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Hindawi Publishing CorporationInternational Journal of Inorganic ChemistryVolume 2011, Article ID 243519, 6 pagesdoi:10.1155/2011/243519

Research Article

Kinetics of Reduction of Colloidal MnO2 by Glyphosate inAqueous and Micellar Media

Uzma Aisha,1 Qamruzzaman,1 and M. Z. A. Rafiquee1, 2

1 Department of Applied Chemistry, Zakir Hussain College of Engineering and Technology, Aligarh Muslim University,Aligarh 202 002, India

2 Higher Institute of Engineering, Hoon, Libya

Correspondence should be addressed to M. Z. A. Rafiquee, [email protected]

Received 24 November 2010; Revised 23 January 2011; Accepted 10 February 2011

Academic Editor: Hakan Arslan

Copyright © 2011 Uzma Aisha et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The kinetics of the reduction of colloidal MnO2 by glyphosate has been investigated spectrophotometrically in an aqueous andmicellar (cetyltrimethylammonium bromide, sodium lauryl sulfate) media. The reaction follows first-order kinetics with respectto colloidal MnO2 in both the aqueous and micellar media. The rate of oxidation increases with increase in [glyphosate] in thelower concentration range but becomes independent at its higher concentrations. The addition of both the anionic (NaLS) andcationic (CTAB) micelles increased the rate of reduction of colloidal MnO2 by glyphosate while the nonionic TX-100 micelles didnot influence the rate of reaction. In both aqueous and micellar media, the oxidation of glyphosate occurs through its adsorptionover colloidal MnO2 surface. The reaction in micellar media was treated by considering the pseudophase model. The values ofreaction rates and binding constants in the presence of micelles were determined.

1. Introduction

Manganese is the eleventh most abundant element in theearth crust and is among the important micronutrients forall micro-organisms. Manganese (III, IV) oxide minerals arethermodynamically stable in the oxygenated environments.These oxide particles in earth crust and in natural water aresusceptible for reduction by humic acid and organics [1–4]. Thus, the humic and organic compounds are degradedinto simpler molecules through the abiotic mechanisms.The oxidising and catalytic properties of manganese oxidehave been limited due to its insolubility under ordinaryconditions. However, in recent years, water-soluble perfectlytransparent manganese dioxide has been prepared, by thereduction of alkaline potassium permanganate solutionthrough γ-irradiation and also, by the reduction of neutral orslightly acidic potassium permanganate solution by sodiumthiosulphate [5–11].

Glyphosate (N-phosphonomethylglycine) is an amino-phosphonic analogue of the natural amino acid, glycine. Itis a postemergence nonselective broad spectrum herbicideextensively used in agriculture for the control of many

annual and perennial weeds [12]. Glyphosate is less toxicthan a number of other herbicides and pesticides, such asthose from the organochlorine family. It is essentially non-toxic to mammals and birds, but fish and invertebrates aremore sensitive to this herbicide. Glyphosate degrades rapidlyin soils under most conditions [13, 14]. In view of thesignificance of manganese dioxide and glyphosate, kineticstudies on the degradation of glyphosate by colloidal MnO2

has been carried out. The influence of ionic (cetyl trimethylammonium bromide and sodium dodecyl sulphonate) andnonionic (triton X-100) surfactants has also been studied.Surfactants are used in pesticide formulations [15–17] toincrease the solubility of pesticide, to stabilize, to controlevaporation or decomposition, and to enhance the effective-ness of pesticides by providing fine spray. The surfactant alsoacts as wetting, dispersing and emulsifying agent. Surfactantsaggregates mimic enzyme structurally and functionally, and,therefore, the study in CTAB and SDS micelles will be helpfulin understanding the mechanism of degradation of herbicidein humic and biological media. The observed data will also behelpful in the determination and interpretation of the fate ofherbicide after its dispersal in the environment.

2 International Journal of Inorganic Chemistry

2. Experimental

2.1. Materials. Glyphosate, N-phosphonomethylglycine (Ex-celcropcarelimited, Mumbai), potassium permanganate(Qualigens, India), and sodium thiosulphate (Qualigens,India), cetyltrimethyl ammonium bromide (99.9% CDH,India), sodium lauryl sulphate (98% CDH, India), TritonX-100 (98% CDH, India), H2SO4(98% CDH, India), aceticacid (Qualigens, India) sodium acetate (Qualigens, India)were used as received. Doubly distilled water was usedthroughout the experimental work.

Stock solutions of glyphosate (= 1.0 × 10−2 mol dm−3),cetyltrimethyl ammonium bromide, sodium lauryl sulphateand Triton X-100 (= 1.0 × 10−2 mol dm−3) were preparedin doubly distilled water. All the other required solutionswere also prepared in deionized water. Colloidal MnO2

was prepared by mixing the standardized solutions ofpotassium permanganate (10 mL, 0.1 mol dm−3) and sodiumthiosulphate (20 mL, 1.88 × 10−2 mol dm−3) as given in theliterature [9]. The resulting brown solution was transparentand obeyed Beer-Lamberts law within the concentrationrange of present studies. pH of the solution was adjustedusing sulphuric acid solution and its measurement was madeusing Elico LI-120 pH meter.

2.2. Kinetic Measurements. Kinetic experiments were carriedout by taking the requisite amounts of aqueous solutions ofglyphosate, colloidal manganese dioxide and surfactant in athree-necked reaction vessel. The reaction vessel was fittedwith a double surface condenser to prevent any evaporation.The reaction vessel was kept in a thermostated water bathat the desired temperature (±0.5◦C). The reaction vesselcontaining the reactants was kept in the water bath forsufficient time to attain the temperature of bath. The reactionwas started with the addition of colloidal MnO2. Theabsorbance was measured by means of a Spectronic 20D+Thermoscientific UV-V is Spectrometer using 1 cm pathlength quartz cuvette. The progress of the reaction was mon-itored by measuring the absorbance at λmax (= 365) becausethe colloidal manganese dioxide showed strong absorptionat this wave length while the absorbance by the complexesof manganese (II) and manganese (III) was negligible at thiswave length. All the kinetic experiments were run under thefirst-order reaction condition in which the concentrationsof surfactant and glyphosate were kept in large excess over[MnO2]. Kinetic experiments were performed under thevarying conditions of temperature and concentrations ofreactants, surfactants and sulphuric acid. The pseudo first-order rate constants for the oxidation of glyphosate werecalculated from the slope of the ln(absorbance) versus time.

3. Result and Discussion

Barrett and McBride [1] reported that glyphosate is degradedin dilute aqueous suspensions of birnessite [MnO2], amanganese oxide common in soil, to yield phosphate ionin solution. The abiotic degradation of glyphosate involvesthe C–P bond cleavage at the manganese oxide surface. Asimilar degradation was observed when colloidal MnO2 was

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

Rat

eco

nst

ant,k

(s−1

)

0 2 4 6 8 10 12

103 [glyphosate] (mol dm−3)

Figure 1: Plot of dependence of rate constant on [glyphosate] at[MnO2] (= 2 × 10−4 mol dm−3), temperature = 30◦C, and pH = 6.

added to glyphosate solution. The brown colour of colloidalMnO2 faded away slowly giving glycine and phosphate at theend of reaction. The presence of glycine and phosphate wastested using ninhydrin reagent and ammonium molybdatesolution, respectively.

The order of reaction in [MnO2] was determined by mea-suring the rate of reaction at different initial concentrationsof colloidal MnO2 in the concentration range from 0.5 ×10−4 to 2.5 × 10−4 mol dm−3 at a fixed concentration ofglyphosate (= 5.0× 10−3 mol dm−3) at pH 6 and temperature30◦C. It was observed that the rate constant decreases withincrease in concentration of colloidal manganese dioxideThis decrease in rate constant may be due to the floc-culation of the colloidal MnO2 particles. As the MnO2

concentration further increases, the rate constants becomeindependent of the initial MnO2 concentration. It wasobserved that the rate constant values were also independentof the initial concentrations of glyphosate, when the initialconcentration of glyphosate was taken excess (tenfolds ormore) over [MnO2] and, therefore, the kinetic experimentswere carried out at lower concentrations of glyphosatealso. The dependence of rate constant on [glyphosate] werecarried 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 kineticsin [glyphosate] at higher concentration and fractional orderat lower concentrations as depicted in Figure 1.

The study on the variation of pH from 4 to 6 at 30◦Cshows that the reaction rate is independent of the pH in thisrange. However, the rate of reaction increased linearly withthe increase in [H2SO4] as shown in Figure 2. The reactionbetween the colloidal MnO2 and glyphosate takes place at thesurface of colloidal manganese dioxide after the adsorptionof glyphosate over the MnO2 surface. The reaction followstwo independent paths namely; path I, which is hydrogenion independent and path II is hydrogen ion dependent. Theoverall reaction rate may be presented through Scheme 1 asfollows.

The rate of oxidation of adsorbed glyphosate at theMnO2 surface is given by

V = kθ[MnO2], (1)

International Journal of Inorganic Chemistry 3

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

Rat

eco

nst

ant,k

(s−1

)

0 2 4 6 8 10 12

104 [hydrogen ion] (mol dm−3)

Figure 2: 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.

140

120

100

80

60

40

20

0

1/ra

teco

nst

ant

(s)

0 1 2 3 4 5 6

10−3 (1/[glyphosate] (mol−1 dm3))

Figure 3: Plot of 1/kobs versus 1/[glyphosate] for [MnO2] (= 2 ×10−4 mol dm−3), temperature = 30◦C, and pH = 6.

where k is the overall rate constant and θ is the extent ofadsorption of glyphosate over MnO2 surface and in terms ofequilibrium constant Kad, it is given by

θ =(Kad[Glyphosate

])

(1 + Kad

[Glyphosate

]) . (2)

On putting the value of θ, corresponding to the aboveScheme, the rate of reaction is given by

V =(k1 + k2

[H3O+])Kad

[Glyphosate

][MnO2]

(1 + Kad

[Glyphosate

]) , (3)

where k1 is rate constant for the hydrogen ion concentrationindependent path and k2 is the rate constant for the hydrogenion concentration dependent path. At low hydrogen ionconcentration, the reaction proceeds through path I andreaction occurring through path II is negligible. Equation (3)can be rewritten in terms of observed rate constant as

kobs = k1Kad[Glyphosate

]

(1 + Kad

[Glyphosate

]) , (4)

1kobs

= 1k1

+1

(k1Kad

[Glyphosate

]) . (5)

Table 1: Values of rate constants, adsorption constant, and bindingconstant for the oxidation of glyphosate by colloidal manganesedioxide.

Rate constant/adsorptionconstant/binding constant

Aqueousmedium

Micellar medium

CTAB SDS

k1 (s−1) 0.0328 — —

k2 (s−1)102 32.248 — —

km (mol dm−3 s−1) — 1.88± 0.37 15.80± 0.07

Kn (mol dm−3) — 12 8

Ks (mol dm−3) — 400 40

Kad (mol dm−3) 1509.455 — —

According to (5), a straight-line plot for 1/kobs versus1/[Glyphosate]) should be obtained. The observed results asgiven in Figure 3 support the proposed mechanism for theplot of 1/kobs versus 1/[Glyphosate]. The values of k1 andKad were obtained from the slope and intercept of the plot,respectively and are given in Table 1.

The rate of reaction at higher hydrogen ion concen-trations mainly occurs via path II in which the adsorbedglyphosate interacts with hydrogen ion and the contributionof hydrogen ion independent path becomes negligible. Thusin presence of sulphuric acid equation (3) can be rewrittenas:

kobs = k2[H3O+], (6)

where it is assumed that Kad[Glyphosate] ≥ 1.Thus, according to (6) a straight-line plot should be

obtained for kobs versus [H−3O+] and the value of k2

(Table 1) can be obtained from the slope.The effect of variation of surfactant concentration on

the rate of degradation of glyphosate by colloidal manganesedioxide was studied at 30◦C by keeping the concentrationsof MnO2, glyphosate constant at 2.0 × 10−4 mol dm−3 and5.0 × 10−3 mol dm−3, respectively. The increase in CTABconcentrations resulted in an increase in rate of reaction butthe rate decreased on further increase in [CTAB] as shownin Figure 4. The maximum value of rate constant was foundto be 0.212 s−1 at [SDS] = 4.0 × 10−4 mol dm−3.The additionof SDS also increased the rate of reaction for the degradationof glyphosate by colloidal manganese dioxide (Figure 4). Theaddition of TX-100 did not influence the rate of reaction.

The variation of rate constant on [MnO2] and[Glyphosate] in the presence of surfactants showed a similarbehaviour as was observed in aqueous medium. Theseobservations indicate that the same mechanism is beingfollowed in both aqueous medium and micellar medium.The increased rate of reaction in the presence of CTAB andSDS may be explained on the basis of pseudophase modelof micelles in which reactions are considered to occur bothin aqueous and micellar pseudophases with varying rateconstants [18–22]. Scheme 2 has been proposed for reactionsoccurring in aqueous and micellar media as follows.

In this scheme, “G” denotes glyphosate “Dn” themicellized surfactant and Ks the binding constant betweensurfactant and glyphosate and Kn is the binding constant


C

O

H

C

O

C

O

H

P

O

C

O

H

P

O

[O O]

C

O

Fast

Fast

(I)

HMnO2

Mn

C

O

H

P

O

+

+

+

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

NH

NH

NH

+

NH

P

O

O O[ ]

H

C

O

C

O

H

P

O

Path I

Path II

OH Mn

Slow

+

NH

+

NH

+

NH3 + HCOOH + Mn2+ + H3PO4

NH3 + HCOOH + Mn2+ + H3PO4 + (MnO2)

OH

(II)Slow

O−

O−

O−

O−

O−

O−O−

O−

O−

O−

O−

O−

O−

OH + (MnO2)n

Kad

n

k1

O• + HMnO2 + (MnO2)n−1

H2O

Kad

OH + (MnO2)n

k2, (H3O+)

CH2•

n−1

n

+

Scheme 1

International Journal of Inorganic Chemistry 5

0.25

0.2

0.15

0.1

0.05

02 4 6 8 10 12

Rat

eco

nst

ant

(s−1

)

103 [surfactant] (mol dm−3)

Figure 4: 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.

Ks

kw km

Products

Kn

++

MnO2mMnO2a + Dn

GmGa + Dn

Scheme 2

between surfactant and MnO2. Subscript a and m denoteaqueous and micellar pseudophase, respectively.

The overall rate can be expressed as

v = kΨ[GT] = k[GT][MnO2T]

= k′w[Gw][MnO2w] + k′m[Gm][MnO2m],(7)

where

[GT] = [Gw] + [Gm],

[MnO2T] = [MnO2w] + [MnO2m],(8)

kΨ = k′w + k′mKs[Dn]1 + Ks[Dn]

. (9)

The pseudofirst-order rate constants in aqueous (k′w) andmicellar (k′m) pseudophase are given by

k′w = kw[Gw],

k′m =km[Gm]

[Dn]= kmmG,

(10)

where mG is mole ratio of micellar bound glyphosate tosurfactant headgroup km and kw are second order rateconstants for reactions occurring in micellar and aqueouspseudophase, respectively.

Equation (9) can now be expressed in terms of secondorder rate constants as

kΨ = kw[Gt] + (kmKn − kw)mG[Dn]1 + Kn[Dn]

. (11)

The values of mG (= [Gm]/[Dn]) were determined byapplying the mass action model for the distribution ofglyphosate in the aqueous and micellar pseudophases usingthe following relationship [18]:

KS[Gm]2 − (KS[Dn] + KS[Gt] + 1)[Gm] + KS[Dn][Gt] = 0.(12)

The fitting values of km, Kn, and KS were obtained fromthe computer program by minimizing the deviation betweenthe simulation and the observed values for kΨ-[CTAB] andkΨ-[SDS] profiles. The values of these parameters are givenin Table 1.

The observed higher rate in CTAB and SDS could beattributed to the binding of both glyphosate and colloidalmanganese dioxide on to the micellar surface. It is evidentfrom the higher Kn values that the colloidal manganesedioxide is adsorbed strongly to both the CTAB and SDSmicelles. The binding of colloidal manganese dioxide isto the cationic micellar surfaces of CTA+ may probablythrough the oxygen atom. The strong adsorption of colloidalmanganese dioxide to SD− may occur via electropositivemanganese atom. In the presence of surfactants, the reactionsmainly occur in the Stern layer of the micelles. Withthe increase in [CTAB] and [SDS] the local molarities ofcolloidal manganese dioxide and glyphosate in the micellarpseudophase increase and, therefore, the rate of oxidationof glyphosate by colloidal manganese dioxide increases. Thedecrease in rate of oxidation at higher CTAB concentrationsmay be due to the increase in CTAB concentration resultsinto the formation of increased number of micelles andcauses in the dilution of micellar bound colloidal manganesedioxide. It, therefore, results into decrease in concentrationsof colloidal manganese dioxide in the micellar pseudophaseand thus, the reduction rate for colloidal manganese dioxideis decreased.

Acknowledgment

Thanking due to the UGC, New Delhi, India for financialsupport in the form of a Major Research project no. F.No.33-278/207(SR) dated February 28, 2008.

References

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