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Experimental and theoretical investigations of CuO-catalyzed ozonation of humic acid
Separation and Purification Technology 134 (2014) 110–116
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Experimental and theoretical investigations of CuO-catalyzed ozonation of humic acid Ozge Turkay ⇑, Hatice Inan, Anatoli Dimoglo Gebze Institute of Technology, Faculty of Engineering, Department of Environmental Engineering, 41400 Gebze, Kocaeli, Turkey
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Article history: Received 16 January 2014 Received in revised form 12 July 2014 Accepted 15 July 2014 Available online 30 July 2014 Keywords: Catalytic ozonation Copper oxide Density Function Theory Humic acid
a b s t r a c t In this study, the efficiency of copper oxide (CuO) as a catalyst in the ozonation process of humic acid (HA) was investigated in both experimental and theoretical respects. Ozonation and catalytic ozonation processes were conducted in a lab setting. HA concentration was determined by measurement of the surrogate organic parameters. The results show that the degradation of HA by catalytic ozonation in the presence of CuO was found to be much more effective than the ozonation process alone. The experimental data was verified by means of theoretical modeling. Density Function Theory (DFT) was used to calculate the decomposition of ozone in the catalytic processes. The reactions on the surface of metal oxides were evaluated with quantum-chemical calculations to explain the mechanisms of catalytic ozonation. Two models of adsorption were investigated: when only O3 is attached to the surface and when O3 and H2O are simultaneously adsorbed by the active center of catalyst. Each is a barrierless reaction, as follows from the calculations mentioned. The result of the first reaction is one oxygen molecule and atomic oxygen being adsorbed on the CuO surface. The second reaction’s final products are O2 and hydroxyl-radicals, which are adsorbed on the CuO surface. These particles behave as powerful oxidizing agents in the further reactions with HA. Comparison of the two mechanisms shows that the second reaction with the water molecule participation is preferable to the first one based on energy levels. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A heterogeneous catalytic ozonation process is used to improve the oxidation of organic compounds by generating free radicals, mainly hydroxyl radicals of high oxidation power, by adding metal oxides [1–3]. Multiple examples of this process have been reported in the last decade, and it has been found favorable for enhancing organic matter degradation. However, its mechanism is quite complicated in that there are three phases (gas, liquid, and solid) composed of ozone gas, aqueous ozone, water, organic matter matrix, and metal oxide. In these systems, surface reactions on the catalyst are vital in determining the mechanism thoroughly. The application of theoretical modeling to the catalytic ozonation process is a new approach to clarify the interactions between substances and catalyst [4]. Density Function Theory (DFT) is a modeling method for the explanation of ground state properties of metals. DFT provides an explanation of the mechanism of organic matter degradation on the molecular level. Quantum-chemical calculation is used to explain the decomposition of ozone molecule and radical generation on the surfaces. Zhai et al. [5] evaluated ⇑ Corresponding author. Tel.: +90 5055051332. E-mail address: [email protected] (O. Turkay). http://dx.doi.org/10.1016/j.seppur.2014.07.040 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
dichloroacetic acid degradation by ZnO-catalyzed ozonation and analyzed the results by DFT. It was shown that the generation of hydroxyl radicals increased, and they were responsible for raising catalytic activity of ZnO. Zhang et al. [6] have reported the atomistic details of NiO, which was used as a catalyst in enhancing the degradation of organic pollutants. In their study, DFT calculations elucidated the adsorption characteristics and electronic structure of the catalytic ozonation process. It was also suggested that the decomposition of ozone and generation of superoxide ions and hydroxyl groups increased with the addition of NiO; in turn, they obtained an enhancement of the degradation of organic pollutants [6]. In the present study, the efficiency of copper oxide (CuO) as a catalyst was investigated in the ozonation process in both experimental and theoretical respects. Experimental studies were carried out in a semi-batch stirred reactor. Ozonation and catalytic ozonation processes were run. Humic acid (HA) was selected as organic matter because of its structure that is recalcitrant to conventional ozonation. Furthermore, disinfection by-products (trihalomethanes and haloacetic acids) may possibly form during the disinfection process due to humic substances contents of surface water [7,8]. The results were expressed in terms of parameters such as dissolve organic matter (DOC), UV absorbance at 254 nm (UV254) and 400 nm (VIS400) to specify the organic matter content
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The nano-powder CuO was provided from Sigma Aldrich. 0.25 g L 1 of CuO was used as a catalyst without further modification in catalytic ozonation experiments. Surface area (0.64 m2 g 1) and pore volume (0.002 cm3 g 1) of Aldrich CuO are detailed elsewhere [10]. Geometry ball and stick structures of CuO are shown in Fig. 1. 2.2. Experimental methods
2. Material and method
Ozone and catalytic ozone experiments were performed in a semi-batch stirred reactor with a volume of 2000 mL (the inside diameter of 12 cm). The reactor was covered with a light-proof material in order to avoid daylight. A schematic representation of the experimental system was given in Fig. 2. Experiments were carried out at natural pH (5.5–6.0) and at a room temperature (23 °C). Ozone was produced from pure oxygen by means of a generator (Sabo Electric, Turkey), and 0.190 mg of ozone was fed into the reactor in a minute after waiting for the concentration to become steady. A potassium iodide solution was used to determine the ozone introduced and in off-gas by the iodometric titration method [11]. The ozone bubbles were continuously dispersed through a porous glass diffuser located at the bottom of the reactor. A complete mixing (400 rpm) was achieved with a magnetic stirrer. Nitrogen gas was used to remove excess ozone at the end of the reaction. Samples were withdrawn at different intervals during 60 min. All samples filtrated through 0.45 lm membrane filter to separate particulates (humic materials and catalyst) prior to analyses. Normalized UV254, VIS400 and DOC values are given respect to time. Three repetitive measurements are presented as mean values, and each error bar represents the standard deviation. Graphs were drawn using Grapher 9 (Golden Software).
2.1. Reagents
2.3. Analytical methods
HA used in this research is commercially available and was supplied from Sigma–Aldrich. Some physical properties of Aldrich HA have been stated elsewhere [9]. A simulation of average organic matter content in surface water (25 mg L 1 of HA solution) was prepared using ultra-pure water. The solution of HA was characterized as follows: 0.474 ± 0.009 cm 1 of UV254, 0.119 ± 0.003 cm 1 of VIS400, 5.86 ± 0.43 mg L 1 of DOC and 5.33 ± 0.16 of pH value.
A total organic carbon (TOC) analyzer (Teledyn-Tekmar Apollo 9000) was used to measure the dissolved organic carbon (DOC) of the samples, as described in Standard Methods 5310 B [12]. A HACH Lange – DR 5000 spectrophotometer was used for the two of absorbance measurements run at wavelengths of 254 nm (UV254) and 400 nm (VIS400), as described in Standard Methods 5910 B [12]. Cyber-scan pH meter was used to determine pH changes.
Fig. 1. Geometry ball and stick structures of elementary cell of CuO (Bravais lattice, face centered monoclinic).
in water. Theoretical modeling (DFT) was employed to provide further evidence for the mechanism of ozonation process. Electron density distribution and reaction mechanism of superoxide radicals formation, which participated in the catalytic ozonation process, were analyzed. The energy values were calculated for frontier orbitals (highest occupied and lowest unoccupied molecular orbitals, HOMO/LUMO) and the energy profiles of each process are presented.
Gas Outlet
Ozone Generator KI Solution Flow meter
Sampling Point Gas Outlet
KI Solution
Diffuser Oxygen Gas
Magnetic Mixer
Fig. 2. Schematic representation of the experimental system.
O. Turkay et al. / Separation and Purification Technology 134 (2014) 110–116
1
1
0.8
0.8
VIS 400 C / C 0
UV 254 C / C 0
112
0.6
0.4
0.2
0.6
0.4
0.2
0
0 0
20
40
60
0
20
40
60
t (min)
t (min) 1
DOC C/C 0
0.8
0.6
0.4
0.2
0 0
20
40
60
t (min) Fig. 3. Degradation of humic acid in terms of UV254 (a), VIS400 (b) and DOC (c) (the straight-line and broken line represents ozonation and catalytic ozonation process, respectively. 25 mg L 1 of HA concentration, 0.25 g L 1 of catalyst dose, initial pH 5.33).
2.4. Theoretical Investigation of the catalytic ozonation process The ozone decomposition and the possibility of active-particle formation on CuO surface during ozonation with and without a catalyst were investigated. In this study, all the molecular orbital calculations were performed by the DFT method. DFT is a quantum-chemistry method first proposed by Hohenberg and Kohn in 1964 [13], which shifts the focal point of a quantum mechanical calculation from the many-particle wave function to the electronic density. DFT is the most up-to-date method for determining quantities derived from energy differences in moderately large (up to several hundreds of atoms) systems. Becke3LYP hybrid functional was used throughout this work. It consists of the nonlocal exchange functional of Becke’s three-parameter set [14] and the nonlocal correlation functional of Lee et al. [15]. All the functions are applied to the self-consistent-field HF densities. Model calculations were made utilizing the RB3LYP-DFT method, taking into account the parameters of the solvation environment. LanL2DZ and 6-311G++(d, p) were taken as basis functions for the calculations [16]. In the frameworks of geometry
optimization, the vibrational frequency calculations showed all stationary points as minima or transition states (TS). Intrinsic reaction coordinate (IRC) calculations were also performed for the TS. 3. Results and discussion 3.1. Catalytic and non-catalytic processes of humic acid HA is an important component of natural organic matter and has high molecular weight compounds of a complex nature [17]. Because of the structural complexity, heterogeneous composition and variable chemical content of HA, its determination is quite difficult. The UV absorbance at 254 nm (UV254) and dissolved organic carbon (DOC) are the surrogate parameters for the determination of HA concentration [18]. Fig. 3 shows degradation of HA through application of ozonation and catalytic ozonation processes. UV254 absorbance represents the aromatic structure of organics in water. The ozonation of HA led to the destruction of its aromatic structure. The UV254 removal efficiency was 87%, with the addition of CuO within 5 min, whereas it was just 9% for simple ozonation
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HO
OH
COOH
O O N OH
O O O O O
HO
H N
NH
OH
OH
O
OH
N O
HO
O
HO
O
HO O
O
HO O O
HO O
HO O O
Fig. 4. Optimized 3D structure of humic acid and places of hydrogen bonds formation.
(a)
(b)
Fig. 5. HOMO/LUMO energy levels: (a) CuO + O3 and (b) CuO + O3 + H2O.
(Fig. 3a–c). After 5 min, an increment was observed in the normalized value of UV254.One of the possible mechanisms for this increment is that HA desorption from catalyst surface after adsorption and oxidation. The general idea behind the mechanism of catalytic ozonation is that molecular ozone and organic molecules diffuse
into the aquatic solution through the catalyst surface [19]. Radical chain reactions start immediately by means of the electron transport process. Active species and ozone molecules decompose into free radicals, which are more reactive than ozone molecules [20,21]. Here,
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Erel, eV IS 0 CuO n+O3
-1 TS -2
-1.85
FS -2.34
-3
CuO nO + O2
(a)
Erel, eV 0
IS
CuO n +O3+H2O
TS -5.58
-4 -8
FS -12.14
-12
CuO n + 2OH + O 2
(b) Fig. 6. Geometry and potential energy profiles of the reactions (initial state (IS), transition state (TS), and final state (FS)).
the catalyst serves as an adsorbent, and most of the reactions take place on its surface. Ozone species can adsorb organic molecules, on the catalyst surface instantly and continue to diffuse towards the inner layer of the catalyst. Then, further reactions take place between organic molecules and ozone species, and oxidized organic molecules are produced as a consequence of these reactions. The reactions go on with subsequent interactions between the new oxidation products and ozone species [21–23]. However, new products, which are bound with weak Van der Waals bonds, can desorb from the catalyst surface easily [24]. They diffuse into the aqueous solution again and may be oxidized by ozone or hydroxyl radicals once more. This increment may be due to desorption of these organic molecules from the catalyst surface. Similar trends were observed in Fig. 3b and c, obviously. Fig. 3b shows value of normalized VIS400 absorbance. VIS400 signifies color-forming substances, and it represents changes in structure via degradation of organic molecule. Initially, the rate of HA degradation was very fast, and then decelerated with time. The highest removal efficiency of HA was obtained as 90% after 10t min of reaction. Even increments were observed at intervals;
the final removal efficiency reached to 90% at the end of the reaction (60 min). The color changes of samples became lighter along the time, during experimental study. The decomposed aromatic molecules of HA change from brown to light yellow. These changes have been reflected in the results over time as decreasing the value of VIS400. As can be seen from Fig. 3c, the catalytic ozonation process was more effective than a simple ozonation process to remove DOC from aqueous HA solution. There is a significant difference between ozonation and catalytic ozonation processes in the first 5 min. In that time, the reduction of DOC was determined as 11% for ozonation alone, while it was 84% for ozonation with the addition of CuO, suggesting more carbon double bonds breaking in humic structure. The differences of DOC removal efficiencies of the two processes (catalytic and non-catalytic) were quite obvious at the end of the reaction, albeit at the same level as observed on UV254 and VIS400. The higher removal efficiencies of VIS400 and UV254 relative to DOC are likely due to formation of new low molecular weight degradation intermediates and their resistance to ozonation (Yu et al., 2005). Based on our previous experience, we run the catalytic ozonation experiments for 60 min. However, sufficient removal of HA was achieved in the first minutes of reaction. A ten-minute application of catalytic ozonation provides rapid degradation, with lower ozone consumption as well as higher organic removal efficiency. We have another possible explanation for the mechanism of this increment. Initial pH of solution was 5.33, and the experiments were run without pH adjustment. A possible reaction involves the reaction between CuO and hydronium ions (CuO + 2H+ ? Cu2+ + H2O). Copper ions begin to accumulate in the solution, which, meanwhile, tends to form complexes with HA easily. This observed removal efficiency of HA within 5 min is likely due to the formation of Cu2+ –HA complex precipitate. When ozone species attack the precipitate, the complex decomposes and HA transfers from precipitate to the bulk solution. Transferred HA molecules may lead to increase in organic matter content of the solution. Optimized 3D structure of HA is shown in Fig. 4. As shown, there are intramolecular hydrogen bonds in the structure, which are formed by oxygen atom and corresponding OH- and NH-groups. Positions of hydrogen bonds that are being formed and may cause further appearance of coordination bonds with Cu2+ are shown by arrows. One can note that strong five- and six-membered chelate cycles are thereby formed.
3.2. Theoretical modeling of CuO-catalyzed ozonation process Few theoretical research studies have been devoted to the processes of catalytic ozonation in solutions. However, the most recent of them [16] should be mentioned, in which the authors attempted to study the mechanism of catalysis on the surface of metal oxides with the help of quantum-chemical calculations [16].
Table 1 Charges on atoms for the different modeling systems. Molecules
CuO
O3 O1
O2
H2O
Atoms
States
Cu
O3
O4
CuO + O3
IS TS FS
1.23 1.12 0.93
0.68 0.75 0.73
0.01 0.27 0.37
0.02 0.08 0.05
0.01 0.15 0.05
CuO + O3 + H2O
IS TS FS
1.21 1.11 0.98
0.66 0.75 0.74
0.01 0.47 0.63
0.02 0.05 0.04
0.01 0.05 0.04
CuO O + H2O
IS TS FS
1.10 1.18 1.15
0.69 0.70 0.80
0.32 0.42 0.41
– – –
– – –
O5
H1
H2
– – –
– – –
– – –
0.16 0.68 0.70
0.08 0.36 0.31
0.08 0.37 0.31
0.70 0.48 0.40
0.36 0.40 0.39
0.34 0.37 0.36
O. Turkay et al. / Separation and Purification Technology 134 (2014) 110–116
To clear up the mechanism of catalytic ozonation of HA, the electron structure of ozone molecule has been studied on the surface of CuO (see Fig. 1). The electron structure of a separate CuO cluster calculations have shown that the energy gap between the HOMO and the LUMO is 1.97 eV. The existence of the gap is important for the donor–acceptor interaction characterization and electron density distribution. The CuO surface charging has an effect on the donor–acceptor properties of the active centers of catalyst in the ozonation reaction into aqueous solution. As calculations show, the increase of positive charge on the CuO surface strengthens acceptor properties of the catalyst, while its donor properties increase in the case of negative charging of CuO. It is mostly important in the case of the ozone molecule adsorption and catalytic activation. Two models of adsorption are given here: (a) only O3 molecule is attached to the surface; (b) O3 and H2O are simultaneously adsorbed by the active center of the catalyst. The aqueous environment of the reaction causes the O3 decomposition and oxygen-containing active particles formation, which participate in the further decomposition of HA. The HOMO and LUMO energy levels and the electron density distribution on the orbitals are given in Fig. 5. As seen from the figure, energy gaps for the two mentioned systems are negligible at the first stage (IS) and equal to 1.28 eV (a) and 0.82 eV (b). The orbitals (HOMO/LUMO) are represented by the wave functions of those atoms that participate in the electron density redistribution in the process of old bonds breakup and new bonds formation in the reacting system. A difference is observed in the TS. HOMO of the [CuO + O3] system includes orbitals of atoms belonging to the active center of catalyst, while LUMO consists of the orbitals of O3. For the model system [CuO + O3 + H2O], its HOMO/LUMO electron densities are evenly distributed on the O3 and H2O atoms of catalyst. At the final stage of the reaction (FS), the electron density is distributed mainly on the HOMO/LUMO of the catalyst atoms. This tells us that the reaction products are stable O2 molecules and OH radicals of which energy levels lie lower than those observed for the system as a whole. The generation of OH radicals was detected by spin-trapping/EPR technique in the catalytic ozonation studies [6,25,26] which were used different catalysts. These studies show that ozonation with the catalyst described stronger signals than that in ozonation. The metal oxide starts the chain reactions to produce OH radicals in the ozonation process. The common idea is that the catalyst accelerates ozone decomposition due to its surface area. We corroborated the given results showing the reactions which took place on CuO surface in Fig. 6. When considering model systems taken for this study, the energy profiles analysis is an important element for understanding the mechanism of the ozone reaction. Fig. 6 shows geometry and potential energy profiles calculated for the reactants (initial state-IS), TS, and FS of the reactions. The barrierless reaction is characteristic of both models. Activation energy for the [CuO + O3] reaction is 1.85 eV, and reaction energy is 2.34 eV. Negative values of the energies reflect its exothermic character. The second model reaction, [CuO + O3 + H2O], is exothermic, and passes with high activation energy (Eact = 5.58 eV). The result of the first reaction is one oxygen molecule and atomic oxygen that is being adsorbed on the CuO surface. Final products of the second reaction are O2 and hydroxyl-radicals, which are adsorbed on the CuO surface. These particles behave as powerful oxidizing agents in the further reactions with HA. Comparison of the two mechanisms shows that the second reaction with the water molecule participation is preferable to the first in terms of energy levels. In addition the two reaction models, an additional reaction between an oxygen atom being the result of O3 decomposition and water molecule on the CuO
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surface [CuO O + H2O] is studied. As calculations show, the barrierless path is also a characteristic of the latter reaction (the activation barrier is 7.35 eV). The reaction energy, DE = E (IS) – E (FS), is 8.16 eV. The results of the reaction are hydroxyl radicals, as in the case of the model reaction [CuO + O3 + H2O]. Analysis of the electron charge distribution on atoms for the studied model reactions is given in Table 1. As seen from Table 1, positive charge on the Cu atom regularly decreases with the increase of negative charge on the O2 atom of ozone molecule, which is bonded with Cu. A high negative charge is observed on the O1 oxygen atoms being on the surface of the CuO catalyst. As to the O3 and O4 atoms of ozone, then their charges change only negligibly in the process of reacting. The water molecule presence in the [CuO + O3 + H2O] system causes considerable electron density redistribution on the molecular node [Cu O2 H1 O5 H2] that is related to the new bonds formation and breaking up of old chemical bonds. High negative charge on the atoms O2 ( 0.74 e¯) and O5 ( 0.70 e¯) in FS signify the formation of HO radical particles. The tendency of the electron density redistribution in the system [CuO O + H2O] is analogous (see Table 1). Thus, the simultaneous O3 and H2O adsorption on the surface of catalyst causes the appearance of strongly polarized bonds in the reacting node and formation of free radical particles, which participate in the further oxidizing process. 4. Conclusions In this study, ozonation processes with and without CuO were applied for the degradation of HA. It was shown that the catalyst addition positively affects the mechanism of ozonation. Thus, organic matters in the solution could be reduced much more effectively by CuO-catalyzed ozonation. In a short time, about 90% of HA degraded in terms of UV254 and VIS400 and about 80% as DOC. It was found that adsorption, desorption, oxidation, and chelating reactions had taken place in solution. To clarify the mechanism of the reactions, theoretical modeling was carried out with the account of experimental results. We used the DFT approach to calculate the mechanism of the ozone decomposition. Donor–acceptor interaction between substance and catalyst was discussed regarding to frontier (HOMO/ LUMO) orbitals. Ozonation catalyzed by CuO was modeled through the two possible reaction systems ([CuO + O3] and [CuO + O3 + H2O]). The latter was found more appropriate for the CuO-catalyzed ozonation with regard to energy level. The surface of the CuO catalyst binds the O3 molecule by means of the electron density transfer from CuO to O3. Then, the adsorbed atomic oxygen reacts with H2O and forms two OH radicals, which initiate the heterogeneous catalytic ozonation of HA. Thus, the surface reactions between ozone and catalyst determine the mechanism of catalytic ozonation. Acknowledgement This research was supported by the Scientific Research Fund of Gebze Institute of Technology (Grant Number 2009A17). References [1] A. Matilainen, M. Sillanpaa, Removal of natural organic matter from drinking water by advanced oxidation processes, Chemosphere 80 (2010) 351–365, http://dx.doi.org/10.1016/j.chemosphere.2010.04.067. [2] D. Li, J. Qu, The progress of catalytic technologies in water purification: a review, J. Environ. Sci. 21 (2009) 713–719, http://dx.doi.org/10.1016/s10010742(08)62329-3. [3] B. Kasprzyk-Hordern, M. Ziolek, J. Nawrocki, Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment, Appl. Catal. B. – Environ. 46 (2003) 639–669, http://dx.doi.org/10.1016/S0926-3373(03)00326-6.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Experimental and theoretical investigations of CuO-catalyzed ozonation of humic acid Ozge Turkay ⇑, Hatice Inan, Anatoli Dimoglo Gebze Institute of Technology, Faculty of Engineering, Department of Environmental Engineering, 41400 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 16 January 2014 Received in revised form 12 July 2014 Accepted 15 July 2014 Available online 30 July 2014 Keywords: Catalytic ozonation Copper oxide Density Function Theory Humic acid
a b s t r a c t In this study, the efficiency of copper oxide (CuO) as a catalyst in the ozonation process of humic acid (HA) was investigated in both experimental and theoretical respects. Ozonation and catalytic ozonation processes were conducted in a lab setting. HA concentration was determined by measurement of the surrogate organic parameters. The results show that the degradation of HA by catalytic ozonation in the presence of CuO was found to be much more effective than the ozonation process alone. The experimental data was verified by means of theoretical modeling. Density Function Theory (DFT) was used to calculate the decomposition of ozone in the catalytic processes. The reactions on the surface of metal oxides were evaluated with quantum-chemical calculations to explain the mechanisms of catalytic ozonation. Two models of adsorption were investigated: when only O3 is attached to the surface and when O3 and H2O are simultaneously adsorbed by the active center of catalyst. Each is a barrierless reaction, as follows from the calculations mentioned. The result of the first reaction is one oxygen molecule and atomic oxygen being adsorbed on the CuO surface. The second reaction’s final products are O2 and hydroxyl-radicals, which are adsorbed on the CuO surface. These particles behave as powerful oxidizing agents in the further reactions with HA. Comparison of the two mechanisms shows that the second reaction with the water molecule participation is preferable to the first one based on energy levels. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A heterogeneous catalytic ozonation process is used to improve the oxidation of organic compounds by generating free radicals, mainly hydroxyl radicals of high oxidation power, by adding metal oxides [1–3]. Multiple examples of this process have been reported in the last decade, and it has been found favorable for enhancing organic matter degradation. However, its mechanism is quite complicated in that there are three phases (gas, liquid, and solid) composed of ozone gas, aqueous ozone, water, organic matter matrix, and metal oxide. In these systems, surface reactions on the catalyst are vital in determining the mechanism thoroughly. The application of theoretical modeling to the catalytic ozonation process is a new approach to clarify the interactions between substances and catalyst [4]. Density Function Theory (DFT) is a modeling method for the explanation of ground state properties of metals. DFT provides an explanation of the mechanism of organic matter degradation on the molecular level. Quantum-chemical calculation is used to explain the decomposition of ozone molecule and radical generation on the surfaces. Zhai et al. [5] evaluated ⇑ Corresponding author. Tel.: +90 5055051332. E-mail address: [email protected] (O. Turkay). http://dx.doi.org/10.1016/j.seppur.2014.07.040 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
dichloroacetic acid degradation by ZnO-catalyzed ozonation and analyzed the results by DFT. It was shown that the generation of hydroxyl radicals increased, and they were responsible for raising catalytic activity of ZnO. Zhang et al. [6] have reported the atomistic details of NiO, which was used as a catalyst in enhancing the degradation of organic pollutants. In their study, DFT calculations elucidated the adsorption characteristics and electronic structure of the catalytic ozonation process. It was also suggested that the decomposition of ozone and generation of superoxide ions and hydroxyl groups increased with the addition of NiO; in turn, they obtained an enhancement of the degradation of organic pollutants [6]. In the present study, the efficiency of copper oxide (CuO) as a catalyst was investigated in the ozonation process in both experimental and theoretical respects. Experimental studies were carried out in a semi-batch stirred reactor. Ozonation and catalytic ozonation processes were run. Humic acid (HA) was selected as organic matter because of its structure that is recalcitrant to conventional ozonation. Furthermore, disinfection by-products (trihalomethanes and haloacetic acids) may possibly form during the disinfection process due to humic substances contents of surface water [7,8]. The results were expressed in terms of parameters such as dissolve organic matter (DOC), UV absorbance at 254 nm (UV254) and 400 nm (VIS400) to specify the organic matter content
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The nano-powder CuO was provided from Sigma Aldrich. 0.25 g L 1 of CuO was used as a catalyst without further modification in catalytic ozonation experiments. Surface area (0.64 m2 g 1) and pore volume (0.002 cm3 g 1) of Aldrich CuO are detailed elsewhere [10]. Geometry ball and stick structures of CuO are shown in Fig. 1. 2.2. Experimental methods
2. Material and method
Ozone and catalytic ozone experiments were performed in a semi-batch stirred reactor with a volume of 2000 mL (the inside diameter of 12 cm). The reactor was covered with a light-proof material in order to avoid daylight. A schematic representation of the experimental system was given in Fig. 2. Experiments were carried out at natural pH (5.5–6.0) and at a room temperature (23 °C). Ozone was produced from pure oxygen by means of a generator (Sabo Electric, Turkey), and 0.190 mg of ozone was fed into the reactor in a minute after waiting for the concentration to become steady. A potassium iodide solution was used to determine the ozone introduced and in off-gas by the iodometric titration method [11]. The ozone bubbles were continuously dispersed through a porous glass diffuser located at the bottom of the reactor. A complete mixing (400 rpm) was achieved with a magnetic stirrer. Nitrogen gas was used to remove excess ozone at the end of the reaction. Samples were withdrawn at different intervals during 60 min. All samples filtrated through 0.45 lm membrane filter to separate particulates (humic materials and catalyst) prior to analyses. Normalized UV254, VIS400 and DOC values are given respect to time. Three repetitive measurements are presented as mean values, and each error bar represents the standard deviation. Graphs were drawn using Grapher 9 (Golden Software).
2.1. Reagents
2.3. Analytical methods
HA used in this research is commercially available and was supplied from Sigma–Aldrich. Some physical properties of Aldrich HA have been stated elsewhere [9]. A simulation of average organic matter content in surface water (25 mg L 1 of HA solution) was prepared using ultra-pure water. The solution of HA was characterized as follows: 0.474 ± 0.009 cm 1 of UV254, 0.119 ± 0.003 cm 1 of VIS400, 5.86 ± 0.43 mg L 1 of DOC and 5.33 ± 0.16 of pH value.
A total organic carbon (TOC) analyzer (Teledyn-Tekmar Apollo 9000) was used to measure the dissolved organic carbon (DOC) of the samples, as described in Standard Methods 5310 B [12]. A HACH Lange – DR 5000 spectrophotometer was used for the two of absorbance measurements run at wavelengths of 254 nm (UV254) and 400 nm (VIS400), as described in Standard Methods 5910 B [12]. Cyber-scan pH meter was used to determine pH changes.
Fig. 1. Geometry ball and stick structures of elementary cell of CuO (Bravais lattice, face centered monoclinic).
in water. Theoretical modeling (DFT) was employed to provide further evidence for the mechanism of ozonation process. Electron density distribution and reaction mechanism of superoxide radicals formation, which participated in the catalytic ozonation process, were analyzed. The energy values were calculated for frontier orbitals (highest occupied and lowest unoccupied molecular orbitals, HOMO/LUMO) and the energy profiles of each process are presented.
Gas Outlet
Ozone Generator KI Solution Flow meter
Sampling Point Gas Outlet
KI Solution
Diffuser Oxygen Gas
Magnetic Mixer
Fig. 2. Schematic representation of the experimental system.
O. Turkay et al. / Separation and Purification Technology 134 (2014) 110–116
1
1
0.8
0.8
VIS 400 C / C 0
UV 254 C / C 0
112
0.6
0.4
0.2
0.6
0.4
0.2
0
0 0
20
40
60
0
20
40
60
t (min)
t (min) 1
DOC C/C 0
0.8
0.6
0.4
0.2
0 0
20
40
60
t (min) Fig. 3. Degradation of humic acid in terms of UV254 (a), VIS400 (b) and DOC (c) (the straight-line and broken line represents ozonation and catalytic ozonation process, respectively. 25 mg L 1 of HA concentration, 0.25 g L 1 of catalyst dose, initial pH 5.33).
2.4. Theoretical Investigation of the catalytic ozonation process The ozone decomposition and the possibility of active-particle formation on CuO surface during ozonation with and without a catalyst were investigated. In this study, all the molecular orbital calculations were performed by the DFT method. DFT is a quantum-chemistry method first proposed by Hohenberg and Kohn in 1964 [13], which shifts the focal point of a quantum mechanical calculation from the many-particle wave function to the electronic density. DFT is the most up-to-date method for determining quantities derived from energy differences in moderately large (up to several hundreds of atoms) systems. Becke3LYP hybrid functional was used throughout this work. It consists of the nonlocal exchange functional of Becke’s three-parameter set [14] and the nonlocal correlation functional of Lee et al. [15]. All the functions are applied to the self-consistent-field HF densities. Model calculations were made utilizing the RB3LYP-DFT method, taking into account the parameters of the solvation environment. LanL2DZ and 6-311G++(d, p) were taken as basis functions for the calculations [16]. In the frameworks of geometry
optimization, the vibrational frequency calculations showed all stationary points as minima or transition states (TS). Intrinsic reaction coordinate (IRC) calculations were also performed for the TS. 3. Results and discussion 3.1. Catalytic and non-catalytic processes of humic acid HA is an important component of natural organic matter and has high molecular weight compounds of a complex nature [17]. Because of the structural complexity, heterogeneous composition and variable chemical content of HA, its determination is quite difficult. The UV absorbance at 254 nm (UV254) and dissolved organic carbon (DOC) are the surrogate parameters for the determination of HA concentration [18]. Fig. 3 shows degradation of HA through application of ozonation and catalytic ozonation processes. UV254 absorbance represents the aromatic structure of organics in water. The ozonation of HA led to the destruction of its aromatic structure. The UV254 removal efficiency was 87%, with the addition of CuO within 5 min, whereas it was just 9% for simple ozonation
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HO
OH
COOH
O O N OH
O O O O O
HO
H N
NH
OH
OH
O
OH
N O
HO
O
HO
O
HO O
O
HO O O
HO O
HO O O
Fig. 4. Optimized 3D structure of humic acid and places of hydrogen bonds formation.
(a)
(b)
Fig. 5. HOMO/LUMO energy levels: (a) CuO + O3 and (b) CuO + O3 + H2O.
(Fig. 3a–c). After 5 min, an increment was observed in the normalized value of UV254.One of the possible mechanisms for this increment is that HA desorption from catalyst surface after adsorption and oxidation. The general idea behind the mechanism of catalytic ozonation is that molecular ozone and organic molecules diffuse
into the aquatic solution through the catalyst surface [19]. Radical chain reactions start immediately by means of the electron transport process. Active species and ozone molecules decompose into free radicals, which are more reactive than ozone molecules [20,21]. Here,
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Erel, eV IS 0 CuO n+O3
-1 TS -2
-1.85
FS -2.34
-3
CuO nO + O2
(a)
Erel, eV 0
IS
CuO n +O3+H2O
TS -5.58
-4 -8
FS -12.14
-12
CuO n + 2OH + O 2
(b) Fig. 6. Geometry and potential energy profiles of the reactions (initial state (IS), transition state (TS), and final state (FS)).
the catalyst serves as an adsorbent, and most of the reactions take place on its surface. Ozone species can adsorb organic molecules, on the catalyst surface instantly and continue to diffuse towards the inner layer of the catalyst. Then, further reactions take place between organic molecules and ozone species, and oxidized organic molecules are produced as a consequence of these reactions. The reactions go on with subsequent interactions between the new oxidation products and ozone species [21–23]. However, new products, which are bound with weak Van der Waals bonds, can desorb from the catalyst surface easily [24]. They diffuse into the aqueous solution again and may be oxidized by ozone or hydroxyl radicals once more. This increment may be due to desorption of these organic molecules from the catalyst surface. Similar trends were observed in Fig. 3b and c, obviously. Fig. 3b shows value of normalized VIS400 absorbance. VIS400 signifies color-forming substances, and it represents changes in structure via degradation of organic molecule. Initially, the rate of HA degradation was very fast, and then decelerated with time. The highest removal efficiency of HA was obtained as 90% after 10t min of reaction. Even increments were observed at intervals;
the final removal efficiency reached to 90% at the end of the reaction (60 min). The color changes of samples became lighter along the time, during experimental study. The decomposed aromatic molecules of HA change from brown to light yellow. These changes have been reflected in the results over time as decreasing the value of VIS400. As can be seen from Fig. 3c, the catalytic ozonation process was more effective than a simple ozonation process to remove DOC from aqueous HA solution. There is a significant difference between ozonation and catalytic ozonation processes in the first 5 min. In that time, the reduction of DOC was determined as 11% for ozonation alone, while it was 84% for ozonation with the addition of CuO, suggesting more carbon double bonds breaking in humic structure. The differences of DOC removal efficiencies of the two processes (catalytic and non-catalytic) were quite obvious at the end of the reaction, albeit at the same level as observed on UV254 and VIS400. The higher removal efficiencies of VIS400 and UV254 relative to DOC are likely due to formation of new low molecular weight degradation intermediates and their resistance to ozonation (Yu et al., 2005). Based on our previous experience, we run the catalytic ozonation experiments for 60 min. However, sufficient removal of HA was achieved in the first minutes of reaction. A ten-minute application of catalytic ozonation provides rapid degradation, with lower ozone consumption as well as higher organic removal efficiency. We have another possible explanation for the mechanism of this increment. Initial pH of solution was 5.33, and the experiments were run without pH adjustment. A possible reaction involves the reaction between CuO and hydronium ions (CuO + 2H+ ? Cu2+ + H2O). Copper ions begin to accumulate in the solution, which, meanwhile, tends to form complexes with HA easily. This observed removal efficiency of HA within 5 min is likely due to the formation of Cu2+ –HA complex precipitate. When ozone species attack the precipitate, the complex decomposes and HA transfers from precipitate to the bulk solution. Transferred HA molecules may lead to increase in organic matter content of the solution. Optimized 3D structure of HA is shown in Fig. 4. As shown, there are intramolecular hydrogen bonds in the structure, which are formed by oxygen atom and corresponding OH- and NH-groups. Positions of hydrogen bonds that are being formed and may cause further appearance of coordination bonds with Cu2+ are shown by arrows. One can note that strong five- and six-membered chelate cycles are thereby formed.
3.2. Theoretical modeling of CuO-catalyzed ozonation process Few theoretical research studies have been devoted to the processes of catalytic ozonation in solutions. However, the most recent of them [16] should be mentioned, in which the authors attempted to study the mechanism of catalysis on the surface of metal oxides with the help of quantum-chemical calculations [16].
Table 1 Charges on atoms for the different modeling systems. Molecules
CuO
O3 O1
O2
H2O
Atoms
States
Cu
O3
O4
CuO + O3
IS TS FS
1.23 1.12 0.93
0.68 0.75 0.73
0.01 0.27 0.37
0.02 0.08 0.05
0.01 0.15 0.05
CuO + O3 + H2O
IS TS FS
1.21 1.11 0.98
0.66 0.75 0.74
0.01 0.47 0.63
0.02 0.05 0.04
0.01 0.05 0.04
CuO O + H2O
IS TS FS
1.10 1.18 1.15
0.69 0.70 0.80
0.32 0.42 0.41
– – –
– – –
O5
H1
H2
– – –
– – –
– – –
0.16 0.68 0.70
0.08 0.36 0.31
0.08 0.37 0.31
0.70 0.48 0.40
0.36 0.40 0.39
0.34 0.37 0.36
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To clear up the mechanism of catalytic ozonation of HA, the electron structure of ozone molecule has been studied on the surface of CuO (see Fig. 1). The electron structure of a separate CuO cluster calculations have shown that the energy gap between the HOMO and the LUMO is 1.97 eV. The existence of the gap is important for the donor–acceptor interaction characterization and electron density distribution. The CuO surface charging has an effect on the donor–acceptor properties of the active centers of catalyst in the ozonation reaction into aqueous solution. As calculations show, the increase of positive charge on the CuO surface strengthens acceptor properties of the catalyst, while its donor properties increase in the case of negative charging of CuO. It is mostly important in the case of the ozone molecule adsorption and catalytic activation. Two models of adsorption are given here: (a) only O3 molecule is attached to the surface; (b) O3 and H2O are simultaneously adsorbed by the active center of the catalyst. The aqueous environment of the reaction causes the O3 decomposition and oxygen-containing active particles formation, which participate in the further decomposition of HA. The HOMO and LUMO energy levels and the electron density distribution on the orbitals are given in Fig. 5. As seen from the figure, energy gaps for the two mentioned systems are negligible at the first stage (IS) and equal to 1.28 eV (a) and 0.82 eV (b). The orbitals (HOMO/LUMO) are represented by the wave functions of those atoms that participate in the electron density redistribution in the process of old bonds breakup and new bonds formation in the reacting system. A difference is observed in the TS. HOMO of the [CuO + O3] system includes orbitals of atoms belonging to the active center of catalyst, while LUMO consists of the orbitals of O3. For the model system [CuO + O3 + H2O], its HOMO/LUMO electron densities are evenly distributed on the O3 and H2O atoms of catalyst. At the final stage of the reaction (FS), the electron density is distributed mainly on the HOMO/LUMO of the catalyst atoms. This tells us that the reaction products are stable O2 molecules and OH radicals of which energy levels lie lower than those observed for the system as a whole. The generation of OH radicals was detected by spin-trapping/EPR technique in the catalytic ozonation studies [6,25,26] which were used different catalysts. These studies show that ozonation with the catalyst described stronger signals than that in ozonation. The metal oxide starts the chain reactions to produce OH radicals in the ozonation process. The common idea is that the catalyst accelerates ozone decomposition due to its surface area. We corroborated the given results showing the reactions which took place on CuO surface in Fig. 6. When considering model systems taken for this study, the energy profiles analysis is an important element for understanding the mechanism of the ozone reaction. Fig. 6 shows geometry and potential energy profiles calculated for the reactants (initial state-IS), TS, and FS of the reactions. The barrierless reaction is characteristic of both models. Activation energy for the [CuO + O3] reaction is 1.85 eV, and reaction energy is 2.34 eV. Negative values of the energies reflect its exothermic character. The second model reaction, [CuO + O3 + H2O], is exothermic, and passes with high activation energy (Eact = 5.58 eV). The result of the first reaction is one oxygen molecule and atomic oxygen that is being adsorbed on the CuO surface. Final products of the second reaction are O2 and hydroxyl-radicals, which are adsorbed on the CuO surface. These particles behave as powerful oxidizing agents in the further reactions with HA. Comparison of the two mechanisms shows that the second reaction with the water molecule participation is preferable to the first in terms of energy levels. In addition the two reaction models, an additional reaction between an oxygen atom being the result of O3 decomposition and water molecule on the CuO
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surface [CuO O + H2O] is studied. As calculations show, the barrierless path is also a characteristic of the latter reaction (the activation barrier is 7.35 eV). The reaction energy, DE = E (IS) – E (FS), is 8.16 eV. The results of the reaction are hydroxyl radicals, as in the case of the model reaction [CuO + O3 + H2O]. Analysis of the electron charge distribution on atoms for the studied model reactions is given in Table 1. As seen from Table 1, positive charge on the Cu atom regularly decreases with the increase of negative charge on the O2 atom of ozone molecule, which is bonded with Cu. A high negative charge is observed on the O1 oxygen atoms being on the surface of the CuO catalyst. As to the O3 and O4 atoms of ozone, then their charges change only negligibly in the process of reacting. The water molecule presence in the [CuO + O3 + H2O] system causes considerable electron density redistribution on the molecular node [Cu O2 H1 O5 H2] that is related to the new bonds formation and breaking up of old chemical bonds. High negative charge on the atoms O2 ( 0.74 e¯) and O5 ( 0.70 e¯) in FS signify the formation of HO radical particles. The tendency of the electron density redistribution in the system [CuO O + H2O] is analogous (see Table 1). Thus, the simultaneous O3 and H2O adsorption on the surface of catalyst causes the appearance of strongly polarized bonds in the reacting node and formation of free radical particles, which participate in the further oxidizing process. 4. Conclusions In this study, ozonation processes with and without CuO were applied for the degradation of HA. It was shown that the catalyst addition positively affects the mechanism of ozonation. Thus, organic matters in the solution could be reduced much more effectively by CuO-catalyzed ozonation. In a short time, about 90% of HA degraded in terms of UV254 and VIS400 and about 80% as DOC. It was found that adsorption, desorption, oxidation, and chelating reactions had taken place in solution. To clarify the mechanism of the reactions, theoretical modeling was carried out with the account of experimental results. We used the DFT approach to calculate the mechanism of the ozone decomposition. Donor–acceptor interaction between substance and catalyst was discussed regarding to frontier (HOMO/ LUMO) orbitals. Ozonation catalyzed by CuO was modeled through the two possible reaction systems ([CuO + O3] and [CuO + O3 + H2O]). The latter was found more appropriate for the CuO-catalyzed ozonation with regard to energy level. The surface of the CuO catalyst binds the O3 molecule by means of the electron density transfer from CuO to O3. Then, the adsorbed atomic oxygen reacts with H2O and forms two OH radicals, which initiate the heterogeneous catalytic ozonation of HA. Thus, the surface reactions between ozone and catalyst determine the mechanism of catalytic ozonation. Acknowledgement This research was supported by the Scientific Research Fund of Gebze Institute of Technology (Grant Number 2009A17). References [1] A. Matilainen, M. Sillanpaa, Removal of natural organic matter from drinking water by advanced oxidation processes, Chemosphere 80 (2010) 351–365, http://dx.doi.org/10.1016/j.chemosphere.2010.04.067. [2] D. Li, J. Qu, The progress of catalytic technologies in water purification: a review, J. Environ. Sci. 21 (2009) 713–719, http://dx.doi.org/10.1016/s10010742(08)62329-3. [3] B. Kasprzyk-Hordern, M. Ziolek, J. Nawrocki, Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment, Appl. Catal. B. – Environ. 46 (2003) 639–669, http://dx.doi.org/10.1016/S0926-3373(03)00326-6.
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