Degradation of tricyclazole by colloidal manganese dioxide in the absence and presence of surfactants

Abstract

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 surfactant cetyl trimethyl ammonium bromide (CTAB) causes flocculation with oppositely charged 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.

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], [2], [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], [5], [6]. In spite of advantageous and unavoidable uses fungicides often contaminate the environment and cause public health problem due to their high toxicity and long persistence [7], [8], [9], [10]. The tricyclazole residue in paddy rice and environments has been monitored and analyzed by a number of investigators [10], [11], [12], [13], [14], [15], [16], [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 °C 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 (MnO₂). 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 MnO₂ 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 MnO₂ is limited due to its insolubility under ordinary conditions. However, in recent years perfectly transparent colloidal solution of MnO₂ has been prepared by the reduction of neutral or slightly acidic potassium permanganate solution by sodium thiosulfate. The water soluble colloidal MnO₂ has successfully been used for the oxidative degradation of a number of substances such as aspartic 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 different research groups. Literature survey reveals that the degradation studies on the oxidative degradation of tricyclazole are very limited and scarce. Recently, Phong et al. [34] have monitored the fate and transport of tricyclazole in paddy field after nursery-box-application and reported the mean half life value to be 11.8 and 305 days in paddy water and surface soil, respectively. They also pointed out that even less than 0.9% of tricyclazole were lost through run off during the monitoring period under 6.3 cm of rain fall. The interaction between degradation of phenonthrene and tricyclzole in soil and soil–mushroom compost has been studied by Liu et al. [35]. In the present investigation studies on the degradation kinetics of tricyclazole by water soluble colloidal MnO₂ have been conducted.

Surfactants are used to lower the surface tension and can act as wetting, foaming, emulsifying and dispersion agents. A surfactant molecule contains at least one polar hydrophilic part and at least one nonpolar hydrophobic part. The coexistence of two opposite types of units inside the same molecule is the origin of local constraints which lead to spontaneous aggregation into microscopic labile structures [36]. These surface active agents in aqueous medium thus self aggregate at the concentration above critical micelle concentration (cmc) to form association colloids known as micelles. Surfactant micelles offer a relatively large microscopic nonpolar environment for solute partition (solubilization) resulting increase in solubility of solute (apparent water solubility) in micellar media in comparision with water solution [37]. Surfactants are therefore very commonly used in pesticide formulation to increase the solubility of pesticides and also to enhance their effectiveness by fine spray. In fact surfactants essentially consist of nonpolar hydrophobic (tail) and polar hydrophilic group (head) groups. According to the nature of latter group the surfactants may be classified as cationic, anionic, nonionic and zwitterionic. In the present investigation, effect of three common surfactants such as cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and Triton X-100 (TX-100) on the degradation kinetics of tricyclazole by colloidal MnO₂ in presence of HClO₄ has been studied. Selection of these surface active agents is based on the criteria of picking one member from each category of cationic, anionic and nonionic compounds as chosen in respective order. In order to generate various activation parameters the study has also been extended at different temperatures. The kinetic data have been analyzed in the light of Arrhenius and Eyring theories and also discussed in terms of different various activation parameters as generated.