Ultrasonics Sonochemistry 21 (2014) 1108–1116

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Ultrasonics Sonochemistry

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Ultrasound-based treatment approaches for intrinsic viscosity reductionof polyvinyl pyrrolidone (PVP)

1350-4177/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.12.013

⇑ Corresponding authors. Tel.: +91 20 26058587; fax: +91 20 26059843 (A.V.Mohod). Tel.: +91 22 33612024; fax: +91 22 33611020 (P.R. Gogate).

E-mail addresses: ashishmohod2004@gmail.com (A.V. Mohod), pr.gogate@ictmumbai.edu.in (P.R. Gogate).

Indrajeet A. Pawar a, Prathmesh J. Joshi a, Akshay D. Kadam a, Nishant B. Pande a, Priyanka H. Kamble a,Shruti P. Hinge a, Barnali S. Banerjee a, Ashish V. Mohod a,⇑, Parag R. Gogate b,⇑a Department of Chemical Engineering, AISSMS College of Engineering, Kennedy Road, Pune 411001, Indiab Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400019, India

a r t i c l e i n f o

Article history:Received 17 July 2013Received in revised form 2 December 2013Accepted 14 December 2013Available online 21 December 2013

Keywords:Polymer degradationPVPIntrinsic viscosityUltrasoundUltraviolet irradiationTitanium dioxide

a b s t r a c t

The present work deals with achieving viscosity reduction in polymer solutions using ultrasound-basedtreatment approaches. Use of simple additives such as salts, or surfactants and introduction of air at vary-ing flow rates as process intensifying parameters have been investigated for enhancing the degradation ofpolyvinyl pyrrolidone (PVP) using ultrasonic irradiation. Sonication is carried out using an ultrasonic hornat 36 kHz frequency at an optimized concentration (1%) of the polymer. The degradation behavior hasbeen characterized in terms of the change in the viscosity of the aqueous solution of PVP. The intrinsicviscosity of the polymer has been shown to decrease to a limiting value, which is dependent on the oper-ating conditions and use of different additives. Similar extent of viscosity reduction has been observedwith 1% NaCl or 0.1% TiO2 at optimized depth of horn and 27 �C, indicating the superiority of titaniumdioxide as an additive. The combination of ultrasound and ultraviolet (UV) irradiation results in asignificantly faster viscosity reduction as compared to the individual operations. A kinetic analysis forthe degradation of PVP has also been carried out. The work provides a detailed understanding of the roleof the operating parameters and additives in deciding the extent of reduction in the intrinsic viscosity ofPVP solutions.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Worldwide production of synthetic polymers is approximately140 million pounds annually. Subjecting polymers to degradation(mostly for achieving a reduction in viscosity) is a significantlyimportant processing step due to a wide variety of applicationsof polymers also requiring specific characteristics. Polymerdegradation techniques based on the use of energy (thermal orradiation), chemical (acid or alkali) and microorganisms or en-zymes are particularly important and applied generally for a rangeof polymers. Pyrolysis, photolysis, biological action and shear canbe effectively used for the degradation of different polymers andthese processes have been widely investigated for polymers suchas poly (ethylene oxide), polyethylene, polypropylene, etc. Pro-longed exposure of solutions of macromolecules to high energywaves (ultrasound) has been shown to produce a permanentreduction in viscosity [1–8]. Schmidt and Rommel [2] firstobserved the permanent reduction in the viscosity of polymer

solutions and attributed it to the breakage of covalent bonds inthe polymer chain.

In recent years, it has been observed that degradation ofpolymers is of primary importance due to widespread uses of thepolymer in many industrial applications. Synthetic polymers suchas poly(glycolic acid), poly(lactic acid) and their copolymers,poly(p-dioxanone), and copolymers of trimethylene carbonateand glycolide have been used in a number of clinical applications[9–12]. The major applications include resorbable sutures, drugdelivery systems and orthopaedic fixation devices such as pins,rods, and screws [13,14].

It has also been observed that many opportunities exist for theapplication of synthetic polymers in the biomedical area, particu-larly, in the field of tissue engineering and controlled drug delivery.In biomedical applications, the criteria for selection of biomaterialsare based on their functional groups, molecular weight, solubility,shape and structure, hydrophilicity/hydrophobicity, lubricity, sur-face energy, water absorption, and erosion mechanism. In tissueengineering, polymeric scaffolds play a unique role in tissue regen-eration and repair. Scaffolds can be fabricated from either syntheticor biological polymers, and may be degradable or non degradable,depending upon the intended use [15]. Biological scaffolds are de-rived from humans and animal tissues whereas synthetic scaffolds

I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116 1109

are derived from synthetic polymers. The degradation of scaffoldscan occur through mechanisms that involve physical, chemical orbiological processes.

Reduction in viscosity would also be required in mechanicalsynthesis of various block and graft copolymers, in the plasticiza-tion of rubber, and in the production of valuable low-molecularweight substances e.g. glucose from natural polymers. Thus, thestudy of degradation is essential in many ways and can be one ofthe practical methods for stabilizing the polymers [16].

Use of ultrasonic irradiation, or in general, cavitation phenome-non has also been looked upon as a promising technology not onlyfor physical processing, but also for chemical processing applica-tions [17,18]. Ultrasound can also be looked upon as an effectivemeans to introduce the desired changes in polymer solutions.Ultrasonic irradiation has a significant effect on polymers in termsof their mechanical, mechano-chemical and morphological proper-ties. Understanding the effect of different operating parameters onthe extent of degradation of polymers, thus, becomes very impor-tant. The polymer degradation can be quantified either by molecu-lar weight distribution or by change in the intrinsic viscosity [19].It is believed, that, ultrasonic degradation, unlike chemical or ther-mal decomposition, is a non-random process, with cleavage takingplace at the center of the polymer chain. It is important to notehere that there is a certain molecular weight of polymer belowwhich degradation does not take place. The polymer degradationin the solution using ultrasonication is due to the motion of thewall of violently collapsing bubble causing movements of solventmolecules around these bubbles. These movements set up largeshear fields due to which the polymer chain get shorter or con-tracted [19,20–26]. In general, as the concentration of polymer de-creases, the viscosity of the solution lowers and thus enhancementin degradation can be accomplished by reducing the correspondingviscosity. When high intensity acoustic energy waves travelthrough a medium, rapid and successive compression and rarefac-tion cycles occur. The decrease in the pressure in the negative pres-sure cycle of the ultrasonic shock wave spontaneously generatessmall cavities. Theses cavities collapse in the positive pressure cy-cle and produce highly turbulent flow conditions and extremelyhigh pressure and temperatures, a process that is better knownas cavitation. It is known that the cavitation in the liquid requiresthat the negative pressure in the rarefaction region of wave func-tion must overcome the natural cohesion forces acting within theliquid. It is difficult to use cavitation in highly viscous liquids,due to strong forces, as it requires waves with greater amplitudeand intensity that limits the collision activity of bubbles. Due toabove fact, dilute solutions of polymers have been generally usedfor the study of ultrasonic degradation [27].

A variety of different models have been proposed to explain theway in which the factors such as frequency, intensity, solvent, tem-perature, nature of dissolved gas, external pressure and molecularmass distribution influence the rate of degradation and finalmolecular mass of the degraded species [28–30]. Akyuz et al.[30] investigated the polymer degradation using ultrasound andcompared different theoretical and phenomenological models ofevolution of the molecular weight during sonication. Typically,these models share the property of an initial rapid drop in molec-ular weight followed by a slowing down of the rate of decrease inthe molecular weight. Different models are proposed to explain thedegradation phenomena based on cavitation induced byultrasound.

Polyvinyl pyrrolidone (PVP), which is also called as Polyvidoneor Povidone, is a water-soluble polymer. PVP is used as a binderin many pharmaceutical tablets and also as an adhesive in gluesticks and hot-melts. It is also used as a special additive for batter-ies, ceramics, fiberglass, inks and also acts as an emulsifier and dis-integrant for solution polymerization. PVP is also used in personal

care products, such as shampoos and toothpastes. In molecularbiology, PVP can be used as a blocking agent during Southern blotanalysis as a component of Denhardt’s buffer. It is also exception-ally good at absorbing polyphenols during DNA purification. Theapplication of PVP depends on the different properties such asmolecular weight, density, viscosity, solubility etc and hence it isimportant to study the polymer degradation in terms of achievingthe viscosity reduction as well as molecular weight and this stepcan be considered as an essential preprocessing requirement forapplications of PVP in any field. Due to large number of applica-tions, PVP is considered as a very important polymer and hencewas considered as the model polymer in the present work. Thereare not many studies in the literature on detailed investigation ofsonochemical degradation of PVP. The work of Akyuz et al. [30]dealt with a fundamental understanding into the degradation pro-cess and reported the comparison of different kinetic models fordegradation of PVP. Taghizadeh et al. [31] investigated the effectof initial molecular weight of PVP on the extent of degradationusing ultrasonic irradiations where the extent of degradation wasquantified using the method of viscometry and a kinetic modelwas developed to estimate the degradation rate constant. It hasbeen reported that the rate constant decreases as the initial con-centration increases. The objective of the present study is to estab-lish new experimental data for the ultrasonic degradation ofpolyvinyl pyrrolidone (PVP) in terms of the optimization of the dif-ferent operating parameters such as depth of horn and operatingtemperature at laboratory scale operation using ultrasonic hornreactor. After the completion of initial laboratory scale optimiza-tion studies, intensification studies using different additives suchas air, sodium chloride, titanium dioxide and surfactant have beencarried out. The work is also first of its kind to report the influenceof additives and combination of US and ultraviolet irradiations forthe degradation of PVP. The extent of degradation has been quan-tified in terms of the change in the intrinsic viscosity of the poly-mer solution, which is a simple method for monitoring the rateof degradation and used generally in monitoring the rate of poly-mer degradation [31–34].

2. Experimental work

2.1. Materials

The material used was polyvinyl Pyrrolidone (PVP), obtainedfrom S. D. Fine Chemicals, Mumbai, India. The molecular weightof polyvinyl pyrrolidone is approximately 125,000 g/mol with val-ues of the constants, k (related to the particular solvent–solutepair) as 1.30 � 10�2 l/kg and a (related to the shape of the solutemolecule) as 0.68, for 86.8% hydrolysis [35]. k and a are theMark-Houwink parameters that relate intrinsic viscosity (g) withthe molecular weight (M), specifically for polymer solution andwhose values are dependent on the particular polymer-solventsystem. The dilute solutions of different chemicals with requiredconcentrations were obtained using demineralized water (DMwater) for experimental studies. Titanium oxide (TiO2), sodiumsulphate (Na2SO4), sodium chloride (NaCl), sodium carbonate(Na2CO3), sodium lauryl sulphate (SLS) were obtained from MerckSpecialties Pvt. Ltd., Mumbai, India.

2.2. Sonochemical reactors

For the present experimental studies, ultrasonic horn (Fig. 1)obtained from M/s Dakshin, Mumbai operating at 36 kHz and arated power dissipation of 120 W has been used. The tip of thehorn is 2 cm in diameter. The actual power dissipated in thesystem was investigated using calorimetric measurements [36],

Fig. 1. Schematic representation of ultrasonic horn.

1110 I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116

and was observed to be equal to 29.3 W giving an energy efficiencyof 24.5%. Calorimetric measurements for each run also indicatedthat the ultrasonic power dissipated in the solution is same foreach of the viscosities and is not affected due to the use of differentadditives over the range of loadings considered in the work.

The solutions of PVP having concentrations were prepared byusing demineralized water and after initial optimization studieswith varying concentration, initial concentration of 1% (w/v) waskept constant for all the remaining experimental runs. The degra-dation was investigated at different operating temperatures andin the presence of different additives. During all the experimentalruns, any external bubbling was not introduced expect for the runsinvolving presence of air. Whenever any solvent or solution is irra-diated by ultrasonic energy, heat is generated and temperature ofthe system increases. In the present work, the desired temperatureof the reaction vessel (glass beaker) was maintained by using an icebath around the glass beaker. For experiments involving the ultra-violet irradiations, a UV lamp was used which was procured fromPhilips India Pvt. Ltd., Mumbai (Model PL-S). Three different lampswith a power rating of 5 W, 9 W and 11 W respectively, have beenused with an objective of investigating the dependency on thepower dissipation of UV tube.

2.3. Experimental methodology

In a typical run for optimization studies, the polymer solutionwas taken in a glass beaker with a maximum capacity of 500 ml.Periodically samples of sonicated solutions were removed for vis-cosity measurements which was carried out using a Ubbelohde vis-cometer (Model 9721-K56) procured from Cole-Parmer Pvt. Ltd.,Mumbai, India. Before the withdrawal of the samples, it was en-sured that the reaction mixture was well mixed. The location ofthe sampling point was near the bottom of the reactor, i.e. awayfrom the location of the ultrasonic horn and the same locationwas used in all experiments for the removal of the sample. Dueto distribution of cavitational activity in the reactor (maximumnear to the horn surface and reduced activity away from the sur-face of the ultrasonic horn), maximum extent of degradation is ob-tained very near to the ultrasonic horn and hence sampling point atthis location should be avoided.

Relative and specific viscosities (gr and gsp, respectively) werecalculated using Eq. (2):

gr ¼ ðt=t0Þ ð1Þ

gsp ¼ gr � 1 ð2Þ

where t and t0 are the efflux time for polymer solution and solvent,respectively.

The intrinsic viscosity [g] values can be related to the specificviscosity and relative viscosity by the Huggins and Kramer equa-tions [37]. The conditions used in this work (a = 0.55 andK = 6.67 � 10�5 l g/l) were adopted on the basis of previous find-ings reported in the literature [38].

Reproducibility of the efflux time measurements was within±0.3 s. Experiments have been also repeated twice to check thereproducibility of the obtained data for the variation of viscosityagainst time for all the sets. It has been observed that experimentalerrors were within ±2%. The g values for the PVP solution at differ-ent parameters were calculated by the one-point intrinsic viscosityequation [39].

g ¼ ½2ðgsp � ln grÞ�0:5=C ð3Þ

where C is concentration of PVP solution in g/l.The variation of either molecular weight or the intrinsic viscos-

ity in the presence of ultrasonic irradiation reflects the ultrasonicdegradation of polymer. The extent of ultrasonic degradation ofpolymer solution has been quantified by using a parameter:

u ¼ ð½g0� � ½g�Þð½g0� � ½g1�Þ

� 100 ð4Þ

where [g] and [g1] is actual intrinsic viscosity and limiting intrinsicviscosity, respectively. g0 is the initial intrinsic viscosity.

For calculating the molecular weight of PVP solution, Mark-Houwink equation has been used which describes the dependenceof the intrinsic viscosity of a polymer on its relative molecularmass (molecular weight) and can be given as follows:

½g� ¼ k �Ma ð5Þwhere [g] is the intrinsic viscosity, k and a are constants, values ofwhich depend on the type of polymer, solvent and the temperatureof viscosity determinations [40].

2.4. Kinetic analysis of polymer degradation process

Baramboim [41] suggested that the kinetics of polymer degra-dation under stress could be expressed as Eq. (6), which is appliedto describe the kinetics of ultrasonic degradation in this work:

dðMt�M1ÞM1

dt¼ k

Mt �M1

M1

� �ð6Þ

where M1 and Mt are the limiting molecular weight and averagemolecular weight at irradiation time t, respectively, and k is the rateconstant of degradation reaction.

By integrating and considering that at t = 0, Mt = M0 (where M0

is the initial molecular weight), Eq. (6) could be expressed as:

Mt ¼ ðM0 �M1Þe�kt þM1 ð7ÞIf the polymer degradation process is monitored in terms of the

intrinsic viscosity of the polymer solution, similar equation interms of intrinsic viscosity can be written as:

gt ¼ ðg0 � g1Þe�kt þ g1 ð8Þ

According to Eq. (8), the magnitude of the rate constant hasbeen calculated for all the runs knowing the initial viscosity andthe limiting intrinsic viscosity of the polymer solution by plottinga graph of ln (A) against time [42] where:

A ¼ ðg0 � g1Þ=ðgt � g1Þ ð9Þ

3. Results and discussion

3.1. Effect of reaction volume

As the reaction volume plays a key role in determining the ex-tent of degradation, the experiments were carried out at different

Fig. 3. Effect of initial concentration of PVP on degradation using ultrasound(sonication time 180 min; volume 200 ml; depth of horn 0.5 cm; 300 K).

I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116 1111

reaction volumes with same power dissipation of the ultrasonicreactor at 1% concentration of PVP. Fig. 2 shows the effect of reac-tion volume on the extent of degradation in terms of the change inintrinsic viscosity at a 1% concentration of PVP solution. The extentof degradation decreases with an increase in the reaction volumeat same supplied ultrasonic power dissipation. After 180 min ofirradiation time, the extent of degradation for 200 ml at 1% concen-tration of PVP solution is two-times higher than that obtained at300, 400 and 500 ml. As it can be perceived that increase in thereaction volume decreases the power dissipation per unit volume,resulting in a corresponding decrease in the cavitational activity.The decreased cavitational activity naturally reduces the extentof degradation of polymer. Sivakumar and Pandit [43] have re-ported similar trends for the effect of power density on the extentof degradation of Rhodamine B whereas Harkal et al. [44] have re-ported similar results for the degradation of polyvinyl alcohol. Ithas been also observed that the degradation rate constant for opti-mum volume, i.e. 200 ml, was 2.6 � 10�2 (min�1).

3.2. Effect of initial concentration of polymer

The effect of initial concentration of PVP on extent of degrada-tion has been studied at different initial concentrations including1%, 1.5%, and 2% and the obtained results have been given inFig. 3. It has been inferred that the extent of degradation was high-er at lower concentration of PVP and hence the lower initial con-centration of 1% has been selected for the further work onparameter optimization and investigating the effect of additives.Similar results in terms of lower extent of degradation at higherconcentrations have been reported earlier [45,46]. The degradationrate constant was also found to be higher (about 3 times) for 1%concentration as compared to the higher concentration of 2.0%.

The observed results can be attributed to the fact that concen-tration of polymer solution decides the viscosity of polymer solu-tion. With an increase in concentration, viscosity also increasesand at high viscosity of solution, the cavitational intensity is lower.Independent bubble dynamics studies have clearly indicated thatan increase in the viscosity results in decrease in the collapse pres-sure generated due to cavitation [47]. In highly viscous solution,the molecules present in the solution are less mobile. Thereforevelocity gradients around the collapsing bubble become smaller,which results in lower extents of degradation. Similar results forthe effect of initial concentration have been reported by Taghi-zadeh et al. [31] considering PVP of different initial molecularweights (160,000, 360,000 and 1,300,000 g/mol) at fixed tempera-ture. It was established that the degradation rate reduced with

Fig. 2. Effect of reaction volume on degradation of PVP using ultrasound (sonicationtime 180 min; 1% PVP; depth of horn 0.5 cm; 300 K).

increasing solution concentration or with an increase in the molec-ular weight of polymer.

3.3. Effect of sonication time on intrinsic viscosity (only sonication)

Fig. 4 shows the change in intrinsic viscosity of PVP solutionwith sonication time at 1% initial concentration of PVP for 200 mlreaction volume, depth of horn as 0.5 cm, and operating tempera-ture as 27 �C. Using only ultrasound, it has been observed that theviscosity of the polymer solution decreases with an increase in theultrasonic irradiation time. The study also shows that thedegradation of molecules continued only to a certain limitingmolecular weight or intrinsic viscosity. Below this intrinsic viscos-ity value, the polymer chain was too short and thus, cleavage at thecenter of the PVP chain did not take place any further. From Fig. 4,it has been observed that the limiting intrinsic viscosity of PVPsolution was 8.50 (l/g), which was achieved after 200 min oftreatment.

A number of different rate models have been proposed for thedegradation of polymers [2,8], but in this study a simple model de-scribed by Eq. (6) was employed. The obtained data was observedto be consistent with Eq. (8) by considering the order of reactionwith respect to molar concentration as 1. The plot of ln (A) versussonication time has been given in Fig. 5. The apparent degradationrate constant can be estimated from the slope of the plot using Eq.(9). Table 1 gives the value of the rate constant obtained from thekinetic studies and it can be seen that the observed initial kinetic

Fig. 4. Effect of sonication time on intrinsic viscosity on degradation of PVP usingultrasound (sonication time 180 min; 1% PVP; volume 200 ml; depth of horn0.5 cm; 300 K).

Fig. 5. Kinetic rate constant for degradation of PVP using ultrasound (sonicationtime 180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm;300 K).

1112 I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116

rate constant for PVP solution degradation using ultrasound alonewas 1.8 � 10�2 min�1.

3.4. Effect of depth of horn

The effect of depth of horn on the extent of degradation hasbeen investigated at a constant power dissipation of the ultrasonichorn. The experiments were conducted for varying depth of hornover the range of 0.5–2 cm for a constant concentration of 1%and volume as 200 ml. Fig. 6 shows the effect of depth of hornon the intrinsic viscosity at different depths of horn. It can be easilyseen from the figure that the rate of decrease in the intrinsic vis-cosity increases with an increase in the depth of horn. The final

Table 1Kinetic rate constant for different sets of PVP degradation.

Sr. no. Parameters Extent of d

1 Reaction volume 200 ml 83.892 Reaction volume 300 ml 68.623 Reaction volume 400 ml 77.254 Reaction volume 500 ml 64.405 Concen. of PVP at 1% 83.896 Concen. of PVP at 1.5% 72.907 Concen. of PVP at 2% 50.058 Air flow rate at15 cm/s2 88.409 Air flow rate at 25 cm/s2 92.9810 Air flow rate at 35 cm/s2 97.6411 NaCl at 1% concen. 83.8912 NaCl at 2% concen. 83.3113 NaCl at 4% concen. 80.4314 NaCl at 6% concen. 70.3815 NaSO4 at 1% concen. 82.2316 NaSO4 at 2% concen. 81.3417 NaSO4 at 4% concen. 76.3418 NaSO4 at 6% concen. 74.3419 Na2CO3 at 1% concen. 80.3420 Na2CO3 at 2% concen. 78.3621 Na2CO3 at 4% concen. 75.4522 Na2CO3 at 6% concen. 72.3423 0.5 cm depth of horn 83.89 (@ 224 1.0 cm depth of horn 83.89 (@ 225 2.0 cm depth of horn 83.89 (@ 126 SLS at 0.1% 79.9927 SLS at 1.0% 64.3028 TiO2 at 0.1% 98.44 (@ 829 TiO2 at 1.0% 95.20 (@ 130 UV only (5 W) 89.52 (@ 831 US + UV (5 W) 89.76 (@ 832 US + UV (9 W) 90.45 (@ 833 US + UV (11 W) 98.86 (@ 8

limiting intrinsic viscosity is marginally affected but the time forreaching the limiting viscosity decreases with an increase in thedepth of the horn. At a depth of 0.5 cm, the required time for reach-ing the limiting viscosity was around 175 min but the time reducedto only 120 min when the depth of horn in the reactor was in-creased to 2 cm. The results can be attributed to the changes inthe flow pattern of the liquid depending on the distance of horntip immersed in the polymer solution. The direct circulationcurrents in the liquid solution are due to the acoustic flow in thereactor when horn is mostly immersed in the liquid solutionwhereas the reverse flow i.e. reflection occurred from the bottomof the reactor also contribute significantly at enhanced depth ofhorn. The extent of mixing in the reactor is also dependent onthe immersion depth of the horn tip [48]. The results reported inthis study are consistent with the results reported for ultrasonicprecipitation of calcium carbonate, which is also controlled bythe physical effects of cavitation phenomena [48]. As it is observedthat 0.5 cm immersion depth of horn gives similar results in termsof reaching the final limiting viscosity, this preferred value of0.5 cm depth of horn based on the efficient operation of currentconfiguration of ultrasonic horn has been kept constant for allthe remaining experiments.

3.5. Effect of air sparging at different flow rate

Experiments were performed to study the introduction of air onviscosity reduction of PVP solution. The air was introduced withthe help of fish pond aerator at different flow-rates. The obtainedresults for effect of varying air flow rate as 15, 25 and 35 cm3/s un-der the conditions of normal temperature and pressure i.e. 1 atmand 25 �C on the extent of degradation of the PVP has beendepicted in Fig. 7. It can be seen from the figure that the rate of vis-cosity reduction is maximum at flow rate of air as 15 cm3/s. It can

egradation (%) after 180 min Kinetic rate constant (min�1)

2.6 � 10�2

1.8 � 10�2

2.4 � 10�2

1.6 � 10�2

2.6 � 10�2

2.2 � 10�2

1.4 � 10�2

3.0 � 10�2

7.1 � 10�2

7.3 � 10�2

2.6 � 10�2

2.9 � 10�2

2.4 � 10�2

1.9 � 10�2

2.8 � 10�2

2.7 � 10�2

2.3 � 10�2

2.2 � 10�2

2.9 � 10�2

2.6 � 10�2

2.3 � 10�2

1.9 � 10�2

00 min) 1.8 � 10-2

20 min) 1.9 � 10�2

80 min) 2.6 � 10�2

2.4 � 10�2

2.3 � 10�2

0 min) 7.3 � 10�2

00 min) 7.1 � 10�2

0 min) 6.9 � 10�2

0 min) 3.1 � 10�2

0 min) 3.2 � 10�2

0 min) 7.3 � 10�2

Fig. 6. Effect of depth of horn on degradation of PVP using ultrasound (sonicationtime 180 min; 1% PVP; volume 200 ml;300 K).

Fig. 7. Effect of air sparging at different flow rate on degradation of PVP usingultrasound (sonication time 180 min; 1% PVP; volume 200 ml; depth of horn0.5 cm; 300 K).

Fig. 8. Effect of different salts on degradation of PVP using ultrasound (sonicationtime 180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm; 300 K).

I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116 1113

be seen from figure that viscosity reduction rapidly falls at lowerflow rate of air and limiting viscosity is reached around 140 minof sonication time. It was also observed that the limiting viscositywas marginally lower at higher flow rate of air i.e.35 cm3/s. The en-hanced rate of decrease in viscosity in the presence of aeration (atlower flow rate) can be attributed to the fact that the presence ofair flow significantly increases the cavitational effect by supplyingmore number of nuclei for the process [49]. It has been observedthat at early time periods, the intrinsic viscosity decreases rapidlyin the presence of air. In absence of external aeration, the degassingeffect of ultrasound ensures that there is less amount of dissolvedgas in the liquid. This gives a lesser quantum of nuclei and hencelowers cavitational intensity resulting in slower rate of decreasein the intrinsic viscosity. At significantly higher flow rates of air,the degradation was observed to be slower attributed to the factthat too much cavitation leads to lower cavitational intensity dueto the cushioning effect.

3.6. Effect of different salts

The effect of presence of salt such as sodium sulphate, sodiumchloride and sodium carbonate used as catalyst on viscosity reduc-tion of PVP solution (1% initial loading) at constant ultrasonicpower has been investigated over varying concentration range of1–6% (by weight) of the salt over a fixed time of treatment as180 min for comparison purpose. Fig. 8 shows the effect of differentconcentration of salts on the extent of degradation of PVP solution

after treatment of 180 min. It has been observed that by addingthe salts (at optimum concentration of 1%), the extent of reductionin the intrinsic viscosity of PVP solution changes significantly. Thisis due to the fact that the presence of salt increases the presence ofpolymer molecules at the site of cavity collapse and also alters thesurface tension, vapour pressure and ionic strength of the aqueousphase [50]. All these factors help in a more violent collapse ofbubbles, resulting in enhanced degradation of polymer. However,the effect is only marginal at higher loadings of salts. It has beenobserved that with an increase in concentration of salts such assodium sulphate, sodium chloride, the extent of viscosity reductiondecreases marginally. The maximum extent of degradation (83.8%)was observed with 1% NaCl at 27 �C constant temperature.Previous studies [51] illustrated that the addition of salt increasesthe ionic strength of the aqueous phase which drives the organicpollutants towards the bubble�bulk interface and gives significanteffects for the hydrophobic compounds. For hydrophilic com-pounds, marginal enhancement due to the presence of NaCl canbe attributed to enhanced cavitational activity due to the changesin physicochemical properties leading to lower attenuation ofsound waves and higher cavitational intensity [52].

3.7. Effect of surfactant on extent of viscosity reduction

The effect of surfactant on the viscosity reduction at constantreaction volume of 200 ml of PVP was investigated for differentconcentrations of surfactant i.e. sodium lauryl sulfate, SLS (0.1%and 1%). The obtained results are illustrated in Fig. 9. It has beenobserved that, at same ultrasonic power supply, the polymer deg-radation is marginally affected at 0.1% concentration of sodiumlauryl sulfate (SLS). The extent of degradation is slightly lower inthe presence of SLS as compared to that obtained in the absenceof SLS, which is also confirmed by the observed value of kinetic rateconstant (Table 1). In addition to the quantification of the viscosityreduction, percentage degradation calculation is very importantdue to the variation in the initial viscosity of the polymer solutionby the presence of surfactant. The extent of degradation of PVP athigher concentration of SLS (1%) loading was about 65% whereasthe extent of degradation of PVP in the absence of SLS was 84%.The observed results can be attributed to the fact that the presenceof surfactant in higher concentration affects the growth of bubbleadversely. Crum [53] investigated the effect of surface active sol-utes on rectified diffusion growth of bubble during cavitation. Itwas established that surfactant molecules are adsorbed at bub-ble/solution interface restricting the growth of the bubbles. Leeet al. [54] also confirmed and observed that surfactant molecules

Fig. 9. Effect of SLS on degradation of PVP using ultrasound (sonication time180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm; 300 K).

Fig. 10. Effect of TiO2 on degradation of PVP using ultrasound (sonication time180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm; 300 K).

Fig. 11. Effect of US and UV irradiation on degradation of PVP using ultrasound(sonication time 180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm; 300 K,5 W of UV tube).

1114 I.A. Pawar et al. / Ultrasonics Sonochemistry 21 (2014) 1108–1116

are adsorbed at the bubble/solution interface affecting the rate ofgrowth of acoustic bubbles. Ashokkumar et al. [55] also reportedthat bubble growth by coalescence pathway is restricted leadingto a relatively slower growth of the bubbles by rectified diffusionpathway. At gas–liquid interface, the thermodynamic stability ofthe solution is critical, whereas adding SLS into the solution resultsin altering the stability of acoustic bubble and the process of intra-molecular energy transfer (macroscopic effects) as well as theintermolecular energy transfer (microscopic effects) during the ra-pid collapse of the bubbles [52,53,56].

3.8. Effect of titanium dioxide on extent of viscosity reduction

The presence of solid particles such as titanium dioxide pro-vides an additional phase in the system which can increase thecavitational activity by allowing more bubbles to form and growinto active cavitation bubbles. Hence, the number of cavitationevents occurring in the reactor is enhanced resulting in a subse-quent enhancement in the cavitational activity. It must be notedthat the presence of solids also have a negative impact on the cavi-tational activity as the solid particles result in scattering of thesound waves thereby decreasing the focused energy transferredinto the system. The net effect of these two phenomena will bedependent on the system in question and hence optimization isthe required before operating parameters are selected for the ac-tual operation [57–59].

The obtained results for effect of TiO2 on viscosity reduction ofPVP are depicted in Fig. 10. It can be seen that intrinsic viscosityreduction is higher and significantly faster at lower concentration(0.1%) as compared to the operation involving only ultrasound. Ithas been observed that the final limiting viscosity is also reducedin the presence of 0.1% loading of TiO2. The observed enhancementcan be attributed to the fact that solid particles provides additionalnuclei which promote cavitational activity [18]. Similar resultswere obtained by Guo et al. [60] for the degradation of 2,4-dinitro-phenol (at 20 mg/l) which showed higher degradation with cupricoxide than in the absence of CuO. Shirgaonkar and Pandit [61] re-ported that the extent of degradation of trichlorophenol was high-er (16.8%) at 0.1 g/l of TiO2 loading as compared to that in theabsence of TiO2 (10.2%).

Results with higher concentration of TiO2 (1%) revealed that therate of viscosity reduction was lower as compared to the use oflower TiO2 concentration (0.1%), although better as compared tothe sonication alone. The observed results can be attributed tothe fact that at higher loadings of solid particles in the system,the incident sound waves are scattered depending on the surface

characteristics and the projected area, resulting in a lower amountof energy dissipated into the system due to the ultrasonic irradia-tion. If the scattering of sound waves is dominant, as the case maybe in the use of TiO2 at higher concentrations, slower viscosityreduction is observed due to lower degree of cavitational effects[58,62].

3.9. Effect of combination of ultrasound (US) and ultravioletirradiations (UV)

The results for this combination approach have been presentedin Fig. 11. It can be seen that the extent of intrinsic viscosity reduc-tion is higher for the combination mode as compared to the indi-vidual operations. Both US and UV irradiations have somepotential to degrade PVP and UV has slightly better efficacy ascompared to the use of ultrasound. Use of UV irradiations with11 W power rating results in faster reduction in the viscositythough the final limiting viscosity is about 10% higher as comparedto the use of ultrasonic irradiations alone. This is due to the factthat the UV irradiation involves abstraction of hydrogen atom fromthe molecule by a radical which is produced from the water mole-cule. These radicals react with oxygen, forming other radicalswhich are responsible for chain scission into smaller chains andother molecules [63]. Thus the presence of ultraviolet radiationsprovides an additional mechanism for polymer degradation.

A more careful observation leads to the fact that when both thetypes of irradiations are operated in combination, the value of the

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limiting viscosity is lowered and at a higher rate indicating thesuperiority of the combined operation. Thus, for the lower viscosityreduction of PVP, use of US and UV irradiation in the combinedmanner would be much better option as compared to individualoperating mode. Quantitatively speaking, maximum extent of deg-radation i.e. 98.8% has been observed at 100 min of operation usingcombination of US and UV whereas in the case of ultrasound oper-ated individually, the extent of degradation was only 83%. Theimportant objective of the present work is to highlight the utilityof combined operation for the degradation of PVP polymeric solu-tion. The obtained results are very important as there have been noprior studies that elucidate the use of combined irradiation ap-proach for the polymer degradation (especially for PVP).

3.10. Effect of power intensity of UV irradiations

Addition of energy via UV photons can produce significantabsorption of energy in the molecule at sufficient levels that canlead to effective breaking of the molecular bonds or effective cut-ting of molecular chains which leads to a reduction in the intrinsicviscosity. The results of the experiments, with varying powerintensity of the UV tube carried out using ultrasonic horn forenhancing the degradation of PVP, are depicted in Fig. 12. The re-sults indicate that intrinsic viscosity decreases with increasingpower intensity. It has been observed that at initial stages the vis-cosity reduction was higher at 5 W. With increasing treatmenttime, the viscosity reduction seems to be similar upto 60 min forall power intensities, and after that the viscosity reduction in-creases rapidly for the case of higher power rating i.e. 11 W. UVradiation in the range of 290–400 nm, when absorbed by polymermolecule, is strong enough to rupture the most chemical bonds.Also it has been said that the essential photochemical reactionsin polymers comprise of the cleavage of main chains, the combina-tion of different macromolecules (cross linking), the generation ofunsaturated groups and cleavage of side groups resulting in forma-tion of volatile products. Generally, the lateral groups are split offin the primary process and further intermediates decomposeinvolving bond rupture in the main chain. Also, the main chainbonds are split leading to a pair of terminal radicals. It is expectedthat the these effects are higher at higher power dissipation levelsand hence the extent of degradation and the rate of polymer break-age will be higher at 11 W as compared to that obtained at 5 W and9 W. Kinetic study revealed that with a change in the power dissi-pation from 5 W to 11 W, the rate constant was found to increasefrom 3.1 � 10�2 to 7.3 � 10�2.

Fig. 12. Effect of power intensity on degradation of PVP using ultrasound(sonication time 180 min; 1% PVP; volume 200 ml; depth of horn 0.5 cm; 300 K,5 W,9 W and 11 W of UV tube).

4. Conclusions

The degradation of polyvinyl pyrrolidone (PVP) has been suc-cessfully demonstrated using ultrasonic irradiations in combina-tion with process intensifying approaches and it has beenconfirmed that the use of ultrasound effectively results in thebreaking of polymer chains. It can be also concluded that the ex-tent of degradation and the viscosity reduction is strongly depen-dent on the operating parameters such as reaction volume, depthof horn, and initial concentration. The molecular weight or intrinsicviscosity decreases with an increase in the sonication time andreaches to a limiting value, below which no further degradation oc-curs. It has been also established that the use of simple additivessuch as salts, titanium dioxide and introduction of air at differentflow rates can enhance the rate of ultrasonic degradation of PVPeffectively. Higher enhancement in rates of degradation is achievedwhen optimized amounts of additives are used. Presence of surfac-tants leads to marginally detrimental effects at lower concentra-tion and in general should not be used as desired additives forprocess intensification. Among the different additives investigatedin the work, titanium dioxide is most effective in depolymerizingthe PVP using ultrasonic horn. It has been observed that the max-imum extent of degradation is obtained by combination of ultra-sound and ultraviolet irradiation. It has been also establishedthat major extent of degradation takes place in the initial phaseof irradiation. This shows that a continuous operation with ultra-sonic horn is indeed possible, which is important for treatmentof large volumes of the streams containing low concentrations ofPVP.

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