INTRODUCTION  

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1. GENERAL INTRODUCTION

The introduction of the thesis entitled “Kinetics of the degradation of

pesticides in the organized media” includes the brief description about the pesticides,

surfactants and their role on the degradation of the pesticides by different oxidizing

agents like colloidal manganese dioxide, chloramine-T and potassium permanganate.

1.1 PESTICIDES AND THEIR CLASSIFICATION

The term pesticide is derived from Latin word cida means “to kill”. It is

defined [1-5] as any substance, preparation or organism used to protect plants, wood

or plants products from harmful organisms; to regulate the growth of plants; to give

protection against the harmful creatures or to render such creatures harmless. Thus the

term pesticide embraces large group of compounds which includes avicides,

chemosterilants, defoliants, fungicides, growth regulators, herbicides, insecticides,

nematicides, piscicides, repellants, rodenticides, surface biocides, wood preservatives

etc. The pesticides have saved millions of lives by controlling human disease vectors,

greatly increasing the yields of agricultural crops and protecting foodstuffs. In India,

agriculture is still the main source of livelihood and the pesticides are among the most

useful tools available to man in order to get the crops properly grown and fruitfully

harvested. Therefore, successful large –scale agriculture requires that the vegetation

should be free from disease and the land reserved for commercial crops should be free

from unwanted pests and weeds growth. Thus pesticides are important component of

high-input farming and are given a share of credit for the green revolution. On the

other hand, usage of pesticides in agriculture has imposed many direct and indirect

undesirable effects on the environment. Hence, there is a strong tendency in our

present civilization to resort to the use of pesticides. Indiscriminate use of pesticides

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like dichlorodiphenyl tricholroethane (DDT), dieldrin and aldrin is of great concern

because these chemicals are passed on from one organism to another through all the

links of food chain and can ultimately be accumulated in the fat of human body [6-

10].

Pesticides usually have high stability, low vapour pressure and very low

solubility in water, but have substantial solubility in oils and fats. For example, the

concentration of DDT in fresh water is about 0.00001 ppm. It tends to become more

concentrated from a tropical level to another due to the process of biological

magnification. As a result of biological magnification, fish and predatory birds are

very susceptible to chlorinated hydrocarbons like DDT, dieldrin and aldrin. Dieldrin

has been found to affect the calcium metabolism in predatory birds and this has

resulted in the reproductive failure. Thus there is a genuine need of study on the fate

of a pesticide after its dispersal in the environment. It is also needed to study the

nature and harmful effect of the various products formed after the degradation of the

pesticide occurring under varying environmental conditions. The pesticides have been

broadly classified into two classes namely chemical pesticides and biopesticides on

the basis of their sources [11-14].

1.1.1 Chemical Pesticides: Chemical pesticides have been classified into three

categories:

(i) Inorganic pesticides: The commonly used inorganic pesticides contain arsenic,

antimony, boron, copper, fluorine, lead, manganese, mercury, selenium,

sulphur, thallium, tin and zinc as their active ingredients [15-20]. Although

these products are not effective for pesticidal use, but being non-biodegradable

in nature, remains persistent for long in the soil. In some cases they have the

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harmful effect on the soil, contaminate the underground water, adversely affect

the quality and fertility of soil and also damage the crops [21-24].

(ii) Organic pesticides: The use of organic pesticides started from 1940 and since

then it has progressed so rapidly and widely that today about 1500 pesticides in

the form of more than 10,000 formulations are available around the world [25-

27]. These chemicals are found to be much successful in controlling the pests

and weeds [28-30]. The general classes of organic pesticides are given in the

Table 1.1.

• Organochlorines: The examples of organochlorines are DDT, BHC,

endosulfan, aldrin, heptachlor, chlordane etc. They are not biodegradable and

remain persistent for long period in the environment [31,32]. They are

insoluble in water but readily soluble in lipids and fats. They are accumulated

in the body fat of mammals and even a low concentration of it may pose

problems in the long run.

• Organophosphates: These include diazinon, dimecron, dimethoate, malathion,

parathion etc. Organophosphates have an advantage of having a non-lipophilic

nature, moderate persistence, systemic nature and more potency than

chlorinated pesticides. They are sparingly soluble in water and biological

fluids. They are highly toxic since they inhibit the activity of enzymes [33,34].

• Carbamates: These include carbaryl, carbofuran, baygon, dinocap etc. They are

relatively more soluble in water than organophosphates and organochlorines.

They have reversible mode of action, less persistence, systemic nature and

adequate potency against crop pests. Normally they do not accumulate in

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tissues and are rapidly excreted when ingested. Thus these pesticides are

considered to be less toxic than organophosphates [35,36].

• Carboxylic acid derivatives: These include TCA (Trichloroacetate), dalapon, 2,

4-D (2, 4-dichlorophenoxy acetic acid), 2, 4, 5-T (2, 4, 5-trichlorophenoxy

acetic acid), MCPA (4-chloro-2-methylphen-oxyacetic acid) etc. They are

considered as a potent source of environmental hazards, especially when their

degradation products remain active in the environment after desired purpose is

over. They are used as herbicides both as agricultural lands and in eradicating

weeds in non-productive areas such as roadsides and electric transmission lines

[37,38].

• Substituted ureas: The substituted ureas such as monuron and diuron form

another class of biocides. These are primarily herbicides. Their solubility in

water is generally low. They do not create problems of persistence and

accumulation due to their facile environmental reactions. These are moderately

unstable [39,40].

• Triazines: The triazines such as atrazine and simazine are used in large

quantities as a pre-emergent herbicide in corn fields. Biologically, dealkylation

and substitution are the major routes of metabolic breakdown. In soils also,

dealkylation appears to be the major route. There is a considerable persistence of

the resulting products in soil [41,42].

• Pyrethroids: This group includes cypermethrin, deltamethrin and fenvalerate

which are commonly used in Indian agriculture. They are very stable in sunlight.

These are effective against agricultural pests [43,44].

• Neem products/Neem based formulations: The versatile neem tree is a wonder

and the limonoids present in it and its products have made it a harmless and

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useful insecticide, bactericide, fungicide, pesticide etc. Neem crudes such as its

kernel crush and oil which are the potential source of bio-actives were

formulated as ready to use dust and water dispersible powder and emulsifiable

concentrate respectively [45,46].

• Others: These include thiocyanates, dinitrophenols, formamides etc.

Thiocyanates have creosote like odours, are relatively safe to use around

humans and animals, and give astonishing quick knockdown of flying insects.

They interfere with cellular respiration and metabolism. The nitrophenols have

been used as ovicides, insecticides, acaricides, herbicides, fungicides and

blossom thinning agents. The formamides comprise a very new, small, but

promising group of insecticides. They are used to control the pests that are

resistant to organophosphates and carbamate [47,48].

(iii) Organic pesticides containing metal ions: These are relatively less familiar as of

other pesticides. The activity of these pesticides depends on the chelating action

of the metal ion as well as the activity of the organic matrix [49,50]. Some of the

organic pesticides containing metal ions are given in the Table 1.2.

1.1.2 Biopesticides: Biological controls of pests or biopesticides have been suggested

as an effective substitute for chemicals. It is the control of harmful pests by

using other pests, plants or any such living body. The controlling agents include

parasites, predators, diseases, protozoa and nematodes that attack pests.

Biopesticides are derived from natural materials such as animals, plants, bacteria

and certain minerals e.g. canola oil and baking soda have pesticidal applications

and are considered biopesticides [51,52].

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1.2 FORMULATIONS OF PESTICIDES

Pesticides are useful as well as harmful or sometimes toxic chemicals

and need a lot of caution and care during handling and application. Pesticides

are available in various formulations [53-55]. The pesticide itself, or the

ingredient which actually has the toxic action, is called the active ingredient.

Pesticide formulation is a combination of the active ingredient and one or more inert

ingredients. Inert ingredients usually are added to improve the effectiveness of the

active ingredient. These inert ingredients usually determine the method of application.

Pesticides are formulated in various ways keeping in view the major

advantages and disadvantages associated with different types of formulations. During

formulations special cares are taken keeping in view of the aspects that affect human

health, plant damage and equipment wear. Pesticides are formulated to improve such

characteristics as handling, persistence on foliage, safety, ease of application and

ability to mix with water. There are many different formulation types. The important

formulations are described as follows:

1.2.1 Dusts : Dusts are dry materials that are made by the combination of the

active ingredient with an inert material that act as a diluent or carrier.

Dust usually contain one to five percent active ingredient and is

designed for dry application. Dusts can be effective in hard to reach

places such as cracks and crevices for cockroach or ant control. Dust

formulation has lost popularity, because of equipment difficulties,

excessive drift and applicator inhalation hazard.

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1.2.2 Sprays : Sprays are materials in which active ingredients are

formulated as a liquid solution. Emulsifying additive is needed for

water insoluble active ingredients.

1.2.3 Wettable Powder : For an active ingredient insoluble in water, the

pesticide may be formulated as a wettable powder. Wettable powder

are dry, finely ground formulations that are similar to dust, but are

mixed with water and an emulsifier is also added to aid in mixing.

Wettable powders typically contain at least 50 percent active

ingredient. These formulations require vigorous and continuous

agitation during application. They are easy to store and fairly easy to

measure and mix. They are easily inhaled if mishandled. They leave

visible residues when they dry which can be a problem on ornamental

plants in high traffic areas.

1.2.4 Dry Flowables or Water Dispersible Granules : Dry flowables or

water dispersible granules are just like wettable powder formulations,

except the active ingredient is prepared as granule-sized, bead-like

particles, not as a powder. Flowables are formulated by impregnating

the active ingredient on a diluent or carrier, such as clay or talc. The

big advantage compared to wettable powders is that water dispersible

granules and dry flowables are easier to measure and don’t pose the

inhalation concern when measuring and mixing. Flowable formulations

are used with pesticide that can be produced only in solid or semi solid

form.

1.2.5 Emulsifiable Concentrates : Emulsifiable concentrates have an active

ingredient that does not dissolve in water. So the active ingredient is

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mixed with at least one petroleum-based oil solvent. To aid in mixing

with water, an emulsifier is added. The emulsifiers are surface-active

agents that promote mixing with water to form a suspension. Emulsifiable

concentrates are the most common formulations used today. They are

relatively easy to handle and measure, little agitation is required to get

them initially mixed or remixed if they settle, not abrasive to most

application running and leave little visible residues on treated surfaces.

1.2.6 Microencapsulated : Microencapsulated pesticides are impregnated

into tiny, slow release plastic beads and mixed into a liquid. This

formulation reduces dermal toxicity and increase residual efficacy.

1.2.7 Wetting Agents or Surfactants : These are used to improve spread of

a spray mixture on foliage. Surfactants are most commonly used to

apply pesticides on plants that have waxy or hairy leaves. Surfactants

help to increase the uptake of chemicals.

1.2.8 Aerosols : The aerosol formulations are self-contained units- bug

sprays. These units contain an inert gas under pressure and when the

trigger is released the formulation is dispersed in very fine droplets.

Because of small droplet size, aerosol formulations have a high drift

potential and an increased risk of inhalation exposure. Aerosols are

most appropriate for use within buildings, although there are types

available for limited outdoor treatment. Insecticidal fogs generated by

heating a mixture of an insecticide in a solvent such as kerosene utilize

a similar principle. The fogs are used outdoors to control certain flying

insects such as mosquitoes and flies.

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1.2.9 Fumigants : Fumigants are volatile pesticides that kill pests with

vapours. Because fumigants are very toxic and replace the air with

toxic gases, they require special precautions and protective equipment

when handling and applying. These can be used only where the gas can

be confined, such as in storage bins, in the soil, within building or

under gas-tight tarps. Fumigants are non-selective and highly toxic to a

wide range of living organisms e.g. nematodes, weeds, fungi and

insects.

1.3 DEGRADATION OF PESTICIDES

The breaking down/transformation of a complex substrate into simpler

products is called degradation. Degradation of pesticides is necessary to

convert them into non-toxic compounds because not all pesticides reaching to

the soil are biodegradable. Thus new methods are currently being set up for

monitoring the degradation of different pesticides and analysis of their

residual products in order to minimize the adverse effects of these

degradation products. However, degradation is detrimental when a pesticide is

destroyed before the target pest has been controlled. Three general types of

pesticides degradation are microbial, chemical and photo degradation.

Pesticides reaching to the soil are acted upon by several physical,

chemical and biological forces. However, physical and chemical forces are

acting upon/degrading the pesticides to some extent, microorganism’s plays

major role in the degradation of pesticides. Many soil microorganisms have

the ability to act upon pesticides and convert them into simple non-toxic

compounds. Microbial degradation [56,57] is the breakdown of pesticide by

fungi, bacteria, and other micro-organisms that use pesticide as a food source.

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Microbial degradation depends not only on the presence of microbes with the

appropriate degradative enzymes, but also on a wide range of environmental

parameters [58]. Most microbial degradation of pesticide occurs in the soil.

Soil conditions such as moisture, temperature, aeration, pH and the amount of

organic matter affect the rate of microbial degradation because of their direct

influence on microbial growth and activity. Optimum temperature, moisture and

organic matter in soil provide congenial environment for the breakdown or

retention of any pesticide added in the soil. The frequency of pesticide

applications can also influence microbial degradation. Rapid microbial

degradation is more likely when the same pesticide is used repeatedly in a

field. Repeated applications can actually stimulate the build up of organisms

effective in degrading the chemical. As the population of these organisms

increases, degradation accelerates and the amount of pesticide available to

control the pest is reduced.

Chemical degradation is the breakdown of pesticide by processes that

do not involve living organism. Besides the physical and chemical properties

of the pesticides, temperature, moisture, pH and type of soil determine the

reaction that would take place. At low temperatures the chemical may be

relatively inactive, but at high temperatures chemical degradation is

accelerated. One of the most common pesticide degradation reaction is

hydrolysis [59,60]. In this breakdown process the pesticides react with water.

Depending on the nature of pesticide, the hydrolysis may occur in acidic

and/or alkaline conditions.

Photo degradation [61,62] is the breakdown of pesticide by light,

particularly the sunlight. It is an abiotic process in the dissipation of

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pesticides where molecular excitation by absorption of light energy results in

various organic reactions, or reactive oxygen species such as O3 and O2

specifically or nonspecifically oxidizes the functional groups in a pesticide

molecule [63]. Photo degradation can destroy pesticide on foliage, on the soil

surface and even in the air. Factors that influence pesticide photo degradation

include the intensity of the sunlight, properties of the application site, and the

properties of the pesticide.

Pesticides [64] pollution of surface water and ground water has been

recognized as a major problem in many countries because of the persistence of

pollutants in aquatic environment and the subsequent potential adverse health effects.

Various treatment processes have been investigated to reduce pesticide concentration

in water and to minimize potential health risks associated with exposure to the

chemicals through consumption of contaminated waters [65]. It has been suggested

that the combination of physical or chemical methods with biological treatment was

likely a feasible option for the detoxification of pesticide waste water [66]. Physical

unit processes such as nanofiltration and reverse osmosis [67-70], slow sand filtration

[71], and activated carbon adsorption [72-74] as well as chemical oxidation such as

ozonation and advanced oxidation processes [75,76], have proven useful in the

removal of pesticide residues during water treatment. However, neither physical nor

chemical approaches alone can achieve the complete removal of pesticides. Physical

processes merely transfer pollutants to another phase, and it is necessary to destroy

rejected or adsorbed pesticides. Chemical oxidation can lead to incomplete

destruction of pesticide molecules and thus, the formation of undesirable by-products.

Therefore, a combination of physical and chemical unit processes is required and

actually employed to ensure the removal of pesticide residues and by-products from

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drinking water. Chemical oxidation is apparently a key technology for solving the

pesticide removal problems. Various chemical oxidants have been evaluated for such

purpose, including chlorine, chlorine dioxide, KMnO4, Ozone (O3) and hydrogen

peroxide. On the other hand, combinations of chemical oxidants (such as O3 and

H2O2), iron salts, semiconductors (titanium dioxide, TiO2), and/or ultraviolet-visible

light(UV-Vis) irradiation yield hydroxyl radicals, which are very powerful oxidants

[77]. Such processes are collectively known as advanced oxidation processes, and

have gained considerable popularity in recent years [78-80]

Barrett et al. [81] investigated the kinetics of degradation of glyphosate by

manganese oxide. They reported the first evidence for an abiotic pathway of

glyphosate and AMPA (aminomethyl phosphonic acid) degradation under

environmentally realistic conditions. Both glyphosate and AMPA degraded at 200C in

dilute aqueous suspensions of birnessite, a manganese oxide common in soils, as

evidenced by the accumulation of orthophosphate in solution over a period of several

days. Rate of degradation was faster at higher temperatures (500C).

The facile degradation of glyphosate by birnessite, involving C-P and C-N

bond cleavage, almost certainly requires bonding of the molecule to the oxide surface,

presumably by inner sphere coordination to Mn(III) or Mn(IV). Electrons are then

transferred to Mn, and the reaction can proceed to some degree in the absence of

dissolved O2. In the process, both Mn2+ and OH- are generated, although release of

Mn2+ into solution may not be detected in aerated systems as the Mn2+ is reabsorbed

or reoxidized by O2 [82]. The proposed mechanism can be represented in Scheme 1.1.

The addition of sulphate to the solution had no marked effect on the reaction rate, as

the trend of accumulating orthophosphate in solution was nearly identical for both the

glyphosate and glyphosate-SO42- treatments. Addition of Cu2+ inhibited degradation.

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As the metal ion complexes strongly with glyphosate, the inhibition can be attributed

to the ability of Cu2+ to limit glyphosate coordination to reactive oxidation sites at the

Mn oxide surface.

Laudien and Mitzner [83] investigated the kinetics and mechanism of

the degradation of phenylureas by hydrolysis in acidic and basic media. In

acidic medium, authors studied the degradation of phenylureas, kinetically in

acidic water-methanol solutions (9:1) at 80oC and 90oC. They reported that

the rate of degradation of phenylureas is not directly proportional to the

hydronium ion concentration, but rises with the acid strength only in solution

of low acidity, whereas it passes through a maximum and decrease again in

stronger acidic solution. They proposed the mechanism for the degradation of

phenylureas under acidic conditions presented in scheme 1.2. In the course of

hydrolytic decomposition, the substrate is first protonated followed by rate

determining attack by water. The tetrahedral intermediate then decomposes to

an amine and a phenyl carbamic acid which again decarboxylates very quickly

under acidic conditions to form the corresponding aniline.

The degradation of phenylureas under basic conditions was studied in

the pH range 12-14 in water- methanol (9:1) solution at 800C and 900C. The

authors [83] observed that the rate of degradation of phenylureas is not

directly proportional to the hydroxide ion concentration, but tends towards a

limiting value at high hydroxide ion concentration. They found that in weakly

basic media the degradation of phenylurea presumably occurs via an addition-

elimination mechanism in which an intermediate hydroxide ion addition

complex is formed. Water acts as a general acid protonates the leaving group

to facilitate its elimination. In stronger alkaline media, the formation of

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PO

O

OH

CH2 NH2+ C

OHCH2Mn3+

O

electron transfer

PO

OH

CH2 NH2+ C

OHCH2Mn2+

O

+ O

Glyphosate

P

O

OH

CH2 NH2+ C

OHCH2

OO+

O

COH

CH2H3C NH+

Sarcosine

P

O

OH

OH

OH

O

COH

CH2HCOOH NH3++

Glycine  

Scheme 1.1 : Mechanism of the degradation of glyphosate by MnO

PhNH2+CO2

H2OPh

N

H

C

N

R

H R'

O

Ph NH C

OH

OH

NHR

R' NHR

R'

Ph

N

N

C

OH

OH

H

Scheme 1.2 : Mechanism of the degradation of phenyurea under acidic

conditions

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conjugate base of phenylurea is caused by its deprotonation of the aryl-NH

group. This conjugate base is inactive towards hydrolysis, as the resonance in

the anion, increases the double bond character of the C-N bond compared

to that in the urea and thereby stabilizes it against cleavage. Besides, the

increased electron density impedes the attack of the hydroxide ion at the carbonyl

carbon to form the intermediate tetrahedral complex. The proposed mechanism for

the degradation of phenylurea under basic conditions has been presented in

Scheme 1.3.

k1

k -1

R'

N

C

R''

O

N

H

Ph

OH

R'

R"N C NH Ph

O

OH

R'

N

C

R''

O

N

Ph

+H2O

R'

R"N C

O

O

PhNH2

k2

Ka

Scheme 1.3 : Mechanism of the degradation of phenylurea under basic conditions

The kinetics of degradation of phenylurea herbicides namely fenuron,

monuron, diuron and chloroxuron were studies by Sabaliuna et al. [84] at

temperatures 64oC and 84oC in alkaline and neutral media. They suggested the

mechanism in which hydroxide ion promoted reactive tetrahedral intermediate

anion is formed. This intermediate can either revert to the starting compound,

decompose to products, or react with a second hydroxide to form a dianion,

prior decomposition to products.

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The poor solubility and the use of micellar media in pesticides and

herbicide formulation prompted many workers to study the effects of

surfactants on the kinetics of hydrolysis of pesticides. Mollet and O’Connor

[85] reported the effects of micelles on the solubilization and decomposition

of 4-methyl and 4-nitrophenylurea. The value of cmc of non-ionic nonyl

phenoxypoly (oxyethylene)-14 ethanol(A-730) and cationic cetyltrimethy-

lammonium bromide (CTAB) were increased on addition of very small

amounts of 4-methyl-and 4-nitrophenylureas. The micelles of A-730, CTAB

and anionic sodium dodecyl sulfate (SDS) have little effect on the rate

constants of hydrolysis of these phenylureas.

Matondo et al. [86] carried out a detailed investigation for the

influence of SDS and CTAB on the kinetics of degradation of phenyl, methyl

and dodecyl N-(4-pyridyl) carbamates in H2O- dioxane solutions. The SDS

micelles reduced the rate of hydrolysis of phenyl and methyl N-(4-pyridyl)

carbamate while the CTAB micelles speeded up the hydrolysis rate of phenyl

carbamates and decreased the rate constant for methyl carbamate. They

explained their results in the presence of surfactants on the basis of

pseudophase model. Inhibition of the rate of hydrolysis of dodecyl (4-pyridyl)

carbamate by SDS and CTAB were attributed to the tensioactive character of

these substances.

The kinetics of degradation of the dicarboximide fungicides namely

procymidone, iprodione, vinclozolin, and chlozolinate were investigated in

the micellar media by Villedieu at al. [87]. They studied in solution

containing various amounts of either SDS, CTAB or three nonionic

surfactants (two C13 alcohols and a copra amine combined with ethoxy

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chains) and compared with the kinetics in aqueous media. For all compounds,

the rate constants observed were slightly reduced by the SDS micellar media,

showing that reactions essentially took place in the aqueous pseudophase. The

CTAB micellar media speeded up the hydrolysis rates with small quantities of

Br− ions in the medium. As the number of Br− ions increases, the rates of

reactions fall. This is characteristic of an ion exchange (OH− and Br−) at the

surface of the CTAB micelles. The presence of non- ionic micelles had little

influence on the rate of hydrolysis of the fungicides. The inhibition in the rate

of dicarboximide ring opening is attributed to micelle-substrate association.

The results were explained by means of the pseudophase kinetic model coupled

with the mechanism of hydrolysis of these fungicides in water.

The kinetics of cleavage of paraxon and parathion pesticides by

cetyltrimethylammonium iodosobenzoate (CTA)IBA were investigated by

Moss et al. [88]. They observed that (CTA)IBA is highly reactive towards

paraxon and parathion. In aqueous solution at pH 9.0 excess (CTA)IBA

mediates their hydrolysis with kobs (max) = 0.014 and 0.0030 s−1 respectively.

Parathion is more hydrophobic than paraxon and consequently binds more

strongly to CTA micelles. This results into the sharp rise of kobs with

increasing surfactant concentration for parathion than paraxon. Another

consequence of the stronger binding of parathion is that kψ (max) is obtained

at a lower surfactant concentration for parathion relative to paraxon.

1.4 SURFACTANTS AND THEIR CLASSIFICATION

The term surfactant is used for a blend of surface active agent [89,90]

consisting of hydrophilic headgroup and hydrophobic hydrocarbon chain.

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Conventional surfactants have one head and one tail e.g. sodium lauryl

sulfate, cetylpyridinium chloride (CPC), sodium bis(2-ethylhexyl)

sulphosuccinate (AOT) etc. Surfactants may act as detergents, wetting agents,

emulsifiers, foaming agents and dispersants because they lower surface

tension and interfacial tensions between solid/liquid or liquid/air interfaces.

They concentrate at air/water interfaces and in water, or similarly strongly

hydrogen-bonded solvents, they self associate at concentrations above the

critical micelles concentration called cmc, to form association colloids,

known as micelle [91,92]. In the lower range of concentrations of surfactant,

micelles are typically spherical with alkyl groups in the interior and radii

slightly larger than extended alkyl chain lengths. But, with increasing

concentrations of surfactant, and on addition of hydrophobic solutes or low

charge density ions, micelles grow and become ellipsoidal. Addition of co-

surfactants and apolar solvents generates microemulsions, a more ordered

structure form with more concentrated surfactant [93]. Association colloids

have interfacial regions containing ionic or polar head groups and ionic and

polar solutes may be incorporated in this region. Considerations of the

dimensions of head groups and apolar tails indicate that the volume of the

interfacial region is approximately half of the total micelles volume.

Solutions of dilute surfactants are isotropic containing submicroscopic

particles, which form a micellar pseudophase distinct from the aqueous

pseudophase. The interfacial region is highly anisotropic.

The unusual properties of aqueous surfactant solutions can be ascribed

to the presence of a hydrophilic head group and a hydrophobic hydrocarbon

chain (or tail) in the molecule. The polar or ionic head group usually interacts

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strongly with an aqueous environment via dipole-dipole or ion-dipole

interactions. On the basis of the charge on the head group or nature of the

polar head group, surfactants can be divided the following categories.

1.4.1 Cationic Surfactants : Surfactants that bears positive charge on its

head group of the amphiphilic molecule and negative charge on its

counter ion on dissolution in the polar solvent are termed as cationic

surfactants. A very large proportion of this class of molecules are based on

quaternary nitrogen compounds such as alkylammonium halides and tetra-

alkylammonium halide. Pyridine and related species such as quinoline,

isoquinoline, pyrazine, and their derivatives form the basis for a wide

class of aryl based quaternary cationic surfactants. Phosphorus is also

quaternarized with alkyl groups to provide some cationic

alkylphosphonium surfactants.

Examples

Cetyltrimethylammonium bromide CH3(CH2)15 +

N (CH3)3Br−

Dodecylamine hydrochloride CH3(CH2)11 +

N H3 Cl−

Dodecylpyridinium chloride

1.4.2 Anionic Surfactants : Alkali alkanoates (or soaps) are well known anionic

surfactants. The ionization of carboxyl group bears the anionic charge on the

amphiphile. Sodium dodecylbenzene sulfonate (SDBS) has effectively

replaced soaps. Alkyl sulfates, alkyl ether sulfates, alkyl sulfonates, secondary

alkyl sulfonates, aryl sulfonates, methylester sulfonates, and sulfonates of

alkylsuccinates are other important classes of anionic surfactants. The fatty

N+

C12H25Cl_

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acids and sulfo compounds include the three most important anionic groups:

corboxylate (-CO2-), sulfate (−OSO3

−), and sulfonate (−SO3−). On the basis of

basicity and phase data their hydrophilicity ranking is given as:

−CO2− >> − SO3

− > −OSO3−

Examples

Potassium laurate CH3 (CH2)10 COO− K+

Sodium dodecyl sulfate CH3 (CH2)11 OSO3− Na+

Sodium dodecylbenzene sulfonate

1.4.3 Nonionic Surfactants : The water-soluble moiety of this type of surfactant

molecule contains non dissociable hydrophilic group, such as alcohol, phenol,

ether, ester, or amide hydroxyl groups. Many nonionic surfactants are

structurally analogous to anionic and cationic surfactants, except that the head

group is uncharged. Most prevalent among the head groups of nonionics are

oligomers of ethylene oxide and saccharides such as glucose and sucrose. The

most widely studied class of alkyl ethylene oxide surfactant, also called

alcohol ethoxylates, is represented as CxEy (where x is the length of the alkyl

chain and y is number of ethylene oxide unit in the head group) e.g. Triton X-

100 (TX-100)

Examples

N, N-dimethyldodecylamine oxide CH3(CH2)11 (CH3)2

Polyoxyethylene monohexadecyl ether CH3(CH2)15 O(CH2CH2O)21H

CH3(CH2)10CH2 SO3Na+_

N

O

  21

Polyethylene glycol (t) – Octylphenyl ether, TX-100

(CH3)3CCH2C(CH2)2 O(CH2CH2O)9.5H

1.4.4 Zwitterionic Surfactants : This type of surfactants can behave as cationic,

anionic or nonionic species depending on the pH of solution. True zwitterions

such as α –amino acids, can become ionized by intramolecular proton transfer.

NH2CH(R) CO2H ↔ N+ H3 CH (R) CO2−

Considerations of phase data and basicity [94] suggest that the relative

hydrophilicities of ammonio zwitterionics decrease in the following order:

ammonio – CO2− >> ammonio – SO3

− > ammonio − OSO3−

The betaines are very important class of zwitterionic surfactants and

include alkylbetaines, amidoalkylbetaines and heterocyclic betaines.

Examples

N − dodecyl − N : N-dimethyl betaine

C12H25N+ (CH3)2CH2COO−

CH3 (CH2)11 N+ (CH3)2CH2−CH2−CH2−SO3−

2, 3 dimethyl-3- dodecyl- 1,2,4- triazolium- 5 thiolate

NCH3

N N _S

CH2(CH2)10CH3

CH3 +

  22

1.4.5 Gemini Surfactants : Gemini surfactant is an amphiphile [95] made up of two

hydrocarbon tails and ionic groups linked by a ‘spacer’ in the sequence :

hydrocarbon tail/ionic group/spacer/ionic group/hydrocarbon tail ( Fig.1.1 ) or

it can be said that a gemini surfactant consists of two surfactant molecules

chemically bonded together by a spacer. The two terminal hydrocarbon tails

can be short or long; the two polar head groups can be cationic, anionic or non

ionic; the spacer can be short or long, flexible or rigid. The gemini surfactant

need not to be symmetrically disposed about the center of the spacer. Gemini

surfactant self-assembles at much lower concentrations and is superior in

surface activity as compared to conventional surfactants.

Examples

C12H25N+(CH3)2(CH2)2OOC(CH2)2COO(CH2)2N+(CH3)2C12H25 .2Br-

C12H25OPO2-O-(CH2)6-O-PO2

-OC12H25.2Na+

1.5 GENERAL PROPERTIES OF SURFACTANTS

1.5.1 The Micelle Formation : In aqueous solution, dilute concentrations of ionic

surfactant behave as normal/strong electrolytes, but at higher concentrations

its behaviour changes. At higher concentrations, the surfactant molecules

aggregate together to form association colloids called micelles. The tendency

of the ionic or the polar part of the molecule is to maintain contact with

aqueous environment while at the same time the hydrophobic alkyl part of

molecules avoids energetically unfavorable contacts with water. Thus, the

formation of micelles in aqueous solution is the result of a compromise

between the tendency for alkyl chains and the hydrophilic property of the head

group. A thermodynamic description of the process of micelle formation

  23

includes a description of both electrolyte and hydrophobic contribution to the

overall Gibbs energy of the system. Hydrocarbon and water are immiscible

and the limited solubility of hydrophobic species in water can be attributed to

the hydrophobic effect. The transfer of the carbon solute from the hydrocarbon

solvent to water is accompanied by an increase in the Gibbs transfer energy

[96]. The decrease in entropy is thought to be the results of the breakdown of

the normal hydrogen bonded structure of water accompanied by the formation

of differently structured water, around the hydrocarbon chain. The presence of

the hydrophobic species promotes an ordering of water molecules in the

vicinity of the hydrocarbon chain. The overall process has the tendency to

bring the hydrocarbon molecule together, which is known as hydrophobic

interaction. Molecular interaction arising from the tendency for the water

molecule to regain their normal tetrahedral structure, and the attractive

dispersion force between hydrocarbon chains, act cooperatively to remove the

hydrocarbon chain from the water leading to an association of hydrophobic

chains [93,97,98].

1.5.2 Critical Micelle Concentration (CMC) : The concentration above which the

surfactant molecules aggregate to form micelles is called critical micelle

concentration or simply ‘cmc’. The values of ‘cmc’ depend upon the nature of

the surfactant, temperature, pressure, and the presence and nature of additives.

A low ‘cmc’ is favorable by increasing the molecular mass of the lipophilic

part of the molecule, lowering the temperature and adding electrolyte.

Surfactants molecular masses typically vary from a few hundred up to several

thousands. The micellization behavior of Gemini surfactant is qualitatively

different from that of conventional ones. The lower cmc can be directly

  24

attributed to the increase in the number of hydrocarbon groups in the

molecule.

The physico-chemical properties of surfactants vary markedly above and

below the cmc value [99-105]. The ionic surfactants (e.g. sodium dodecyl sulfate,

SDS) behave as strong electrolyte at concentrations below the cmc value. The

physico-chemical properties such as conductivities, electromotive force measurements

resemble those of a strong electrolyte. At concentrations, above the cmc value, these

properties change dramatically, indicating the occurrence of a highly co-operative

association process. Thus, the cmc value of a surfactant can be obtained from the plot

of an appropriate physico-chemical property versus the surfactant concentration [106-

108] (Fig. 1.2). At cmc, a break in the plot is observed.

Micelles are often considered as static structures of spherical aggregates of

surfactant molecules. However, micelles are in dynamic equilibrium with surfactant

monomers in the bulk, which are constantly being exchanged with the surfactant

molecules in the micelles. The equilibrium between monomer and aggregate is

established within a few milliseconds. At equilibrium the number of micelles formed

in a given time is equal to the micelles disintegrated in the same time. Additionally,

the micelles themselves are continuously disintegrating and reforming. The surface

layer of micelle resembles concentrated electrolyte solution with a dielectric constant

lower than that of the bulk water. The micellar phase is less polar than water and the

ionic micelles have a polarity near to that of pure ethanol even at the Stern’s layer

[109-111].

In case of ionic micelles, electrical charge on a micelle is neutralized by

counter ions in the electrical double layer around it as shown in Figure 1.3. The first

layer immediately adjacent to its surface is called Stern’s layer [112]. In this layer, the

  25

counter ions are adsorbed so strongly that there is no thermal agitation and they

migrate together with the colloids micelle in an electrical field. According to the most

widely accepted model, the headgroups of surfactant molecules are situated in this

layer. The remainder of the double layer is diffused and called Gouy-Chapman layer

since the ions are diffused into the bulks solution as a consequence of the thermal

motion. The decrease in counter ion concentration with the distance from the micellar

surface has an exponential form [113-116].

1.5.3 The Krafft and Cloud Points : The temperature above which a sharp increase

in solubility of micelle-forming surfactant is observed is called as the Krafft

point. Below the Krafft point the solubility of the surfactant is too low and the

surfactant molecules form the micelles. Above the Krafft point a relatively

large amount of surfactant can be dispersed in micelles and solubility

increases greatly. Above the Krafft point maximum reduction in surface or

interfacial tension occurs at the cmc because the cmc then determines the

surfactant monomer concentration [117]. The non-ionic surfactants do not

exhibit Krafft points. The solubility of non-ionic surfactants decreases with the

increase in temperature and they may begin to lose their surface active

properties. The temperature above which cloudiness or turbidity appears due

to the separation of a surfactant rich phase of swollen micelles is called cloud

point. The addition of ionic surfactants increases the cloud points of their non-

ionic counterparts, this increase being dependent on the composition of the

mixed micelle. The surface charge of the micelle also contributes towards the

increase in cloud point. Therefore, increasing electrostatic repulsion between

the micelles through adsorbing charged particles may reduce their tendency to

coalescence responsible for clouding.

  26

1.5.4 Surface Tension, Elasticity and Rheology : The adsorption of surfactants at

a surface or interface modifies the physical properties of the interface. It is

sometimes very important in all types of natural phenomena and industrial

processing operations. The surfactant molecules decrease the interfacial

tension on adsorption at an interface. Related to these effects are the Gibbs and

Marangoni surface elasticities [118-120] and the surface dilational viscosity

[118,121]. Dynamic surface tensions, and related phenomena, are important in

areas such as photography, where the dynamic surface tension is monitored to

prevent film deformation and irregularities, in crop protection products where

wettability rate is vital for pesticide spreading on leaves, biological processes,

and in paper and textile production. A high interfacial viscosity reduces the

rate of droplet/bubble coalescence and thereby increases emulsion or foam

stability [122-125]. The mixed surfactant adsorption may sometimes cause

very viscous surfaces [126]. The addition of a small amount of non-ionic

surfactant to a solution of anionic surfactant can enhance foam.

1.6 APPLICATIONS OF SURFACTANTS

The unique properties of surfactant that includes the interfacial tension

lowering, emulsification, suspension stabilization, as a delivery vehicle, or in

promoting foam stability, provide a vast scope for its use in different disciplines

ranging from cleansing agent to drug delivery vehicles. The four basic surfactant

applications technology groups are agrochemicals, consumer products, industrial

processes and resources. The construction related applications of surfactant are

asphalt, cement and wallboard. Surfactants are widely used in crop protection

products i.e., pesticides, herbicides, fungicides and insecticides [127-130]. The

formulation of pesticides is significant in terms of product stability and product

  27

performance. The current trend in crop protection is towards products that are more

potent, safer to user, having less impact on the environment, more convenient to use

and improved efficiency of the applied products. In the case of sprayed products,

colloid and interface science impacts all aspects of application. Spray droplets impact

the leaf surface, creating a foliar deposit from which the pesticide moves into the leaf

or contacts the pest.

In pesticide formulations, the inert ingredients usually are added to improve

the effectiveness of the active ingredient. Surfactants are the inert ingredient and

increase the efficiency of main ingredient. These inert ingredients usually determine

the method of application. The most important formulations are dusts, sprays,

wettable powders, flowables, emulsifiable concentrates, granules, baits and

encapsulated pesticides. Surfactants are used in pesticide formulations as wetting

dispersing, emulsifying, solubilizing and bio enhancement agent. Wetting is a primary

function of surfactant in pesticide formulations. It is incorporated into the formulation

to facilitate satisfactory surface coverage of the agent. It is also used to increase the

uptake of chemicals. The other important application of surfactants is in pesticides

formulations [131] where they are used to increase the solubility of the pesticide in

aqueous medium, to stabilize the pesticide by controlling evaporation or

decomposition (above cmc), or to enhance the effectiveness of the pesticide by

providing the fine spray (below cmc). Surfactant is also used as emulsifying agent and

allows insoluble pesticides to be applied in water to spray easily.

Surfactants are involved in the production of many common food items

[132,133] and can be found in the extraction of cholesterol, solubilization of oils,

liquor emulsification, prevention of component separation, and solubilization of

essential nutrients. Alkali surfactant polymers are used to mobilize oil in oil wells.

  28

Gemini surfactants are used in skincare formulations, anti-pollution protocols,

analytical separations, nano-scale technology, biotechnology, enhanced oil recovery

and as paint additives [134]. They have also been used in the synthesis of new

mesoporous zeolites for catalysis and adsorption applications [135]. Cosmetic

formulations are dependent on new formulation techniques [136,137] for emulsion,

particularly for storage properties. Emulsion and microemulsion are characterized by

fine droplet size which is highly stable. Microemulsions are useful for creating a clear

formulation of oil in water. Good cosmetic formulations also yield good skin-product

interactions and therefore good penetration of active ingredient into the skin layers.

Detergents are widely used in biochemistry and biotechnology. They help in

solubilizing molecules by dissociating aggregates and unfolding proteins e.g. SDS,

CTAB. Detergents are key reagents for the extraction of protein from the cells and

tissues. They disorganize the membrane’s lipidic bilayer and solubilize proteins. In

electrophoresis, proteins are classically treated with SDS to denature the native

tertiary and quaternary structures, allowing the separation of proteins according to

their molecular weight. Detergents have also been used to decellularise organs. This

process maintains a matrix of proteins that preserves the structure of the organ and

often the microvascular network. The process has been successfully used to prepare

organs such as the liver and heart for transplant in rats [138].

An emulsion is used in petroleum industry after primary and secondary cycles

of oil recovery [139-144]. Chemicals may be injected to drive out additional oil in an

enhanced oil recovery process, which may involve creating in situ emulsion in the

reservoir. The applications of surfactants in the petroleum industry area are quite

diverse and have a great practical importance [145,146]. Surfactant used as wettability

alteration, enhanced micro displacement of oil, stabilizing foams for mobility control

  29

or foam drilling fluids, separation and flotation aids in oil sands processing, heavy oil

transportation as aqueous emulsion, enhanced aquifer remediation and breaking of oil

emulsions.

Micelles exert large rate effects upon organic reactions and can in principle

discriminate between different reactions, depending upon their charge type or

molecularity. The rate of bimolecular E2 reactions involving OH− in aqueous solution

is largely enhanced by cationic surfactants while at the same time it is inhibited by

anionic micelles [147]. The SN reactions are generally inhibited strongly by cationic

micelles and less strongly accelerated by anionic micelles. It is, therefore, relatively

easy to observe micellar control of product formation. An ionic surfactant or phase-

transfer catalyst can also be immobilized by binding it to an insoluble resin. The uses

of micelles in chemical analysis are rapidly increasing [148]. Analytical reactions are

carried out typically on a small scale and are based on spectrophotometry.

1.7 KINETIC TREATMENT OF REACTIVITIES IN MICELLAR

MEDIA

Ionic colloidal assemblies e.g., micelles, microemulsions, hemimicelles

(solids), bilayers and vesicles are believed to mimic for biological system [93]. It

possesses structural similarities between globular proteins and spherical micelles and

analogies between the catalytic effects of enzymes and functional micelles between

catalysis and phase transfer catalysis. For these reasons numerous investigators [149-

153] have focussed their attention towards the reactions occurring in micellar media.

A number of important thermodynamic and kinetic studies of organic

reactions especially with pesticides and herbicides have been performed in

micellar solutions [154-160]. Aqueous micelles and other associated colloids

have been observed to influence reaction rates and equilibria and

  30

concentration, or depletion of reactants in the interfacial region [99,161-165].

The interfacial region differs from water as a reaction medium [166-169],

which can affect rates of spontaneous reactions. These effects depend upon

the transfer of substrate from water to micelles, the reaction mechanism and

properties of the interfacial region, i.e., local charge, polarity and water

content. Usually, spontaneous reactions that are accelerated by decrease in

solvent polarity e.g., anionic decarboxylation and dephosphorylation, are

accelerated by micelles and most spontaneous hydrolysis that are accelerated

by increase in solvent polarity are inhibited in the presence of surfactants.

The major problem in modelling chemical reactivity in many surfactant

systems has been to describe the effect of surfactant and counter ion

concentration and counter ion type on the distribution of ionic reactants.

Several models have been developed in which most of the kinetic results in

micellar media have been interpreted satisfactorily with the help of Poisson-

Boltzman Equation (PBE) [170-173] model and Pseudophase Ion Exchange

(PIE) [174-177] model.

1.7.1 Poisson-Boltzman Equation Model (PBE) : In this model, the electrostatic

interactions are assumed to attract counter ions to the micellar surface but

repel co-ions, and ion distributions are estimated by solving the Poisson-

Boltzman equation in the appropriate symmetry. A simple formalism that has

been applied kinetically involves the assumption in which micelles are

spherical and monodisperse. If ions are treated as point charges, which interact

only coulombically with the micelle, ion specificity is lost, and a term was

therefore added for non-coulombic interactions which should be small for

hydrophilic, high charge density ions and larger for polarizable ions [178,179].

  31

These specifically interacting ions should neutralize ionic head groups and

reduce the micellar charge density. Therefore, inter-ionic competition is

introduced and this treatment fits kinetic data for ions e.g., Cl− or Br−, which

are reactive, as in SN2 reactions, or are inhibiting reactions of other ions [180].

It also fits ionic distributions, based on NMR spectrometry, and explains why

Cl− and Br− displace ions such as OH−, whereas very hydrophilic ions such as

OH− or SO42− are ineffective in displacing Cl− and Br−, even when they are in

very large excess.

The treatment involves disposable parameters e.g., the term for ion

specific interactions and the size of the reaction region at the micellar surface.

The assumption of a “smooth” micelle [181] and neglect of finite ion

correlation and size [182] are over simplification in this model and one

expects it to fail if micelles become highly polydisperse. The PBE treatment

predicts that the counter ions and co-ions are respectively in negative and

positive concentration gradients extending radially from the surface. An

increase in ionic concentration markedly reduces both positive and negative

concentration gradients.

1.7.2 Pseudophase Ion Exchange Model (PIE) : In the pseudophase ion exchange

model, the totality of the micelles in solution is treated as a separate

pseudophase uniformly distributed in the aqueous phase. Above cmc, a

micellar-catalyzed bimolecular reaction is described in terms of the

distributions of reactants between the micellar and aqueous

pseudophases. The reaction occurs in both aqueous and micellar

pseudophases according to the following scheme:

  32

k'w k'm

Scheme 1.4 : Reaction in aqueous and micellar pseudophases

In this scheme k'w and k'm are first order rate constant, and (Dn) is the

micellized surfactant (given by [surfactant]Total - [cmc]) [99,151].

Corresponding to the scheme 1.4, the overall first order rate constant is given

by:

]D[K1]D[K'k'kk

ns

nsmw

++

=ψ (1.1)

Where, KS is called binding constant and is given by:

]][D[S][SK

nw

ms =

(1.2)

The subscript w and m denotes aqueous and micellar pseudophases,

respectively and quantities in squared bracket are concentrations in terms of

total solution volume. The values of kψ increases or decreases with [Dn] to a

constant value of kobs = k′m, when substrate is fully micellar bound. The

treatment also fits inhibition of bimolecular reactions with dilute hydrophilic,

co-ions which remain in water while micelles incorporate hydrophobic

substrate [93,100,150]. Kinetic values of Ks generally agree with those

Sw Sm

Products

Water pseudophase

Micellar pseudophase

  33

estimated or measured directly. The values of k′m/ k′w are consistent with this

difference in polarity, based on kinetic solvent effect [161].

This treatment fits a great deal of data for the kinetics of simple,

spontaneous, unimolecular or water catalyzed reactions. The treatment also

fits for the inhibition of bimolecular reactions with dilute, hydrophilic, co-

ions, which remain in water while micelles incorporate hydrophobic

substrates. The equaton (1.1) could be rearranged in the form similar to

Lineweaver- Burk equation (1.3).

( ) )cmc]D([K'k'k1

'k'k1

'kk1

nswmwmw −−+

−=

−ψ

(1.3)

The equation (1.3) permits calculation of k′m and Ks, provided that k′w

is known [151]. These equations (1.1 and 1.3) have been used extensively and

provided the basis for quantitative analysis of micellar rate effects.

The equations (1.1 and 1.3) generally fail to explain the micelles

catalyzed bimolecular reactions. According to equation (1.1) the first-order

rate coefficient should reach a constant, limiting value at high surfactant

concentration when the substrate is fully micellar bound. But in most of the

non-solvolytic bimolecular reactions the rate- [surfactant] profile shows a rate

maxima behavior. The influence of the surfactant concentrations on rate can

be treated quantitatively by taking into account the distribution of both

reactants between water and micelles. This is done by expressing the first

order rate constant k′w and k′m in terms of the second-order rate constants in

water and micelles, and reactant concentrations in each pseudophase

[101,183-186]. The concentration of reactant in micellar pseudophase can be

  34

given in terms of mole ratio. However, this approach does not allow direct

comparison of second order rate constants in aqueous and micellar

pseudophases. The first order rate constants are written in equations (1.4) and

(1.5) in terms of second order rate constants, kw and km for reaction of a

reagent Y.

k′w = kw [Yw] (1.4)

k′m = SYm mk (1.5)

Where, the mole ratio, [ ] ( )cmc]D[/Ym nmSY −= .

By combining the equations (1.1) (1.4) and (1.5), expressions (1.6) and

(1.7) are obtained.

kw[Yw] + kmKs mSY[Dn]

1 + KS[Dn]kψ =

                                                 (1.6)

[ ] [ ]( )cmc]D[K1

YKkYk

ns

msmww

−++

= (1.7)

Addition of surfactant leads to binding of both reactants to micelles,

and usually increases the reaction rate. Eventually, however, increase in

surfactant concentration dilutes the reactants in the micellar pseudophase and

the rate falls. This behavior supports the original assumption that substrate in

one micelle does not react with reactant in another, and that equilibrium is

maintained between aqueous and micellar pseudophases. In addition, widely

observed inhibition by inert electrolytes is ascribed to competition between

reactive and inert counter ions. The ionic concentration in the interfacial

surface region is given by

  35

( )α−= 1NM / Vm (1.8)

Where α is the micellar fractional ionization [162,187]. Based on α-

value in the range 0.2-0.4 and Vm = 0.2 M−1, ionic concentration at the surface

can be much higher than that in dilute aqueous electrolyte. Romsted extended

this relation to mixed ion system by assuming that counter ions Y and N,

compete as at ion- exchange resin [101,103].

wm NY + YNK mw NY +

The equilibrium constant for the concentration, Y and N between the

micellar and aqueous psedophases are given by

[ ][ ][ ] [ ]mw

wmYN YN

YNK = (1.9)

Provided that α is constant, the mass-balance relation allow kψ to be

related to concentrations of surfactant and reactive and inert electrolyte in

terms of YNK , α, rate constants. Values of Y

NK , follow the Hofmeister series and

agree with non kinetic evidence on ionic competition [188]. Low charge

density ions displace high-charge density hydrophilic ions and the

approximate anion affinity sequence is [161,162]

−−−−−−− ≈>>>≈≈ OHFAcOClNBrNO 33

The PIE model fits variations of kψ with concentrations of surfactant,

reactive and inert ions and predicts the dependence of kψ on substrate

hydrophobicity. PIE treatment fails when reactive ion is very hydrophilic,

e.g., OH− or F− and is in high concentration [189,190].

  36

The PIE treatment does not predict satisfactorily the behavior of kψ -

[surfactant] profile in case of very hydrophilic reactive ions e.g., OH−, F− and

in high concentrations. The situation is different for reactions in functional

micelles e.g., CTAOH or CTAF. With fully bound substrate, kψ increases

sharply with increasing total concentrations of OH− or F− [189-191]. Several

treatments have been proposed that retain ion-exchange model, but without

the concept of a constant value of α which controls counter ion concentration

at the micelle-water interface.

Bunton [189] assumed that the ionic- binding follows a Langmuir

isotherm to explain the variation of kψ with [CTAOH] and added [OH−]. The

ionic equilibria for X− in CTAX is given by

[ ] [ ] [ ]( )−−

−

−=

mnw

mX XDX

]X['K (1.10)

If K′x is large the concentration of X− in the micelle is approximately

independent of [XT], i.e., α is approximately constant, but if K′x is small, this

concentration increases with the total concentration. This so-called mass

action treatment fit kinetic data and can be extended to system in which ions

compete [192-194].

Hall [195] discussed failure of the PIE treatment in terms of transition

state theory and thermodynamics of interactions between solutes and

association colloids, in particular between ions and ionic micelles. This

treatment analyzes overall reaction rates by treating the activated complex as

being in equilibrium with reactants, follows the Eyring rates equation. The

activated complex in the bulk aqueous medium should therefore be in

  37

equilibrium with that in the micelles, although its lifetime is very short in

terms of rates of solute transfer between water and micelles. Solutes enter

micelles with diffusion-controlled rates [195] whereas the lifetime of the

activated complex is that of a stretching vibration. Attack of a small

nucleophilic anion, upon a nonionic substrate generates a larger anion, and

reaction rates depend upon interactions of these species with their

environment. Depending upon reaction mechanism there may be

compensation between interactions of the initial and transition states. In this

situation km and kw should be similar in many bimolecular reactions, as

indicated by the pseudophase treatment.

An important point in the transition state model is the demonstration

that when the only counter ion is the reactive ion form of the rate-[surfactant]

profile depends upon the fractional micellar ionization, α [195]. If α is small

as with CTABr, kobs, with fully bound substrate, will not increase

significantly with increasing [counter ion], but if α is larger, as with CTAOH,

kobs should increase markedly with increasing [counter ion], as is found [196].

1.8 OXIDIZING AGENTS

1.8.1 Colloidal Manganese Dioxide : Manganese is the eleventh most abundant

element in the earth crust and is among the important micronutrients for all

micro-organisms. Manganese does not occur in the environment as a pure

metal, but is a component of more than 100 minerals, including sulfides,

oxides, carbonates, silicates, phosphates and borates. Manganese exists in

variable oxidation states between −3 to +7. The most common oxidation

states of manganese are +2, +3, +4, +6 and +7. The important manganese

  38

compounds are manganous chloride (MnCl2), manganous sulfate (MnSO4),

manganese tetroxide (Mn3O4), manganese dioxide (MnO2) and potassium

permanganate (KMnO4).

Manganese dioxide is the blackish or brown solid and occurs naturally as the

mineral pyrolusite, which is the main ore of manganese. Manganese dioxide is a kind

of attractive inorganic material, and material scientists have made great efforts on the

synthesis of manganese dioxide [197-201]. Although heterogeneous reaction

conditions are suitable for synthetic chemistry, the insolubility of MnO2 in water has

limited its analytical application. However, the formation of soluble manganese (IV)

complexes with various inorganic and organic species has been explored [202,203].

Under certain conditions, permanganate can be reduced to give a reasonably stable,

soluble form of manganese dioxide [204-210]. Perez-Benito et al [211] treated

permanganate with a variety of reducing agents in near-neutral aqueous conditions to

produce transparent dark-brown solution containing colloidal manganese dioxide,

which did not precipitate during several months of investigation. Few methods for the

synthesis of different forms of manganese dioxide are:

(i) The hydrothermal-electrochemical method [212] has been used for the

synthesis of manganese dioxides from acidic MnSO4 solutions. This method

produced manganese dioxides of either the β- or γ-structure type. At pH 1 and

1300C, pure β-MnO2 was formed; this was the first time that this structure has

been obtained electrochemically. At 113 or 920C, mixtures of β- or γ-MnO2

were obtained, and the β-MnO2 content decreased with temperature.

(ii) Manganese dioxide nanocomposite foams (Fig. 1.4) [213] were prepared via a

novel and facile one-step method using high internal phase emulsion as

templates. In this method aqueous phase containing CaCl2.2H2O or KMnO4

  39

was gradually added into the, continuous phase, which consisted of a definite

amount of styrene, neopentyl glycol diacrylate as a crosslinker, span 80 and

AOA (12-acryloxy-9-octadecenoic acid) as surfactants, and isopropanol as a

scavenger for hydroxyl radicals. The presence of manganese dioxide was

identified by XPS pattern (X-ray photoelectron spectroscopy), XRD pattern

(X-ray powder diffraction) and FT-IR spectra (Fourier transform infra-red

spectroscopy) of the resulting composites.

(iii) Nano sized manganese dioxide was synthesized using hydrothermal and co-

precipitation methods [214] by the reduction of hydrogen peroxide and

potassium permanganate in respective methods. Characterization of the

synthesized material was carried out by Elemental Analysis, XRD, Thermal

Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA), FT-IR

and Scanning Electron Microscopy (SEM). SEM analysis showed that the

manganese dioxide prepared by hydrothermal method is made of spherical

nanoparticles with sizes ranging from 15-30nm and the MnO2 prepared by co-

precipitation method showed nanospheres of 20-100nm.

(iv) Preparation of colloidal MnO2 was reported by Perez-Benito et al

[205,206,211]. They prepared dark brown colloidal manganese dioxide by

mixing potassium permanganate solution and sodium thiosulphate solution in

appropriate amount. Colloidal MnO2 solution was prepared by reducing

KMnO4 with a stoichiometric amount of Na2S2O3 according to the reaction:

8MnO4- + 3S2O3

2- + 2H+ = 8MnO2 + 6SO42- + H2O

A 2-L volumetric flask was filled with water to around ¾ of its capacity. 20.0

mL of Na2S2O3 (1.88 x 10-2 M) solution was added to it. Then, 5.0 mL of KMnO4

(0.200 M) solution was added slowly and each addition was followed by

  40

homogenization by gentle shaking. The mixture was finally diluted to 2 L with more

water. The colloidal solution so prepared remained perfectly transparent and remained

stable for several months.

Colloidal MnO2 has been used extensively as an oxidizing agent [215-217]

and catalytic agent [218] for the oxidation of both inorganic and organic compounds.

The transparent solutions of colloidal MnO2 got importance because of their

widespread participation as intermediate or reaction products in most permanganate

oxidations [219], being actively involved in the mechanism as auto-catalyst in many

cases [220]. It has been established that water-soluble colloidal manganese dioxide

has the advantage over the water insoluble forms. The aqueous solution of colloidal

MnO2 is unstable in acidic medium and has found extensive use as an oxidant in the

aqueous neutral medium [221]. Manganese (IV) is known to form a number of water

soluble complexes with organic and inorganic ligands [222-225]. Colloidal MnO2-

organic acid reactions have been studied in some depth focusing on the

determinations of kinetic parameters [207,226,227]. A brief summary of the work

done on the colloidal MnO2 as an oxidant is presented herewith.

Perez-Benito et al. reported kinetic studies of the reduction of colloidal MnO2

by manganese II [211], formic [227] and oxalic [207] acids in acidic media. Studies

on the coagulation behavior of colloidal MnO2 by divalent cations both in the absence

and presence of a complexing ligand (L-histidine) has been reported [228]. The

colloidal MnO2 particles were reported to be highly sensitive to the presence of free

divalent cations in the solution. They used coagulation of colloidal MnO2 to

determine the apparent equilibrium constants for the complexation of these cations by

bulky ligands. The efficiency as coagulating agents increased in the order Mg2+ < Ni2+

  41

< Zn2+ < Cu2+ <Co2+ < Mn2+. It was found that with the increase in pH, the

coagulating concentration of each divalent cation was also increased.

Joaquin F. Perez-Benito [229] carried out the oxidation of glycine by

permanganate ion in near-neutral aqueous solutions. The progress of reaction was

monitored with a UV-vis Spectrophotometer at two different wavelengths in order to

observe the decay of the oxidant (MnO4-, at 526 nm) and the formation of one of the

reaction products (colloidal MnO2, at 418 nm). The reaction performed using

phosphate buffer resulted in the stabilization of MnO2 as a soluble colloid during the

kinetic runs. On the basis of experimental data, it was suggested that out of the two

conjugated buffer ions H2PO4-/HPO4

2- the acidic one (H2PO4-) is the predominant

species responsible for the stabilization of MnO2 through the formation of hydrogen

bonds between the MnO2 oxygen atoms and the hydrogen atoms from phosphate ions.

Phosphate ions have a notable inhibition effect on the autocatalytic reaction pathway

of the reaction. The reaction showed an autocatalytic behavior, but the kinetic plots

deviated from the differential rate law usually employed for the study of autocatalytic

reactions. The experimental data suggest that these deviations might be caused by the

competitive adsorption of both phosphate ions and glycine on the surface of the MnO2

colloidal particles.

Tuncay et al. [230] studied the reaction of colloidal MnO2 and formic acid at

25oC in aqueous perchloric acid solution in the presence of surface active agents.

They found that the CTAB micelles caused flocculation of colloidal MnO2 whereas

SDS micellar system was found to be ineffective on the reaction rate. No noticeable

effect of SDS on the reaction rate was observed, even at high concentrations. But the

reaction rate was found to be accelerated by non-ionic micelles of Triton X-100.

  42

Kabir-ud-Din et al. [231] carried out a detailed investigation on the influence

of Triton X-100 on the kinetics of oxidation of d-glucose by colloidal manganese

dioxide spectrometrically. The reaction proceeds through the adsorption of d-glucose

on the surface of the colloidal particles. The results show that the rate of reaction at

initial stage (non autocatalytic path) increases with increasing the concentrations of d-

glucose, H+ and temperature. The values of kobs were determined and the Arrhenius

equation (Eq.1.11) was used to calculate the activation energy (Ea).

log kobs log AEa

2.303 RT= (1.11)

The Eyring equation was used to calculate other activation parameters such as

ΔH≠ and ΔS≠. They observed that the transition state is well structured and highly

solvated on the basis of the large negative values of ΔS≠. Non-ionic TX-100 micelles

also increased the rate of reaction, which indicated that the surfactant enhances the

concentration of d-glucose at the surface of colloidal MnO2. The authors suggested

the occurrence of hydrogen bonding between the five –OH groups of d-glucose and

the ether-oxygen of the polyoxyethylene chains of TX-100. Due to the presence of a

number of donor atoms in one TX-100 molecules, multiple hydrogen bonding may

take place and the number of bound d-glucose molecules increases. The H-bonding

also occur between MnO2 sols and hydrophilic part (polar ethylene oxide) of the TX-

100 molecules, and called it adsorption. This surfactant thus helps in bringing the

reactants in close proximity and to orient the reactants in a manner suitable for the

redox reaction. The decrease in rate at higher [TX-100] could be due to the ‘dilution

effect’: continuous increase in [TX-100] produces micelles and progressively more

and more substrate (d-Glucose) gets associated to the micellar phase. This segregation

deactivates the substrate as d-glucose in one micelle cannot react with MnO2 (onto

  43

which TX-100 is adsorbed). Therefore rate of reaction decreases. Kabir-ud-Din et al.

[232] also investigated kinetics of the reduction of colloidal manganese dioxide by D-

fructose in acidic perchlorate media at different temperatures. The authors reported

the formation of an adsorption complex between d-fructose and MnO2 on the basis of

their observed results. The complex decomposes in a rate-determining step, leading to

the formation of a free radical, which again reacts with the colloidal MnO2 in a

subsequent fast step to yield the final products. The overall reaction can be

represented as:

O

CH2OH

MnO2

H

2H+ Mn(II)

HCHO 2H2O

+

++

+ OH

OH

+

O

The reaction rate was decreased on addition F-, P2O74- and Mn2+ ions to the

reactants solution. It is well established that fluoride and pyrophosphate ions form

complexes with Mn (III) [233,234] as an intermediate during the reduction of

colloidal MnO2 by d-Fructose. The decreasing rate constant may be due to complex

formation between Mn (III) and F- and P2O74- ions. Mn (II) binds with the negative

site of colloidal MnO2 and prevents the binding of d-fructose, thereby, resulting into

the decrease of reaction rate.

Kabir-ud-Din et al. [235] studied kinetics of the redox reaction between

colloidal MnO2 and glycolic acid by monitoring the decay in the absorbance of

colloidal MnO2 in absence and presence of surfactants. Anionic SDS has no effect,

non-ionic surfactant TX-100 catalyzed the reaction and experiments were not possible

in presence of cationic surfactant CTAB due to the precipitation of MnO2. The

catalytic effect of ionic and non-ionic surfactants on reaction rates of bimolecular

reactions is due to the association/incorporation through electrostatic/hydrophobic and

  44

hydrogen bonding interactions between the reactants within a small volume of the

self-assemblies [236-238]. As TX-100 is a non-ionic surfactant, any type of

electrostatic interaction is not possible. Also, MnO2 and glycolic acid have no

hydrophobicity. Thus, only hydrogen bonding is left to play its role for the catalytic

action of TX-100. Wan-Xin et al. [239] reported an efficient method of solid-state

synthesis of diazenecarboxamide azo compounds by /using active manganese dioxide

supported on acidic silica gel. They oxidize aryl-substituted semicarbazides into their

corresponding N-aryl-2-phenyldiazenecarboxamides. The solid-state synthesis

method has been widely used in a variety of organic syntheses in recent years. This

method has many advantages, such as high efficiency and selectivity, easy separation

and purification, mild reaction conditions, energy-saving etc. It has been found that

oxidation process was slowed down considerably in the absence of silica gel and the

reaction remained incomplete. The oxidizing ability of active MnO2 supported on

silica gel is stronger in the presence of acid than in the presence of base.

Andrabi and Khan [240] carried out the detailed kinetics of oxidation of some

sulphur- and non-sulphur containing amino acids by the water soluble colloidal

MnO2. The studied amino acids were glutathione, cysteine, glycine and glutamic acid.

Glutathione is oxidized to disulphide with the reduction of MnO2 to Mn(II). Glycine

and glutamic acid were not oxidized by the colloidal MnO2, but the other structural

unit, cysteine, was found to be susceptible to oxidation by the same oxidant under

similar experimental conditions. They studied and elaborated the role of –NH2, -

COOH, -SH groups present in the carbon chain of the amino acids on the reactivity

with colloidal MnO2. They observed that the reactivity of –SH group was higher than

–NH2 and –COOH groups.

  45

1.8.2 Chloramine-T (CAT) : Chloramine-T (N-chloro-p-toluenesulfonamide) is a

N-chlorinated and N-deprotonated sulfonamide having versatile oxidizing

properties [241-245] at nearly neutral pH (typically 8.5). CAT is a strong

oxidizing compound with antiviral, bactericidal and fungicidal properties. It

has widespread uses in a broad range of practices, including in the fields of

medical, dental, veterinary, food processing and agricultural [246].

Chloramine-T is a water soluble reagent and can be used as a source of

electrophilic chlorine in organic synthesis. At pH 7-8, CAT hydrolyses and

releases hypochlorite (ClO-) (equation 1.12), which is an effective oxidizing

agent forming ICl upon contact with radioiodide. CAT has to be stored in

darkness because it decomposes if exposed to light [247]. It is naturally

dechlorinated to para-toluene sulfonamide (p-TSA). Chloramine-T reacts with

iodide ion in solution and results in oxidation with subsequent formation of a

reactive, mixed halogen species, ICl (equation 1.13). ICl rapidly reacts with

any sites in target molecules that can undergo electrophilic substitution

reaction resulting in incorporation of iodine.

CH3C6H4SO2NClNa + H2O CH3C6H4SO2NH2 + NaOCl (1.12)

CH3C6H4SO2NClNa + H++I- CH3C6H4SO2NH2 + ICl (1.13)

Sharma et al. [248] carried out the kinetic studies on catalytic effect of CTAB

on the oxidation of triethylene glycol by Chloramine-T in acidic medium resulting in

the formation of corresponding aldehyde. It was found that with increase in hydrogen-

ion concentration, the rate of reaction increased. Increase in rate with increase in

perchloric acid concentration may be attributed to the following equilibria:

ArSO2NCl- + H+ ↔ ArSO2NHCl

ArSO2NHCl + H2O ↔ ArSO2NH2 + HOCl

  46

Rate of reaction was extremely slow in absence of CTAB. The addition of

CTAB in the reaction system caused the rate of reaction to increase due to interactive

localization of the reacting species in the relatively small volume of the micelles

compared to the bulk solution and also due electrostatic attraction between polar

triethylene glycol molecule and the micelle. On increasing Cl- concentration rate of

reaction increases. The cause of acceleration in the reaction rate is because of the

generation of molecular chlorine, this is due to the interaction between a Cl- and a Cl+

produced by the hydrolysis of N-Cl bond or the added Cl- may coordinate with the

protonated oxidant. The reaction shows positive temperature effect and obeys

Arrhenius relationship.

The kinetics of oxidation of metochlopramide (MCP) with CAT in HClO4

medium at 313 K was studied by Meenakshi et al. [249] where CH3C6H4SO2NHCl

have been assumed to be the reactive oxidizing species. The stoichiometry of MCP

with CAT is found to be 1:2. The addition of reduction product of CAT has no

significant effect on the reaction rate. The oxidation products were identified as 2-(4-

amino-5-chloro-2-methoxybenzamide) acetic acid and diethyl amine.

1.8.3 Potassium permanganate : Permanganate ion has been for a long time as one

of the most versatile and widely employed oxidizing agents [250,251]. In the

biological field, permanganate ion is used as a chemical probe in many

biological essays [252]. The applicability of permanganate ion in green

chemistry as an environmentally friendly oxidant has been enhanced by the

development of a method for the recycling of the product manganese dioxide,

which can be converted back into permanganate or, alternatively, used in

organic synthesis as a mild oxidant or in the manufacture of catalysts, batteries

or pigments [253]. Soluble forms of colloidal manganese dioxide are formed

  47

as products in many permanganate reactions carried out in near-neutral

aqueous solutions, and quite often they play an important role in autocatalytic

[254,255] and oscillating [256-264] reactions. They are also involved as

detectable intermediates in some permanganate reactions carried out in acidic

solutions such as the much studied permanganate-oxalic reactions [265].

Permanganate reactions with amino acids [229] are of particular interest

because they show autocatalytic patterns both in acidic [266,267] and in neutral media

[268]. The reactions of permanganate ion with seven α-amino acids in aqueous

KH2PO4/K2HPO4 buffers at two different wavelengths (526 and 418 nm) were carried

out [269]. All of the reactions studied were autocatalyzed by colloidal MnO2, with the

contribution of the autocatalytic reaction pathway decreasing in the order glycine > l-

threonine > l-alanine > l-glutamic acid > l-leucine > l-isoleucine > l-valine. The

activation enthalpy of the nonautocatalytic pathway showed a strong, positive

dependence on the standard Gibbs energy for the dissociation of the protonated amino

group of the α-amino acid.

1.9 STATEMENT OF THE PROBLEM

The pesticides have become one of the essential components of modern

civilization. Pesticides are used to control the human disease vectors and to protect the

agricultural crops, food stuffs, household items, furnitures, etc from pests and rodents.

The indiscriminate usage of pesticides in agriculture and houses has imposed many

direct and indirect undesirable effects on the environment. These chemicals are

sometimes passed on from one organism to another through all the links of food chain

and can ultimately be accumulated in the fat of human body through the process of

biological magnification. Our soil and water are contaminated with thousands of

  48

commonly and rarely used pesticides. The exact fate, biotic and abiotic reactions of

these pesticides has not been fully studied and understood. In order to get an insight

into the abiotic reactions of some of the pesticides in the environment, the present

work has been undertaken to study the degradation of some of the commonly used

pesticides and herbicides. The oxidation of glyphosate, isoproturon, cartap, 2,4-D and

metribuzin were carried out using MnO2, a component of soil, Chloramine –T and

KMnO4 ( common oxidant used in green chemistry). The studies were also carried

out in the micellar media to elaborate the role of amphiphilc molecules in the

degradation process of pesticides. Surfactants are usually used in pesticide

formulations to increase the solubility of pesticide, to stabilize pesticides by

controlling the evaporation or decomposition processes. Surfactant molecules

aggregate to form association colloids and mimic enzyme structurally and

functionally. The study in the surfactant media will be helpful in understanding the

mechanism of degradation of pesticide in humic and biological system and can be

useful to predict the fate of pesticides after its dispersal in the environment. Therefore,

a thorough kinetics on the degradation of pesticides have been done and described in

this thesis entitled "Kinetics of Degradation of Pesticides in the Organized Media".

The work has been divided in the following seven Chapters:

The Chapter 1 under the heading 'Introduction' describes briefly about the

pesticides, its classification, formulations employed during its application and the

literature survey of the degradation reactions of commonly used pesticides after its

dispersal in the environment. The 'Introduction' also includes description about the

surfactants, their classification, properties and applications especially in the

formulations and in kinetics. The nature and reactions of oxidizing agents like MnO2,

chloramine –T and KMnO4 with pesticides have also been discussed in this Chapter.

  49

Chapter 2 outlines the sources of the chemicals used during the experimental

work, their supplier and purity. The experimental methods, instruments used and

conditions employed during the kinetics runs have been given in detail here.

The Chapter 3 has been divided in two parts namely; (a) Kinetics of oxidation

of glyphosate by colloidal MnO2 in aqueous and micellar media and (b) Kinetics of

oxidation of glyphosate in the absence and presence of sodium dodecyl sulfate

micelles in HClO4 medium. The details of the kinetics of degradation of glyphosate

by colloidal MnO2 and Chloramine-T in the aqueous and micellar media are

discussed. Glyphosate is degraded in dilute aqueous solution of colloidal MnO2. This

degradation of glyphosate involves the C-P bond cleavage at the manganese oxide

surface. As a result, the brown colour of colloidal MnO2 faded slowly giving glycine

and orthophosphate at the end of reaction. The presence of glycine and

orthophosphate were tested using ninhydrin reagent and ammonium molybdate

solution, respectively.

The oxidation of glyphosate by chloramine-T gave glycine (tested using

ninhydrin reagent) and para-toluene sulfonamide.

The Chapter 4 deals with the kinetics of oxidation of isoproturon by colloidal

MnO2 in the aqueous and micellar media. The colloidal MnO2 slowly oxidizes

isoproturon to give carbon dioxide with dimethylamine and 4-isopropylaniline. The

kinetics of the oxidation of isoproturon by colloidal MnO2 followed simple first order

kinetics with respect to MnO2 and the values of the rate constant increased with the

increase in concentrations of isoproturon. The oxidation of isoproturon by the

colloidal MnO2 occurs at its surface after the adsorbtion. The rate of degradation of

isoproturon by colloidal MnO2 increased with the increase in [CTAB] and [SDS]

  50

initially, and then the rate decreased on further increasing the CTAB or SDS

concentration. The enhancement in rate of reaction in the presence of CTAB and SDS

micelles has been explained on the basis of pseudophase model of micelles. The

reaction occurs in the aqueous and micellar pseudophases with different values of

rate.

The Chapter 5 describes about the kinetics of oxidation of Cartap by colloidal

MnO2 in aqueous and in the presence of micellar media of CTAB and SDS. The

brown colour of the colloidal MnO solution slowly fades away when added to the

solution of cartap hydrochlor2ide. It is oxidized by colloidal manganese dioxide to

give dihydronereistoxin, with the evolution of carbondioxide and ammonia.

In the Chapter 6, the kinetics of oxidation of 2,4-D by colloidal MnO2 in

aqueous and micellar media has been discussed. The colloidal MnO2 oxidizes slowly

2,4-D molecules to give 2,4-dichlorophenol and formic acid with the evolution of

carbon dioxide. The initial rates method demonstrated that the value of rate constant is

independent of the initial concentrations of colloidal MnO2 and the reaction followed

zero order kinetics in [2,4-D] at higher concentrations and fractional order in the

lower concentration range. The effect of variation of surfactant concentration (SDS

and TX-100) on the rate of degradation of 2,4-D by colloidal manganese dioxide was

studied at 30±0.5 0C by keeping the concentrations of colloidal MnO2 and 2,4-D

constant at 2.0x10-4 mol dm-3 and 5.0x10-3 mol dm-3, respectively. The increase in

SDS concentrations initially increased the reaction rate but further increase in [SDS]

resulted in the decrease in rate of reaction and became almost constant with increase

in [SDS]. Similarly, the studies carried out at different concentrations of TX-100

increased the rate of degradation of 2,4-D in the lower concentration range and then

reached to a maximum value and, thereafter, decreased with the increase in the

  51

concentration of TX-100. The same mechanism of reaction was followed in both the

micellar and aqueous media as evident from the results of the kinetic studies

performed at different concentrations of [MnO2] and [2,4-D] in the absence and

presence of surfactant (TX-100 and SDS).

The kinetics of oxidation of metribuzin by KMnO4 in aqueous and micellar

media has been studied and are discussed in Chapter 7. The value of pseudo first order

rate constants was found to be independent on [KMnO4] and the values of rate

constants were found to be dependent on [metribuzin]. The values of rate constant for

the oxidation of metribuzin by KMnO4 were found to be dependent of the

concentrations of H2PO4--HPO4

2-. An increase of the phosphate concentration resulted

in a notable decrease of the absorption coefficient and the size of the MnO2 colloidal

particles. There is a competitive adsorption of both phosphate ions and metribuzin on

the surface of the MnO2 colloidal particles, resulting in an inhibition of the

autocatalytic reaction pathway of the permanganate-metribuzin reaction by phosphate

ions.The variation of TX-100 concentration on the rate of degradation of metribuzin

by potassium permanganate was observed to be increased with the increase in TX-100

concentrations initially, then reached to a plateau value at higher [TX-100] at both the

wavelengths i.e. 418 nm and 526 nm. The revelant references to the work are cited

after Chapter 7 under the heading ‘REFERENCES’.

  52

Table 1.1: Chemical classification of pesticides

Chemical type Example Structure Typical Action

Organochlorines P,P'–DDT Cl CH

CCl3

Cl

Insecticide

Organophosphates Parathion

P – OC2H5O

NO2C2H5O

S

Insecticide

Carbamates Carbaryl OCONHCH3

Insecticide

Carboxylic acid derivatives 2, 4–D Cl

OCH2COOHCl

Herbicide

  53

Substituted ureas Diuron

NHCN (CH3)2

Cl

Cl

O

Herbicide

Triazines Simazine

NHC2H5H5C2HNN

C

NC

N

C

Cl

Herbicide

Prethroids Cypermethrin

OC

CH O

CN

C

O

CH CH CCl2

CH3 CH3

Insecticide

Organometallics Phenylmercury

acetate HgOCOCH3

Fungicide

 

 

  54

Thiocyanates Lethane-60

CH3(CH2)10 C – OCH2CH2 – SCN

O Insecticide

Phenols Dinitrocresol CH3

NO2

ONaNO2

Insecticide

Formamides Chlordimeform Cl N = HCN (CH3)2

CH3

Insecticide

Neem Products Nimbidin (Azadirachtin)OTg

H

COOCH3

O

COOCH3

OCOOCH3

OH

OO

O

OH

OTg OCOC

CH3

CHCH3

Insecticide

  55

Table 1.2: Chemical structure and mode of action of some organic pesticide

containing metal ion.

Pesticides Properties

1. Alloxydim-sodium

C–OCH3

ONaC

H3C

H3C

O

C3H7

N–O–CH3–CH=CH3

O=

(a) colourless crystals (b) 2250-2560 > 1630 (c) selective systemic herbicide

2. Aluminium Phosethyl

AlP

H

O

O–C2H5O3

(a) colourless crystals (b) 5800>3200 (c) systemic fungicide

3. Azocyclotin

NN

NSn

(a) colourless crystals (b) 99 > 1000 (c) contact acaricide

4. Calcium Cynamide Ca = N – C N

(a) Grey powder (b) 76.5- (b) herbicide fungicide and defoliant

5. Dikegulac-sodium CH3H3C

OO

COONa

CH3H3C

OO

O

 

(a) colourless crystals (b) 31000, > 2000 (c) systemic plant Growth regulator

6. Disodium methanearsonate

CH3 AS

ONa

ONa

O

 

(a) colourless crystals (b) 1000, - (c) Selective post- emergence herbicide

  56

7. Fenaminosulf

N = N – SO3Na(CH3)2N

(a) Yellow brown crystalline powder (b) 60, 100 (c) Seed and soil fungicide

8. Fentinacetat

ACOSn

(a) Colourless crystal (b) 125-160, 450 (c) Non systemic leaf- fungicide, algicide and molluscicide

9. Ferbam

H3CS

H3CN – C – S – Fe

3

(a) Black powder (b) 4000-17000,- (c) Protective leaf- fungicide

10. Mancozeb

CH2 – N – C – S –Mn, Zn

CH2 – N – C – S –

SHX

(X > 1)

(a) Grayish-yellow powder (b) 5000, > 10000 (c) Protective leaf- fungicide

11. Maneb

CH2 – N – C – S –Mn

CH2 – N – C – S –

SH

H S

X(X > 1) 

(a) Yellow amorphous powder (b) 7500, > 5000 (c) Protective leaf- fungicide

12. 2-methoxy ethylmercurychloride CH3O – CH2 – CH2 – Hg – Cl

(a) Colourless crystals (b) 570, – (c) Systemic fungicide

13. Metham-sodium H3C – N – C – SNa

H S

(a) Colourless crystals (b) 820, 97 (c) Nematicide, fungicide, insecticide & herbicide.

  57

14 Methyl-metiram

3

H3C – C – N – C – S –

H SH

H3C – N – C – S – Zn(NH3) –

SH

H3C – C – N – C – S –

H2C – N – C – S –

H S

SHH

X(X > 1)

(a) Pale yellow powder (b) 1540, – (c) Protective leaf fungicide & Acaricide

15. Metiram

X(X > 1)

SH

H2C – N – C – S –

H2C – N – C – S –

H S

H2C – N – C – S – Zn(NH3) –

SH

H2C – N – C – S –

3

(a) yellow powder (b) 10000, > 2000 (c) Protective leaf- fungicide

16. Nabam

H2C – N – C – S – Na

H2C – N – C – S – Na

S

SH

H  

(a) Colourless crystals (b) 395, – (c) Protective leaf- fungicide & algicide

17. Naplatam

NH

C

O

ONa

C O

(a) White crystals (b) 1900, – (c) Selective pre- emergence herbicide

18. Phenyl mercury acetate

Hg – O – COCH3

(a) Pale yellow powder (b) 50-100, – (c) Eradicative Fungicide

19. Potassium cyanate K – N = C = O

(a) Colourless crystals (b) 841, – (c) Herbicide

20. Propineb

X(X > 1)

Zn

H SH

SH

H2C – N – C – S –

H3C – C – N – C – S –

(a) Pale yellow powder (b) 8500, > 1000 (c) Protective leaf- fungicide

  58

21. Zineb

H2C – N – C – S –

H2C – N – C – S –

H S

H S

Zn

X(X > 1)

(a) Light coloured powder (b) > 5200, > 10000 (c) Protective leaf- Fungicide

22. Ziram

N – C – S –

S

Zn

2

H3C

H3C

(a) White powder (b) 1400, > 20000 (c) Protective leaf- fungicide & repellant

(a) Physical appearance

(b) Active oral LD50 (rats) and acute dermal LD50 (rats) in mg/kg

(c) Mode of action

 

Fig. 1.2

Fig.

: Variation

ion

.1.1 : Struct

n of physical

59

Spacern

ture of a gem

 

 

l properties

ion

mini surfact

with surfac

Tail

tant

ctant concen

ntration

 

FFig. 1.3 : M

io

odel of a ty

ns (×), the h

ypical ionic

head group

60

c micelles sh

( ) and the

howing the

e hydrocarb

location of

bon chain (

f the counte

)

er

 

F

Fig. 1.4 : Synnthesis proccedure of ma

61

anganese diioxide nano

ocomposite ffoams