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Accepted Manuscript Nitrobenzene degradation in aqueous solution using ozone/ cobalt supported activated carbon coupling process: a kinetic approach Asma Abdedayem, Monia Guiza, Francisco Javier Rivas Toledo, Abdelmottaleb Ouederni PII: DOI: Reference:

S1383-5866(17)30556-7 http://dx.doi.org/10.1016/j.seppur.2017.05.004 SEPPUR 13715

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

16 February 2017 28 April 2017 2 May 2017

Please cite this article as: A. Abdedayem, M. Guiza, F.J.R. Toledo, A. Ouederni, Nitrobenzene degradation in aqueous solution using ozone/ cobalt supported activated carbon coupling process: a kinetic approach, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.05.004

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Nitrobenzene degradation in aqueous solution using ozone/ cobalt supported activated carbon coupling process: a kinetic approach Asma Abdedayem1*, Monia Guiza1, Francisco Javier Rivas Toledo2, Abdelmottaleb Ouederni1 Research Laboratory Process Engineering and Industrial Systems (LRGPSI), National School of Engineers of Gabes, University of Gabes, Street Omar Ibn El Khattab, 6029 Gabes, Tunisia 2 Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, 06071 Badajoz, Spain 1

*Correspondingauthor: Full adress: National School of Engineers of Gabes, University of Gabes, Street Omar Ibn El Khattab, 6029 Gabes, Tunisia Email: [email protected]*

Abstract In this study, a new supported catalyst (Co/OSAC) based on olive stones activated carbon (OSAC) was developed using the wetness impregnation method. A kinetic study was performedto investigate the effect of ozone/supported catalyst coupling process on nitrobenzene degradation in aqueous solution. Several analytical techniques, such asNitrogen adsorption-desorption at 77K, XRD, XPS and SEMEDX analyses were used to characterize the new catalyst. The effect of adding t-BuOH as radical scavengers was also investigated. Characterization results showed that a highly distribution of cobalt nanoparticles on the surface of OSCA was obtained by using impregnation method. Results were modelled by a global first-order model (R2> 0.99).Co/OSAC exhibited a high activity in oxidation of NB compared to ozone alone and OSAC. The total organic carbon (TOC) removal proved that Co/AC/O3combined process increases significantly the mineralization of NB with about 65% in 20 min compared to 45% and 24% using O3/OSAC and ozonation alone respectively.The kinetic contribution of radical mechanisms was estimated by using tertbutanol as radical scavenger. It was demonstrated that NB degradation is mainly due to radical generation promoted on the surface of the catalyst. Keywords: ozonation, cobalt supported catalayts, activated carbon, couplingprocess, nitrobenzene.

1. Introduction Recently, with the increasing varietyof manufactured products, the water pollution has become a global issue ofconcerns [1, 2]. The use of biological process for water treatment involves several problems [3]. Furthermore, the presence of toxic refractory molecules with low biodegradability characters prevents the use of such treatments [3, 4]. It is, therefore, urgent to achieve new effective treatment processes to resolve these problems. Advances in wastewater treatment have led to the development of new methods known by advanced oxidation processes (AOPs) [5]. AOPs can be generally defined as aqueous phase oxidation methods based on the generation of highly reactive species such as hydroxyl radicals (OH°) [6]. These radicals react rapidly with molecules present in water in an unselective way [7]. The application of AOPs such as ultrasonic, photo-oxidation, photocatalytic oxidation plasma, Fenton, photo-Fenton, wet oxidation, and ozone/ultraviolet (UV) has concerned many investigations for the degradation and mineralization of a wide rangeof organic compounds [8]. Due to their complexity and high costs, these processes are rarely used as a possible solution for water treatment [8]. Ozonation process appears as an appropriate AOP for water treatment [9]. Ozone is well-known as a powerful oxidant; however it reacts slowly with some organic compounds such as inactivated aromatic compounds [10] and rarely leads to total mineralisation [1]. Many researches are focused on developing catalysts in order to provide fast degradation and high mineralization level by ozone decomposition on high active species [11]. The combination of ozonation process with other agents such as UV, H2O2, or homogeneous catalyst (Mn +2, Fe+3, Fe+2, Ag+, Zn+2, and Co+2) can lead to an important degradation efficiency [11, 12, 13]. However, in this case, when dissolved metal salts in aqueous solutions or metal oxides were used as catalysts a secondary polluting problem can be exist [10]. Since, these catalysts have to be removed after the oxidation reaction of the organic compounds [10]. Therefore, these technologies are rarely selected as promising methods [8].

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Actually, to increase the efficiency of simple ozonation process, heterogeneous catalytic ozonation processes, as a promising AOP, have been investigated [14]. Among frequently effective heterogeneous catalysts used in heterogeneous ozonation processmainly include: Zeolithe [15] Al2O3 [16], TiO2 [17] and AC [18]. The coupling of ozone and activated carbon was proven to be an effective method to degrade organic contaminants [19, 20]. The use of activated carbon can promote the decomposition of dissolved ozone into free hydroxyl radicals (•OH) [21, 22, 23]. Thus, in the presence of activated carbon, ozonation process can lead to the oxidation of micropollutants either by through molecular ozone reaction or with generated •OH [17]. The basic functional groups (e.g., chromene, pyrone and pyrrole) on the surface of the AC, metals in the structure and/or on its surface, or the electrons in the graphene planes were suggested as the possible active sites during ozonation [24, 25]. In this process, carbon can act as an adsorbent, a reactive support, and free-radical initiator [20]. Nevertheless, to furthermore improve the AC catalytic activity and stability as catalyst to be used on catalytic ozonation, AC was used as a catalyst support for metal as well as for their oxides [10]. AC exhibits excellent physical and chemical characteristics for supported catalyst preparation used for the oxidation of organic compounds [6, 10]. Activated carbon is stable under both acidic and alkaline environments and is a relatively cheap material [26]. It can be synthesized from low cost materials (agricultural industrial wastes) [26]. Transition metals catalysts such as Fe, Mn, Ru, Ce, Co, Ni are gaining prominence, nowadays, to be supported on AC [14]. Synthesized catalyst has been applied to increase the ozone decomposition and generated highly reactive free radicals [8]. Both the catalyst and the support play important roles in heterogeneous catalysis [27]. Supported catalyst is common used in the oxidation of a great number of organic compounds [1, 28] such as nitrobenzene (NB) [10, 26, 29]. This molecule is considered to be highly toxic; unfortunately, it is resistant to oxidation by biological treatment processes due to its carcinogenesis and mutagenesis nature [29]. It is also considered as a hard biodegradable compound and inhibitor for activated sludge [30]. This important class of industrial chemicals is widely used in the synthesis of many products, including dyes, polymers, pesticides, and explosives [31]. The release of these compounds into the environment is the result of their widespread application and improper disposal [10]. The oxidation of nitrobenzene was achieved using various supported catalyst [26]. In a previous study, O3/Cu/AC coupling was proven to be efficient methods to remove NB [10]. These preliminary experimental results proved that copper supported catalyst was very effective for enhancing the degradation of refractory organic pollutants. 82% removal of TOC was achieved in aqueous solution. This study is focused on the treatment of bio-recalcitrant organic compound by coupling processes involving new supported cobalt catalyst. Nitrobenzene was selected as the target organic component to be treated by the coupled process. It is resistant to be treated with ozone molecule with low reaction rate constant and highly reactive with generated hydroxyl radicals [32]. Cobalt supported catalyst was prepared using wetness impregnation method over olive stones activated carbon (OSAC) prepared by physical process.The effect of cobalt catalyst on the coupling O3/AC process performance was investigated. The presence of radical scavengers on the process rate was also assessed to highlight the mechanism of ozone decomposition during catalytic ozonation reaction of NB.

2. Materials and methods 2.1. Olive stones activated carbon preparation In this study, activated carbon (OSAC) was prepared from crushed olive stones in the size range of 1.25- 3.00 mm by physical processes according tothe method developed by Souad Najjar et al. [33]. Initially, raw material was abundantly washed with hot distilled water to remove any impurities then dried at ambient temperature. Thereafter, olive stones were thermally carbonized under a continuous nitrogen flow. The carbonization temperature was fixed at 600°C for two hours. The char obtained from the first step (100 m2/g of specific area measured by adsorption of N2 at 77K) was then activated in the same furnace using steam as oxidizing agent at 850°C for 8h. The elemental analysis of the stones is summarized in table 1. The high carbon and low ash contents make olive stone a good starting material in the production of porous activated carbons [33]. The produced OSAC was washed several times by Milli-Q water and then dried to be ready for using. Table 1Elemental analysis of olive stones [33] Element C Composition (wt.%)

47.3

H

O

N

Ash

6.1

45.85

0.075

1.86

%O was obtained by difference

2.2. Preparation of supported catalyst

2

Wetness impregnation method with respect to the pore volume was used to prepare cobalt supported on activated carbon catalyst (Co/OSAC). A solution of Co(SO4)7H2O (Merkmillipore) was used as precursor for catalyst preparation. Samples were, firstly, sieved to obtain a particle size in the range of 0.6-0.8mm. Co/OSAC catalyst was synthesized by immersing OSAC in a cobalt sulfate solution for 24h at ambient temperature. The solution concentration was adjusted to reach 5wt.% of metal (0.05g of cobalt/1g of OSAC) without attaining solution saturation. After impregnation samples were dried overnight at room temperature then heated at 550°C for 2 h under nitrogen atmosphere. 2.3. Catalysts characterization 2.3.1.

Specific surface area and textural properties

Specific surface area and pores characteristics of activated carbon were determined by nitrogen adsorption and desorption isotherms at 77.7 K with an automatic Sorptiometer Quantachrome Autosorb automatic apparatus (iQ2-C Series). Prior to measurement, the sample was out-gassed at 120°C under vacuum for 12h to ensure a clean and dry surface, with no loosely held adsorbed species. The specific surface areas SBET of different samples were determined using the Brunauer–Emmett–Teller (BET) method. The total pore volumesVtotwere estimated from the volume of adsorbed N2 at the maximum relative pressure close to P/P0=1 (at 0.95 relative pressure). Micro and mesopore volumes (Vmic and Vmes respectively) and the external surface area (Sext) were determined by means of the t-method. Finally, the pore size distributions (PSD) were determined using Density Functional Theory (DFT) using Plus Software (provided by Quantachrome Instrument Corporation). 2.3.2.

XRD analysis

To identify the state of the fixed cobalt on activated carbon, the developed catalysts were analysed using a high-resolution X-ray diffraction (XRD) apparatus Bruker D8 ADVANCE (Cu-Kα radiation λ= 1.5406 Å) equipped with linear detector type VANTEC. XRD pattern was recorded for 2Ө in the range of 10- 100° at a step of 0.01° for 0.1s per step. 2.3.3.

XPS analysis

To identify the nature, the distribution and amount of surface oxygen groups of activated carbon and cobalt supported activated carbon catalyst, X-ray photoelectron spectrum (XPS) was used. XPS experiments were performed with a K-Alpha Thermo Scientific with monochromatic Al Kα X-rays source at 1486.68 eV with a spot size of 400 μm, the base pressure inside the analysis chamber was 12×10-7mbar. For the survey spectra an energy step size of 1.0 eV and pass energy of 200 eV were used. For the individual element spectra, C1s, O1s and Co2p an energy step size of 0.1eV and a pass energy of 40.0 eV were used. For the deconvolution of peaks a Shirley type background was used. Peaks were readjusted to a combination of Gaussian and Lorentzian functions using the XPS Peak 4.1 software. 2.3.4.

Catalysts morphology determination

Catalysts morphologies and micrographic shapes of catalysts macroporosity were visualized by SEMEDX carried out on SDD Apollo X instrument.

3. Experimental Setup Nitrobenzene degradation was performed in a 1-L gas–liquid stirred reactor of controlled temperature equipped with a mechanically stirrer to favour mass transfer between the three phases and to limit AC attrition. In each experiment, the reactor was filled with 1L of a nitrobenzene aqueous solution (10mg/L), and 1 g of OSAC or Co/OSAC was then added. The reactor was continuously fed with an ozonated oxygen stream. The temperature in the reactor was controlled and kept constant at 25°C. pH was adjusted and kept constant at 7 by using HCl or NaOH solution. Agitation was maintained constant at 300rpm in order to keep the reactor content perfectly mixed. Ozone was produced in a pure oxygen stream using an ozone generator with a controlled power to achieve variable O3 concentrations. The aqueous ozone concentration was determined by the Indigo Carmin method [34] while the gaseous ozone concentration was determined by the iodometric method [35]. The experimental set up is shown in Fig.1. The residual concentrations of nitrobenzene samples taken regularly from the reactor at various reaction times were measured by UV spectrophotometry absorption at a wavelength of 267 nm using a Shimatzu 1700 UV visible spectrophotometer. The degree of mineralization was followed by a Shimadzu TOC 5000A analyzer. Ionic short chain organic and inorganic anions were monitored by

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ionic chromatography (Metrohm 881 Compact Pro ionic) connected to MagIC NETTM software. In the samples taken for NB detection, Na2SO3 (0.1M) solution was added to stop the ozonation reaction. For comparative purposes, both adsorption on OSAC and Co/OSAC and simple ozonation experiments (without material) were performed in the same system, under identical experimental conditions. 3.1. Kinetic studies The kinetic study of NB removal over OSAC and Co/OSAC was carried out with and without the presence of tert-butanol as radical scavenger. The coupling process may involve synergetic mechanism at the solid–liquid interface of catalysts and in the bulk liquid phase. The overall reaction kinetic includes homogenous and heterogeneous reaction and can be modelled using a global pseudo-firstorder model given by the following equation: 

 k

d NB dt

overall

NB 

(1)

Where koverall is the global kinetic reaction constant (min -1); [NB] is the concentration of NB (mg/L).The oxidation of NB via ozonation process in the presence of catalyst is considered as the combination of different mechanism: direct reaction and indirect reaction involving radicals generation through ozone decomposition. Both reactions take place in the bulk solution and on the surface of the catalyst. The main global oxidation mechanism can be described as follow: Homogeneous reactions: NB  O 3

NB  OH

k

ox 1   products



(2)

k

ox 2   products

(3)

kox1 and kox2 are the kinetic oxidation constants of direct and indirect reaction respectively (min-1). Heterogeneous reactions: S

S

S

cat

cat

cat

 NB

k

ads   products

 NB  O

(4)

k

3

 NB  OH

ox 4   products



(5)

k

ox 5   products

(6)

Where {Scat−NB} represent the sorbed NB on the surface of the catalyst groups, kads is the kinetic constant of NB adsorption (min−1), kox4 is the kinetic constant of ozone oxidation (direct reaction) of NB on the catalysts (OSAC or Co/OSAC) surfaces (L mg−1min−1), kox5 is the kinetic constant of hydrxyl radicals oxidation (indirect reaction) initiated by O3-catalysts interactions and t is the time (min). The concentration of both O3 and HO•was considered constant during experiments. Therefore, koverall can be defined as the combination of the homogeneous and heterogeneous reactions: k overall  k hom  k heter

(7)

Where khom is the kinetic constant of the reactions taking place in the bulk solution (min-1) and kheter is the kinetic constant of the reactions in the presence either of OSAC and Co/OSAC (min-1). The value of khom can be determined from the experiments of ozonation alone and kheter is obtained by difference from the Eq.2. Eq. 2 can be written as follows: k

k

hom

k

heter 

ox 1

O3   k ox 2 OH  

k ads  k ox 3  O3   k ox 4 OH

(8) 



(9)

4

Heterogeneous reactions contribution on the degradation kinetic of NB removal can be evaluated using the following Eq: 

heter

k  k homo (%)  overall  100

(10)

k overall

For experiments realized in the presence of t-BuOH, hydroxyl radicals are scavenged. Eq. 1, of NB degradation kinetics can be modelled as follow: 

 k

d NB dt

k

obs

k

ox 1

obs

NB 

O3   k ox 2 OH    k ads

(11)

(12)

After determination of kobs, the kinetic contribution of radical reactions can be estimated:



OH 

k  k obs (%)  overall  100

(13)

k overall

4. Results and discussion 4.1. AC and Co/OSAC characterization 4.1.1.

Specific surface area and textural properties

Adsorption-desorption isotherms of nitrogen at 77K on OSAC and Co/OSAC supported catalyst are shown in Fig.2. As it can be observed from Fig. 2, the raw OSAC fits the typical type I/IV isotherm according to the IUPAC classification, which is characteristic of microporous materials with a contribution of mesoporosity [37]. Moreover, the presence of a broad knee at the relative pressure of P/P0 = 0.4, suggests the presence of micropores [6]. The isotherm indicates the presence of a type-H4 hysteresis loop on the desorption curve indicative of some significant contributions of mesoporosity to their final porous structure [36]. When cobalt was incorporated on the surface of the activated carbon, fairly similar adsorption-desorption isotherm was found indicating that the global texture of the carbon does not show any regular trend. Thus, the carbon internal texture has not been modified after cobalt impregnation. The textural properties deduced from the adsorption-desorption isotherms are illustrated in Table.2. Prepared OSAC showed a microporeous structure respected to the micropores volume which represents 83.5% of the total pore volume with a mean diameter from 2.2 nm. This was in agreement with the type of the adsorption-desorption isotherm. However, the Co/OSAC catalyst presents fairly low surface area as compared to the OSAC due to the blockage of some pores attributed to metals nanoparticles penetration into the carbon matrix which obstructed some micropores [10, 37]. Furthermore, when metals are fixed on the surface of the activated carbon, some modifications would be appreciated. There is an increment on the external surface area of the OSAC. This indicates that cobalt nanoparticles were especially clustered on the external surface of the OSAC. In addition, pHpzc of both used materials are determined and depected in Table 2. The pHpzc of OSAC indicates its basic character which can be favorable for ozone decomposition during degradation process. Also, considering the Co/OSAC pHpzc, it can be seen that after adding cobalt on the surface of the OSAC, the pHpzc increases indicating an ion-exchange process for cobalt fixation on the surface of the OSAC. This parameter points out the favorable application of both OSAC and Co/OSAC in ozone/catalyst process. The pore size distrubution of the samples is depicted in Fig.3. It can be seen that the DFT (Density Functional Theory) plots are bimodal for both catalysts, with a large proportion of ultramicropores smaller than 0.7 nm. Two maximum widths can bedistinguished at 0.56 and 0.7nm, characterizing a heterogeneous porosity distribution. Consequently, it is interested to note that, the impregnated cobalt species do not significantly change the activated carbon texture.

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Table 2 Textural properties and chemical composition of the catalysts Sample SBET Vtot Vmic Vmes Sext D(A°) 2

3

3

3

2

Bulk Coa/ wt.%

Surface Cob/ wt.%

pHpzc

(m /g)

(cm /g)

(cm /g)

(cm /g)

(m /g)

OSAC

734.17

0.407

0.34

0.06

109.42

22.2

_

_

7.3

Co/OSAC

721.92

0.423

0.35

0.073

123.6

23.6

0.50

0.52

7.7

4.1.2.

XRD analysis

X-ray diffractograms for both olive stones activated carbon and cobalt supported activated carbon catalysts are illustrated in Fig.4. The XRD spectra show a multiple peaks spread over the 2Ө range. Two broad diffraction peaks appears at 23° and 43° confirm the carbon structure of samples [38, 39, 40, 41] with well alignment of carbon layer planes [42]. These results prove the amorphous nature of the compound being analyzed related to activated carbon [38, 39, 42]. This is in a good agreement with nitrogen physisorption results (Table 2). In addition, there are tow reflections on (002) and (100) indicate the presence of graphitelike carbon d(002) [43, 44]. In the case of raw OSAC there is a sharp peak seen at 29.4° indicating the presence of a crystalline component in the sample [38]. However, after cobalt modifications of OSAC supported material, this peak was totally disappeared. This observation can be attributed to the cobalt species fixation instead of crystalline component present on the surface of the carbon. Furthermore, a small peak appeared at 58.97° associated to Co3O4 [45, 46], due to the few amount of supported cobalt particles deposed on the surface of the porous support and the high dispersion of metal oxides species. No extra peaks attributed to cobalt species were detected in the curve of Co/OSAC. The previous statement indicates the highly dispersion of cobalt species on the surface of the catalyst resulting on stronger interaction between cobalt and OSAC surface [47]. Thus, no peaks corresponding to metallic cobalt were detected which was difficult to be reduced to metal cobalt form [47]. These informations indicate that the addition of cobalt led to a decrease in AC crystallinity [48] which can enhance the contact with the surface area of the catalyst especially the access to the active site which could favor for ozone decomposition. As a conclusion, wetness impregnation was found to be an appropriate method for the preparation of well-dispersed nano-catalyst on the surface of AC. 4.1.3.

XPS analysis

The XPS spectra in the (C1s), (O1s) spectral regions for all the samples are shown in Figure.5. Two distinct peaks can be identified in the XPS spectra of both OSAC (Fig.5 (A)) and cobalt supported catalysts Fig.5 (B) corresponding to carbon (C1s) and oxygen (O1s). In the case of Co/OSAC catalysts another spectra was also obtained corresponding to Co2p. The C1s spectra have been resolved into five separated symmetric peaks of Gaussian-Lorentzian type [6]. In addition, the main peak at 284.6eV was associated to graphitic or aromatic carbon. Another peak was identified at the higher energy, 291eV, corresponding to П-П* transitions in aromatic rings [48]. Three more peaks, centered at 286eV,287.3eV and 288.6 eV can be related respectively to C-O bonds of phenol or esters groups; C=O bond in carbonyl groups and with –COOC- bond characteristic of carboxylic, anhydride or ester groups [49]. The XPS analysis highlights, also, the higher concentration of oxygen functional groups on the surface of the catalysts. Four peaks of O (1s) have been deconvoluted at 530.7, 532.1, 533.3 and 535.3eV. These peaks have been associated respectively to C=O bonds characteristic of carbonyl and/or quinone groups, C-O bonds characteristic of phenol, lactone, anhydride and/or ether groups, and O=C-OH corresponding to carboxylic groups and to chemisorbed oxygen or water [24, 25]. Concerning the XPS spectrum of the Co2p region, the curves indicate the presence of two main bands at 781.4eV and 797.5eV, characteristic to Co(II)resulting from the spin-orbit splitting related to Co 2p3/2 and Co 2p1/2 respectively [50]. In addition, the weak satellites peaks were observed at around 786.3eV and 803.5eV; confirm the presence of Co3O4 on the surface of the catalyst [24]. These results are in agreement with XRD data. Surface oxygen groups distribution according to XPS deconvolution of C1s and O1s spectral region are summarized in Table3. Interestingly, C1s profile was the same in OSAC and supported catalysts. However, considering the O1s, the impregnation of OSAC by cobalt particles leads to a drop in the concentration of acid groups (peak3: 533.3 eV) with an increment on the ether and hydroxyl groups (peak 532.1eV) form 20.75% to 36.56%. As a consequence, the activated carbon acidity was also decreased which suggested that cobalt particles were fixed by an ion exchange mechanism between the carbon surface and cobalt ions in

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solution. A variety of interactions between cationic species and the activated carbon surface might occur, such as the formation of surface complexes, ion-exchange process, and redox reactions [10]. Previous investigations have demonstrated that the presence of oxygenated groups on the surface of the AC favors metal adsorption through the formation of metal complexes [10]. In general, this process is considered as an ion-exchange reaction with the participation of strong acidic surface groups [10, 51], which could be described by the following mechanism:

surface  nH   M n  (aq )

surface  M n   nH  (aq )

(14)

In addition, there is a decrement on physisorbed water (peak 535.5eV) evaporated during the calcination step of the catalyst. Table 3 XPS Results: Surface Composition forOSAC and Co/OSAC Element

peak

C1s C1s C1s C1s C1s

1 2 3 4 5

O1s O1s O1s O1s

1 2 3 4

Functional groups graphitic, aromatic (C-C) C in hydroxyl, ethers (C-OH, C-O-C) C in carbonyl (C=O) C in COOR (R ) H or alkyl)    * transitions in aromatic or phisorbed water carbonyl, quinone (C=O) hydroxyl, ethers (C-OH, C-O-C) anhydride, lactone, carboxylic acids Chemisorbed H2O or O2

Bending Energy 284.6 286 287.3 288.6 291 530.7 532.1 533.3 535.5

OSAC 43 10 3.2 10.1 17.4

Atomic % Co/OSAC 50 5.3 5.3 9.5 13.6

18.73 20.75 21.2 17.8

15.33 36.56 13.48 4.1

4.1.4. SEM-EDX analysis SEM micrographs of the OSAC and Co/OSAC catalyst were obtained trying to identify some evidence modifications. Fig. 6 represents both micrographs, where some differences in the external appearance of the OSAC and Co/OSAC can be observed. From Fig.6 (a) it can be clearly seen that the surface of activated carbon was filled with a large and developed irregular cavities, indicating the influence of the activation agent on the development of AC porosity [52]. These cavities can be considered as main channels to get the inner surface of OSAC throughout micropores [52]. This high porosity of activated carbon provides a large surface area for attachment of metals. As a consequence cobalt species could be easily loaded and distributed on OSAC surface area. This can be an advantage for catalytic ozone decomposition. In the case of Co/OSAC (Fig.6(b)), the photograph shows a large and well-developed porosity of the carbon as compared to OSAC, and some cobalt particles distributed in the inner surface. This result proves that the major amount of cobalt species was fixed on the outersurface of activated carbon. This developed porosity was due to the heat treatment of carbon during calcination step of catalyst preparation. Fig.6 (c) shows the EDX analysis and Co element of Co/OSAC. Cobalt was clearly identified indicating the successfully cobalt loading onto the carbon. Inset of Fig.6 (c) presents the cobalt mapping on the materials, in which green bright spots represented the cobalt elements in the sample. The cobalt mapping of Co/OSAC strongly suggested that cobalt was almost homogeneously distributed on the support surface. This result supports the conclusion of the good metal dispersion prouven by XRD analysis. 4.2. Catalytic ozonation of nitrobenzene 4.2.1.

Nitrobenzene degradation mechanism

Comparative kinetic experiments of nitrobenzene degradation were carried out in the same conditions using different processes. Nitrobenzene concentration profiles under these conditions are displayed in the Fig.7. The lightly difference in NB adsorption on OSAC and Co/OSAC was due to the loading of cobalt nanoparticle on OSAC surface, which reduced the surface area and pore volume as confirmed by textural analysis (table2). Also, it can be noticed from Fig.7 that the presence of OSAC enhances the removal rate of NB by ozonation leading to higher NB conversion than those obtained during simple ozonation alone and adsorption process. The addition of activated carbon promotes the removal of NB from 58% to 77% in 20 min, as compared with ozone alone. Therefore, activated carbon plays the

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rolenot only as a catalyst but also as adsorbent. Previous researches suggested that activated carbon had evident catalytic activity in the ozonation process, since the presence of activated carbon can accelerates dissolved ozone decomposition into highly active free radicals such as OH° and O- radicals [53] which improves significantly the degradation efficiency of ozonation process. Comparing the curves of O3/Co/OSAC with simple ozonation, it can be seen that the removal of NB was greatly enhanced by coupling ozone and the Co/OSAC catalyst simultaneously. As comparison with OSAC, the degradation efficiency of NB using Co/OSAC catalyst was significantly improved.By using 5w.% of cobalt loaded on activated carbon as catalyst, the final NB removal increased to 99% in 20 min as compared to other process. As mentioned in the characterization section, Co/OSAC exhibited lower BET surface area compared with OSAC. Therefore, the higher activity of Co/OSAC is attributed to the presence of Co species, which increases the activity of OSAC in the degradation of NB during the present catalytic ozonation process. The obtained result implicates that the addition of Co in OSAC plays an important role in the degradation process. The surface of Co/OSAC potentially has more . active sites, which enhances the decomposition of ozone with large amount of OH radicals generation indicating the strong synergistic effects. The experimental data were fitted to an apparent first-order kinetic law with respect to NB concentration to determine the rate constate for each applied process (R2> 0.98). For the adsorption results, kinetic modelisation of NB adsorption proves that the pseudofirst order model fits quit the experimental results compared to other models. Fitting results are summurized in table 8. It can be noteced that the rate constante of NB degradation after adding Co/OSAC catalyst was 4.5 times higher than ozone alone and 3 times higher than using OSAC as catalyst.This demonstrates the great interest O3/Co/OSAC coupling process. Table 8 Experimental determination of rate kinetics constants of NB degradation by different processes Process O3 O3/OSAC O3/Co/OSAC -1 0.046 0.069 0.206 koveall(min ) 2 0.988 0.990 0.992 R The contribution of heterogeneous reaction in the kinetic degradation of NB was investigated by determiningδhetero. The results are summarized in table9. Without adding radical scavenger, the NB degradation was significantly improved through the heterogeneous reaction contribution using Co/OSAC and OSAC. This indicates that the degradation mechanism of NB removal was mostlyrealized on the surface of the catalyst and not on the bulk of the solution. Comparing the contribution of Co/OSAC to OSAC, it is clear that cobalt supported catalyst improved significantly the heterogeneous degradation of NB in aqueous solution compared to OSAC. Thus, adding metals, enhances the hydroxyl radicals. Table 9 Kinetic contribution of heterogeneous reactions on NB degradation O3 Process O3/OSAC heter 33.33  (%)

O3/Co/OSAC 77

4.3. Catalytic degradation of NB in presence of t-BuOH Hence, in order to further investigate the possible role played of hydroxyl radicals in the catalytic ozonation process, the effects of organic radical scavengers (tert-butanol) on NB removal in ozonation with and without catalysts were investigated. As reported, t-BuOH reacts rapidly with •OH radicals with a rate constant of 6.108 M−1s−1, while its reacts slowly with ozone molecule with a rate constant of 3.10−3 M−1 s−1 [39]. From Fig.8, it is clear that the addition of t-BuOH inhib the degradation of NB by catalytic ozonation. Thus, the direct ozonation pathway can not reach NB degradation in aqueous solution. This evidence suggested that the degradation of NB proceeded mostly via a radical pathway in the bulk water. Other supplementary mechanisms can exist during NB degradation. Supported metals have an important role during ozonation process. Fonatnieret al. suggested that anextremely reactive complex was formedbetween metal and ozone molecule to react with organic molecules [54, 55]. For that, cobalt nanoparticles with its multivalence form played an essential role in the catalytic activity and the electron transfer between the loaded cobalt oxides and the ozone molecules promoted the generation of •OH [14]. In addition, the presence of metals on the surface of activated carbon would further enhance its catalytic activity by creating new active sites on the catalyst surface [17]. Table10 illustrates the influence of t-BuOH on NB removal using the different used processes. These results

8

imply a reduction of NB removal rate due to the presence of t-BuOH. The presence of t-BuOH reduces ozone decomposition, acting as a radical scavenger. The determined rate constant was significantly smaller than the kinetic constants previously obtained (without t-BuOH). Moreover, the only effects taking place in this case were adsorption and direct ozonation in the bulk liquid and on the catalyst surface. Therefore, NB degradation mainly occurs by indirect reaction in the presence of OH° radicals during the coupling processes [20]. Table 10 Experimental determination of rate kinetics constantsof NB degradation by different processes in the presence of t-BuOH Process O3 O3/OSAC O3/Co/OSAC 0.001 0.041 0.046 kobs(min-1) 0.988 0.988 0.994 R2 To confirm that the degradation of NB is mainly achieved by radical mechanisms during treatment by ozonation process the value of δHO• was determined. Results are presented in Table 10. The δHO• values obtained in the case of simple ozonation was 100%. This result indicates that radical mechanism is the main way for NB degradation. For OSAC and Co/OSAC radical contribution were 41% and 78% respectively.Therefore, in both cases, the degradation kinetics of NB is mainly due to the indirect reaction way through hydroxyl radicals generation on the surface of the catalysts with the contribution of the adsorption process compared to O3 alone. Moreover, the value of δHO• in the case of OSAC is lower than for Co/OSAC. It appears that the functional groups on the surface of the used catalyst influence significantly ozone decomposition on OH°. According to XPS analysis OSAC presents more acidic surface groups then the Co/OSAC. The high amount of these groups on OSAC leads to adecreamenton the removal of NB. In addition, the great efficiency of Co/OSAC as catalyst compared to OSAC could also be explained by the high microporous volume and important external surface which favor intraparticle diffusion. Table 11 Kinetic contribution of hydroxyl radicals on NB degradation Process O3 O3/OSAC 100 40.57 OH   (%)

O3/Co/OSAC 77.67

Total organic carbon (TOC) removal by different processes was experimentally investigated: O3 alone, O3/OSAC, O3/Co/OSAC. The results are illustrated in Fig.9. It can be observed that activated carbonsupported cobalt enhances the kinetics and the yield of nitrobenzene mineralization by ozone. These results show that the simple activated carbon also enhances the ozonation process of nitrobenzene, in a lesser extent better than in the presence of cobalt. 65% of TOC removal was achieved using in the presence of cobalt supported catalyst compared to 45% in the presence of OSAC and it is only 24% using O3 alone. 4.3.1.

Identification of the main ozonation short chain products of NB

Before reaching the mineralization, the oxidation of nitrobenzne proceeds through a complex reaction system generating some intermediate compounds, known by complete oxidation to CO2 and H2O. During the degradation of NB by simultaneous adsorption/ozonation process, many low-molecularweight organic acids generated and the compounds remaining in solution were analyzed by IC. Fig.10 shows the evolution of the concentrations of related ions. It can be seen that the main organic acids found were: oxalic acid, formic acid and succinic acid which were responsible of the final TOC detected in the reaction. Furthermore, a part from the organic product, nitrate ions were detected pointing out the presence of nitrobenzene molecule during ozonation process. It is interesting to note that ozonation alone can not reach the nitrobenzene molecule degaradtion which is proved by the big amount of nitrate ions present in the solution after ozonation treatment (ozone alone) as compared to O3/OSAC and O3/Co/OSAC. This result indicates again the refractory character of nitrobenzene via ozone molecule and proves the important effect of adding catalyst for ozonation process. Concerning organic products, it can be seen that the combination of adsorption and ozonation process with OSAC increases the miniralization rate of organic generated during the ozonation process compared to ozonation alone. As well, after adding cobalt naoparticles on the surface of the carbon, the miniralization capacity of ozonation process was increased showed by TOC removal results. Hence, the radical route increases reaction rates without changing the nature of the byproducts. The reason of the higher radical efficiency is probably due to the less selectivity of this reaction pathway [56].

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4.3.2.

Cobalt leaching and catalyst stability

From the most important characteristics of the prepared catalyst are its minimal deactivation and reusability. The experiments were carried out to investigates the stability of the Co active phase on the prepared catalyst in the presence of acidic short chain genrated by-product (as seen in Fig. 10).The experiments were performedin semi-contineous conditions described in the experimental section.Fig.11. shows the obtained results. After each reaction, the catalyst was recovered from the reactor by filtration and dried over night in an oven at 60°C. The catalyst was reused in consecutive NB degradation runs. As seen from the Fig.11, Co/OSAC catalyst activity showed an acceptable stability. It can be seen that the performance of the Co/OSAC catalyst is similar for all catalyst runs and the catalyst retained its catalytic activity after 3-times reuse and there are no reduction of NB removal as compared to other process. Some researchers suggest that the oxidation of contaminatsby catalytic ozonation has been enhanced after reuse, due to the chemical functional groups modification of the catalyst after ozonation [ 25, 57, 58].

5. Conclusions The feasibility of using supported catalyst on olive stones activated carbon in ozonation process was studied in this paper, in order to develop a new hybrid process for the treatment of refractory molecules by t he combination ozone/supported catalyst process. The results showed that Co/OSAC catalyst prepared by impregnation method was more efficient for catalytic ozonation activity than OSAC alone in the degradation of NB. Co/OSAC catalyst could also enhance the TOC removal efficiency. Hence, the TOC removal with Co/OSAC catalyst at 20 min could reach 65%, while 45% with OSAC catalyst, 24% using ozone alone . The advantage of Co/OSAC catalyst was attributed to the fact that cobalt increased the generation of OH°, which could react with organic compounds and intermediates to form oxidized products rapidly.The realised adsorption experiments on OSAC and Co/OSAC proved that the degradation of NB during the ozoantion process was due to the oxiation by the genrated hydroxyl radical. The addition of tert-butanol, a well-known OH° scavenger, confirm that nitrobenzene was oxidized primarily by OH° during ozonation process.

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Nomenclature OSAC: Olive stones activated carbon Co/OSAC: cobalt supported on olive stones activated carbon NB: nitrobenzene BET: Brunauer–Emmett–Teller PHpzc: TOC: total organic carbon SBET: specific surface area determined by BET method (m2/g) Sext: External specific surface area (m2/g) Vtot: total volume of pore (cm3/g) Vmic: micropore volume (cm3/g) Vmes: mesopore volume (cm3/g) D: average micropore diameter (A˚´ ) T: temperature (K) or (°C) t: time (min) koverall: the overall kinetic consatnte rate (min-1) khomo: the kinetic constant rate of homogeneous reaction (min-1) khetro: the kinetic constant rate of heterogeneous reaction(min-1), kads: the kinetic constant rate of adsorption reaction(min-1), k1: the kinetic constant rate of directe oxidation(molecular ozone) in homogeneous phase(min-1), k2: the kinetic constant rate of indirecte oxidation(by OH°) in homogeneous phase(min-1) k3: the kinetic constant rate of directe oxidation(molecular ozone) in heterogeneous phase(min-1) k4: the kinetic constant rate of indirecte oxidation(by OH°) in heterogeneous phase(min-1) heter : the contrivbution of heterogenous recation in the NB ozonation reaction  

OH



: hydroxyl radicals contribution in the NB degradation

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R2: correlation coefficients kint: intraparticle diffusion rate constant (mg.g -1.min-0.5) [NB]: nitrobenzene concentration (mg/L) [O3]: ozone concentration (mg/L) mOSAC: mass of activated carbon (g) mCo/OSAC: mass of copper supported on activated carbon (g) t-BuOH: Tert-butanol

Figure captions Fig1. Experimental setup Fig.2.adsorption desorption isotherm of N2 at 77K on OSAC and Co/OSAC Fig.3. Pore size distribution of OSAC and Co/OSAC Fig4. XRD diffractogram of OSAC and Co/OSAC Fig.5. XPS analysis of OSAC (A) and Co/OSAC (B) Fig.6. SEM-EDX analysis of OSAC and Co/OSAC: (a): OSAC, (b): Co/OSACand (c): EDX of Co/OSAC Fig.7. NB removal using different process ([O3] =10ppm, [NB]=10ppm, pH=7, T=25°C, mOSAC=1g, mCo/OSAC=1g) Fig.8.NB removal using different processwith the presence of t-BuOH as radical scavenger ([O3]=10ppm, [NB]=10ppm, pH=7, T=25°C, mOSAC=1g, mCo/OSAC=1g) Fig.9. TOC removal by different process ([O3]=25ppm, [NB]=10ppm, pH=7, T=25°C, mOSAC=1g, mCo/OSAC=1g) Fig.10 Evolution of short-chain organic in different process for NB degradation ([O3]=25ppm, [NB]=10ppm, pH=7, T=25°C,mOSAC=1g, mCo/OSAC=1g) Fig. 11. Degradation of NB in multiple use of Co/OSAC catalyst ([O3]=10ppm, [NB]=10ppm, pH=7, T=25°C,mOSAC=1g, mCo/OSAC=1g)

Figure 1

Figure 2

14

Figure 3

Figure 4

15

Figure 5

(A)

(B)

Figure 6

16

Figure 7

1 0.9

(C0-C)/C0

0.8 0.7

O3

0.6

O3+AC O3+Co/AC

0.5

AC

0.4

Co/AC

0.3 0.2 0.1 0 0

20

40

60

80

100

time (min)

Figure 8

17

1 0.9 0.8 0.7 O3+t-BuOH

(C0-C)/C0

0.6 0.5

O3+AC+t-BuOH

0.4 O3+Co/AC+t-BuOH

0.3 0.2 0.1 0 0

20

40

60

80

100

120

time (min)

Figure 9

(TOC0-TOC)/TOC0

100 90 80 70 60 50 40 30 20 10 0 O3/Co/OSAC

O3/OSAC

O3

Figure 10

18

Figure 11

19

Highlights

1. Prepapartion and characterization of new cobalt supported catalyts based on olive stones activated carbon using wetness impregnation method. 2. A high catalyts distribution on the surface of the carbon has been ontained. 3. Cobalt supported catalyt show the high catalytic perfomance as catalyst for ozonation process. 4. The oxidation mechanism proved that the degradation of nitrobenzene was globaly due to the hydroxyl radicals generation.

Graphical abstract

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