Catalytic degradation of chlorinated organic pollutants over CexFe1−xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocomposites at mild conditions

Accepted Manuscript Catalytic degradation of chlorinated organic pollutants over CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocomposites at mild conditions Manju Kurian, Christy Kunjachan, Asha Sreevalsan PII: DOI: Reference:

S1385-8947(16)31275-X http://dx.doi.org/10.1016/j.cej.2016.09.039 CEJ 15749

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

3 June 2016 12 August 2016 7 September 2016

Please cite this article as: 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, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.09.039

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Catalytic degradation of chlorinated organic pollutants over CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocomposites at mild conditions Manju Kurian*, Christy Kunjachan, Asha Sreevalsan Department of Chemistry, Mar Athanasius College, Kothamangalam-686666, India.

Address, Department of Chemistry, Mar Athanasius College, Kothamangalam, India. PIN 686666

E-mail ID: [email protected] Tele fax: 91485 2822512

1

Catalytic degradation of chlorinated organic pollutants over CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocomposites at mild conditions Abstract Oxidation of chlorinated organic pollutants (COPs) like 4-chlorophenol (4-CP), 2,4dichlorophenol (2,4-DCP), 2,4-dichlorophenoxy acetic acid (2,4-D) using aqueous hydrogen peroxide (30% v/v) over CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocatalysts was studied. The catalysts were characterised by temperature programmed reduction (H2-TPR) and desorption (NH3-TPD and CO2-TPD) techniques. The reducing power of ceria increased on iron doping as revealed by TPR analysis. CexFe1-xO2 were effective catalysts for Wet Peroxide Oxidation of 4CP, 2,4-DCP and 2,4-D pollutants. 4-CP (500mg/L) was completely degraded with 37.38% TOC and 58.75% COD removal after 90 min at 70℃ using Ce0.25Fe0.75O2 catalyst. Complete 2,4-DCP (250mg/L) conversion with 21.60% TOC and 42.44% COD removal was observed at 45 min over Ce0.5Fe0.5O2 catalyst at 50℃. 100% catalytic oxidative conversion of 2,4-D (250mg/L) with 23.64% TOC and 45.78% COD removal was achieved at 70℃ at 60 min. Atomic Absorption Spectrometry (AAS) indicated the extent of leaching of iron from the catalytic structure to be negligible. The mixed oxides were reusable and stable on consecutive uses as indicated by X-ray diffraction (XRD) and surface area measurements. The catalytic efficiency was retained after five successive runs. Keywords: Ce-Fe oxides, temperature programmed reduction and desorption, catalytic wet peroxide oxidation, chlorinated organic pollutants.

1. Introduction

Phenolic compounds are a major class of environmental pollutants and are potential human carcinogens. They are widely used in the manufacturing processes of plastics, dyes, drugs, pesticides and papers. Their disposal may contaminate soil and water. Because of increasing public health concerns and stricter regulations on treatment and disposal, it becomes more important to develop newer and efficient methods for removing these compounds from wastewaters [1-5]. The reported methods for removing phenolics from wastewaters include 2

activated carbon adsorption [6], microbial degradation [7], chemical oxidation [8], corona discharge [9], enzymatic degradation [10] etc. 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4DCP) and 2,4-dichlorophenoxy acetic acid (2,4-D) have been recognised as priority pollutants by United States Environmental Protection Agency (USEPA) since 1976 and by the European Decision 2455/2001/EC [11-14]. Taking all phenols together, US EPA has set a non-enforceable Human Health Water Quality standard of 2100 ppb [15]. 2,4-dichlorophenol is a precursor to the manufacture of the widely used herbicide 2,4-dichlorophenoxy acetic acid and is also the major transformation product of 2,4- D caused by solar photolysis and also microbial activities in soil or natural water. 2,4-DCP is also a water disinfection byproduct and is produced during incineration of municipal waste [16]. 2,4-dichlorophenoxyacetic acid is used as a herbicide and a plant growth regulator, which control the broad-leaf weeds in agriculture, and control the woody plants along roadsides and railways. 2,4-D has been proved to cause damage to the vital organs of human and animals, for instance, kidneys and liver at high doses. The maximum contaminant level of 2,4- D is set as 0.07 mg/L in drinking water by EPA [17].

The toxic and bio-resistant organo chlorine compounds in aqueous systems need to be transformed into harmless species. Various abatement techniques including biological, thermal and chemical treatments have been developed in the last few years for the detoxification of organic pollutants. Biological oxidation requires longer retention time and is not suitable for high concentrations of pollutants or for persistent pollutants. Among chemical techniques, catalytic wet peroxide oxidation (CWPO) appears to be a promising field of study. It has been reported to be effective for the near ambient degradation of soluble organic pollutants, because it can provide a nearly complete degradation [18]. Active, economical and stable catalysts would play a critical role in the degradation of organic pollutants in CWPO. Various metal incorporated heterogeneous catalysts including modified clays and metal oxides have been tested for lowering the temperature and time of the catalytic wet air oxidation of phenol and substituted phenols [19]. These include Fe0/CeO2 composites [20], Pd/C [21], Pd/Fe bimetallic nanoparticles [22], Ce-V composite oxides [23] etc. Our aim was to develop a simplified method for abatement of non-biodegradable organic wastewater pollutants using CexFe1-xO2 (x - 0, 0.25, 0.5, 0.75, 1) nanocatalysts by Wet Peroxide Oxidation of 4-CP, 2,4-DCP and 2,4-D. Iron doping to ceria based solid solution can show a remarkable enhancement on the textural properties and oxygen 3

storage-release ability [24,25]. Effect of different reaction variables on reaction rate was also studied and a suitable kinetic model has been proposed. The reusability of the catalysts was examined using leaching studies by AAS and structural examination by XRD and surface area measurements.

2. Experimental 2.1. Chemicals

Cerium(III) nitrate hexahydrate (Ce(NO3)3.6H2O) by Aldrich Chemical Co. Inc. (St. Louis, MO, USA),

iron(III) nitrate nonahydrate (Fe(NO3)3.9H2O) and ammonia (Merck Chemicals,

Mumbai, India) were used for preparation of oxide catalysts. For catalytic activity studies 4chlorophenol and 2,4-dichlorophenol (Loba Chemie) 2,4-dichlorophenoxy acetic acid (Himedia Laboratories), hydrogen peroxide, n-butanol, silver sulphate, mercury sulphate, sulphuric acid, ferrous ammonium sulfate and potassium dichromate (Merck Chemicals, India) were used as obtained. 2.2. Characterisation of prepared catalysts CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) nanocatalysts were prepared by coprecipitation [26]. Temperature Programmed Reduction experiments were performed in a Micromeritics TPx system using 10% H2 in He flowing at 5ml/min. Experiments were carried out in the range of 30850℃ at a heating rate of 5℃/min. Total acidity was evaluated by temperature-programmed desorption of ammonia/carbon dioxide using a Micromeritics TPx system. Before NH3/CO2 desorption, the sample was pre-treated under He flow of 10ml/min at 400℃. NH3/CO2 adsorption was performed under ambient conditions by flowing 10% NH3 in He/10% CO2 in He over the oxide until saturation and then desorption of NH3/CO2 by temperature-programmed treatment under He from 30-850℃ at a heating rate of 10℃/min. X-ray diffractograms of the reused catalysts were obtained using a Rigaku MiniFlex 600 X-ray diffractometer using CuKα radiation. Phase identification was carried out by comparison with JCPDS database cards. The average crystallite size was determined by Scherrer equation: D= Kλ/(βcosθ) where D is the average crystallite size, K the shape factor (0.89),  the wavelength of the incident X-rays (1.5418Å),  the diffraction angle, and  h, k,l the full width at half maximum in radian of the 4

Bragg peak corrected using the corresponding peak in micron-sized powder. Specific surface area measurements were done by nitrogen physisorption at 77K using a Micromeritics Gemini VII instrument. The specific areas of the samples were determined according to the standard Brunauer-Emmett-Teller (BET) procedure using nitrogen adsorption taken in the relative equilibrium pressure interval. 2.3. Catalytic activity studies

Catalytic activity experiments were performed by placing the reaction mixture containing 50ml of 500mg/L 4-CP / 250mg/L 2,4-DCP / 250mg/L 2,4-D solutions,

500mg/L mixed oxide

catalyst and requisite amounts of hydrogen peroxide in a 100ml round bottom flask connected to a condenser and immersed in a thermo stated oil bath and agitating with a magnetic stirrer at 150 rpm. At specific intervals, aqueous sample of 3ml was withdrawn and filtered immediately by Whattman grade1 filter paper to remove the catalyst particles before analysis. The samples were quantitatively analysed using Perkin Elmer Clarus 580 Gas Chromatograph equipped with an Elite-5 capillary column. The results are expressed as percentage conversion of 4-CP, 2,4DCP and 2,4-D. The extent of oxidation and total organic carbon removal was measured using COD measurements with standard dichromate method and Shimadzu TOC-L analyzer respectively. Removal percentage of chemical oxygen demand (COD) was calculated as {[COD]0 – [COD]t/[COD]0}100 where [COD]0 and [COD]t are CODs at initial and at time t respectively. The residual amount of peroxide was back calculated by dichrometry. Iron leaching was quantified by Perkin Elmer Analyst 700 Atomic Absorption Spectrometer. The error percentage between the results of analyses is less than 5%. The active species trapping experiments were carried out by adding 200mM/L n-butanol (∙OH scavenger) to the 4-CP / 2,4DCP / 2,4-D degradation solution in the presence of catalyst. The reaction intermediates of 4-CP, 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 pH was measured with a EUTECH digital pH meter. All experiments were repeated and averages are reported.

3. Results and discussions 3.1. Catalyst characterisation

5

XRD and TEM analyses of CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) reveal the presence of nanoparticles in all oxides beyond solid solution limit as detailed in our previous publication. Average crystallite sizes calculated from the corresponding diffraction peaks using DebyeScherrer equation ranges from 12.8 to 32.29 nm. The TEM images confirm the nanometric size of the prepared catalysts and average crystallite sizes are in the range of 12-65 nm [26]. BET surface area and pore volume of synthesised composites are in the range of 30-10 m2/g and 0.004-0.01cm3/g. Raman analysis indicate that iron incorporated ceria lattice have improved surface reducibility as a result of oxygen vacancy formation and gradual shrinkage of the unit cell. FT-IR spectra reveal the characteristic Ce-O stretching and crystalline water absorption of synthesised nanoparticles. Thermal analysis data shows these oxides to be thermally stable [26]. TPR profiles of CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1) catalysts are shown in Fig. 1. For the composition, x = 0, one peak at 354oC and a twin peak at 592℃ was observed. The reduction of Fe2O3 has been reported to occur by a stepwise process in such a manner that Fe2O3 is reduced to Fe3O4, then FeO and finally to Fe0. The peak centred at 354℃ can be attributed to the reduction of Fe2O3 to Fe3O4 while the twin peaks at 592℃ corresponds to the reduction of Fe3O4 to FeO and FeO to Fe0 respectively. [27, 28]. Pure CeO2 exhibits two main peaks at 477℃ and around 730℃ which are attributed to the reduction of surface lattice oxygen and bulk oxygen respectively. The reduction of oxygen in ceria at around 730℃ gives characteristic peak with H2 consumption of 737mol/g. Successive iron doping to ceria causes the bulk oxygen reduction on ceria shift to lower temperatures with drastic decrease in H2 consumption at 730℃. Three reduction peaks at 354, 489 and 704℃ can be observed for Ce-Fe mixed oxides profiles. It could be sequentially ascribed to the reduction of Fe2O3 to Fe3O4, FeO to Fe0 and the reduction of bulk ceria respectively, based on the TPR peaks of pure Fe2O3 and ceria. Synergistic interaction between Ce and Fe in the mixed solid solution results in lowering of reduction temperature. It should be noted that hydrogen consumption during the TPR measurements increases with increase in Fe content and the reduction peak shift to lower temperature on Fe doping. This phenomenon reveals the chemical interaction of iron species and ceria in the complex oxide as indicated by Fourier Transform Raman Analysis [26]. This enhances the reducibility of Ce-Fe mixed oxides increasing its potential use as oxidation catalysts.

6

Fig. 1. TPR profile of CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1)

The acid-base surface characterisation of solid catalyst experimented by NH3–TPD and CO2-TPD are shown in Figs. 2 and 3 respectively. The characteristic peaks of these profiles can be assigned to their desorption temperatures indicating the strength of surface sites [29]. For all catalysts, both weak and moderate acid-base sites are observed. However, strong acid-base sites on the surface are not exhibited by any of these catalysts. The broad desorption pattern indicates a large distribution of acid-base sites of different strength. With increase in iron doping in CexFe1-xO2, the amount of weak acid sites decreases while medium strength sites remain more or less same. Low temperature peaks are Bronsted acid sites and are attributed to the acidic protons on surface hydroxyl groups. Weak basic sites also decrease with increase in iron doping as indicated by a shift of desorption peak towards lower temperature. However the moderately strong base sites increase with increase of doping of iron.

7

Fig. 2. NH3-TPD profile of CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1)

Fig. 3. CO2 - TPD profile of CexFe1-xO2 (x: 0, 0.25, 0.5, 0.75, 1)

3.2. Catalytic oxidation studies by 4-CP, 2,4-DCP and 2,4-D The degradation and removal of 4-CP, 2,4-DCP and 2,4-D by catalytic wet peroxide oxidation were investigated using CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) catalyst systems and the results of 8

TOC and COD removal are shown in Figs. 4-7. The extent of removal followed the order of 4CP>2,4-DCP>2,4-D for CeO2 and Ce0.75Fe0.25O2, 4-CP>2,4-D>2,4-DCP for Fe2O3, 2,4-D>4CP>2,4-DCP for Ce0.25Fe0.75O2 and 2,4-DCP>2,4-D>4-CP for Ce0.5Fe0.5O2 (Fig. 4). It is interesting to see that iron oxide and doped catalysts with higher Fe2O3 percentages are effective in the degradation of these pollutants.

Fig. 4.Comparison of removal % of 4-CP (500mg/L), 2,4-DCP (250mg/L) and 2,4-D (250mg/L) over CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites (500mg/L) in the presence of H2O2 oxidant. CWPO of 4-CP using CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) catalysts are shown in Fig. 5. All catalysts show complete CP removal at 70℃. The oxidative removal efficiency of ceria enhances with iron doping and TOC removal percentage increases from 22.14 to 60.99% and COD removal percentage from 45.50 to 85.50%. The extent of removal of 4-CP, TOC and COD follow

the

order

Ce0.25Fe0.75O2>Ce0.5Fe0.5O2>Ce0.75Fe0.25O2>Fe2O3, >CeO2.

Catalytic

degradation of 2,4-dichlorophenol is illustrated in Fig. 6. Pure ceria exhibits 79% DCP removal, 49.50% COD removal and 21.54% TOC removal at 120 minutes of the reaction which increases as the amount of doped iron increases. Ce0.5Fe0.5O2 shows 100% DCP removal at 45 minutes of 9

reaction with COD and TOC removal of 67.98% and 69% respectively at 120 minutes reaction. Using pure iron oxide catalyst, DCP removal percentage after 120 minutes of the reaction is 79.62% with COD and TOC removal as 50.05% and 21.99%. The extent of removal of 2,4-DCP, TOC and COD by different CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) catalysts by CWPO follow the order of Ce0.5 Fe0.5O2>Ce0.25Fe0.75O2>Ce0.75Fe0.25O2>Fe2O3>CeO2. Figure 7 depicts the degradation of 2,4-D using CWPO by different CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) catalysts. Here also iron doping increases the catalytic removal efficiency. 2,4-D removal increases from 69.69% to 100% with TOC and COD removal percentage as 37.37% and 75.98% respectively. The efficiency of catalysts follows the order of Ce0.25Fe0.75O2> Ce0.5Fe0.5O2> Fe2O3 > Ce0.75Fe0.25O2> CeO2. Significant surface adsorption could not be observed in the peroxide oxidation of the pollutants using CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) catalyst systems. Analysis of the results indicates that the catalytic property of ceria is improved substantially by doping with iron. The reaction intermediates of chlorinated organics identified by GC-MS analysis are phenol, benzoquinone and 2,5-hexane dione. GC-MS data clearly evidence the absence of chlorinated intermediate organic compounds. The decrease in pH of the reaction mixture indicates the complete mineralization of the pollutants into HCl and H2O. The active hydroxyl radical trapping experiments were done by adding n-butanol to the reaction suspension where it act as an ∙OH scavenger. As can be seen in Table 1 the degradation reaction is inhibited by the addition of scavenger. The degradation percentage is reduced and the peaks of 4-CP, 2,4-DCP and 2,4-D can be detected in GC with the presence of radical scavenger suggesting the mechanism to be free radical mechanism. Therefore a possible reaction mechanism in the catalytic degradation of these chlorinated organics (COPs) is proposed as follows [20]: Fe2+ + Ce4+

Fe3+ + Ce3+

(1)

Fe2+/Ce3+ + H2O2

Fe3+/Ce4+ + ∙OH + OH-

(2)

OH- +

Fe2+/Ce3+ + ∙OH

(3)

H2O2

(4)

Fe3+/Ce4+

∙OH + ∙OH

10

In the initial step one electron transfer from Fe2+/Ce3+ to H2O2 as well as from OH- to Fe3+/Ce4+ via the reactions (2) and (3) produce highly reactive ∙OH fragment. Fe2+/Ce3+ is regenerated via reaction (3) with hydroxyl ion (OH-). Reaction (4) indicates the termination process by scavenging effect of ∙OH by excess hydroxyl radicals. Hydroxyl radicals act as active species which efficiently degrade the COPs as shown in equations (5-8). COPs + H2O2

Intermediates

COPs + ∙OH

Intermediates

Intermediates

+ H2O

Intermediates + ∙OH

+ H2O

(5) (6)

CO2 + H2O + Cl-

(7)

H2O + Cl-

(8)

CO2 +

Fig. 5. Removal of 500mg/L 4-CP with 500mg/L CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites and

0.13mol/L H2O2 at 70℃.

11

Fig. 6. Removal of 250mg/L 2,4-DCP with 500mg/L CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites and 0.06mol/L H2O2 at 70℃.

12

Fig. 7. Removal of 250mg/L 2,4-D with 500mg/L CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites and

0.06mol/L H2O2 at 70℃. The application of the first-order kinetics to the catalytic wet peroxide oxidation reaction of 4CP, 2,4-DCP and 2,4-D over CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) in terms of both GC and COD removal was done according to the equation: Ct = C0 e 

or 

log Ct = log C0 – (.) t where C0 and Ct are concentrations / COD of 4-CP, 2,4-DCP and 2,4-D initially and at time, t respectively, and k is the first order rate constant [30]. The linear fit of log Ct versus t can be obtained with slope as - (k/2.303) and the values of k calculated from the slopes are given in Fig. 8. A linear trend is obtained with time indicating the degradation to be first order with respect to reactant concentration as well as with COD. It could be observed that first order rate constants for

4-CP, 2,4-DCP and 2,4-D degradation are higher or almost same for Ce-Fe mixed oxide

catalysts than cerium and iron oxide.

(a)

(b)

13

(c)

(e)

(d)

(f)

Fig. 8. (a) & (b) Kinetic study of 500mg/L 4-CP degradation with 500mg/L Ce0.25Fe0.75O2,

0.13mol/L H2O2 at 70℃ by GC and COD analysis respectively. (c) & (d) Kinetic study of 250mg/L 2,4-DCP degradation with 500mg/L Ce0.5Fe0.5O2, 0.06mol/L H2O2 at 50℃ by GC and COD analysis respectively. (e) & (f) Kinetic study of 250mg/L 2,4-D degradation with 500mg/L Ce0.25Fe0.75O2, 0.06mol/L H2O2 at 70℃ by GC and COD analysis respectively. 3.3. Effect of reaction parameters on CWPO

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Catalytic wet peroxide oxidation reaction of 4-CP, 2,4-DCP and 2,4-D were studied with reference to reaction parameters such as temperature, oxidant concentration, DCP concentration, catalyst dosage and time by choosing the model catalyst as Ce0.5Fe0.5O2 for 2,4-DCP and Ce0.25Fe0.75O2 for 4-CP and 2,4-D. Experiments were carried out with initial concentrations of pollutants (4-CP - 0.5g/L, 2,4-DCP - 0.25g/L and 2,4-D - 0.25g/L) with

0.5g/L catalyst.

Concentration of H2O2 was fixed according to the stoichiometric equation for complete degradation to CO2 and H2O as 0.06 mol/L for 2,4-DCP and 2,4-D and 0.13mol/L for 4-CP without changing the initial pH. Optimization of reaction conditions for the CWPO of 4-CP, 2,4DCP and 2,4-D was carried out by changing any one of the reaction variable while keeping all others constant (Tables 1-4). Effect of increasing oxidant concentration from 0, 0.03, 0.07, 0.1, 0.13 to 0.16 mol/L for degradation of 4-CP shows complete removal efficiency in all dosages except in its absence (Table 1). TOC removal % shows a concomitant increase from 1.96 to 37.38% with further decrease to 11.68% on increasing oxidant concentration to 0.16mol/L. With 2,4-DCP, COD and TOC removal % increases from 28.76 to 100% and 10.01 to 22.53% respectively. Here also further increase in concentration of oxidant to 0.1mol/L decreases the conversion to 95.47% along with TOC removal to 22.12%. At higher concentrations of oxidant, generally the degradation is higher because more ∙OH radicals are available for the oxidation. The scavenging effect of ∙OH is not observed at moderate amounts of H2O2 due to slow agitation of the reaction mixture at low temperatures, reducing the auto scavenging effect. But at excess peroxide, reaction rate decreases due to the lack of proper decomposition of H2O2 as ∙OH radical and auto scavenging effect. 4-CP is removed completely at all temperatures under study, but maximum TOC (37.38%) and COD (58.75%) removal is observed at 70℃ (Table 2). For 2,4-DCP, complete conversion could be observed at 70℃ with 27.61% TOC and 64.05% COD. 2,4-D is degraded completely at 70℃ and 80℃ with TOC removal % as 23.64, 36.51and COD removal % as 45.78 and 75.21 respectively. In all cases the rate of removal of 4-CP, 2,4-DCP and 2,4-D increases rapidly with increase in temperature. This may be due to the more kinetically active hydroxyl radicals causing faster degradation.

15

Effect of catalyst dosage on CWPO was checked with different catalyst loadings of 0, 0.25, 0.5, 0.75, 1, 1.25 g/L. 4-CP is degraded completely at all catalyst doses (Table 3). With 4CP TOC removal % increases from 2.99 to 37.38%, on increasing catalyst dosage from 0 to 0.5g/L. Further increase of catalyst amount reduces the TOC removal % of 4-CP. When the catalyst dosage is increased from 0 to 0.5g/L, TOC removal increases from 27.14 to 100% for 2,4-DCP which slightly decreases with further increase in catalyst loading. Almost 2% reduction in removal could be observed for further increase in catalyst amount. For 2,4-D degradation a gradual increase in conversion with increase in catalyst amount from 35.21% to 100% at 60min reaction was observed with TOC removal % increasing from 12.41 to 35.54%. As discussed earlier the CWPO of 4-CP, 2,4-DCP and 2,4-D occur through OH∙ free radical mechanism, generated by the interaction of active oxygen species on the catalyst and H2O2. Hence an increase in catalyst amount generates more OH∙ radicals and enhances conversion. The ineffectiveness of oxidative degradation of 4-CP and 2,4- DCP with increased catalyst loading can be due to two factors; (i) an optimum amount of catalyst at which the hydroxyl radical generation is saturated, and (ii) beyond certain limit ∙OH radical formation is controlled by the decomposition of H2O2 to other products, which might have resulted a steady decrease in the oxidation process [31]. The effect of initial concentration of 4-CP, 2,4-DCP and 2,4-D on catalytic peroxide oxidation was studied by taking five different concentrations of 0.25, 0.5, 0.75, 1 and 1.25 g/L (Table 4). 4CP shows decrease of conversion rate from 100 to 15.61% with increase in CP concentration at 90min. TOC removal increases from 7.83 to 55.94% with increase of CP concentration from 0.25 to 1g/L which then decreases to 16.04% for 1.25g/L of 4-CP. On increasing the 2,4-DCP concentration from 250mg/L to 1.25g/L, DCP removal percentage decreases from 100% to 21.12% and TOC removal % increases from 22.60 to 25.72% at 45 min reaction. For 2,4-D, a decrease from 100 to 42.12% conversion with TOC removal % 23.64 to 27.72% could be observed at 60 min reaction. The TOC removal percentage increases with increase of initial reactant concentration due to the fact that with the enhancement of initial organic concentration, the intermediate concentration increases, which restrain the 4-CP, 2,4-DCP and 2,4-D from adsorbing on catalytic surface.

16

4-CP removal% in terms of GC, TOC and COD

2,4-DCP removal% in terms of GC, TOC and COD

H2O2 conc. as reaction variable (mol/L) 0

GC

TOC

COD

GC

1.96

6.75

H2O2 conc. as reaction variable (mol/L) 28.76 10.01 19.54 0

GC

37.5

H2O2 conc. as reaction variable (mol/L) 0

0.03

100

8.40

20

0.04

54.31 17.40 35.44 0.04

72.15 18.25 36.44

0.07

100

30.79 36.25 0.045

73.55 19.39 38.26 0.06

100

23.64 45.78

0.1

100

27.25 51.25 0.06

100

27.61 64.05 0.08

100

23.26 46.24

0.13

100

37.38 58.75 0.08

100

22.62 42.44 0.1

95.47 22.62 44.04

0.16

100

11.68 30

100

22.53 44.81 0.12

95.01 22.10 42.25

0.13 + n-

32.50 -

-

butanol

0.1

TOC

0.06 + n- 25.47 butanol

Table 1. Effect of H2O2 concentration

2,4-D removal% in terms of GC, TOC and COD

COD

-

TOC

COD

34.76 6.01

14.54

0.06 + n- 29.87 -

-

butanol

on the removal of 500mg/L 4-CP with 500mg/L

Ce0.25Fe0.75O2, 0.13mol/L H2O2 and 250mg/L 2,4-DCP/2,4-D with 500mg/L Ce0.5Fe0.5O2 composite, 0.06mol/L H2O2 at 70℃. Temperature 4-CP removal% in terms as reaction of GC, TOC and COD variable (℃) GC TOC COD

2,4-DCP removal% in terms of GC, TOC and COD GC TOC COD

2,4-D removal% in terms of GC, TOC and COD GC

TOC

COD

Room temperature 50

100

9.83

6.25

71.92

16.34

35.22

61.87

15.24

30.04

100

13.67

18.75

90.57

21.60

42.44

87.51

20.87

40.25

60

100

9.19

16.25

91.23

23.41

48.00

90.47

21.24

43.50

70

100

37.38

58.75

100

27.61

64.05

100

23.64

45.78

80

100

39.20

60.75

100

70.33

82.42

100

36.51

75.21

Table 2. Effect of temperature on the removal of 500mg/L 4-CP with 500mg/L Ce0.25Fe0.75O2, 0.13mol/L H2O2, 250mg/L 2,4-DCP/2,4-D with 500mg/L Ce0.5Fe0.5O2 composite, 0.06mol/L H2O2. Catalyst concentration as reaction variable (mg/L) 0

4-CP removal% in terms of GC, TOC and COD GC

TOC

COD

2,4-DCP removal% in terms of GC, TOC and COD GC TOC COD

40.82

2.99

8.5

27.14

8.64

20.57

2,4-D removal% in terms of GC, TOC and COD GC

TOC

COD

35.21

12.41

14.07 17

250

100

13.67

30

95.62

20.59

40.51

85.67

20.24

38.45

500

100

37.38

58.75

100

27.61

64.05

100

23.64

45.78

750

100

36.53

51.25

100

26.06

60.04

100

26.06

60.04

1000

100

17.53

37.5

96.75

39.17

72.50

100

32.17

64.50

1250

100

17.50

38.25

98.99

37.54

70.59

100

35.54

72.59

Table 3. Effect of Ce0.25Fe0.75O2 catalyst concentration on the removal of 500mg/L 4-CP with 0.13mol/L H2O2 at 343K and Ce0.5Fe0.5O2 catalyst concentration on the removal of 250mg/L 2,4DCP/2,4-D with 0.06mol/L H2O2 at 70℃.

Reactant concentration as reaction variable (mg/L) 250

4-CP removal% in terms of GC, TOC and COD GC

TOC

COD

2,4-DCP removal% in terms of GC, TOC and COD GC TOC COD

2,4-D removal% in terms of GC, TOC and COD

100

7.83

41.66

100

27.61

64.05

100

23.64

45.78

500

100

37.38

58.75

99.01

22.73

44.45

80.62

20.73

41.45

750

100

37.32

26.66

58.50

23.65

48.66

68.50

23.65

47.66

1000

42.03

55.97

16.17

55.62

26.02

61.05

54.62

26.02

53.05

1250

15.67

16.04

8.57

21.12

25.72

61.05

42.12

27.72

54.05

GC

TOC

COD

Table 4. Effect of 4-CP concentration on the removal with 250mg/L Ce0.25Fe0.75O2 composite and 0.13mol/L H2O2 and 2,4-DCP/2,4-D concentrations on the removal with 250mg/L Ce0.5 Fe0.5O2 composite and 0.06mol/L H2O2 at 70℃. 3.4. Stability and reusability of the catalyst XRD and AAS analyses were experimented to display the stability of the catalysts. PXRD pattern of CexFe1-xO2 remain almost same before and after the reaction (Fig. 9). Ceria exhibit cubic fluorite structure, Fe2O3 has typical XRD pattern of hematite, α-Fe2O3 (hexagonal) and for CexFe1-xO2, the diffraction peaks can be indexed to cubic fluorite structure when x≥0.5. As x value in CexFe1-xO2 decreases, the XRD peak arising from hematite appears. Average crystallite sizes calculated from the corresponding diffraction peaks using Debye-Scherrer equation ranges from 11 to 32 nm which remains same in the range of experimental error after the reaction. Leaching of iron is minimum for Ce-Fe oxides than pure Fe2O3 for 2,4-D degradation. Amount

18

of leached iron is negligible for all catalysts and it can be concluded that they also retain the structural stability (Table 5).

Fig. 9. PXRD pattern of CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites before and after CWPO of 4-CP, 2,4-DCP and 2,4-D. Catalyst

Average

Surface

Pore

%Removal

%Removal

%Removal

Average

composition

crystalline size,

area

volume

of 4-CP

of 2,4-DCP

of 2,4-D

crystalline size, D

D (nm)

m2/g

cm3/g

Leaching of Fe (ppm)

(nm)

Debye-scherrer

Debye-scherrer

method (fresh)

method

4-CP

2,4-DCP

19

2,4-D

(After reaction) CeO2

18.06

26

0.0107

91.57

79

66.46

17.43

-

-

-

Ce0.75Fe0.25O2

12.80

30

0.0124

92.48

81.68

69.69

12.97

5.044

4.292

1.412

Ce0.5Fe0.5O2

13.13

24

0.0098

97.23

100

98.12

11.29

7.013

3.418

1.771

Ce0.25Fe0.75O2

16.98

30

0.0122

100

86.44

100

15.41

6.001

4.507

1.049

Fe2O3

32.29

12

0.0048

91.47

79.62

95.6

31.05

4.780

4.846

4.182

Table 5. Stability and activity of CexFe1-xO2 (x = 0, 0.25, 0.5, 0.75, 1) composites towards CWPO of 4-CP, 2,4-DCP and 2,4-D. Reusability was checked for five consecutive runs in the degradation of pollutants using Ce0.5Fe0.5O2 and Ce0.25Fe0.75O2 as model catalysts. Between each run, the catalysts were separated from the reaction solution by filtration, washed several times with deionized water and finally by acetone to remove adsorbed particles on the catalyst surface and dried at 200℃ in an air oven.

Tables 5-7 shows the data on recycling and reusability studies of the catalysts

Ce0.25Fe0.75O2 and Ce0.5Fe0.5O2. After fourth recycle 100% 4-CP, 89.5% DCP and 92.01% 2,4-D removal is retained. This indicates that the catalyst remains active after five consecutive runs. PXRD pattern of the recycled catalyst compared with the freshly synthesized catalyst (Fig. 10) indicate that the catalysts retain phase purity. Average crystallite size remains more or less the same. The specific surface areas of recycled catalysts remain almost same for each recycle in the case of catalysts used for degradation of 2,4-DCP and 2,4-D. But in the case of 4-CP degradation, surface area of Ce0.25 Fe0.75O2 catalyst decreases after reuse may be due to the the removal of fine particles in grain boundaries along with benzoquinone intermediate. The data on leached iron concentration indicate that the total leached iron is minimum without any toxicity issues which also evidence a heterogeneous catalytic mechanism for the reaction. Catalyst

No.

of Leaching

cycles

of Fe

% removal of 4-CP

COD

TOC

(ppm)

Average

SBET

crystallite

(m2/g)

size D(nm)

Ce0.25Fe0.75O2

First use

3.690

100

58.75

37.38

16.98

Recycle 1

3.569

100

55

37.05

7.36

Recycle 2

3.121

100

42

34.04

10.58

Recycle 3

2.276

100

40

24.33

11.04

Fresh

27.80

Recycle 3 18.57

20

Recycle 4

2.436

100

42

26.72

9.248

Recycle 4 11.45

Table 6. Reusabiliy of Ce0.25Fe0.75O2 catalyst; Reaction conditions: 4-CP- 500mg/L, H2O20.13mol/L, catalyst - 500mg/L, temperature- 70℃.

Fig. 10. PXRD pattern of Ce0.5 Fe0.5O2 before and after catalytic reusability studies of 2,4-DCP; Reaction conditions: DCP- 250mg/L, H2O2- 0.06mol/L, catalyst - 500mg/L, temperature- 70℃. Catalyst

Ce0.5Fe0.5O2

No.

of Leaching of % removal of

Average crystallite SBET

cycles

Fe (ppm)

DCP

COD

TOC

size D(nm)

(m2/g)

First use

6.773

100

42.44

21.62

13.13

23.96

Recycle 1

4.199

98.61

40.56

20.05

13.83

23.51

Recycle 2

2.169

95.51

39.11

19.52

13.18

33.82

Recycle 3

2.074

92.67

39.11

18.67

17.56

22.56

Recycle 4

1.908

89.50

37.54

17.48

13.31

21.40

Table 7. Reusabiliy of Ce0.5 Fe0.5O2 catalyst; Reaction conditions: DCP- 250mg/L, H2O20.06mol/L, catalyst - 500mg/L, temperature- 70℃.

Catalyst

No. cycles

of Leaching of

% removal of Fe 2,4-D

COD

TOC

Average

SBET

crystallite

(m2/g) 21

(ppm) Ce0.25Fe0.75O2

First use

size D(nm)

2.158

100

45.78

23.64

16.98

Recycle 1 0.200

100

44.65

25.25

16.25

Recycle 2 0.841

100

42.58

20.01

17.01

Recycle 3 1.086

100

41.50

16.61

18.20

Recycle 4 0.672

92.01

40.45

14.36

24.365

First use

27.80

Recycle 4 30.56

Table 8. Reusabiliy of Ce0.25Fe0.75O2 catalyst; Reaction conditions: 2,4-D- 250mg/L, H2O20.06mol/L, catalyst - 500mg/L, temperature- 70℃.

Conclusions CexFe1-xO2 oxides exhibit high catalytic activity towards wet peroxide oxidation of chlorinated

organic pollutants under study. Heterogeneous catalytic free radical mechanism is observed for the reactions. The mineralization efficiency in terms of TOC and COD abatement of 120 minutes reactions with prepared Ce-Fe oxides indicate the formation of sustainable intermediates. The maximum catalytic efficiency is observed for Ce0.25Fe0.75O2 catalyst for 4-CP and 2,4-D and Ce0.25Fe0.75O2 for 2,4-DCP in the CWPO. Catalysts remain active for five consecutive runs and show fewer tendencies to leach. CexFe1-xO2 oxides could be good alternatives for pretreatment of chlorinated organic contaminants to reduce the effluent toxicity. Acknowledgment Financial support from Kerala State Council for Science, Technology and Environment, India is gratefully acknowledged.

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25

Highlights •

TPR study indicates the redox nature of the CeFe nanocomposites.

•

Iron doping on ceria increases the catalytic efficiency of the wet oxidation.

•

CeFe nanocomposites are stable and reusable for consecutive runs.

•

Iron leaching from the catalyst structure was negligible.

26