Treatment of acetamiprid insecticide from artificially contaminated water by colloidal manganese dioxide in the absence and presence of surfactants

Abstract

Acetamiprid is one of the most important pesticides and is effective against a number of insects. The increasing use of insecticides in the agricultural field is associated with a significant risk to water resources and aquatic systems. Thus the degradation of such compounds, after fulfillment of their insecticidal role, is essential to eliminate or minimize the contamination of water. The degradative treatment of acetamiprid insecticide from artificially contaminated water 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. The degradation kinetics has been observed to be first-order with respect to MnO₂ while fractional-order in both acetamiprid and HClO₄. The anionic surfactant, sodium dodecyl sulfate (SDS) has been observed to be ineffective. On the other hand the reaction in the presence of cationic surfactant, cetyltrimethyl ammonium bromide (CTAB) could not be followed as well because it possesses a positive charge opposite to that of colloidal MnO₂ causing flocculation and therefore could not be studied further. However, the addition of non-ionic surfactant, polyethylene glycol tert-octylphenyl ether (TX-100) accelerates the reaction rate. The catalytic effect of TX-100 has been discussed in the light of the available mathematical model. The kinetic data have been exploited to generate the various activation parameters for the oxidative degradation of acetamiprid by colloidal MnO₂ in the absence and presence of non-ionic surfactant, TX-100.

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Introduction

Acetamiprid, (E)-N-[(6-chloro-3-pyridyl) methyl]-N′-cyano-N-methyl-acetamidine, is a systematic insecticide belonging to the neonicotinoid group. It shows excellent efficacy against aphids, leafhoppers, whiteflies, thrips, leaf beetles, leaf miner moths, termites, etc. in a wide range of crops such as okra, gram, mustard, strawberry, cotton, tea, etc.^1–8 In spite of advantageous and unavoidable use insecticides often contaminate the environment and cause public health problems due to their high toxicity and long persistence. Clinical studies have indicated that the human intake of acetamiprid causes physiological disturbance resulting in nausea, vomiting, muscle weakness, hypothermia, convulsions, tachycardia, hypotension, etc.⁹ Because of high water solubility (4.25 g dm⁻³ at 25 °C) this insecticide has large potential to contaminate the water resources. Thus, agricultural applications of acetamiprid are associated with the significant risk to the environment and human health as well. Hence the treatment of the compound is essential to eliminate or at least minimize its negative impacts.

The possible treatment of a pesticide is based on the splitting of molecules by photochemical, biological and chemical processes. The photochemical degradation of acetamiprid in soil has been studied several investigators.^10–13 The degradation studies on acetamiprid have also been conducted by different investigators in different regions and locations under varying climatic conditions and temperatures.^14,15 Although the chemical degradation process would be simple and economical for the treatment of waste water contaminated by acetamiprid, a literature survey reveals that so far no systematic efforts have been made to conduct the study in this direction. It is well known that many organic substances undergo chemical decomposition in the presence of manganese compounds and especially its dioxide (MnO₂).¹⁶ In fact manganese is 12^th most abundant element in the earth's crust and available in variable amounts depending upon region and location. The MnO₂ particles present in the earth's crust and natural water have potential for oxidative degradation of humic and organic substances including pesticides. However, the oxidizing power of MnO₂ is limited due to its insolubility under ordinary conditions. Fortunately, Perez-Benito and co-workers^17–19 in their pioneer work have developed a method to prepare a water soluble form of perfectly transparent colloidal MnO₂ by employing the redox reaction between potassium permanganate and sodium phosphate. The water soluble form of colloidal MnO₂ is advantageous over its insoluble form due to the higher oxidizing power and feasibility of monitoring the reaction by conventional UV-visible spectrophotometry. The water soluble colloidal MnO₂ has successfully been used for the oxidative degradation studies of a number of pesticides and other organic compounds such as formic acid,^18,20 D-fructose,²¹ D-glucose,²² glycyl-glycine,²³ glycolic acid,²⁴ DL-malic acid,²⁵ mandelic acid,²⁶ metribuzin,²⁷ oxalic acid,^19,28 thiourea²⁹ and tricyclazole³⁰ by employing UV-visible spectrophotometric technique. As so far the degradation of acetamiprid by colloidal MnO₂ has not been studied the present investigation has been planned with the prime objective to conduct measurements in this direction. In the present study the degradation kinetics of acetamiprid insecticide in artificially contaminated water by colloidal MnO₂ in acidic medium (HClO₄) has been thoroughly investigated. HClO₄ was used as the acidifying agent due to the non-complexing nature of perchlorate ion, ClO₄⁻.²⁹ Since surfactants are commonly used in pesticide formulation to increase the solubility of pesticides and also to enhance their effectiveness, it was also considered worth to extend the study in the presence of three common surfactants such as cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and polyethylene glycol tert-octylphenyl ether (TX-100). A selection of these surface active agents is based on the criteria of picking the one member from each category of cationic, anionic and non-ionic compounds as chosen in respective order. The kinetic data have been analyzed in the light of Arrhenius and Eyring theories and different activation parameters have also been determined.

Experimental

Materials

Laboratory reagent grade acetamiprid (Parijat Industries, India), scintillation grade TX-100 (CDH, India) and analytical reagent grade each potassium permanganate and sodium thiosulfate (Qualigens, India), perchloric acid (Merck, Germany), H₂SO₄ (Qualigens, India), CTAB and SDS (CDH, India) were used in the present investigation. Following analytical reagent grade salts: LiCl, NaCl, KCl, NH₄Cl, MgCl₂, CaCl₂, BaCl₂ (each from SRL, India) and SrCl₂ (CDH, India) were used for the characterization of colloidal MnO₂. The water used was purified by deionization followed by double distillation. This doubly distilled, deionized water was used throughout the experimental studies.

Preparation and characterization of colloidal MnO₂

The stock solution of colloidal MnO₂ was prepared by the method described by Perez-Benito and coworkers.^17–20 The preparation involves the reduction of potassium permanganate by sodium thiosulfate in dilute H₂SO₄ medium (pH 4.7), according to the following stoichiometry:

8MnO₄⁻ + 3S₂O₃²⁻ + 2H⁺ → 8MnO₂ + 6SO₄²⁻ + H₂O (1)

The required volume of sodium thiosulfate solution (20.0 cm³, 1.88 × 10⁻² mol dm⁻³) was slowly added to standard solution of potassium permanganate (10.0 ml, 0.1 mol dm⁻³) and the reaction mixture was then diluted to 1 dm³ in a standard flask. In this way the solution prepared to be dark brown transparent and remained stable for over a month.

A method based on Rayleigh's scattering law was used to check the formation of colloidal particles. According to this law the absorbance (A) due to scattering of light by a solution of colloidal particle is inversely proportional to the fourth power of the wavelength (λ).³¹ The plot between log A and log λ shown in Fig. 1 is linear with a slope of 4.19, which is slightly greater than the theoretical value of 4.0. The fulfillment of Rayleigh's law is indicative of the spectrum due to scattering of light by colloidal particles instead of absorption of light by non-colloidal species.³²

Fig. 1 Plots of log A versus log λ ([MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³) and temperature = 30 °C.

The formation of water soluble colloidal particles of MnO₂ was also independently confirmed by adding the minimum amount of different electrolytes necessary for their precipitation.¹⁷ For this purpose appropriate amount of the different electrolytic salts containing monovalent and divalent cations, viz., LiCl, NaCl, KCl, NH₄Cl, MgCl₂, CaCl₂, SrCl₂ and BaCl₂ were mixed into the solution with constant stirring. Further details of the method have been described elsewhere.³⁰ With each electrolyte, the appearance of brownish precipitate in the reaction mixture was observed, which indicated the formation of water soluble form of colloidal MnO₂.

These observations confirm that water soluble form of MnO₂ as prepared from KMnO₄ and Na₂S₂O₃ is in the form of colloidal species.

Kinetic measurements

The absorption spectrum of aqueous KMnO₄ solution shown in Fig. 2 has a peak (λ_max) at 525 nm. With the addition of Na₂S₂O₃ solution, the band gradually disappeared with the appearance of a new single broad band maximum at 360 nm. The new stabilized spectrum is due to formation of colloidal MnO₂ and therefore wavelength of 360 nm was chosen to execute the kinetic runs. Kinetic experiments were performed by taking a requisite quantity of aqueous solution of acetamiprid in a reaction vessel kept in a thermostated water bath. The reaction vessel was allowed to remain in the water bath for sufficient time to attain the desired temperature with an accuracy of ±0.5 °C. The kinetic studies were carried out by adding the calculated volume of colloidal solution of MnO₂, HClO₄ and surfactants corresponding to their selected concentrations. The progress of the reaction was monitored spectrophotometrically. The absorbance of unreacted MnO₂ in the reaction mixture was taken by UV-visible spectrophotometer (Perkin Elmer, Model-lambda 25, USA) at an optimized wavelength of 360 nm (i.e. λ_max = 360 nm). The fulfillment of Beer's law was checked and found to be validated in an experimental concentration range (6.0 × 10⁻⁵ to 1.6 × 10⁻⁴ mol dm⁻³) of MnO₂. The measurements were taken under the varying conditions of concentrations (acetamiprid, MnO₂, HClO₄ and surfactants) and temperature (20–50 °C).

Fig. 2 Absorption spectra of KMnO₄ (•) and colloidal MnO₂ (o) i.e. reaction product of KMnO₄ and Na₂S₂O₃.

Results and discussion

General consideration

All the measurements were formulated under the pseudo-first-order reaction conditions in which concentrations of acetamiprid and HClO₄ were taken in large excess over MnO₂. The pseudo-first-order rate constants were calculated from the slope of log A versus time plot. The plot of log A versus time at a typical fixed concentration of acetamiprid (5.0 × 10⁻³ mol dm⁻³), MnO₂ (8.0 × 10⁻⁵ mol dm⁻³) and HClO₄ (6.0 × 10⁻⁴ mol dm⁻³) at 30 °C shown in Fig. 3 is represented by a straight line with r² = 0.989. This indicates that the reaction obeys first-order kinetics with respect to MnO₂ under the adopted reaction conditions.

Fig. 3 Plot of log(absorbance) versus time for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

Effect of concentrations of acetamiprid, MnO₂ and HClO₄ on the reaction rate

The dependence of the rate of reaction on the concentration of acetamiprid has been studied by conducting the kinetic measurements at the varying concentration of acetamiprid (1.0 × 10⁻³ to 1.5 × 10⁻² mol dm⁻³) keeping the concentrations of MnO₂ (8.0 × 10⁻⁵ mol dm⁻³) and HClO₄ (6.0 × 10⁻⁴ mol dm⁻³) constant at a typical temperature of 30 °C. Under the said conditions, the values of observed rate constant (k_obs) so obtained are plotted against the concentration of acetamiprid in Fig. 4. This figure clearly indicates that the variation is nonlinear and there is a continuous increase in the values of the rate constant with increasing concentration of acetamiprid up to 1.0 × 10⁻² mol dm⁻³ beyond which the rate constants tend to remain constant. The plot between log k_obs and log[acetamiprid] is linear (Fig. 5) with a slope of 0.315 (r² = 0.996) indicating that the reaction is fractional-order (i.e. 0.315) with respect to acetamiprid.

Fig. 4 Effect of [acetamiprid] on k_obs for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

Fig. 5 Effect of log[acetamiprid] on log k_obs for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

In this context, it is worth relevant to notice that the oxidative degradation of similar pesticides such as metribuzin²⁷ and tricyclazole³⁰ by colloidal MnO₂ has been observed to obey fractional-order reaction kinetics. These findings are also in conformity with the observation of fractional-order reaction kinetics for the oxidative degradation of a number of other organic compounds such as D-fructose,²¹ glycyl-glycine,²³ glycolic acid²⁴ and thiourea²⁹ by colloidal MnO₂ in HClO₄ medium.

The effect of concentration of HClO₄ (in the range of 2.0 × 10⁻⁴ to 1.2 × 10⁻³ mol dm⁻³) was also studied by conducting a series of kinetic measurements at the fixed concentrations of acetamiprid (5.0 × 10⁻³ mol dm⁻³) and MnO₂ (8.0 × 10⁻⁵ mol dm⁻³) at a constant temperature of 30 °C. The plot of k_obs vs. [HClO₄] shown in Fig. 6 clearly indicates that rate constant increases with increase in [HClO₄] throughout the entire concentration range. The linear plot (Fig. 6) gives a positive intercept on k_obs axis indicating the existence of acid independent and acid dependent paths. The nature of rate constant with acid concentration has further resolved in a better way by making a double logarithmic plot between log k_obs and log[HClO₄] (Fig. 7). This plot is also linear with a slope of 0.236 (r² = 0.988) which is indicative of fractional order dependence of the degradation of acetamiprid with respect to [HClO₄]. On the basis of above observations the rate (ν) of degradation of acetamiprid by colloidal MnO₂ in the HClO₄ medium may be represented by the following experimental rate law equation:

ν = −d[MnO₂]/dt = (k_I + k_D[HClO₄]^0.236)[acetamiprid]^0.315[MnO₂] (2)

where, k_I and k_D are rate constants for the acid independent and dependent paths, respectively.

Fig. 6 Effect of [HClO₄] on k_obs for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, temperature = 30 °C).

Fig. 7 Effect of log[HClO₄] on log k_obs for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, temperature = 30 °C).

The acid independent reaction path taking place may presumably occur by the adsorption of acetamiprid on the colloidal particles of MnO₂ followed by reaction between adsorbed acetamiprid molecule and one of the MnO₂ molecules pertaining to the colloidal surface leading to the reaction as represented by the following plausible mechanism:

where, (MnO₂)_x and O-acetamiprid represent colloidal MnO₂ and intermediate oxidative degrades of acetamiprid, respectively.

On the other hand, the acid dependent path comprises with the adsorption of two hydrogen ions, in addition to acetamiprid molecule, on the colloidal surface of MnO₂ leading to the degradation of acetamiprid by the following plausible mechanism:

Effect of surfactants

In the present investigation commonly used surfactants such as CTAB (cationic), SDS (anionic) and TX-100 (non-ionic) have been used to study their role on the degradation kinetics of acetamiprid. The concentrations of each surfactant were selected above its critical micelle concentration (cmc). The cmc values of the chosen surfactants, i.e. CTAB, SDS and TX-100 in the presence of HClO₄ under experimental conditions were determined by electrical conductivity measurements and found to be 5.2 × 10⁻⁴, 4.6 × 10⁻³ and 1.9 × 10⁻⁴ mol dm⁻³, respectively. The effect of concentrations of surfactants on the rate constant has been studied at the temperature of 30 °C by keeping the concentrations of MnO₂ (8.0 × 10⁻⁵ mol dm⁻³), acetamiprid (5.0 × 10⁻³ mol dm⁻³) and HClO₄ (6.0 × 10⁻⁴ mol dm⁻³) constant. It has been observed that SDS (4.7 × 10⁻³ to 8.0 × 10⁻³ mol dm⁻³) has no effect on the value of the rate constant. This is due to repulsion between anionic micellar aggregates of SDS (the micelles have a net negative charge due to – OSO₃⁻) and the negatively charged colloidal MnO₂. In this context, it is worth relevant to mention here that it has been well established that the water soluble colloidal MnO₂ in aqueous media is stabilized by adsorption of anions resulting negative charge on its particles.^17–19 On the other hand the reaction in the presence of CTAB (5.3 × 10⁻⁴ to 8.0 × 10⁻³ mol dm⁻³) could not be followed as well because it possesses the positive charge opposite to that of colloidal MnO₂ causing flocculation and thereby the turbidity of reaction mixture was observed. It is presumably due to adsorption of cationic CTAB on the surface of negatively charged colloidal particles of MnO₂ resulting in neutralization of their electrostatic charge and precipitation occurs. The problem of flocculation has also been observed by a number of earlier investigators for the redox reactions involving colloidal MnO₂ as oxidant in the presence of CTAB.^20,23–26 However, the addition of non-ionic surfactant, TX-100 enhances the rate of reaction. The plots between log(absorbance) versus time in the presence of different concentration of TX-100 (2.0 × 10⁻⁴ to 8.0 × 10⁻³ mol dm⁻³) were also observed to be linear, as in absence of surfactants, which confirms that the reaction is also first-order with respect to MnO₂ in presence of TX-100. Thus, in other words, the order of reaction with respect to MnO₂ remains the same as that observed in the absence of surfactants. The values of the pseudo-first-order rate constant (k_Ψ) in presence of TX-100 are drawn against [TX-100] in Fig. 8. There is a continuous increase in the rate constant with increasing concentration of TX-100 up to a concentration of about 4.0 × 10⁻³ mol dm⁻³ beyond which it tends to become stagnant. Obviously the catalytic effect is more pronounced in lower concentration range.

Fig. 8 Effect of [TX-100] on k_Ψ for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

While studying the kinetics of the reaction between colloidal manganese dioxide and formic acid in aqueous perchloric acid solution, Tuncay et al.²⁰ have developed an empirical model to explain the catalytic effect of the surfactant, TX-100. Later on this model has successfully been used to describe the role of TX-100 for the oxidative degradation of a number of compounds by colloidal MnO₂ by different investigators.^22–27,30 According this concept the rate constant (k_Ψ) in the presence of the surfactant TX-100 has to obey the following relationship:

1/(k_Ψ − k_obs) = a + b/[TX-100] (7)

where a and b are empirical constants. Thus, according to the Tuncay concept a plot of 1/(k_Ψ − k_obs) versus 1/[TX-100] should be linear, which was also observed in the present case (Fig. 9) with a = 512 s and b = 8.034 mol dm⁻³ s (r² = 0.991).

Fig. 9 Plot of 1/(k_Ψ − k_obs) versus 1/[TX-100] for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

In order to further explain the role of TX-100, an alternative empirical equation was also suggested by Tuncay et al.²⁰

log k_Ψ = c log[TX-100] + d (8)

where c and d are empirical constants. The plot between log k_Ψ and log[TX-100] (Fig. 10) was linear which resulted the value of c = 0.096 and d = −2.978 (r² = 0.987). The numerical values of these empirical constants strongly depend on the nature and concentration ranges of the reactants as well as reaction conditions.

Fig. 10 Plot of log[TX-100] versus log k_Ψ for the degradation of acetamiprid by colloidal MnO₂ (reaction conditions: [acetamiprid] = 5.0 × 10⁻³ mol dm⁻³, [MnO₂] = 8.0 × 10⁻⁵ mol dm⁻³, [HClO₄] = 6.0 × 10⁻⁴ mol dm⁻³, temperature = 30 °C).

The validity of eqn (7) and (8) confirms that the oxidative degradation of acetamiprid by MnO₂ in presence of TX-100 obeys Tuncay model. The catalytic effect of TX-100 is presumably due to increasing adsorption of reactants in the presence of this surfactant. According to Tuncay et al.²⁰ the driving force for the adsorption of TX-100 is the hydrogen bond formation between polar ethylene oxide and the surface of MnO₂ particles.

Effect of temperature on rate constant and determination of activation parameters

A series of kinetic experiments was also performed at different temperatures in the range of 20–50 °C at the fixed concentrations of acetamiprid (5.0 × 10⁻³ mol dm⁻³), MnO₂ (8.0 × 10⁻⁵ mol dm⁻³) and HClO₄ (6.0 × 10⁻⁴ mol dm⁻³). The values of the rate constant, so obtained at different temperatures are used to calculate the activation energy of the process. The variation of log k_obs against 1/T has been observed to obey the following linear equation which indicates that the system obeys the Arrhenius relationship:

log k_obs = −1114/T + 0.299 (r² = 0.995) (9)

The value of the activation energy (E_a) as calculated from the slope of the above equation is listed in Table 1. In order to realize whether the reaction mechanism is associative or dissociative, the entropy of activation is necessarily required. The values of entropy of activation (ΔS^#) along with other thermodynamic activation parameters such as enthalpy of activation (ΔH^#) and free energy of activation (ΔG^#) may be calculated from following Eyring equation:

log(k_obs/T) = −(ΔH^#/2.3026RT) + log(k_B/h)) + (ΔS^#/2.3026R) (10)

Table 1 Activation parameters for the degradation of acetamiprid by colloidal MnO₂^a

Activation parameters	Values
	Aqueous	TX-100
a Reaction conditions: [acetamiprid] = 5.0 × 10^−3 mol dm^−3, [MnO2] = 8.0 × 10^−5 mol dm^−3, [HClO4] = 6.0 × 10^−4 mol dm^−3) and [TX-100] = 4.0 × 10^−3 mol dm^−3.
Ea/kJ mol^−1	21.33	17.95
ΔH^#/kJ mol^−1	18.78	15.40
ΔS^#/J K^−1 mol^−1	−247.78	−255.42
ΔG^#/kJ mol^−1	93.85	92.78

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In the above equation k_B, R and h represent Boltzman, gas and Plank's constants, respectively. The plot of log k_obs against 1/T has also been observed to follow the following linear equation which shows that the degradation process obeys the Eyring theory:

log(k_obs/T) = −980.9/T − 2.623 (r² = 0.994) (11)

The values of different activation parameters for the degradation of acetamiprid by colloidal MnO₂, as calculated from the slope and intercept of eqn (11) are given in Table 1. In similar manner the values of different activation parameters in presence of surfactant TX-100 have also been calculated and included in Table 1. This table clearly highlights that the contribution of entropy factor is much larger than the enthalpy factor towards the free energy of activation i.e. overall process controlling parameter. A large negative value of the entropy of activation points out the formation of highly ordered associative transition state complex during the degradation process of acetamiprid by colloidal MnO₂ in the presence of HClO₄. Therefore the transition state in the region of activated complex has a more ordered rigid structure than the reactants in the ground state. Further a lower value of activation energy in the presence of surfactant is due to a high catalytic effect of surfactant which provides an alternative path of the lower activation barrier for the reaction.

Conclusions

The kinetic studies for the oxidative degradation of acetamiprid by colloidal MnO₂ in acidic medium have successfully been performed in the absence and presence of surfactants. The rate constants have been determined as a function of the concentrations of acetamiprid, MnO₂ and HClO₄ under the pseudo-first-order reaction conditions. The order of the reaction has been observed to be fractional order in both acetamiprid and HClO₄ under the pseudo-first-order reaction condition with respect to MnO₂. On the basis of variation of the rate constant with the concentration of reactants, rate law equation was established. Effect of common surfactants, namely, CTAB, SDS and TX-100 on the degradation kinetics of acetamiprid by colloidal MnO₂ has also been studied. It has been observed that CTAB causes flocculation while SDS has no considerable effect on the reaction kinetics. However, the significant catalytic role of TX-100 has been observed, which has been discussed and explained in terms of mathematical models of Tuncay et al. The kinetic data as obtained in the present investigation have successfully been exploited to generate different activation parameters for the oxidative degradation of acetamiprid by colloidal MnO₂ in acidic medium.

Acknowledgements

The authors are grateful to the Chairman, Department of Applied Chemistry, Aligarh Muslim University for providing necessary laboratory facilities. One of the authors (Qamruzzaman) is also thankful to the University Grants Commission, New Delhi for the award of Maulana Azad National Fellowship.

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