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On the efficiency of cobalt zinc ferrite nanoparticles for catalytic wet peroxide oxidation of 4-chlorophenol
Journal of Environmental Chemical Engineering 2 (2014) 63–69
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On the efficiency of cobalt zinc ferrite nanoparticles for catalytic wet peroxide oxidation of 4-chlorophenol Manju Kurian *, Divya S. Nair Department of Chemistry, Mar Athanasius College, Kothamangalam 686666, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 August 2013 Received in revised form 23 November 2013 Accepted 26 November 2013
Cobalt substituted zinc ferrite nanoparticles prepared by sol–gel auto combustion method are efficient catalysts for the wet peroxide oxidation of 4-chlorophenol. Complete degradation of 4-chlorophenol occurs within one hour with all catalysts under study. 90% of the target compound is removed with oxidant concentration as low as 1 ml. Varying the catalyst dosage, it is found that direct correlation between the amount of catalyst present and extent of degradation of 4-chlorophenol is observed, ruling out the heterogeneous–homogeneous mechanism. The catalysts are reusable and the activity of the catalysts remains almost the same after four successive runs. The extent of iron leaching is fairly low after five consecutive cycles indicating the mechanism to be heterogeneous. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Wet peroxide oxidation 4-Chlorophenol Heterogeneous mechanism
Introduction The quality of water has become a priority issue for all countries of the world regardless of social and economic development. Last several decades have witnessed a growth in stringent regulations for the control of pollution in water resources of mother earth. This has stimulated the development of newer and efficient methods for water and wastewater treatment [1,2]. Synthetic pesticides have been popular with farmers because of their wide spread availability, simplicity in application, efficacy and economic returns. But, they also have huge environmental costs. Because they can be transported by wind and water, most pollutants generated in one country can affect people and wildlife far from where they are used and released. Many synthetic pesticides are based on chlorophenols, which are a group of priority toxic pollutants listed by the US EPA in the Clean Water Act [3] and by the European Union Decision in view of the risk factor for human health and environment due to their high toxicity and low biodegradability. They are also released into water as wastes generated from industrial activities like petrochemical, pharmaceutical, wood preserving and plastic use [4]. They can also be produced by environmental degradation of more complex molecules like chlorophenoxy acetic acids and chlorobenzenes. In humans, developmental, behavioural, neurologic, endocrinal, reproductive and immunologic adverse health effects have been linked to these compounds as they are considered to act as
* Corresponding author. Tel.: +91 485 2822512; fax: +91 485 2822512. E-mail addresses: [email protected], [email protected] (M. Kurian). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.11.026
uncouplers to oxidative dephosphorylation [5]. Within an isomeric group of chlorophenols, those with chlorine in 4th position are more toxic than others. Various abatement technologies including biological, thermal and chemical treatments have been developed in the last few years for the detoxification of organic pollutants [1,2]. Among these techniques, advanced oxidation processes (AOPs) appear to be a promising field of study for the effective removal of CPs from water. AOPs include several techniques like ozonation, fenton, photo-fenton, photo catalysis and wet oxidation [6–8]. Catalytic wet oxidations, which are technologies based on an initial formation of hydroxyl radicals that further oxidize the organic matter has been the favoured green chemistry path to convert chlorinated organic compounds to benign products [9,10]. They are degradative processes where attack by reactive oxygen species results in the overall oxidation of an organic pollutant via intermediate products [11]. In wet air oxidation (WAO), the degradation rate is strongly limited by the mass transfer of molecular oxygen from the gas to the liquid phase. Wet peroxide oxidation (WPO) takes advantage of using hydrogen peroxide as the liquid oxidant, which avoids gas–liquid mass transfer limitations. The severe operation conditions of WAO make it more capital intensive whereas WPO demands a lower capital. Also the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen allowing the reaction to be performed at conditions close to the ambient ones. Water is the only by-product formed and the oxidant is inexpensive. Also, aqueous hydrogen peroxide is a stable reagent, provided it is handled and stored in the proper manner. The mechanistic pathway suggested by Li et al. for wet peroxide oxidation process, involves the following reactions
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[12]. (1) OH radical formation: the OH radical formation occurs when hydrogen peroxide decomposes over heterogeneous or homogenous (M) species present in the system. (2) Chain reactions and oxidation of the organic compounds: at this stage, the organic compounds are oxidized to less complex molecules by means of the hydroxyl radical. (3) Final reactions: the chain reactions end when the hydroperoxide formed during the chain reactions reacts with the organic compounds yielding the formation of alcohols or when it decomposes to ketones and eventually to acids. The low weight organic acids will eventually be converted to end products like CO2 and H2O. In WPO, hydroxyl radicals can be produced from H2O2 using homogeneous methods such as Fenton reaction [13,14] or heterogeneous catalysts like metal oxides, clays, supported metal catalysts, etc. [10,15]. Application of homogeneous Fenton reaction demands acidification of the water (optimal pH around 3) and involves additional contamination of the treated water by iron. Heterogeneous catalytic wet peroxide oxidation has several advantages over the classical homogeneous Fenton like processes such as enhancement of catalytic activity, lack of secondary pollution and the possibility of reuse of catalyst in successive cycles. Ferrites are ceramic ferromagnetic materials with general chemical formula MFe2O4, where M represents a metallic cation like Fe, Mn, Mg, Ni, Co, Zn, Cu, Al or a mixture of these. They crystallize into the spinel structure in which, the sites occupied by the cations are of two types: tetrahedral (A) and octahedral (B) sites. Based on the partial occupancy of these sites, spinels are divided into three; (a) normal spinel in which M occupies A sites and Fe, B sites, (b) inverse spinel in which M occupies B sites together with half of Fe atoms, the other half being on A sites and (c) mixed or random spinel in which both M and Fe occupy both A and B sites. The type of cations and their distribution between two interstitial sites in ferrites can be tuned, resulting in many interesting magnetic and redox properties [16]. The mixed ferrospinels are treated as solid solutions obtained by the substitution with other cations. Lattice parameter changes with cation distribution, obeying Vegard’s law, which is a linear relation existing between the lattice parameters and the composition of the solid solution [17]. Recently ferrite research has been shifted towards developing these materials in nanometric scales as the performance in their conventional bulk preparation routes is reaching their limits due to higher electrical conductivity and domain wall resonance. There are several reports on spinel ferrites in the literature based on studies of magnetic properties. They have well established catalytic properties also for reactions like decomposition of alcohols and hydrodesulphurization of petroleum crude. Potential applications of nanoferrites in catalytic processes of redox nature are expected due to the presence of transition metals with variable oxidation states and chemical stability. The present study evaluates the effectiveness of mixed ferrite nanoparticles for the degradation of 4-chlorophenol using peroxide oxidation in aqueous phase. CoxZn1 xFe2O4 nanoparticles where (x = 0, 0.25, 0.5, 0.75 and 1) are prepared by sol–gel auto combustion method. The prepared catalysts are characterized by X-ray diffraction, X-ray fluorescence and transmission electron microscopy. Liquid phase catalytic peroxide oxidation of 4chlorophenol over the prepared ferrite nano particles under mild conditions is studied in detail using both gas chromatography and chemical oxygen demand measurements. The effect of different reaction variables like time, temperature, 4-chlorophenol concentration, peroxide concentration, catalyst concentration, etc. is also studied. The reusability of the catalysts is investigated for five consecutive cycles and the possibility of iron leaching is checked using atomic absorption spectroscopy. Based on the products obtained, a plausible reaction mechanism is also suggested.
Materials and methods Pure (99.9%) Fe(NO3)39H2O, Co(NO3)26H2O, Zn(NO3)26H2O and ethylene glycol from Merck, India were used as starting materials without further purification. The mixed spinels of CoxZn1 xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) were prepared by the sol–gel auto combustion method, using ethylene glycol as the gelating agent. Required stoichiometric ratio of Fe(NO3)39H2O, Co(NO3)26H2O and Zn(NO3)26H2O for various compositions of the mixed ferrites were dissolved in minimum amount of ethylene glycol at room temperature and mixed together. The solution was heated at 60 8C to obtain a wet gel, which when dried at 120 8C, self-ignited to form a fluffy powder. The powder was calcined at 800 8C for 4 h to achieve transformation into spinel phase. The ferrite powder was then sieved through 90 mm mesh. The prepared ferrite compositions are ZnFe2O4, Co0.25Zn0.75Fe2O4, Co0.5Zn0.5Fe2O4, Co0.75Zn0.25Fe2O4 and CoFe2O4. Phase identification of the prepared particles was performed using Bruker AXS D8 Advance X-ray diffractometer with Cu Ka (l = 1.5406 A˚) as the radiation source. The average crystallite size (D) of the ferrite particles was determined from X-ray diffractograms using the Debye–Scherer formula: D(hkl) = 0.9 l/b cos u, where D(hkl) is the average crystallite size (nm), l is the wavelength (nm), b is the full width at half maximum (radian) and u is the Bragg angle. The lattice parameter ‘a’ was calculated using the relation, a = d(h2 + k2 + l2)1/2. Since each primitive unit cell of the spinel structure contains 8 molecules, X-ray density (dx) was calculated according to the relation dx = 8 M/Na3, where N is Avogadro number and M is molecular weight of the sample. The measured density or bulk density (dB) was calculated from mass and bulk volume of the sample pellets using the formula, dB = m/ pr2h, where m is the mass of the sample taken, r is the radius of the sample pellet and h is the height of the pellet. From the bulk density and X-ray density, porosity P of the ferrite nano particle was determined using the relation, P = 1 (dB/dX). The stoichiometry of the prepared catalysts was verified using Bruker PIONEER model X-ray fluorescence studies. The particle size and morphology of various nanoparticles were confirmed using PHILIPS Model CM 200 Transmission Electron Microscope with a resolution of 2.4 A˚. Catalytic wet peroxide oxidation of 4-chlorophenol was carried out in a 250 ml two necked round bottomed flask immersed in a water bath with magnetic stirring. The flask was equipped with a running water condenser to maintain the reactor content inside the flask. A thermometer immersed in the water bath ensures the specified temperature. In a typical run, the amount of 4chlorophenol and catalyst was fixed as 1000 ppm and hydrogen peroxide concentration as 48 ppm. The reaction mixture was heated up to 70 8C prior to the initiation of the reaction. At specified intervals, liquid samples were periodically withdrawn from the reaction mixture and quantitatively analyzed using Perkin Elmer Clarus 580 Gas Chromatograph equipped with an Elite-5 capillary column and flame ionization detector. The products were identified by mass spectral analysis on a Varian 1200L Single Quadrupole GC/MS using He as the carrier gas. The extent of degradation is expressed as percentage of total chlorophenol transformed into products. The extent of oxidation was also studied using COD measurements employing standard dichromate method. The error percentage between the results of analysis is less than 5%. The influence of various reaction variables like nature of the catalyst and its dosage, substrate concentration, reaction temperature, time, oxidant concentration, etc. was studied in detail. The reusability of the catalyst was checked for five consecutive cycles. The amount of iron leached during this was quantified using Perkin Elmer Analyst 700 Atomic Absorption Spectrophotometer.
[(Fig._1)TD$IG]
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Results and discussions Characterization of cobalt zinc ferrite nanoparticles Recording X-ray diffraction (XRD) pattern of powdered polycrystalline samples by powder diffractometer method has many applications like qualitative phase analysis, quantitative phase analysis, determination of unit cell parameters, study of preferred orientation and determination of particle size. The X-ray diffractograms of cobalt zinc ferrites agree closely with the standard values given in the JCPDS data cards (80-2377, 791150) (Fig. 1). Well defined and broad diffraction peaks corresponding to characteristic crystallographic planes of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of the spinel structure can be observed in the figure. The strongest reflection comes from the (3 1 1) plane, which corresponds to cubic spinel structure. Additional peaks corresponding to the oxides of iron, cobalt or zinc is not observed, confirming the single phase spinel formation. The mixed ferrospinels are treated as solid solutions obtained by the substitution of cobalt ferrites with zinc ions. In interpreting the XRD profile of such substitutions, the substitutional site can be considered as occupied by a composite atom made up of appropriate portions of the involved atoms, statistically [18,19]. Hence the pattern of the substituted samples is expected to be intermediate between those of the pure substances. The mixed ferrites give almost identical XRD patterns with those of cobalt ferrite proving that zinc substitution does not change the site occupancy. The different parameters calculated from XRD profile is tabulated in Table 1. The average crystallite size of the cobalt zinc ferrite systems is found to be in the range of 14.4–25.3 nm. The substitution of zinc in CoFe2O4 corresponds to decrease in particle size, which suggests the formation of a compositionally homogeneous solid solution. This was probably due to the reaction condition, which favoured the formation of new nuclei preventing further growth of particles when the zinc concentration was increased. Lattice parameter increases linearly within the range 8.32–8.43 A˚ with increase in concentration of zinc, obeying Vegard’s law, which is a linear relation existing between the lattice parameters and the composition of the solid solution [17]. The observed lattice parameter is higher for ZnFe2O4 which could be attributed to the replacement of smaller sized iron cation by larger sized zinc cation. As a result, the size of the unit cell is increased. X-ray density, bulk density and porosity of different compositions do not show any significant change with zinc content. Transmission electron micrographs of the prepared Co1 xZnxFe2O4 samples are given in Fig. 2. The particles are roughly spherical with a narrow distribution in grain size in the range of 10–20 nm. The size obtained from X-ray diffraction profile is generally larger than the size obtained from transmission electron micrography (TEM) because X-ray intensity is proportional to the average particle volume, while the particle size obtained from TEM images represents the average radius. Moreover, the size distributions obtained from TEM images are restricted to primary particles and do not include secondary particles formed by
Fig. 1. X-ray profile of CoxZn(1
x)Fe2O4
(x = 0.0, 0.25, 0.5, 0.75, 1.0).
epitaxial attachment. Secondary particles are formed from integer multiples of primary particles and can have a significant influence on the volume distribution, even at relatively low concentration [20]. The well-defined rings in the selected area electron diffraction (SAED) patterns given in the inset can be assigned to the crystal planes of the ferrites indicating the established crystallinity. The elemental composition of the synthesized ferrites are important in the sense that the nature and concentration of the different elements present, can determine the surface morphology which in turn can affect the catalytic activities of the compounds. The experimental and the expected values regarding the stoichiometry of different cobalt zinc ferrites using X-ray fluorescence studies are tabulated in Table 2. The data proves that the stoichiometry of the prepared ferrite samples is in good agreement with the desired composition of the compounds. Effect of reaction variables The main objective of this experimental research is to evaluate the efficiency of mixed nanoferrites towards wet peroxide oxidation of organic pollutants taking 4-chlorophenol as the model compound. With this view, the reaction variables like temperature, 4-chlorophenol concentration, catalyst concentration and oxidant concentration, which can affect the oxidation activity of the catalysts to a great extent, was optimized using Co0.75Zn0.25Fe2O4 as the reference catalyst. The influence of the initial concentration of 4-chlorophenol in water was studied by keeping the amount of catalyst as 500 mg/l, hydrogen peroxide as 4 ml and time as 75 min. 25 ml solutions of 4-chlorophenol with concentration ranging from 0.75 to 2 g/l was used. These concentrations were selected because the De Minimis
Table 1 Data on crystallite size (D), lattice parameter (a), X-ray density (dx), bulk density (dB) and porosity (P) of the prepared ferrite samples obtained from X-ray diffractograms. Sample
Crystallite size D (nm)
Lattice parameter a (A˚)
X-ray density dx (g/m3)
Bulk density dB (g/m3)
Porosity P (%)
CoFe2O4 Co0.75Zn0.25Fe2O4 Co0.5Zn0.5Fe2O4 Co0.25Zn0.75Fe2O4 ZnFe2O4
25.3 17.9 17.4 14.5 14.4
8.32 8.35 8.38 8.39 8.43
5.411 5.389 5.368 5.385 5.337
3.970 4.051 4.030 4.059 4.018
26.64 24.84 24.92 24.63 24.71
[(Fig._2)TD$IG]
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Fig. 2. TEM images of CoxZn(1
level (the minimum threshold for which a conformity determination must be performed, for various criteria pollutants) for 4chlorophenol specified by US EPA is 0.1% [3]. The results of the present study at 70 8C are given in Table 3. As expected, complete degradation of 4-chlorophenol occurs at lower concentrations. Increase in the target compound concentration decreases the
x)Fe2O4
(x = 0.0, 0.25, 0.5, 0.75, 1.0).
degradation rate. These results are supported by COD measurements. A series of wet peroxide oxidations were performed in the temperature range of 25–70 8C keeping the amount of catalyst as 500 mg/l and hydrogen peroxide as 4 ml. The influence of reaction temperature on chlorophenol conversion and COD removal in one
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Table 2 Elemental composition of CoxZn1 xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) samples from X-ray fluorescence. Compound
Experimental value
Expected value
Zn
Co
Fe
O
Zn
Co
Fe
O
ZnFe2O4 Co0.25Zn0.75Fe2O4 Co0.5Zn0.5Fe2O4 Co0.75Zn0.25Fe2O4 CoFe2O4
0.99 0.73 0.54 0.24 0
0 0.26 0.52 0.73 1.09
1.99 1.96 1.94 1.95 1.92
4 4.01 4 3.97 4
1 0.75 0.5 0.25 0
0 0.25 0.5 0.75 1
2 2 2 2 2
4 4 4 4 4
hour is given in Table 3. It can be seen that cobalt zinc ferrite system is highly efficient in removing the target pollutant even at ambient temperature. Complete removal of 4-chlorophenol occurs with 95% reduction in COD at room temperature. The results prove that cobalt zinc ferrite nanoparticles can act as efficient catalysts for catalytic wet peroxide oxidation reactions. The oxidant dose is a critical feature in wet peroxide oxidation processes since H2O2 consumption is a determining component of the treatment cost. The effect of oxidant dosage was determined by changing the amount of oxidant in the range of 1–5 ml and keeping the amount of catalyst as 500 mg/l. Table 3 depicts the results in one hour at 70 8C. It can be observed that degradation of 4chlorophenol is almost independent of oxidant concentration. Even when the peroxide concentration is as low as 1 ml, above 90% of the target compound is removed. However, only 40–55% of the COD is reduced at lower oxidant concentrations, which increases sharply at higher amounts of the oxidant. Complete degradation of target pollutant occurs with oxidant concentrations above 3 ml and 96.3% reduction in COD is obtained with oxidant concentrations of 4 ml. High concentrations of hydrogen peroxide can accelerate self-decomposition of the oxidant, reducing the conversion of target compound. Several authors have reported decrease in the reaction rate at high oxidant concentrations [21]. However, this effect is not observed in the present case. The results confirm the superior activity of cobalt zinc ferrite nanoparticles for catalytic wet peroxide oxidation reactions. The concentration of catalyst used for the liquid phase wet peroxide oxidation is an important parameter in determining the Table 3 Effect of reaction variables in the wet peroxide oxidation of 4-chlorophenol. Reaction variable
4-Chlorophenol conversion (%)
Reduction of COD (%)
4-chlorophenol concentration (g/l)
0.75 1.0 1.25 1.75 2.0
95.3 100 79.1 69.5 51.6
95.0 96.3 69.1 66.6 56.9
Temperature (8C)
Room temp 40 50 60 70
100 100 100 100 100
54.5 68.2 76.3 88.7 96.3
H2O2 concentration (30% v/v) (ml)
1 2 3 4 5
92.4 99.6 100 100 100
42.8 55.5 68.8 96.3 98.2
Catalyst concentration (mg/l)
0 300 400 500 600 700
60.1 75.7 98.9 100 100 100
10.2 14.6 76.3 96.3 96.9 97.7
Reaction conditions: catalyst – Co0.75Zn0.25Fe2O4, H2O2 (30% v/v) – 4 ml, catalyst – 500 mg/l, chlorophenol – 1 g/l (25 ml), temperature – 70 8C.
efficiency. The results of the studies on the influence of catalyst concentration at 70 8C in one hour, by keeping the amount of 4chlorophenol as 1000 ppm and peroxide concentration as 4 ml are given in Table 3. In the control experiment in the absence of catalyst, only 60.1% of 4-chlorophenol was degraded with 10.2% reduction in COD values. This signifies the importance of the catalyst used in the complete degradation of 4-chlorophenol. It can be seen from the table that there is a direct correlation between the amount of catalyst present and the extent of degradation of 4chlorophenol. More than 75% of the target pollutant is degraded at catalyst concentrations as low as 300 mg/l. However, the reduction in COD is only about 15% indicating that organic reaction intermediates are persisting in the system. When the catalyst concentration is increased to 500 mg/l, complete degradation of 4chlorophenol is obtained, with a simultaneous decrease in the COD of the solution. Complete reduction of COD could be achieved by using 700 mg/l of the prepared nano ferrite particles. The dependence of efficiency on catalytic dosage indicates that the process runs on the surface of the catalyst. In many reports, there was a negative dependence of rate on catalyst concentration indicating that termination steps occur on the catalyst surface [22,23]. However, this is not observed in the present case. Considering that the experiments are carried out over nanoparticles, it can be assumed that the outer surface of the catalyst grain is used and hence the high reaction rate. Wet peroxide oxidation of 4-chlorophenol The present study aims to evaluate the effectiveness of mixed ferrite nanoparticles for the degradation of 4-chlorophenol using peroxide oxidation in aqueous phase. The catalytic activity of different compositions of cobalt zinc ferrite systems towards the degradation of 4-chlorophenol with hydrogen peroxide was studied by keeping the amount of catalyst as 500 mg/l, hydrogen peroxide as 4 ml and time as 75 min. The results of the investigation by gas chromatographic and COD analyses at 70 8C are presented in Fig. 3. Complete degradation of 4-chlorophenol occurs within 60 min over all catalysts under study indicating the superior activity of the prepared systems. Progressive substitution of cobalt increases the oxidation activity of ZnFe2O4. Reduction in COD is a more important parameter than chlorophenol conversion obtained from GC since COD measures the extent of abatement of the contaminant as well as the intermediates and/or products. The results of COD measurements echoes the results obtained by gas chromatographic analysis. Wet peroxide oxidation of chlorophenols has been reported to occur over several catalysts. AlFe pillared clays gave 70% removal of 4-chlorophenol over 1 g/m3 catalyst at 50 8C when a 125 ppm aqueous solution was used [9]. 2 mM solution of 4-chlorophenol was completely degraded using hydrotalcite/clay composite using 40 mM H2O2 at 40 8C [10]. Complete removal of 4-chlorophenol occurred at 100 8C with 2.5 cm3 of H2O2 and an initial concentration of 500 ppm after 90 min in the absence of a catalyst [24]. Hence it can be safely concluded that cobalt zinc ferrites exhibit enhanced activity towards the catalytic oxidation of 4-chlorophenol at higher
[(Fig._3)TD$IG]
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68
Chlorophenol/COD removal %
100
Table 4 Stability of catalyst towards wet peroxide oxidation.
80
60
40
% Removal of 4-chlorophenol
% Reduction of COD
Amount of Fe leached (ppm)
1 2 3 4 5
100 100 95.85 90.31 90.7
96.3 77.8 72.6 76.5 75.8
1.26 1.16 3.26 3.76 4.50
Reaction conditions: catalyst – Co0.75Zn0.25Fe2O4, H2O2 (30% v/v) – 4 ml, catalyst – 500 mg/l, chlorophenol – 1 g/l (25 ml), temperature – 70 8C.
20
0
No. of cycles
15
30
45
60
75
90
105
120
Time (minutes) CoFe2O4
Co0.75Zn0.25Fe2O4
Co0.5Zn0.5Fe2O4
Co0.25Zn0.75Fe2O4.
Zn Fe2O4
CoFe2O4.
Co0.75Zn0.25Fe2O4.
Co0.5Zn0.5Fe2O4.
Co0.25Zn0.75Fe2O4.2
ZnFe2O4.
Fig. 3. Wet peroxide oxidation of 4-chlorophenol over CoxZn(1 0.5, 0.75, 1.0).
x)Fe2O4
(x = 0.0, 0.25,
concentrations of the target compound using lower amounts of the oxidant compared to literature. Initial induction period attributed to the time necessary to reach a critical concentration of organic radicals is not significant during the present study. Several compounds like naphthalene, flourene, acenaphthalene, chrysene, etc. have been reported as products in wet peroxide oxidation of 4-chlorophenol in literature [25]. Zhou et al. reported benzoquinone and the chlorinated benzoquinones as intermediate products [9]. In our study, triacetone peroxide is the major intermediate identified by GC–MS analysis. Other products observed in traces are acetone, 4-hydroxyl-4-methyl-2-pentanone, 2,5-hexane dione, xylenes and benzoquinone. The presence of triacetone peroxide can be due to the conversion of acetone by excess hydrogen peroxide which forms the polymer in the presence of acidic catalysts like ferrites. Thus, the target pollutant is completely removed as simple organic intermediates which in turn are oxidized completely and this underlines the superior activity of the catalyst. Stability of the catalyst The catalyst stability is of primary importance in industrial applications. The presence of strongly adsorbed reaction products or by products can block part of the surface producing deactivation [26]. Hence, the reusability of the prepared systems has been checked by retaining a catalyst batch and using it for several cycles after drying at 100 8C in an air oven after each cycle. The catalyst and reactants were placed in the reactor, following the procedure previously described. The results are tabulated in Table 4. It is interesting to note that the catalyst remains active after five successive runs. The catalyst removed 90.7% of the added 4chlorophenol even on its fifth cycle. Zhang et al. reported 10% loss of catalytic activity after the fourth cycle over magnetic nanoparticles (MNPs) [27]. 97% retention in catalytic activity was reported by Zhou et al. over Cu–Ni–Al hydrotalcite with decrease in TOC conversions (28%) after five runs [28]. Leaching of iron Continuous and gradual leaching of metal ions from any heterogeneous catalyst can lead to potential toxicity issues and this limits the suitability of the catalyst for the reaction. Leaching studies are also valuable since they can provide insights to the
mechanism of the reaction, whether homogeneous or heterogeneous. The iron concentration in the liquid phase after one hour reaction was repeatedly measured using atomic absorption spectrometer and the results are presented in Table 4. For the catalyst under study, the amount of iron leached out into the reaction mixture is about 1 ppm for two consecutive runs. This increases slightly to 3 ppm in the consecutive cycles. Thus it can be concluded that iron leaching is fairly low after 5 consecutive cycles indicating the mechanism to be essentially heterogeneous. Conclusions WPO appears to be a promising technology for the removal of chlorophenols (CPs) from wastewaters since complete removal of the target compound was achieved when working under not so severe conditions of temperature (70 8C, 4 ml of H2O2). Cobalt zinc ferrite nanoparticles act as efficient catalysts for the oxidation of 4-chlorophenol in the presence of hydrogen peroxide as oxidant. Complete removal of 4-chlorophenol occurs at ambient temperature. There is a direct correlation between the amount of catalyst present and the extent of degradation of 4-chlorophenol indicating that the process runs on the surface of the catalyst. Studies on the stability of the catalyst show that the activity of the catalysts remains almost the same after five successive runs. The extent of Fe leaching is fairly low after five consecutive cycles indicating that the mechanism remains heterogeneous. Acknowledgement Financial assistance from the Department of Science and Technology, India through Fast Track Scheme for Young Scientists is gratefully acknowledged. References [1] M. Stoyanova, G. Christoskova, M. Georgieva, Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst, Appl. Catal. A: Gen. 248 (2003) 249–259. [2] N. Li, C. Descorme, M. Besson, Catalytic wet air oxidation of aqueous solution of 2chlorophenol over Ru/zirconia catalysts, Appl. Catal. B: Environ. 71 (2007) 262– 270. [3] Emergency Planning and Community Right to Know Section 313, US EPA, EPA 745-B-95-004, 1994. [4] L.H. Kaith, W.A. Telliard, Priority pollutants. I: A perspective view, Environ. Sci. Technol. 13 (1979) 416–423. [5] H. Terada, Uncouplers of oxidative phosphorylation, Environ. Health Perspect. 87 (1990) 213–218. [6] S. Caudo, G. Centi, C. Genovese, S. Perathoner, Copper- and iron-pillared clay catalysts for the WHPCO of model and real wastewater streams from olive oil milling production, Appl. Catal. B: Environ. 70 (2007) 437–446. [7] E. Chamarro, E. Maeco, S. Esplugas, Use of Fenton reagent to improve organic chemical biodegradability, Water Res. 35 (2001) 1047. [8] E. Psillakis, D. Mantzavinos, Enhancement of biodegradability of industrial wastewaters by chemical oxidation pre-treatment, J. Chem. Technol. Biotechnol. 79 (2004) 431–454. [9] S. Zhou, C. Gu, Z. Qian, J. Xu, C. Xia, The activity and selectivity of catalytic peroxide oxidation of chlorophenols over Cu–Al hydrotalcite/clay composite, J. Colloid Interface Sci. 357 (2011) 447–452.
M. Kurian, D.S. Nair / Journal of Environmental Chemical Engineering 2 (2014) 63–69 [10] C. Catrinescu, D. Arsene, C. Teodosiu, Catalytic wet hydrogen peroxide oxidation of para-chlorophenol over Al/Fe pillared clays (AlFePILCs) prepared from different host clays, Appl. Catal. B: Environ. 101 (2011) 451–460. [11] M. Kurian, M. Joy, D. Raj, Hydroxylation of phenol over rare earth exchanged iron pillared montmorillonites, J. Porous Mater. 19 (2012) 633–640. [12] L. Li, C. Peischi, F.G. Earnest, Generalized kinetic model for wet oxidation of organic compounds, AIChE J. 37 (1991) 1687–1697. [13] G. Kwon, D.S. Lee, N. Kang, J. Yoon, Characteristics of p-chlorophenol oxidation by Fenton’s reagent, Water Res. 33 (9) (1999) 2110–2118. [14] M. Lu, Oxidation of chlorophenols with hydrogen peroxide in the presence of goethite, Chemosphere 40 (2000) 125–130. [15] M. Kurian, R. Babu, Iron aluminium mixed pillared montmorillonite and the rare earth exchanged analogues as efficient catalysts for phenol oxidation, J. Environ. Chem. Eng. 1 (2013) 86–91. [16] J.H. Wang, Y.C. Liu, D.C. Liu, Y.W. Yu, F.B. Guo, Effect of electromagnetic field strength on Ni0.35Zn0.65Fe2O4 as performed by combustion synthesis, J. Magn. Magn. Mater. 321 (2009) 3646–3649. [17] X. Whisto, Ray Methods, Wiley, New York, 1991. [18] M. Kurian, D.S. Nair, Effect of preparation conditions on nickel zinc ferrite nanoparticles: a comparison between sol–gel auto combustion and co-precipitation methods, J. Saudi Chem. Soc. (2013), http://dx.doi.org/10.1016/j.jscs.2013.03.003. [19] L.S. Dent Glasser, Crystallography and Its Applications, Van Nostrand Reinhold, New York, 1977.
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[20] S. Ayyappan, S. Mahadevan, P. Chandramohan, M.P. Srinivasan, J. Philip, B. Raj, Influence of Co2+ ion concentration on the size, magnetic properties, and purity of CoFe2O4 spinel ferrite nanoparticles, J. Phys. Chem. C 114 (2010) 6334–6340. [21] S. Hamoudi, A. Sayari, K. Belkacemi, L. Bonneviot, F. Larachi, Catalytic wet oxidation of phenol over PtxAg1 xMnO2/CeO2 catalysts, Catal. Today 62 (2000) 379–388. [22] A. Sadana, J.R. Katzer, Involvement of free radicals in the aqueous-phase catalytic oxidation of phenol over copper oxide, J. Catal. 35 (1) (1974) 140–152. [23] M. Kurian, S. Sugunan, Wet peroxide oxidation of phenol over mixed pillared montmorillonites, Chem. Eng. J. 115 (2006) 139–146. [24] V.G. Molina, M. Lopez Arias, M. Florczyk, E. Chamarro, S. Esplugas, Wet peroxide oxidation of chlorophenols, Water Res. 39 (2005) 795–802. [25] K.S. Lin, H.P. Wang, Byproduct shape selectivity in supercritical water oxidation of 2-chlorophenol effected by CuO/ZSM-5, Langmuir 16 (2000) 2627–2631. [26] T. Mallat, A. Baiker, Oxidation of alcohols with molecular oxygen on platinum metal catalysts in aqueous solutions, Catal. Today 19 (1994) 247–283. [27] J. Zhang, J. Zhuang, L. Gao, Y. Zhang, N. Gu, J. Feng, D. Yang, J. Zhu, X. Yan, Decomposing phenol by the hidden talent of ferromagnetic nanoparticles, Chemosphere 73 (2008) 1524–1528. [28] S. Zhou, Z. Qian, T. Sun, J. Xu, C. Xia, Catalytic wet peroxide oxidation of phenol over Cu–Ni–Al hydrotalcite, Appl. Clay Sci. 53 (2011) 627–633.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
On the efficiency of cobalt zinc ferrite nanoparticles for catalytic wet peroxide oxidation of 4-chlorophenol Manju Kurian *, Divya S. Nair Department of Chemistry, Mar Athanasius College, Kothamangalam 686666, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 August 2013 Received in revised form 23 November 2013 Accepted 26 November 2013
Cobalt substituted zinc ferrite nanoparticles prepared by sol–gel auto combustion method are efficient catalysts for the wet peroxide oxidation of 4-chlorophenol. Complete degradation of 4-chlorophenol occurs within one hour with all catalysts under study. 90% of the target compound is removed with oxidant concentration as low as 1 ml. Varying the catalyst dosage, it is found that direct correlation between the amount of catalyst present and extent of degradation of 4-chlorophenol is observed, ruling out the heterogeneous–homogeneous mechanism. The catalysts are reusable and the activity of the catalysts remains almost the same after four successive runs. The extent of iron leaching is fairly low after five consecutive cycles indicating the mechanism to be heterogeneous. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Wet peroxide oxidation 4-Chlorophenol Heterogeneous mechanism
Introduction The quality of water has become a priority issue for all countries of the world regardless of social and economic development. Last several decades have witnessed a growth in stringent regulations for the control of pollution in water resources of mother earth. This has stimulated the development of newer and efficient methods for water and wastewater treatment [1,2]. Synthetic pesticides have been popular with farmers because of their wide spread availability, simplicity in application, efficacy and economic returns. But, they also have huge environmental costs. Because they can be transported by wind and water, most pollutants generated in one country can affect people and wildlife far from where they are used and released. Many synthetic pesticides are based on chlorophenols, which are a group of priority toxic pollutants listed by the US EPA in the Clean Water Act [3] and by the European Union Decision in view of the risk factor for human health and environment due to their high toxicity and low biodegradability. They are also released into water as wastes generated from industrial activities like petrochemical, pharmaceutical, wood preserving and plastic use [4]. They can also be produced by environmental degradation of more complex molecules like chlorophenoxy acetic acids and chlorobenzenes. In humans, developmental, behavioural, neurologic, endocrinal, reproductive and immunologic adverse health effects have been linked to these compounds as they are considered to act as
* Corresponding author. Tel.: +91 485 2822512; fax: +91 485 2822512. E-mail addresses: [email protected], [email protected] (M. Kurian). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.11.026
uncouplers to oxidative dephosphorylation [5]. Within an isomeric group of chlorophenols, those with chlorine in 4th position are more toxic than others. Various abatement technologies including biological, thermal and chemical treatments have been developed in the last few years for the detoxification of organic pollutants [1,2]. Among these techniques, advanced oxidation processes (AOPs) appear to be a promising field of study for the effective removal of CPs from water. AOPs include several techniques like ozonation, fenton, photo-fenton, photo catalysis and wet oxidation [6–8]. Catalytic wet oxidations, which are technologies based on an initial formation of hydroxyl radicals that further oxidize the organic matter has been the favoured green chemistry path to convert chlorinated organic compounds to benign products [9,10]. They are degradative processes where attack by reactive oxygen species results in the overall oxidation of an organic pollutant via intermediate products [11]. In wet air oxidation (WAO), the degradation rate is strongly limited by the mass transfer of molecular oxygen from the gas to the liquid phase. Wet peroxide oxidation (WPO) takes advantage of using hydrogen peroxide as the liquid oxidant, which avoids gas–liquid mass transfer limitations. The severe operation conditions of WAO make it more capital intensive whereas WPO demands a lower capital. Also the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen allowing the reaction to be performed at conditions close to the ambient ones. Water is the only by-product formed and the oxidant is inexpensive. Also, aqueous hydrogen peroxide is a stable reagent, provided it is handled and stored in the proper manner. The mechanistic pathway suggested by Li et al. for wet peroxide oxidation process, involves the following reactions
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[12]. (1) OH radical formation: the OH radical formation occurs when hydrogen peroxide decomposes over heterogeneous or homogenous (M) species present in the system. (2) Chain reactions and oxidation of the organic compounds: at this stage, the organic compounds are oxidized to less complex molecules by means of the hydroxyl radical. (3) Final reactions: the chain reactions end when the hydroperoxide formed during the chain reactions reacts with the organic compounds yielding the formation of alcohols or when it decomposes to ketones and eventually to acids. The low weight organic acids will eventually be converted to end products like CO2 and H2O. In WPO, hydroxyl radicals can be produced from H2O2 using homogeneous methods such as Fenton reaction [13,14] or heterogeneous catalysts like metal oxides, clays, supported metal catalysts, etc. [10,15]. Application of homogeneous Fenton reaction demands acidification of the water (optimal pH around 3) and involves additional contamination of the treated water by iron. Heterogeneous catalytic wet peroxide oxidation has several advantages over the classical homogeneous Fenton like processes such as enhancement of catalytic activity, lack of secondary pollution and the possibility of reuse of catalyst in successive cycles. Ferrites are ceramic ferromagnetic materials with general chemical formula MFe2O4, where M represents a metallic cation like Fe, Mn, Mg, Ni, Co, Zn, Cu, Al or a mixture of these. They crystallize into the spinel structure in which, the sites occupied by the cations are of two types: tetrahedral (A) and octahedral (B) sites. Based on the partial occupancy of these sites, spinels are divided into three; (a) normal spinel in which M occupies A sites and Fe, B sites, (b) inverse spinel in which M occupies B sites together with half of Fe atoms, the other half being on A sites and (c) mixed or random spinel in which both M and Fe occupy both A and B sites. The type of cations and their distribution between two interstitial sites in ferrites can be tuned, resulting in many interesting magnetic and redox properties [16]. The mixed ferrospinels are treated as solid solutions obtained by the substitution with other cations. Lattice parameter changes with cation distribution, obeying Vegard’s law, which is a linear relation existing between the lattice parameters and the composition of the solid solution [17]. Recently ferrite research has been shifted towards developing these materials in nanometric scales as the performance in their conventional bulk preparation routes is reaching their limits due to higher electrical conductivity and domain wall resonance. There are several reports on spinel ferrites in the literature based on studies of magnetic properties. They have well established catalytic properties also for reactions like decomposition of alcohols and hydrodesulphurization of petroleum crude. Potential applications of nanoferrites in catalytic processes of redox nature are expected due to the presence of transition metals with variable oxidation states and chemical stability. The present study evaluates the effectiveness of mixed ferrite nanoparticles for the degradation of 4-chlorophenol using peroxide oxidation in aqueous phase. CoxZn1 xFe2O4 nanoparticles where (x = 0, 0.25, 0.5, 0.75 and 1) are prepared by sol–gel auto combustion method. The prepared catalysts are characterized by X-ray diffraction, X-ray fluorescence and transmission electron microscopy. Liquid phase catalytic peroxide oxidation of 4chlorophenol over the prepared ferrite nano particles under mild conditions is studied in detail using both gas chromatography and chemical oxygen demand measurements. The effect of different reaction variables like time, temperature, 4-chlorophenol concentration, peroxide concentration, catalyst concentration, etc. is also studied. The reusability of the catalysts is investigated for five consecutive cycles and the possibility of iron leaching is checked using atomic absorption spectroscopy. Based on the products obtained, a plausible reaction mechanism is also suggested.
Materials and methods Pure (99.9%) Fe(NO3)39H2O, Co(NO3)26H2O, Zn(NO3)26H2O and ethylene glycol from Merck, India were used as starting materials without further purification. The mixed spinels of CoxZn1 xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) were prepared by the sol–gel auto combustion method, using ethylene glycol as the gelating agent. Required stoichiometric ratio of Fe(NO3)39H2O, Co(NO3)26H2O and Zn(NO3)26H2O for various compositions of the mixed ferrites were dissolved in minimum amount of ethylene glycol at room temperature and mixed together. The solution was heated at 60 8C to obtain a wet gel, which when dried at 120 8C, self-ignited to form a fluffy powder. The powder was calcined at 800 8C for 4 h to achieve transformation into spinel phase. The ferrite powder was then sieved through 90 mm mesh. The prepared ferrite compositions are ZnFe2O4, Co0.25Zn0.75Fe2O4, Co0.5Zn0.5Fe2O4, Co0.75Zn0.25Fe2O4 and CoFe2O4. Phase identification of the prepared particles was performed using Bruker AXS D8 Advance X-ray diffractometer with Cu Ka (l = 1.5406 A˚) as the radiation source. The average crystallite size (D) of the ferrite particles was determined from X-ray diffractograms using the Debye–Scherer formula: D(hkl) = 0.9 l/b cos u, where D(hkl) is the average crystallite size (nm), l is the wavelength (nm), b is the full width at half maximum (radian) and u is the Bragg angle. The lattice parameter ‘a’ was calculated using the relation, a = d(h2 + k2 + l2)1/2. Since each primitive unit cell of the spinel structure contains 8 molecules, X-ray density (dx) was calculated according to the relation dx = 8 M/Na3, where N is Avogadro number and M is molecular weight of the sample. The measured density or bulk density (dB) was calculated from mass and bulk volume of the sample pellets using the formula, dB = m/ pr2h, where m is the mass of the sample taken, r is the radius of the sample pellet and h is the height of the pellet. From the bulk density and X-ray density, porosity P of the ferrite nano particle was determined using the relation, P = 1 (dB/dX). The stoichiometry of the prepared catalysts was verified using Bruker PIONEER model X-ray fluorescence studies. The particle size and morphology of various nanoparticles were confirmed using PHILIPS Model CM 200 Transmission Electron Microscope with a resolution of 2.4 A˚. Catalytic wet peroxide oxidation of 4-chlorophenol was carried out in a 250 ml two necked round bottomed flask immersed in a water bath with magnetic stirring. The flask was equipped with a running water condenser to maintain the reactor content inside the flask. A thermometer immersed in the water bath ensures the specified temperature. In a typical run, the amount of 4chlorophenol and catalyst was fixed as 1000 ppm and hydrogen peroxide concentration as 48 ppm. The reaction mixture was heated up to 70 8C prior to the initiation of the reaction. At specified intervals, liquid samples were periodically withdrawn from the reaction mixture and quantitatively analyzed using Perkin Elmer Clarus 580 Gas Chromatograph equipped with an Elite-5 capillary column and flame ionization detector. The products were identified by mass spectral analysis on a Varian 1200L Single Quadrupole GC/MS using He as the carrier gas. The extent of degradation is expressed as percentage of total chlorophenol transformed into products. The extent of oxidation was also studied using COD measurements employing standard dichromate method. The error percentage between the results of analysis is less than 5%. The influence of various reaction variables like nature of the catalyst and its dosage, substrate concentration, reaction temperature, time, oxidant concentration, etc. was studied in detail. The reusability of the catalyst was checked for five consecutive cycles. The amount of iron leached during this was quantified using Perkin Elmer Analyst 700 Atomic Absorption Spectrophotometer.
[(Fig._1)TD$IG]
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65
Results and discussions Characterization of cobalt zinc ferrite nanoparticles Recording X-ray diffraction (XRD) pattern of powdered polycrystalline samples by powder diffractometer method has many applications like qualitative phase analysis, quantitative phase analysis, determination of unit cell parameters, study of preferred orientation and determination of particle size. The X-ray diffractograms of cobalt zinc ferrites agree closely with the standard values given in the JCPDS data cards (80-2377, 791150) (Fig. 1). Well defined and broad diffraction peaks corresponding to characteristic crystallographic planes of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of the spinel structure can be observed in the figure. The strongest reflection comes from the (3 1 1) plane, which corresponds to cubic spinel structure. Additional peaks corresponding to the oxides of iron, cobalt or zinc is not observed, confirming the single phase spinel formation. The mixed ferrospinels are treated as solid solutions obtained by the substitution of cobalt ferrites with zinc ions. In interpreting the XRD profile of such substitutions, the substitutional site can be considered as occupied by a composite atom made up of appropriate portions of the involved atoms, statistically [18,19]. Hence the pattern of the substituted samples is expected to be intermediate between those of the pure substances. The mixed ferrites give almost identical XRD patterns with those of cobalt ferrite proving that zinc substitution does not change the site occupancy. The different parameters calculated from XRD profile is tabulated in Table 1. The average crystallite size of the cobalt zinc ferrite systems is found to be in the range of 14.4–25.3 nm. The substitution of zinc in CoFe2O4 corresponds to decrease in particle size, which suggests the formation of a compositionally homogeneous solid solution. This was probably due to the reaction condition, which favoured the formation of new nuclei preventing further growth of particles when the zinc concentration was increased. Lattice parameter increases linearly within the range 8.32–8.43 A˚ with increase in concentration of zinc, obeying Vegard’s law, which is a linear relation existing between the lattice parameters and the composition of the solid solution [17]. The observed lattice parameter is higher for ZnFe2O4 which could be attributed to the replacement of smaller sized iron cation by larger sized zinc cation. As a result, the size of the unit cell is increased. X-ray density, bulk density and porosity of different compositions do not show any significant change with zinc content. Transmission electron micrographs of the prepared Co1 xZnxFe2O4 samples are given in Fig. 2. The particles are roughly spherical with a narrow distribution in grain size in the range of 10–20 nm. The size obtained from X-ray diffraction profile is generally larger than the size obtained from transmission electron micrography (TEM) because X-ray intensity is proportional to the average particle volume, while the particle size obtained from TEM images represents the average radius. Moreover, the size distributions obtained from TEM images are restricted to primary particles and do not include secondary particles formed by
Fig. 1. X-ray profile of CoxZn(1
x)Fe2O4
(x = 0.0, 0.25, 0.5, 0.75, 1.0).
epitaxial attachment. Secondary particles are formed from integer multiples of primary particles and can have a significant influence on the volume distribution, even at relatively low concentration [20]. The well-defined rings in the selected area electron diffraction (SAED) patterns given in the inset can be assigned to the crystal planes of the ferrites indicating the established crystallinity. The elemental composition of the synthesized ferrites are important in the sense that the nature and concentration of the different elements present, can determine the surface morphology which in turn can affect the catalytic activities of the compounds. The experimental and the expected values regarding the stoichiometry of different cobalt zinc ferrites using X-ray fluorescence studies are tabulated in Table 2. The data proves that the stoichiometry of the prepared ferrite samples is in good agreement with the desired composition of the compounds. Effect of reaction variables The main objective of this experimental research is to evaluate the efficiency of mixed nanoferrites towards wet peroxide oxidation of organic pollutants taking 4-chlorophenol as the model compound. With this view, the reaction variables like temperature, 4-chlorophenol concentration, catalyst concentration and oxidant concentration, which can affect the oxidation activity of the catalysts to a great extent, was optimized using Co0.75Zn0.25Fe2O4 as the reference catalyst. The influence of the initial concentration of 4-chlorophenol in water was studied by keeping the amount of catalyst as 500 mg/l, hydrogen peroxide as 4 ml and time as 75 min. 25 ml solutions of 4-chlorophenol with concentration ranging from 0.75 to 2 g/l was used. These concentrations were selected because the De Minimis
Table 1 Data on crystallite size (D), lattice parameter (a), X-ray density (dx), bulk density (dB) and porosity (P) of the prepared ferrite samples obtained from X-ray diffractograms. Sample
Crystallite size D (nm)
Lattice parameter a (A˚)
X-ray density dx (g/m3)
Bulk density dB (g/m3)
Porosity P (%)
CoFe2O4 Co0.75Zn0.25Fe2O4 Co0.5Zn0.5Fe2O4 Co0.25Zn0.75Fe2O4 ZnFe2O4
25.3 17.9 17.4 14.5 14.4
8.32 8.35 8.38 8.39 8.43
5.411 5.389 5.368 5.385 5.337
3.970 4.051 4.030 4.059 4.018
26.64 24.84 24.92 24.63 24.71
[(Fig._2)TD$IG]
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Fig. 2. TEM images of CoxZn(1
level (the minimum threshold for which a conformity determination must be performed, for various criteria pollutants) for 4chlorophenol specified by US EPA is 0.1% [3]. The results of the present study at 70 8C are given in Table 3. As expected, complete degradation of 4-chlorophenol occurs at lower concentrations. Increase in the target compound concentration decreases the
x)Fe2O4
(x = 0.0, 0.25, 0.5, 0.75, 1.0).
degradation rate. These results are supported by COD measurements. A series of wet peroxide oxidations were performed in the temperature range of 25–70 8C keeping the amount of catalyst as 500 mg/l and hydrogen peroxide as 4 ml. The influence of reaction temperature on chlorophenol conversion and COD removal in one
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Table 2 Elemental composition of CoxZn1 xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) samples from X-ray fluorescence. Compound
Experimental value
Expected value
Zn
Co
Fe
O
Zn
Co
Fe
O
ZnFe2O4 Co0.25Zn0.75Fe2O4 Co0.5Zn0.5Fe2O4 Co0.75Zn0.25Fe2O4 CoFe2O4
0.99 0.73 0.54 0.24 0
0 0.26 0.52 0.73 1.09
1.99 1.96 1.94 1.95 1.92
4 4.01 4 3.97 4
1 0.75 0.5 0.25 0
0 0.25 0.5 0.75 1
2 2 2 2 2
4 4 4 4 4
hour is given in Table 3. It can be seen that cobalt zinc ferrite system is highly efficient in removing the target pollutant even at ambient temperature. Complete removal of 4-chlorophenol occurs with 95% reduction in COD at room temperature. The results prove that cobalt zinc ferrite nanoparticles can act as efficient catalysts for catalytic wet peroxide oxidation reactions. The oxidant dose is a critical feature in wet peroxide oxidation processes since H2O2 consumption is a determining component of the treatment cost. The effect of oxidant dosage was determined by changing the amount of oxidant in the range of 1–5 ml and keeping the amount of catalyst as 500 mg/l. Table 3 depicts the results in one hour at 70 8C. It can be observed that degradation of 4chlorophenol is almost independent of oxidant concentration. Even when the peroxide concentration is as low as 1 ml, above 90% of the target compound is removed. However, only 40–55% of the COD is reduced at lower oxidant concentrations, which increases sharply at higher amounts of the oxidant. Complete degradation of target pollutant occurs with oxidant concentrations above 3 ml and 96.3% reduction in COD is obtained with oxidant concentrations of 4 ml. High concentrations of hydrogen peroxide can accelerate self-decomposition of the oxidant, reducing the conversion of target compound. Several authors have reported decrease in the reaction rate at high oxidant concentrations [21]. However, this effect is not observed in the present case. The results confirm the superior activity of cobalt zinc ferrite nanoparticles for catalytic wet peroxide oxidation reactions. The concentration of catalyst used for the liquid phase wet peroxide oxidation is an important parameter in determining the Table 3 Effect of reaction variables in the wet peroxide oxidation of 4-chlorophenol. Reaction variable
4-Chlorophenol conversion (%)
Reduction of COD (%)
4-chlorophenol concentration (g/l)
0.75 1.0 1.25 1.75 2.0
95.3 100 79.1 69.5 51.6
95.0 96.3 69.1 66.6 56.9
Temperature (8C)
Room temp 40 50 60 70
100 100 100 100 100
54.5 68.2 76.3 88.7 96.3
H2O2 concentration (30% v/v) (ml)
1 2 3 4 5
92.4 99.6 100 100 100
42.8 55.5 68.8 96.3 98.2
Catalyst concentration (mg/l)
0 300 400 500 600 700
60.1 75.7 98.9 100 100 100
10.2 14.6 76.3 96.3 96.9 97.7
Reaction conditions: catalyst – Co0.75Zn0.25Fe2O4, H2O2 (30% v/v) – 4 ml, catalyst – 500 mg/l, chlorophenol – 1 g/l (25 ml), temperature – 70 8C.
efficiency. The results of the studies on the influence of catalyst concentration at 70 8C in one hour, by keeping the amount of 4chlorophenol as 1000 ppm and peroxide concentration as 4 ml are given in Table 3. In the control experiment in the absence of catalyst, only 60.1% of 4-chlorophenol was degraded with 10.2% reduction in COD values. This signifies the importance of the catalyst used in the complete degradation of 4-chlorophenol. It can be seen from the table that there is a direct correlation between the amount of catalyst present and the extent of degradation of 4chlorophenol. More than 75% of the target pollutant is degraded at catalyst concentrations as low as 300 mg/l. However, the reduction in COD is only about 15% indicating that organic reaction intermediates are persisting in the system. When the catalyst concentration is increased to 500 mg/l, complete degradation of 4chlorophenol is obtained, with a simultaneous decrease in the COD of the solution. Complete reduction of COD could be achieved by using 700 mg/l of the prepared nano ferrite particles. The dependence of efficiency on catalytic dosage indicates that the process runs on the surface of the catalyst. In many reports, there was a negative dependence of rate on catalyst concentration indicating that termination steps occur on the catalyst surface [22,23]. However, this is not observed in the present case. Considering that the experiments are carried out over nanoparticles, it can be assumed that the outer surface of the catalyst grain is used and hence the high reaction rate. Wet peroxide oxidation of 4-chlorophenol The present study aims to evaluate the effectiveness of mixed ferrite nanoparticles for the degradation of 4-chlorophenol using peroxide oxidation in aqueous phase. The catalytic activity of different compositions of cobalt zinc ferrite systems towards the degradation of 4-chlorophenol with hydrogen peroxide was studied by keeping the amount of catalyst as 500 mg/l, hydrogen peroxide as 4 ml and time as 75 min. The results of the investigation by gas chromatographic and COD analyses at 70 8C are presented in Fig. 3. Complete degradation of 4-chlorophenol occurs within 60 min over all catalysts under study indicating the superior activity of the prepared systems. Progressive substitution of cobalt increases the oxidation activity of ZnFe2O4. Reduction in COD is a more important parameter than chlorophenol conversion obtained from GC since COD measures the extent of abatement of the contaminant as well as the intermediates and/or products. The results of COD measurements echoes the results obtained by gas chromatographic analysis. Wet peroxide oxidation of chlorophenols has been reported to occur over several catalysts. AlFe pillared clays gave 70% removal of 4-chlorophenol over 1 g/m3 catalyst at 50 8C when a 125 ppm aqueous solution was used [9]. 2 mM solution of 4-chlorophenol was completely degraded using hydrotalcite/clay composite using 40 mM H2O2 at 40 8C [10]. Complete removal of 4-chlorophenol occurred at 100 8C with 2.5 cm3 of H2O2 and an initial concentration of 500 ppm after 90 min in the absence of a catalyst [24]. Hence it can be safely concluded that cobalt zinc ferrites exhibit enhanced activity towards the catalytic oxidation of 4-chlorophenol at higher
[(Fig._3)TD$IG]
M. Kurian, D.S. Nair / Journal of Environmental Chemical Engineering 2 (2014) 63–69
68
Chlorophenol/COD removal %
100
Table 4 Stability of catalyst towards wet peroxide oxidation.
80
60
40
% Removal of 4-chlorophenol
% Reduction of COD
Amount of Fe leached (ppm)
1 2 3 4 5
100 100 95.85 90.31 90.7
96.3 77.8 72.6 76.5 75.8
1.26 1.16 3.26 3.76 4.50
Reaction conditions: catalyst – Co0.75Zn0.25Fe2O4, H2O2 (30% v/v) – 4 ml, catalyst – 500 mg/l, chlorophenol – 1 g/l (25 ml), temperature – 70 8C.
20
0
No. of cycles
15
30
45
60
75
90
105
120
Time (minutes) CoFe2O4
Co0.75Zn0.25Fe2O4
Co0.5Zn0.5Fe2O4
Co0.25Zn0.75Fe2O4.
Zn Fe2O4
CoFe2O4.
Co0.75Zn0.25Fe2O4.
Co0.5Zn0.5Fe2O4.
Co0.25Zn0.75Fe2O4.2
ZnFe2O4.
Fig. 3. Wet peroxide oxidation of 4-chlorophenol over CoxZn(1 0.5, 0.75, 1.0).
x)Fe2O4
(x = 0.0, 0.25,
concentrations of the target compound using lower amounts of the oxidant compared to literature. Initial induction period attributed to the time necessary to reach a critical concentration of organic radicals is not significant during the present study. Several compounds like naphthalene, flourene, acenaphthalene, chrysene, etc. have been reported as products in wet peroxide oxidation of 4-chlorophenol in literature [25]. Zhou et al. reported benzoquinone and the chlorinated benzoquinones as intermediate products [9]. In our study, triacetone peroxide is the major intermediate identified by GC–MS analysis. Other products observed in traces are acetone, 4-hydroxyl-4-methyl-2-pentanone, 2,5-hexane dione, xylenes and benzoquinone. The presence of triacetone peroxide can be due to the conversion of acetone by excess hydrogen peroxide which forms the polymer in the presence of acidic catalysts like ferrites. Thus, the target pollutant is completely removed as simple organic intermediates which in turn are oxidized completely and this underlines the superior activity of the catalyst. Stability of the catalyst The catalyst stability is of primary importance in industrial applications. The presence of strongly adsorbed reaction products or by products can block part of the surface producing deactivation [26]. Hence, the reusability of the prepared systems has been checked by retaining a catalyst batch and using it for several cycles after drying at 100 8C in an air oven after each cycle. The catalyst and reactants were placed in the reactor, following the procedure previously described. The results are tabulated in Table 4. It is interesting to note that the catalyst remains active after five successive runs. The catalyst removed 90.7% of the added 4chlorophenol even on its fifth cycle. Zhang et al. reported 10% loss of catalytic activity after the fourth cycle over magnetic nanoparticles (MNPs) [27]. 97% retention in catalytic activity was reported by Zhou et al. over Cu–Ni–Al hydrotalcite with decrease in TOC conversions (28%) after five runs [28]. Leaching of iron Continuous and gradual leaching of metal ions from any heterogeneous catalyst can lead to potential toxicity issues and this limits the suitability of the catalyst for the reaction. Leaching studies are also valuable since they can provide insights to the
mechanism of the reaction, whether homogeneous or heterogeneous. The iron concentration in the liquid phase after one hour reaction was repeatedly measured using atomic absorption spectrometer and the results are presented in Table 4. For the catalyst under study, the amount of iron leached out into the reaction mixture is about 1 ppm for two consecutive runs. This increases slightly to 3 ppm in the consecutive cycles. Thus it can be concluded that iron leaching is fairly low after 5 consecutive cycles indicating the mechanism to be essentially heterogeneous. Conclusions WPO appears to be a promising technology for the removal of chlorophenols (CPs) from wastewaters since complete removal of the target compound was achieved when working under not so severe conditions of temperature (70 8C, 4 ml of H2O2). Cobalt zinc ferrite nanoparticles act as efficient catalysts for the oxidation of 4-chlorophenol in the presence of hydrogen peroxide as oxidant. Complete removal of 4-chlorophenol occurs at ambient temperature. There is a direct correlation between the amount of catalyst present and the extent of degradation of 4-chlorophenol indicating that the process runs on the surface of the catalyst. Studies on the stability of the catalyst show that the activity of the catalysts remains almost the same after five successive runs. The extent of Fe leaching is fairly low after five consecutive cycles indicating that the mechanism remains heterogeneous. Acknowledgement Financial assistance from the Department of Science and Technology, India through Fast Track Scheme for Young Scientists is gratefully acknowledged. References [1] M. Stoyanova, G. Christoskova, M. Georgieva, Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst, Appl. Catal. A: Gen. 248 (2003) 249–259. [2] N. Li, C. Descorme, M. Besson, Catalytic wet air oxidation of aqueous solution of 2chlorophenol over Ru/zirconia catalysts, Appl. Catal. B: Environ. 71 (2007) 262– 270. [3] Emergency Planning and Community Right to Know Section 313, US EPA, EPA 745-B-95-004, 1994. [4] L.H. Kaith, W.A. Telliard, Priority pollutants. I: A perspective view, Environ. Sci. Technol. 13 (1979) 416–423. [5] H. Terada, Uncouplers of oxidative phosphorylation, Environ. Health Perspect. 87 (1990) 213–218. [6] S. Caudo, G. Centi, C. Genovese, S. Perathoner, Copper- and iron-pillared clay catalysts for the WHPCO of model and real wastewater streams from olive oil milling production, Appl. Catal. B: Environ. 70 (2007) 437–446. [7] E. Chamarro, E. Maeco, S. Esplugas, Use of Fenton reagent to improve organic chemical biodegradability, Water Res. 35 (2001) 1047. [8] E. Psillakis, D. Mantzavinos, Enhancement of biodegradability of industrial wastewaters by chemical oxidation pre-treatment, J. Chem. Technol. Biotechnol. 79 (2004) 431–454. [9] S. Zhou, C. Gu, Z. Qian, J. Xu, C. Xia, The activity and selectivity of catalytic peroxide oxidation of chlorophenols over Cu–Al hydrotalcite/clay composite, J. Colloid Interface Sci. 357 (2011) 447–452.
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