analysis The adsorptionedesorption isotherm of CeO2, Ce0.4Fe0.6O2 and Fe2O3 catalysts are presented in Fig. 3(a). All the samples indicated type IV adsorption isotherm with hysteresis loop (at the relative pressure P/Po > 0.5), characteristic of mesoporous materials [36]. The pore size distributions (PSD) of various samples are presented in Fig. 3(b). PSD indicated the presence of pores with wide distribution ranging from 3 to 9 nm for CeO2 and 9e12 nm for Fe2O3. Almost, uniform PSD (3e5 nm) was attained for Ce0.4Fe0.6O2 mixed oxide. The BrunauereEmmetteTeller (BET) specific surface areas of various catalysts along with pore volume are listed in Table 1. Pure CeO2 exhibited low specific surface area of 20 m2/g and pore volume of 0.0897 cc/g. Surface area and pore volume were found to increase with increasing Fe content and reached a maximum of 149 m2/g and 0.2693 cc/g, respectively for Ce0.4Fe0.6O2 mixed oxide. These findings showed that mixed oxides exhibited the porous structure with relatively high specific surface area and pore volume. Such architecture is useful for heterogeneous catalysis as it provides more active sites for decomposition of pollutant molecules [37]. 3.1.3. FT-IR spectroscopic measurements FT-IR spectra of CeO2, Ce0.4Fe0.6O2 and Fe2O3 catalysts calcined at 400 C are presented in Fig. 4. The FT-IR spectra of pure CeO2 exhibited a band at 560 cm1 corresponding to CeeO vibrations [38]. Whereas, pure Fe2O3 showed two strong bands at 540 cm1
Fig. 2. (a) XRD pattern of various samples (b) low angle region from 25 to 35 .
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Table 1 Structural parameters of various Ce1xFexO2 mixed oxides. Sample
Unit cell Crystallite Lattice size (nm)a parameter volume (Å)a (Å3)a
Fe2O3 19b Ce0.2Fe0.8O2 10, 17b Ce0.4Fe0.6O2 Ce0.5Fe0.5O2 Ce0.6Fe0.4O2 Ce0.8Fe0.2O2 CeO2
11 12 13 14 21
a
from from from from
b c d
Calculated Calculated Calculated Calculated
5.043b 5.229, 5.047b 5.332 5.347 5.355 5.385 5.416
Average Specific Total pore particle surface volume size (nm)c area (cc/g)d (m2/g)d
351b 35 ± 143, 355b 14 ± 37 ± 152 17 ± 153 20 ± 154 23 ± 156 28 ± 159 45 ±
(111) peak of CeO2. (110) peak of Fe2O3. FE-SEM images. N2-adsorption/desorption.
5 2, 3 1 3 3 2 4
35 135
0.176 0.252
149 109 97 94 20
0.283 0.256 0.269 0.256 0.089
and 464 cm1, which can be assigned to FeeO bonds in the internal structure of Fe2O3 [39]. In Ce0.4Fe0.6O2 mixed oxide FeeO band at 540 cm1 was shifted to 510 cm1 and second band at 464 cm1 decreased to 430 cm1. This red shift (decrease in frequency) was observed due to increase in lattice parameter from Fe2O3 (5.043 Å) to Ce0.4Fe0.6O2 (5.069 Å), which is correlating with the previous literature [40]. Decreased intensity of bands in Ce0.4Fe0.6O2 mixed oxide strongly supported the interaction between CeO2 and Fe2O3. 3.1.4. Thermogravimetric analysis Thermogravimetric (TG) and differential thermal gravimetric (DTG) spectra of non-calcined Ce0.4Fe0.6O2 mixed oxide are shown in Fig. 5. As it can be seen from figure, the TG and DTG plots depicted three main weight change areas. First weight loss in the range 50e120 C can be attributed to desorption of water adsorbed at the catalyst surface by hydrogen bonding. During the second stage a large DTG peak was observed in the range 200e350 C,
Fig. 3. (a) N2 adsorptionedesorption isotherm (b) Pore size distribution.
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Fig. 7(aec). Statistical analysis of TEM micrograph illustrates that these mixed oxides are having the particles with average diameter of 16, 8 and 6 nm, respectively. These results are in good agreement with XRD analysis. Corresponding EDS (Fig. 7(d)) confirmed 40:60 M ratio of Ce/Fe for Ce0.4Fe0.6O2 mixed oxide. 3.2. Parameter optimization Wastewater obtained from paper industry exhibited higher organic load in terms of COD and color (Table 2). The influence of various reaction variables i.e. pH (3.0e8.0), reaction temperature (40e90 C), reaction time (0.5e2.5 h), catalyst dose (0.5e2 g/L) and Ce/Fe mole ratio was estimated and the results are presented below.
Fig. 4. FT-IR spectra of various samples.
Fig. 5. TG and DTG curve of non-calcined Ce0.4Fe0.6O2.
associated with the formation of oxides due to condensation reactions of metal hydroxyl residual groups [41]. The weight loss of the precursor during oxide formation (13%) was very near to that of calculated value. Third weight loss (3%) from 420 to 480 C was due to the further oxidation of Fe2O3 to Fe3O4 [42].
3.1.5. Morphological studies The microstructures of Ce1xFexO2 mixed oxides were investigated by FE-SEM and TEM, the results obtained are as shown in Fig. 6 and Fig. 7, respectively. It can be seen from micrographs that the particles were in nano-meter range. FE-SEM micrograph of pure CeO2 showed the average particle size to be 45 nm (Fig. 6(a)) which decreased to 28 nm (Fig. 6(b)) for Ce0.8Fe0.2O2 mixed oxide. Increase in Fe content brought considerable decrease in average particle size, as given in Table 1. This may be due to the inhibition of aggregation of CeO2 nano-crystals during the crystallization process in presence of Fe [43]. Precipitation of Fe2O3 as a separate phase results in nano-particles with two different sizes for Ce0.2Fe0.8O2 mixed oxide (Fig. 6(f)). TEM micrographs of CeO2, Ce0.4Fe0.6O2 and Fe2O3 along with representative SAED pattern are presented in
3.2.1. Effect of pH The pH of solution is an important parameter for catalysis, as it affects catalyst charge that governs the adsorption of organics on catalyst surface [44]. The influence of pH was studied using several solutions with determined pH, without any modifications or control of the pH during the process. The experiments were performed in pH range 3.0e8.0, using 1 g/L Ce0.4Fe0.6O2 catalyst at 70 C for 2 h. The results obtained in terms of percent removal of COD and color, are presented in Fig. 8(a). The maximum removal of COD (65%) and color (80%) was obtained at pH 3, which decreased slightly at pH 4 (COD 63% and color 78%); after that the removal efficiency dropped. In acidic medium the surface of catalyst was positively charged (isoelectric point of CeO2 ¼ 6.7; Fe2O3 ¼ 8.4) which facilitated the adsorption of OH and anionic organic pollutants present in paper industry. In alkaline medium due to repulsion between the negatively charged catalyst surface and OH/anionic pollutants, low reduction efficiency was obtained [45]. Therefore, the initial pH strongly affected the reaction and pH 4 was selected as optimum value for further studies. 3.2.2. Effect of temperature To study the effect of temperature within the range 40e90 C, CWAO tests were performed using 1 g/L Ce0.4Fe0.6O2 catalyst at pH 4 for 2 h Fig. 8(b) presents the removal of COD and color with an increase in reaction temperature. The raise of temperature from 40 to 50 C produces only a slight increase in removal efficiencies. After that removal efficiency increased greatly and the maximum removal was attained at 90 C. This could be due to the accelerated supercharging of small particles with oxygen under oxidizing environment with increasing temperature [46]. 3.2.3. Effect of reaction time The optimization of reaction time was done within time interval of 0.5e2.5 h under treatment conditions; 1 g/L Ce0.4Fe0.6O2 catalyst, 90 C, pH 4. A rapid decrease in COD and color with increase in treatment time upto 2 h was observed and there after reached a nearly constant value (Fig. 8(c)). The wastewater contain large amount of organic pollutants and decrease in the rate of degradation after 2 h may be due to the adsorption of organic matter on catalyst surface, which hinders the oxidation process by decreasing the oxygen supply to catalyst surface and increasing competition for active sites between the reaction intermediates and organic matter [47,48]. Hence, a 2 h reaction time was selected as optimum for further experiments. 3.2.4. Effect of catalyst dose The influence of catalyst concentration within the reaction system was studied by varying the Ce0.4Fe0.6O2 catalyst dose at 90 C, pH 4 for 2 h. The COD and color removal efficiency increased appreciably with the catalyst concentration of 1 g/L (Fig. 8(d)).
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Fig. 6. FE-SEM (aeg) images of various catalysts: (a) CeO2 (b) Ce0.8Fe0.2O2 (c) Ce0.6Fe0.4O2 (d) Ce0.5Fe0.5O2 (e) Ce0.4Fe0.6O2 (f) Ce0.2Fe0.8O2 (g) Fe2O3.
Fig. 7. TEM image and SAED pattern of (a) CeO2 (b) Ce0.4Fe0.6O2 (c) Fe2O3 (d) EDS of Ce0.4Fe0.6O2.
Table 2 Characteristics of wastewater. Parameter
Value (average)
COD (mg/L) Color (mg PteCo/L) BOD5 (mg/L) pH
865 2768 234 7.6
± ± ± ±
32.14 114.46 12.84 0.14
Further increase in amount of catalyst brought only a slight increase in removal efficiency. This observation leads to the premise that catalyst dose of 1 g/L was optimum for the treatment studies.
3.2.5. Effect of mole ratio Fig. 8(e) presents the percent removal of COD and color with variation of cerium to iron mole ratio (90 C, pH 4, 1 g/L, 2 h). Pure CeO2 demonstrated very low removal of COD and color as compared to mixed oxide catalysts. The catalytic activity augmented with gradual increase of Fe content and attained a maximum value of 74% COD and 82% color reduction with Ce0.4Fe0.6O2 mixed oxide under optimum conditions. These results are in good agreement with XRD and BET analysis. It has been observed that pH of solution increased to 6.4 after reaction with Ce0.4Fe0.6O2 catalyst. This increase in pH could be attributed to
generation of hydroxyl radicals [49]. The initial biodegradability index (BOD5/COD ratio) of wastewater was low i.e. 0.27 which further increased to 0.47 after treatment. According to literature, the biodegradability index should be at least 0.40 for complete biodegradation [50]. Results revealed that Ce0.4Fe0.6O2 mixed oxides exhibited good catalytic activity for CWAO of wastewater at milder operating conditions. There are three possible reasons for increased activity of porous Fe modified ceria. Firstly, the substitution of Fe into CeO2 lattice leads to the formation of mixed oxides with decreased particle size, high surface area and high pore volume, resulting into high availability of active sites as well as increased adsorption of organic compounds onto the catalyst surface [51,37]. Second mechanism is the coupled semiconductor mechanism. CeO2 and Fe2O3 are n-type and p-type semiconductor, respectively; coupling of these semiconductors alters the electronic properties (Ceþ4eFeþ3 and Ceþ3eFeþ2) in mixed oxides [52]. Third, the incorporation of Feþ3 into CeO2 lattice create the oxygen vacancies therefore increased the OSC [15]. 3.3. Reusability of the catalyst To test the reusability of Ce0.4Fe0.6O2 catalyst, 5 treatment cycles were conducted with the same catalyst. The used catalyst was
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Fig. 8. Effect of various operating variables on COD and color removal (a) pH (b) Temperature (c) Time (d) Dose (e) mole ratio (f) catalyst recycling.
removed, then washed three times and dried for 12 h at 110 C before each run. Meanwhile, COD and color removal efficiency was measured. After three cycles the COD removal efficiency decreased from 74 to 69%, while color removal efficiency declined from 82 to 77% (Fig. 8(f)). These results depicted that the catalyst can be reused efficiently for three cycles and after that removal efficiency decreased considerably.
4. Conclusion Mesoporous Ce1xFexO2 mixed oxides were prepared by means of co-precipitation and applied for CWAO of paper industry wastewater. A significant improvement in treatment efficiency was observed by doping Feþ3 atoms into ceria lattice to form mixed oxides. Highest COD (74%) and color (82%) removal was achieved with Ce0.4Fe0.6O2 mixed oxide at the optimized conditions
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(temperature 90 C, atmospheric pressure, pH 4, catalyst dose 1 g/L and reaction time 2 h). The highest catalytic activity could be ascribed to its relatively high surface area, high pore volume and low crystallite size. In addition, the catalyst can be reused for 3 successive runs, without significant loss of activity. Increased biodegradability index from 0.27 to 0.47 indicated the removal of non-biodegradable component and provided a desirable pretreatment approach for biological treatment. Therefore, the Ce1xFexO2 mixed oxides were found to be promising catalysts for CWAO of wastewater under milder operating conditions. Acknowledgment The research grant for this study, provided by Ministry of Human Resource and Development, Government of India is gratefully acknowledged. References [1] D. Pokhrel, T. Viraraghavan, Treatment of pulp and paper mill wastewater a review, Sci. Total Environ. 333 (2004) 37e58. [2] V. Peck, R. Daley, Toward a ‘greener’ pulp and paper industry, Environ. Sci. Technol. 28 (1994) 524e527. [3] B. Karrasch, O. Parra, H. Cid, M. Mehrens, P. Pacheco, R. Urrutia, C. Valdovinos, C. Zaror, Effects of pulp and paper mill effluents on the microplankton and microbial self-purification capabilities of the Biobío River, Chile. Sci. Total Environ. 359 (2006) 194e208. [4] M. Ali, T.R. Sreekrishnan, Aquatic toxicity from pulp and paper mill effluents: a review, Adv. Environ. Res. 5 (2001) 175e196. [5] T. Leiviska, H. Nurmesniemi, R. Poykioc, J. Ramo, T. Kuokkanen, J. Pellinen, Effect of biological wastewater treatment on the molecular weight distribution of soluble organic compounds and on the reduction of BOD, COD and P in pulp and paper mill effluent, Water Res. 42 (2008) 3952e3960. €nder, S. Arayici, H. Barlas, Advanced treatment of pulp and paper mill [6] Z.B. Go wastewater by nanofiltration process: effects of operating conditions on membrane fouling, Sep. Purif. Technol. 76 (2011) 292e302. [7] S. Ciputra, A. Antony, R. Phillips, D. Richardson, G. Leslie, Comparison of treatment options for removal of recalcitrant dissolved organic matter from paper mill effluent, Chemosphere 81 (2010) 86e91. [8] J. Herney-ramirez, A.M.T. Silva, M.A. Vicente, C.A. Costa, L.M. Madeira, Degradation of acid orange 7 using a saponite-based catalyst in wet hydrogen peroxide oxidation: kinetic study with the Fermi's equation, Appl. Catal. B Environ. 101 (2011) 197e205. [9] W. Zhan, X. Wang, D. Li, Y. Ren, D. Liu, J. Kang, Catalytic wet air oxidation of high concentration pharmaceutical wastewater, Water Sci. Technol. 67 (2013) 2281e2286. [10] M. Han, Y. Chen, F. He, L. Yu, Treatment of oily wastewater with WPO and CWO, J. Kor. Chem. Soc. 58 (2014) 68e71. [11] A. Rodrıguez, G. Ovejero, M.D. Romero, C. Dıaz, M. Barreiro, J. Garcia, Catalytic wet air oxidation of textile industrial wastewater using metal supported on carbon nanofibers, J. Supercrit. Fluids 46 (2008) 163e172. Sze , J. Kristo f, E. les, [12] A.M. Hosseini, A. Tungler, Z. Schay, S. Szabo si, Comparison of precious metal oxide/titanium monolith catL. Szentmiklo alysts in wet oxidation of wastewaters, Appl. Catal. B Environ. 127 (2012) 99e104. [13] R.A. Sheldon, J.K. Kochi, Metal-catalyzed Oxidation of Organic Compounds, Acad. Press, London, 1981. [14] A. Rodriguez, J. Garcia, G. Ovejero, M. Mestanza, Wet air and catalytic wet air oxidation of several azodyes from wastewaters: the beneficial role of catalysis, Water Sci. Technol. 60 (2009) 1989e1999. [15] E. Aneggi, C. de Leitenburg, A. Trovarelli, On the role of lattice/surface oxygen in ceria-zirconia catalysts for diesel soot combustion, Catal. Today 181 (2012) 108e115. [16] A. Tschope, D. Staadt, R. Birringer, J.Y. Ying, Catalytic properties of nanostructured metal oxides, synthesized by inert gas condensation, Nanostruct. Mater. 9 (1997) 423e432. [17] J. Guzman, S. Carrettin, J.C. Fierro-Gonzalez, Y. Hao, B.C. Gates, A. Corma, CO oxidation catalyzed by supported gold: cooperation between gold and nanocrystalline rare-earth supports forms reactive surface superoxide and peroxide species, Angew. Chem. Int. 44 (2005) 4778e4781. [18] J. Xu, J. Harmer, G. Li, T. Chapman, P. Collier, S. Longworth, S.C. Tsang, Size dependent oxygen buffering capacity of ceria nanocrystals, Chem. Commun. 46 (2010) 1887e1889. [19] A. Trovarelli, C. de Leitenburg, G. Dolcetti, Design better cerium-based oxidation catalysts, Chemtech 27 (1997) 32e37. [20] A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J.E. Fallah, L. Hilarie, F. Leonormand, E. Quemere, G.N. Sauvion, O. Touret, Reduction of CeO2 by hydrogen, magnetic susceptibility and Fouriertransform infrared, ultraviolet and X-ray photoelectron spectroscopy
measurements, J. Chem. Soc. Faraday Trans. I 87 (1991) 1601e1609. [21] G. Zhang, M. Liu, Effect of particle size and dopant on properties of SnO2-based gas sensors, Sens. Actuators B 69 (2000) 144e152. [22] M.A.S. Quintanilla, J.M. Valverde, A. Castellanos, Adhesion force between fine particles with controlled surface properties, AIChE J. 52 (2006) 1715e1728. [23] G.D. Panagiotou, T. Petsi, K. Bourikas, A.G. Kalampounias, S. Boghosian, C. Kordulis, A. Lycourghiotis, Interfacial impregnation chemistry in the synthesis of molybdenum catalysts supported on titania, J. Phys. Chem. C 114 (2010) 11868e11879. [24] F.J. Perez-Alonso, M.L. Granados, M. Ojeda, T. Herranz, S. Rojas, P. Terreros, J.L.G. Fierro, M. Gracia, J.R. Gancedo, Relevance in the Fischer-Tropsch synthesis of the formation of Fe-O-Ce interactions on iron-cerium mixed oxide systems, J. Phys. Chem. B 110 (2006) 23870e23880. [25] T. Zhang, P. Hing, H. Huang, J. Kilner, Densification, microstructure and grain growth in the CeO2-Fe2O3 system (0¼
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis, characterization and catalytic wet air oxidation property of mesoporous Ce1xFexO2 mixed oxides Anushree, Satish Kumar*, Chhaya Sharma Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano-scaled Ce1xFexO2 mixed oxides were synthesized and characterized. The catalytic activity was assessed for CWO of paper industry wastewater. Surface area, pore volume and crystallite size affected the catalytic activity. Ce0.4Fe0.6O2 exhibited highest efficiency in catalytic wet oxidation. Biodegradability index increased appreciably after treatment.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 21 January 2015 Accepted 21 February 2015 Available online 26 February 2015
Nano-structured Ce1xFexO2 mixed oxides with different Fe loading have been synthesized via coprecipitation route and were evaluated as heterogeneous catalysts for catalytic wet air oxidation of wastewater. The synthesized catalysts were characterized by XRD, FT-IR, TGA, N2 adsorption/desorption, FE-SEM, TEM and EDS. The influence of initial pH, reaction temperature, reaction time and catalyst dose has been evaluated for the removal of COD and color from paper industry wastewater. The catalytic performance of Ce1xFexO2 mixed oxides was found to be superior to those of individual oxides of Ce and Fe. The molar ratio of Ce/Fe markedly influenced the activity of mixed oxides. Ce0.4Fe0.6O2 mixed oxide exhibited maximum COD (74%) and color (82%) removal under optimum experimental conditions. The biodegradability of wastewater increased from an initial level of 0.27e0.47 after treatment. The highest catalytic activity of Ce0.4Fe0.6O2 mixed oxide was strongly influenced by its high surface area, high pore volume, small particle size and high dispersion of Fe2O3 into ceria lattice. © 2015 Elsevier B.V. All rights reserved.
Keywords: Mesoporous Ce1xFexO2 mixed oxide Nano-catalyst Wastewater
1. Introduction The paper industry generates considerable amount of wastewater which can adversely affect the receiving aquatic environment through slime production, scum formation, toxicity and esthetical issues, as it has low BOD (biochemical oxygen demand),
* Corresponding author. E-mail address: [email protected] (S. Kumar). http://dx.doi.org/10.1016/j.matchemphys.2015.02.034 0254-0584/© 2015 Elsevier B.V. All rights reserved.
high COD (chemical oxygen demand), high color and low biodegradability (ratio of BOD5/COD) values [1]. This is due to the presence of more than 200e300 organic compounds and approximately 700 organic and inorganic compounds, i.e. chlorinated compounds, suspended solids, fatty acids, tannins, stillbenes, resin acids, lignin and its derivatives, sulfur and sulfur compounds etc. [2,3]. Various methods including precipitation, flocculation, adsorption and reverse osmosis have been widely studied to remove these compounds, but they require post-treatments to dispose the separated contaminants. The biological processes are
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most commonly used for full-scale secondary treatment of paper industry wastewater but their efficiency is very low (45e60% COD removal), as some of the pollutants are not biodegradable [4,5]. Therefore, other technologically advanced method is required in order to decrease the non-biodegradable components prior to biological treatment [6,7]. Among various advance oxidation processes (AOP's), the catalytic wet air oxidation (CWAO) is an extensively used chemical technique [8e10]. Rodríguez et al. [11] studied Cu/CNF catalyst for liquid phase oxidation of textile effluent at 160 C, 8.7 bar pressure and 6.6 g/L catalysts loading. Hosseini et al. [12] reported the Ru/Ir oxide coated Ti monolith for wet oxidation of pharmaceutical wastewater at 230 C, 50 bar pressure for 5 h. Although the removal efficiencies were good, but harsh operating conditions were the major problem. In the view of above examples, the application of CWAO process for wastewater treatment at milder operating conditions is of great interest. It has been reported that wet air oxidation (WAO) occur through the chain autocatalytic-oxidation process with the help of dissolved oxygen [13,14]. Therefore, increased amount of dissolved oxygen greatly influences the WAO process. CeO2 has enormous applications in heterogeneous catalysis due to its redox properties (Ce4þ/ € pe et al. [16] Ce3þ) and high oxygen storage capacity (OSC) [15]. Tcho suggested that increased activity of CeO2 nano-crystals is due to the existence of a more reactive type of oxygen. Later on Guzman et al. [17] confirmed these species to be superoxides (O 2 ), which could be formed when an electron trapped at reduced ceria surface (Ce3þ site), is transferred to an adsorbed O2 molecule. The reaction can be written as follows: Ce(III) þ O2(g) / Ce(IV) þ O 2 (a) Xu et al. [18] also detected that the oxygen storage capacity (OSC) for small nano-crystals was high for fully covered (supercharged) crystals with attached O2 molecules in the form of superoxide ions. Usually pure CeO2 is not used because of its rapid sintering at higher temperatures [19,20]. Therefore, the study of mixed CeO2 systems is of great interest. Cation dopants with oxidation states lower than þ4 leads to formation of oxygen vacancies. Size and concentration of oxygen vacancies depends on the size and oxidation state of dopant, respectively [21,22]. Various ceria-based systems have been investigated and it was observed that doped ions influence the catalytic properties by forming an active phase at the interface of catalyst [23]. Particular attention was given to CeeFe mixed oxides, because ceria and iron oxides both are of great importance in catalysis [24]. Fe2O3 interact with CeO2 and densify it by increasing oxide ion vacancies, which provide higher oxygen storage capacity [25,26]. Ironecerium mixed oxide was used successfully as a photocatalyst for the degradation of phenol, methylene blue, and congo red [27]. It has been reported as an effective catalyst for total oxidation of CO and CH4 [28]. CO, NOx, and C3H8 were removed from air using Pd/FexCe1xOy as an efficient three way catalyst [29]. Magnetic, nano-scaled Fe/Ce composite was found to be an efficient fenton like heterogeneous catalyst for degradation of 4chlorophenol [30]. Nano-crystalline CeeFe mixed oxides were reported to be a successful catalyst for CO oxidation [31]. The present work aimed to investigate the role of Fe2O3 in modifying the surface and catalytic properties of CeO2 at nanoscale. Various characterization techniques were employed in order to study the physicochemical properties of synthesized catalysts. These catalysts were studied for wet oxidation of paper industry wastewater at milder operating conditions. The influence of various reaction variables have been investigated followed by studies for reusability of the catalyst.
2. Experimental 2.1. Material and methods All the chemicals used for catalyst preparation were of analytical grade. Combined wastewater after primary clarifier was procured from a nearby paper industry. The pH of wastewater was adjusted with 1 M NaOH or 1 M H2SO4 solutions. The wastewater was characterized before and after treatment in accordance with standard methods [32]. COD was determined by closed reflux titrimetric method and color was measured at 465 nm using a UV-VIS double beam spectrophotometer (Analytic Jena Model SPEKOL 2000). BOD5 was determined by measuring dissolved oxygen before and after incubation at 20 C for 5 days. 2.2. Catalyst synthesis A series of Ce1xFexO2 (x ¼ 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) mixed oxides, as well as pure CeO2 and Fe2O3 were prepared by a facile coprecipitation method. The metal nitrate solutions i.e. Ce(NO3)3.6H2O and Fe(NO3)3.9(H2O) were stirred sufficiently to obtain a homogeneous solution. A solution of NaOH (0.5 M) was added drop wise to the mixture. When, the pH value was increased to ~10, the resulting solution was maintained at 70 C with continuous stirring for 2 h. The obtained slurry was washed with distilled water, followed by drying at 110 C and calcination at 400 C in air for 4 h. 2.3. Catalyst characterization Powder X-Ray diffraction (PXRD) for phase identification was carried out in the scanning angle (2q) range from 20 to 80 using diffractometer (Bruker AXS Diffractometer D8) with Cu Ka radiation (l ¼ 0.15418 nm) at a scan speed of 2 /min. Average crystallite size of the catalysts were calculated using the scherrer equation and the lattice parameter were estimated by means of standard indexation method. Fourier transform infrared spectroscopy (FT-IR) was employed as an additional probe to confirm the presence of inorganic species. FT-IR spectra were collected on a spectrophotometer (Perkin Elmer FT-IR C91158, resolution 4 cm1) using KBr pellet method in transmission mode from 800 to 400 cm1. Sample was first mixed with KBr at a ratio of 1/9, followed by pressing of the mixture into a transparent pellet. N2 adsorptionedesorption measurements were recorded (Quantachrome ASiQwin™) with previous vacuum-degasification of sample at 120 C for 12 h. The specific surface area of catalysts was calculated using the BET method, pore size distribution was analyzed by DFT method, and the pore volume was assessed from the adsorbed amount of nitrogen. The weight loss measurement was carried out thermo gravimetrically (EXSTAR TG/DTA 6300) from 25 to 800 C at heating rate of 10 C min1 in dry air. The microstructure of prepared catalysts was analyzed by field emission scanning electron microscopy (FE-SEM, Quanta 200F, 20 kV) and transmission electron microscopy (TEM, Tecnai G2 STWIN, 200 kV). The samples were prepared by gold coating method for FE-SEM analysis. Samples for TEM analysis were prepared by ultrasonication in ethanol followed by evaporating the drop of resultant suspension onto a carbon-coated copper grid. Elemental analysis of samples was carried out with energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, 51 XMX 1005). 2.4. Catalytic wet air oxidation The experiments were carried out in a three necked glass reactor operating at a constant temperature under atmospheric pressure. The experimental set-up used for CWAO of wastewater is
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Fig. 1. Experimental set-up for catalytic wet oxidation.
shown in Fig. 1. The wastewater was adjusted to desired pH and transferred to the glass reactor. After catalyst addition the aqueous suspension was stirred with excess oxygen. After completion of reaction, mixture was allowed to settle and supernatant solution was collected for further analysis.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD analysis XRD patterns of Ce1xFexO2 mixed oxides along with pure CeO2 and Fe2O3 are presented in Fig. 2(a). XRD patterns illustrate the effect of incorporation of Fe3þ into ceria matrix with varied composition. The diffraction pattern for cubic CeO2 match well with the characteristic peaks at 2q values of 28.5 , 33 , 47.4 , 56.3 , corresponding to (111), (200), (220), (311) crystal planes, respectively (JCPDS 81-0792). In pure Fe2O3 all reflections were characteristic of tetragonal hematite structure with peaks at 2q values of 33.1, 35.6 , 49.4 , 54 , 62.4 , 64 , corresponding to (104), (110), (024), (116), (214), (300) crystal planes, respectively (JCPDS 860550). In mixed oxides no peaks corresponding to Fe2O3 phase were found in samples upto 60 mol% Fe content. A weak diffraction peak at 2q value of 35.4 corresponding to (110) crystal plane of Fe2O3
225
was obtained in Ce0.2Fe0.8O2 mixed oxide. It showed the successful incorporation of Fe into ceria lattice upto Ce0.4Fe0.6O2, while in case of Ce0.2Fe0.8O2 precipitation of Fe2O3 takes place as a separate phase. No diffraction peaks from impurities were observed confirming the successful synthesis of mixed oxides. In Ce0.8Fe0.2O2, characteristic ceria peak (111) shifted to higher angles (28.5e28.7 ) and the shifting increased with increasing Fe content. This shifting is shown in the low angle region from 25 to 35 (Fig. 2(b)). Similar findings have also been reported by Sirichaipraserta et al. [33]. The average crystallite size and lattice parameter of CeO2 and Fe2O3 were calculated from (111) and (110) diffraction peaks, respectively (Table 1). The peak width increased with increasing Fe content, which lead to decrease in crystallite size. The lattice parameter of CeO2 (5.416 Å) decreased linearly with increasing Fe content, indicating the lattice constraint due to smaller ionic radius of Feþ3 (0.64 Å) than that of Ceþ4 (1.01 Å) and Ceþ3 (1.11 Å). This lattice constraint induces the generation of oxygen vacancies [34,35]. 3.1.2. N2 adsorptionedesorption analysis The adsorptionedesorption isotherm of CeO2, Ce0.4Fe0.6O2 and Fe2O3 catalysts are presented in Fig. 3(a). All the samples indicated type IV adsorption isotherm with hysteresis loop (at the relative pressure P/Po > 0.5), characteristic of mesoporous materials [36]. The pore size distributions (PSD) of various samples are presented in Fig. 3(b). PSD indicated the presence of pores with wide distribution ranging from 3 to 9 nm for CeO2 and 9e12 nm for Fe2O3. Almost, uniform PSD (3e5 nm) was attained for Ce0.4Fe0.6O2 mixed oxide. The BrunauereEmmetteTeller (BET) specific surface areas of various catalysts along with pore volume are listed in Table 1. Pure CeO2 exhibited low specific surface area of 20 m2/g and pore volume of 0.0897 cc/g. Surface area and pore volume were found to increase with increasing Fe content and reached a maximum of 149 m2/g and 0.2693 cc/g, respectively for Ce0.4Fe0.6O2 mixed oxide. These findings showed that mixed oxides exhibited the porous structure with relatively high specific surface area and pore volume. Such architecture is useful for heterogeneous catalysis as it provides more active sites for decomposition of pollutant molecules [37]. 3.1.3. FT-IR spectroscopic measurements FT-IR spectra of CeO2, Ce0.4Fe0.6O2 and Fe2O3 catalysts calcined at 400 C are presented in Fig. 4. The FT-IR spectra of pure CeO2 exhibited a band at 560 cm1 corresponding to CeeO vibrations [38]. Whereas, pure Fe2O3 showed two strong bands at 540 cm1
Fig. 2. (a) XRD pattern of various samples (b) low angle region from 25 to 35 .
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Table 1 Structural parameters of various Ce1xFexO2 mixed oxides. Sample
Unit cell Crystallite Lattice size (nm)a parameter volume (Å)a (Å3)a
Fe2O3 19b Ce0.2Fe0.8O2 10, 17b Ce0.4Fe0.6O2 Ce0.5Fe0.5O2 Ce0.6Fe0.4O2 Ce0.8Fe0.2O2 CeO2
11 12 13 14 21
a
from from from from
b c d
Calculated Calculated Calculated Calculated
5.043b 5.229, 5.047b 5.332 5.347 5.355 5.385 5.416
Average Specific Total pore particle surface volume size (nm)c area (cc/g)d (m2/g)d
351b 35 ± 143, 355b 14 ± 37 ± 152 17 ± 153 20 ± 154 23 ± 156 28 ± 159 45 ±
(111) peak of CeO2. (110) peak of Fe2O3. FE-SEM images. N2-adsorption/desorption.
5 2, 3 1 3 3 2 4
35 135
0.176 0.252
149 109 97 94 20
0.283 0.256 0.269 0.256 0.089
and 464 cm1, which can be assigned to FeeO bonds in the internal structure of Fe2O3 [39]. In Ce0.4Fe0.6O2 mixed oxide FeeO band at 540 cm1 was shifted to 510 cm1 and second band at 464 cm1 decreased to 430 cm1. This red shift (decrease in frequency) was observed due to increase in lattice parameter from Fe2O3 (5.043 Å) to Ce0.4Fe0.6O2 (5.069 Å), which is correlating with the previous literature [40]. Decreased intensity of bands in Ce0.4Fe0.6O2 mixed oxide strongly supported the interaction between CeO2 and Fe2O3. 3.1.4. Thermogravimetric analysis Thermogravimetric (TG) and differential thermal gravimetric (DTG) spectra of non-calcined Ce0.4Fe0.6O2 mixed oxide are shown in Fig. 5. As it can be seen from figure, the TG and DTG plots depicted three main weight change areas. First weight loss in the range 50e120 C can be attributed to desorption of water adsorbed at the catalyst surface by hydrogen bonding. During the second stage a large DTG peak was observed in the range 200e350 C,
Fig. 3. (a) N2 adsorptionedesorption isotherm (b) Pore size distribution.
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Fig. 7(aec). Statistical analysis of TEM micrograph illustrates that these mixed oxides are having the particles with average diameter of 16, 8 and 6 nm, respectively. These results are in good agreement with XRD analysis. Corresponding EDS (Fig. 7(d)) confirmed 40:60 M ratio of Ce/Fe for Ce0.4Fe0.6O2 mixed oxide. 3.2. Parameter optimization Wastewater obtained from paper industry exhibited higher organic load in terms of COD and color (Table 2). The influence of various reaction variables i.e. pH (3.0e8.0), reaction temperature (40e90 C), reaction time (0.5e2.5 h), catalyst dose (0.5e2 g/L) and Ce/Fe mole ratio was estimated and the results are presented below.
Fig. 4. FT-IR spectra of various samples.
Fig. 5. TG and DTG curve of non-calcined Ce0.4Fe0.6O2.
associated with the formation of oxides due to condensation reactions of metal hydroxyl residual groups [41]. The weight loss of the precursor during oxide formation (13%) was very near to that of calculated value. Third weight loss (3%) from 420 to 480 C was due to the further oxidation of Fe2O3 to Fe3O4 [42].
3.1.5. Morphological studies The microstructures of Ce1xFexO2 mixed oxides were investigated by FE-SEM and TEM, the results obtained are as shown in Fig. 6 and Fig. 7, respectively. It can be seen from micrographs that the particles were in nano-meter range. FE-SEM micrograph of pure CeO2 showed the average particle size to be 45 nm (Fig. 6(a)) which decreased to 28 nm (Fig. 6(b)) for Ce0.8Fe0.2O2 mixed oxide. Increase in Fe content brought considerable decrease in average particle size, as given in Table 1. This may be due to the inhibition of aggregation of CeO2 nano-crystals during the crystallization process in presence of Fe [43]. Precipitation of Fe2O3 as a separate phase results in nano-particles with two different sizes for Ce0.2Fe0.8O2 mixed oxide (Fig. 6(f)). TEM micrographs of CeO2, Ce0.4Fe0.6O2 and Fe2O3 along with representative SAED pattern are presented in
3.2.1. Effect of pH The pH of solution is an important parameter for catalysis, as it affects catalyst charge that governs the adsorption of organics on catalyst surface [44]. The influence of pH was studied using several solutions with determined pH, without any modifications or control of the pH during the process. The experiments were performed in pH range 3.0e8.0, using 1 g/L Ce0.4Fe0.6O2 catalyst at 70 C for 2 h. The results obtained in terms of percent removal of COD and color, are presented in Fig. 8(a). The maximum removal of COD (65%) and color (80%) was obtained at pH 3, which decreased slightly at pH 4 (COD 63% and color 78%); after that the removal efficiency dropped. In acidic medium the surface of catalyst was positively charged (isoelectric point of CeO2 ¼ 6.7; Fe2O3 ¼ 8.4) which facilitated the adsorption of OH and anionic organic pollutants present in paper industry. In alkaline medium due to repulsion between the negatively charged catalyst surface and OH/anionic pollutants, low reduction efficiency was obtained [45]. Therefore, the initial pH strongly affected the reaction and pH 4 was selected as optimum value for further studies. 3.2.2. Effect of temperature To study the effect of temperature within the range 40e90 C, CWAO tests were performed using 1 g/L Ce0.4Fe0.6O2 catalyst at pH 4 for 2 h Fig. 8(b) presents the removal of COD and color with an increase in reaction temperature. The raise of temperature from 40 to 50 C produces only a slight increase in removal efficiencies. After that removal efficiency increased greatly and the maximum removal was attained at 90 C. This could be due to the accelerated supercharging of small particles with oxygen under oxidizing environment with increasing temperature [46]. 3.2.3. Effect of reaction time The optimization of reaction time was done within time interval of 0.5e2.5 h under treatment conditions; 1 g/L Ce0.4Fe0.6O2 catalyst, 90 C, pH 4. A rapid decrease in COD and color with increase in treatment time upto 2 h was observed and there after reached a nearly constant value (Fig. 8(c)). The wastewater contain large amount of organic pollutants and decrease in the rate of degradation after 2 h may be due to the adsorption of organic matter on catalyst surface, which hinders the oxidation process by decreasing the oxygen supply to catalyst surface and increasing competition for active sites between the reaction intermediates and organic matter [47,48]. Hence, a 2 h reaction time was selected as optimum for further experiments. 3.2.4. Effect of catalyst dose The influence of catalyst concentration within the reaction system was studied by varying the Ce0.4Fe0.6O2 catalyst dose at 90 C, pH 4 for 2 h. The COD and color removal efficiency increased appreciably with the catalyst concentration of 1 g/L (Fig. 8(d)).
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Fig. 6. FE-SEM (aeg) images of various catalysts: (a) CeO2 (b) Ce0.8Fe0.2O2 (c) Ce0.6Fe0.4O2 (d) Ce0.5Fe0.5O2 (e) Ce0.4Fe0.6O2 (f) Ce0.2Fe0.8O2 (g) Fe2O3.
Fig. 7. TEM image and SAED pattern of (a) CeO2 (b) Ce0.4Fe0.6O2 (c) Fe2O3 (d) EDS of Ce0.4Fe0.6O2.
Table 2 Characteristics of wastewater. Parameter
Value (average)
COD (mg/L) Color (mg PteCo/L) BOD5 (mg/L) pH
865 2768 234 7.6
± ± ± ±
32.14 114.46 12.84 0.14
Further increase in amount of catalyst brought only a slight increase in removal efficiency. This observation leads to the premise that catalyst dose of 1 g/L was optimum for the treatment studies.
3.2.5. Effect of mole ratio Fig. 8(e) presents the percent removal of COD and color with variation of cerium to iron mole ratio (90 C, pH 4, 1 g/L, 2 h). Pure CeO2 demonstrated very low removal of COD and color as compared to mixed oxide catalysts. The catalytic activity augmented with gradual increase of Fe content and attained a maximum value of 74% COD and 82% color reduction with Ce0.4Fe0.6O2 mixed oxide under optimum conditions. These results are in good agreement with XRD and BET analysis. It has been observed that pH of solution increased to 6.4 after reaction with Ce0.4Fe0.6O2 catalyst. This increase in pH could be attributed to
generation of hydroxyl radicals [49]. The initial biodegradability index (BOD5/COD ratio) of wastewater was low i.e. 0.27 which further increased to 0.47 after treatment. According to literature, the biodegradability index should be at least 0.40 for complete biodegradation [50]. Results revealed that Ce0.4Fe0.6O2 mixed oxides exhibited good catalytic activity for CWAO of wastewater at milder operating conditions. There are three possible reasons for increased activity of porous Fe modified ceria. Firstly, the substitution of Fe into CeO2 lattice leads to the formation of mixed oxides with decreased particle size, high surface area and high pore volume, resulting into high availability of active sites as well as increased adsorption of organic compounds onto the catalyst surface [51,37]. Second mechanism is the coupled semiconductor mechanism. CeO2 and Fe2O3 are n-type and p-type semiconductor, respectively; coupling of these semiconductors alters the electronic properties (Ceþ4eFeþ3 and Ceþ3eFeþ2) in mixed oxides [52]. Third, the incorporation of Feþ3 into CeO2 lattice create the oxygen vacancies therefore increased the OSC [15]. 3.3. Reusability of the catalyst To test the reusability of Ce0.4Fe0.6O2 catalyst, 5 treatment cycles were conducted with the same catalyst. The used catalyst was
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Fig. 8. Effect of various operating variables on COD and color removal (a) pH (b) Temperature (c) Time (d) Dose (e) mole ratio (f) catalyst recycling.
removed, then washed three times and dried for 12 h at 110 C before each run. Meanwhile, COD and color removal efficiency was measured. After three cycles the COD removal efficiency decreased from 74 to 69%, while color removal efficiency declined from 82 to 77% (Fig. 8(f)). These results depicted that the catalyst can be reused efficiently for three cycles and after that removal efficiency decreased considerably.
4. Conclusion Mesoporous Ce1xFexO2 mixed oxides were prepared by means of co-precipitation and applied for CWAO of paper industry wastewater. A significant improvement in treatment efficiency was observed by doping Feþ3 atoms into ceria lattice to form mixed oxides. Highest COD (74%) and color (82%) removal was achieved with Ce0.4Fe0.6O2 mixed oxide at the optimized conditions
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(temperature 90 C, atmospheric pressure, pH 4, catalyst dose 1 g/L and reaction time 2 h). The highest catalytic activity could be ascribed to its relatively high surface area, high pore volume and low crystallite size. In addition, the catalyst can be reused for 3 successive runs, without significant loss of activity. Increased biodegradability index from 0.27 to 0.47 indicated the removal of non-biodegradable component and provided a desirable pretreatment approach for biological treatment. Therefore, the Ce1xFexO2 mixed oxides were found to be promising catalysts for CWAO of wastewater under milder operating conditions. Acknowledgment The research grant for this study, provided by Ministry of Human Resource and Development, Government of India is gratefully acknowledged. References [1] D. Pokhrel, T. Viraraghavan, Treatment of pulp and paper mill wastewater a review, Sci. Total Environ. 333 (2004) 37e58. [2] V. Peck, R. Daley, Toward a ‘greener’ pulp and paper industry, Environ. Sci. Technol. 28 (1994) 524e527. [3] B. Karrasch, O. Parra, H. Cid, M. Mehrens, P. Pacheco, R. Urrutia, C. Valdovinos, C. Zaror, Effects of pulp and paper mill effluents on the microplankton and microbial self-purification capabilities of the Biobío River, Chile. Sci. Total Environ. 359 (2006) 194e208. [4] M. Ali, T.R. Sreekrishnan, Aquatic toxicity from pulp and paper mill effluents: a review, Adv. Environ. Res. 5 (2001) 175e196. [5] T. Leiviska, H. Nurmesniemi, R. Poykioc, J. Ramo, T. Kuokkanen, J. Pellinen, Effect of biological wastewater treatment on the molecular weight distribution of soluble organic compounds and on the reduction of BOD, COD and P in pulp and paper mill effluent, Water Res. 42 (2008) 3952e3960. €nder, S. Arayici, H. Barlas, Advanced treatment of pulp and paper mill [6] Z.B. Go wastewater by nanofiltration process: effects of operating conditions on membrane fouling, Sep. Purif. Technol. 76 (2011) 292e302. [7] S. Ciputra, A. Antony, R. Phillips, D. Richardson, G. Leslie, Comparison of treatment options for removal of recalcitrant dissolved organic matter from paper mill effluent, Chemosphere 81 (2010) 86e91. [8] J. Herney-ramirez, A.M.T. Silva, M.A. Vicente, C.A. Costa, L.M. Madeira, Degradation of acid orange 7 using a saponite-based catalyst in wet hydrogen peroxide oxidation: kinetic study with the Fermi's equation, Appl. Catal. B Environ. 101 (2011) 197e205. [9] W. Zhan, X. Wang, D. Li, Y. Ren, D. Liu, J. Kang, Catalytic wet air oxidation of high concentration pharmaceutical wastewater, Water Sci. Technol. 67 (2013) 2281e2286. [10] M. Han, Y. Chen, F. He, L. Yu, Treatment of oily wastewater with WPO and CWO, J. Kor. Chem. Soc. 58 (2014) 68e71. [11] A. Rodrıguez, G. Ovejero, M.D. Romero, C. Dıaz, M. Barreiro, J. Garcia, Catalytic wet air oxidation of textile industrial wastewater using metal supported on carbon nanofibers, J. Supercrit. Fluids 46 (2008) 163e172. Sze , J. Kristo f, E. les, [12] A.M. Hosseini, A. Tungler, Z. Schay, S. Szabo si, Comparison of precious metal oxide/titanium monolith catL. Szentmiklo alysts in wet oxidation of wastewaters, Appl. Catal. B Environ. 127 (2012) 99e104. [13] R.A. Sheldon, J.K. Kochi, Metal-catalyzed Oxidation of Organic Compounds, Acad. Press, London, 1981. [14] A. Rodriguez, J. Garcia, G. Ovejero, M. Mestanza, Wet air and catalytic wet air oxidation of several azodyes from wastewaters: the beneficial role of catalysis, Water Sci. Technol. 60 (2009) 1989e1999. [15] E. Aneggi, C. de Leitenburg, A. Trovarelli, On the role of lattice/surface oxygen in ceria-zirconia catalysts for diesel soot combustion, Catal. Today 181 (2012) 108e115. [16] A. Tschope, D. Staadt, R. Birringer, J.Y. Ying, Catalytic properties of nanostructured metal oxides, synthesized by inert gas condensation, Nanostruct. Mater. 9 (1997) 423e432. [17] J. Guzman, S. Carrettin, J.C. Fierro-Gonzalez, Y. Hao, B.C. Gates, A. Corma, CO oxidation catalyzed by supported gold: cooperation between gold and nanocrystalline rare-earth supports forms reactive surface superoxide and peroxide species, Angew. Chem. Int. 44 (2005) 4778e4781. [18] J. Xu, J. Harmer, G. Li, T. Chapman, P. Collier, S. Longworth, S.C. Tsang, Size dependent oxygen buffering capacity of ceria nanocrystals, Chem. Commun. 46 (2010) 1887e1889. [19] A. Trovarelli, C. de Leitenburg, G. Dolcetti, Design better cerium-based oxidation catalysts, Chemtech 27 (1997) 32e37. [20] A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J.C. Lavalley, J.E. Fallah, L. Hilarie, F. Leonormand, E. Quemere, G.N. Sauvion, O. Touret, Reduction of CeO2 by hydrogen, magnetic susceptibility and Fouriertransform infrared, ultraviolet and X-ray photoelectron spectroscopy
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