Naik et al 2008 - Photocatalytic degradation of the herbicide pendimethalin using nanoparticles of BaTiO3- TiO2 prepared by gel to crystalline conversion method - A kinetic approach

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<p>This article was downloaded by: [McGill University Library]</p><p>On: 09 January 2013, At: 04:46</p><p>Publisher: Taylor & Francis</p><p>Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,</p><p>37-41 Mortimer Street, London W1T 3JH, UK</p><p>Journal of Environmental Science and Health, Part B:</p><p>Pesticides, Food Contaminants, and Agricultural Wastes</p><p>Publication details, including instructions for authors and subscription information:</p><p>http://www.tandfonline.com/loi/lesb20</p><p>Photocatalytic degradation of the herbicide</p><p>pendimethalin using nanoparticles of BaTiO3/TiO2</p><p>prepared by gel to crystalline conversion method: A</p><p>kinetic approach</p><p>Lakshmipathi Naik. Gomathi Devi a & Gantigaiah Krishnamurthy a</p><p>a Department of Post Graduate Studies in Chemistry, Bangalore University, Bangalore, India</p><p>Version of record first published: 20 Sep 2008.</p><p>To cite this article: Lakshmipathi Naik. Gomathi Devi & Gantigaiah Krishnamurthy (2008): Photocatalytic degradation of</p><p>the herbicide pendimethalin using nanoparticles of BaTiO3/TiO2 prepared by gel to crystalline conversion method: A kinetic</p><p>approach, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 43:7,</p><p>553-561</p><p>To link to this article: http://dx.doi.org/10.1080/03601230802234351</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions</p><p>This article may be used for research, teaching, and private study purposes. Any substantial or systematic</p><p>reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to</p><p>anyone is expressly forbidden.</p><p>The publisher does not give any warranty express or implied or make any representation that the contents</p><p>will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should</p><p>be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,</p><p>proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in</p><p>connection with or arising out of the use of this material.</p><p>Journal of Environmental Science and Health Part B (2008) 43, 553–561</p><p>Copyright C Taylor & Francis Group, LLC</p><p>ISSN: 0360-1234 (Print); 1532-4109 (Online)</p><p>DOI: 10.1080/03601230802234351</p><p>Photocatalytic degradation of the herbicide pendimethalin</p><p>using nanoparticles of BaTiO3/TiO2 prepared by gel to</p><p>crystalline conversion method: A kinetic approach</p><p>LAKSHMIPATHI NAIK. GOMATHI DEVI and GANTIGAIAH KRISHNAMURTHY</p><p>Department of Post Graduate Studies in Chemistry, Bangalore University, Bangalore, India</p><p>Photocatalytic degradation of the herbicide, pendimethalin (PM) was investigated with BaTiO3/TiO2 UV light system in the presence</p><p>of peroxide and persulphate species in aqueous medium. The nanoparticles of BaTiO3 and TiO2 were obtained by gel to crystallite</p><p>conversion method. These photo catalysts are characterized by energy dispersive x-ray analysis (EDX), scanning electron microscope</p><p>(SEM), x-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) adsorption isotherm and reflectance spectral studies. The</p><p>quantum yields for TiO2 and BaTiO3 for the degradation reactions are 3.166 Einstein m−2 s−1 and 2.729 Einstein m−2 s−1 and catalytic</p><p>efficiencies are 6.0444 × 10−7 mg−2h−1L2 and 5.403 × 10−7 mg−2h−1L2, respectively as calculated from experimental results. BaTiO3</p><p>exhibited comparable photocatalytic efficiency in the degradation of pendimethalin as the most widely used TiO2 photocatalyst. The</p><p>persulphate played an important role in enhancing the rate of degradation of pendimethalin when compared to hydrogen peroxide.</p><p>The degradation process of pendimethalin followed the first-order kinetics and it is in agreement with Langmuir-Hinshelwood</p><p>model of surface mechanism. The reason for high stability of pendimethalin for UV-degradation even in the presence of catalyst and</p><p>oxidizing agents were explored. The higher rate of degradation was observed in alkaline medium at pH 11. The degradation process was</p><p>monitored by spectroscopic techniques such as ultra violet-visible (UV-Vis), infrared (IR) and gas chromatography mass spectroscopy</p><p>(GC-MS). The major intermediate products identified were: N-propyl-2-nitro-6-amino-3, 4-xylidine, (2, 3-dimethyl-5-nitro-6-hydroxy</p><p>amine) phenol and N-Propyl-3, 4-dimethyl-2, 6-dinitroaniline by GC-MS analysis and the probable reaction mechanism has been</p><p>proposed based on these products.</p><p>Keywords: Photocatalytic degradation; pendimethalin; TiO2; BaTiO3; spectroscopic analysis.</p><p>Introduction</p><p>Pendimethalin is a selective herbicide used to control many</p><p>grass and broadleaf weeds in crops such as corn, cotton,</p><p>peanuts, potatoes, rice, sorghum, sunflower, and tobacco.</p><p>It is used as both pre-emergence and early post-emergence.</p><p>Pendimethalin is strongly adsorbed by a large variety of</p><p>soils.[1−3] Binding capacity of Pendimethalin on the soil in-</p><p>creases in the presence of soil organic matter and the pres-</p><p>ence of clay enhances its binding. It is least soluble in wa-</p><p>ter, and thus will not leach appreciably in most soils.[1−3]</p><p>Pendimethalin is absorbed by plant roots and shoots, and</p><p>inhibits cell division and cell elongation.[1] Pendimethalin</p><p>is not absorbed by the leaves of grasses, and only very small</p><p>Address correspondence to L. Gomathi Devi, Department of Post</p><p>Graduate Studies in Chemistry, Central College Campus, Dr. B.R.</p><p>Ambedkar Veedi, Bangalore University, Bangalore-560 001, In-</p><p>dia. E-Mail: gomatidevi naik@yahoo.co.in</p><p>Received December 10, 2007.</p><p>amounts are taken up by plants from the soil. Residues on</p><p>crops at harvest are usually below detectable levels (0.05</p><p>ppm).[1]</p><p>The effect of pendimethalin on the green alga Protosiphon</p><p>botryoides was reported[4] as the specific growth rate, cell</p><p>number and chlorophyll-A level significantly decrease with</p><p>increasing pendimethalin concentrations, while protein and</p><p>carbohydrate contents increase significantly. On the other</p><p>hand, photosynthetic activity decreases whereas dark res-</p><p>piration increases with high pendimethalin concentrations.</p><p>Increasing nitrate and phosphate levels led to a decrease in</p><p>cell number and chlorophyll A at higher concentrations of</p><p>pendimethalin.</p><p>Pendimethalin is Synonymous with: N-(1-Ethylpropyl)-</p><p>3,4-dimethyl-2,6-dinitrobenzenamine; N-(1-Ethylpropyl)-</p><p>2,6-dinitro-3,4-xylidine; Herbadox; Pay-off; Penoxalin;</p><p>Penoxaline; Penoxyn; Phenoxalin; Prowl; Stomp, Stomp-</p><p>300D; Stomp-300E; Tendimethalin , AC 92553. It is an eye</p><p>irritant. Ashton and Monaco[1] reported that it is toxic and</p><p>has LD50 (96 h) for rainbow trout and bluegill sunfish of</p><p>138 µg/L and 199 µg/L, respectively. The acute oral LD50</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>554 Devi and Krishnamurthy</p><p>of the technical product for male and female rats is 1,250</p><p>and 1050 mg/kg, respectively.[2]</p><p>The removal of pesticides from runoff water from agricul-</p><p>tural lands is a challenging area of environmental pollution</p><p>control. Several techniques such as biological treatments,</p><p>adsorption on activated carbon, application of destructive</p><p>techniques leading to decomposition of pollutant molecules</p><p>etc., are found to be less effective. A very promising</p><p>technique like advanced oxidation process (AOP), which</p><p>includes heterogeneous photocatalysis, is being used effi-</p><p>ciently in this pursuit. This allows the complete mineral-</p><p>ization of a great variety of organic compounds present</p><p>especially at very low concentration in ppm or ppb levels.[5]</p><p>The photo catalysis with a semiconductor catalyst such as</p><p>TiO2, BaTiO3 etc., is based on the formation of electron-</p><p>hole pairs under irradiation of light of appropriate wave-</p><p>length. The electrons leads to the formation of superoxide</p><p>anions in the presence of oxygen[6] and the holes oxidize</p><p>the adsorbed organic substrates or react with water leading</p><p>to formation of hydroxyl radicals, which are very efficient</p><p>oxidizing agent</p><p>leading to mineralization of herbicides.[7]</p><p>Titanium dioxide is a very active photocatalyst stable in</p><p>most chemical environments, inexpensive and of low bio-</p><p>logical toxicity. These properties justify its use in several in-</p><p>dustrial applications, including water remediation through</p><p>the degradation of organic and inorganic contaminants.</p><p>Many TiO2 formulations are developed and used as cata-</p><p>lysts. These photocatalysts exhibit different photo efficien-</p><p>cies. In general TiO2 is the most studied photochemically</p><p>active catalyst.[8]</p><p>However in the present research work, an attempt has</p><p>been made to use another wide band gap semiconductor</p><p>BaTiO3, a perovskite material, as a photocatalyst. Semi-</p><p>conductor materials tested so far suffer from one prob-</p><p>lem or another and their efficiencies are 1–6% or less. The</p><p>only semiconductor photocatalyst proven to be stable in an</p><p>aqueous environment are wide band gap oxides (Eg >3 eV).</p><p>In this context TiO2, the most studied and best responsive</p><p>material, is taken and a comparative study is made by using</p><p>a perovskite titanate (BaTiO3).</p><p>The purpose of this study was to investigate the photo-</p><p>catalytic degradation of the herbicide, pendimethalin, using</p><p>nanoparticles of BaTiO3/TiO2. Suitable experimental con-</p><p>ditions for the degradation of pendimethalin in contami-</p><p>nated water were determined and the intermediate degra-</p><p>dation products were identified.</p><p>Materials and methods</p><p>Materials</p><p>Pendimethalin is an orange-yellow crystalline solid belong-</p><p>ing to the dinitroaniline family The pesticide sample of 95%</p><p>pure pendimethalin was obtained from Rallis India Lim-</p><p>ited, an Agrochemical Research Center, Bangalore, India.</p><p>The pesticide sample was used as obtained. The structure of</p><p>NHCH(CH2CH3)2</p><p>CH3</p><p>CH3</p><p>NO2</p><p>NO2</p><p>Fig. 1. Structure of pendimethalin.</p><p>pendimethalin is given in the Figure 1. It has the Molecular</p><p>formula C13H19N3O4 and the Formula weight is 281.31.</p><p>The chemicals such as titanium tetrachloride, am-</p><p>monium hydroxide, sulphuric acid, barium hydroxide,</p><p>ammonium persulphate, hydrogen peroxide, hydrochloric</p><p>acid and sodium hydroxide used in this research are all E-</p><p>Merck chemicals. The solutions are prepared using double</p><p>distilled water.</p><p>Photoreactor and light source</p><p>The Photocatalytic degradation experiments are carried out</p><p>in a reaction cell of circumference 34 cm of 1 liter capacity</p><p>with the exposure area of 92 cm2. A medium pressure, 125</p><p>watt mercury vapor lamp is used as a light source whose</p><p>wavelength peaks in the range 350 to 400 nm. The photon</p><p>flux is found to be 7.75 mW/cm2as determined by ferriox-</p><p>alate actionometry. The light source is designed for the di-</p><p>rect exposure into the reaction solution with inset cooling</p><p>fan. The experimental set up is associated with a magnetic</p><p>stirrer. The reaction vessel is mounted on the magnetic stir-</p><p>rer exposed directly to the light at a distance of 28.5 cm</p><p>from the lamp.</p><p>Photocatalytic degradation procedure</p><p>Photocatalytic degradation of PM has been carried out un-</p><p>der UV-light in the presence of TiO2/BaTiO3 catalyst sus-</p><p>pensions. The reaction solution of pendimethalin in 10 ppm</p><p>concentration was prepared using distilled water in 400 mL</p><p>quantity. About 300 mg of catalyst powder was suspended</p><p>in the solution. The reaction solution in all the experiments</p><p>was stirred for 5–10 min at the rate of 120 rpm, before the</p><p>start of illumination to facilitate the mixing and to establish</p><p>adsorption equilibrium. The experimental temperature was</p><p>maintained around 25 ± 3◦C. A series of photodegradation</p><p>experiments under different conditions was performed. The</p><p>concentration of PM remaining at any given time was mon-</p><p>itored by UV-visible spectrophotometric analysis. The in-</p><p>termediates formed during the degradation were analyzed</p><p>by IR and GC-MS techniques.</p><p>Results and discussion</p><p>Photocatalyst preparation and characterization</p><p>The semiconductor photocatalyst powders of TiO2 and</p><p>BaTiO3 are prepared by gel to crystalline conversion</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>Degradation of pendimethalin using BaTiO3/TiO2 555</p><p>method to yield nanoparticles as reported earlier.[9,10]</p><p>BaTiO3 is obtained by reaction of gels of hydrated tita-</p><p>nia with barium hydroxide as reported by Padmini et al.[10]</p><p>The recovered solids of both the catalysts were oven dried</p><p>at 105 to 120◦C. Thus the obtained fine-grained powders of</p><p>TiO2 and BaTiO3 were further heat treated at 600◦C and</p><p>450◦C respectively for about 4 hours.</p><p>The X-ray diffractograms for the finely ground</p><p>TiO2/BaTiO3 photocatalyst powders were obtained using</p><p>Phillips PW 1050/70/76 X-ray diffractometer which is op-</p><p>erated at 30 kV and 20 mA. CuKα radiation was used with</p><p>a nickel filter. The scanning range employed was 2θ = 5◦–</p><p>85◦. The normal scanning speed was 2◦/min and the check</p><p>speed being 2 mm/min. The crystallite size is obtained us-</p><p>ing Scherrer’s equation relating the pure diffraction breadth</p><p>(half-band width) to crystallite size normal to plane hkl.</p><p>Half bandwidth depends on crystallite size and micro strain</p><p>in the lattice.[11] The calculated crystallite size of anatase</p><p>TiO2 is 17.93 nm and that of BaTiO3 is 19.24 nm. The</p><p>particle size is obtained by SEM analysis of the samples</p><p>using JEOL (JSM- 840 A) scanning electron microscope.</p><p>The range of average particle size for TiO2 obtained from</p><p>the SEM analysis is 57–251 nm and that for BaTiO3 is 85–</p><p>286 nm.</p><p>The band gap energies were calculated using UV-Visible</p><p>diffused reflectance study, which reveals the λmax for anatase</p><p>TiO2 at 413 nm and that for BaTiO3 at 372 nm. The corre-</p><p>sponding band gaps were 3.0 eV and 3.33 eV respectively.</p><p>The energy dispersive X-ray (EDX) analysis was done</p><p>using a JSM-840A EDX analyzer. The Ti/O atom % was</p><p>33.25/ 66.75 and that of Ba/Ti/O is 19.63/20.25/60.12.</p><p>The result shows the perfect matching of atom percentage</p><p>composition of TiO2 and BaTiO3. The BET method of</p><p>analysis has been adopted for determining the surface area</p><p>and pore size distribution of these catalyst powders for</p><p>which NOVA-1000 High gas sorption Analyzer (Quanta</p><p>chrome corporation, USA) version 3.70 was used. The</p><p>normal and specific surface areas obtained for TiO2 are</p><p>8.41 m2 and 50.54 m2/g and that for BaTiO3 were 9.64 m2</p><p>and 34.14 m2/g respectively. The average pore diameters</p><p>were found to be 147.18 Å and 104.95 Å, respectively.</p><p>Catalytic efficiency and quantum yield</p><p>The catalytic efficiency of TiO2 and BaTiO3 for the degra-</p><p>dation of pendimethalin is determined using the Equa-</p><p>tion 1. Different amounts of catalysts were taken keeping</p><p>the concentration of PM and other experimental conditions</p><p>constant.</p><p>Ceff = Apparent rate constant for the degradation of PM</p><p>Concentration of the catalyst</p><p>= k(mg−1h−1l)</p><p>Ccat(mgl−1)</p><p>Table 1. Experimentally calculated catalytic efficiency and</p><p>quantum yields for TiO2 and BaTiO3.</p><p>Catalytic efficiency Quantum yield</p><p>Photocatalyst (in mg−2h−1L2) (in Einstein m−2s−1)</p><p>TiO2 6.0444 × 10−7 3.166</p><p>BaTiO3 5.403 × 10−7 2.729</p><p>Where Ceff is the catalytic efficiency, k is the rate con-</p><p>stant and Ccat is concentration of the catalyst. The catalytic</p><p>efficiencies for TiO2 and BaTiO3 are given in the Table 1.</p><p>The quantum yield () is determined using the Equation</p><p>2, under similar experimental conditions as follows.</p><p> = Rate of a chemical reaction</p><p>Intensity of light absorbed</p><p>= v</p><p>I</p><p>Einstein m−2s−1</p><p>Where ‘v’ is the rate of chemical reaction and I is the inten-</p><p>sity of the light used. There is often a lower limit for the true</p><p>quantum yield (number of molecules reacted divided by the</p><p>number of photons absorbed) due to the reflection of UV-</p><p>light or scattering by the catalyst particles.[12] The quantum</p><p>yield values calculated for the degradation of pendimethalin</p><p>in the presence of both the catalysts are given (Table 1).</p><p>Influence of electron acceptors</p><p>The photocatalytic degradation experiment was carried out</p><p>in the presence of the oxidizing agent alone (in the absence</p><p>of catalyst and UV-light irradiation) which shows negligi-</p><p>ble degradation.</p><p>Efficient degradation takes place in the</p><p>presence of catalyst and oxidizing agents under UV light.</p><p>The oxidizing agents used were H2O2 and (NH4)2S2O8</p><p>which act as electron acceptors. They undergo photoly-</p><p>sis to give hydroxyl and persulphate radicals. They inhibit</p><p>the recombination of photogenerated electron-hole pairs</p><p>thereby enhancing the degradation rate. The .OH radicals</p><p>formed during illumination are extremely potent oxidizing</p><p>agents having the oxidation potential of 2.80 V. Some of</p><p>the reactions of ammonium persulphate in the heteroge-</p><p>neous system under UV-light illumination are given by the</p><p>Equations 3–6.</p><p>(NH4)2S2O8 → 2NH+</p><p>4 + S2O2−</p><p>8</p><p>S2O2−</p><p>8 + e−</p><p>CB → SO2−</p><p>4 + SO−</p><p>4</p><p>SO−</p><p>4 + eCB− → SO2−</p><p>4</p><p>SO−</p><p>4 + H2O → SO2−</p><p>4 + OH + H+</p><p>The SO.−</p><p>4 is a strong oxidant (E0 = 2.6 eV) involved in</p><p>different reactions such as abstraction of hydrogen atom,</p><p>adding to aromatic compounds and removing one elec-</p><p>tron from certain neutral molecules[13] and also acts as a</p><p>trap for photogenerated electrons as shown in the above</p><p>schemes.</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>556 Devi and Krishnamurthy</p><p>The mechanism most commonly accepted for the pho-</p><p>tolysis of H2O2 is the cleavage of the molecules into hy-</p><p>droxyl radicals with a quantum yield of two .OH radicals</p><p>formed per quantum of radiation absorbed[14] as shown in</p><p>Equation 7.</p><p>H2O2+hν → 2 · OH</p><p>H2O2+e− → OH + OH−</p><p>H2O2+OH → HO2. + H2O</p><p>The H2O2 can capture conduction band electron accord-</p><p>ing to Equation 8, producing hydroxyl radical, and thereby</p><p>hinders the electron-hole recombination process leading to</p><p>an increase in the rate of photocatalytic degradation up to</p><p>certain concentration. A further increase in the concentra-</p><p>tion of H2O2 has no effect on the rate of degradation and</p><p>reaches a constant value. This is due to quenching of .OH</p><p>by H2O2 itself according to the Equation 9, and formation</p><p>of perhydroxyl radical. The perhydroxyl radical is a less</p><p>reactive species than .OH.[13]</p><p>The degradation rate remains constant above 100 ppm</p><p>of hydrogen peroxide but with ammonium persulphate it</p><p>increases as concentration increases and comparatively a</p><p>higher rate is observed in the presence of ammonium per-</p><p>sulphate. This could be due to the pH of a solution which</p><p>influences the band edge positions of the semiconductor.</p><p>In the presence of hydrogen peroxide the pH of the solu-</p><p>tion was about 6.7, which is almost neutral, whereas the</p><p>pH with ammonium persulphate was 6.0. It can also be</p><p>due to different oxidation potentials (E0) of H2O2 (1.78</p><p>V) and (NH4)2S2O8 (2.01 V).[15] The illumination periods</p><p>for complete mineralization of pendimethalin with TiO2</p><p>and BaTiO3 catalysts in the presence of oxidants are given</p><p>(Table 2). The data given shows that the degradation time</p><p>with (NH4)2S2O8 at ∼1 × 103 ppm concentration is only 2</p><p>h in the presence of both the catalysts but, it is more than</p><p>6 h with H2O2 at similar concentration. Though the ef-</p><p>ficient degradation is achieved at higher concentrations of</p><p>(NH4)2S2O8, the optimum concentration of about 100 ppm</p><p>is used because, the use of very high concentration loses its</p><p>practical meaning.</p><p>Table 2. Degradation time for the experiments with different</p><p>initial pH conditions.</p><p>Degradation time (h)</p><p>pH TiO2 BaTiO3</p><p>2 8 9</p><p>5 10 10</p><p>8 10.5 7.25</p><p>11 4 .75</p><p>Effect of pH</p><p>The photocatalytic degradation experiments were con-</p><p>ducted at different pHs ranging from 2 to 11. The experi-</p><p>mental runs carried out at pH 2 (adjusted using 0.1M HCl),</p><p>pH 5.6 (as such without adjustment) and pH 11.0 (adjusted</p><p>with 0.1M NaOH solution) in TiO2/(NH4)2S2O8 system</p><p>takes 7, 9 and 4 hours of irradiation for complete min-</p><p>eralization respectively. The gradual decrease in the pH</p><p>was observed during the process of irradiation in all the</p><p>pH experiments. The highest rate of degradation in the</p><p>alkaline medium at pH = 11 is due to the formation of</p><p>surface-bound .OH radicals and also due to the highest</p><p>rate of .OH radical attack on the substrate molecules (1.3 ×</p><p>10−8M−1s−1). The other reasons may be the shift in band</p><p>edge positions of semiconductor catalyst to more reduc-</p><p>ing potential on increasing pH and also the surface charge,</p><p>which determines the adsorption of the species on to the</p><p>catalyst surface. A negligible influence on the rate of degra-</p><p>dation is observed in the pH range 6.5–8 (pHZPC of TiO2)</p><p>due to neutral surface charges on TiO2.</p><p>The pH of BaTiO3 suspension in pendimethalin solution</p><p>in the presence of (NH4)2S2O8 and H2O2 are 8.0 and 8.6</p><p>respectively. The high pH in BaTiO3 suspensions can be</p><p>attributed to the presence of more surface hydroxyls and</p><p>more negative surface charges on the catalyst surface. The</p><p>degradation rate increases with increase in pH. The rate</p><p>is found to be high at pH 11 with ammonium persulphate.</p><p>The higher rate of SO.−</p><p>4 formation and its hydrogen abstrac-</p><p>tion is responsible for the observed higher rate of degrada-</p><p>tion in the presence of (NH4)2S2O8. The degradation time</p><p>for the experiments at different pH conditions are given</p><p>(Table 3).</p><p>Rate of degradation process</p><p>The degradation reaction is basically followed by the dis-</p><p>appearance of PM by UV-Vis absorbance spectrum. From</p><p>Table 3. Time taken for the complete mineralization of herbicide</p><p>in the experiments with different concentrations of H2O2 and</p><p>(NH4)2S2O8.</p><p>Degradation time (h) with</p><p>Concentration of</p><p>(NH4)2S2O8/H2O2 TiO2 BaTiO3</p><p>50 ppm H2O2 11 12</p><p>100 ppm H2O2 7.25 7.5</p><p>200 ppm H2O2 6 7.5</p><p>500 ppm H2O2 6 7.25</p><p>1000 ppm H2O2 6 7</p><p>50 ppm (NH4)2S2O8 10 11</p><p>100 ppm (NH4)2S2O8 6 6.25</p><p>228.2 ppm (NH4)2S2O8 4 5</p><p>456.4 ppm (NH4)2S2O8 3 3.5</p><p>1141 ppm (NH4)2S2O8 2 2</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>Degradation of pendimethalin using BaTiO3/TiO2 557</p><p>Fig. 2. The plot of C/C0 versus the degradation time in which the curves a-g correspond to various degradation experiments: a)</p><p>is for the experiment without catalyst and oxidizing agents, b) for the experiment with TiO2 catalyst, c) for the experiment with</p><p>TiO2 and ammonium persulphate, d) for the experiment with TiO2 and Hydrogen peroxide, e) for the experiment with BaTiO3</p><p>catalyst, f) for the experiment with BaTiO3 catalyst and ammonium persulphate and g) for the experiment with BaTiO3 and hydrogen</p><p>peroxide.</p><p>the absorbance the concentration of PM remaining at any</p><p>given time is determined by using the calibration curve. The</p><p>residual concentration C/C0, at different time intervals is</p><p>calculated. Figure 2 is a plot of C/C0 versus irradiation</p><p>time, where C0 is the initial concentration and C is the con-</p><p>centration at any instant of time. The time taken for the</p><p>degradation of pendimethalin (for >90% of degradation)</p><p>in the absence of catalyst and oxidizing agent is very high,</p><p>about 30 h. In the presence of catalyst the residual concen-</p><p>tration decreases at a faster rate. The rate of degradation is</p><p>much faster in the presence of oxidizing agents (NH4)2S2O8</p><p>and H2O2.</p><p>The degradation rate is insignificant when only oxi-</p><p>dizing agent is used (in the absence of catalyst). A lin-</p><p>ear decrease in the concentration of PM is observed in</p><p>the presence of oxidizing agent and catalyst. The rate of</p><p>degradation in TiO2/(NH4)2S2O8 and TiO2/H2O2 sys-</p><p>tems are 0.304 h−1 and 0.248 h−1 respectively and with</p><p>BaTiO3/(NH4)2S2O8 and BaTiO3/H2O2 are 0.278 h−1 and</p><p>0.227 h−1 respectively. However the complete mineraliza-</p><p>tion of pendimethalin takes a prolonged irradiation. As</p><p>per the results, it is due to the presence of a greater more</p><p>number of substitutes such as amino, methyl and nitro</p><p>groups and formation of a number of sequential stable in-</p><p>termediates during the irradiation. A general reaction for</p><p>photocatalytic degradation of pendimethalin can be given</p><p>by the Equation 10.</p><p>C13H19N3O4(aq) + 161/2O2</p><p>hν + TiO2/BaTiO3</p><p>−−−−−−−−−→ [Intermediates]</p><p>[Intermediates] → 13CO2 + 6H2O + 2HNO2 + NH4OH</p><p>Order of the degradation process</p><p>The photocatalytic degradation of pendimethalin follows</p><p>the first-order kinetics</p><p>with respect to its concentration in</p><p>the presence of ammonium persulphate in which the con-</p><p>centration of the latter is kept constant. The rate of degra-</p><p>dation is found to vary in the manner as that in the simple</p><p>Langmuir–adsorption isotherm, Equation 11, as shown in</p><p>the Figure 3.</p><p>Surface reactions involving single-adsorbed molecules</p><p>are the unimolecular reactions, which can be considered</p><p>in terms of the Langmuir adsorption mechanism. The rate</p><p>‘v’ is proportional to the θ , fraction of surface covered and</p><p>0</p><p>1</p><p>2</p><p>3</p><p>4</p><p>5</p><p>6</p><p>7</p><p>8</p><p>9</p><p>10</p><p>0 5 10 15 20 25 30</p><p>[PM]</p><p>R</p><p>a</p><p>te</p><p>o</p><p>f</p><p>d</p><p>e</p><p>g</p><p>ra</p><p>d</p><p>a</p><p>ti</p><p>o</p><p>n</p><p>,v</p><p>(</p><p>h</p><p>-1</p><p>)</p><p>Fig. 3. Plot of rate of degradation versus concentration of</p><p>pendimethalin (PM).</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>558 Devi and Krishnamurthy</p><p>Fig. 4. Plot of log v0 versus log C0 in which the slope of the straight</p><p>line gives order of the degradation reaction.</p><p>thus is</p><p>v = kdegθ = kdegKadsC</p><p>1 + KadsC</p><p>Where C is the concentration of pendimethalin, kdeg is the</p><p>apparent degradation rate constant and Kads is the equi-</p><p>librium adsorption constant. The dependence of rate on C</p><p>is shown in Figure 3. The degradation of PM follows the</p><p>first-order kinetics at lower concentrations of PM, where</p><p>the slope of the line approximates to 1. While at higher</p><p>concentrations it follows the zero-order kinetics, where the</p><p>slope is almost zero. In terms of Langmuir-Hinshelwood</p><p>(L-H) model,[16,17] this means that at lower concentrations</p><p>the number of active sites on the catalyst surface is not the</p><p>limiting factor for the degradation rate when the surface is</p><p>sparsely covered. Therefore the rate is proportional to the</p><p>concentration of the substrate/oxidants and hence it fol-</p><p>lows the first order kinetics. But at higher concentrations</p><p>all the catalytic sites are occupied and further increase in the</p><p>concentration does not affect the efficiency of the degrada-</p><p>tion process and hence the rate remains constant (follows</p><p>Fig. 5. Plot of reciprocal rate versus reciprocal concentration.</p><p>The linearity of the plot obeys the Langmuir- Hinshelwood</p><p>mechanism.</p><p>Table 4. Kinetic parameters for the degradation of pendimethalin.</p><p>[PM] [APS] Velocity</p><p>(ppm) (ppm) (ppm h−1) k (h−1) K (ppm−1)</p><p>5 100 4.01</p><p>10 100 3.629</p><p>20 100 6.428 0.111 × 102 0.1234</p><p>30 100 7.236</p><p>50 100 8.65</p><p>PM: Pendimethalin.</p><p>APS: Ammoniumpersulfate.</p><p>zero-order kinetics). The order of the degradation reaction</p><p>of PM is also studied with respect to the concentration of</p><p>(NH4)2S2O8 keeping the substrate concentration as con-</p><p>stant, which also supports the first-order kinetics.</p><p>The plot of logarithmic velocity against the logarithmic</p><p>concentration gives a straight line (Fig. 4) and the slope</p><p>of which gives the order of the reaction (first order). The</p><p>Equation 11 can be rewritten in a linear form as Equation</p><p>12.</p><p>C</p><p>v</p><p>=</p><p></p><p>1</p><p>kdeg</p><p> </p><p>1</p><p>Kads</p><p>+ C</p><p></p><p>Further, these data are consistent with the linear plot</p><p>of 1/v0 versus 1/C0 as shown in Figure 5. The values of</p><p>kdeg and Kads obtained from the plot are given (Table 2).</p><p>Fig. 6. UV-Visible spectra for the samples at different time inter-</p><p>vals during the degradation of PM in the experiment with catalyst</p><p>(0.750 g/L) and ammonium persulphate (100 ppm).</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>Degradation of pendimethalin using BaTiO3/TiO2 559</p><p>Fig. 7. IR spectrum for the samples at different time intervals of</p><p>irradiation in the experiment PM with catalyst(0.750 g/L) and</p><p>ammonium persulphate (100 p pm). The spectra (a), (b) and (c)</p><p>corresponds to the samples at 0h, 3h and 6h of irradiation.</p><p>The linearity of the plot implies the Langmuir-Hinshelwood</p><p>mechanism for the first-order kinetics. The order of the re-</p><p>action with respect to (NH4)2S2O8 at constant [PM] also</p><p>follows the first–order kinetics. The slope obtained is 0.8,</p><p>which can be approximated to first order. The kinetic data</p><p>for the degradation of PM is given (Table 4).</p><p>Spectroscopic analysis</p><p>UV-visible spectral analysis</p><p>The disappearance of pendimethalin during irradiation</p><p>is followed by UV-Visible absorption spectrum. The</p><p>UV-visible spectrum of initial sample of pendimethalin has</p><p>four prominent characteristic absorption bands at 205 nm,</p><p>240 nm, 285 nm and at 440 nm, which corresponds to E-</p><p>band (aromatic π → π∗ transition), K-band due to the</p><p>presence of NO2 group on the aromatic ring (π → π∗ tran-</p><p>sition), B-band is due to the benzenoid ring and R- band</p><p>(due to n → π∗ transitions) respectively. Initial spectrum</p><p>before irradiation shows less intense peaks due to the strong</p><p>adsorption of PM molecules on the catalyst surface. This</p><p>can be ascribed for the greater electrostatic attraction of the</p><p>catalyst surface and the herbicide molecules at initial pH to</p><p>accomplish the adsorption equilibrium.</p><p>Table 5. Intermediate products with ion masses identified by gas</p><p>chromatography-mass spectrometry GC-MS analysis.</p><p>Sl. No. Ion mass Intermediate product</p><p>( a) M+-253</p><p>( b) M+-223</p><p>( c) M+-178</p><p>( d) M+-197</p><p>( e) M+-181</p><p>( f) M+-181</p><p>Further on UV-light illumination the intensity of the ab-</p><p>sorption bands increases gradually due to desorption and</p><p>the band at 440 nm is shifted towards shorter wavelength</p><p>(hypsochromic or red shift). On continuation of irradiation</p><p>the band at 240 nm decreases in intensity, consequently</p><p>there is a decrease in yellow color of the solution visibly,</p><p>which shows the cleavage of NO2 group and at the same</p><p>time the band at 205 nm increases in intensity. The NO2</p><p>group may be converted into NH2 or OH group which</p><p>may sequentially involve in imidazole ring formation. The</p><p>yellow color of the compound completely disappears in</p><p>about 4h of irradiation. The band at 205 nm appeared as</p><p>the maximum at 4h of irradiation, which can be attributed</p><p>to the formation of hydroxy compound (n → π*) during</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>560 Devi and Krishnamurthy</p><p>Fig. 8. Reaction mechanism of photocatalytic degradation.</p><p>the process of irradiation. The appearance of this interme-</p><p>diate peak is very rapid (formed by 1h of irradiation) in</p><p>the presence of oxidizing agent and catalyst (Fig. 6). No</p><p>characteristic absorption bands could be detected after 6h</p><p>of illumination, which implies the complete mineralization</p><p>of pendimethalin.</p><p>IR spectral analysis</p><p>Identifying the intermediates based on the functional</p><p>groups is done by IR spectral analysis. The samples were</p><p>withdrawn during the process of irradiation and centrifuged</p><p>to separate solid catalyst particles. Further the samples were</p><p>extracted in to a non-aqueous solvent medium using spec-</p><p>troscopic grade chloroform. The concentrated solutions</p><p>were used for recording of IR spectra (Fig. 7). The initial</p><p>sample spectrum possess the peaks at: 3478 cm−1 which</p><p>corresponds to N H stretching in the PM molecule,</p><p>1738 cm−1 corresponds to N O of nitro group and the peak</p><p>at 1636 cm−1 is due to N H bending of secondary amine.</p><p>The peaks in the range 1533–1315 cm−1 are attributed to</p><p>C N bending vibrations. The sample spectrum after 3h</p><p>of irradiation has peak at 3462 cm−1, which may be due to</p><p>N H or O H stretching vibration, 1729 cm−1 could be due</p><p>to C N vibration. The peaks in the range 1248–1109 cm−1</p><p>may be due to O H bending/C O stretching/C N bend-</p><p>ing vibrations of the intermediate products. It indicates the</p><p>absence of the NO2 group and the presence of the hy-</p><p>droxy group. The spectrum for the sample at 6 h of irradia-</p><p>tion gives the peaks of negligible intensity, which shows the</p><p>degradation of pendimethalin.</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p><p>Degradation of pendimethalin using BaTiO3/TiO2 561</p><p>GC-MS analysis</p><p>GC-MS analysis has been carried out for the confir-</p><p>mation of the intermediate products formed during the</p><p>degradation process. The mass spectrum shows the base</p><p>peak of the parent compound, which corresponds</p><p>to m/z</p><p>value 280. The sample after 1 h has shown the prod-</p><p>uct (a) at the m/z value 253 corresponds to N-Propyl-</p><p>3,4-dimethyl-2,6-dinitroaniline and the ion peak at m/z</p><p>value 223 which corresponds to N-propyl-2-nitro-6-amino-</p><p>3,4-xylidine (product- b). The gas chromatography (GC)</p><p>spectrum for the sample after 3 h of irradiation has</p><p>shown the molecular ions at the m/z values 178, 197</p><p>and 181, which correspond to the products 5,6-dimethyl-</p><p>7-nitrobenzimidazole (product- c), (2,3-dimethyl-5-nitro-</p><p>6-hydroxy amine) phenol (product-d) and 2,3-dimethyl-</p><p>5-nitro-6-aminophenol (product-e) respectively. The m/z</p><p>value 181 may also corresponds to 2-amino-6-nitro-3,4-</p><p>xylidine (product-f). These intermediates are also substan-</p><p>tiated from IR and UV-Visible spectral analysis. The sample</p><p>after 6 h of irradiation has shown no major peaks implying</p><p>the degradation of pendimethalin. The intermediate prod-</p><p>ucts with the ion mass are given (Table 5). Probable reaction</p><p>mechanism is proposed based on the above intermediate</p><p>products is given in the Figure 8.</p><p>Conclusion</p><p>The wide band gap, perovskite semiconductor-BaTiO3 has</p><p>exhibited comparable photocatalytic efficiency as the most</p><p>widely used TiO2 photocatalyst in the degradation of</p><p>pendimethalin. The persulphate played an effective influ-</p><p>ence in enhancing the rate of degradation of pendimethalin</p><p>when compared to hydrogen peroxide for both the cata-</p><p>lysts. The alkaline pH 11 is found to be the most favor-</p><p>able condition for faster degradation of pendimethalin.</p><p>Pendimethalin shows a greater stability towards complete</p><p>mineralization due to its molecular stability and formation</p><p>of number of sequential stable intermediate products dur-</p><p>ing irradiation.</p><p>The degradation process follows the first-order kinetics</p><p>with substrate as well as ammonium persulphate with both</p><p>the catalysts and it follows the Langmuir-Hinshelwood</p><p>mechanism. The N-propyl-2-nitro-6-amino-3,4-xylidine,</p><p>(2,3-dimethyl-5-nitro-6-hydroxy amine) phenol and N-</p><p>Propyl-3,4-dimethyl-2,6-dinitroaniline are identified as the</p><p>major intermediate degradation products. This research</p><p>work presents another efficient photo catalyst BaTiO3 in</p><p>the process of photo catalysis. 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Catal. 1988, 111, 264–</p><p>272.</p><p>D</p><p>ow</p><p>nl</p><p>oa</p><p>de</p><p>d</p><p>by</p><p>[</p><p>M</p><p>cG</p><p>il</p><p>lU</p><p>ni</p><p>ve</p><p>rs</p><p>it</p><p>y</p><p>L</p><p>ib</p><p>ra</p><p>ry</p><p>]</p><p>at</p><p>0</p><p>4:</p><p>46</p><p>0</p><p>9</p><p>Ja</p><p>nu</p><p>ar</p><p>y</p><p>20</p><p>13</p>