Catalytic peroxide oxidation of persistent chlorinated organics over nickel-zinc ferrite nanocomposites

Journal of Water Process Engineering 16 (2017) 69–80

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Catalytic peroxide oxidation of persistent chlorinated organics over nickel-zinc ferrite nanocomposites Divya S. Nair, Manju Kurian ∗ Research Centre in Chemistry, Mar Athanasius College, Kothamangalam 686 666, India

a r t i c l e

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Article history: Received 20 September 2016 Received in revised form 24 December 2016 Accepted 25 December 2016 Keywords: Ni-Zn ferrite nanocomposites Catalytic peroxide oxidation 2,4-Dichlorophenol 2,4-Dichlorophenoxy acetic acid Reaction kinetics Catalyst stability

a b s t r a c t Catalytic wet peroxide oxidation over nickel-zinc ferrite nanocomposites offers a novel way for the effective removal of Priority Toxic Pollutants (2,4-dichlorophenol and 2,4-dichlorophenoxy acetic acid) in water under mild reaction conditions. The rate of catalytic oxidation reaction was influenced by the concentration of pollutant, catalyst and oxidant, temperature and catalyst stability. The oxidation ability and acid-base properties of composites were studied by Temperature Programmed Reduction (TPR-H2 ) and desorption (TPD-NH3 and TPD-CO2 ) techniques. Zinc doping increased the oxidizing power and surface acidity of nickel ferrite composites which in turn increased the catalytic efficiency. Complete removal of 2,4-dichlorophenol (DCP) and 2,4-dichlorophenoxy acetic acid (2,4-D) was achieved within 75 and 90 min with 84/73% and 83/70% of COD/TOC removal respectively at 343 K. Zinc rich compositions of Znx Ni1-x Fe2 O4 were found to be more effective for the destructive removal of pollutants and the composite with composition x-0.75 showed the highest activity. Kinetic study revealed that peroxide oxidation reaction followed a first order kinetic model with rate constant and activation energy of 2.44 × 10−2 min−1 /13.26 kJ/mol, 3.16 × 10−2 min−1 /14.98 kJ/mol respectively for DCP and 2,4-D. The results of five consecutive catalytic runs revealed the excellent stability and recyclability of the composite as evident from X-Ray Diffraction, Brunauer Emmet Teller and leaching studies from Atomic Absorption Spectrophotometry (AAS). Catalyst leaching increased with temperature, catalyst amount and dopant concentration. Reactive species trapping experiments with n-butanol indicated a heterogeneous free radical mechanism. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Chlorinated phenols and phenoxy acid herbicides have been used on large scale in agricultural sector to control the growth of weeds in vegetation. Among these, 2,4-dichlorophenoxy acetic acid (2,4-D), is used up in an uncontrolled manner because of its easy availability and simplicity in application. 2,4-D is highly mobile and persistent in aqueous media and is very difficult to decomposed due to its chemical stability and non-biodegradability [1–3]. It is considered to be carcinogenic and is one of the widely known endocrine disrupting chemical [4]. Because of its high affinity, it can be easily transferred into water and biomagnified through food chain, causing damage to the vital organs of humans and animals. These toxic compounds are therefore listed by both the US-EPA Clean Water Act and the European Union Decision 2455/2001/EC. The World Health Organization (WHO), recommended its maxi-

∗ Corresponding author. E-mail address: [email protected] (M. Kurian). http://dx.doi.org/10.1016/j.jwpe.2016.12.010 2214-7144/© 2016 Elsevier Ltd. All rights reserved.

mum allowable concentration in drinking water as 70 ␮g/L [5]. 2,4-dichlorophenol (DCP) is the prime precursor for the manufacture of 2,4-D and so is the major transformation product resulted by the solar photolysis and microbial action to 2,4-D in soil or natural water [6]. Therefore, the degradation of 2,4-D and other related derivatives has been a serious environmental concern. In recent years, Advanced Oxidization Processes (AOPs) employing hydrogen peroxide as oxidizing agent has exhibited great potential as an environment friendly and sustainable treatment technology for the degradation of toxic organic compounds [7–9]. Because of the limitations of homogeneous Fenton reaction, a variety of hydrogen peroxide based heterogeneous catalytic reactions over iron based catalysts have been developed and experimented. In recent years the catalytic efficiency of iron based oxide compounds that are heterogeneous in nature have been experimented for the oxidative degradation of toxic chlorinated organics without pH adjustment [10–12]. Studies revealed that AB2 O4 type spinel ferrites are very stable and active catalysts towards the decomposition of hydrogen peroxide and destructive removal of pollutants. Though several reports are available showing the

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catalytic applications of spinel ferrites for different organic transformation reactions, to the best of our knowledge the catalytic power of spinel ferrites are not much exploited for environmental applications. So far we have published the catalytic applications of nickel, cobalt, manganese and zinc substituted mixed ferrite nanocomposites for environmental remediation [13–15]. In spite of this, much effort is still needed to explore the doping effect of different metals on their catalytic improvements, degradation mechanism, activation energy changes, catalyst recyclability and stability against leaching to make the heterogeneous Fenton process more suitable for wide scale practical applications. The present study aims at the catalytic application of different compositions of zinc doped nickel ferrite nanocomposites (Znx Ni1-x Fe2 O4 , x-0.0, 0.25, 0.5, 0.75, 1.0) for liquid phase oxidative destruction of 2,4-dichlorophenol (DCP) and 2,4-dichlorophenoxy acetic acid (2,4-D) under mild operating conditions. The influence of reaction parameters including reaction temperature, catalyst composition and its dosage, oxidant concentration, effect of dopant on catalyst stability and reusability were investigated. Different from our previous research outcomes [13–15], the effect of dopant on the oxidation potential and surface acidity of the catalyst was studied in detail using Temperature Programmed Desorption of ammonia (TPD-NH3 ) and Temperature Programmed Reduction (TPR-H2 ) using Micromeritics ChemiSorb 2750 Pulse Chemisorption system. Also, the effect of temperature, catalyst dosage and dopant concentration towards catalyst deactivation was studied in detail. Based on the structural identification of transformation intermediates of DCP and 2,4-D, a plausible mechanism was also proposed. 2. Materials and methods 2.1. Chemicals used for catalytic evaluation 2,4-Dichlorophenol (Loba Chemie), 2,4-dichlorophenoxy acetic acid (Himedia Laboratories Ltd), hydrogen peroxide and n-butanol from Merck, India, were used for catalytic reaction studies. AgSO4 , HgSO4 , H2 SO4 , ferrous ammonium sulphate and K2 Cr2 O7 from Merck India were used for COD measurements. KMnO4 (Merck India) was used for the estimation of residual hydrogen peroxide. The stock solutions of DCP and 2,4-D were prepared by dissolving suitable amount in deionized water and all the chemicals were used as such without any further purification. 2.2. Catalyst preparation and characterization Different compositions of Znx Ni1-x Fe2 O4 (x-0.0, 0.25, 0.50, 0.75, 1.0) nanocomposites were prepared by sol-gel auto combustion method [13] and characterized by different physicochemical tools such as X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray Fluorescence (XRF) and Fourier Transform Infra-Red spectral (FTIR) studies. Phase identification and purity of the prepared composites were performed using Bruker AXS D8 Advance ˚ as the radiation X-ray diffractometer with CuK␣ (␭ = 1.5406 A) source. The particle size, distribution of nanoparticles and surface morphology were confirmed using pH ILIPS Model CM 200 Transmission Electron Microscope with a resolution of 2.4 A˚ with the help of Image J software. Stoichiometry of prepared catalysts was verified using Bruker PIONEER model X-ray fluorescence spectrometer. The Fourier Transform Infra-Red spectral studies were carried out in KBr medium using Thermo Nicolet, Avatar 370 model FTIR spectrometer in the range of 400–4000 cm−1 with a resolution of 4 cm−1 . The synthesis and characterization of the nanoferrite composites were reported in our previous publication [13]. The TPR analysis was carried out to study the redox nature of the cata-

lyst, in a stream of hydrogen and argon with a flowing rate of 50 ml/min. The amount of hydrogen consumption during reduction was estimated with a thermal conductivity detector. For TPD-NH3 and CO2 analyses were conducted to study the acid-base properties. For that samples were pre-treated under He flow of 10 ml/min at 400 ◦ C. NH3 /CO2 adsorption was carried out under standard condition by flowing 10% NH3 /He and 10% CO2 /He over the ferrite composite till saturation and then desorption of NH3 /CO2 by temperature-programmed analysis under constant He flow from 30 to 800 ◦ C with a heating rate of 10 ◦ C/min. The specific surface area of nanocomposites was performed by N2 adsorption measurements on a Micromeritics Gemini VII instrument after degassing the sample at 300 ◦ C under vacuum. X-ray diffractograms of the reused catalysts were analysed and phase identification was carried out by comparison with JCPDS data cards and the average crystallite size was calculated by Debye-Scherrer equation. 2.3. Experimental procedure for catalytic wet peroxide oxidation (CWPO) of 2,4-dichlorophenol (DCP) and 2,4-dichlorophenoxyacetic acid (2,4-D) Prior to oxidation experiments, the possibility of adsorption of pollutants on catalyst surface was checked and chance of removal via adsorptive way was completely ruled out. All experiments were carried out in a 250 ml two necked RB with 25 ml of pollutant stock solution (1 g/L DCP and 0.3 g/L 2,4-D). Afterwards, this was placed on a magnetic stirrer at a fixed temperature for two hours with a stirring speed of 230 rpm. To initiate the reaction, definite amount of catalyst and hydrogen peroxide according to the stoichiometry of reactants were added. At every 15 min regular interval of time, samples were withdrawn from the reaction mixture, filtered and analysed. The effect of each reaction variables on the degradation rate was studied by changing that variable while keeping all other parameters as constant. The initial pH of reaction mixture at the initial stage was respectively 6.2 and 4.3 for DCP and 2,4-D. The active species trapping experiments were carried out using 200 mM/L nbutanol (• OH scavenger) during the WPO reaction under the same conditions. All experiments were conducted in triplicate to observe the reproducibility. The progress of removal of DCP and 2,4-D from aqueous medium was analysed periodically using PerkinElmer Clarus 580 Model GC equipped with an Elite-5 capillary column and Flame Ionization Detector (FID). The pH of the reaction system was checked during the course of reaction using a EUTECH digital pH meter. COD measurements were carried out by standard dichromate method and the reduction in COD was calculated as {[COD]0 –[COD]t /[COD]0 }100 where [COD]0 and [COD]t are at initial and at a time t respectively. The residual amount of peroxide was studied at its higher concentrations using permanganometry, since hydrogen peroxide can interfere with the COD value by consuming oxidizing agent. The COD contribution due to the presence of residual peroxide results in overestimation of COD by 0.47 mg/L and its amount is removed from the actual COD value [16]. To substantiate GC and COD results, reaction progress was analysed via the reduction in Total Organic Carbon (TOC) using Shimadzu TOC-L pH 200 analyser. The oxidation intermediates of DCP and 2,4-D were identified by GC–MS analysis on a Varian 1200 l Single Quadruple spectrometer using Helium as the carrier gas. The error percentage between the results of analyses is less than 5%. 2.4. Recycling and leaching studies To evaluate the reusability of zinc doped nickel ferrite composites in catalytic applications, the used catalyst was collected immediately by filtration at the end of each catalytic run, washed with deionized water and then with acetone to remove organ-

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Table 1 Data on crystalline size and surface are of Znx Ni1-x Fe2 O4 (x-0.0, 0.25, 0.5, 0.75, 1.0) nanocomposites. Catalyst

Crystalline size (D) nm

BET Surface area, m2 /g

Pore size nm

Pore volume cm3 /g

ZnFe2 O4 Ni0.25 Zn0.75 Fe2 O4 Ni0.5 Zn0.5 Fe2 O4 Ni0.75 Zn0.25 Fe2 O4 NiFe2 O4

14.4 26.3 32.8 33.8 39.6

2.97 7.5 4.2 3.8 6.6

1.659 1.657 1.659 1.653 1.659

0.0012 0.0031 0.0017 0.0012 0.0027

ics from catalyst’s surface pores, dried under vacuum and then used again for the next catalytic run under the same conditions. Leaching of the catalyst at the end of the oxidation process was quantified using PerkinElmer Analyst 700 Atomic Absorption Spectrophotometer. Phase analysis of the catalyst after each successive catalytic runs was studied using X-Ray Diffraction method and textural changes if any was checked by BET surface area analyser (Micromeritics Gemini VII). 3. Results and discussions 3.1. Characterization of Zn-Ni ferrite composites by XRD, TEM, XRF and FTIR studies XRD studies confirmed the spinel structure of Znx Ni1-x Fe2 O4 (x0.0, 0.25, 0.50, 0.75, 1.0) nanocomposites with good crystallinity. The average crystallite size of Nix Zn1-x Fe2 O4 (x-0.0, 0.25, 0.50, 0.75, 1.0) estimated from the Scherrer equation was found in the range 14.4-39.6 nm. TEM images yields the particle size in the range 14.2–32.5 nm which nearly complements with the XRD results. FTIR spectra again confirmed the spinel structure by revealing two absorption bands in the range 400–450 cm−1 and 550–600 cm−1 respectively due to the vibrations of oxide ions with the divalent and trivalent metal ions in octahedral and tetrahedral sites in the spinel lattice. Stoichiometry of different ferrite catalysts were verified by XRF studies and the obtained results are in good agreement with the theoretical values [13]. 3.2. BET surface area analysis The surface properties of zinc substituted nickel ferrite nanocomposites in terms of surface area are summarized in Table 1. It is clear from the table that the surface area of doped nickel ferrite depends on dopant concentration to some extent. The surface area (SBET ) of un-substituted nickel and zinc ferrite nanocomposites possess highest and lowest surface area values6.6 and 2.97 m2 /g among the series. The surface area of nickel ferrite decreased (3.8 m2 /g) with doping of zinc by x-0.25. Increase in zinc content (x-0.5) increased the surface area to 4.8 m2 /g and possess maximum value (7.5 m2 /g) when zinc composition was 0.75. This observation was opposite to the general trend that, surface area normally increased when crystallite size decreased. The pore sizes and pore volumes of all the compositions of zinc doped nickel ferrite nanocomposites also varied in an irregular manner. 3.3. Oxidation property by temperature programmed reduction studies (TPR-H2 ) TPR profiles of zinc substituted nickel ferrite composites are presented in Fig. 1. The strength of metal-oxygen bonds and the amount of suitable oxygen species are revealed from reduction temperature and signal intensity respectively. The reduction peak area is directly affected by the amount of reducible species in the catalyst. In the present case, for all ferrite composites the optimum temperature for reduction is extended between300 and 900 ◦ C. The TPR profiles of all composites are broad and it could be consid-

Fig. 1. TPR profile of Znx Ni1-x Fe2 O4 (x-0, 0.25, 0.5, 0.75, 1.0) nanocomposites.

ered as a superposition of reduction transitions of Ni2+ , Fe3+ and Zn2+ ions. Reduction patterns shows that the reduction of Fe3+ to Fe takes place in two steps giving two corresponding peaks in the specified range. The peak appearing at higher temperature could be assigned to the continuous reduction of metal ions. During the first stage of reduction, iron is reduced from Fe3+ state to an intermediate state Fe3+/2+ which appear in the temperature range 350 and 650 ◦ C. In the second stage Fe3+/2+ is converted to Fe via Fe2+ and then to Fe which is nearly in agreement with other reports [17]. It has been noted that the TPR profiles of Zn0.5 Ni0.5 Fe2 O4 and Zn0.25 Ni0.75 Fe2 O4 are almost same in intensity and broadness which can be ascribed to their similar crystallite size. But as the composition of zinc increases, the consumption of hydrogen for the redox change of iron is increased from 1.913 to 2.684 mmol/g. This clearly indicates a structural transformation from inverse spinel structure of nickel ferrite to a normal spinel structure as a result of zinc doping. Velinov et al. related the position of the temperature maxima for the reduction peak with the particle size of the material [17]. The reduction profile of all zinc substituted nickel ferrite materials except ZnFe2 O4 , are found to be less broadened and shifted to lower temperatures which indicate easier reduction at lower temperatures indicative of the oxidation power of nickel ferrite composite. The intensity of un-substituted nickel ferrite was high compared to other composites and this was probably because of its interference with the reduction curve of Fe3+ /Fe2+ /Fe redox change. This analysis confirms the surface occupancy of iron ions as indicated from FTIR spectral studies. 3.4. Acid-base properties by temperature programmed desorption studies (TPD-NH3 /CO2 ) Temperature programmed desorption analysis of ammonia (TPD-NH3 ) is an appropriate tool to find out the total acidity, nature of acid sites and its distribution for a solid catalyst. For simpler quantitative interpretations, the desorption profiles of ferrite nanocomposites are partitioned to three temperature ranges to

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Fig. 2. TPD-NH3 profile for Znx Ni1-x Fe2 O4 (x-0, 0.25, 0.5, 0.75, 1.0) nanocomposites. Fig. 4. Optimization of reaction variables for the WPO of DCP over Znx Ni1-x Fe2 O4 (x-0.0, 0.25, 0.5, 0.75, 1.0) system.

Fig. 3. TPD-CO2 profile for Znx Ni1-x Fe2 O4 (x-0, 0.25, 0.5, 0.75, 1.0) nanocomposites.

200 ◦ Cis

classify the surface acidity i.e. from room temperature to treated as weakly acidic (Bronsted sites), 201–500 ◦ Cas moderately acidic, and 501–800 ◦ Cas strongly acidic (Lewis sites) [18]. From Fig. 2, it can be noted that all ferrite composites are moderately acidic. Among the series, nickel ferrite possess least acidity as envisaged from the amount of desorbed ammonia in the temperature range of 300–400 ◦ C. The number and strength of moderately acidic sites in nickel ferrite increased from 0.1325 to 1.0462 mmol/g with zinc doping. The cumulative acidity of Zn0.25 Ni0.75 Fe2 O4 (0.4718 mmol/g) andZn0.5 Ni0.5 Fe2 O4 (0.4801 mmol/g) are seen to be almost same. The Lewis acid behaviour of the composite increased slowly and the strong acid sites are even more broadly distributed in the case of un-substituted zinc ferrite material. In conclusion zinc doping to the inverse spinel lattice of nickel ferrite composite increases the surface acidity. This can be attributed to the presence of more number of Fe3+ ions in octahedral sites, which are more peripheral contributing to the surface acidity. The remarkable increase in surface acidity of zinc doped ferrite catalysts can best be correlated with its behaviour as a good heterogeneous Fenton like catalyst. The surface basicity possessed by different compositions of nickel-zinc ferrite nanocomposites was studied and characterized (Fig. 3). As can be seen from the desorption profile of nickel ferrite, two less intense peaks were observed between 30 and 150 and 300–500 ◦ C corresponding to weak and medium basic sites respectively. On increasing zinc concentration, basicity decreased and the samples exhibited only one peak between 30 and 150 ◦ C which can be attributed to the amount of carbon dioxide physisorbed on the catalyst surface.

Fig. 5. Optimization of reaction variables for the WPO of 2,4-D over Znx Ni1-x Fe2 O4 (x-0.0, 0.25, 0.5, 0.75, 1.0) system.

3.5. Optimization of reaction variables for the CWPO of DCP and 2,4-D The optimization of reaction variables is very important in order to carry out it in an effective and economical way. Optimization studies were conducted thoroughly with respect to all reaction variables such as reaction time, reaction temperature, oxidant concentration, pollutant concentration, catalyst dosage and its composition. Zn0.75 Ni0.25 Fe2 O4 composite was selected from Znx Ni1-x Fe2 O4 (x-0.0, 0.25, 0.5, 0.75, 1.0) system as model catalyst. 3.5.1. Effect of reaction temperature To study the effect of temperature on the removal of DCP and 2,4-D, reactions were performed at five different temperatures (298, 313, 323, 333, 343 and 353 K) with 1:12 and 1:15 molar ratio of 1 g/L DCP: H2 O2 and 0.3 g/L of 2,4-D: H2 O2 respectively over 0.5 g/L catalyst. Figs. 4 b and 5 b depicts the relationship between temperature and the removal of DCP and 2,4-D along with COD and TOC removal in 75 and 90 min respectively. The activity of the catalyst was increased gradually with temperature and shows its maximum activity at 343 K. As temperature increases from 298 to 333 K, the rate of removal of DCP increased from 13.31% to 56.42% and complete removal was achieved at 343 K with 80.41% and 67.10% of reduction in COD and TOC, whereas the rate of removal of 2,4-D was higher compared to DCP under the same reaction conditions. The removal rate of 2,4-D increased from 17.7% to 79.1% when temperature was raised to 333 K. Complete removal was observed at 343 K

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Table 2 Data on rate constant and activation energy for catalysed and non-catalysed peroxide oxidation of DCP and 2,4-D. Temperature (K)

298 313 323 333 343 Non-catalysed reaction

DCP

2,4-D

Rate constant (min−1 )

Activation energy Ea (kJ/mol)

Rate constant (min−1 )

Activation energy Ea (kJ/mol)

0.0016 0.0025 0.0036 0.0125 0.0244 0.0020

13.26

0.0073 0.0103 0.0163 0.0293 0.0316 0.0032

14.98

22.92

26.14

Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst (Zn0.75 Ni0.25 Fe2 O4 )–0.5 g/L, time – 120 min.

of removal of COD/TOC. This decrease in the rate of oxidation with higher catalyst loads may be because the optimum efficiency of the catalysts might have been reached at the minimum loading itself [13]. There by the possibility of catalyst leaching at higher dosages could effectively be controlled.

Fig. 6. Effect of temperature in the kinetics of WPO of DCP (a) and 2,4-D (b) Reaction conditions (a): DCP–1 g/L, DCP: H2 O2 –1:12, Zn0.75 Ni0.25 Fe2 O4 – 0.5 g/L, pH-5.2–7.03; (b): 2,4-D-0.3 g/L, 2,4-D: H2 O2 –1:15, Zn0.75 Ni0.25 Fe2 O4 – 0.5 g/L, pH-4.13–7.2.

with 74.4% of COD reduction and 62.3% of TOC removal. At temperatures beyond 343 K, catalyst activity decreased and 343 K was opted as suitable for oxidation studies of DCP and 2,4-D. Increase in temperature facilitates the self-scavenging of hydroxyl radicals and thereby rate of removal decreased. The degradation kinetics of DCP and 2,4-D at all temperatures are shown in Fig. 6. Linear shape of the kinetic curves reveals that the degradation follow a first-order kinetic model. With increase in temperature from 298 K to 343 K, the first order rate constant for the oxidation of DCP and 2,4-D also increased significantly from 1.6 × 10−3 to 2.4 × 10−2 min−1 and 7.3 × 10−3 to 3.2 × 10−2 min−1 (Table 2). The increase in activity with increasing temperature could be attributed by Arrhenius behaviour of the reaction. 3.5.2. Effect of catalyst dosage The effect of catalyst dosage in the oxidative degradation of DCP and 2,4-D was investigated by varying the catalyst dosages as 0, 0.3, 0.5, 0.7, 1 and 1.5 g/L keeping other variables constant. Figs. 4d and 5d shows the correlation between catalyst concentration and rate of removal of 1 g/L DCP and 0.3 g/L 2,4-D with 1:12 and 1:15 ratio with oxidant at 343 K. A blank run conducted without catalyst yield only 11.46% and 24.72% of removal of DCP and 2,4-D respectively. With 0.3 g/L of catalyst dose, DCP and 2,4-D removal rate enhanced to 61.46% and 62.5% respectively. 60.59%/51.84% and 49.3%/36.4% of reduction in COD/TOC respectively for DCP and 2,4-D was observed. Maximum activity could be observed with 0.5 g/L of catalyst which resulted in complete removal of DCP and 2,4-D within 75 and 90 min respectively with a reduction of 80.41/67.1% and 74.4/62.3% COD/TOC respectively. When the amount of catalyst exceeds 0.5 g/L, the rate of oxidation of both pollutants decreased significantly with a removal of DCP and 2,4-D by60.48% and 41.3% respectively with 54.89/50.85% and 30.8/27.6%

3.5.3. Effect of pollutant concentration Figs. 4a and 5a depicts the kinetics of WPO of different concentrations of DCP (0.75, 1, 1.25, 1.5, 1.75, 2 g/L) and 2,4-D (0.1, 0.3, 0.5, 0.7, 1, 1.25 g/L) solutions according to their stoichiometry with hydrogen peroxide over 0.5 g/L catalyst at 343 KIt can be seen that under a given experimental condition, complete removal of DCP was obtained with 0.75, 1 and 1.25 g/L concentrations with increase in COD and TOC removal from 73.28% to 80.41% and 62.04% to 67.10% respectively. Similar trend was observed with increase in 2,4-D concentration from 0.1 to 0.3 g/L, but excellent COD and TOC removal was achieved with a concentration of 0.3 g/LAs seen from the graph, further increase in the initial concentration of DCP from 1.5–2 g/L and 2,4-D from 0.5 to 1.25 g/L, removal rate reduced continuously to 43.19% and 23.1% with 30.59%/25.53% and 18.2/12.9% of COD/TOC reduction respectively. The increase in substrate concentration may result in greater adsorption on the catalyst surface there by suppressing active sites for effective reaction. This phenomenon minimizes the decomposition of H2 O2 molecules to strong oxidative hydroxyl radicals over the catalyst surface which in turn retarded the rate of CWPO of DCP and 2,4DCP. Thus, the COD and TOC removal rates significantly decrease under the aforementioned experimental conditions. 3.5.4. Effect of oxidant concentration Figs. 4c and 5c depicts the effect of different stoichiometry between DCP/2,4-D with H2 O2 (DCP: H2 O2 varied as 1:0, 1:12, 1:14, 1:16, 1:18 and 1:20, 2,4-D: H2 O2 as 1:0, 1:15, 1:17, 1:19, 1:20 and 1:22). When the oxidant dosage is zero, the removal of DCP and 2,4-D was respectively 0.20% and 1.71%pointing out that the adsorption of pollutant molecules over the catalyst surface was fairly low as indicated by their low surface area values. Hence the removal mechanism can be confidently ascribed to the activated complex theory of catalysis. In both cases complete removal was resulted with the minimum stoichiometry of reactants itself with high reduction in COD (80.41% and 74.4% respectively for DCP and 2,4-D) and TOC (72.14% and 62.3% for DCP and 2,4-D). It can be seen from Fig. 4c, that the removal of DCP was decreased with higher oxidant concentrations, COD and TOC removal decreased to 72.85% and 52.81% respectively in 75 min. The increase in reaction rate at higher oxidant concentrations can be explained due to the easy availability of hydroxyl radicals that result from the decomposition of hydrogen peroxide over the catalyst surface. The decrease in COD reduction was attributed to the contribution from residual peroxide and in order to avoid that, the minimum stoichiometric ratio was maintained for DCP (1:12) and 2,4-D (1:15) throughout the study with which highest COD and TOC removals were obtained. The rate

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Fig. 7. Catalytic efficiency of NiFe2 O4 in the kinetics of WPO of DCP and 2,4-D. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst – 0.5 g/L, time – 120 min, Temperature–343 K.

Fig. 8. Catalytic efficiency of Zn0.25 Ni0.75 Fe2 O4 in the kinetics of WPO of DCP and 2,4-D. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst – 0.5 g/L, time – 120 min, Temp–343 K.

of removal of DCP and 2,4-D decreased to 31.2% and 61.8% respectively at very high oxidant concentrations. This is because excess hydrogen peroxide may react with the reactive hydroxyl radicals and/or by its auto scavenging effect as given by (Eqs. (7) and (9)) and this inhibits the rate of degradation of target compound [12,13]. 3.6. Effect of zinc doping on the catalytic activity of nickel ferrite nanocomposite towards the abatement of DCP and 2,4-D CWPO of DCP and 2,4-D were performed under optimized conditions and the results of the investigation on the effect of zinc on the catalytic activity of nickel ferrite nanocomposites based on GC, COD and TOC are presented in Figs. 7–11. All compositions especially the zinc rich composites were found to be more active for the complete removal of DCP and 2,4-D under mild reaction conditions. In the case of ZnFe2 O4 catalyst, an induction period was observed towards the oxidation of DCP as clearly evident from the results 10.53% of removal occurred at 90 min of the reaction which then increased to54.48% in 120 min with 52.8% and 50.1% of reduction in COD and TOC. Towards 2,4-D, the activity of ZnFe2 O4 was almost same in the initial stages but later the removal rate increased to 84.27% within 120 min with 40.35% and 37.8% of reduction in COD and TOC respectively. In the case of un-substituted nickel ferrite, 90.15% of DCP and 52.09% of 2,4-D removal was achieved with

Fig. 9. Catalytic efficiency of Zn0.5 Ni0.5 Fe2 O4 in the kinetics of WPO of DCP and 2,4D. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst – 0.5 g/L, time – 120 min, Temperature–343 K.

Fig. 10. Catalytic efficiency of Zn0.75 Ni0.25 Fe2 O4 in the kinetics of WPO of DCP and 2,4-D. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst – 0.5 g/L, time – 120 min, Temperature–343 K.

63.7/54.9% and 49.7/34.7% of COD/TOC reduction respectively. The catalytic activity of nickel ferrite gradually increased by zinc substitution. In the case of Zn0.25 Ni0.75 Fe2 O4 nanocomposite, the rate of removal of DCP reached its completion in 120 min with a reduction of 67.8% and 56.7% in COD and TOC respectively, whereas towards the oxidative 2,4-D,79.62% of removal was obtained with 74.7% and 36.2% of reduction in COD and TOC respectively. With Zn0.5 Ni0.5 Fe2 O4 complete destruction of DCP was obtained with reduction of COD and TOC by 65.53% and 64.32% respectively. The rate of removal and reduction in COD were found to be almost same for 2,4-D also (67.57%) but the reduction in TOC was less (42.8%). This is probably due to the presence of some stable intermediate compounds of 2,4-D oxidation. Increase in zinc composition by 0.75% (Zn0.75 Ni0.25 Fe2 O4 ), resulted in complete DCP removal within 75 min and the percentage of reduction in COD and TOC increased to 83.82% and 72.6% respectively. Its activity was found to be nearly the same towards 2,4-D also with its complete removal in 90 min with 83.5% and 70.1% of reduction in COD and TOC respectively. The time taken by the catalyst for the complete removal of 2,4-D was less in comparison with that of DCP at moderate concentrations of zinc, but at higher concentrations the rate of oxidation of DCP was increased. Though complete removal of target pollutants was achieved in less than 90 min, mineralization did not reach more than 85% upon two hours reaction. This clearly indi-

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Fig. 11. Catalytic efficiency of ZnFe2 O4 in the kinetics of WPO of DCP and 2,4-D. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst – 0.5 g/L, time – 120 min, Temperature–343 K.

75

Fig. 12. Scavenger effect on CWPO of DCP (a) and 2,4-D (b) over Zn0.75 Ni0.25 Fe2 O4 composite. Reaction conditions: 25 ml DCP (1 g/L), 25 ml 2,4-D (0.3 g/L), catalyst (Zn0.75 Ni0.25 Fe2 O4 ) – 0.5 g/L, time – 120 min, Temperature–343 K.

3.7. Reaction mechanism, kinetics and activation energy changes

cates the presence of simple but stable intermediate compounds as transformation products of DCP and 2,4-D during wet oxidation. The catalytic activity of a catalyst can be explained mainly in terms of its surface area and by its chemical structure. The catalytic effectiveness of zinc substituted nickel ferrite composites are less compared to cobalt and manganese substituted zinc ferrite [12,13]. The reduced catalytic behaviour can very well be explained by the surface area parameter. The surface area possessed by mixed spinels of zinc and nickel was very small in contrast to other mixed ferrites [18]. From Table 1, it is clear that surface area of nickel ferrite catalyst decreased first with zinc doping and then increased gradually till x-0.75. The un-substituted zinc ferrite possesses the least surface area. Relative to their surface area, the catalytic performances of different composites of nickel and zinc ferrite nanocomposites were also found to be in the following decreasing order, Zn0.75 Ni0.25 Fe2 O4 > Zn0.5 Ni0.5 Fe2 O4 > Zn0.25 Ni0.75 Fe2 O4 > NiFe2 O4 > ZnFe2 O4 . High surface area implies more interactions with reactant molecules. The chances of interaction of pollutant molecules over the catalyst surface are neglected according to our previous results. Another possibility for the reaction is that hydrogen peroxide decomposition occur on the catalyst surface producing highly reactive hydroxyl radicals. Higher the surface area, more reactive radicals are produced accelerating the reaction. The ease of the reaction is also influenced greatly by the metal ions present on the surface of spinel structured ferrite nanocomposites. In spinel ferrites the octahedral sites, rather than tetrahedral sites are more exposed towards the outer surface and hence the octahedral cations are considered to be more responsible for catalytic activity [27–32]. The redox stability of the metal present in the octahedral site contributes greatly towards the catalytic efficiency. NiFe2 O4 has an inverse spinel structure ([Fe2 3+ Zn2+ ]tet [Ni2+ Fe2 3+ ]oct O4 2− ); its octahedral sites are occupied by Ni2+ and Fe3+ ions. Iron is redox active (Fe3+ /Fe2+ /Fe0 ) whereas nickel has no stable redox pair so the catalytic activity mainly attributed to the presence of iron. When zinc doping occurs, Fe3+ ions are redistributed among tetrahedral and octahedral sites and more Fe3+ ions are forced to occupy in octahedral sites resulting in a normal spinel structure of zinc ferrite ([Zn2+ ]tet [Fe2 3+ ]oct O4 2− ) .As the number of redox active metal ions increases the oxidation property also increases there by improving its activity towards oxidation reactions. TPR analysis results provide strong evidence to prove the increase in oxidation property of nickel ferrite composites with zinc substitution.

It is widely accepted that the hydrogen peroxide based catalytic oxidation reactions generally proceeds by the attack of hydroxyl radicals. The complete oxidation of 2,4-dichlorophenol and 2,4-dichlorophenoxy acetic acid using hydrogen peroxide can be illustrated by the following equations, C6 H4 OCl2 + 12H2 O2 → 13H2 O + 6CO2 + 2HCl

(1)

C8 H6 O3 Cl2 + 15H2 O2 → 17H2 O + 8CO2 + 2HCl

(2)

Although no clear mechanism can be given for the CWPO of these kinds of chlorinated organics, our investigation on hydroxyl radical trapping experiments using n-butanol revealed a radical based mechanism for the oxidation of DCP and 2,4-D. So the explanations presented by Pardeshi et al. for the generation of hydroxyl radicals by catalytic decomposition of hydrogen peroxide might be applicable in our case also [23]. Fig. 12 display the inhibitory effect of n-butanol in the removal of DCP and 2,4-D under the optimized reaction conditions. As shown in figure, the addition of n-butanol caused a significant reduction in the extent of removal of DCP and 2,4-D respectively from 100% to 14.7%, and 100% to 19.8%clearly indicating that the removal of these kinds of pollutants are done by hydroxyl radicals generated through an intra-molecular electron transfer in metal-hydrogen peroxide complex [19]. Generation of hydroxyl radicals facilitated by the ease of formation of metalhydrogen peroxide complex and its decomposition by Eqs. (3) and (4). The oxidative decomposition reaction then initiated and propagated by hydroxyl radicals. Even in the presence of hydroxyl radical scavenger, some removal occurred which shows the presence of some other reactive species like hydro peroxide radical as reported by Pardeshi et al. [20]. All other possibilities of side reactions are illustrated in the following equations. M2+ + H2 O2 → M2+ · · ·H2 O2 (Formation of activated complex) (3) M2+ · · ·H2 O2 → M3+ + OH− +• OH (Dissociation step)

(4)

M3+ + H2 O2 → M2+ + • OOH + H+

(5)

2+

M

+ HOO• /O2 •

→ M

3+

+ H2 O2

•

(6)

OH + H2 O2 → HO2 + H2 O

(7)

HOO• /O2 •

(8)

HO•

+

+

HO•

HOO• /O2 •

→ H2 O2

→ H2 O2 [11–13]

HO• + chlorinated organics → benign products

(9)

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Fig. 13. Arrhenius plots for catalysed and non-catalysed reactions of DCP and 2,4-D.

A linear plot of ln(C/Co ) against time reveals that the decomposition kinetics of DCP and 2,4-D follow substantially a first order kinetics with the equation, ln (C/Co ) = k·t where k is the rate constant and t is time (Fig. 6). It was also noted that at higher temperature (343 K), the oxidation of DCP was slightly deviated from first order kinetics, whereas 2,4-D oxidation kinetics exactly follow first order rate equation. It demonstrates that the rate of catalytic oxidation of DCP and 2,4-D over Zn0.75 Ni0.25 Fe2 O4 is respectively about 12.2 and 9.88 times faster than their corresponding noncatalysed reactions. It was found that the first order rate constant was higher for DCP than 2,4-D. The results of rate constant for the catalysed reactions at different temperatures are presented in Table 2. The activation energies (Ea) of the catalysed peroxide oxidation of DCP and 2,4-D by zinc-nickel ferrite composite were estimated from linear Arrhenius plots (Fig. 13) and the data are given in Table 2. As it can be seen from the table, the catalyst had a significant role in lowering the activation energy of the reaction in comparison with its non-catalysed route. The activation energy obtained from our investigation was comparable to those results obtained over metal oxide catalysts [21]. As given in the table, the activation energy for WPO of DCP and 2,4-D are respectively 22.9 and 26.1 kJ/mol and it was reduced to 13.26 and 14.98 kJ/mol respectively in the presence of catalyst. Kusmierek et al., calculated the activation energy for the oxidative degradation of 2-chlorophenol by persulfate solution containing Fe2+ and Cu2+ ions in the range 50–66 kJ/mol. Zhao et al. reported the reaction activation energy for the decomposition of hydrogen peroxide between 47–43 kJ/mol

under microwave irradiation. In comparison with these reports, the obtained activation energy values are significantly low for both the catalysed and non-catalysed reactions [22,23]. 3.7.1. Degradation pathway for mineralization of DCP and 2,4-D The intermediates of oxidative degradation of DCP and 2,4-D over Zn0.75 Ni0.25 Fe2 O4 /H2 O2 system at 343 K were identified by GC–MS. WPO of DCP and 2,4-D has been reported to occur over catalysts like Pd-Fe nanoparticles [24,25], Fe0 /CeO2 composite [26], etc. Wang et al. reported the dechlorination of DCP followed by the transformation of intermediate products 2-chlorophenol and 4-chlorophenol ending with the final product of phenol [24]. Chaliha et al. presented a multistep pathway of destruction of DCP proceeding via the formation of 2,6-dichloro-1,4-benzoquinone and 2-chloro-1,4-dihydroxybenzene which then dechlorinated to 1,4-hydroxybenzene and to simple carboxylic acids [6]. Based on GC–MS data, 4-chlorophenol, phenol, xylenes, benzoquinone, 4-hydroxy 4-methyl 2-pentanone, 2,5-hexane dione, triacetone peroxide and acetone were found as intermediates during the degradation of DCP in the present case. The presence of some stable intermediates was also evident from the TOC value that about 27.4% of organic content was retained in the aqueous system. After four hours of reaction, it was reduced to 1.42% which means that complete mineralization is possible with this catalyst. In the case of 2,4-D, it was found that 2,4-dichlorophenol, 4-chlorophenol, phenol, benzoquinone and acetone are the major intermediate compounds. The decrease in pH of reaction medium clearly indicates the formation of organic acids during the decomposition

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Fig. 14. Plausible reaction pathway of CWPO of DCP over Ni1-x Znx Fe2 O4 nanocomposites. Reaction condition: DCP–1 g/L, DCP: H2 O2 –1:12, Zn0.75 Ni0.25 Fe2 O4 – 0.5 g/L, pH5.2–7.03.

reaction of 2,4-D. But the TOC removal was 62.5% in two hours of WPO reaction which then increased significantly after three hours and reaches its completion after five hours with excellent COD and TOC reduction. A plausible reaction mechanism for the complete mineralization of DCP and 2,4-D via an oxidative way is shown in Figs. 14 and 15 respectively. The change in reaction pH was again strong proof for the reaction progress and complete mineralization of pollutants. Though the reaction mechanism was explained by Fenton’s way, which is possible at lower pH range but by considering the applicability of the reaction as a small scale remedy for the abatement and complete minimization of toxicity environmental pollutants, the reaction pH was not changed. Probably the surface acidity of the catalyst enhanced and facilitates the decomposition of hydrogen peroxide to generate reactive hydroxyl radicals. In the case of WPO of DCP, the initial pH of the reaction mixture was 5.23 which increased gradually to 6.78–7.03 at the end of the reaction, whereas for 2,4-D oxidation the initial pH was 4.13 which increased to 7.2 ruling out the chances of secondary pollution as in Fenton’s catalysis [10]. 3.8. Catalyst deactivation 3.8.1. Effect of temperature, catalyst amount and dopant concentration on catalyst deactivation Catalyst deactivation via leaching is normally influenced by different reaction conditions such as reaction temperature, catalyst composition and its dosage. So a detailed investigation was conducted to show the dependence of catalyst deactivation with the reaction variables and the research results are presented in Table 3. The leaching tendency was also studied well during reusability experiments of peroxide oxidation reactions of DCP and 2,4-D and are summarized in Table 4. Leaching of metal ions from the catalyst surface was zero at lower temperatures (298, 313 and 323 K) which slightly increased to 0.052 ppm at 343 K. Leaching of iron increased

considerably with further increase in temperature and highest leaching (3.64 ppm) was observed at 353 K. Though the amount of metal ions that leached (ppm) into reaction mixture increased with catalyst loads but was fairly low in comparison with other reported results [18]. Leaching was found to be high with 2,4-D compared to those measured with DCP. Dopant has a significant role in providing structural integrity to the composite. It was interesting to note that zinc doping decreased the leaching nature of nickel ferrite composite. As the composition of zinc increased from 0.0 to 0.75, amount of iron leached in to the reaction system decreased from 0.462 to 0.051 ppm, whereas the un-substituted zinc ferrite exhibited the highest leaching (2.64 ppm) among the series. So from these results by choosing good dopant at its right composition (Zn0.75 Ni0.25 Fe2 O4 ) and at suitable temperature (343 K), the problem of catalyst deactivation can be minimized effectively. The leaching studies also provide valuable information regarding the mechanism of reaction, whether the reaction is homogeneous or heterogeneous [27,28]. From Tables 3 and 4, it can be concluded that the leaching of metals was very less even after five consecutive cycles indicating a heterogeneous catalytic mechanism for the peroxide oxidation of pollutants. The catalysts can be easily separated from the reaction system by physical sedimentation or magnetic means after completion of reaction. Also the weight of the catalyst remains almost the same after the reaction. 3.9. Catalyst reusability and its heterogeneity Application of heterogeneous catalysts in waste water treatment demands that they should be stable against metal ion leaching under operating conditions. Continuous and gradual leaching results in deactivation of the catalyst and leads to secondary pollution in water. So it is very important to study the reactive life time of the catalyst and its potential for reactivation for the next reaction cycle. In addition to total leaching, reusability of the catalyst is a

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Fig. 15. Plausible reaction pathway of CWPO of 2,4-D over Ni1-x Znx Fe2 O4 nanocomposites. Reaction conditions: 2,4-D-0.3 g/L, 2,4-D: H2 O2 –1:15, Catalyst – 0.5 g/L, pH4.13–7.2. Table 3 Leaching tendency of catalyst during WPO of DCP with respect to temperature, catalyst dosage and dopant concentration. Temperature (K)

Leaching of Fe (ppm)

Catalyst dosage (g/L)

Leaching of Fe (ppm)

298 313 323 343 353

0 0.001 0 0.051 3.64

0.1 0.3 0.5 0.7 1

0.024 0.047 0.051 0.7 1.0

critical feature for a heterogeneous catalyst for long term uses [28]. Therefore in order to check the reusability, the catalyst was separated from the reaction mixture between each cycle by filtration,

Leaching of Fe with Zn concentration Znx Ni1-x Fe2 O4 x

Fe

1.0 0.75 0.50 0.25 0.0

2.64 0.051 0.176 0.284 0.462

washed several times and dried as detailed in the experimental part and then reused. The magnetic property of ferrite nanocomposites is likely to be helpful for its better and easy retrieval. It can be noted

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Table 4 Stability and leaching features of Zn0.75 Ni0.25 Fe2 O4 catalyst in the WPO of 1 g/L DCP and 0.3 g/L of 2,4-D over 0.5 g catalyst at 343 K. No. of cycles

First use Recycle 1 Recycle 2 Recycle 3 Recycle 4 Recycle 5

DCP

2,4-D

Leaching of Fe (ppm)

Removal of DCP

0.051 0.699 1.140 1.171 1.170 1.151

100 100 100 93.41 90.23 90.04

Reduction in

COD

TOC

80.41 78.37 76.52 76.05 75.98 72.54

67.10 57.13 56.25 55.74 50.52 49.68

Leaching of Fe (ppm)

Removal of 2,4-D

0.084 0.475 0.293 1.399 2.012 2.399

100 100 96.32 90.56 81.52 77.56

Reduction in

COD

TOC

74.4 57.68 57.34 56.84 52.1 48.6

62.3 40.25 32.86 31.02 30.40 23.04

Table 5 Data on average crystallite size (D) and lattice parameter (a) of catalyst during five consecutive reaction cycles of WPO of DCP and 2,4-D. No. of cycles

First use Recycle 1 Recycle 2 Recycle 3 Recycle 4 Recycle 5

DCP

2,4-D

crystallite size D (nm) (XRD)

SBET (m2 /g)

crystallite size D nm (XRD)

SBET (m2 /g)

26.30 26.01 25.62 23.93 23.04 22.37

7.55 7.59 7.58 7.52 7.44 7.34

26.30 27.01 27.52 27.73 28.04 28.37

7.55 7.32 7.25 7.18 7.24 7.20

Reaction conditions (a): DCP–1 g/L, DCP: H2 O2 –1:12, Zn0.75 Ni0.25 Fe2 O4 – 0.5 g/L, pH-5.2–7.03; (b): 2,4-D-0.3 g/L, 2,4-D: H2 O2 –1:15, Zn0.75 Ni0.25 Fe2 O4 – 0.5 g/L, pH-4.13–7.2.

from Table 4 that the catalytic efficiency of zinc doped nickel ferrite catalyst towards the peroxide oxidation of DCP was decreased after second cycle. Removal rate was decreased to 93.41% in the third cycle and then the activity remains almost constant for the remaining cycles. But it was investigated that the catalyst was stable enough to provide more than 90% removal of DCP even in its fifth cycle. However the reduction in COD and TOC were reduced respectively from 80.41 and 67.1% to 72.54 and 49.68%. The activity of the catalyst was found to be almost same for the removal of 2,4-D in its second use and after that performance level consistently decreased. The results are displayed in Table 4. But more than 90% of removal was achieved in its second and third cycles. Then the reaction rate drops to 77.5% in the fifth cycle with reduction of 48.6% of COD and 23.04% of TOC. The reduction in degradation rate in the successive runs may be due to the deposition of carbonaceous compounds in the active sites of catalyst’s surface in addition to metal leaching. Also the easy recovery and fairly low leaching tendency of the catalyst were strong proofs for defining the heterogeneity of the catalysed wet peroxide oxidation reaction. 3.10. Structural and surface studies of the catalyst during catalytic runs It is crucial to determine whether the catalyst surface is affected by the reaction conditions. Therefore we analysed the catalyst structure and its surface by XRD and BET tools. X-ray diffraction patterns of the catalyst were taken after each catalytic run with DCP and 2,4-D in order to check any change in the crystallinity and spinel phase of the catalyst. Fig. 16. It depicts the XRD profiles for the catalyst during five repeated cycles of WPO reaction. It was seen from the XRD pattern that, even after five consecutive cycles, the spinel phase of the ferrite catalyst was retained as such with similar crystallinity as that of fresh sample. The intensity of peaks and its position was still in close agreement with the standard 2␪ values. But the average size of ferrite composite undergoes slight variation (Table 5). In the case of WPO of DCP, the value of crystallite size decreased continuously from 26.3 nm to 22.37 nm. However, after its reaction with 2,4-D the crystallite size was found to increase from 26.3 nm to 28.37 nm.

Fig. 16. XRD patterns of Zn0.75 Ni0.25 Fe2 O4 catalyst before and after catalytic studies. Reaction conditions: Catalyst-0.5 g/L, 2,4-D: H2 O2 -1:15, 2,4-D–25 ml (0.3 g/L), Time–90 min, Temperature–343 K, pH-4.13–7.2.

We also analysed the surface area of the catalyst by nitrogen adsorption analysis after each cycle. Table 5 depicts the changes in surface area during each catalytic run for DCP and 2,4-D. It is clear from the table, in both the cases the surface area of catalysts decreased slowly. This decrease in surface area during reaction may be due to the deposition of hydrocarbons as a result of charring. 4. Conclusions Zinc substituted nickel ferrite nanocomposites exhibits excellent catalytic activity towards the oxidative destruction of 2,4-dichlorophenol and 2,4-dichlorophenoxy acetic acid at mild conditions. A strong effect of reaction parameters such as temperature, oxidant concentration, pollutant concentration, catalyst dosage, pollutant to oxidant molar ratio and time has been observed and has a direct influence on the rate of oxidation reaction. Zinc substitution in nickel ferrite increased oxidation ability and Zn0.75 Ni0.25 Fe2 O4 composite was found to be very active among the series in giving complete removal of DCP and 2,4-D within 75 and

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90 min respectively. Catalytic activity of nickel doped zinc ferrite nanocomposites can be directly correlated to their acidic behaviour and redox capacity. Kinetic studies indicated a first order reaction mechanism for both the oxidation reactions and it was found that the catalysed reaction was respectively 12.2 and 9.8 times faster over the catalyst than the non-catalysed reaction. Leaching and reusability studies showed that the catalyst has excellent chemical and structural stability with negligible leaching which also indicate a heterogeneous mechanism for the catalysed peroxide oxidation. Deactivation of catalyst was strongly correlated to reaction temperature, dopant concentration and its dosage. Thus nickel-zinc ferrite composites could be effectively applied as a good alternative for the detoxification chlorinated organic compounds. Acknowledgement Junior Research Fellowship to Ms. Divya S. Nair from University Grants Commission, New Delhi, India is gratefully acknowledged. References [1] L.P. Cardoso, R. Celis, J. Cornejo, J.o.B. Valim, Layered double hydroxides as supports for the slow release of acid herbicides, J. Agric. Food Chem. 54 (2006) 5968–5975. [2] D. Chaara, F. Bruna, M. Ulibarri, K. Draoui, C. Barriga, I. Pavlovic, Organo/layered double hydroxide nanohybrids used to remove non-ionic pesticides, J. Hazard. Mater. 196 (2011) 350–359. [3] J. Cornejo, R. Celis, L. Cox, M. Hermosin, in: F. Wypych, K. Satyanarayana (Eds.), Clay Surfaces: Fundamentals and Applications, Academic Press, Amsterdam, 2004, p. 36, pp. 247–266. [4] U. Akpan, B. Hameed, Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid by Ca–Ce–W–TiO2 composite photocatalyst, Chem. Eng. J. 173 (2011) 369–375. [5] L. Ding, X. Lu, H. Deng, X. Zhang, Adsorptive removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solutions using MIEX resin, Ind, Eng. Chem. Res. 51 (2012) 11226–11235. [6] M. Kurian, C. Kunjachan, A. Sreevalsan, Catalytic degradation of chlorinated organic pollutants over CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocomposites at mild conditions, Chem. Eng. J. 308 (2017) 67–77. [7] R. Munter, Advanced oxidation processes—current status and prospects, Proc. Estonian Acad. Sci. Chem. 50 (2001) 59–80. [8] M. Kurian, C. Kunjachan, Cex V1-x O2 (x: 0, 0.25–1) nanocomposites as efficient catalysts for degradation of 2,4 dichlorophenol, J. Environ. Chem. Eng 4 (2016) 1359–1366. [9] M.E. Suarez-Ojeda, A. Fabregat, F. Stuber, A. Fortuny, J. Carrera, J. Font, Catalytic wet air oxidation of substituted phenols: temperature and pressure effect on the pollutant removal, the catalyst preservation and the biodegradability enhancement, Chem. Eng. J. 132 (2007) 105–115.

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