Introduction and Objecfives

1 . General

The improper and uncontrolled disposal of cheniicals from chemical,

pharmaceutical and other it~dustries_ dyestuffs from textile industries, increased use of

pesticides in the agricultural fields elc have resulted in the contamination of both

ground and surface water to 1 great extent. Contamination of drinking water is a

major problcni faced in different parts of tlie world. These pollutants include mainly

chlorinated organic derivatives, chlorinated solvents, nitro derivatives of benzene and

phenol, pesticides like triazini: derivatives and azodye derivatives. Most of these

compounds are not degraded by direct exposure to sunlight due to their \\zeak

absorption in tlie visible rang?. Majority of these colnpounds are stable and can

persist for sufficiently long tim': once 'they are ejected into the environment.

Thc traditional methods used for water treatment such as adsorption on

activated charcoal, flocculatior~ and filtration are inefficient to degrade most of these

organic compounds and hence for their conlplete removal from water. Adsorption on

charcoal is not so effective since it results in the generation of solid sludge which then

havc to be disposed off. Most of thesc compounds are found to be resistant to

biodegradation. Biological treatment is ineffcctive in lnost cases ibr wastewater

trcatmcnt due to the toxic nat.lre of thc clicmicals. Chlorinating is another method

used for the purification of' water, but one of the disadvantages of this method is tlie

generation of chlorinated cleri\atives, which are found to be niore toxic. Hence, this

method is not preferred for the treatment of wastewater.

Ilecently, researcli is being concentrated for thc development of new

technologies for the safe destruction of the pollutants. Oxidative degradation of

organic contaminants in water using hydroxyl radical ('OH) is believed to be an

efficient technique that can be used for the detoxification of water pollutants. I-Ience

tlic reactions that generate '01-1 in solutioii state at room temperature have gained

trcmcndous attention and such techniques are generally termed as Advanced

Oxidation Processes (AOPs).

1.1 Advanced Oxidation I'rocess (AOP)

Advanced oxidation xocess , by definition, is a process that involve the

gcncration ol' reactive ositlizing agenl. the 'OIH. in sufiicient yield \vhicli can be

utilized lor wastewater treatnlent.'" The main ail11 of an AOI' is to degrade toxic

organic pollutants fiom the ppb or pptn range lo non detectable litiiits without the

gcncration of hazardous secondary wastc. The coinrnonly used Advanced Oxidation

I'rocesses include photodissociation of' I~ydrogeii peroxide (H~O~IUV),"-' photolysis

of ozone ( O ~ I U V ) , ' ~ ~ Fetilon reaction ( I : ~ ~ ' / H ~ O ~ ) , ' ~ ~ ' Photo-l'entoti reaction

( 1 ' c ~ ' 1 1 - 1 2 0 2 / ~ ~ ) , ~ ~ ~ ~ ~ photoca:alysis (T~o~IUV) ,"~" radiation cheniical

and sonochemical method."." Each method has its own merits and limitations, a i d

the efficiency of each proces: depends oil the ease with \vliich it can be operated, the

cost ef'ectiveness, thc extent of lnineralizatio~i achieved and finally tile reaction time.

Most of these mctliotls are ef ic t i \ ,c in bringitig about the complete oxidation of

organic pollutants, thus achie,+ing the lowest level of pollution.

1 . 1 . 1 I'hotodissociation ot'hytlrogcn pcroride (I.12021UV)

'OH can be produced by the photolysis of aqueous solutions of H202 using

IJV-light (iL < 254 nm) in acidic: or neutral p ~ . '

L1202 has relatively weak at~sorption in the UV region and has an absorption

coefficient of about 19.6 M-' :m-' at 254 nm. Several secondary reactions are also

possible during I-120:IUV oxidaria11 as shown helow,"

'01-1 + 1-1202 H02' + Hz0 (1.2)

k - 2.7 10' M-I s - ~

1 H 0 2 ' + 1k102' ---, l-IzO2 + 0: ( 1.3)

1 = 3 x 10' bl~ ' s-'

1-102' + 'OH -* IIzO + 0? (1.4)

k- 6.0 x 10' M.' S-'

wlicrc k is thc rate constant. In this system, ~ h c organic compound is osidizcd niainly

by '014. I-Iowevcr, a icsscr extent of oxidation can occur by the direct absorption of

UV-light as well.

Thc concentration of the organic compounds and H202 must be opti~iiized to

get a maximum absorption of' light by H202. This means the overall efficiency of this

proccss is determined by several fjctors like concentration of 1-1202, intensity of UV-

light, chemical structure of tile substrates elc. This technique can effectively be ~ ~ s e d

lor tile degradation of a \-arie.:y of organic pollutants. 25-29

1.1.2. I'hotolysis of ozonc (OJIIJV)

Molecular ozone is highly selective in its reaction towards organic substrates

with ratc constants of tl?e order oi' I - 1 0' h 4 ~ ' s ~ l . 111 certain cases ozonolysis results in

the formation of a variety of :nter~nediates which are not conlpletely mineralized.

Ilcncc, there is more intcrest in the production of 'OH from ozone by different

methods such as photolysis csf ozone (0;IUV) and decomposition of ozone by

liydrogcn perox~tic (0,/I-I2O2).

Ozone absorbs in the LiV region with absorption maximum around 254 nni

both in gaseous and in aqueous mediuin. In gaseous phase enriched with water

vapor, pliotolysis of ozone results in thc decomposition of ozone into 0 1 and 0'.

Combination of 0' with a water molecule leads to 2 'OM as shown,"

'01-1 bcing lcss selcctivc ill its reaction tow~irds organic substrates can rapidly oxidize

compounds which arc very stablc to ozone, into inorganic

1.1.3. I~ccomposition of ozonc initiated by Hz02 (03/H202)

It is reported that hydrcge~i peroxide can initiate the decompositio~i of ozone

in aqueous medium by single electron t ran~fer . '~ The initiating species is the

hydroperoxide ion (H02-) fornled by the dissociation of hydrogen peroxide. The

dccornposition rate of ozone by I-120z is found to increase with increase in pH. The

dccornposition of ozone proccctls through the following mechanisms,'

--- I-1202 i I-120 ---- 1 lo2- + 1 ~ ~ 0 ' ( I . 7 )

k = 2.8 x 10%-'s- '

0 3 + HO2- -- + 'OH i 02'- + 0 2 (1.8)

k = 2.2 x lo6 M-I S-'

0, + 0 2 . - ----+ 0,'- + 0 2 (1.9)

k = 1.6 x lo9M- 's - '

0;- + Hz0 -- + 'OH + OH-+ o2 (1.10)

This technique is also reported to be used for the degradation of several organic

pol~utants. '~

1.1.4. Photocatalysis (Ti02!UV)

TiOz mediated photooxidation of organic con~pounds has gained lot of attention

in rccent years. Irradiation of Ti02 particles in aqueous medium with photons of

cnergy hygher than or equal t ' ~ the band gap energy, Eg (i.e., the energy gap between

the valence band and the conduction band) results in the excitation of electrons from

the valence band (VB) to the conduction band (CB), thereby creating holes and

clcctrons on thc surface of thc scrniconductor particle.34

I'lie holes can oxidize the hydroxyl group or water inolecules adsorbed on the surface

forming '01-1

h'vl3 + (OH-).,ds ---+ 'OH.,ds (1 12)

h'vil + (k120)ads - 'OHa& + Hf (1.13)

The electrons are trapped by G2 adsorbed on the surface forming superoxide radical

anion, 0 2 ' -

e.ci3 + 0 2 ---* 02'- (1.14)

7'he excited state conduction b;ind electrons and valence band holes can also undergo

recolnbination and dissipate thc: input encrgy as heat.

h'v13 + c'cu -L heat (1.15)

The fast recombination of electron-hole pair reduces the efficiency of this system.

I~lowever, this can he prevented by using luore reactive electron scavengers like

hydrogen peroxide.

?'hc '014 generated cat- oxidizc the pollutants at the solid-liquid interface. I t

is reported that the organic c31npounds can also be oxidized by the valence band

holes which are powerful oxidants ( +1.0 lo 3.5 V vs NHE depending on the

semiconductor and the p ~ ) . ' 5

0 1 - 1 + Compound Oxidized product (1.16)

h'vu + Compound Oxidized product (1.17)

The conduction band eleclrons are also good reductants (+0.5 to -1.5 V vs NHE)."

TiO2 has an absorption maxi~nuln around 340 nm which makes this system applicable

for waste water treatment since photolysis can be performed at higher wavelength and

also with sunlight. There are :nany reports on the mineralization of several organic

watcr pollutants using this t e c l ~ n i ~ u e . ~ " ' ~ '

1.1.5. Fenton reaction ( I ~ ~ ' + / M ~ O ~ )

l'he reaction of fcrrous iron and liydrogen peroxide, known as Fenton's reagent,

reported by 1-1.5.1-1 I'cnton in 1394 is one of the simple and efficient iuethods for the

generation of '013 in aqueous medium."

Fe2+ + I-1202 -- + Fe" + 'OH + OH- (1.18)

k = 7 6 w1 s-'

Scveral secondary reactions arc also possible in Fcnton reaction as s l ~ o w n , ' ~

'01-1 + 1 - 1 ~ 0 ~ -- -+ 1-120 + H02' (1.19)

k = 2.7 x ~o 'M. ' s-'

'013 + i;e2'~ --+ ~ e ~ + + OH- (1.20)

k = 3 x 10' M" s-I

l:e3' + 1H20z ----, ~ e ' + + 1-1' + H02' (1.21)

k = 1 x 10-2 b1.1 s- '

~ e " + t I02 ' -4 FC" + O2 + EI' (1.22)

k = 3.3 x 10' M-' s-'

The 'OH thus produced can react with organic substrates forming intermediate

radicals, which can be either xidized by Fe" or reduced by ~ e ' + forming stable

pl.oducts. The reaction pathway may be summarized as follows,"'.'"

RH + '019 -- + I<' + f-120

R' + I:~'+ -- + ~ ~ ~ d u c t - ~ e ~ '

R' + ~ e ~ ' ----+ product + F'e3'

R' +'01H -- + 11-011

The rate constant fvr the abovc reactions are substrate specific. Fenton

reaction have been efficiently used for the destruction o i several organic water

14-50 poliulants. An advantagi: of this reaction is that it can also be utilized for soil

treatment" and also for the treatment of dye waste water."

However, there are some reports on the production of a ferry1 intermediate

[1:e=012' as the oxidizing species in Fenton r e a c t i ~ n . ~ ~ - j " It is argued that some or

most of the 'OH produced in Fenton reaction nlay remain bound at the iron center,

either as [Fe ...' OH]" or the [ F ~ = o ] ~ ' intermediate. It is proposed that these

intermediates have oxidizin;; power similar to but distinguishable from free 'OH.

However, in addition to the huge llunlber of earlier references, recent ESR

n~easurenients have also s l i ~ ~ w n that li-ce 'Oil is the key oxidizing intermediate in

Fenton r e a c t i o ~ l . ~ ~ . ' ~ As ti is debale continues, i t can be noticed from the literature that

the involvctiicnt of '011, to a greater estent, depends on the nature of the substrates

and the reaction conditions and therefore, one has to clearly establish the favorable

conditions where 'OH will be the key intermediate.

1.1.6. lihoto-Fenton reaction. ( F ~ " / H ~ o ~ / u v )

The oxidizing power of Fenton reaction is reported to be highly enhanced by

irradiation using UV or UVIVIS light. This enhancement in the efficiency is due to

the photoreduction of ~2' to Fe2+ ions in presence of water (eqn. 1.27)",j8 which can

produce new 'OW with H202 (eqn. 1.18).

hv ~ e " + H 2 0 -- + Fe2' + 'OH + 1-1' (1.27)

I'hoto-Fenton reaction is optimum at low pH where almost 50% of Fe(II1) exist as

I:~(OH)'' complex, which is the major photoactive species responsible for the

production of '013. The photo-Fenton reaction has been effectively used for the

degradation of several pesticides and organic pollutants.j7'6'

Recently there are reports stating the involvenlent of an additional

intermediate in photo-fen to:^ reaction which is proposed to be the fenyl species

(I:C'~=O) which can also oxidize organic species. 54.62 - ~ime-resolved laser flash

photolysis studies has revca1,:d that the complex [FeN ' -00~]* ' formed between HzOz

and Ee(Il1) act as the precursor for ferry1 moiety.62

1.1.7 I'hotoproduction of '014 from Fe(II1)-hydroxy complex

I'hotolysis of' aqueous solution of ferric ion in acidic pH itself can act as an

ei'licient method for the produc:tioil of 'OM. 'OH is formed by the photodissociation

of Fe(ll1)-hydroxy complex, I:~(oH)*+ in acidic p ~ . 6 3 , h 4 111 acidic solutions

hydroiysis of Fc(ll1) results in the following equi~ibriurn,~"

FC" + HzO -.--A ---- F ~ ( o I I ) ~ + + H'~ (1.30)

Kt = 2.7 x 10.) mol I''

-L Fe3+ + 2H20 -- F e ( 0 ~ ) ~ ' + 2 ~ ' (1.3 1)

where KI and Kz are the equilibrium constants. Faust and ~ o i ~ n e ~ ~ calculated the

hydrolytic speciation of Fe(lI1: based on the equilibrium constant of Fe(II1)-hydroxy

complex, and found that ~ : e ( ~ l - i ) ~ + is thc predominant photoactive species in the pH

range 2.5-5. This complex constitutes about 92% of the monomeric Fe(II1)-hydroxy

Aquated ferric ion u11d1:rgo ligand 10 metal charge transkr tliereby rhe ligand

is oxidizcd and the ineval gets reduced." 111 purc water, hydrated ferric ion nlainly

I:c(oI-I)~' complex undergo phatoinitiated electron transfer from OH- to Fe(lI1) in the

cxcited state, forming 'Of1 and ferrous ion as shown below,63

The molar extinction c0efficier.t for F~(oH)*' at 313 nm is 1760 M-' cm-I and the

quantum yield for the photolysi:; of this complex was found to be in between 0.12 and

0.1 4.(lJ

An advantage of ferric mediated reactions is that, the chat-ge transfer band of

this specics strongly overlaps with the solar UV spectrum (290-400 nm) thereby

lnaking th& photolysis of this cotnplex possible by solar irradiatioi~."rhe half life for

Ihe pl~ololysjs of i:c(01~1j'* cornplex in sunligl~t was determined to bc 20 min. The

absorption band of this comple:.; extends upto 400 run and therefore irradiation can be

performed at longer wavelengt l~s . '~ The rate of formation of 'OH by the photolysis of

aqueous solution of ferric ion was calculated using competition kinetic inethod and

thc value was found to be in thc: range 0.50-0.67 x 1 0 . ~ M.' ~nin. ' in acidic pH 66

The generation of 'OH by the photolysis of Fe(II1)-hydroxy complex has great

implication in atmosphcric cl-,emistry. Photodissociation of F~(oH)*+ complex is

proposed to be the main sourc: for 'OH in rain drops.67 It was reported by Faust and

I-loigne," "that i:e(014)*+ constilutc the dominant Fe(II1)-hydroxy coinplex present in

clouds and fog in the pt1 range 2.5-5, which is also the characteristic pH of clouds

and fog and that photolysis of this complcx is responsible for the generation 'OH.

1'e(01-1)~+ species is present in atmospheric water at concentration greater than 0.1 phl

sincc soluble iron specics is a .najor constituent of polluted atmosphere.

. . l his reaction has been used for the degradation of several pollutants. 68-70

.l'here are also reports that organic compounds can be degraded by another

mechanism which is proposed to occur by photoinitiated electron transfer between the

organic ligand and the ferric ion in the ferric-organo complex, formed between the

organic compound and ferric ion." This rcsults in the formation of an organic free

radical and 17e(lI) wl~iclh is S~lrther oxidized in prcsclicc of' oxygen. .file i i iecha~~ism is

rcprcscntcd as follows.

Fc(lI1) + org Fe(II1)-org complex (I 24)

11v Fe(ll1)-org complex Fe(I1) + org radical (1.35)

Fc(1I) i- org radical t 0 2 ----+ Fe(ll1) + oxidized org (1.36)

l'llis tncchanism is proposed as the main pathway for the removal of atrnosplieric

oxalic and a-keto acids." However, a recent photochemical study of aqueous ferric

chloridc using pulsed laser re~ea led the involvement of 'OH as the main reactive

spccics ibr the dcgradntion of'x!,lidincs i n aqueous nietiium."

1.1.8. Iladiation chemic;~l tccl-~nique

Radiation chelnical rechnique is a clean method for the it7 situ generation of

'01-1 by the action of ionizing radiation on water. Radiation chemistry of water is one

of' tlic most extensively studied iieltl. 7he passage of high energy ioniziiig radiation

~hrough watcr or dilute aquecus solutions initially produces electrons, positively

cliarged water ions and excited water molecules. Tlic duration of this process is of

thc order of lo-' ' sec or less. The various processes occurring in the radiolysis of

water is schematically represented in ~ c h c l n e 1.1 .73 Some of the radicals produced

within thc . s ~ ~ u u (spur refers to a small volu~iie of material in which the deposition of

radiation energy has produced a small number of excited and ionized molecules)"

react togctlicr formii~g molecul,rr products like lHZ and H202 . l'he possible reactions

occurring within the is S~,OWII in table 1 . I

Reactive Molecular radicak products

I DifSusioll of radicals and molecular products. out of the spur Y 1

Schcmc 1 . 1 Schematic representation of the radiolysis of water

.She chemical yield in radi;itio11 chemist~y is expressed by the term G-value

which is defined as the amount of product formed (expressed as moiec~~lcs ) per 100

cV of absorbed energy and this can bc converted into the SI unit by multiplying by a

factor of 0.10364 p mol J - I . Tlie G-value of radicals and molecular products forliied

- ~

in the radiolysis of water " is !;ive11 in table 1 .?.

'l'ablc 1 . 1 : Reactions occurrin;: within the Spur

Table 1.2: The G-value of radicals and molecular products by the radiolysis of

water

0.27

. 1-1 i

0.062

112 0.047

1-1202 1 0.072 I OH-acl 0.05

lii 0.32

1.1.8.1 I'rimary radical spccics: The primary radical species produced by the

radiolysis of water include 'OH, llydrated electron (C,,,) and hydrogen atoni. The

'01-1 is a powerful oxidizing agent while e-;,q is reducing in nature. H'act as oxidizing

or reducing agent depending or, the pH. -She optical absorption spectra of these three

species are given i t ? ligurc I . I

Figure 1.1. Absorption spectra of e-,,, 'OH, ' H. (taken from ref. 75)

'l'lie properties of each radical species are discussed in detail below.

( I . The /?y(lrr~tefl electrutz (c-,,,J

I-lydrated electron fornied during radiolysis of water was identified using

pulse radiolysis technique. liydratcd electron is identified as an electroii surrounded

by four water molecules and carry a unit negative charge. The important properties

of hydrated e~ec t ron '~ is summirized in table 1.3

In water e-,q can undergo recon-(bination or react with solutes or with protons in acidic

mcdium as shown below,

e-,q + e-aq ---+ H, + 2OH- (1.37)

e-,,, + I-]* - 'H (I 3 8 )

e-,, react with solutes predoniirantly by electron attachment as shovni

S"-l e-aq + Sn -+ (1.39)

Table 1.3: Properties of hydrated e lcctro~~

I Wavelength of maximum absorption 715 nni i 1 Molar absorptivity at 7 15 nm 1.85 x lo4 M-' cm-I

1 Standard reduction poten~ial

I . . 1 D ~ f f u s ~ o n coefficient

In presence of oxygen e-,, is scavenged to form 02'-

e.,, + O2 -- 02'-

I Radius of charge distribution

I-lydrogen alom is a1 important reducing agent in acidic solutions with

rctluction potential of aboi11 -2.33 V (pi1 = 0). 'l'lie diffusion coeflicicnt of hydrogen

0.25-0.3 nm

atom is about 8 x 10' cm2 s-' . It absorbs weakly in the UV region, hence the rate

constant for its reaction is del-ermined by following the rate of foni~ation of tlie

intermediate product using pulse radiolysis technique or by competition kinetics

where two solutes compete :o r the radicals and one of the products c& be

dctermined.

One of the important rextions of hydrogen atom is its reaction with molecular

oxygen forming hydropcroxide radical, H 0 2 '

'H + 0 2 --+ HOz' (1.41)

With organic substrates hydrogen atom reacts by H-atom abstraction and by addition.

c. Tlze Itydroxyl rrrdicril ('OH)

Thc properties and gei1i:ral reactions of 'OH are discussed ill section 1.2

1.1.8.2 Restricted radical source

For many radiation chi:mical studies, it is more preferable to have a system

with totally oxidizing or reducing conditions rather than the mixture of all the

radicals. 'fliis can be achievcA by tlie inter conversion of primary radicals or by tlie

conversion of primary radical!; into a single kind of secondary radicals or by the use

of ccr~ain specific scavengers.

(I. Oxirlizitzg conr1itiott.r

The nlost convenient ni:tliod for obtaining a totally oxidizi~ig condition is by

saturating the solution with N20, which converts hydrated electron into 'OH as

7 6 shown,

H20 -- 'OH, c - ~ ~ , 'H, H202, H2, Ht (1.44)

NzO + c-,, '01-1 + 013- + N2 (1.45)

k = 9.1 x lo9 M.'s''

In N20 saturated solution there will be about 10% H atoms and 90% 'OH due to the

slow reaction between N 2 0 and hydrogen atom. Another method involves the use of

hydrogen peroxide

e-,, + H202 ----+ 'OH + OH- (1.46)

k = I .3 x 10" M.' s-'

NzO is more preferred since NzO and its product Nz are inert towards free ratlical

attack.

6. Kerluci~~g condit io~~s

A reducing condition can be achicved by scavenging 'OH by [err-butanol 76as shown.

'OH + (CI-I,)3COH -- (CH,)2'CH2COH + H20 (1.17)

k = j 108 M-1 s-'

1-1 atom can also be scavenged by terr-butailol: but with low efficiency due to the low

rate constant for the reaction (k == 8 x 10' M-' s-I)

11 numbcr of sccondary radical!; such as CIl,'CIIOW, 'CH20H, SO4'-, Clz'-, 1 2 ' e/c

can also be generated by the reaztion of primary radicals with appropriate solutes.

1.1.8.3 I'ulse and stcady state radiolysis

Iladiation induced chemical changes involve the generation of short-lived

reactive intermediates anti the formation of stable products. The kinetics of the

transient formation, its decay and the spectral characteristics o f the intermediate are

generally studied using pulse radiolysis technique. The analysis of the stable end

products after y or X-ray irradiation by normal analytical techniques is generally

ki~own as stcady state radiolysii technique.

(1. I'rilse Rrrdiolysis

The properties of short-lived intermetiiates formed during radiolysis can be

studied using pulse radiolysis. The delivery of ionizing radiation in the form of short

pulses to a chemical systcm rt:sults in a non-equilibrium system, with the production

of sufficiently high concentration of reactive intermediates which are then illo~litored

as a function of limc.'"hort pulse time is required since the rate constants for the

reaction of these intermediates are very high (10l0 M-'S.'). Hence, the half-life for

most of these reactions is in the il~icrosecond range. The high-energy electron

sources used in pulse radio1y:jis include linear accelerator, van de Graaff accelerator

and Febetron.

In pulse radiolysis, nanosecond or microsecond pulse durations are used

depending on the reactions pt:rformcd. Experiment is carried out by directing a short

clcctron pulse into a cell cor~taining the sample. The build up and decay of'

intcrmcdiates arc rno~iitorcd using various detection techniques such as optical

cibsorption, conductolnetry. E1t:ctron Spin Resonance (ESR), Ranian spectroscopy

~ ind liaylcigh light s~a t t c r i ng . ' ~ For most of the radical rcactiolis optical absorption is

uscd, f h i s tcchniquc is appl.cable to most of tlie transients or combination of

t a s i t s . ?'I1c dil'fcre~icc il l optical absorption spcctra of the starting material and

the intermediate is the basis of optical absorption detection. Since the intermediate

radical normally absorbs at longer wavelength than the parent compound, it can be

easily detected using this tcclmique. When the spectrum of the transient is known, a

particular wavclcngth is selec~ed and tlie change in absorption is recorded as a

lunction of time.74 The build.-up or decay of the transient can be monitored on an

oscilloscope and the data is proc:essed using a computer.

h. Sfeutly Slate Rurliolysis.

60 Steady statc radiolysis i:j performed by irradiating the substrate using a Co-

;i-source or X-rays and tile end products are analyzed using various a~lalytical

tcc11ni~ucs .~" Commonly uscd icchniques are I-ligh Pressurc Liquid Chromatoyrapliy

(I-II'LC), Gas Chron~atograpliy (GC), Gas Cliromatography-Mass Spectrometry (GC-

.\/is), Liquid Chromatography-Mass Spcctro~iietry (LC-MS) ere. The yield of the

products formcd is represented in terms of G-value (section 1.1.8). A clear idea about

the products togctlicr with tlic ~:#uise radiolysis data for a radical reaction helps radical

chcmisls to elucidate a cornpletc reaction mechanism for tlie reaction of a particular

radical with any substrate molec:ule.

1 . 9 Sonochemical method

Sonochemistry is emerging as an AOP for the destruction of hazardous

organic p o l l ~ t a n t s . ~ ~ ~ ~ ' Sonochemistry is concerned with the chemical changes

induced in a systcm when subj~:cted to ~lltrasonic waves. Ultrasound is conlposed of

acoustic waves with Srequcncy higher than the audible human frequency about 16

kt lz ,"bad is typically associaled with thc frequency range of 20 kHz to 500 MHz.

Ultrasound is generally classified depending on whether they are low-po\\,er high-

Srcc1uency or liigl?-power lou-ficqucncy \wiles. Ultrasonic waves having frequencies

between 1 and 10 MI-Iz and power in the milliwatt range are described as dinyr7osiic

uliru.sozind. Thesc \raves clo not alter the state of the matter through which it is

transmitted and hence i t is mainly used for non-destructive purposes and medical

tllagnosis. Ultrasonic waves \v~ th frcqucncies that span fronl 20 to 100 kHz with

power ranging from several hundred to thousands of watts are known as power

iillru.\ound and they alter the state of the medium and is preferred for sonochen~ical

applications. 91.92

The chemical effects of ultrasound are due to the phenomenon of c~cozrstic

cuvi/uiion. Ultrasound is transmitted through an aqueous i ned i~~n? as waves

consisting of alternatc conipres~sion and rarefaction cycles, and this causes localized

increase and decrease in the tc~tal pressure of the medium. At any time, a specific

region will experience tile suyicrposition of' two types of pressures, Pa: due to the

acoustic waves (applied acoustic pressure) and PI,, the hydrostatic pressure. The total

pressure at any given instant t: is given by the equation ( 1 . 4 8 ) ~ ~ , ~ )

where I', = PA sin2nft, wliere is the pressure amplitude, and f is the frequency of

the acoustic wave.

During rarefaction, the net pressure exerted on the molecules is negative. If

the rarefaction cycle is very powerful, the negative pressure developed will be

sulliciently strong to overcolne the liquid cohesive force and as a result, the

molecules will bc torn apan from each other creating sn~all voids or cavities within

thc liquid, ie., cavitation bubbles \+ i l l be fornied. These bubbles grow to a critical

resonant size during the succ~ssive compression/rareSaction cycles and during tlie

growing cycle they gct fillet1 with tlie dissolved gases or by the vapor from the liquid.

Further compression leads to :he violent collapse of the bubbles with the release of

huge amount of energy.94"'

Tlic collapse of tlic bubble occurs in a short period of time less tllan tlie

<1(>,97 - compression cyclc. rlie collapse oi'the bubble was estimated to occur in a time

period of - 3.5 ps." As the ~ubb les collapse, the contents get heated more or less

adiabatically depending on thl: thermodynamic properties of the vapor mixture. This

results in the generation of res.ction sites where tlie temperature can reach the order of

thousands of degree ol' Kelvin and pressure of the order of hundreds of

atmospheres. 99.1 00 . rernperatures of the order of 5000 K and pressures of the order of

100 atni at the time of bubble collapse have been reported. 101,102 Temperatures of the

intcrPdcial, region is reporled to bc oS the order of 2000 K.'" I t is also reported that

103 collapsc or the bubble prodi~ces shock waves and electrical discharges. At this

extreme conditions, water vapor is thermolytically cleaved inside the cavitation

bubble forming highly reactive radical species, the 'OH and 'H. 104-106

1-120 -))) + 'OH + '1-1 (1.49)

A coniprclicnsive nintlicn~aticzl analysis of cavitational bubble evolution includilig

thc effect oS chemical transibrmations within the bubble has been recently reported

by Colussi el. UI.""

The radical species produced can recombine, react with other gaseous species

present in the cavity, or can dil'fuse into thc interface and react with solute r~iolecules

at or near the iliterf'acial rcgior. All illustratioll of the cavitation process is shown in

figure 1.2.

'01-1 i '1-1 --- + H20 (1.50)

2 ' O H - + Hz02 (1.5 1)

2 '1-1 -----+ EI 2 (1.52)

In presence of 0 2 , 'H get sc;~vtnged ibrming I-I02'as shown below2'

'13 + 0 2 ----+ HOz' (1.41)

It was observed that in the absence of any reactive volatile solutes, tlie amount of 'H

reaching tlie bulk phase is very small comparctl to that of 'OH. This can be due to the

recombination of '1-1 in the gas phase fonning H2 (eqn.l . j?) , and also due to its

conversion into 'OII in tlic pyr3lytic region (eqn 1.53). 109-111

High ~(essures and IernpBiBIuIas

/bad lo lhermolyrir h (he cevily

Tho bulk rolulion 1s Mmpasod ol solvenl and rubrlrsle molecules

Figure 1.2. A schematic represe:itatioil of tile cavitation process (taken froill ref. 108)

The concentration of 'C31I in the iiiterfacial region is estimated to be (10.'

h ' ~ ) . ~ ~ ~ - ~ ~ ~ I-Ience a major portion of these radicals undergo recombinatio~l in this

rcgion forming Hz02 (cqn. 1.51). In the presence of any 'OM scavengers part of the

radicals will be trapped resulting in lower yield of H202. A portion of the 'OH will

pcnctratc illto the bulk inedi~iin ;md react with the dissolved solutes. Hydrophobic

compou~lds like 11011-ionic orgaric solutes can accumulate in the boundary zone and

can rcact with thc '01-1 in this boundary layer. 111 tile case of volatile solutes. a large

portion of 'OI-I is scavenged frc~m the reactive site thereby reducing thc number of

'01-1 reaching the boundary regicn."'?'he rate of' production of 'OH and H202 during

sonolysis is determined by the final temperature and pressure within the bubblc2 '

Since the life time of the: active radicals produced during sonolysis is larger

than the lifetime of the cavitatian bubble, these species can migrate into the bulk

lnediuln and is available for chemical reactio~i. 116-118 A steady state concentration of

~.cactive radical species call thus be generated by co~itinuous irradiation with

ultrasound.

1.1.9.1 C;ivit;~tion

IS the negative pressure developed during the rarefaction cycle is sufficiently

large i t can ovcrcomc the intermolecular fvrce holding the liquid resulting in the

formation of voids or bubbles ~ i l h i n the liquid. This process is known as ca\,itatioii.

Any bubble created during the larefaction phase or initially present in the liquid will

grow in size during the successive cycles. During the compression cycle all bubbles

will bc made to contract or col1s.pse. However, if during its growth, any gas or vapor

has diffused into the bubblc, cc~mplete collapse may not occur and the bubble may

oscillate in the applied field. There are two distinct types of bubbles, bubbles which

collapse completely ( /~.unsici~i) and those which oscillate and exist for a considerable

pcriod of time (slubie). Wheth-r the bubbles will collapse or oscillate depends on

several factors such as Leniperatllre, acoustic amplitude, frequency, external pressure,

bubble size, gas type and contenl. The chemical effects of ultrasound is basically due

to the prcscnce of two types olcavitation, jiiible cuvitiilion and rr.unsier~/ crrvirtilio,~.~'

(1. Sluhle C(ivi1~1tio11

Stable cavities are bubl~les which exist for several acoustic cycles and

oscillate around a meal? radius in the sound I i e ~ d . ~ ~ , ' ~ The time scale over which they

exist is sufficiently long so that mass diffusion of gas into and out of the cavitation

bubblcs occur. If thc mass trans.'er across the gas-liquid interface is not equal, it can

result in bubble growth. The process by which lnicrobubbles undergo growth in a

liquid is tcrrned as rectified tlifhsion. In the rarefaction cycle, the gas diffuses from

the liquid into the bubble while in the compression cycle the gas diffuses out of the

bubble into the liquid. Since the surface area during the rarefaction cycle is very

large, the inward diffusion of gas is greater and this leads to the expansion of the

bubble. The bubble can undergo expansion urltil it reaches its resonance fl-equency.

j;,,, given by the expression"!

whcre I<, is the radius of rlic bubble at rcsouance and y is the specific heat ratio o f the

dissolved gas. As the bubbles grow the acous~ic and environmental conditions o f t h e

ri~cdium may changc and the bubbles can be transfornled into a transient bubble and

undergo collapse."

The wall motion of a stable bubble in an acoustic field is given by the Rayleigh-

Plcsset cquation, 93

.. Where R = dR/dt = velocity of the cavity wall R is the acceleration of the cavity

wall, R, is the radius of the bubble at its rcst (equilibrium) position, a is the surface

1cnsiori of thc liqi~id, r1 is 111~ viscosity of the liquid, I>,, is tile vapour pressure of the

liquid, p is thc density of tlic licluid, I',, is the atmospheric (hydrostatic) pressure, P,, is

tllc applied a c o ~ ~ s t i c pressure and K is thv polytropic il~clcs of the gas.

I). 7icr11sietzt Crrvifclfio~z

Transient cavities are voids or vapor filled bubbles formed by sound waves

with intensities in cxccss of IC W They exist only for a few acoustic cycles.

during this time they undergo a rapid growth to a radius twice or thrice its original

sizc (- 150-200 pm) during tlie rarefaction cycle. On con~pression, these bubbles

undcrgo violent and rapid col:apse (1-10 11s) generati~ig temperature of the order of

thousands of Kelvin and pressures of the order of hundreds of a t ~ n s . During the

lifetime of the transient bubbles, it is assumed that there is no time for diffusion of the

gases into and out of the huk)bles and therefore there will be no cushioning effect

during bubblc collapse, thereby resulting in higher cavitational events.93

Assuming the collapst: of the bubble as adiabatic process, ~ o l t i n ~ k " ~ and

121 Ncppiras: ~ l y n n ' ~ " and separ;ltely by Nepplras gave the following expression for

the calcula t jo~~ 01' tlie tcmpc.ature (Y;,,,,) and pressure (P,,,,,) produced within the

bubble at the moment oftotal collapse

whcrc TO is the ambient (expc:rimcntal) tcmpzrature, P is tlie pressure inside the

bubblc at its maxirnun~ size (a:;sumed to be equal to the vapor pressure (P,.) of the

liquid), P,, is the pressure in the liquid at the moment of transient collapse, and K is

he polytropic ratio of the tiissolved gas.93

For transient cavitation f o r ~ i ~ e d from stable one, the temperatilrc and pressure

gencratcd during collapse \\..ill t)e lower conlpared to the collapse of transient bubbles

initiated by high frequency ultrasound, due to the presence of vapor which cushions

the collapse.

1.1.9.2 Theory

There exist two theorie:; to explain the chemical effects due to cavitation: the

hol ,spo1 theory and the eleclricr~l rheory. The hot spot theory states that when the

bubbles cavitate, localized hot spots are created which reach temperature and pressure

in excess of 5000 K and 500 atm." According to this theory the possible reaction

sites consist o f the gaseous phase inside the cavitation bubble, the thin liquid layer

surrounding the cavity a i d the bulk solution. ESR and spin trap studies revealed that

thc solvent vapor aiid the ambient gases can decoinpose into free radicals in the

bubble. Volatilc solutes at high concentration can diffuse into the bubble and

undergo tl~ermolysis into SI-ec: radicals inside the bubble. ?'lie type of reactions

occurri~ig insidc the gaseou:; phase include isotope exchange, conlbustio~l and

d c c ~ m ~ o s i t i o n . ~ ~ 7'hc tcrnpera.urc in thc iii~erf,aciai region is estimated to reach ~ ~ p t o

2000 K during bubble collapse: and superci-itical reactions are reported to occur in this

region. ' 2 2 I'yrolytic reactions are also possible in the gas liquid interface when

solutes are present in high concentration. At low solute concentration they undergo

reaction by scavenging the radicals produced in the gaseous phase.92 It is reported

that about 10% of the reactive free radicals formed inside the cavitation bubble

diffuses into the bulk mediuiii. Therefore in the bulk medium the solute is degraded

by free radical rcaction.. The hof spol theory is the widely accepted theory.

The elecrricul iheory postulates that an electric charge is created on the

surFdce of the cavitation bubble which can produce enormous electric field gradient

across tlic bubble which can cause bond breakage upon collapse.

1 . I .9.3. F;~ctors affecting cavitation.

'She reaction condition:; strongly influence the intensity of cavitation, which

directly affect the rate of sonochemical reactions. They include temperature, external

pressure, irradiation Sreqiiciicy, acoustic power and ultrasonic intensity. Another

Sactor which effect cavitation i: the presence and nature of dissolved gas.

(I. Temperature

The rate of sonochemical reactions is enhanced by lowering the temperature

of tlic system. Increase in reaction temperature increases the equilibrium vapor

pressure of the system thercb:: leading to the formation of cavitation bubbles with

higli vapor contcnt. The prese.lcc of vapor in the bubble cushions the collapse of the

bubble, thereby reducing the ultraso~lic energy produced during cavitational event.

1.owering the tcinpcraturc of the system lowers the solvent vapor pressur-e and

thereby increases the intensity of cavitational collapse. Thus higher so~locheniical

cffcct is produced at lowel- tenlperature when the majority of the bubble content is

9 1.92 gas.

h. Exjiterncrl Pressure

Increase in tile cxtcrntil pressure Pi, leads to an increase in tlie cavitatioll

threshold and the intensity of bubble collapse. Since PI, - P, > 0 at higlier external

pressure, the magnitude of'tlle ;~coustic pressure Pa applied to the system must also be

increased in ordcr to produce bubble growth during rarefaction. This can be achieved

by increasing the intensity of ihe ultrasonic waves since it will generate large values

of P, ( I x PA^ ; Pa = PA sin 27t f t ) making Ph - Pa < 0. In that P,,, ( the pressure in the

bubble at the time of collapse ) is = I-'], + Pa and hence increasing the value of Ph can

lead to more rapid and violcllt collapse (eqn.1 .57).'2,93

r. Solvent

l'or cavitation to occur, tile negatibe pressure in the rarefaction cycle sliould

overcome the intermolecular force holding the liquid intact. Therefore, for high

viscoiis liquids or for licjuids .with high surface tension where the cohesive force are

stronger, high intensity ultrascinic waves are recluired ibr cavitatiori to occur. Vapor

prcssurc of' thc solvcnt is a:~othcr factor that affects the intensity of cavitation.

Cavitation occurs more reatlily in solvents with high vapor pressure, but the

efliciency of collapse will br lower due to tlie cushioning effect produced by tlie

1. I r r ud i~ r t i u~~ f reyue~ lcy

' fhc frequency of ultr;~sc~untl has a signilicant ct'fect on the critical size of the

ca\.itatioi? b~ihblc, \vhicll lins a strong inlluciicc on the cavitatioli event. A: \,cry h i ~ h

Il-ccluurcics ( I -10 MI 12) thc ei'il:ct o l the cavilation is reducecl which is attributed due

to tlic Sollowing reasons ( I ) ti12 negative pressure developed during the rarefaction

cyclc of the sound wave is insufficient in its duration and/or intensity to initiate

cavitation or (2) the compl-ession cyclc occurs at a faster rate than the time required

Sor the bubble to c o ~ l a ~ s e . " ~ ~ " '

Most of the sonocheini~;al reactions carried out in the past were perfornled

using sound waves with frequency between 20 and 50 kHz. Low frequency

ultrusound produces more violent collapse leading to high localized temperature and

j)i-essurc at thc cavitation s i t e4 ' Recently i t Lvas Sound that high ii.equency (100 kl-iz-

1 MI-lz) increases the rate of sonocheinical degradation, which can be directly related

lo the availability ol' 'Oli in tile solution. i\t higher frequency, altho~tgli the

cavitalion is less violent, the nuniber of free radicals in the systetl? is increased

hccai~sc :hcrc arc morc cavitational cvcnts ;iiitl licnce more ciiaiices for the productio~i

of free radicals." Petrier ei 01. proposed that at high frequency, the lifetime of the

bubble is shorter (3 x 10.' s a t 5 I 4 kI-lz and 10.' s at 20 kHz) and therefore more '01-1

can escape the caviration buhble before iindergoing recombiriation or an)- other

rcaclion inside the bubblc.12' ']'he degradation of phenol was reported to occur at a

rate about six tiincs faster for reaction performed at 487 kHz compared to 20 k ~ z . "

I'hc sonochcmical oxidation 01 ' iodide in presence of air was reported to occur at a

faster rate (3 1 times greatcr) when operated at 900 kHz compared to 20 k1-Iz." Hua

and 1-IoSSmann dctermincd th~: concentraiio~~ of 'OH and hydrogen peroxide at

tliSScrent frequencies and observed that the yields of 'OH and H202 are higher at

higiicr frequency. 21

e. I'resence of clissolvecl gcrs

For sonochemical reaction to occur, there should be dissolved gases in the

system. Dissolved gas act as nucleation sites for cavitation thereby making the

initiation of cavitational event more easy." The temperature produced during the

bubblc collapsc depends on the specific heat ratio of the dissolved gas (eqn. 1.56).

For gascs with high polytropic: constant, higher temperature will be produced during

coiiapsc according to eq~~at ior i (1.56). Hence monoatomic gases are more preferred

~ h a n diatomic gases and polyatomic gases for sonochemical reactions. ?'he specific

heat value of the commonly u:ed baciiground gases is given in tablc l . . ~ . ' ' ~

Table 1.4. Ratio of specific heats of different gases

Air

Sonochemical efficiency is also iniluenced by the thermal conductivity and

the solubility of the background gas. Thermal conductivity of the gas determines

how long the temperature is sustained inside the bubble. For reactions occurring

inside the bubble, gases with lower thermal conductivity gives higher chen~ical yield

compared to gases with higher thermal conductivity. Gases which are highly soluble

in the reaction mixture reduc,: the cavitation effect since the bubble fornled will

dissolvc before its collapse." Depending on the nature of the background gas,

djffercnt reactions occur inside the cavitation bubble as given below.

ij flrgon .sulurulion

In the case of saturat on with incst gases i t is proposed that these gases

penetrating into the cavity can contribute to the transfer of electron excitation energy

to watcr molecules12s as showr~,

Ar - ) - Ar* (1 5 8 )

As' +- 1-120 . IH20* + 11r (1.59)

I-120* -- 'H + 'OM (1.60)

Using ESR Spin trap technique it was reported that under argon saturation the

radicals that cscape into the st,lution are 'OH and 'H. 101,105 Saturation with argon is

reported to favor pyrolytic reaction due to higher temperature produced during

collapse.

ilj Oxygo1 .sulul~ciiivl7

Under oxygen saturation, Oz is themmolytically cleaved inside the cavitation

bubble ibrming atomic oxygen. The possible reactions inside the bubble are

sumlnarized b e ~ o d ' " ~

0 2 -1)) + 20. (1.61)

'1-1 + 0 2 -- + 1-102' (1.41)

0' + I-Iz - 'OH + 13' (1.62)

0' + H02' 'OH + O2 (1.63)

1302' + WO:' -- 1-12024-02 (1.64)

ESR-spin trap studies 104,105 under oxygen saturation gave evidence only for 'OH

escaping thc bubble due to tlic scavenging of '1-1 by 0 2 (eqn 1.41). Under oxygen

saturation; the yield of hgdr3gen peroxide formed is higl~er due to an additional

pathway fbr the formation of 1-iz02 (eqn. 1.64).

iii) A'iirogo~ .xuluruiioi?

Dissolved Nz is decomposed inside the cavitation bubble forming atomic

nitrogen. Atomic nitrogen undergo reaction with 'OH, 'H and 0 2 fbnning nitrates

and nitrites. The possible rcactions occurring inside the bubble is given b e ~ o w ~ ' ~ ~ ~

N2 -1)) + 2N'

N + 'Otl ---+ NO* + I3

NO' + '013 --+ FINO2

NO' + '01-1 -- + NO2 + 'H

Nz + 0' -* N' i- NO'

N' + NO' -+ N? .- 0'

2NO' + 1 1 2 0 2 -+ IlKOz + H N 0 j

N O ' + N O ' -- -+ NzO T 0'

1 . I .9.4 Rcaction Zoncs

I3SR and spill trap stuclies using volatile and nonvolatile solutes have shown

that threc rcgions of sonochemical activity exist in sonicated ~ ~ s t e r n s . " ~ ~ h e three

zones ofsonochemical reactiorl is shown in figure 1.3.

Gaseous region of cavuation bubble

Bulk Liquid phase Zone 2

Gas-liquid transition region

Figure 1.3. Three reaction zones of sonochemical reactions (taken from ref .91)

Zone 1 . The holguscousplt~s~? inside ilte cavitutiort bubble

Free radicals and atoms formed in the hot gaseous phase can also initiate

reactions in this zone. These reactions reseinble the one occurring in flame and shock

tubes. The reaction occurring in this phase has the highest ratc (about lo4 A4 min-')

;rmoi~g the various reactions initiated by ultrasound.12' In this zone volatile solutes

and solvents undcrgo pyrolytic reaction due to the extreme temperature and pressure

produced in the vapor phase of the collapsing cavitation bubble.

Zone 2. Tile grrs -lic/nid i~rferjircr

'This zonc consist of the thin layer of superheated liquid surrounding the

gaseous phase. Non volatile solutcs such as acetate anion and polymers are

pyrolytically decomposed in this region. From the nature of the pyrolysis products it

is concluded that the tc~uperaturc in the interfacial region is higher than the critical

temperature of water and tkerefore the solution in this region is possibly in a

supercritical state. Supercritical reaction is reported to occur in this region122 along

with free radical attack and pyrolytic reaction.

Zone 3 . Tile bulk plzrtse

'The rcaction occurrini; in this rcgioil depends on the frequency and the nature

of thc solutcs. It is reported that about 10% of the radicals produced inside the

cavitation bubble diffuscs into the bulk phase."7 The nonvolatile solutes in this

phase is ii~ainly degraded by the '01-1 attack.

1 . 1 . Hcaction pat11w;iys

In sonocilcmical rcaction tile degl.cidation of organic pollutants can proceed

tiirougil several mecilanisms depending on tile nature of tile pollutants. Higi~ly

volatile solutes sucil as CC;l4 and HIS readily diffuses into tile cavitation bubble

during its growing cycle and nndergo pyrolytic reactions witilin tile vapor pilase of

tile collapsing Tile products obtained after sonolysis of volatile

co~npounds resembles tilose obtained during pyrolysis and combustion reaction. For

rcactions occurring inside tile cavitation bubbles, tile nature of tile dissolved gases ilas

3 strong influence on tile kinetics. Low-volatile solutes on tile otileriland, are

degraded mainly by tile attack of 'OH in tile interfacial region and also in tiie b ~ ~ l k

pilasc. In tile casc of reactions occurring in tiie interfacial region tile presence of

radical scavengers can affect tile extent oS organic destruction. Free radical reactions

arc Inore predominant a t low solute concentrations wiiereas pyrolytic and

supercritical water reactions i n tiic interfacial region arc more predominant at iligil

solute concentrations, l'iius, depending on tile pilysical properties of tile solute, it can

be simultaneously or scqucntially degraded botii in tiie gas pilase and in tiie interfacial

or in tile bulk mcdium.

1 . 1 . I0 Ozone ISonolysis

Tile combination of ozone and ultrasound is an efficient metilod to increase

tiic yicld of 'ON. Sonication in presence of ozone results in tile tilermolytic cleavage

of ozone in tile vapor pilase of tile cavitation bubble forming atomic oxygen and 0 2 as

128-130 s i~own,

0 3 -))I --> 02 + o(+)

Atomic oxygen reacts with u.atcr forming '01-1

o ( ~ P ) + I H ~ O -- + 2 . 0 ~

The efficiency of sonccheniical degradation reaction in presence of ozone is

liighcr due to the increasc in the yield of 'OM produced in the vapor phase. The

sonochcmical degradation of methyl (err-butyl ether (MTBE) in presence of ozone

was found to be elihanccd by a factor of 1.5-3.9 depending on the initial MTBE

c o n c e n ~ r a t i o n . ~ ~ ~ This metl~od .xzs ;11so found to be effective for the oxidation of nitro

derivatic~es o f phenol and benz,-nes.lz9 Sonolysis in presence of ozone was applied for

the degradation of na tu r~ l organic matter and 91% decrease in the total organic

carbon content was achieved a:ter a reaction time of about 60 min.'"

1.2 The hydroxyl radical ('OH): Properties and general reactions

'01-1 is a sliort liveti, arid powerful oxidizing agent with oxidation potential of

about 2.8 V in acidic solutioii.

A comparison of thc oxidation potential of 'OH with few other oxidants are given i n

table 1.4.

Table 1.4: Oxidation I'otcntial of Some Oxidants

I I Species ) Oxidation Potential (V)

'OH

0'-

0 3

1 1 2 0 2

- 1 . - 1 '01-1 has a wcak optical absor>tion spectrum in the UV region with E = 370 M LIII

at 260 nm. 1-lence direct determination of its reaction rate constants with solutes

based on its absorption decay is diCficult. In nlost of the cases the intermediate solute

radical formed from its attack has well defined absorption above 300 lun and hence

its reaction rate can be measured by following the absorption build up of the

intermcdiatcs using pulse raliolysis. Rate constant can also be determined using

competition kinetic method using a suitable 'OH scavenger.'j6

'01-1 can react non-selccti\~ely with nlost organic compounds at relatively high

rclrction rate, with ratc const;tnt of the ordcr of 1 0 ~ - 1 0 ' ~ M-' s-l. It undergoes reaction

mainly by four reaction path\vays.'"7"

11. Electron trrr~lsfer

It can undergo electron t ra~sfe r reaction (one-electron oxidation) with anions and

with ccrtain mctal ions.

'OH + X- -+ X' + OH- (1.76)

h. fIydrogetz abstraction

I-lydrogen abstraction occurs similar to that of hydrogen atom but with a higher rate.

'OH + RtI - 11' + H20 (1.77)

c. Arl(1ition reactiotz

Addition reac~ion predcminates with n-electron systems mainly ethylenic or

aromatic compounds

'01-1 + I<]-[ ----+ I~IR'OH

(I. Displ(icentenf reucliofz

'01-1 + R11 + 11' -c 1101-1

1.3 OBJECTIVES

Pollution of both ground and surface waters by the presence of toxic organic

pollutants is a serious problem faced in difierent parts of the world. The conventio~lal

water lreat~nent methods are not effective for the removal of these pollutants. Hence,

in recent years there is a growing interest in the development of new methodologies

for the degradation of toxic water pollutants. Advanced Oxidation Processes (AOPs)

I~avc emerged as a major waste water treatment teclinology which can oxidize most of

the hazardous organic compo~nds . This technology, based on the involvement of a

highly reactive radical, the 'OM, react non-selectively with most of the organic

compounds Icadii~g to tlieir deg:raclation.

Dcsign of any such process is possible only tllrougli a complete understandins

of thc underlying chemistry. Therefore, in this thesis an attempt is made to

investigatc the kinctic and meci~anistic aspects of the reaction of 'OF1 wit11 variety

ol'organic compounds leading to its complete degradation in aqueous medilirll using

t l~rce major AOPs - the photo:ysis of Fe(ll1) hydroxy complex, the radiolysis and

sonolysis oS water.

1.3.1 I'hotoproduction of hy~lroxyl radicals from Fe(1II)-hydroxy complex: A

quantitative assessment and determination of its reaction rate constants with

some substituted benzenes and biomolecules.

It is known that photolysis of Fe(I11)-hydroxy complexes can produce 'OH at

acidic p13s."~" This system has been widely employed for the degradation of organic

contanlinants in aqueous meiium. The predominant photo-active Fe(II1)-hydroxy

conipiex is the Fe(0f1)~' complex, which exists in the pH range 2.5-5.0 and is the

main source for '01-1 (eqn. 1.3:1-1.33). Based on this principle, a considerable attempt

11as been nlatlc to invcstig;r~c the degradation of organic pollutants such as

chlorophcnols and triu~ines.".~' 1-lowever, an alternate mechanism is also proposed

Sor thc degradation of organic compounds without the involvement of'OI3. 7'11is is

suggested as a rcsult of a direct electron transfer from the organic ligand to Fe(1II) in

the cxcitcd Fe(ll1)-organo complex formed between Fe(II1) and the organic

compound, resulting in the formation of Fe(I1) and an organic radical which is

further oxidized in prcsence of oxygen leading to its degradation (eqn. 1.34-1.36)."

Although the above: two mechanisms would ultimately lead to the

dccomposition of' organic n~oleculcs, it is obvious that there exists quite some

ambiguity in the exact reaction mechanism oi'the above process. Therefore, our main

aim was to reinvestigate the pliotolysis of Fe(II1)-hydroxy colliples in order to

cjuantitatively assess the involvement of '01-1 in aqueous niediu~ii using some of its

wcll known typical reactions. Both su~ilight and UV-light were used for the

photolysis. 7'hc selected typical reactioiis ir-iclude ( I ) formation of HCIIO from

mc~haiiol arid (2) formation oi'n TDA-i<cacti\,e Substance ('rB11-RS) from 2'-dsosy-

D-ribose. The results obtaincti with tllese reactions were compared with those from

radiatio~i chemical methods wl~ich is considered as a clean source of 'OH.

In order to design any AOl's, it is important to know the second-order rate

constant for the reaction of 'OH with organic systems. Since the magliit~lde of the

second-order rate constants art: generally very high (> 10' M" s-'), the rate constant

determination is normally done by pulse radioiysis which is a direct and the best

known mcthod for such stildies. Howevel-, due to the expensive nature, its use is

often limited and is not avail;lble to many radical chemists. Therefore, competition

kinetic methods provide an alternative to pulse radiolysis for rate constant

determination for 'OH using a hiown 'OM scavenger whose product can be easily

monitored by normal analyticil techniques. Therefore, an attempt is made to develop

a simple and efficient niethod for the determination of rate constants for the reaction

oS'Ol l wit11 several substitutcd benzenes and some hiomolecules by the photolysis of

aqueous ferric perchlorate at low pH using UV-light and sunlight, coupled with

deoxyribose-thiobarbituric acid assay using competition kinetic method. The values

determined using this 1neth3d was compared with those obtained using pulse

radiolysis.

1.3.2 Oxidative degradation of triazine derivatives in aqueous medium: A

radiation a n d photochemical s tudy

. f r i , ':one . . dcrivativcs occ.:r as a major pollutant in the environn~ent due to their

continual use as hcrbicides. D(:gradation studies oi'triazine dcr-ivatives are important

]'or thcir rc11:oval from the environment as a water pollutant. Direct pliotolysis of

trixzinc dcrivatives are normal,y difficult due to their weak absorption of light with

wa\~cIcngth greater than 220 nln. Decompositio~l of these derivatives using 'OH has

gained considerable attention in recent years. 'There are several reports on the

photochemical decomposition studies of triazine derivatives but very little is known

about their degradation using ionizing radiation which has enormous potential in the

19 detoxification of organic poilu~.ants. The radiation chemical studies make use of the

rcaction of' both oxidising ('014) and reducing (e;,,;) radicals produced by the

interaction of ionizing radiatioils such as y rays or high encrgy electron beam, ~ v i r l ~

waler. Further more, the exac: bimolecuiar rate constants of the reaction of 'OH with

triazines are not seen to be reported. 'Shcreibre, in the present work, pulse radiolysis

\vas cal-I-icd out to iletcrr~~illc tile bimoicculai- ratc constants for the reaction of 'OH

with some triazine derivatives and to investigate the nature of the intermediates at

near ncutral p1-I. Garn~na radiolysis was performed to investigate the oxidative

dccomposi~ion proiilc of lev;, concentrations (10" M) of l,3,5-triazine (T), 2,4,6-

trimcthoxy-I,3,5-triazine (Tb4T) and 2,4-dioxohexahydro-1,3,5-triazine (DHT) in

aqueous medium. I'hotoctiernical degradation in presence of ferric perchlorate was

also investigated with DI-I'T at low p1-I in order to compare the results obtained with

radiation chemical method.

1.3.3. Sonochemical degratlation of azobenzcne and its derivatives

Azo derivatives constitute about 50% of all commercial dyes used and is

cmploycd in a wide range of processes ranging from textile to paper industry. The

major sources of dycs in the environment are effluents from the textile industry.

Most of the dyes are found to be resistant to normal waste water treatment processes

and scvcral AOi's have becn ~ s e d for their relnoval. Azo conlpounds often become

toxic to organisms after anac:robic reduction and cleavage of the azo bond into

carcinogenic amino comp0unc.s vitr intestinal anaerobic bacteria. Therefbre, in the

present work we performed the sonochemical degradation of azobenzene and a group

of monoazo dyes (methyl orange, o-methyl red andp-methyl red) in aqueous solution

aficr saturation with different gases.

(I. Kufe enlzr~ncement vicl Fetrtorr rerrctiotl

During sotiolysis a c:onsiderablc portion of the 'Of1 produced undergo

rccornbination within the bubble and in the interfacial region forriling I-l:Oz. Since

'01~1 is (I more po\vcrfirl oxidant this recon~bination reduces the efficiency of

sonolysis for the degradation studies. Many efforts have been devoted to improve

the efficiency of sonocl~en~ical reactions, considering that a substantial amount of the

energy c~ilploycd in genet-ating the radicals is not effectively cor~vcrted into an

optimum yield of the desired products. ?'he reconibination of 'OH in the gas pliase

within the bubbles and in solution are two of the major processes that limit the

aii1ount of rcactivc radicals ;.ccessible to the target molecules. The sonochemically

gcneratcd 14202 in most case:; is not able to react with the colupounds and eventually

ilccomposcs. ~l 'hercibre~ the addition of FcS0.j to the trcntcd solutions is stildied i i i tllc

pl-cscnt work as a way to en11;ince the amount of 'OH available in solution. The

classical I'en~on reaction bccomes a secondary source of 'OH recovering part of its

chcrnical activity, othorwisc 10s. in the production of relatively large aniounts of'H?Oz

during sonication. The enhancement in the reaction rate is quantitatively accounted

by a simple kinetic model.

I . Synergistic sotzo1ysis/ozot1oiysis of rrzobetzzet~e rrtzrl tnetlryl orrrtlge

Azo dycs are resistant to cliemical and photochemical degradation, and are

~rclativcly stable under norinal wastewater trearineiit conditions. As a consequence,

scvcral advancetl oxidation irethods were tested for their coniplete degradation.

'l'licsc tccliniqiics dccolorizc tlic cflluents, lowcring dye concentration to sub-ppni

Icvels, but Sail howcvcr to ac1ii1:ve a high degree of mineralization to CO?. Therefore

ive st~itlied the combined effect of ultrasonic irradiation and ozonation

(sonolysisiozonolysis), for different Ozi03 ratios for the degradation of azobenzene

(AR) and inethyl orange (MO) in aqueous solution. In order to confirm the cheniical

synergism in the cornbincd sonolysislozonolysis treatment the conibined technique

was also applied to two re1ativt:ly stable byproducts (nitrobenzene and benzoquinone)

of sonolysis under inert gas saturation. The role played by H02' and 0 2 ' in the

sonolysis of beiizoquinone is arialyzed through a siniple kinetic model.