Phsicochemical Treatment of Hazardous Wastes - Chapter 11 pot

The degradation of organic pollutants mayresult from combustion, supercritical water oxidation, and oxidation by rad-icals such as hydroxyl radicals and hydrogen radicals.. The reactions

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Sonolysis

11.1 Introduction

The sonochemical effects produced by sonolysis are due to the phenomenon

of cavitation, which is the nucleation of bubbles in a liquid under the ence of ultrasound Sonolysis is based on the fundamental concepts andtheory involved in sonochemistry; the historical perspective of sonochemis-try in Table 11.1 provides an insight into the discovery and understanding

influ-of fundamental processes in sonolysis When a liquid influ-of relatively high vaporpressure and dynamic tensile strength (such as water) is exposed to high-frequency ultrasonic waves (a few to several hundred kilohertz), acousticcavitation in the liquid will occur The cavitation process includes the for-mation, growth, and implosive collapse of small gas bubbles Cavitation byultrasound is accompanied by high temperature (2000 to 2500 K) and highpressure (hundreds of atmospheres), which are responsible for the degrada-tion of organic pollutants Therefore, sonolysis can degrade organic pollut-ants to CO2 and H2O or convert them to compounds that are less harmfulthan the original compounds The degradation of organic pollutants mayresult from combustion, supercritical water oxidation, and oxidation by rad-icals such as hydroxyl radicals and hydrogen radicals Sonolysis was found

to be efficient and economical to decontaminate industrial organics beforethey are discharged into aquatic ecosystems Therefore, the applications ofultrasound to destroy organic pollutants have increased significantly in thepast decade

11.2 Fundamental Processes in Sonochemistry

11.2.1 Physical Processes

Sonochemistry is defined as the chemical effects produced by ultrasonicwaves Ultrasound, with frequencies roughly between 15 kHz and 10 MHz,has a drastic effect on chemical reactions It is the most important

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424 Physicochemical Treatment of Hazardous Wastes

phenomenon that produces a sonochemical effect on chemical reactions Thisphenomenon proceeds as follows A sound wave impinging on a solution

is merely a cyclic succession of compression and expansion phases imparted

by mechanical vibration During the solution expansion phase, small filled bubbles are formed due to weak points in the solution, primarily attrapped gas pockets on particulate surfaces These bubbles grow and contract

vapor-in response to the expansion and compression phases of the cycle, tively Because the surface area of the bubble is greater during the expansionphase than during the compression phase, growth of the bubble is greaterthan the contraction, resulting in an increase in the average bubble size overmany cycles Over time, the bubble reaches a critical size depending on theultrasonic frequency, whereupon the pressure of the vapor within the bubblecannot withstand the external pressure of the surrounding solution Theresult is a violent collapse of the bubble, with high-velocity jets of solution

respec-TABLE 11.1

Historical Perspective of Sonochemistry

1867 Early observations of cavitation by Tomlinson and Gernez

1880 Discovery of the piezoelectric effect

1883 Earliest ultrasonic transducer by Galton

1895 Cavitation as phenomenon recognized and investigated on propeller blades

1917 First mathematical model for cavitational collapse predicting enormous

local temperatures and pressures by Rayleigh

1927 Publication of first paper on chemical effects of ultrasound (Richards and

Loomis, 1927) 1933–1935 Observation of sonoluminescence effects

1933 Reports on reductions in viscosity of polymer solutions by ultrasound

1943 First patent on cleaning by ultrasound (German Patent No 733.470)

1944 First patent on emulsification by ultrasound (Swiss Patent No 394.390) 1950s Intensification of cavitation and ultrasound research; increasing number of

applications using ultrasound

1950 Effect of ultrasound on chemical reactions involving metals (Renaud, 1950)

1953 First review on the effects of ultrasound (Barnartt, 1953)

1986 First international meeting on sonochemistry

1990 Foundation meeting of the European Society of Sonochemistry

Source: Adapted from the European Society of Sonochemistry.

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It is estimated that 4 ¥ 108 bubbles/s/m3 are produced The bubbles are

on the order of 10 to 200 mm in diameter, and they are short lived, with alifetime near 10 ms; therefore, the bulk characteristics of the solution remainrelatively unaffected, but the implosion of the bubble causes enormous localeffects For example, the temperature of the vapor within the bubble hasbeen estimated to reach as high as 5000 K with a concomitant pressure near

1000 atm Due to the extremely high temperatures created during the process,

a cooling system generally needs to be included in the design of sonolysisreactors The principal result of these conditions in an aqueous solution isthe breakdown of water vapor in the bubble into hydrogen and hydroxylradicals This essentially transforms the bubble into a microreactor, whereinteresting chemistry can take place If organic species are also present inthe water subjected to ultrasonic waves, it is expected that degradation willoccur, ultimately to complete mineralization The extreme conditions created

by acoustic cavitation initiate three distinct destruction pathways for organiccontaminants: oxidation by hydroxyl radicals, supercritical water oxidation,and pyrolysis It has been proposed that pyrolytic mechanisms dominate athigh solute concentrations while hydroxyl radical attack dominates at lowsolute concentrations From the view of pure physics, the effects of sonolysis

on aqueous solutions can be described by three fundamental concepts insonochemistry The first phenomenon is compression and rarefaction, thesecond is cavitation, and the third is microstreaming

11.2.1.1 Compression and Rarefaction

A rapid movement of fluids caused by a variation of sonic pressure subjectsthe solvent to compression and rarefaction This movement can be described

as a motion that alternatively compresses and stretches the molecularstructure within the cavitation process This rapid movement of fluids is

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caused by variation of the sonic pressure Locally, the rarefaction phases ofacoustic pressure wave generate gas and microbubbles (Sochard et al., 1998)

11.2.1.2 Cavitation

Cavitation is a three-step process consisting of nucleation, growth, and lapse of gas- or vapor-filled bubbles in a body of liquid The instantaneouspressure at the center of a collapsing bubble has been theoretically estimated

col-to be about 75,000 psi The temperature has been similarly estimated col-to reach

a value as high as 13,000°F Due to this local high temperature and pressure,

it has been well recognized that cavitation can enhance the rate of a chemicalreaction Under such extreme conditions, the solvent and solute moleculefragmentizes to generate small pieces such as reactive free radicals The freeradicals may further precede some secondary chemical reactions

11.2.1.3 Microstreaming

Microstreaming is an event in which large amounts of vibrational energyare put into a small volume with little heating Furthermore, microstreamingconstitutes an unusual type of fluid flow associated with velocity, tempera-ture, and pressure gradients (Laborde et al., 1998)

11.2.1.4 Cavitation Temperatures Probed by EPR

High temperatures generated due to the diffuse energy produce hot spots

in the liquid When well-defined reactions due to ultrasound were studied,Suslick et al (1986) determined the temperature of the imploding cavity to

be 5500°C and the pressure to be around 500 atm according to namic principles This short-lived hot spot, with heating and cooling ratesgreater than 109 K/s, is the source of sonochemistry The reactions that takeplace at the gas–liquid interface of the bubbles are similar to combustion.The semiclassical model of the temperature dependence of the kinetic isotopeeffect for H and D atom formation was used to estimate the effective tem-perature of the hot cavitation regions in which H and D atoms are formed

thermody-by ultrasound-induced pyrolysis of water molecules (Misik et al., 1995) Thecollapsing microbubbles, filled with dissolved gas (i.e., argon) and solventvapor, are the reaction microchambers in which solvent vapor can be pyro-lyzed, thus producing radicals that undergo further chemical reactions.The physical properties of the supercritical fluid differ from those of thebulk liquid One of the most notable changes is the lower dielectric constant

of polar solvents such as water which allows the accumulation of polarity solutes at this interface This explains the crucial role of the hydro-phobicity of solutes during reactions in the solution Thermolysis as well asradical abstraction reactions occur in this region A temperature of approx-imately 800 K was determined for the interfacial region surrounding the

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Although OH radicals are produced by sonolysis of water, the ing spin adducts could not be detected using PBN due to the very short half-life of the PBN/OH adduct in aqueous solutions at neutral and slightly acidicpHs According to the Rice–Herzfeld mechanism, the primary pyrolysis step

correspond-is cleavage of the weakest bonds in the molecule, such as C–N (~85 kcal/mol) or C–C bonds (~80 kcal/mol)

In an isotope study by Misik et al (1995), H2O, D2O, or a 1:1 mixture ofboth (1.7 mL) containing the spin trap was added to a Pyrex test tube, whichwas fixed in the center of a sonication bath with a frequency of 50 kHz Thetemperature of the coupling water was 25°C The sample was sealed with arubber septum and bubbled with argon through a Teflon tube attached to afine needle (argon flow rate was 50 mL/min) for 5 min before and duringsonication The time of sonolysis was kept at minimum (typically 45 s) tominimize decay of the spin adduct during sonolysis After sonication, theelectron paramagnetic resonance (EPR) spectrum of the sample was mea-sured After each experiment, the pHs in the samples were measured andfound to be within a range of 6.7 ± 0.3 in all experiments Immediately aftersonication, the samples were transferred to EPR quarts cells, and acquisition

of the spectrum typically started within 1 min after sonication A Varian E9X-band spectrometer with a 100-kHz modulation frequency and a micro-wave power of 20 mW was used to record the spectra

The temperature dependence of the kinetic deuterium isotope effect forthe homolytic cleavage of the O–H and O–D bonds of the water moleculewas used to estimate the temperature of the region in which this processoccurs The temperature dependence of the kinetic isotope effect has beenused previously to study the temperatures of different sonochemical regions

in organic liquids.In the semiclassical treatment, quantum mechanical neling, which may contribute at lower temperatures, is not considered andthe ratio of k H/k D for O–H or O–D bond homolytic cleavage is determinedinternally by the zero-point energy difference of the initial states, and thedifference of the zero-point energies of the transition states is neglected Thezero-point energy difference of the ground states (1.24 kcal/mol) was deter-mined from the infrared frequencies of H2O and D2O vapor:

tun-k H /k D= exp{1.24 kcal mol–1/RT} (11.1)The value of kH/k D calculated from Equation (11.1) is 8.09, 1.87, and 1.23 at

298 K, 1000 K, and 3000 K, respectively The intramolecular isotope effect(the k H/k D ratio from HOD) is equal to the intermolecular isotope effect

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within the semiclassical approximation The equations below show thereactions of radical formation by O–H and O–D bond cleavage and H and

D trapping in 1:1 molar mixtures of H2O and D2O exposed to ultrasound:

et al (1999) to develop a reaction kinetic model of water sonolysis

11.2.2 Chemical Processes

Although it is generally agreed that the origin of sonochemical effects lies

in the bubble collapses and subsequent radical formation, the actual waythese collapses achieve a sonochemical effect has been explained by twodifferent main theories One concept is the electrical theory, which assumesthat the extreme conditions associated with collapse are due to an intenseelectrical field where the collapse is fragmentative On the other hand, thethermal theory or “hot-spot” theory considers that the collapses are quiteadiabatic Here, the resulting internal pressures and temperatures are so highthat vapor molecules dissociate, giving rise to free radical which, whenreleased in the liquid, can react with other species (Sochard et al., 1998) Water vapor is pyrolyzed to OH radicals and hydrogen atoms, and gas-phase pyrolysis and/or combustion reactions of volatile substances dis-solved in water occur As a result, interfacial regions exist between thecavitation bubbles and the bulk solution Since the temperature in theseregions is lower than in the bubbles, a temperature gradient is present inthis region Locally condensed •OH radicals in this region have beenreported Bulk solutions at ambient temperature might undergo reactions of

OH radicals or hydrogen atoms that survive migration from the interface

At the same time, the role of supercritical water during cavitation may play

an important role in this region

Supercritical water is a phase of water that exists above its critical ature and pressure (647 K and 221 atm, respectively) This unique state ofwater has different density, viscosity, and ionic strength properties thanwater under ambient conditions Because the solubility of organic

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Sonolysis 429

contaminants increases significantly in supercritical water, these organic

spe-cies are brought into close proximity with the oxidant, usually oxygen from

dissolved air Oxidation rate is therefore several magnitudes higher than wet

air oxidation During sonolysis, it has been proposed that supercritical water

is present in a small, thin shell around the bubble This mode of destruction

is expected to be secondary in importance because the fraction of water in

the supercritical state is estimated to be on the order of 0.0015 parts out of

100 parts of water Alternatively, the volume of the gaseous bubble is

esti-mated to be 2 ¥ 104 times greater than the volume of the thin supercritical

water shell surrounding the bubble The value of supercritical water may be

limited to increasing the solubility of the organic contaminant near the

bub-ble interface for radical attack The possibub-ble occurrence of supercritical water

oxidation in the sonochemical reactor, however, may be one reason to justify

fast degradation of organic compounds without O2

Pyrolysis is defined as the thermal destruction of a compound in the

absence of oxygen The high temperatures attained within the bubbles are

well above the temperatures required to destroy organic materials This

mechanism, however, requires the compound to be present in the vapor

phase within the bubble Compounds with higher vapor pressures will have

a higher vapor concentration inside the bubble It is expected then that

pyrolysis will be more prevalent as the vapor pressure of the contaminant

increases During collapse of the bubble, organic species present within the

bubble interior would clearly degrade, but because bubble implosion occurs

due to the influx of a jet stream of the surrounding liquid it may not be

necessary for the organic contaminants to be initially present inside the

bubble for degradation to occur This implosion scenario is analogous to the

injection of contaminated liquid directly into the hot reaction zone Several

parameters such as frequency applied have been found to influence the

cavitation process Following are the most important parameters that

• External temperature and pressure

Strong oxidation as well as reduction reactions have been observed due to

generation of H and OH radicals The main primary chemical process in the

sonolysis of water is the thermal dissociation of water to hydrogen atoms

and hydroxyl radicals:

H O2 ÆH•+•OH

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In the sonolysis of pure water under argon, the formation rates of hydrogen

radical and hydrogen peroxide are estimated to be 10.7 and 10.0 mM min–1,

respectively In an oxygen or air atmosphere, hydrogen radicals will react

with oxygen as follows:

(11.8)

11.2.2.1 H 2 –O 2 Combustion in Cavitation Bubbles

Hart and Henglein (1986) discovered that typical flame reactions occur when

ultrasonic waves at intensities sufficient to produce cavitation are

propa-gated through water containing a gas or a mixture of gases These reactions

are brought about by temperatures of several 1000 K that exist in the

com-pression phase of oscillating or collapsing gas bubbles The yields of such

gas-phase reactions in many cases are substantially higher than the yields

of reactions occurring in the liquid phase

In early studies on the formation of hydrogen peroxide by ultrasound in

water under various mixtures of oxygen and hydrogen, it has been found

that the yield depends on the composition of the mixture in the complex

manner The intermediates during the formation of hydrogen peroxide are

free radicals and free atoms, and the question arises whether the radicals

can escape from the cavitation bubbles into the bulk solution

The gas consumption determinations were carried out during sonolysis

by direct addition of the H2O2 mixture from a syringe to the gas phase to

keep gas pressure constant The rate of H2O2 formation as a function of the

composition of the gas atmosphere under which the water has been

insonated can be observed Under pure oxygen, H2O2 was formed at a rate

of 17 mM/min No H2O2 was formed upon insonation under pure

hydro-gen The rate of gas consumption is much higher than that of H2O2

forma-tion Therefore, the rate of gas consumption is a function of the composition

of the gas atmosphere

The H2O2 combustion into flames is a branched chain reaction, •H and •O

atoms and •OH and •HO2 radicals are the intermediates, and generally the

chains are very long The combustion in the cavitation bubbles occurs via

short chains, at a maximum rate of 220 mM/min Ozone in oxygen bubbles

decomposes at a rate of about 1 mM/min, and nitrous oxide in argon bubbles

decomposes at a rate of about 300 mM/min A rough estimate of the chain

length can be obtained using the ratio of the yields of gas consumption and

hydrogen peroxide formation H2O2 is formed according to the following

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Sonolysis 431

(11.10) (11.11)

It must also be taken into consideration that part of the H2O2 that reachesthe bulk solution is decomposed, as •OH and HO• radicals may escape the2

hot spots and react with H2O2 in the bulk solution according to the known mechanism:

(11.12) (11.13)

In addition to these destructive reactions, the radicals may also form H2O2

molecules in the bulk solution The conditions under which the H2–O2 bustion occurs in the cavitation bubbles are quite different from those exist-ing in flames The yields of H2O2 and HO2 ∑ first increase with increasing H2

com-concentration as more •H atoms are formed and fewer OH radicals areavailable that could destroy HO2 ∑ radicals via Equation (11.14):

(11.14)This may be due to the fact that the temperature in the compressed cavitybubbles is lower in H2-saturated water than in O2, H2O2, or air due to thehigh thermal conductivity of hydrogen From the solubilities of hydrogenand oxygen, the concentrations of these two gases in the liquid are calculated

to be in the molar ratio of 2:1 at an 80:20 composition of the gas atmosphere.The tiny cavitation bubbles are not in thermodynamic equilibrium with thesurrounding solution; that is, the gaseous composition does not correspond

to that of the gas atmosphere above the insonated liquid The second imum of the yield occurs when the cavitation bubbles contain the H2–O2

max-mixture in a 2:1 ratio In the presence of Fe2+ or Cu2+ as scavengers for •OHand HO2 ∑, the H2O2 yield of 16 mM/min is practically the same as in theabsence of these solutes Although appreciable amounts of •OH and HO2 ∑

radicals are found in the solution, there seems to be no change in the H2O2

yield It thus seems that as much H2O2 is destroyed by the radicals in thebulk solution as is formed there

In the second maximum of the yields, the H2O2 yield in the absence ofradical scavengers is even greater than in the presence of the scavengers Inthis solution, no destruction of H2O2 occurs, and this result is understood interms of the absence of •OH radicals Roughly 50% of the H2O2 is formed inthe hot spots; the other 50% of H2O2 is formed by HO2 ∑ radicals in the bulksolution

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11.3 Degradation of Organic Pollutants in Aqueous Solutions

Sonolysis of organic pollutants in water involves research on how and whatdifferent factors influence the efficiency of sonolysis For example, it hasbeen proven that the frequency applied is a very important factor influ-encing the degradation rate: the higher the frequency applied, the higherthe resulting removal efficiency Moreover, current research is concentrated

on optimizing the effects of sonolysis These optimization techniques can

be useful for compliance with environmental laws, pollution prevention,and remediation of aqueous wastes The degradation of different organiccompounds by ultrasound and the combination of sonolysis and otheradvanced oxidation processes such as combining ozonolysis and sonolysisare also very effective

In the treatment of hazardous wastewater, ultrasonic radiation candecompose water vapor molecules in the bubbles into free radicals such

as hydroxyl (OH), hydrogen (H), and hydroperoxyl (HO2) Evidence forthe formation of free radicals by ultrasound in aqueous solution hasrecently been demonstrated The hydroxyl radical is particularly reactivewith carbon–chlorine and carbon–carbon double bonds and is capable ofcleaving the aromatic ring The primary mechanism is hydroxyl radicaloxidation The severe conditions are enough to break down water vaporwithin the bubble into hydrogen and hydroxyl radicals, but the highlyreactive nature of these radicals prevents a long travel path-length into thesolution; therefore, only organic molecules present within the bubble orvery near the bubble surface will be destroyed in this fashion The simul-taneous production of the hydrogen radical indicates that reductive path-ways may also be available for the destruction of organic pollutants

Hydrogen peroxide will also be produced by radical combination of twohydroxyl radicals, even though the amount may be too small to be signifi-cant The addition of hydrogen peroxide can increase free-radical concentra-tion in the solution Local ultrasound intensities can be affected by severalfactors, such as water level in the ultrasonic tank, position of reaction vessel

in the tank, shape of reaction vessel, and solvent level in the reaction vessel.This variation in ultrasound intensity can lead to differences in the progress

of the chemical reaction Hydrogen peroxide and hydrogen gas are nowconsidered to be the principal products formed when the intensity of ultra-sonic waves is strong enough to create cavitation propagated through water.The liquid must contain a monoatomic gas such as argon or a diatomic gassuch as oxygen for cavitation to occur Hydrogen atoms, oxygen atoms,hydroxyl radicals, and perhydroxyl radicals are believed to be the interme-diates in the production of hydrogen peroxide The products obtained whenwater is sonicated have been found to be dependent on the acoustical power,the insonation cell, temperature, external pressure, and dissolved gaspresent

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Sonolysis 433

11.3.1 Phenol

Petrier et al (1994) compared sonochemical degradation of phenol at 20 and

487 kHz The intermediates of the ultrasonic degradation were identified by

comparing their retention times through high-performance liquid

chroma-tography (HPLC) and by electron spin resonance (ESR) Hydrogen peroxide

concentration has been determined iodometrically The H2O2 determination

was not affected by other products that might have formed during the

sonochemical reaction Reactions were performed at 25°C in a cylindrical

jacketed glass cell, and the temperature was monitored with a temperature

probe immersed in the reacting medium A 487-kHz wave was emitted, and

the system was driven by a high-frequency power supply The 20-kHz

irra-diations were carried out with commercial equipment from Branson The

ultrasonic power dissipated into the reactors was estimated by the

calori-metric method The same ultrasonic power (30 W) was delivered at each run

Exposure of 200 mL of phenol solution to ultrasound at 20 or 487 kHz

resulted in a higher rate of loss for the higher frequency In two cases,

hydroquinone (HQ), catechol (CC), and benzoquinone (BQ) were detected

as primary intermediates of the degradation process When the reactor was

closed, analysis of the atmosphere showed CO2 as the only final gaseous

product At 20 kHz, 2% of the theoretical carbon amount was recovered in

the gaseous phase after 300 min of treatment; 15% was found for the same

irradiation time at 487 kHz Initial rates of degradation have been determined

when the proportion of degraded phenol does not surpass 40% (k20kHz = 1.12

¥ 10–6 M/min) The rate is dependent on the initial concentration, which

reaches a value limit of 1.84 ¥ 10–6M/minat 20 kHz and 11.6 ¥ 10–6M/min

at 487 kHz Nevertheless, whatever the theoretical model describing the

origins of the molecular activation (thermal and/or electrical), the place

where the molecules are brought to an exited state and dissociate is the

interior of the bubble of cavitation, which is filled with gas and vapor In

the case of water saturated with air, the first step appears to be cleavage of

water and the dioxygen molecule:

Inside the bubble or in the liquid shell surrounding the cavity, these radicals

can combine in various ways or react with the gases and vapor present,

leading to the detection of H2O2:

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•OOH + •OOH Æ H2O2 + O2 (11.20)

The main fraction of the H2O2 formed during water sonolysis seems to

come from the OH and OOH radicals, which combine in the bubble or in

the region surrounding the bubble of cavitation in the absence of substrate

The amount of H2O2 produced at each of the two frequencies was

deter-mined The concentration of hydrogen peroxide increases linearly vs time

and the rate of formation was found higher at 487 kHz (4.9 ¥ 10–6 M/min)

than at 20 kHz (0.75 ¥ 10–6 M/min) The hydrogen peroxide yield decreases

when the phenol concentration increases, assuming that the first step of

the phenol degradation results from •OH radical reaction at a site close

to the surface of the bubble Sonochemical phenol degradation, which

proceeds more rapidly at high frequency than at low frequencies, can be

related to improved generating rate of •OH in the solution at higher

frequency The more important cavitational effects occur when the

fre-quency of the ultrasonic wave is equal to the resonance frefre-quency of the

bubble At 487 kHz, when cavitation is located at the gas–liquid interface

no formation of particles results from erosion of the emitting surface To

bring water into cavitation requires more energy at 487 kHz than at 20

kHz, and the threshold of cavitation is lower for water saturated with air

than degassed water

As the ultrasonic frequency continued to increase, phenol degradation was

most effective at 600 kHz, while little effect was observed at 19.5 kHz (Petrier

et al., 1994) The pH in the aqueous solution fell during the ultrasonic

irra-diation due to the formation of nitrous and nitric acids from nitrogen in the

air and dissolved air The increase of the degradation rate constant (k) was

observed at initial pH values in the vicinity of neutrality The ultrasonic

degradation rate of phenol was enhanced by the presence of Fe2+ On the

other hand, the degradation rates were little affected if Ag+, Cu2+, and Ni2+

coexisted The coexistence of ferrous ions seemed to have an effect similar

to that of Fenton’s reagent, which is to act as an effective oxidant of a wide

variety of organic substances because of the formation of hydrogen peroxide

during insonation

Enterazi et al (2003) described the degradation of phenol in a cylindrical

ultrasonic apparatus operating at 35 kHz The reaction rates were compared

to those obtained from devices operating at 20 and 500 kHz The rate of

phenol destruction was higher at 500 kHz than at 35 or 20 kHz; however,

the addition of hydrogen peroxide and copper sulfate proceeded more

effi-ciently at 35 kHz, with 30% faster reaction times compared to operation at

500 kHz The intermediate organic compounds degraded much faster at 35

kHz in the presence of hydrogen peroxide and copper sulfate than at 500

kHz

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Sonolysis 435

11.3.2 Monochlorophenols

Serpone at al (1994) studied three chorophenols under pulse sonolytic ditions (frequency, 20 kHz; power, 50 W) in air-equilibrated aqueous media.These phenols were totally transformed to dechlorinated, hydroxylatedintermediate products via first-order kinetics in about 10 hr for 2-CPOH and3-CPOH and about 15 hr for 4-CPOH; rate constants for the disappearance

con-of these phenols were (4.8 ± 0.4) ¥ 10–3/min, (4.8 ± 0.5) ¥ 10–3/min, and (3.3

± 0.2) ¥ 10–3/min, respectively, for approximately 80 mM initial concentration.The kinetics show two regimes: a low-concentration regime that is zeroorder in [CPOH]I and a second regime at higher concentrations where therate displays saturation-type kinetics reminiscent of Langmuir-type behavior

in solid/gas systems It suggests that the reaction takes place in the solutionbulk at low concentrations of chlorophenol, while at the higher concentra-tions the reaction occurs predominantly at the gas bubble–liquid interface.Chlorophenols are decomposed and dechlorinated almost quantitatively toform hydroxylated aromatic intermediate products; subsequently, specieswith fewer carbon atoms remained undetectable under these conditions Ultrasonic irradiation of aqueous solutions of the chlorophenols was car-ried out with a Vibra Cell Model VC-250 direct immersion ultrasonic horn(Sonics & Materials; Newtown, CT) operated at a frequency of 20 kHz with

a constant power output of ~50 W (the actual insonation power at thesolution was 49.5 W, and the power density was 52.1 W/cm2) Reactionswere done in a glass sonication cell (4.4 cm i.d by 10 cm), similar to the onedescribed by Suslick (1988) The temporal course of the sonochemical pro-cesses was monitored by HPLC

Ultrasonic irradiation (~50 W/cm2) of a 100-mL air-equilibrated aqueoussolution of 4-chlorophenol resulted in the first-order disappearance of thephenol, accompanied after a 1-hr delay by the first-order growth of Cl– The

pH of the isonated solution dropped gradually from the initial value of 5.1

to 3.5 after 11 hr Sonolysis of the aqueous solution of 3-chlorophenol showed

an induction period of ~90 min following which its concentration decreasedvia first-order kinetics The pH of the insonated solution of 2-chlorophenoldecreased at first to 4.9 and then recovered to its near initial value until 9

hr of insonation, when it dropped abruptly to pH 4.4 and remained constant.The initial drop in pH that occurred during the induction period was alsoobserved for the first-order disappearance of the phenol It therefore suggeststhat there are various possible sites where reactions may occur in sonochem-istry

Combined EPR and spin-trapping studies showed that solvent vapor andambient gases (e.g., air) decompose to atoms or free radicals in the gaseousbubble interior Water vapor is thermally dissociated into •OH radicals, H•

atoms, and •O atoms The latter interconvert with •OH radicals at the highpressures in the cavity and recombine in the cooler interfacial region to form

O2 and H2O2 The power dependence in the sonolytic transformation of aphenolic aqueous solution was found to be the first order

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Monochlorophenols represent an important class of environmental waterpollutants of moderate toxicity to mammalian and aquatic life; they possessrelatively strong organoleptic effects, with taste thresholds of 0.1 mg/L(ppb).Their principal sources are the natural degradation of chlorinated herbicides(e.g., chlorophenoxyacetic acids), chlorination of phenolic substances inwaste effluents, and chlorine treatment of drinking water Of the three chlo-rophenols examined here, 3-chlorophenol exhibited the greatest resistance

to sonolysis

Except for the cavity interior, the possible location for oxidation of thesesubstrates by direct or sensitized photolysis, flash photolysis, ultraviolet/peroxide, or irradiated semiconductor (SC) particulates suggests that thesonochemical oxidation process physically mirrors the heterogeneous pho-tocatalytic process such as UV/TiO2

11.3.3 2-Chlorophenol

Ku et al (1997) detected no formation of H2O2 during the sonication process

in pure water and suggested that the decomposition of 2-chlorophenol due

to oxidation of H2O2 is negligible Many researchers have found that thedecomposition rate by sonolysis decreased with increasing solution pH val-ues The decomposition of 2-chlorophenol may take place in the gaseous(bubbles), film, and bulk solution regions by either pyrolysis or free radicalattack Because the temperature in the gaseous region can increase up to

5000 K, pyrolysis of the organics most likely happens in the gaseous regionrather than in the film region (about 1000 K) Free-radical attack of 2-chlo-rophenol is assumed to be minimal in the bulk solution region because thefree radicals generated in the cavitation bubbles are barely transferred to theroom-temperature bulk solution region through the much higher tempera-ture film region For neutral and acidic solutions, molecular 2-chlorophenolspecies in the bulk solution region diffuse into the film region, and part ofthe molecular species may even evaporate into the gaseous (bubbles) regionfrom the gas–liquid interface

Depending upon the speciation of an organic pollutant at various pHs,degradation may take place at different regions of the microbubbles This isbecause the solubility or vapor pressure varies greatly according to theneutral molecule or ionized species At neutral and acidic solutions (pH <8), the molecular species predominates and the ionic species is predominant

in the alkaline solution for pH greater than 9 These two 2-chlorophenolspecies may possess different reaction behaviors during the sonication pro-cess For neutral and acidic solutions, molecular 2-chlorophenol species inthe bulk solution region diffuse into the film region, and part of the molecularspecies may even evaporate into the gaseous (bubble) region from thegas–liquid interface Thus, the overall decomposition of 2-chlorophenol can

be attributed to the pyrolysis, and free-radical attack occurs in both thegaseous and film regions On the other hand, the ionic species of 2-chlo-

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concentration suggest that the complete mineralization may occur via theformation of organic intermediates.

The presence of dissolved oxygen in aqueous solution was reported toplay a very important role in the generation of highly oxidative hydroxylfree radicals; therefore, the free-radical attack in both gaseous and filmregions is strongly influenced by the dissolved gas presented in aqueoussolutions Under uncontrolled conditions, the temperature of solution might

FIGURE 11.1

Reaction regions in the microbubble at various pHs (From Ku, Y et al., Water Res., 31(4), 929–935,

1997 With permission.)

Bulk Solution Region

Film Region

Pyrolysis Pyrolysis Gaseous

Region

Gaseous Region

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be increased during sonication because of the transformation of pressure

into heat Many researchers have mentioned very controversial results

regarding the effect of solution temperature on the decomposition of organics

by sonication

11.3.4 Chlorinated C 1 and C 2 Volatile Organic Compounds

Bhatnagar and Cheung (1991) investigated sonochemical destruction of

chlo-rinated C1 and C2 volatile organic compounds in dilute aqueous solution

Methylene chloride (CH2Cl2), chloroform (CHCl3), carbon tetrachloride

(CCl4), 1-2-dichloroethane (C2H4Cl2), trichloroethylene (C2HCl4), and

per-chloroethylene (C2Cl4) were exposed to 20-kHz ultrasound in a batch reactor

to determine the efficacy of ultrasonic process in the destruction of

undesir-able compounds in water Aqueous mixtures of two or more volatile

compounds were also sonicated to investigate the potential of sonication in

more realistic situations and to study the kinetics of individual halomethanes

in the presence of other halomethanes The initial concentration of VOCs

ranged from 350 to 50 mg/L in water and decreased with time as a result

of sonication

When dilute aqueous solution of C1 compounds and C2 compounds were

exposed to ultrasound under ambient conditions, the concentration

decreased exponentially vs sonication time according to first-order

kinet-ics pH control must be adjusted for optimizing the rate of sonochemical

destruction for chlorinated compounds The values of the first-order rate

constants for the compounds were essentially unchanged by the presence

of other reacting species The rate constant for CH2Cl2 in the mixture was

0.0345 and 0.0458/min for CCl4 Sufficient cavitation bubbles and free

radicals were produced to oxidize all of the volatile organic components

of the mixture Conventionally, the reaction rate changing with

tempera-ture can be described by the Arrhenius equation The reaction could take

place inside the cavitation bubble, where the temperature and pressure

are so high that the solute molecule breaks down, or in the bulk liquid

phase, where the free radicals generated as a result of high-intensity

ultrasonic waves oxidize the target molecule In the absence of any solute,

these radicals lead to the generation of H2O2 If the reaction takes place

in the cavitation bubbles and all the experimental conditions such as

reaction vessel size, temperature, power supplied, and pressure remain

unchanged, the destruction of the compound can be expressed by the

first-order rate equation:

-d[CxHyClz ]/dt = k[C xHyClz] (11.21)where CxHyClz is the target compound A fit to the experimental data for all

the C1 compounds indicates first-order kinetics The rate constants are listed

in Table 11.2

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Sonolysis 439

It seems that the cavities enclose a vapor of the solute because of the highvapor pressure of these compounds The primary reaction pathway for thesecompounds appears to be the thermal dissociation in the cavities The acti-vation energy required to cleave the bond is provided by the high temper-ature and pressure in the cavitation bubbles This leads to the generation ofradicals such as hydroxyl radical, peroxide radical, and hydrogen radical.These radicals then diffuse to the bulk liquid phase, where they initiatesecondary oxidation reactions The solute molecule then breaks down as aresult of free-radical attack The oxidation of target molecules by free radicals

in the bulk liquid phase under normal operating pressures and temperaturescan be presented by a second-order rate equation:

-d[C]dt = k1[C][OH] + k2[C][H] + k3[C][H2O2] (11.22)

where C is the concentration of the target compound

The concentration of free radicals in the equation is a function of the powerinput by sonication C2 compounds probably disintegrate by both the pyrol-ysis type of reaction in the cavitation bubble and free-radical attack in theliquid phase Physical operating conditions such as steady-state temperatureand initial pH of the solution were found to have little effect upon thedestruction rate of the compound The simplicity and flexibility along withthe high efficiency of destruction indicate the potential of a sonochemical-based process to become a competitive technology for water treatment

11.3.5 Pentachlorophenate

The ultrasonic wave effect on the degradation of pentachlorophenate (PCP)

at 530 kHz was studied by Petrier et al (1992) PCP was chosen as a model

TABLE 11.2

First-Order Rate Constants for Sonochemical Destruction of VOCs

Compound

k (min –1 )

Estimated Error (min –1 ) Norm/N data

Pvap (20°C) (Torr)

Source: Bhatnagar, A., and Cheung H.M., Environ Sci Technol., 26, 1481, 1991 With permission.

Trang 18

because it belongs to the class of aromatic halides that have to be removedfrom wastewater It is still a widely used compound for wood treatment andexhibits high toxicity The system is operated at 530 kHz with a 20-W high-frequency electrical power source Ultrasonic intensity determined by a cal-orimetric method was 1.06 W/cm2 PCP (0.5 M) was prepared as stock

concentrated solution from equal molar pentachlorophenol and NaOH.Experiments were performed at pH = 7 in 10–3-M phosphate buffer medium

with 10–4 M PCP solutions The irradiated volume (100 mL) was continuously

bubbled with gas through a gas dispersion fritted disk at a 25-mL/min flowrate

When PCP solution (10–4 M) under continuous air bubbling is subjected

to ultrasound effects, the characteristic absorption bands decrease and thetreatment leads to a complex mixture of products Carbon–chlorine bondsare rapidly cleaved, and after a 150-min sonication time, 90% of the chlorine

is recovered in the solution as chloride ions PCP transformation in aeratedsolution occurs together with nitrite and nitrate formation Carbon dioxide

is a product of PCP degradation, and it has long been recognized as aninhibitor for sonochemical reactions

Sonochemical reactions are strongly affected by ambient gas because thetemperature inside the collapsing bubble is in close relationship with thepolytropic ratio (Cp/Cv) and the thermal conductivity of the gas In addition,reactions with gases such as O2, N2, and CO2 are directly affected by the hightemperatures reached during the collapse of the bubbles As a consequence,reactive species available for PCP degradation and their rate of productionwill depend on the nature of the gas

Under argon bubbling, the degradation is faster than under air or oxygen;

no PCP is detected after 50-min sonication From these experiments, it waspostulated that CO2 is transformed to CO as observed in high-temperaturechemistry because of the high temperature inside the cavitation bubble

11.3.6 para-Nitrophenol

Kotronarou et al (1991) detected temperatures on the order of 2000 K at the

gas/liquid interfacial region Sonochemical reactions are characterized bythe simultaneous occurrence of pyrolysis and radical reactions, especially athigh solute concentrations Any volatile solute will participate in the formerreactions because of its presence inside the bubbles during the oscillations

or collapse of the cavities In the solvent layer surrounding the hot bubble,both combustion and free radical reactions are possible

The ultrasonic irradiation of aqueous solution of PNP was carried out with

a Branson 200 sonifier that was operated at 20 KHz and with an output ofelectrical power of 84 W Reactions were performed in a stainless-steel,continuous-flow reaction cell operated in the batch mode All reactions were

carried out with air-saturated solutions, and the concentration of quinone (p-BQ) was determined spectrophotometrically Hydrogen peroxide

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