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Oxidation of organic pollutants in aqueous solutions by nanosized copper oxide catalysts
Applied Catalysis B: Environmental 85 (2009) 207–211
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Oxidation of organic pollutants in aqueous solutions by nanosized copper oxide catalysts Tal Ben-Moshe, Ishai Dror *, Brian Berkowitz Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2008 Received in revised form 16 July 2008 Accepted 24 July 2008 Available online 31 July 2008
The catalytic activity of copper oxide nanoparticles was investigated for the removal of organic pollutants in aqueous solutions, using hydrogen peroxide as an oxidant. Complete degradation of both alachlor and phenanthrene was achieved after 20 min. The kinetics of the reaction was found to be pseudo-first-order with respect to the pollutant. The influence on the reaction kinetics of different catalyst samples, consisting of the same material but of different origin and different particle properties, was examined. The effects of several factors such as irradiation, oxidant concentration, ionic strength and pH on the reaction were also investigated. The catalysis is not photo-induced and can be performed without UV–vis irradiation. In particular, an optimal oxidant concentration was determined for the studied system. The presence of salts was found to inhibit the alachlor degradation rate. The addition of high concentrations of oxidant or salt results in pseudo-zero-order kinetics. However, NaCl at very high concentrations (>1 M) was found to cause a dramatic increase in reaction rate. The catalysis is efficient over a wide range of pH values. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nano CuO Oxidation Alachlor Hydrogen peroxide
1. Introduction The problem of the release of toxic and persistent organic pollutants into the aquatic environment has drawn much attention in recent years, and is considered one of the most urgent challenges facing environmental scientists today. Hazardous organic pollutants such as pesticides, polychlorinated biphenyls (PCBs), halogenated organic solvents and polycyclic aromatic hydrocarbons (PAHs) are discharged from industrial and wastewater treatment plants and contaminate groundwater and surface water. One of the most promising techniques to eliminate pollutants is complete oxidative mineralization by heterogeneous photocatalysis [1–4]. Heterogeneous photocatalysis involves the use of a suspension of semiconductor powder (usually metal oxides) as a catalyst for the reaction. The process is photo-induced and requires irradiation by UV–vis light for the activation of the catalyst. This technique in considered particularly attractive due to its ability to completely oxidize many organic compounds without the formation
* Corresponding author at: Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel. Tel.: +972 8 9344230; fax: +972 8 9344124. E-mail address: [email protected] (I. Dror). 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.07.020
of hazardous byproducts. It can also be used to remove inorganic compounds, heavy metals, bacteria and viruses from water. Heterogeneous photocatalysis is part of a family of techniques called advanced oxidation processes (AOPs) [5,6] which also includes ozonation, Fenton’s reagent, and direct irradiation of the solution. These processes involve in situ generation of active radical oxidants such as hydroxyl or superoxide radicals. Nanosized particles are considered particularly attractive for catalysis due to their high reactivity, attributed to enhanced surface area [7,8]. The most common catalyst is titanium dioxide [9–11]. It is an efficient catalyst for the oxidation of many compounds in combination with UV irradiation. Other semiconductors such as ZnO [12,13], Fe2O3 [14], and CdS [15] have also been shown to degrade contaminants. In this study we examine the activity of copper oxide (CuO) nanoparticles as catalysts for the degradation of organic pollutants in water. Copper oxide is a semiconductor with an energy band gap of 1.21–1.5 eV [16]. It is known as an efficient catalyst for gas-phase reactions [17], especially for the oxidation of carbon monoxide [18–20]. There are very few studies that examine unsupported copper oxide nanoparticles in aqueous solutions. Copper oxide particles were used for the degradation of nitrophenols [21], for the production of H2O2 [22] and for bacteria inactivation [23].
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2. Experimental 2.1. Materials Alachlor (93.5%) was received from Agan Chemical Manufacturers. H2O2/H2O (30%, v/v), sodium hydroxide pellets (99%), potassium chloride (99.5%) and copper(II) chloride dihydrate were purchased from Merck, Germany. Copper oxide, ammonium carbonate (99.999%), copper acetate monohydrate (99%), phenanthrene (HPLC 97%), n-hexane (HPLC), hydrochloric acid (37%), sodium chloride (99.5%), calcium chloride (98.5%), sodium fluoride (99%), sodium bromide (99%), potassium iodide (99%), copper(I) chloride and neocuproin were purchased from Aldrich. A second type of copper oxide nanoparticles (99%) was received from NaBond Ltd. Deionized water (18.2 MV cm) was used in all experiments. 2.2. Synthesis of CuO nanoparticles Copper oxide nanoparticles were prepared according to the procedure described in Ref. [24]. Aqueous ammonium carbonate solution (0.3 M) was added to aqueous copper acetate monohydrate solution (0.05 M) at a 1:1 molar ratio. The precipitate was separated from the liquid by centrifugation and washed with water and ethanol and dried at 60 8C in air. The precipitation was then annealed at 400 8C. 2.3. Characterization of CuO nanoparticles Three types of copper oxide nanoparticles were used in the experiments. Two commercial samples the first purchased from Aldrich (CuO-1) the second received from NaBond (CuO-2) and a third type was synthesized in the lab (CuO-3). The structure and morphology of the particles were characterized by transmission electron microscope (Philips CM120 operating at 120 KV) and Xray diffraction analysis (Rigaku ULTIMA III diffractometer using Cu Ka radiation at 40 kV and 40 mA). N2 adsorption study was used to obtain BET surface area of the particles. The measurement was carried out on a Quantachrome NOVA 1000 instrument at 77 K. The average particle size in aqueous solution was determined using a NanoSight LM20 nanoparticle detection and analysis system.
tigated. The same procedure was repeated with ultrasonic irradiation instead of stirring in order to test the effect of mixing technique on the reaction rate. To check whether or not the catalysis is photo-induced, the same experiment was performed in complete darkness. The pH effect on the reaction was studied by adjusting the initial pH by addition of HCl or NaOH to the reaction mixture. The effect of H2O2 concentration was examined by addition of varying volumes of H2O2 to the reaction mixture. The ionic strength effect was studied by addition of different amounts of NaCl or CaCl2 to the reaction mixture. The efficiency of three types of copper oxide nanoparticle catalysts was tested by following the kinetic degradation of alachlor for a reaction system containing each of the samples. Analysis of reaction products was done using GC/MS (Varian 3800 Saturn 2000) equipped with VF-5 (25 m length, 0.25 mm inner diameter, 0.25 mm film thickness) capillary column. The GC carrier gas was helium, at a flow rate of 1 mL/min. The GC temperature program was as follows: 100 8C for 1 min; temperature ramp of 20 8C/min to 280 8C for alachlor and 80 8C for 1 min; temperature ramp of 15 8C/min to 240 8C; 240 8C for 1.33 min; temperature ramp of 30 8C/min to 300 8C for phenanthrene. 3. Results and discussion 3.1. Characterization of CuO nanoparticles The properties of the copper oxide samples used in the experiments are presented in Table 1. The average particle size in solution was determined by the NanoSight detection system and verified by comparison to TEM imaging (for CuO-1 and CuO-3) and manufacturer information (for CuO-1 and CuO-2). The particles are spherical with a broad size distribution. No clear image of sample CuO-2 could be obtained due to aggregation of the sample on the grid. The different samples have similar average particle size; however their surface areas are different. This may be due to different porosities of samples, which were prepared by different processes. The XRD spectra of the different types of particles are presented in Fig. 1, indicating similar composition and structure among the three samples. 3.2. Measurement of catalytic activity of CuO nanoparticles
2.4. Measurement of catalytic activity of CuO nanoparticles Copper oxide nanoparticles (0.2 g for alachlor or 0.1 g for phenanthrene) were suspended in an aqueous solution (200 mL) containing 30 mg/L of alachlor or 0.5 mg/L of phenanthrene. Subsequently 2 mL of H2O2 were added. The reaction mixture was stirred in a 250-mL reactor for 30 min at room temperature under ambient fluorescent lighting. Samples of the reaction solution at different times were extracted by n-hexane. The extracts were analyzed by GC/MS to determine the concentration of the organic pollutant. The experiments were done in triplicate, and control experiments consisting of reaction mixtures without (a) both catalyst and oxidizing agent, (b) without catalyst, and (c) without oxidizing agent were performed. The reaction mixture was filtered at the end of each experiment and tested for the presence of copper ions using the neocuproin reagent. The absorbance of the Cu–neocuproin complex was measured in a Cary 100 UV–vis spectrophotometer against a blank sample [25]. To determine whether or not copper ions act as catalysts in the reaction, the experiment was repeated using the same concentration of Cu+ or Cu2+ ions from CuCl or CuCl2 as catalysts. The effects of several factors such as irradiation, oxidant concentration, ionic strength and pH on the reaction were inves-
CuO-1 nanoparticles showed significant catalytic activity in the presence of an oxidizing agent. After 3 min, conversion of 47.6% of the alachlor and 39% of the phenanthrene was achieved. Complete degradation was achieved after 18 min for alachlor and 21 min for phenanthrene, as demonstrated by Figs. 2 and 3, respectively. The concentration of pollutant was plotted vs. time. The plots decay exponentially, suggesting pseudo-first-order kinetics with respect to the pollutants according to the equation C = A exp( kt), where C is the concentration of the pollutant, k is the rate constant, t is the time and A is constant. The measured rate constants are 0.224 0.008 min 1 for alachlor and 0.183 0.010 min 1 for phenanthrene. In the control experiments, without the addition of catalyst, no change in concentration was observed for Table 1 CuO samples used for the experiments Sample description
Commercial CuO sample (Aldrich) Commercial CuO sample (NaBond) Synthesis of CuO sample
Notation used
Surface area (m2 g
CuO-1 CuO-2 CuO-3
24 82 7
1
)
Average particle size (nm) 29 28 30
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Fig. 1. XRD spectra of the different CuO samples.
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Fig. 3. Degradation of phenanthrene in aqueous solution using H2O2 as oxidizing agent and CuO nanoparticles as catalyst.
concentration of copper ions in the solution at the end of the experiment was determined to be 1.5 10 4 M. In a separate experiment the same concentration of Cu+ or Cu2+ ions was added to the reaction mixture without addition of nanoparticles. No substantial degradation of alachlor was observed, indicating that copper ions in solutions do not act as catalysts in the reaction.
alachlor and a small change was observed for phenanthrene. The change in phenanthrene concentration may be attributed to evaporation or sorption, rather than degradation. For related systems where TiO2 and UV irradiation are used, the reaction takes place even without addition of oxidant, as it can generate the reactive oxidant species from oxygen dissolved in water. However in this study, no change in concentration could be observed in systems containing CuO without addition of hydrogen peroxide. To compare the degradation efficiency of different samples of copper oxide nanoparticles, the experiments were repeated for CuO-2 and CuO-3 samples. The results are presented in Fig. 4. Sample CuO-1 demonstrated the strongest catalytic activity. In contrast, sample CuO-2 showed mild catalytic activity. After 30 min, about 20% conversion was achieved. The plot appears to be linear, suggesting zero-order kinetics with respect to alachlor. Sample CuO-2 has the highest surface area as a powder. However, because sample CuO-2 was seen to aggregate strongly when present in aqueous solution, the surface area in solution is smaller, leading to a decrease in the number of accessible active sites. For CuO-3 complete degradation was achieved after 30 min. The shape of the plot is linear at the beginning but changes to exponential decay after about 15 min, indicating a transfer from zero-order to first-order kinetics during the experiment. This change may be the result of increased numbers of active radical species and/or catalyst active sites available for reaction. It was suggested previously that the active species in the reaction are Cu+ ions on the surface of the particles [21,23]. The
Metal oxide nanoparticles (especially TiO2) are known to catalyze oxidation reactions by photo-induction and thus require UV–vis light irradiation. In this work, the type of catalysis was
Fig. 2. Degradation of alachlor in aqueous solution using H2O2 as oxidizing agent and CuO nanoparticles as catalyst.
Fig. 4. Degradation of alachlor in aqueous solution using H2O2 as oxidizing agent and different samples of CuO nanoparticles as catalyst.
3.3. Effect of ultrasonic irradiation Stirring of the reaction mixture is essential for dispersion of the particles. It has been suggested previously that ultrasound irradiation can enhance the reaction rate by creation of additional active radical species and by increasing the particle surface area [26,27]. In this work, magnetic stirring and ultrasonic irradiation were compared for the catalytic degradation of alachlor. After 3 min, 46.6% conversion was achieved for magnetic stirring and 47.6% was achieved for ultrasonic irradiation. In both cases complete degradation was achieved after 18 min. The reaction rates in both cases were similar (0.210 0.009 and 0.224 0.008 min 1), suggesting that ultrasonic irradiation does not affect the reaction rate. 3.4. Effect of light
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investigated by repeating the experiment in complete darkness. The reaction rates are similar with and without light. After 3 min, conversion of 47.6% with and 46.3% without light was achieved. In both cases complete decomposition was achieved after 18 min, suggesting that the catalysis is not photo-induced for CuO. In the experiments described here, copper oxide requires addition of hydrogen peroxide as oxidant. 3.5. Effect of oxidant concentration The effect of the concentration of H2O2 has been investigated previously for nanosized TiO2 [28–30]. When nano TiO2 is used as a catalyst, addition of an oxidizing agent was found to be unnecessary for the degradation reaction. At low concentrations, H2O2 has an enhancing effect on the reaction rate, which can be attributed to the formation of hydroxyl radicals due to the reaction of H2O2 with conduction band electrons, and the increased concentration of available holes for oxidation. However, there is an optimal concentration of H2O2, and addition of excess of H2O2 results in a decrease in reaction rate. The degradation results for different concentrations of H2O2 are presented in Fig. 5. For oxidant concentrations of 0.15 and 0.25 M, a linear decrease in alachlor concentration is observed, suggestion pseudo-zero-order kinetics. These kinetics may be caused by a low concentration of radicals due to increased recombination and scavenging of radicals, for example by the reaction, or by competitive adsorption of hydrogen peroxide to the catalyst active sites. After 3 min, 7.8% conversion for 0.15 M and 15.7% for 0.25 M was achieved. The optimal concentration appears to be 0.05 M, with a rate constant of 0.292 0.004 min 1, 60% conversion after 3 min and complete degradation after 18 min. For low concentrations (0.01 M) a pseudo-first-order reaction is observed; however the rate constant, 0.119 0.006 min 1, is smaller than for the optimal concentration. Conversion of 37.3% was achieved after 3 min.
Fig. 6. Effect of CaCl2 concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
Several studies have investigated the effect of ionic strength on related oxidation reactions catalyzed by nanometal oxides. Common inorganic ions present in natural and industrial waters such as Cl , NO3 , HCO3 , Ca2+, Na+ and Mg2+ have been tested for various pollutants [28]. The concentration of the ions in the vicinity of the nanoparticles depends on the pH of the solution. At pH lower than the point of zero charge (PZC) the particles carry a positive charge, so that there will be a higher concentration of anions and
lower concentration of cations in their vicinity. At pH higher than PZC, the concentration of cations will be higher. It is expected that the presence of ions will inhibit the reaction due to competition with the organic compound for the active radical species, as well as through competitive adsorption on the CuO active sites. In some cases anions are capable of oxidizing the pollutants directly, but this process is slower than oxidation by HO. In most of the experiments reported, small or no decreases in reaction rates were observed for anions in low concentration (up to 0.1 M). For several cations (Fe3+, Cu2+) an increase was observed at low concentration (up to 0.01 M). This increase can be attributed to the ability of the ions to trap electrons, thus preventing the recombination of electron–hole pairs, and to the creation of addition radical species [31]. To test the effect of ionic strength on the catalytic activity of CuO for the system studied here, two common groundwater salts were used, CaCl2 and NaCl. The results are presented in Figs. 6 and 7, respectively. The experiments were performed at pH 5.5–5.8 which is below the point of zero charge of CuO, 9.5 [32]. At this pH the particles are positively charged and therefore it is expected to find mainly negative ions surrounding them. As discussed in Section 3.2, the reactions usually take place on the surface of the nanoparticles or in their immediate vicinity, so that the negative ions will be the ones affecting the catalysis. For CaCl2, a decrease in reaction rate with addition of salt is observed. After 21 min 56.7%, 33% and 41% conversion of alachlor
Fig. 5. Effect of oxidant concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
Fig. 7. Effect of NaCl concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
3.6. Effect of ionic strength
T. Ben-Moshe et al. / Applied Catalysis B: Environmental 85 (2009) 207–211
was achieved for 0.01, 0.1 and 1 M, respectively. Higher concentrations are associated with a transition from pseudofirst-order to pseudo-zero-order kinetics with respect to alachlor. This transition may be caused by radical scavenging by the ions, or by competitive adsorption of ions to the catalyst surface. For NaCl, addition of low salt concentrations (up to 0.1 M) seems to inhibit the reaction. For concentrations of 0.01–0.1 M there is an approximately linear decrease in alachlor concentration, indicating a pseudo-zero-order reaction. However for a concentration of 1 M, a dramatic increase in reaction rate is observed (0.896 0.008 min 1). This rate is faster than without the addition of salt (0.024 0.008 min 1). For 5 M the rate is slower than for 1 M (0.327 0.032 min 1) but still faster than without the addition of salt. After 3 min 32%, 8.6%, 9.3%, 93% and 66.7% conversion was achieved for 0.001, 0.01, 0.1, 1 and 5 M, respectively. Complete degradation was achieved after 30, 6 and 15 min for 0.001, 1 and 5 M, respectively.
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photo-induced and does not require any type of irradiation. The kinetics of the reaction is pseudo-first-order with respect to the degraded organic compound. The optimal conditions for the reaction were found to be 0.05 M of hydrogen peroxide and pH value 4–9. Low concentrations of salt inhibit the reaction; however high concentrations of NaCl increase the rate of the reaction dramatically.
Acknowledgments The support of research grants from the Sussman Family Center of Environmental Research and from Mr. and Mrs. Michael Levine is gratefully acknowledged.
References 3.7. Effect of pH The effect of pH on the catalysis efficiency for nanoparticles can be divided into two stages [28,31,33]: the first stage is the effect of pH on the adsorption of the target organic pollutants to the surface of the catalyst. The reaction takes place on the surface of the nanoparticles or in its vicinity (Section 3.2), and the adsorption is therefore an important step in the reaction. It was shown for many pollutants that adsorption is higher at low pH values. This is explained by the fact that at lower pH, the surface of the catalyst is more positively charged. The pollutants are partly ionized in water and therefore there is an electrostatic attraction between the nanoparticles and the molecules. The second stage is the degradation process itself. For this process both acidic and basic pH values were found to be favorable for the reaction, with neutral values being less favorable. The formation of active species such as HO or HO2 is a necessary step in the reaction and is favorable in acidic or basic pH. There is an extensive series of reactions that can generate these active species [29,34,35]. In this study the effect of pH was examined by adjusting the initial pH of the reaction mixture in the range 3–11.6. It was observed that at pH higher than the PZC (10 or higher) there was little degradation over 30 min, indicating that the reaction is more efficient when the particles are positively charged, resulting in better adsorption of organic molecules and enhanced formation of active radical species. In addition, during the reaction the CO2 concentration in the solution increases. At high pH values the dominant species will be carbonate or bicarbonate ions, which are known as radical scavengers [36], while at low pH carbonic acid becomes the major product. For lower pH values, pseudo-firstorder kinetics was observed. At the lowest value, 2.9, the rate of the reaction is significantly slower than at higher values. This may be due to the addition of Cl ions (HCl was added to adjust the pH). The optimal pH range for the degradation appears to be 4–9. 4. Conclusions Copper oxide nanoparticles are efficient catalysts for the degradation of alachlor and phenanthrene. The catalysis is not
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Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Oxidation of organic pollutants in aqueous solutions by nanosized copper oxide catalysts Tal Ben-Moshe, Ishai Dror *, Brian Berkowitz Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2008 Received in revised form 16 July 2008 Accepted 24 July 2008 Available online 31 July 2008
The catalytic activity of copper oxide nanoparticles was investigated for the removal of organic pollutants in aqueous solutions, using hydrogen peroxide as an oxidant. Complete degradation of both alachlor and phenanthrene was achieved after 20 min. The kinetics of the reaction was found to be pseudo-first-order with respect to the pollutant. The influence on the reaction kinetics of different catalyst samples, consisting of the same material but of different origin and different particle properties, was examined. The effects of several factors such as irradiation, oxidant concentration, ionic strength and pH on the reaction were also investigated. The catalysis is not photo-induced and can be performed without UV–vis irradiation. In particular, an optimal oxidant concentration was determined for the studied system. The presence of salts was found to inhibit the alachlor degradation rate. The addition of high concentrations of oxidant or salt results in pseudo-zero-order kinetics. However, NaCl at very high concentrations (>1 M) was found to cause a dramatic increase in reaction rate. The catalysis is efficient over a wide range of pH values. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nano CuO Oxidation Alachlor Hydrogen peroxide
1. Introduction The problem of the release of toxic and persistent organic pollutants into the aquatic environment has drawn much attention in recent years, and is considered one of the most urgent challenges facing environmental scientists today. Hazardous organic pollutants such as pesticides, polychlorinated biphenyls (PCBs), halogenated organic solvents and polycyclic aromatic hydrocarbons (PAHs) are discharged from industrial and wastewater treatment plants and contaminate groundwater and surface water. One of the most promising techniques to eliminate pollutants is complete oxidative mineralization by heterogeneous photocatalysis [1–4]. Heterogeneous photocatalysis involves the use of a suspension of semiconductor powder (usually metal oxides) as a catalyst for the reaction. The process is photo-induced and requires irradiation by UV–vis light for the activation of the catalyst. This technique in considered particularly attractive due to its ability to completely oxidize many organic compounds without the formation
* Corresponding author at: Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel. Tel.: +972 8 9344230; fax: +972 8 9344124. E-mail address: [email protected] (I. Dror). 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.07.020
of hazardous byproducts. It can also be used to remove inorganic compounds, heavy metals, bacteria and viruses from water. Heterogeneous photocatalysis is part of a family of techniques called advanced oxidation processes (AOPs) [5,6] which also includes ozonation, Fenton’s reagent, and direct irradiation of the solution. These processes involve in situ generation of active radical oxidants such as hydroxyl or superoxide radicals. Nanosized particles are considered particularly attractive for catalysis due to their high reactivity, attributed to enhanced surface area [7,8]. The most common catalyst is titanium dioxide [9–11]. It is an efficient catalyst for the oxidation of many compounds in combination with UV irradiation. Other semiconductors such as ZnO [12,13], Fe2O3 [14], and CdS [15] have also been shown to degrade contaminants. In this study we examine the activity of copper oxide (CuO) nanoparticles as catalysts for the degradation of organic pollutants in water. Copper oxide is a semiconductor with an energy band gap of 1.21–1.5 eV [16]. It is known as an efficient catalyst for gas-phase reactions [17], especially for the oxidation of carbon monoxide [18–20]. There are very few studies that examine unsupported copper oxide nanoparticles in aqueous solutions. Copper oxide particles were used for the degradation of nitrophenols [21], for the production of H2O2 [22] and for bacteria inactivation [23].
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2. Experimental 2.1. Materials Alachlor (93.5%) was received from Agan Chemical Manufacturers. H2O2/H2O (30%, v/v), sodium hydroxide pellets (99%), potassium chloride (99.5%) and copper(II) chloride dihydrate were purchased from Merck, Germany. Copper oxide, ammonium carbonate (99.999%), copper acetate monohydrate (99%), phenanthrene (HPLC 97%), n-hexane (HPLC), hydrochloric acid (37%), sodium chloride (99.5%), calcium chloride (98.5%), sodium fluoride (99%), sodium bromide (99%), potassium iodide (99%), copper(I) chloride and neocuproin were purchased from Aldrich. A second type of copper oxide nanoparticles (99%) was received from NaBond Ltd. Deionized water (18.2 MV cm) was used in all experiments. 2.2. Synthesis of CuO nanoparticles Copper oxide nanoparticles were prepared according to the procedure described in Ref. [24]. Aqueous ammonium carbonate solution (0.3 M) was added to aqueous copper acetate monohydrate solution (0.05 M) at a 1:1 molar ratio. The precipitate was separated from the liquid by centrifugation and washed with water and ethanol and dried at 60 8C in air. The precipitation was then annealed at 400 8C. 2.3. Characterization of CuO nanoparticles Three types of copper oxide nanoparticles were used in the experiments. Two commercial samples the first purchased from Aldrich (CuO-1) the second received from NaBond (CuO-2) and a third type was synthesized in the lab (CuO-3). The structure and morphology of the particles were characterized by transmission electron microscope (Philips CM120 operating at 120 KV) and Xray diffraction analysis (Rigaku ULTIMA III diffractometer using Cu Ka radiation at 40 kV and 40 mA). N2 adsorption study was used to obtain BET surface area of the particles. The measurement was carried out on a Quantachrome NOVA 1000 instrument at 77 K. The average particle size in aqueous solution was determined using a NanoSight LM20 nanoparticle detection and analysis system.
tigated. The same procedure was repeated with ultrasonic irradiation instead of stirring in order to test the effect of mixing technique on the reaction rate. To check whether or not the catalysis is photo-induced, the same experiment was performed in complete darkness. The pH effect on the reaction was studied by adjusting the initial pH by addition of HCl or NaOH to the reaction mixture. The effect of H2O2 concentration was examined by addition of varying volumes of H2O2 to the reaction mixture. The ionic strength effect was studied by addition of different amounts of NaCl or CaCl2 to the reaction mixture. The efficiency of three types of copper oxide nanoparticle catalysts was tested by following the kinetic degradation of alachlor for a reaction system containing each of the samples. Analysis of reaction products was done using GC/MS (Varian 3800 Saturn 2000) equipped with VF-5 (25 m length, 0.25 mm inner diameter, 0.25 mm film thickness) capillary column. The GC carrier gas was helium, at a flow rate of 1 mL/min. The GC temperature program was as follows: 100 8C for 1 min; temperature ramp of 20 8C/min to 280 8C for alachlor and 80 8C for 1 min; temperature ramp of 15 8C/min to 240 8C; 240 8C for 1.33 min; temperature ramp of 30 8C/min to 300 8C for phenanthrene. 3. Results and discussion 3.1. Characterization of CuO nanoparticles The properties of the copper oxide samples used in the experiments are presented in Table 1. The average particle size in solution was determined by the NanoSight detection system and verified by comparison to TEM imaging (for CuO-1 and CuO-3) and manufacturer information (for CuO-1 and CuO-2). The particles are spherical with a broad size distribution. No clear image of sample CuO-2 could be obtained due to aggregation of the sample on the grid. The different samples have similar average particle size; however their surface areas are different. This may be due to different porosities of samples, which were prepared by different processes. The XRD spectra of the different types of particles are presented in Fig. 1, indicating similar composition and structure among the three samples. 3.2. Measurement of catalytic activity of CuO nanoparticles
2.4. Measurement of catalytic activity of CuO nanoparticles Copper oxide nanoparticles (0.2 g for alachlor or 0.1 g for phenanthrene) were suspended in an aqueous solution (200 mL) containing 30 mg/L of alachlor or 0.5 mg/L of phenanthrene. Subsequently 2 mL of H2O2 were added. The reaction mixture was stirred in a 250-mL reactor for 30 min at room temperature under ambient fluorescent lighting. Samples of the reaction solution at different times were extracted by n-hexane. The extracts were analyzed by GC/MS to determine the concentration of the organic pollutant. The experiments were done in triplicate, and control experiments consisting of reaction mixtures without (a) both catalyst and oxidizing agent, (b) without catalyst, and (c) without oxidizing agent were performed. The reaction mixture was filtered at the end of each experiment and tested for the presence of copper ions using the neocuproin reagent. The absorbance of the Cu–neocuproin complex was measured in a Cary 100 UV–vis spectrophotometer against a blank sample [25]. To determine whether or not copper ions act as catalysts in the reaction, the experiment was repeated using the same concentration of Cu+ or Cu2+ ions from CuCl or CuCl2 as catalysts. The effects of several factors such as irradiation, oxidant concentration, ionic strength and pH on the reaction were inves-
CuO-1 nanoparticles showed significant catalytic activity in the presence of an oxidizing agent. After 3 min, conversion of 47.6% of the alachlor and 39% of the phenanthrene was achieved. Complete degradation was achieved after 18 min for alachlor and 21 min for phenanthrene, as demonstrated by Figs. 2 and 3, respectively. The concentration of pollutant was plotted vs. time. The plots decay exponentially, suggesting pseudo-first-order kinetics with respect to the pollutants according to the equation C = A exp( kt), where C is the concentration of the pollutant, k is the rate constant, t is the time and A is constant. The measured rate constants are 0.224 0.008 min 1 for alachlor and 0.183 0.010 min 1 for phenanthrene. In the control experiments, without the addition of catalyst, no change in concentration was observed for Table 1 CuO samples used for the experiments Sample description
Commercial CuO sample (Aldrich) Commercial CuO sample (NaBond) Synthesis of CuO sample
Notation used
Surface area (m2 g
CuO-1 CuO-2 CuO-3
24 82 7
1
)
Average particle size (nm) 29 28 30
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Fig. 1. XRD spectra of the different CuO samples.
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Fig. 3. Degradation of phenanthrene in aqueous solution using H2O2 as oxidizing agent and CuO nanoparticles as catalyst.
concentration of copper ions in the solution at the end of the experiment was determined to be 1.5 10 4 M. In a separate experiment the same concentration of Cu+ or Cu2+ ions was added to the reaction mixture without addition of nanoparticles. No substantial degradation of alachlor was observed, indicating that copper ions in solutions do not act as catalysts in the reaction.
alachlor and a small change was observed for phenanthrene. The change in phenanthrene concentration may be attributed to evaporation or sorption, rather than degradation. For related systems where TiO2 and UV irradiation are used, the reaction takes place even without addition of oxidant, as it can generate the reactive oxidant species from oxygen dissolved in water. However in this study, no change in concentration could be observed in systems containing CuO without addition of hydrogen peroxide. To compare the degradation efficiency of different samples of copper oxide nanoparticles, the experiments were repeated for CuO-2 and CuO-3 samples. The results are presented in Fig. 4. Sample CuO-1 demonstrated the strongest catalytic activity. In contrast, sample CuO-2 showed mild catalytic activity. After 30 min, about 20% conversion was achieved. The plot appears to be linear, suggesting zero-order kinetics with respect to alachlor. Sample CuO-2 has the highest surface area as a powder. However, because sample CuO-2 was seen to aggregate strongly when present in aqueous solution, the surface area in solution is smaller, leading to a decrease in the number of accessible active sites. For CuO-3 complete degradation was achieved after 30 min. The shape of the plot is linear at the beginning but changes to exponential decay after about 15 min, indicating a transfer from zero-order to first-order kinetics during the experiment. This change may be the result of increased numbers of active radical species and/or catalyst active sites available for reaction. It was suggested previously that the active species in the reaction are Cu+ ions on the surface of the particles [21,23]. The
Metal oxide nanoparticles (especially TiO2) are known to catalyze oxidation reactions by photo-induction and thus require UV–vis light irradiation. In this work, the type of catalysis was
Fig. 2. Degradation of alachlor in aqueous solution using H2O2 as oxidizing agent and CuO nanoparticles as catalyst.
Fig. 4. Degradation of alachlor in aqueous solution using H2O2 as oxidizing agent and different samples of CuO nanoparticles as catalyst.
3.3. Effect of ultrasonic irradiation Stirring of the reaction mixture is essential for dispersion of the particles. It has been suggested previously that ultrasound irradiation can enhance the reaction rate by creation of additional active radical species and by increasing the particle surface area [26,27]. In this work, magnetic stirring and ultrasonic irradiation were compared for the catalytic degradation of alachlor. After 3 min, 46.6% conversion was achieved for magnetic stirring and 47.6% was achieved for ultrasonic irradiation. In both cases complete degradation was achieved after 18 min. The reaction rates in both cases were similar (0.210 0.009 and 0.224 0.008 min 1), suggesting that ultrasonic irradiation does not affect the reaction rate. 3.4. Effect of light
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investigated by repeating the experiment in complete darkness. The reaction rates are similar with and without light. After 3 min, conversion of 47.6% with and 46.3% without light was achieved. In both cases complete decomposition was achieved after 18 min, suggesting that the catalysis is not photo-induced for CuO. In the experiments described here, copper oxide requires addition of hydrogen peroxide as oxidant. 3.5. Effect of oxidant concentration The effect of the concentration of H2O2 has been investigated previously for nanosized TiO2 [28–30]. When nano TiO2 is used as a catalyst, addition of an oxidizing agent was found to be unnecessary for the degradation reaction. At low concentrations, H2O2 has an enhancing effect on the reaction rate, which can be attributed to the formation of hydroxyl radicals due to the reaction of H2O2 with conduction band electrons, and the increased concentration of available holes for oxidation. However, there is an optimal concentration of H2O2, and addition of excess of H2O2 results in a decrease in reaction rate. The degradation results for different concentrations of H2O2 are presented in Fig. 5. For oxidant concentrations of 0.15 and 0.25 M, a linear decrease in alachlor concentration is observed, suggestion pseudo-zero-order kinetics. These kinetics may be caused by a low concentration of radicals due to increased recombination and scavenging of radicals, for example by the reaction, or by competitive adsorption of hydrogen peroxide to the catalyst active sites. After 3 min, 7.8% conversion for 0.15 M and 15.7% for 0.25 M was achieved. The optimal concentration appears to be 0.05 M, with a rate constant of 0.292 0.004 min 1, 60% conversion after 3 min and complete degradation after 18 min. For low concentrations (0.01 M) a pseudo-first-order reaction is observed; however the rate constant, 0.119 0.006 min 1, is smaller than for the optimal concentration. Conversion of 37.3% was achieved after 3 min.
Fig. 6. Effect of CaCl2 concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
Several studies have investigated the effect of ionic strength on related oxidation reactions catalyzed by nanometal oxides. Common inorganic ions present in natural and industrial waters such as Cl , NO3 , HCO3 , Ca2+, Na+ and Mg2+ have been tested for various pollutants [28]. The concentration of the ions in the vicinity of the nanoparticles depends on the pH of the solution. At pH lower than the point of zero charge (PZC) the particles carry a positive charge, so that there will be a higher concentration of anions and
lower concentration of cations in their vicinity. At pH higher than PZC, the concentration of cations will be higher. It is expected that the presence of ions will inhibit the reaction due to competition with the organic compound for the active radical species, as well as through competitive adsorption on the CuO active sites. In some cases anions are capable of oxidizing the pollutants directly, but this process is slower than oxidation by HO. In most of the experiments reported, small or no decreases in reaction rates were observed for anions in low concentration (up to 0.1 M). For several cations (Fe3+, Cu2+) an increase was observed at low concentration (up to 0.01 M). This increase can be attributed to the ability of the ions to trap electrons, thus preventing the recombination of electron–hole pairs, and to the creation of addition radical species [31]. To test the effect of ionic strength on the catalytic activity of CuO for the system studied here, two common groundwater salts were used, CaCl2 and NaCl. The results are presented in Figs. 6 and 7, respectively. The experiments were performed at pH 5.5–5.8 which is below the point of zero charge of CuO, 9.5 [32]. At this pH the particles are positively charged and therefore it is expected to find mainly negative ions surrounding them. As discussed in Section 3.2, the reactions usually take place on the surface of the nanoparticles or in their immediate vicinity, so that the negative ions will be the ones affecting the catalysis. For CaCl2, a decrease in reaction rate with addition of salt is observed. After 21 min 56.7%, 33% and 41% conversion of alachlor
Fig. 5. Effect of oxidant concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
Fig. 7. Effect of NaCl concentration on the catalytic degradation of alachlor using CuO nanoparticle catalysts in an aqueous solution.
3.6. Effect of ionic strength
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was achieved for 0.01, 0.1 and 1 M, respectively. Higher concentrations are associated with a transition from pseudofirst-order to pseudo-zero-order kinetics with respect to alachlor. This transition may be caused by radical scavenging by the ions, or by competitive adsorption of ions to the catalyst surface. For NaCl, addition of low salt concentrations (up to 0.1 M) seems to inhibit the reaction. For concentrations of 0.01–0.1 M there is an approximately linear decrease in alachlor concentration, indicating a pseudo-zero-order reaction. However for a concentration of 1 M, a dramatic increase in reaction rate is observed (0.896 0.008 min 1). This rate is faster than without the addition of salt (0.024 0.008 min 1). For 5 M the rate is slower than for 1 M (0.327 0.032 min 1) but still faster than without the addition of salt. After 3 min 32%, 8.6%, 9.3%, 93% and 66.7% conversion was achieved for 0.001, 0.01, 0.1, 1 and 5 M, respectively. Complete degradation was achieved after 30, 6 and 15 min for 0.001, 1 and 5 M, respectively.
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photo-induced and does not require any type of irradiation. The kinetics of the reaction is pseudo-first-order with respect to the degraded organic compound. The optimal conditions for the reaction were found to be 0.05 M of hydrogen peroxide and pH value 4–9. Low concentrations of salt inhibit the reaction; however high concentrations of NaCl increase the rate of the reaction dramatically.
Acknowledgments The support of research grants from the Sussman Family Center of Environmental Research and from Mr. and Mrs. Michael Levine is gratefully acknowledged.
References 3.7. Effect of pH The effect of pH on the catalysis efficiency for nanoparticles can be divided into two stages [28,31,33]: the first stage is the effect of pH on the adsorption of the target organic pollutants to the surface of the catalyst. The reaction takes place on the surface of the nanoparticles or in its vicinity (Section 3.2), and the adsorption is therefore an important step in the reaction. It was shown for many pollutants that adsorption is higher at low pH values. This is explained by the fact that at lower pH, the surface of the catalyst is more positively charged. The pollutants are partly ionized in water and therefore there is an electrostatic attraction between the nanoparticles and the molecules. The second stage is the degradation process itself. For this process both acidic and basic pH values were found to be favorable for the reaction, with neutral values being less favorable. The formation of active species such as HO or HO2 is a necessary step in the reaction and is favorable in acidic or basic pH. There is an extensive series of reactions that can generate these active species [29,34,35]. In this study the effect of pH was examined by adjusting the initial pH of the reaction mixture in the range 3–11.6. It was observed that at pH higher than the PZC (10 or higher) there was little degradation over 30 min, indicating that the reaction is more efficient when the particles are positively charged, resulting in better adsorption of organic molecules and enhanced formation of active radical species. In addition, during the reaction the CO2 concentration in the solution increases. At high pH values the dominant species will be carbonate or bicarbonate ions, which are known as radical scavengers [36], while at low pH carbonic acid becomes the major product. For lower pH values, pseudo-firstorder kinetics was observed. At the lowest value, 2.9, the rate of the reaction is significantly slower than at higher values. This may be due to the addition of Cl ions (HCl was added to adjust the pH). The optimal pH range for the degradation appears to be 4–9. 4. Conclusions Copper oxide nanoparticles are efficient catalysts for the degradation of alachlor and phenanthrene. The catalysis is not
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