Degradation of tricyclazole by colloidal manganese dioxide in the absence and presence of surfactants Qamruzzaman, Abu Nasar * Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India 1. Introduction Among the cereal grains, rice is an important staple food for the major part of the world. During recent years rice consumption has been increased tremendously in many countries. In order to fulfill the increasing demand of rice the use of pesticides, though harmful in many aspects, cannot be avoided. 5-Methyl-1,2,4-triazolo (3,4- b) benzothiazole (tricyclazole) is a very popular and one of the most common pesticides employed for its fungicidal activity in the plantation of paddy rice. It is used to control rice blast disease, caused by the fungus pyricularia oryzae, in both transplanted and direct seeded paddy rice [1–3]. Tricyclazole is advantageous over other rice blast fungicides because it provides long term protection during the entire growth period as it has long effectiveness which ruled out the requirement of multiple applications [1]. It is readily absorbed by plant roots and translocated to leaves, where it provides residual disease control. It inhibits the synthesis of 1,8- dihydroxy naphthalene (DNH) melanin, which is responsible for the rice blast disease. Among the commercially used melanin inhibitors, tricyclazole has been observed to be one of the most effective fungicides [4–6]. In spite of advantageous and unavoid- able uses fungicides often contaminate the environment and cause public health problem due to their high toxicity and long persistence [7–10]. The tricyclazole residue in paddy rice and environments has been monitored and analyzed by a number of investigators [10–17]. It has been observed that this fungicide is quite persistent in the soil-water system with half life from 4 to 17 months in laboratory and about 6 months in the field [17]. Padovani et al. [17] have also reported that tricyclazole is not easily hydrolyzed in the environment and is stable up to 51 8C without volatilization. Therefore application of tricyclazole in agricultural field is associated with significant risk to aquatic system and water resources. Thus the treatment of tricyclazole, which can be executed by degradation of its molecules, is essential to eliminate or minimize its negative effect. In fact fate of a pesticide in soil is governed by its transformation process associated with the decomposition of molecules by chemical reaction. It is well known that humic and organic substances including pesticides have been known to undergo degradation in presence of manganese compounds and especially its dioxide (MnO 2 ). In fact manganese is 12th most abundant element in earth’s crust and available from 7 to 9000 ppm depending upon region with an average value of 440 ppm [18]. The MnO 2 particles present in earth’s crust and natural water are susceptible for reduction by humic and organic substances and pesticides as well. In fact oxidizing power of MnO 2 is limited due to its insolubility under ordinary conditions. Journal of Industrial and Engineering Chemistry 20 (2014) 897–902 A R T I C L E I N F O Article history: Received 15 October 2012 Accepted 2 June 2013 Available online 19 June 2013 Keywords: Kinetics Tricyclazole Surfactants Pseudo-first order Manganese dioxide A B S T R A C T The kinetics of the degradation of tricyclazole by water soluble colloidal MnO 2 in acidic medium (HClO 4 ) has been studied spectrophotometrically in the absence and presence of surfactants. The experiments have been performed under the pseudo-first-order reaction conditions with respect to MnO 2 . To determine the rate constant as functions of [tricyclazole], [MnO 2 ] and [HClO 4 ], the pseudo-first-order reaction conditions have been maintained throughout the entire kinetic runs. The degradation has been observed to be first-order with respect to MnO 2 while fractional-order in both tricyclazole and HClO 4 . The anionic surfactant, sodium dodecyl sulfate (SDS) has been observed to be ineffective whereas nonionic surfactant, Triton X-100 (TX-100) accelerates the reaction rate. However, the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) causes flocculation with oppositely charged colloidal MnO 2 and therefore could not be studied further. The catalytic effect of TX-100 has been discussed in the light of the mathematical model proposed by Tuncay et al. [25]. The kinetic data have been exploited to generate the various activation parameters for the oxidative degradation of tricyclazole by colloidal MnO 2 . ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +91 9415842475; fax: +91 0571 2700528. E-mail address: [email protected](A. Nasar). Contents lists available at SciVerse ScienceDirect Journal of Industrial and Engineering Chemistry jou r n al h o mep ag e: w ww .elsevier .co m /loc ate/jiec 1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.06.020
Transcript
Journal of Industrial and Engineering Chemistry 20 (2014) 897–902
Degradation of tricyclazole by colloidal manganese dioxide in theabsence and presence of surfactants
Qamruzzaman, Abu Nasar *
Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India
A R T I C L E I N F O
Article history:
Received 15 October 2012
Accepted 2 June 2013
Available online 19 June 2013
Keywords:
Kinetics
Tricyclazole
Surfactants
Pseudo-first order
Manganese dioxide
A B S T R A C T
The kinetics of the degradation of tricyclazole by water soluble colloidal MnO2 in acidic medium (HClO4)
has been studied spectrophotometrically in the absence and presence of surfactants. The experiments
have been performed under the pseudo-first-order reaction conditions with respect to MnO2. To
determine the rate constant as functions of [tricyclazole], [MnO2] and [HClO4], the pseudo-first-order
reaction conditions have been maintained throughout the entire kinetic runs. The degradation has been
observed to be first-order with respect to MnO2 while fractional-order in both tricyclazole and HClO4.
The anionic surfactant, sodium dodecyl sulfate (SDS) has been observed to be ineffective whereas
nonionic surfactant, Triton X-100 (TX-100) accelerates the reaction rate. However, the cationic
colloidal MnO2 and therefore could not be studied further. The catalytic effect of TX-100 has been
discussed in the light of the mathematical model proposed by Tuncay et al. [25]. The kinetic data have
been exploited to generate the various activation parameters for the oxidative degradation of
tricyclazole by colloidal MnO2.
� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry
jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec
1. Introduction
Among the cereal grains, rice is an important staple food for themajor part of the world. During recent years rice consumption hasbeen increased tremendously in many countries. In order to fulfillthe increasing demand of rice the use of pesticides, though harmfulin many aspects, cannot be avoided. 5-Methyl-1,2,4-triazolo (3,4-b) benzothiazole (tricyclazole) is a very popular and one of themost common pesticides employed for its fungicidal activity in theplantation of paddy rice. It is used to control rice blast disease,caused by the fungus pyricularia oryzae, in both transplanted anddirect seeded paddy rice [1–3]. Tricyclazole is advantageous overother rice blast fungicides because it provides long term protectionduring the entire growth period as it has long effectiveness whichruled out the requirement of multiple applications [1]. It is readilyabsorbed by plant roots and translocated to leaves, where itprovides residual disease control. It inhibits the synthesis of 1,8-dihydroxy naphthalene (DNH) melanin, which is responsible forthe rice blast disease. Among the commercially used melanininhibitors, tricyclazole has been observed to be one of the mosteffective fungicides [4–6]. In spite of advantageous and unavoid-
1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer
http://dx.doi.org/10.1016/j.jiec.2013.06.020
able uses fungicides often contaminate the environment and causepublic health problem due to their high toxicity and longpersistence [7–10]. The tricyclazole residue in paddy rice andenvironments has been monitored and analyzed by a number ofinvestigators [10–17]. It has been observed that this fungicide isquite persistent in the soil-water system with half life from 4 to 17months in laboratory and about 6 months in the field [17].Padovani et al. [17] have also reported that tricyclazole is not easilyhydrolyzed in the environment and is stable up to 51 8C withoutvolatilization. Therefore application of tricyclazole in agriculturalfield is associated with significant risk to aquatic system and waterresources. Thus the treatment of tricyclazole, which can beexecuted by degradation of its molecules, is essential to eliminateor minimize its negative effect. In fact fate of a pesticide in soil isgoverned by its transformation process associated with thedecomposition of molecules by chemical reaction. It is well knownthat humic and organic substances including pesticides have beenknown to undergo degradation in presence of manganesecompounds and especially its dioxide (MnO2). In fact manganeseis 12th most abundant element in earth’s crust and available from7 to 9000 ppm depending upon region with an average value of440 ppm [18]. The MnO2 particles present in earth’s crust andnatural water are susceptible for reduction by humic and organicsubstances and pesticides as well. In fact oxidizing power of MnO2
is limited due to its insolubility under ordinary conditions.
ing Chemistry. Published by Elsevier B.V. All rights reserved.
Fig. 1. Plots of log (cmin) versus cationic radius (r+) ([MnO2] = 6.0 � 10�5 mol dm�3,
temperature = 30 8C, all electrolytes in the chloride form).
Qamruzzaman, A. Nasar / Journal of Industrial and Engineering Chemistry 20 (2014) 897–902898
However, in recent years perfectly transparent colloidal solution ofMnO2 has been prepared by the reduction of neutral or slightlyacidic potassium permanganate solution by sodium thiosulfate.The water soluble colloidal MnO2 has successfully been used forthe oxidative degradation of a number of substances such asaspartic acid [19], oxalic acid [20,21], D-fructose [22], glycyl-glycine [23], formic acid [24,25], glycolic acid [26], mandelic acid[27], L-methionine [28], DL-malic acid [29], glycyl-leucine [30],ascorbic acid [31], metribuzin [32], methomyl [33] etc. by differentresearch groups. Literature survey reveals that the degradationstudies on the oxidative degradation of tricyclazole are verylimited and scarce. Recently, Phong et al. [34] have monitored thefate and transport of tricyclazole in paddy field after nursery-box-application and reported the mean half life value to be 11.8 and 305days in paddy water and surface soil, respectively. They alsopointed out that even less than 0.9% of tricyclazole were lostthrough run off during the monitoring period under 6.3 cm of rainfall. The interaction between degradation of phenonthrene andtricyclzole in soil and soil–mushroom compost has been studied byLiu et al. [35]. In the present investigation studies on thedegradation kinetics of tricyclazole by water soluble colloidalMnO2 have been conducted.
Surfactants are used to lower the surface tension and can act aswetting, foaming, emulsifying and dispersion agents. A surfactantmolecule contains at least one polar hydrophilic part and at leastone nonpolar hydrophobic part. The coexistence of two oppositetypes of units inside the same molecule is the origin of localconstraints which lead to spontaneous aggregation into micro-scopic labile structures [36]. These surface active agents in aqueousmedium thus self aggregate at the concentration above criticalmicelle concentration (cmc) to form association colloids known asmicelles. Surfactant micelles offer a relatively large microscopicnonpolar environment for solute partition (solubilization) result-ing increase in solubility of solute (apparent water solubility) inmicellar media in comparision with water solution [37]. Surfac-tants are therefore very commonly used in pesticide formulation toincrease the solubility of pesticides and also to enhance theireffectiveness by fine spray. In fact surfactants essentially consist ofnonpolar hydrophobic (tail) and polar hydrophilic group (head)groups. According to the nature of latter group the surfactants maybe classified as cationic, anionic, nonionic and zwitterionic. In thepresent investigation, effect of three common surfactants such ascetyl trimethyl ammonium bromide (CTAB), sodium dodecylsulfate (SDS) and Triton X-100 (TX-100) on the degradationkinetics of tricyclazole by colloidal MnO2 in presence of HClO4 hasbeen studied. Selection of these surface active agents is based onthe criteria of picking one member from each category of cationic,anionic and nonionic compounds as chosen in respective order. Inorder to generate various activation parameters the study has alsobeen extended at different temperatures. The kinetic data havebeen analyzed in the light of Arrhenius and Eyring theories and alsodiscussed in terms of different various activation parameters asgenerated.
2. Experimental
2.1. Materials
Commercial grade tricyclazole (GSP Crop Science, India),scintillation grade TX-100 (CDH, India) and analytical reagentgrade each perchloric acid (Merck, Germany), CTAB (CDH, India),SDS (SRL, India) potassium permanganate and sodium thiosulfate(Qualigens, India) were used in the present investigation.Following analytical reagent grade electrolytic salts: LiCl, NaCl,KCl, NH4Cl, MgCl2, CaCl2 and BaCl2 (each obtained from SRL, India)and SrCl2 (CDH, India) were used for the characterization of
colloidal MnO2 solution. Doubly distilled deionized water was usedthroughout the experimental studies.
2.2. Preparation and characterization of colloidal MnO2
The water soluble colloidal MnO2 solution was prepared by themethod described by Perez-Benito and co-workers [21,24]. Thepreparation involves the reduction of potassium permanganate bysodium thiosulfate in aqueous medium according to the followingstoichiometry:
8MnO�4 þ 3S2O2�3 þ 2Hþ! 8MnO2 þ 6SO2�
4 þ H2O (1)
The required volume of sodium thiosulfate solution (20.0 ml,1.88 �10�2 mol dm�3) was slowly added to standard solution ofpotassium permanganate (10.0 ml, 0.1 mol dm�3) and the reactionmixture was then diluted to 1 dm3 in a standard flask. In this waythe solution prepared was dark brown transparent and remainedstable for over a month.
The formation of water soluble colloidal particles of MnO2 waschecked by adding the minimum amount of different electrolytesnecessary for their precipitation [38]. For this purpose appropriateamount of different types of electrolytes containing monovalentand divalent ions, viz., LiCl, NaCl, KCl, NH4Cl, MgCl2, CaCl2, BaCl2
and SrCl2 were mixed to the solution with stirring. With eachelectrolyte, appearance of precipitate in the reaction mixture wasobserved at a typical concentration which indicated the presenceof water soluble form of colloidal particles of MnO2. According toPerez-Benito et al. [38] the precipitation of MnO2 is due to theadsorption of electrolytic cations on its negatively chargedcolloidal particles. The negative logarithm of the minimumconcentration of different chlorides (cmin) necessary for theprecipitation of MnO2 has been plotted against respective cationicsize (r+) in Fig. 1. This figure indicates that the coagulationefficiency of salts increases with increase of charge as well as sizeof the cations. This result is in conformity with the findings ofearlier investigators [31,38,39].
An alternative method based on the Rayleigh’s scattering lawwas also used to further confirm the formation of colloidalparticles. According to this law the absorbance (A) due toscattering of light by a solution of colloidal particles is inverselyproportional to the fourth power of the wavelength (l) [40]. Theplot between log A and log l at the fixed concentration of MnO2
(6.0 � 10�5 mol dm�3) was linear with a slope of 4.2, which isslightly greater than the theoretical value of 4.0. The fulfillmentRayleigh’s law is indicative of spectrum due to scattering of light bycolloidal particles instead of absorption of light by non-colloidalspecies [41]. Thus on the basis of above observations one canconclude that the water soluble MnO2 prepared as above is in theform of colloidal species.
-0.6
-0.5
-0.4
-0.3
0 8 16 24 32
log
(abs
orba
nce)
Time (min)
Fig. 2. Plot of log (absorbance) versus time for the degradation of tricyclazole by
Qamruzzaman, A. Nasar / Journal of Industrial and Engineering Chemistry 20 (2014) 897–902 899
2.3. Kinetic measurements
Kinetic experiments were performed by taking requisitequantity of aqueous solution of tricyclazole in a reaction vesselkept in a thermostatized water bath. The reaction vessel wasallowed to remain in the water bath for sufficient time to attain thedesired temperature with an accuracy of �0.5 8C. The kineticstudies were carried out by adding the calculated amount of colloidalsolution of MnO2, HClO4 and surfactants. The progress of the reactionwas monitored spectrophotometrically. The absorbance of unreactedMnO2 in the reaction mixture was taken by UV-Visible spectropho-tometer (Perkin Elmer, Model – lambda 25) at an optimizedwavelength of 360 nm (i.e. lmax = 360 nm). The fulfillment of Beer’slaw was checked and found to be validated in experimentalconcentration range (4.0 � 10�5–1.4 � 10�4 mol dm�3) of MnO2.The measurements were taken under the varying conditions ofconcentrations (tricyclazole, MnO2, HClO4 and surfactants) andtemperature (25–60 8C).
3. Results and discussion
3.1. General consideration
All the measurements were formulated under the pseudo-first-order reaction conditions in which concentrations of tricyclazoleand surfactants were taken in large excess over MnO2. The pseudo-first-order rate constants were calculated from the slope of log(absorbance) versus time plot. The plot of log (absorbance) versustime at a typical fixed concentrations of tricyclazole(6.0 � 10�3 mol dm�3), MnO2 (6.0 � 10�5 mol dm�3) and HClO4
(6.0 � 10�4 mol dm�3) at 30 8C shown in Fig. 2 is represented bystraight line with r2 = 0.9893. Thus the reaction is first order withrespect to MnO2 under the adopted reaction conditions. In similarway experiments were also performed in the presence ofsurfactants.
3.2. Effect of concentrations of tricyclazole, MnO2 and HClO4 on the
reaction rate
The dependence of the rate of reaction on the concentration oftricyclazole has been studied by conducting the kinetic measure-ments at the varying concentration of tricyclazole (1.0 � 10�3–1.0 � 10�2 mol dm�3) keeping the concentrations of MnO2
(6.0 � 10�5 mol dm�3) and HClO4 (6.0 � 10�4 mol dm�3) constantat a typical temperature of 30 8C. Under the said conditions, thevalues of observed rate constant (kobs) so obtained are plottedagainst the concentration of tricyclazole in Fig. 3. This figure clearlyindicates that the variation is nonlinear, which is supposed to bepassed through origin on extrapolation. The rate constantincreases in a regular and continuous manner up to a certainconcentration of tricyclazole (about 6.0 � 10�3 mol dm�3) beyondwhich it tends to remain almost constant on further increase of theconcentration of tricyclazole. The plot between log kobs and log[tricyclazole], in increasing zone of kobs with concentration, islinear (Fig. 4) with a slope of 0.70 (r2 = 0.9783) indicating thefractional order with respect to tricyclazole. This result is inconformity with the observation of fractional order for theoxidative degradation of a number of compounds such as D-fructose [22], glycyl-glycine [23], glcyl-leucine [30], metribuzin[32], methomyl [33] etc. by colloidal MnO2 in HClO4 medium.
In order to understand the nature of reaction with respect tovariation in concentration of MnO2, the rate constants have beendetermined at different initial concentration of MnO2 ranging from4.0 � 10�5 to 1.4 � 10�4 mol dm�3. The concentrations of tricycl-zole (6.0 � 10�3 mol dm�3) and HClO4 (6.0 � 10�4 mol dm�3),and temperature (30 8C) have been kept constants. The
pseudo-first-order rate constant obtained at different concentra-tion of MnO2 are plotted in Fig. 5. This figure clearly shows that therate constant decrease with increasing concentration of MnO2. Thesimilar observation of decrease of rate constant with increasingconcentration of MnO2 under pseudo-first-order reaction condi-tions has also been reported for the oxidative degradation of anumber of compounds such as aspartic acid [19], oxalic acid[20,21], glycyl-glycine [23], glycyl-leucine [30], metribuzin [32],methomyl [33]. The continuous and regular decrease of rateconstant with increasing concentration of MnO2 is due toflocculation of its particles.
In the present investigation HClO4 was used as acidifying agent.To prevent any possible precipitation of colloidal MnO2 (asdiscussed in Section 2.2) no buffering agents were added to
2.6
3
3.4
3.8
0.3 0.6 0.9 1.2 1.5
104 k o
bs(s
-1)
104 [MnO2] (mol dm-3 )
Fig. 5. Effect of [MnO2] on kobs for the degradation of tricyclazole by colloidal
Qamruzzaman, A. Nasar / Journal of Industrial and Engineering Chemistry 20 (2014) 897–902900
maintain the pH of the medium. It has also been suggested to avoiduse of even buffer to fix the pH of micellar solutions [42,43].Therefore, in order to ruled out any possible change in behavior ofmicelles, the concentration of HClO4 was kept constant(6.0 � 10�4 mol dm�3) without using any buffering agent in allthe above measurements. Furthermore to see the effect ofconcentration of HClO4 (in range of 1.0 � 10�4–1.2 � 10�3 mol dm�3) on the rate constant a series of kinetic runswas also carried out at the fixed concentration of tricyclazole(6.0 � 10�3 mol dm�3) and MnO2 (6.0 � 10�5 mol dm�3) at aconstant temperature of 30 8C. The plot of kobs against [HClO4]as shown in Fig. 6 is linear with a positive intercept on kobs axis.Thus the degradation of tricyclazole by MnO2 comprises with acidindependent and acid dependent paths. The slope of linear plot oflog kobs versus log [HClO4] has been calculated to be 0.17(r2 = 0.9608). Thus there is fractional order dependence oftricyclazole degradation with respect to [HClO4] in acid dependentpath. On the basis of above observations and findings the rate (n) ofthe degradation of tricyclazole under pseudo-first-order reactionconditions with respect to MnO2 may be given in term of thefollowing rate law equation:
n ¼ � d½MnO2�dt
¼ ðkI þ kD½Hþ�0:17Þ½tricyclazole�0:70½MnO2� (2)
where, kI and kD are rate constants for hydrogen ion concentrationin acid independent and dependent paths, respectively.
The acid independent reaction path taking place may presum-ably occur by the adsorption of tricyclazole on the colloidalparticles of MnO2 followed by reaction between adsorbedtricyclazole molecule and one of the MnO2 molecules pertaining
to the colloidal surface leading to the reaction as represented bythe following plausible mechanism:
where (MnO2)x and O-TRC represent colloidal MnO2 and oxidativedegrades of tricyclazole, respectively.
On the other hand, the acid dependent path comprises with theadsorption of two hydrogen ions, in addition to tricyclazolemolecule, on the colloidal surface of MnO2 leading to thedegradation of tricyclazole by the following plausible mechanism:
In the present investigation commonly used surfactants such asCTAB, SDS and TX-100 have been chosen. The effect of concentra-tions of surfactants on the rate constant has been studied at thetemperature of 30 8C by keeping the concentration of MnO2
(6.0 � 10�5 mol dm�3), tricyclazole (6.0 � 10�3 mol dm�3) andHClO4 (6.0 � 10�4 mol dm�3) constant. It has been observed thatSDS has no effect on the value of rate constant. This is due torepulsion between anionic micellar aggregates of SDS (the micelleshave net negative charge due to –OSO3
�) and the negativelycharged colloidal MnO2. In this context it is worth relevant tomention here that it has been well established that the watersoluble colloidal MnO2 in aqueous medium is stabilized byadsorption of anions resulting negative charge on its particle[38]. Thereby the reaction in presence of CTAB could not befollowed because it possesses positive charge opposite to that ofcolloidal MnO2 causing flocculation. Intense turbidity with theclear appearance of precipitate has been observed in the reactionmixture. The problem of flocculation has also been observed by anumber of earlier investigators during their studies on a redoxreaction involving colloidal MnO2 as oxidant in presence of CTAB[23,25–27,29,30]. However, addition of non-ionic surfactant, TX-100 enhances the rate of reaction. The plots between log(Absorbance) versus time were linear in presence of differentconcentration of TX-100 (1.0 � 10�4–5.0 � 10�3 mol dm�3) whichconfirms that the reaction is also first-order with respect to MnO2
in presence of TX-100. Thus, in other words, the order of reactionwith respect to MnO2 remains the same as that observed inabsence of surfactants. The values of pseudo-first-order rateconstant (kc) in presence of TX-100 are drawn against [TX-100] inFig. 7. The rate constant in presence of TX-100 increases withincreasing concentration of TX-100 throughout the entire range asshown in Fig. 7. However, the catalytic effect is more pronouncedin lower concentration range. The enhancement of the observedrate constant has been explained in terms of increasing solubili-zation and association or adsorption of the reactive species(oxidant and reductant) on the surfactant TX-100 [19,27,29,30].The catalytic effect of TX-100 can be presented in term of thefollowing mathematical model developed by Tuncay et al. [25].
log kc ¼ a log ½TX-100� þ b (3)
where a and b are empirical constants. According to the Tuncaymodel the plot of log kc versus log [TX-100] should be linear,
3
6
9
12
0 1.4 2.8 4.2 5.6
104
k Ψ(s
-1)
103 [TX-100] (mol dm-3 )
Fig. 7. Effect of [TX-100] on kc for the degradation of tricyclazole by colloidal
Qamruzzaman, A. Nasar / Journal of Industrial and Engineering Chemistry 20 (2014) 897–902 901
which was observed in the present case (Fig. 8) with a = 0.265 andb = �2.233 (r2 = 0.9612).
In order to further explain the role of TX-100, an alternativeempirical equation was also suggested by Tuncay et al. [25]:
1
kc � kobs¼ c þ d
½TX-100� (4)
where c and d are empirical constants. The plot between 1/(kc � kobs) and 1/[TX-100] (Fig. 9) was linear which resulted thevalue of c = 655.7 s and d = 0.842 mol dm�3 s (r2 = 0.9710). Thevalidity of Eqs. (3) and (4) confirms that the oxidative degradationof tricyclazole by MnO2 in presence of TX-100 obeys Tuncay model.
3.4. Effect of temperature on rate constant and determination of
activation parameters
A series of kinetic experiments were performed at differenttemperatures in the range of 25–60 8C at the fixed concentrationsof tricyclazole (6.0 � 10�3 mol dm�3), MnO2
(6.0 � 10�5 mol dm�3) and HClO4 (6.0 � 10�4 mol dm�3). Thevalues of rate constant so obtained at different temperatures areused to calculate the activation energy of the process. The variationof log kobs against 1/T has been observed to obey the followinglinear relationship which indicates that the system obeysArrhenious relationship:
log kobs ¼ � 669:5
T � 1:1158ðr2 ¼ 0:9892Þ (5)
The value of activation energy (Ea) as calculated from the slopeof the above equation is listed in Table 1. In order to realizewhether the reaction mechanism is associative or dissociative theentropy of activation is necessarily required. The values of entropyof activation (DS#) along with other thermodynamic activationparameters such as enthalpy of activation (DH#) and free energy ofactivation (DG#) as calculated from following Eyring equation:
logk
T¼ � DH
2:3026RTþ log
kB
hþ DS
2:3026R(6)
are also presented in Table 1. In the above equation kB, R and h
represent Boltzman, gas and Plank’s constants, respectively. Apositive value of Ea (and also that of DH# as DH# = Ea � RT) is due toincrease of rate constant with increase of temperature (as clearlyindicated from the differential form of Arrhenius equation:Ea ¼ RT2ðd ln kobs=dTÞ). Since free energy of activation is acombined factor of enthalpy and entropy of activations(DG# = DH# � TDS#), a still much higher numerically positivevalue of DG# is due to large negative value of DS#. In fact the rateconstant for a forward reaction depends only on the increase of freeenergy of activation on going from initial state to activated stateand not on any free energy changes occurring after acquiring theactivation state [44]. A large negative value of entropy of activationpoints out the formation of highly ordered associative transitionstate complex during the degradation process of tricyclazole bycolloidal MnO2. Moreover, the activation parameters as listed inTable 1 also highlight that the entropy factor plays dominating roleover enthalpy factor.
4. Conclusions
The kinetic studies for the oxidative degradation of tricyclazoleby colloidal MnO2 in acidic medium have successfully beenperformed in the absence and presence of surfactants. The rateconstants have been determined as function of the concentrationsof tricyclazole, MnO2 and HClO4 under the pseudo-first-orderreaction conditions. The order of the reaction has been observed tobe first order in MnO2 and fractional order in both tricyclazole andHClO4. On the basis of variation of the rate constant following ratelaw equation has been developed:
Qamruzzaman, A. Nasar / Journal of Industrial and Engineering Chemistry 20 (2014) 897–902902
n ¼ � d½MnO2�dt
¼ ðkI þ kD½Hþ�0:17Þ½tricyclazole�0:70½MnO2�
Effect of common surfactants, namely, CTAB, SDS and TX-100 onthe degradation kinetics of tricyclazole by colloidal MnO2 has alsobeen studied. It has been observed that CTAB causes flocculationwhile SDS has no considerable effect on the reaction kinetic.However, significant catalytic role of TX-100 has been noticedwhich has been discussed and explained in terms of mathematicalmodel of Tuncay et al.
The kinetic results have also been used to generate variousactivation parameters associated with the degradation of tricy-clazole by colloidal MnO2.
Acknowledgements
The authors are grateful to the Chairman, Department ofApplied Chemistry, Faculty of Engineering and Technology, AligarhMuslim University for providing necessary laboratory facilities.One of the authors (Qamruzzaman) is also thankful to theUniversity Grants Commission, New Delhi for the award ofMaulana Azad National Fellowship.
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