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Nanophotocatalysis using immobilized titanium dioxide nanoparticle
Materials Research Bulletin 42 (2007) 797–806 www.elsevier.com/locate/matresbu
Nanophotocatalysis using immobilized titanium dioxide nanoparticle Degradation and mineralization of water containing organic pollutant: Case study of Butachlor Niyaz Mohammad Mahmoodi a,*, Mokhtar Arami a,b, Nargess Yousefi Limaee a, Kamaladin Gharanjig a,b, Farahnaz Nourmohammadian a a
Colorant Manufacturing and Environmental Science Department, Institute for Colorants, Paints and Coatings, Tehran, Iran b Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran Received 10 January 2006; received in revised form 28 August 2006; accepted 30 August 2006 Available online 12 October 2006
Abstract In this paper, the degradation and mineralization of Butachlor in aqueous solution by nanophotocatalysis using immobilized TiO2 nanoparticles were investigated. Butachlor (N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide) is a persistent organic pollutant in agricultural soil and watercourses. A simple and effective method was used for immobilization of titanium dioxide nanoparticles. UV–vis and Ion Chromatography (IC) analyses were employed to obtain the details of the photocatalytic degradation and mineralization of Butachlor. The effects of operational parameters such as H2O2, inorganic anions (NO3, Cl and SO42) and pH were investigated. The lack of any absorbance in 254 nm was indicative of the complete degradation of aromatic intermediates. The mineralization of Butachlor was evaluated by monitoring of the formed inorganic anions (NO3 and Cl). Butachlor is effectively degraded following first order kinetics model. Results show that the immobilized titanium dioxide nanoparticle photocatalysis is an effective method for treatment Butachlor from contaminated water. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Organic compounds; A. Inorganic compounds; A. Thin films; D. Catalytic properties
1. Introduction The presence of highly biorecalcitrant organic contaminants in the hydrosphere due to industrial and intensive agricultural activities is of particular concern for the freshwater (surface and groundwater), coastal and marine environments [1]. With increasing amounts of chemical herbicides, contaminants in a water stream have become an important issue of worldwide concern because of the potential health hazards associated with the entry of these compounds into the food chain of humans and animals [2–4]. Also, the possible human health implications associated with the use of these waters for drinking are generally difficult to assess [5]. The Butachlor, N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide, is applied in agricultural fields to control weeds. Butachlor is a persistent pollutant in agricultural soil and watercourses [6]. Cultivation systems of a variety of crops * Corresponding author. Tel.: +98 21 22956126; fax: +98 21 22947537. E-mail address: [email protected] (N.M. Mahmoodi). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.08.031
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have tended to rely increasingly on the use of a number of agrochemicals for the maintenance of regular agricultural production. However, their residues sometimes affect the ecosystems and resulted in the pollution of crops [7]. In view of this, it is advisable to develop new technologies that promote the easy degradation of these biorecalcitrant organic compounds. A promising way to perform the mineralization of these type of substances is the application of advanced oxidation processes (AOP), that are characterized by the in situ production of hydroxyl radicals under mild experimental conditions [1]. Nanotechnology can provide us ways to purify the water resources by utilizing semiconductor nanoparticles as catalysts. Furthermore, the utilization of light and ultrasound to activate such nanoparticles opens up new ways to design green oxidation technologies for environmental remediation [8]. At heterogeneous nanophotocatalysis, semiconductor can act as sensitizer for light-reduced redox processes due to their electronic structure, which is characterized by filled valence band and an empty conduction band. When a photon with energy of hn matches or exceeds the band gap energy, Eg, of the semiconductor, an electron, ecb, is promoted from the valence band, vb, into the conduction band, cb, leaving a hole, hvb+ behind. Excited state conduction band electrons and valence band holes can recombine and dissipate the input energy as heat, get trapped in meta-stable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. The hvb+ is a strong oxidant, which can either oxidize a compound directly, or react with electron donors like water or hydroxide ions to form hydroxyl radicals, which react with pollutants such as dyes. Hydroxyl radicals react with organic pollutants leading to the total mineralization of most of them [9]. In large scale applications, the use of suspended nanoparticle requires the separation and recycling of catalyst from the treated water prior to the discharge, which is a time-consuming and expensive process. In addition, the depth of penetration of UV light is limited because of strong absorptions by both catalyst particles and dissolved pollutants [10]. Above problems can be avoided by immobilization of photocatalyst over suitable supports. Thus, the use of immobilized photocatalysts is gaining importance in the elimination of pollutants from water. The photocatalytic degradation and mineralization of dyes using immobilized TiO2 nanoparticle has already been established in our laboratory [11–15]. In the present article, Butachlor as a persistent herbicide pollutant in agricultural soil and watercourses was used as a model compound for photocatalytic degradation and mineralization. The objectives of the present study are (a) immobilization of titanium dioxide nanoparticles using simple and effective method, (b) the feasibility of TiO2 nanoparticle photocatalytic degradation and mineralization of Butachlor, in bench scale (5 L), (c) the identification of the main aliphatic carboxylic acids intermediates by IC technique and degradation of these intermediates, (d) the optimization of the operational parameters of the degradation process and (e) complete degradation of aromatic intermediates. 2. Experimental 2.1. Reagents Butachlor (95% purity) was obtained from SinoHarvest Corp. (China). The chemical structure and properties of Butachlor were shown in Fig. 1 and Table 1, respectively. HCOONa, Na2C2O4, Na2SO4 and NaNO3, NaCl, NaHCO3, Na2CO3 and H2O2 were purchased from Merck. Titanium dioxide nanoparticle (Degussa P25) was utilized as a photocatalyst. Its main physical data are as follows: average primary particle size around 30 nm, purity above 97% and with 80:20 anatase to rutile.
Fig. 1. The chemical structure of Butachlor.
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Table 1 Properties of Butachlor Parameter
Butachlor
IUPAC name Class Usage Empirical formula Formula weight (g/mol)
N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide Herbicide Weed control C17H26ClNO2 311
2.2. Degradation method of Butachlor Photocatalytic degradation processes were performed using a 5 L solution containing specified concentration of Butachlor. The solutions were first agitated under gentle air in the dark for 30 min to reach equilibrated condition. The initial concentration of Butachlor was 0.064 mM. The photocatalytic degradation processes were carried out at 298 K and natural pH (6). Samples were withdrawn from sample point at certain time intervals and analyzed for degradation. 2.3. Immobilization of titanium dioxide nanoparticle Several methods were used for the immobilization of titanium dioxide particles [16,17]. In this study, a simple and effective method was used for immobilization of TiO2 nanoparticles as follows: inner surfaces of reactor walls were cleaned with acetone and distilled water to remove any organic or inorganic material attached to or adsorbed on the surface and was dried in the air. A pre-measured mass of TiO2 nanoparticle was attached on the inner surfaces of reactor walls using a thin layer of a UV resistant polymer. Immediately after preparation, the inner surface reactor wall – polymer – TiO2 system was placed in the laboratory for at least 60 h for complete drying of the polymer [11–15]. 2.4. Photocatalytic reactor Experiments were carried out in a batch mode immersion rectangular immobilized photocatalytic reactor made of Pyrex glass, which is shown in Fig. 2. The radiation source was two UV-C lamps (15 W, Philips). A water pump and air pump were utilized for the transferring and aeration of polluted solution, respectively.
Fig. 2. Scheme of immobilized TiO2 nanoparticle photocatalytic reactor for photocatalytic degradation process.
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2.5. Analyses of Butachlor Degradation of Butachlor were checked and controlled by measuring the absorbance of Butachlor solutions at 265 nm at different time intervals by UV–vis CECIL 2021 spectrophotometer. Also, absorbance measurements of the samples at 254 nm were taken as an indication of the aromatic compounds content in the solution [18]. Ion Chromatograph (METROHM 761 Compact IC) was used to determine formate, acetate, oxalate, Cl and NO3 ions formed during the degradation and mineralization of Butachlor using a METROSEP Anion Dual 2, flow 0.8 mL/ min, 2 mM NaHCO3/1.3 mM Na2CO3 as eluent, temperature 20 8C, pressure 3.4 MPa and conductivity detector. pH meter (Hach) was used to determine the pH of solutions. 3. Results and discussions 3.1. Hydrogen peroxide effect Hydrogen peroxide is an important factor on the photocatalytic degradation processes, depending on its concentration and nature of reductants. The reduction of H2O2 at the conduction band of photocatalyst would produce hydroxyl radicals [19]. Fig. 3 shows the unconverted fraction of Butachlor (C/C0) versus irradiation time when different H2O2 concentrations were used. It is shown to be exponential to time at each concentration of H2O2. This means that the first order kinetics relative to Butachlor is operative. The correlation coefficient (R2) and degradation rate constants (k, min1) of Butachlor for the various H2O2 concentrations were shown in Table 2. Apparently, as H2O2 concentration increases from 0 to 3.5 mM, the degradation rate is greatly enhanced because more hydroxyl radicals are formed at higher hydrogen peroxide concentrations in solution. However, when H2O2 concentration is larger than 3.5 mM, the degradation rate of Butachlor slows down. This can be explained by the scavenging effect when using a higher H2O2 concentration on the further generation of hydroxyl radicals in aqueous solution. The overall stoichiometry for photocatalytic mineralization of Butachlor without H2O2 can be written as TiO2 ;hn
þ C17 H26 ClNO2 þ 47 2 O2 ! 17CO2 þ NO3 þ Cl þ 12H2 O þ 2H
(1)
40% of Butachlor was degraded after 80 min of irradiation time (Fig. 3). Also, the overall stoichiometry for photocatalytic mineralization of Butachlor in the presence of H2O2 can be written as TiO2 ;hn
þ C17 H26 ClNO2 þ 20O2 þ 7H2 O2 ! 17CO2 þ NO 3 þ Cl þ 19H2 O þ 2H
(2)
On the basis of this equation, 7 mol of H2O2 are theoretically needed to completely degrade 1 mol of Butachlor. In our case, the optimal [H2O2]/[Butachlor] molar ratio equals 55 mol. This is much larger than the theoretical value of 7, and it can be implied that an excess amount of H2O2 is needed to reach the maximum degradation of Butachlor. It
Fig. 3. Photocatalytic degradation of Butachlor with different concentration of hydrogen peroxide at different time intervals of irradiation (Butachlor: 0.064 mM, pH 6).
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Table 2 Parameters (k and R2) for the effect of different H2O2 concentrations on the degradation rate of Butachlor H2O2 (mM)
k (min1)
R2
0 1.5 2.5 3.5 4.5 5.5
0.007 0.0152 0.0204 0.0349 0.0371 0.0394
0.997 0.986 0.996 0.987 0.984 0.992
should be pointed out that it has been assumed in Eq. (2) that oxygen can play a dominant role in the destruction rate of Butachlor. If we assume that H2O2 can be used as the dominant oxidant, the required H2O2 will be 47 mol for 1 mol of Butachlor, according to the following chemical reaction: TiO2 ;hn
þ C17 H26 ClNO2 þ 47H2 O2 ! 17CO2 þ NO 3 þ Cl þ 59H2 O þ 2H
(3)
Therefore, our optimal [H2O2]/[Butachlor] molar ratio of 55 is only higher than the theoretical value. 3.2. Inorganic anion effect The occurrence of dissolved inorganic salts such as NaCl, NaNO3 and Na2SO4 is rather common in environmental natural water resources. These substances may compete for the active sites on the TiO2 surface or deactivate the photocatalyst and, subsequently, decrease the degradation rate of the target molecule. A major drawback resulting from the high reactivity and non-selectivity of OH is that it also reacts with non-target compounds present in the background water matrix, i.e. inorganic anions present in water. This results in a higher OH demand to accomplish the desired degree of degradation, or complete inhibition of advanced oxidation rate and efficiency [20,21]. To consider how the presence of dissolved inorganic anions on the photocatalytic degradation rate of Butachlor, we have chosen the NaCl, NaNO3 and Na2SO4 salts. The same amount (2 mM) of these salts was used. Fig. 4 shows the effects of anions on the photocatalytic degradation rate of Butachlor. The parameters k (rate constant) and R2 (correlation coefficient) of degradation process are shown in Table 3. Of the anionic species studied (NaCl, NaNO3 and Na2SO4), chloride exhibited the strongest inhibition effect followed by nitrate. The observed detrimental effect on the photocatalytic degradation of Butachlor obeyed the following order: SO42 < NO3 < Cl. Inhibition effects of anions can be explained as the reaction of positive holes and hydroxyl radical with anions, that behaved as h+ and OH scavengers resulting prolonged contaminant removal [20,21]. 3.3. Solution pH effect Since Butachlor to degrade can be at different pHs in aqueous solutions, comparative experiments were performed at different pH values (2–10). Also, the pH of solution is an important parameter in reaction taking place on
Fig. 4. The effect of anions on the photocatalytic degradation rate constants of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, salt: 2 mM).
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Table 3 Parameters (k and R2) for the effect of different anions on the degradation rate of Butachlor Anion
k (min1)
R2
No anion Sulfate Nitrate Chloride
0.0349 0.0171 0.0123 0.0105
0.987 0.988 0.987 0.986
semiconductor particle surfaces, since it influences the surface charge properties of the photocatalyst. The point of zero charge (pzc) is at pHpzc = 6.8 for the TiO2 particles [22–24]. The TiO2 surface is positively charged in acidic media (pH < 6.8). Therefore, an electrostatic attraction exists between the positively charged surface of the TiO2 and anionic species. As the pH of the system increases, the number of negatively charged sites increased. A negatively charged surface site on the TiO2 does not favor the adsorption of anionic species due to the electrostatic repulsion. However, the interpretation of pH effects on the efficiency of the photocatalytic degradation process is a difficult task, because the different reaction mechanisms such as hydroxyl radical attack, direct oxidation by positive hole and direct reduction by the electron in the conducting band can contribute to Butachlor degradation. The importance of each one depends on the substrate nature and pH [24–26]. As shown in Fig. 4 and Table 3, the sulfate anion has smallest inhibition effect on the degradation rate. Thus, H2SO4 and NaOH were used to pH adjustment of Butachlor solutions. The effect of pH on the degradation of Butachlor is shown in Fig. 5. The parameters k (rate constant) and R2 (correlation coefficient) of degradation process are shown in Table 4. 3.4. Degradation and mineralization of Butachlor Due to the complexity of the reactions occurring in photocatalytic treatments, it is difficult to indicate an exhaustive reaction scheme explaining the formation of all formed intermediates [26]. Destruction of the Butachlor should be evaluated as an overall degradation process, involving the degradation of both the Butachlor and its intermediates. Degradation of the aromatic intermediates was shown in Fig. 6. The lack of any absorbance in 254 nm after 140 min of irradiation time was indicative of the complete aromatic intermediates degradation [18]. Further hydroxylation of aromatic intermediates leads to the cleavage of the aromatic ring resulting in the formation of oxygen-containing aliphatic compounds [27]. Formate, acetate and oxalate were detected as important aliphatic carboxylic acid intermediates during the degradation of Butachlor (Fig. 7). The formation of carboxylic acids initially increased with the illumination time, and then dropped. Carboxylic acids can react directly with holes generating CO2 according to the ‘‘photo-Kolbe’’ reaction [28]: RCOO þ TiO2 ðhvb þ Þ ! TiO2 þ R þ CO2
(4)
Also, the photocatalytic mineralization of Butachlor implies the appearance of inorganic products, mainly anions, since hetero-atoms are generally converted into anions in which they are at their highest oxidation degree.
Fig. 5. The effect of pH on the photocatalytic degradation rate constants of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM).
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Table 4 Parameters (k and R2) for the effect of different pH values on the degradation rate of Butachlor pH
k (min1)
R2
2 4 6 10
0.0255 0.0345 0.0349 0.0175
0.982 0.987 0.987 0.990
Fig. 6. Disappearance of the aromatic intermediates during the photocatalytic degradation of Butachlor at 254 nm (A0 and A are initial and final absorbance, respectively, Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
Fig. 7. Formation and disappearance of aliphatic carboxylic acids in the solution during the photocatalytic degradation of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
Fig. 8. Evolution of NO3 and Cl anions in the solution during the photocatalytic degradation of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
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Fig. 9. The chemical structure of butenchlor.
Fig. 10. The chemical structure of diethatyl.
The Butachlor degradation leads to the conversion of organic carbon into harmless gaseous CO2 and that of N and Cl heteroatoms into inorganic ions, such as NO3 and Cl, respectively. Mineralization of Butachlor is reported for an irradiation period of 240 min. The formation of NO3 and Cl from Butachlor mineralization was shown in Fig. 8. The quantity of NO3 ions released (0.046 mM) is lower than that expected from stoichiometry (0.064 mM from Eq. (3)) indicating that N-containing species remain adsorbed in the photocatalyst surface or most probably, that significant quantities of N2 and/or NH3 have been produced and transferred to the gas phase. N2 evolution constitutes the ideal case for a decontamination reaction involving totally innocuous nitrogen-containing final product [20]. Also, the quantity of chloride ions released (0.06 mM) is lower than that expected from stoichiometry (0.064 mM from Eq. (3)) indicating that chloride remains adsorbed in the photocatalyst surface. 4. Conclusions Butachlor could be successfully degraded and mineralized by nanophotocatalysis in an immobilized TiO2 nanoparticle photocatalytic reactor using hydrogen peroxide. The degradation rate for Butachlor goes through a maximum when the concentration of the hydrogen peroxide increases from 0 to 3.5 mM and then it does not show appreciable change. Chloride exhibited the strongest inhibition effect on the Butachlor degradation followed by nitrate. The photocatalytic degradation kinetics follows a first order model for Butachlor. The formation of carboxylic acid intermediates (acetic, formic and oxalic) initially increased with the illumination time, and then dropped due to direct reaction with holes and generation of CO2 according to the photo-Kolbe reaction. Mineralization of Butachlor is identified by production of inorganic anions (chloride and nitrate). Thin-film coating of photocatalyst may resolve the problem of suspension system of Butachlor degradation. Nanophotocatalysis by immobilized titanium dioxide nanoparticle in the presence of hydrogen peroixde is able to treatment of Butachlor
Fig. 11. The chemical structure of pretilachlor.
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Fig. 12. The chemical structure of metolachlor.
Fig. 13. The chemical structure of acetochlor.
Fig. 14. The chemical structure of delachlor.
Fig. 15. The chemical structure of dimethchlor.
Fig. 16. The chemical structure of propisochlor.
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Fig. 17. The chemical structure of propachlor.
from polluted waters without using high pressure of oxygen or heating. Hence, this technique may be a viable one for treatment of large volume of water polluted by Butachlor. This work has discussed the photocatalytic degradation and mineralization experiments of only one chloroacetanilide herbicide (Butachlor) but this method could be used for the degradation of other chloroacetanilide herbicides such as butenchlor, diethatyl, pretilachlor, metolachlor, acetochlor, delachlor, dimethchlor, propisochlor and propachlor that have very similar chemical structures with Butachlor (Figs. 9–17, respectively). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
M.J. Farre, M.I. Franch, S. Malato, J.A. Ayllon, J. Peral, X. Domenech, Chemosphere 58 (2005) 1127–1133. M.S. Kim, C.S. Ryu, B.W. Kim, Water Res. 39 (2005) 525–532. O.R. Pal, A.K. Vanjara, Sep. Purif. Technol. 24 (2001) 167–172. E. Gonzalez-Pradas, M. Villafranca-Sanchez, A. Gallego-Campo, D. Urena-Amate, M. Fernandez-Perez, J. Chem. Technol. Biotechnol. 74 (1999) 49. L. Fava, P. Bottoni, A. Crobe, E. Funari, Chemosphere 41 (2000) 1503–1508. Y.L. Yu, Y.X. Chen, Y.M. Luo, X.D. Pan, Y.F. He, M.H. Wong, Chemosphere 50 (2003) 771–774. M. Karavangeli, N.E. Labrou, Y.D. Clonis, A. Tsaftaris, Biomol. Eng. 22 (2005) 121–128. P.V. Kamat, D. Meisel, C.R. Chim. 6 (2003) 999–1007. M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. A.K. Ray, A.A.C.M. Beenackers, AIChE J. 44 (2) (1998) 477–483. N.M. Mahmoodi, M. Arami, N. Yousefi Limaee, N. Salman Tabrizi, Chem. Eng. J. 112 (1–3) (2005) 191–196. N.M. Mahmoodi, M. Arami, N. Yousefi Limaee, N. Salman Tabrizi, J. Colloid Interf. Sci. 295 (1) (2006) 159–164. N.M. Mahmoodi, M. Arami, N. Yousefi, Limaee, J. Hazard. Mater. 133 (1–3) (2006) 113–118. N.M. Mahmoodi, M. Arami, J. Photochem. Photobiol. A: Chem. 182 (2006) 60–66. N.M. Mahmoodi, M. Arami, N. Yousefi Limaee., K. Gharanjig, F. Doulati Ardejani, Colloid Surf. A: Physicochem. Eng. Aspects. 2006, in Press. J.G. Yu, X.J. Zhao, Q.N. Zhao, Thin Solid Films 379 (2000) 7–14. H.G. Yu, S.C. Lee, C.H. Ao, J.G. Yu, J. Cryst. Growth 280 (2005) 612–619. D. Georgiou, P. Melidis, A. Aivasidis, K. Gimouhopoulos, Dyes Pigments 52 (2002) 69–78. C.M. So, M.Y. Cheng, J.C. Yu, P.K. Wong, Chemosphere 46 (2002) 905–912. I.K. Konstantinou, T.A. Albanis, Appl. Catal. B: Environ. 49 (2004) 1–14. I. Arsalan-Alaton, Color. Technol. 119 (2003) 345–353. F. Zhang, J. Zhao, T. Shen, H. Hidaka, E. Pelizzetti, N. Serpone, Appl. Catal. B: Environ. 15 (1998) 147–156. J.G. Yu, H.G. Yu, B. Cheng, X.J. Zhao, J.C. Yu, W.K. Ho, J. Phys. Chem. B. 107 (2003) 13871–13879. B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. B89 (2002) 303–317. W.Z. Tang, Z. Zhang, H. An, M.O. Quintana, D.F. Torres, Environ. Technol. 18 (1997) 1–12. I.K. Konstantinou, T.A. Albanis, Appl. Catal. B: Environ. 42 (2003) 319–335. K. Tanaka, S.M. Robledo, T. Hisanaga, R. Ali, Z. Ramli, W.A. Bakar, J. Mol. Catal. A: Chem. 144 (1999) 425–430. A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.M. Hermann, Appl. Catal. B: Environ. 31 (2001) 145–157.
Nanophotocatalysis using immobilized titanium dioxide nanoparticle Degradation and mineralization of water containing organic pollutant: Case study of Butachlor Niyaz Mohammad Mahmoodi a,*, Mokhtar Arami a,b, Nargess Yousefi Limaee a, Kamaladin Gharanjig a,b, Farahnaz Nourmohammadian a a
Colorant Manufacturing and Environmental Science Department, Institute for Colorants, Paints and Coatings, Tehran, Iran b Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran Received 10 January 2006; received in revised form 28 August 2006; accepted 30 August 2006 Available online 12 October 2006
Abstract In this paper, the degradation and mineralization of Butachlor in aqueous solution by nanophotocatalysis using immobilized TiO2 nanoparticles were investigated. Butachlor (N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide) is a persistent organic pollutant in agricultural soil and watercourses. A simple and effective method was used for immobilization of titanium dioxide nanoparticles. UV–vis and Ion Chromatography (IC) analyses were employed to obtain the details of the photocatalytic degradation and mineralization of Butachlor. The effects of operational parameters such as H2O2, inorganic anions (NO3, Cl and SO42) and pH were investigated. The lack of any absorbance in 254 nm was indicative of the complete degradation of aromatic intermediates. The mineralization of Butachlor was evaluated by monitoring of the formed inorganic anions (NO3 and Cl). Butachlor is effectively degraded following first order kinetics model. Results show that the immobilized titanium dioxide nanoparticle photocatalysis is an effective method for treatment Butachlor from contaminated water. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Organic compounds; A. Inorganic compounds; A. Thin films; D. Catalytic properties
1. Introduction The presence of highly biorecalcitrant organic contaminants in the hydrosphere due to industrial and intensive agricultural activities is of particular concern for the freshwater (surface and groundwater), coastal and marine environments [1]. With increasing amounts of chemical herbicides, contaminants in a water stream have become an important issue of worldwide concern because of the potential health hazards associated with the entry of these compounds into the food chain of humans and animals [2–4]. Also, the possible human health implications associated with the use of these waters for drinking are generally difficult to assess [5]. The Butachlor, N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide, is applied in agricultural fields to control weeds. Butachlor is a persistent pollutant in agricultural soil and watercourses [6]. Cultivation systems of a variety of crops * Corresponding author. Tel.: +98 21 22956126; fax: +98 21 22947537. E-mail address: [email protected] (N.M. Mahmoodi). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.08.031
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have tended to rely increasingly on the use of a number of agrochemicals for the maintenance of regular agricultural production. However, their residues sometimes affect the ecosystems and resulted in the pollution of crops [7]. In view of this, it is advisable to develop new technologies that promote the easy degradation of these biorecalcitrant organic compounds. A promising way to perform the mineralization of these type of substances is the application of advanced oxidation processes (AOP), that are characterized by the in situ production of hydroxyl radicals under mild experimental conditions [1]. Nanotechnology can provide us ways to purify the water resources by utilizing semiconductor nanoparticles as catalysts. Furthermore, the utilization of light and ultrasound to activate such nanoparticles opens up new ways to design green oxidation technologies for environmental remediation [8]. At heterogeneous nanophotocatalysis, semiconductor can act as sensitizer for light-reduced redox processes due to their electronic structure, which is characterized by filled valence band and an empty conduction band. When a photon with energy of hn matches or exceeds the band gap energy, Eg, of the semiconductor, an electron, ecb, is promoted from the valence band, vb, into the conduction band, cb, leaving a hole, hvb+ behind. Excited state conduction band electrons and valence band holes can recombine and dissipate the input energy as heat, get trapped in meta-stable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. The hvb+ is a strong oxidant, which can either oxidize a compound directly, or react with electron donors like water or hydroxide ions to form hydroxyl radicals, which react with pollutants such as dyes. Hydroxyl radicals react with organic pollutants leading to the total mineralization of most of them [9]. In large scale applications, the use of suspended nanoparticle requires the separation and recycling of catalyst from the treated water prior to the discharge, which is a time-consuming and expensive process. In addition, the depth of penetration of UV light is limited because of strong absorptions by both catalyst particles and dissolved pollutants [10]. Above problems can be avoided by immobilization of photocatalyst over suitable supports. Thus, the use of immobilized photocatalysts is gaining importance in the elimination of pollutants from water. The photocatalytic degradation and mineralization of dyes using immobilized TiO2 nanoparticle has already been established in our laboratory [11–15]. In the present article, Butachlor as a persistent herbicide pollutant in agricultural soil and watercourses was used as a model compound for photocatalytic degradation and mineralization. The objectives of the present study are (a) immobilization of titanium dioxide nanoparticles using simple and effective method, (b) the feasibility of TiO2 nanoparticle photocatalytic degradation and mineralization of Butachlor, in bench scale (5 L), (c) the identification of the main aliphatic carboxylic acids intermediates by IC technique and degradation of these intermediates, (d) the optimization of the operational parameters of the degradation process and (e) complete degradation of aromatic intermediates. 2. Experimental 2.1. Reagents Butachlor (95% purity) was obtained from SinoHarvest Corp. (China). The chemical structure and properties of Butachlor were shown in Fig. 1 and Table 1, respectively. HCOONa, Na2C2O4, Na2SO4 and NaNO3, NaCl, NaHCO3, Na2CO3 and H2O2 were purchased from Merck. Titanium dioxide nanoparticle (Degussa P25) was utilized as a photocatalyst. Its main physical data are as follows: average primary particle size around 30 nm, purity above 97% and with 80:20 anatase to rutile.
Fig. 1. The chemical structure of Butachlor.
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Table 1 Properties of Butachlor Parameter
Butachlor
IUPAC name Class Usage Empirical formula Formula weight (g/mol)
N-butoxymethyl-2-chloro-20 ,60 -diethylacetanilide Herbicide Weed control C17H26ClNO2 311
2.2. Degradation method of Butachlor Photocatalytic degradation processes were performed using a 5 L solution containing specified concentration of Butachlor. The solutions were first agitated under gentle air in the dark for 30 min to reach equilibrated condition. The initial concentration of Butachlor was 0.064 mM. The photocatalytic degradation processes were carried out at 298 K and natural pH (6). Samples were withdrawn from sample point at certain time intervals and analyzed for degradation. 2.3. Immobilization of titanium dioxide nanoparticle Several methods were used for the immobilization of titanium dioxide particles [16,17]. In this study, a simple and effective method was used for immobilization of TiO2 nanoparticles as follows: inner surfaces of reactor walls were cleaned with acetone and distilled water to remove any organic or inorganic material attached to or adsorbed on the surface and was dried in the air. A pre-measured mass of TiO2 nanoparticle was attached on the inner surfaces of reactor walls using a thin layer of a UV resistant polymer. Immediately after preparation, the inner surface reactor wall – polymer – TiO2 system was placed in the laboratory for at least 60 h for complete drying of the polymer [11–15]. 2.4. Photocatalytic reactor Experiments were carried out in a batch mode immersion rectangular immobilized photocatalytic reactor made of Pyrex glass, which is shown in Fig. 2. The radiation source was two UV-C lamps (15 W, Philips). A water pump and air pump were utilized for the transferring and aeration of polluted solution, respectively.
Fig. 2. Scheme of immobilized TiO2 nanoparticle photocatalytic reactor for photocatalytic degradation process.
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2.5. Analyses of Butachlor Degradation of Butachlor were checked and controlled by measuring the absorbance of Butachlor solutions at 265 nm at different time intervals by UV–vis CECIL 2021 spectrophotometer. Also, absorbance measurements of the samples at 254 nm were taken as an indication of the aromatic compounds content in the solution [18]. Ion Chromatograph (METROHM 761 Compact IC) was used to determine formate, acetate, oxalate, Cl and NO3 ions formed during the degradation and mineralization of Butachlor using a METROSEP Anion Dual 2, flow 0.8 mL/ min, 2 mM NaHCO3/1.3 mM Na2CO3 as eluent, temperature 20 8C, pressure 3.4 MPa and conductivity detector. pH meter (Hach) was used to determine the pH of solutions. 3. Results and discussions 3.1. Hydrogen peroxide effect Hydrogen peroxide is an important factor on the photocatalytic degradation processes, depending on its concentration and nature of reductants. The reduction of H2O2 at the conduction band of photocatalyst would produce hydroxyl radicals [19]. Fig. 3 shows the unconverted fraction of Butachlor (C/C0) versus irradiation time when different H2O2 concentrations were used. It is shown to be exponential to time at each concentration of H2O2. This means that the first order kinetics relative to Butachlor is operative. The correlation coefficient (R2) and degradation rate constants (k, min1) of Butachlor for the various H2O2 concentrations were shown in Table 2. Apparently, as H2O2 concentration increases from 0 to 3.5 mM, the degradation rate is greatly enhanced because more hydroxyl radicals are formed at higher hydrogen peroxide concentrations in solution. However, when H2O2 concentration is larger than 3.5 mM, the degradation rate of Butachlor slows down. This can be explained by the scavenging effect when using a higher H2O2 concentration on the further generation of hydroxyl radicals in aqueous solution. The overall stoichiometry for photocatalytic mineralization of Butachlor without H2O2 can be written as TiO2 ;hn
þ C17 H26 ClNO2 þ 47 2 O2 ! 17CO2 þ NO3 þ Cl þ 12H2 O þ 2H
(1)
40% of Butachlor was degraded after 80 min of irradiation time (Fig. 3). Also, the overall stoichiometry for photocatalytic mineralization of Butachlor in the presence of H2O2 can be written as TiO2 ;hn
þ C17 H26 ClNO2 þ 20O2 þ 7H2 O2 ! 17CO2 þ NO 3 þ Cl þ 19H2 O þ 2H
(2)
On the basis of this equation, 7 mol of H2O2 are theoretically needed to completely degrade 1 mol of Butachlor. In our case, the optimal [H2O2]/[Butachlor] molar ratio equals 55 mol. This is much larger than the theoretical value of 7, and it can be implied that an excess amount of H2O2 is needed to reach the maximum degradation of Butachlor. It
Fig. 3. Photocatalytic degradation of Butachlor with different concentration of hydrogen peroxide at different time intervals of irradiation (Butachlor: 0.064 mM, pH 6).
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Table 2 Parameters (k and R2) for the effect of different H2O2 concentrations on the degradation rate of Butachlor H2O2 (mM)
k (min1)
R2
0 1.5 2.5 3.5 4.5 5.5
0.007 0.0152 0.0204 0.0349 0.0371 0.0394
0.997 0.986 0.996 0.987 0.984 0.992
should be pointed out that it has been assumed in Eq. (2) that oxygen can play a dominant role in the destruction rate of Butachlor. If we assume that H2O2 can be used as the dominant oxidant, the required H2O2 will be 47 mol for 1 mol of Butachlor, according to the following chemical reaction: TiO2 ;hn
þ C17 H26 ClNO2 þ 47H2 O2 ! 17CO2 þ NO 3 þ Cl þ 59H2 O þ 2H
(3)
Therefore, our optimal [H2O2]/[Butachlor] molar ratio of 55 is only higher than the theoretical value. 3.2. Inorganic anion effect The occurrence of dissolved inorganic salts such as NaCl, NaNO3 and Na2SO4 is rather common in environmental natural water resources. These substances may compete for the active sites on the TiO2 surface or deactivate the photocatalyst and, subsequently, decrease the degradation rate of the target molecule. A major drawback resulting from the high reactivity and non-selectivity of OH is that it also reacts with non-target compounds present in the background water matrix, i.e. inorganic anions present in water. This results in a higher OH demand to accomplish the desired degree of degradation, or complete inhibition of advanced oxidation rate and efficiency [20,21]. To consider how the presence of dissolved inorganic anions on the photocatalytic degradation rate of Butachlor, we have chosen the NaCl, NaNO3 and Na2SO4 salts. The same amount (2 mM) of these salts was used. Fig. 4 shows the effects of anions on the photocatalytic degradation rate of Butachlor. The parameters k (rate constant) and R2 (correlation coefficient) of degradation process are shown in Table 3. Of the anionic species studied (NaCl, NaNO3 and Na2SO4), chloride exhibited the strongest inhibition effect followed by nitrate. The observed detrimental effect on the photocatalytic degradation of Butachlor obeyed the following order: SO42 < NO3 < Cl. Inhibition effects of anions can be explained as the reaction of positive holes and hydroxyl radical with anions, that behaved as h+ and OH scavengers resulting prolonged contaminant removal [20,21]. 3.3. Solution pH effect Since Butachlor to degrade can be at different pHs in aqueous solutions, comparative experiments were performed at different pH values (2–10). Also, the pH of solution is an important parameter in reaction taking place on
Fig. 4. The effect of anions on the photocatalytic degradation rate constants of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, salt: 2 mM).
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Table 3 Parameters (k and R2) for the effect of different anions on the degradation rate of Butachlor Anion
k (min1)
R2
No anion Sulfate Nitrate Chloride
0.0349 0.0171 0.0123 0.0105
0.987 0.988 0.987 0.986
semiconductor particle surfaces, since it influences the surface charge properties of the photocatalyst. The point of zero charge (pzc) is at pHpzc = 6.8 for the TiO2 particles [22–24]. The TiO2 surface is positively charged in acidic media (pH < 6.8). Therefore, an electrostatic attraction exists between the positively charged surface of the TiO2 and anionic species. As the pH of the system increases, the number of negatively charged sites increased. A negatively charged surface site on the TiO2 does not favor the adsorption of anionic species due to the electrostatic repulsion. However, the interpretation of pH effects on the efficiency of the photocatalytic degradation process is a difficult task, because the different reaction mechanisms such as hydroxyl radical attack, direct oxidation by positive hole and direct reduction by the electron in the conducting band can contribute to Butachlor degradation. The importance of each one depends on the substrate nature and pH [24–26]. As shown in Fig. 4 and Table 3, the sulfate anion has smallest inhibition effect on the degradation rate. Thus, H2SO4 and NaOH were used to pH adjustment of Butachlor solutions. The effect of pH on the degradation of Butachlor is shown in Fig. 5. The parameters k (rate constant) and R2 (correlation coefficient) of degradation process are shown in Table 4. 3.4. Degradation and mineralization of Butachlor Due to the complexity of the reactions occurring in photocatalytic treatments, it is difficult to indicate an exhaustive reaction scheme explaining the formation of all formed intermediates [26]. Destruction of the Butachlor should be evaluated as an overall degradation process, involving the degradation of both the Butachlor and its intermediates. Degradation of the aromatic intermediates was shown in Fig. 6. The lack of any absorbance in 254 nm after 140 min of irradiation time was indicative of the complete aromatic intermediates degradation [18]. Further hydroxylation of aromatic intermediates leads to the cleavage of the aromatic ring resulting in the formation of oxygen-containing aliphatic compounds [27]. Formate, acetate and oxalate were detected as important aliphatic carboxylic acid intermediates during the degradation of Butachlor (Fig. 7). The formation of carboxylic acids initially increased with the illumination time, and then dropped. Carboxylic acids can react directly with holes generating CO2 according to the ‘‘photo-Kolbe’’ reaction [28]: RCOO þ TiO2 ðhvb þ Þ ! TiO2 þ R þ CO2
(4)
Also, the photocatalytic mineralization of Butachlor implies the appearance of inorganic products, mainly anions, since hetero-atoms are generally converted into anions in which they are at their highest oxidation degree.
Fig. 5. The effect of pH on the photocatalytic degradation rate constants of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM).
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Table 4 Parameters (k and R2) for the effect of different pH values on the degradation rate of Butachlor pH
k (min1)
R2
2 4 6 10
0.0255 0.0345 0.0349 0.0175
0.982 0.987 0.987 0.990
Fig. 6. Disappearance of the aromatic intermediates during the photocatalytic degradation of Butachlor at 254 nm (A0 and A are initial and final absorbance, respectively, Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
Fig. 7. Formation and disappearance of aliphatic carboxylic acids in the solution during the photocatalytic degradation of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
Fig. 8. Evolution of NO3 and Cl anions in the solution during the photocatalytic degradation of Butachlor (Butachlor: 0.064 mM, H2O2: 3.5 mM, pH 6).
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Fig. 9. The chemical structure of butenchlor.
Fig. 10. The chemical structure of diethatyl.
The Butachlor degradation leads to the conversion of organic carbon into harmless gaseous CO2 and that of N and Cl heteroatoms into inorganic ions, such as NO3 and Cl, respectively. Mineralization of Butachlor is reported for an irradiation period of 240 min. The formation of NO3 and Cl from Butachlor mineralization was shown in Fig. 8. The quantity of NO3 ions released (0.046 mM) is lower than that expected from stoichiometry (0.064 mM from Eq. (3)) indicating that N-containing species remain adsorbed in the photocatalyst surface or most probably, that significant quantities of N2 and/or NH3 have been produced and transferred to the gas phase. N2 evolution constitutes the ideal case for a decontamination reaction involving totally innocuous nitrogen-containing final product [20]. Also, the quantity of chloride ions released (0.06 mM) is lower than that expected from stoichiometry (0.064 mM from Eq. (3)) indicating that chloride remains adsorbed in the photocatalyst surface. 4. Conclusions Butachlor could be successfully degraded and mineralized by nanophotocatalysis in an immobilized TiO2 nanoparticle photocatalytic reactor using hydrogen peroxide. The degradation rate for Butachlor goes through a maximum when the concentration of the hydrogen peroxide increases from 0 to 3.5 mM and then it does not show appreciable change. Chloride exhibited the strongest inhibition effect on the Butachlor degradation followed by nitrate. The photocatalytic degradation kinetics follows a first order model for Butachlor. The formation of carboxylic acid intermediates (acetic, formic and oxalic) initially increased with the illumination time, and then dropped due to direct reaction with holes and generation of CO2 according to the photo-Kolbe reaction. Mineralization of Butachlor is identified by production of inorganic anions (chloride and nitrate). Thin-film coating of photocatalyst may resolve the problem of suspension system of Butachlor degradation. Nanophotocatalysis by immobilized titanium dioxide nanoparticle in the presence of hydrogen peroixde is able to treatment of Butachlor
Fig. 11. The chemical structure of pretilachlor.
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Fig. 12. The chemical structure of metolachlor.
Fig. 13. The chemical structure of acetochlor.
Fig. 14. The chemical structure of delachlor.
Fig. 15. The chemical structure of dimethchlor.
Fig. 16. The chemical structure of propisochlor.
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Fig. 17. The chemical structure of propachlor.
from polluted waters without using high pressure of oxygen or heating. Hence, this technique may be a viable one for treatment of large volume of water polluted by Butachlor. This work has discussed the photocatalytic degradation and mineralization experiments of only one chloroacetanilide herbicide (Butachlor) but this method could be used for the degradation of other chloroacetanilide herbicides such as butenchlor, diethatyl, pretilachlor, metolachlor, acetochlor, delachlor, dimethchlor, propisochlor and propachlor that have very similar chemical structures with Butachlor (Figs. 9–17, respectively). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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