The kinetics of the reduction of colloidal MnO2 by glyphosate has been investigated spectrophotometrically in an aqueous and micellar (cetyltrimethylammonium bromide, sodium lauryl sulfate) media. The reaction follows first-order kinetics with respect to colloidal MnO2 in both the aqueous and micellar media. The rate of oxidation increases with increase in [glyphosate] in the lower concentration range but becomes independent at its higher concentrations. The addition of both the anionic (NaLS) and cationic (CTAB) micelles increased the rate of reduction of colloidal MnO2 by glyphosate while the nonionic TX-100 micelles did not influence the rate of reaction. In both aqueous and micellar media, the oxidation of glyphosate occurs through its adsorption over colloidal MnO2 surface. The reaction in micellar media was treated by considering the pseudophase model. The values of reaction rates and binding constants in the presence of micelles were determined.
Manganese is the eleventh most abundant element in the earth crust and is among the important micronutrients for all micro-organisms. Manganese (III, IV) oxide minerals are thermodynamically stable in the oxygenated environments. These oxide particles in earth crust and in natural water are susceptible for reduction by humic acid and organics [
Glyphosate (N-phosphonomethylglycine) is an aminophosphonic analogue of the natural amino acid, glycine. It is a postemergence nonselective broad spectrum herbicide extensively used in agriculture for the control of many annual and perennial weeds [
Glyphosate, N-phosphonomethylglycine (Excelcropcarelimited, Mumbai), potassium permanganate (Qualigens, India), and sodium thiosulphate (Qualigens, India), cetyltrimethyl ammonium bromide (99.9% CDH, India), sodium lauryl sulphate (98% CDH, India), Triton X-100 (98% CDH, India), H2SO4 (98% CDH, India), acetic acid (Qualigens, India) sodium acetate (Qualigens, India) were used as received. Doubly distilled water was used throughout the experimental work.
Stock solutions of glyphosate (= 1.0 × 10−2 mol dm−3), cetyltrimethyl ammonium bromide, sodium lauryl sulphate and Triton X-100 (= 1.0 × 10−2 mol dm−3) were prepared in doubly distilled water. All the other required solutions were also prepared in deionized water. Colloidal MnO2 was prepared by mixing the standardized solutions of potassium permanganate (10 mL, 0.1 mol dm−3) and sodium thiosulphate (20 mL, 1.88 × 10−2 mol dm−3) as given in the literature [
Kinetic experiments were carried out by taking the requisite amounts of aqueous solutions of glyphosate, colloidal manganese dioxide and surfactant in a three-necked reaction vessel. The reaction vessel was fitted with a double surface condenser to prevent any evaporation. The reaction vessel was kept in a thermostated water bath at the desired temperature (±0.5°C). The reaction vessel containing the reactants was kept in the water bath for sufficient time to attain the temperature of bath. The reaction was started with the addition of colloidal MnO2. The absorbance was measured by means of a Spectronic 20D+ Thermoscientific UV-V is Spectrometer using 1 cm path length quartz cuvette. The progress of the reaction was monitored by measuring the absorbance at
Barrett and McBride [
The order of reaction in [MnO2 ] was determined by measuring the rate of reaction at different initial concentrations of colloidal MnO2 in the concentration range from 0.5 × 10 −4 to 2.5 × 10−4 mol dm−3 at a fixed concentration of glyphosate (= 5.0 × 10−3 mol dm−3) at pH 6 and temperature 30°C. It was observed that the rate constant decreases with increase in concentration of colloidal manganese dioxide This decrease in rate constant may be due to the flocculation of the colloidal MnO2 particles. As the MnO2 concentration further increases, the rate constants become independent of the initial MnO2 concentration. It was observed that the rate constant values were also independent of the initial concentrations of glyphosate, when the initial concentration of glyphosate was taken excess (tenfolds or more) over [MnO2] and, therefore, the kinetic experiments were carried out at lower concentrations of glyphosate also. The dependence of rate constant on [glyphosate] were carried out in the concentration range 2 × 10−4 − 1 × 10−2 mol dm−3 at 2 × 10−4 mol dm−3 MnO2 concentration. It was observed that the reaction followed zero order kinetics in [glyphosate] at higher concentration and fractional order at lower concentrations as depicted in Figure
Plot of dependence of rate constant on [glyphosate] at [MnO2] (= 2 × 10−4 mol dm−3), temperature = 30°C, and pH = 6.
The study on the variation of pH from 4 to 6 at 30°C shows that the reaction rate is independent of the pH in this range. However, the rate of reaction increased linearly with the increase in [H2SO4] as shown in Figure
Plot of rate constant versus [H+] at [glyphosate] (= 5.0 × 10−3 mol dm−3), [MnO2] (= 2 × 10−4 mol dm−3) and Temperature = 30°C.
The rate of oxidation of adsorbed glyphosate at the MnO2 surface is given by
Values of rate constants, adsorption constant, and binding constant for the oxidation of glyphosate by colloidal manganese dioxide.
Rate constant/adsorption constant/binding constant | Aqueous medium | Micellar medium | |
CTAB | SDS | ||
0.0328 | — | — | |
32.248 | — | — | |
— | |||
— | 12 | 8 | |
— | 400 | 40 | |
1509.455 | — | — |
Plot of
The rate of reaction at higher hydrogen ion concentrations mainly occurs via path II in which the adsorbed glyphosate interacts with hydrogen ion and the contribution of hydrogen ion independent path becomes negligible. Thus in presence of sulphuric acid equation (
Thus, according to (
The effect of variation of surfactant concentration on the rate of degradation of glyphosate by colloidal manganese dioxide was studied at 30°C by keeping the concentrations of MnO2, glyphosate constant at 2.0 × 10−4 mol dm−3 and 5.0 × 10−3 mol dm−3, respectively. The increase in CTAB concentrations resulted in an increase in rate of reaction but the rate decreased on further increase in [CTAB] as shown in Figure
Plot for the dependence of rate constant on [CTAB] (•) and [SDS] (▪) at [MnO2] (= 2 × 10−4 mol dm−3), [glyphosate] (= 5.0 × 10−3 mol dm−3), temperature = 30°C, and pH = 6.
The variation of rate constant on [MnO2] and [Glyphosate] in the presence of surfactants showed a similar behaviour as was observed in aqueous medium. These observations indicate that the same mechanism is being followed in both aqueous medium and micellar medium. The increased rate of reaction in the presence of CTAB and SDS may be explained on the basis of pseudophase model of micelles in which reactions are considered to occur both in aqueous and micellar pseudophases with varying rate constants [
In this scheme, “
The overall rate can be expressed as
The pseudofirst-order rate constants in aqueous (
Equation (
The fitting values of
The observed higher rate in CTAB and SDS could be attributed to the binding of both glyphosate and colloidal manganese dioxide on to the micellar surface. It is evident from the higher
Thanking due to the UGC, New Delhi, India for financial support in the form of a Major Research project no. F.No.33-278/207(SR) dated February 28, 2008.