Sonochemical Degradation of Rhodamine B Using Oxidants, Hydrogen Peroxide/Peroxydisulfate/Peroxymonosulfate, with Fe 2+ Ion: Proposed Pathway and Kinetics

ORIGINAL ARTICLE

Sonochemical Degradation of Rhodamine B Using Oxidants, HydrogenPeroxide/Peroxydisulfate/Peroxymonosulfate, with Fe2 + Ion:

Proposed Pathway and Kinetics

Ashok Babu Kurukutla,1 Panneer Selvam Sathish Kumar,2

Sambandam Anandan,2 and Thirugnanasambandam Sivasankar1,*

Departments of 1Chemical Engineering and 2Chemistry, National Institute of Technology, Tiruchirappalli, Tamilnadu, India.

Received: July 7, 2014 Accepted in revised form: September 12, 2014

Abstract

This article addresses the sonochemical degradation of Rhodamine B (RB), a recalcitrant textile organic dye.The relative influence of extent of radical production by the cavitation bubbles and radical scavenging (orconservation) on the overall degradation of RB was assessed. Degradation of RB at different experimentalconditions, such as pH, gases (air, argon [Ar], oxygen, and nitrogen), hydrogen peroxide (H2O2), perox-ymonosulfate (PMS), peroxydisulfate (PDS), ferrous sulfate, and novel Fenton-like reagents (H2O2 + ferroussulfate, PDS + ferrous sulfate, and PMS + ferrous sulfate), was studied. Experimental results revealed thatsonochemical degradation of RB is governed by the extent of utilization of HOc radicals. Ar-bubbledsolution that produces higher HOc radicals gave a higher color-removal rate than other gases. Further,higher color removal was observed at solution pH of 3. Of all the experimental conditions studied, novelFenton-like reagent, that is, PMS + ferrous sulfate-added solution, gave complete color removal. Miner-alization study also revealed that higher removal of total organic carbon was attained at the condition ofPMS with ferrous sulfate (pH 3) than with other experimental conditions. This result has been attributed tosynergistic effects of HOc radicals and sulfate radicals providing effective interaction with the dye mole-cule. Degradation intermediates of RB through LCMS/GCMS analysis were also provided to support thedegradation pathway.

Key words: novel Fenton-like reaction; peroxymonosulfate; Rhodamine B; TOC removal; ultrasound

Introduction

The relationship of industrial activity and environ-mental pollution is a serious topic and matter of great

concern in modern times. Wastewater discharged from in-dustrial units create large problem for conventional treatmentunits in the entire world. The release of this wastewater innatural environment is not only hazardous to aquatic life, butin many ways affects the human beings through mutagensfrom water (AlHamedi et al., 2009). The major environ-mental pollutants come from the textile, leather, paper, andpharmaceutical industries (Behnajady et al., 2008; Mehrdadand Hashemzadeh, 2010; Mehrdad et al., 2011). The textile

industries use variety of dyes (Yang and McGarrahan, 2005)and there are 20–30 different groups of dyes based on thechemical structure or chromophores that are available in themarket. Anthraquinone, phthalocyanine, triarylmethane, andazo dyes are the most important groups that are largely usedin textile industry.

Azo dyes are a group of compounds bearing the functionalgroup R-N = N-R0. Among the dyes, the most commonly usedare the reactive azo dyes. Moreover, these dyes are the mostproblematic pollutants of textile wastewaters. Rhodamine B(RB) is one of the most important xanthane dyes that is usedin a variety of industries, such as paper, textile, food stuffs,and dye lasers, thereby the wastewater discharged fromthese industries causes color to water streams (Lee et al.,2013). The disposal of these colored wastewaters poses amajor problem for the industry due to stringent environ-mental regulations on disposal standards as well as a threat tothe aquatic environment posing color and obstructing the

*Corresponding author: Department of Chemical Engineering,National Institute of Technology, Tiruchirappalli 620015, Tamilnadu,India. Phone: + 91-431-2503131; Fax: + 91-431-2500133; E-mail:[email protected]

ENVIRONMENTAL ENGINEERING SCIENCEVolume 32, Number 2, 2014ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2014.0328

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photosynthesis process in water streams. It is reported thatRB is carcinogenic and could induce skin, eye, gastrointesti-nal tract, and respiratory tract irritations (Tang et al., 2012).The chemical structure of RB dye is given above (Fig. 1).

In general, conventional treatment systems, such as ad-sorption on activated carbon, coagulation by a chemicalagent, or reverse osmosis, were used to treat this type ofwastewater. The afore-mentioned methods are nondestruc-tive and, hence, the disposal of the recovered/concentratedpollutant (after these treatment methods) poses a major threatto the environment (Mehrdad and Hashemzadeh, 2010).Nowadays, oxidation technologies are used as the alternatesfor nondestructive methods as they mineralize the pollutantsthrough reaction with the hydroxyl radicals produced in thesolution. These hydroxyl radicals are short-lived and highlyreactive species that react nonselectively with organic matterpresent in wastewater and are referred as advanced oxidationprocesses (AOPs). These AOPs use any one of the processes,for example, ultraviolet radiation, hydrogen peroxide (H2O2),ozone, Fenton’s reagent, ultrasound, or a combination ofthese processes (Vogelpohl and Kim, 2004; Rasoulifardet al., 2011). Fenton and photo-Fenton reactions have theadvantage of being very fast and efficient, but they only workin acidic solutions and need extraneous iron as reported byAlHamedi et al. (2009).

In recent years, considerable interest has been shown onthe application of ultrasound as a possible alternative fortreating textile wastewater (Chiha et al., 2010; Merouaniet al., 2010a, 2010b; Mishra and Gogate, 2010; Wang et al.,2010a, 2010b; Ahmedchekkat et al., 2011; Pang et al.,2011b). Earlier studies had shown that ultrasound treatmentof pollutants can significantly reduce the treatment time andthe amount of catalyst/additives (Sivakumar and Pandit,2001; Wang et al., 2008a, 2008b; Gayathri et al., 2010; Tiongand Price, 2012). The Fenton’s reagent is a combination of anoxidizing reagent (H2O2) and a catalyst (usually iron) thatyields hydroxyl radicals (cOH) upon reaction with each other.The cOH radical is the primary oxidizing chemical speciesgenerated with Fe2 + initiating the decomposition of H2O2 inan acidic environment. The amount of hydroxyl radicals(cOH) would be increased in Fenton process in combinationwith ultrasound due to its transient cavitation action. Effec-tive utilization of (cOH) would be facilitated through intensemicromixing resulted out of ultrasonic mechanisms, such asmicroturbulence and microstreaming (Chakma and Mo-holkar, 2013). When the Fenton processes were combinedwith ultrasound (sono-Fenton), 99% efficiency was achievedin a very short time for Acid Black 1 wastewater at theconditions of 40-kHz ultrasound frequency, 0.025 mM Fe2 + ,

and 8.0 mM H2O2. With various initial concentrations (20,100, and 200 mg/L) of carbofuran, the degradation efficien-cies were increased from 60%, 42%, and 25% for Fentonprocess to 99%, 63%, and 39% for sono-Fenton process at theidentical studied conditions (Ying-Shih et al., 2010).

Fenton-like processes, for example, 4A-zeolite supporteda-Fe2O3 (Fe-4A) prepared by hydrothermal-calcinationmethod, were effectively used along with H2O2 to degradeOrange II using sono-Fenton process. This system was pro-ven to work under neutral conditions, providing higher re-active oxide production to degrade the dye leading to 92.5%removal in 80 min of sono-Fenton reaction time (Chen et al.,2010). H2O2 in combination with FeOOH representing aFenton-like reaction to degrade para-chlorobenzoic acidproved to give faster reaction rate (Neppolian et al., 2004). Inthe same way, Fenton-like degradation of RB using H2O2

with iron metal oxides (II and III) proven to have resulted inan efficient catalytic oxidation reaction (Xue et al., 2009). Itis reported that the reaction had followed pseudo first-orderkinetics and H2O2 with iron (III) oxide provided higher de-composition of RB at neutral pH.

Similarly, H2O2 with three heterogeneous copper catalysts—CuO, Cu/Al2O3, and CuO-ZnO/Al2O3—under ultrasoundprovided the synergistic effect to give higher p-chlorophenolremoval at 25�C and 100 W of ultrasound power (Kim et al.,2007). However, the Cu/Al catalyst has proven to give highertotal organic carbon (TOC) removal along with H2O2 due tohigher catalyst dispersion. Heterogeneous Fenton-like deg-radation using H2O2/CuFe-ZSM-5 to degrade Rhodamine 6Gin water showed 100% color removal at low pH of 3.4 after45 min of reaction time though the TOC elimination was only51.8% after 2 h of reaction time (Dukkanci et al., 2010).Prihod’ko et al. (2011) studied the degradation of RhodamineG dye using Fe-exchanged zeolites (Fe-ZSM-5 and Fe-USY)/H2O2 as materials for catalytic wet peroxide oxidation.Complete color removal of the dye is enabled with Fe-ZSM-5/H2O2 at a reaction time of 150 min and the TOC removal is80% at near-ambient temperature (323 K) and quasi-neutralpH (4.9). The effect of various anions in the presence ofultrasound for the degradation of Acid Black 1 had providedan indication that different oxides would behave in a differ-ent manner with the pollutants. The degree of degradationwith anionic oxides has followed the order as SO2�

3 >CH3COO� >Cl� > CO2�

3 >HCO�3 >SO2�4 >NO�3 (Sun

et al., 2007; Pang et al., 2011a).In this present study, effort has been put to increase the

efficiency of ultrasonic treatment of RB dye using a novelhomogeneous Fenton-like reagent. Initial experimentalstudies were focused on parametric variations, such as initialpH of the aqueous solution, dosage of H2O2, type of oxidants(peroxydisulfate [PDS] and peroxymonosulfate [PMS]), andFenton’s reagent. To understand the mechanism of sono-chemical degradation of RB dye, experiments, such as RBdye solution saturated with respective gas nuclei (oxygen[O2], argon [Ar], nitrogen [N2], and air) and sodium chlorideof varied concentration in RB dye solution, were performed,respectively. Based on the results of these studies, the ex-periments have been directed to intensify the ultrasonictreatment of RB with homogeneous Fenton-like reagent(Fe2 + + PDS and Fe2 + + PDS) in addition to conventionalFenton’s reagent (Fe2 + + H2O2) and subsequently, appliedthe Fenton-like reagent process to varied RB concentration

FIG. 1. Chemical structure of Rhodamine B dye.

2 KURUKUTLA ET AL.

in order to make the process feasible for field application.Mineralization study through TOC and LCMS/GCMS ana-lyses was also performed and presented.

Experimental

Materials

Chemicals, such as RB (Loba Chemie), conc. H2SO4

(Merck), NaOH (Merck), H2O2 (30%; Merck), NaCl (Merck),PMS (Merck), PDS (Merck), and ferrous sulfate (Merck), wereused as procured. Double-distilled water is used for preparing allthe solutions. Gases, such as Ar, O2, and N2, are of 99.99%purity and were used for bubbling the aqueous liquid medium.In the case of air, compressed air is used for bubbling. Allexperiments were performed with an ultrasound probe (Sonics

& Materials; VCX 500) that operates with a frequency of20 kHz and delivers a variable output power of up to 500 W. Thediameter of the probe tip is 13 mm.

Experimental procedure

All the degradation experiments were carried out in a 120-mL-jacketed borosilicate glass reactor. The reaction volumetaken is 100 mL with the initial RB concentration of2.08 · 10 - 5 mol/L unless otherwise specified. The sonica-tion (or reaction) time of the dye solution is 60 min for allthe experiments. The ultrasound probe is set for 100-W-deliverable power for all the experiments. The color removalof the dye is followed at different time intervals of 15, 30, 45,and 60 min. The temperature of the reaction medium is

FIG. 2. Color removal of RBwith ultrasound treatment alone(RB: 2.08 · 10 - 5 mol/L, solutionpH 6.7, and temperature 25�C). (a)Color-removal profile and UV-visspectrum with reaction time and (b)first-order color-removal kinetics.RB, Rhodamine B.

ULTRASOUND-ASSISTED NOVEL FENTON TREATMENT OF RHODAMINE B 3

maintained constant during sonication by means of a circu-lating water bath ( Julabo; Model: ED-5). The initial pH of thesolution is varied using either 0.05 N sulfuric acid or 0.05 Nsodium hydroxide solution. The pH of the aqueous medium ismonitored using full-featured multiparameter instrument(YSI, Inc.; Model: Professional Plus). In the case of gasbubbling experiments, the RB solution is bubbled with therequired gas for 5 min at the rate of 5 LPM in order to saturatethe solution with the respective gas nuclei.

Analysis

Peak absorbance of RB was measured at 554-nm wave-length using UV-vis spectrophotometer ( JASCO, V-570).The percentage color removal of the RB at different timeintervals is calculated from the precalibrated chart withknown RB concentrations. TOC content of the initial andtreated RB solutions is analyzed by direct injection of thesamples into a TOC analyzer (Shimadzu; TOCvcph) calibratedwith standard solutions of potassium hydrogen phthalate. Thedegradation-intermediate formation was studied using LC-MS. LC-MS analysis was performed for untreated RB andFenton’s-reagent-treated RB in the negative-ion mode on aliquid chromatography–ion trap mass spectrometer (LCQFleet; Thermo Fisher Instruments Limited). The analysis wasperformed using a C18 column (150 · 4.6 mm i.d., 5 m) andthe separation was carried out using an ammonium phosphate(0.1%, v/v) as buffer solution with methanol as eluent sol-vent and at a flow rate of 1 mL/min. The compound detectionwas done using a diode array detector at the wavelength of550 nm. GCMS analysis was performed for Fenton-like-reagent-treated RB (PDS + ferrous sulfate and PMS + ferroussulfate) in a gas chromatograph (Bruker; GC45X-GC-44)combined with a mass spectrometer (Flame Ionization De-tection [FID] Detector). Programmed Temperature Vapor-izing injector was used with 1 mL/min flow and the analysiswas performed on column DB-WAX (30 m · 0.25 mm and0.25lm).

Results and Discussion

The difficult part of wastewater treatment is dealing withlower pollutant concentration. Bringing in the effective in-teraction between the pollutant and produced radicals is themost challenging part in all kinds of AOPs. There are twoways by which the interaction between organic moleculesand HOc radicals can be made. One way is by increasing thenumber of HOc radicals in the bulk medium, thus makinghigher probability of interaction with an organic molecule.The other way is to bring an organic molecule near to thebubble–bulk interface so that the produced radicals wouldreact immediately with the organic molecule. Effort has beenput to overcome this challenge using different experimentaltechniques for the degradation of RB using ultrasound and theresults were discussed in the following sections.

Degradation of RB with ultrasound alone

Degradation of 2.08 · 10 - 5 mol/L RB aqueous solutionwith ultrasound alone is shown in Fig. 2a and b. The colorremoval is about 27% with simple ultrasound and performedwith aqueous solution pH (i.e., 6.7), which means that theinitial pH of the aqueous solution is unaltered. The observedlow color-removal rate with ultrasound alone is because of

the poorer interaction between HOc radical and RB molecule.RB molecule is characterized by higher solubility (*15 g/Lat 20�C) and lower vapor pressure (1.89E-19 mmHg at 25�C),which makes it to remain in bulk liquid medium. Due to thenonavailability of RB molecule at the point of HOc radicalproduction (i.e., the bubble–bulk interface), the producedHOc radicals recombine with Hc radicals to form H2O again.The graph drawn between sonication time and ln(c/c0) islinear, indicating that sonochemical treatment of RB followspseudo first-order kinetics. The rate constant for RB aqueoussolution with simple ultrasonic treatment is found to be5.13 · 10 - 3 min - 1.

Effects of initial solution pH

Sonochemical degradation of 2.08 · 10 - 5 mol/L RB withvarying initial solution pH is shown in Fig. 3. The degradationof RB increased with decrease in initial solution pH. Themaximum color removal of 32% is attained for initial solutionpH of 3 and lower color removal of 12% is attained for initialsolution pH of 11. It is evident that at acidic pH the RB willexists in molecular form and hence migrates toward the surfaceof the cavitation bubble where the generated HOc radicalseffectively oxidize molecular form of RB to undergo miner-alization (Vajnhandl and Marechal, 2007). In addition, undernormal condition there is a maximum possibility that theamount of radicals produced out of the cavitation bubble willintend to recombine to form water again and this possibility isreduced when the RB molecule remains near the surface of thecavitation bubble and this is slightly achieved with decreasedpH of the pollutant aqueous solution. Also, the pKa value ofRB is 4.2 (Zhang et al., 2011), which further adds to the claimthat at initial solution pH of 3 (i.e., below pKa value) thesolubility of the RB lowers and would remain in molecularform. Since the cavitation bubble and the molecular form ofRB were hydrophobic in nature, both would come close toeach other. Due to this, the RB molecule would be readilyavailable for HOc radical attack produced out of the cavitationbubble upon ultrasound irradiation. The produced HOc radicalwill break the complex structure of RB and it will open up thearomatic ring that will eventually undergo further cleavagewith continuous ultrasound irradiation under acidic condition.The rate constant values for initial solution pH of 3 and 12were 6.09 · 10- 3 min - 1 and 2.05 · 10- 3 min - 1, respectively.

FIG. 3. Effect of the initial pH (RB: 2.08 · 10 - 5 mol/Land temperature 25�C).

4 KURUKUTLA ET AL.

Effects of H2O2

To enhance the degradation of RB, experiments wereperformed by adding varied amounts of H2O2, a strong oxi-dant, which will further produce additional HOc radicalsupon irradiation of ultrasound. As the initial concentration ofthe pollutant taken for the study is rather low (2.08 · 10 - 5

mol/L), the interaction between the pollutant and HOc radicalplays a major role in terms of degradation. Hence, addingH2O2 should increase the interaction between HOc radicalsand RB molecules to give higher degradation. When H2O2 isadded to aqueous medium, it splits into two HOc radicalsunder the ultrasonic irradiation (Chand et al., 2009). Hence,increasing the HOc radicals in the liquid medium could in-crease the percentage degradation of RB, which is evidentfrom Fig. 4. It needs to be mentioned that these experimentswere performed without altering the initial solution pH (i.e.,6.7). Increasing H2O2 concentration has increased the RB dyedecoloration up to 2.64 · 10 - 2 mol/L and it got lowered withfurther increase of H2O2 concentration (i.e., for 3.54 · 10 - 2

and 4.4 · 10- 2 mol/L). The maximum color removal of 28% isattained when 2.64 · 10- 2 mol/L of H2O2 is added with rateconstant value of 5.55 · 10- 3 min- 1. Beyond 2.64 · 10- 2

mol/L of H2O2, the scavenging of HOc radicals with excessproduction of Hc radicals and HOc radicals would exist toform H2O and H2O2 again without undergoing oxidationreaction (Saritha et al., 2007).

Effects of PMS and PDS

Effect of sulfate peroxides, such as PMS and PDS, on thecolor removal of RB was studied. As like the H2O2 the PMSand PDS were reported to produce radicals, which will en-hance the oxidation reaction (Maruthamuthu and Neta, 1977;Fernandez et al., 2004; Zhao et al., 2010; Chen et al., 2012;Olmez-Hanci and Arslan-Alaton, 2013; Hao et al., 2014).The study of experiments with the additions of PMS and PDSwill further increase the degradation efficiency of RB andmay enlighten the location of degradation of RB that is in thebulk liquid medium. As it is intended to make a comparisonof the effect of these peroxides over H2O2 on the color-re-moval efficiency and the location of degradation, experi-mental condition chosen was aqueous solution of pH 3 and

2.64 · 10 - 2 mol/L of peroxides (H2O2, PDS, and PMS), forwhich higher color removal is achieved for H2O2. The RBdye concentration remains the same for these experiments(i.e., 2.08 · 10 - 5 mol/L). Figure 5 shows that the percentagecolor removal has increased significantly with the addition ofboth PMS and PDS when compared with H2O2. PMS-addedsolution gave highest color removal of 95%, while it is 83%and 46% for PDS- and H2O2-added solution. The same isreflected in their rate constant values, 36.14 · 10- 3 min - 1

(PMS), 22.13 · 10- 3 min - 1 (PDS), and 10.04 · 10 - 3 min - 1

(H2O2). It is proven that HOc radicals have the higher redoxpotential and would be able to attack organic molecules toundergo faster dissociation. But sulfate radical anion (SO4c - )is found to have greater redox potential when dissociatedfrom either PMS or PDS (E0 ranges from 2.00 to 3.1 eV) thanHOc radicals dissociated from H2O2 (E0 = 1.76 eV) and wasexperimentally studied by Fernandez et al. (2004). It is wellknown that, upon ultrasound irradiation, H2O2 would yieldtwo hydroxyl radicals [Eq. (1)] (Olmez-Hanci and Arslan-Alaton, 2013). In the similar manner, ultrasonic irradiation ofPDS would yield two sulfate ions [Eq. (2)]. But the unsym-metric PMS would be yielding one sulfate radical anion(SO4c- ) and one hydroxyl radical HOc [Eq. (3)]. PMS issupposed to produce synergistic effect of H2O2 and PDS. Theproduced sulfate radicals react with water molecule yieldingHOc radicals [Eq. (4)] thus increasing the number of availableHOc radicals for the oxidation reaction.

Based on the reactions [Eqs. (1)–(4)], the following justi-fication could be presumed. In the case of H2O2-added so-lution, though two HOc radicals are produced, all thoseproduced might not effectively attack the RB molecule due toits high reactivity and short half-life period. Some mightrecombine again and some might react with hydrogen radical(produced out of cavitation process) to form water molecule.Moreover, at initial solution pH 3 all the RB molecules mightnot be in molecular form and the short-lived radicals mightnot reach all the RB in the bulk liquid medium. Also, theradical production in H2O2-added solution is a one-stepprocess. In the case of PDS, two SO4c- radicals would beproduced and those radicals react with water to form HOcradicals, which is a two-step process. As indicated earlier in

FIG. 5. Effect of peroxides (RB: 2.08 · 10 - 5 mol/L,H2O2/PDS/PMS: 2.64 · 10 - 2 mol/L, initial pH 3, and tem-perature 25�C). PMS, peroxymonosulfate; PDS, peroxy-disulfate.

FIG. 4. Effect of hydrogen peroxide concentration (RB:2.08 · 10 - 5 mol/L, solution pH 6.7, and temperature 25�C).

ULTRASOUND-ASSISTED NOVEL FENTON TREATMENT OF RHODAMINE B 5

this section that the redox potential of SO4c- radicals ishigher than HOc radicals, the oxidation reaction get enhancedinitially with SO4c- radical directly [Eq. (2)] and if not thenthrough the second process [Eq. (4)]. Hence, with PDS theinitial attack on RB would be by SO4c- radicals and then byHOc radicals. This might have accounted to higher color re-moval with PDS than H2O2. In the case of PMS, HOc andSO4c- radicals [Eq. (3)] are produced one at a time. Therewould be initial attack on RB by both the radicals. Later onunreacted SO4c- radicals again form HOc radical [Eq. (4)] thatadds up additional oxidation reaction on RB dye molecule.Hence, with PMS there is three-way radical attack on RB dyemolecule leading to highest color removal than H2O2 and PDS.

H2O2

)))/ 2HO� [Eq: (1)]

S2O82� )))/ 2SO4

�� [Eq: (2)]

HSO�5)))

/HO� þ SO��4 [Eq: (3)]

SO4�� þH2O

)))/ SO4

2� þHO� þH þ [Eq: (4)]

Effect of novel Fenton-like reagents

This study is performed to intensify the color-removal rateand to identify the best-suited oxidant (H2O2, PMS, and PDS)with Fe2 + , that is, novel Fenton-like reagent. For this, ex-periments were performed with 1 mmol/L concentration ofFe2 + in combination with 2.64 · 10 - 2 mol/L of individualperoxides (H2O2, PMS, and PDS) at initial solution pH of 3and for 60 min of sonication time. The color-removal rate hasfurther increased drastically with these conditions as shownin Fig. 6. Complete color removal (99%) of 2.08 · 10 - 5 mol/L RB has been observed with novel Fenton-like reagent (i.e.,PMS + Fe2 + ) after 2 min of ultrasound irradiation followed

by 86% color removal for PDS + Fe2 + and *50% color re-moval for H2O2 + Fe2 + . Their respective rate constants were49.80 · 10- 2 min - 1, 45.14 · 10- 2 min - 1, and 13.67 · 10- 2

min - 1. This further shows that an increase in the production ofHOc and SO4c - radicals has immensely increased the color-removal rate of RB. The radical production of ferrous sulfatewith peroxides upon ultrasound irradiation is given in Equa-tions (5)–(7). The PMS + Fe2 + -added solution has the advan-tage over other two (PDS + Fe2 + and H2O2 + Fe2 + ) that all thereactions [Eqs. (5)–(7)] would occur leading to increased rateof continuous production of radicals (both HOc and SO4c - ).

H2O2þFe2þ )))/Fe3þ þOH � þHO� [Eq: (5)]

S2O82� þFe2þ )))

/Fe3þ þ 2SO4�� [Eq: (6)]

HSO�5 þFe2þ )))/Fe3þ þHO� þ SO4

�� [Eq: (7)]

Effect of initial dye concentration

The color-removal study with increasing initial dye con-centrations was performed with 1 mmol/L concentration ofFe2 + in combination with 2.64 · 10 - 2 mol/L of PMS at so-lution initial pH of 3 and for 60 min of sonication time. Theexperiments were conducted for three different RB concen-trations of 1.04 · 10 - 4, 2.08 · 10 - 4, and 1.04 · 10 - 3 mol/Land the results were shown in Fig. 7. The results indicate thatthe optimized condition helps in removal of color at a fasterrate for lower dye concentration and as the initial dye con-centration increases the color-removal rate decreases. It isobvious that, as the number of RB molecules increases to avery large extent with increased initial dye concentration,there would not be sufficient HOc and SO4c - radicals avail-able to degrade all the RB molecules and hence, there is areduction in the color-removal rate of RB at higher concen-tration. The number of HOc and SO4c - radicals producedwould be the same irrespective of the initial RB dye con-centration as the concentration of Fenton-like reagent usedwas the same. With the condition studied, 1.04 · 10 - 4 (rate

FIG. 6. Effect of Fenton-like reagents (RB: 2.08 · 10 - 5

mol/L, initial pH 3, H2O2/PDS/PMS: 2.64 · 10 - 2 mol/L,Fe2 + : 1 mmol/L, and temperature 25�C).

FIG. 7. Effect of initial dye concentration with Fenton-like reagent (PMS: 2.64 · 10 - 2 mol/L, Fe2 + : 1 mmol/L,initial pH 3, and temperature 25�C).

6 KURUKUTLA ET AL.

constant is 10.08 · 10 - 2 min - 1) and 2.08 · 10 - 4 mol/L (rateconstant is 7.52 · 10 - 2 min - 1) RB-treated solutions resultedin the complete color removal whereas for 1.04 · 10 - 3 mol/L(rate constant is 1.38 · 10 - 2 min - 1), 63% of color removalwas attained, respectively. Although the near-complete colorremoval is attained for concentrations of 1.04 · 10 - 4 and2.08 · 10 - 4 mol/L, it needs to be mentioned here that thecolor removal does represent complete degradation of thepollutants and the number of intermediates formed wouldvary with dye concentrations and the 60-min sonication timewould not be sufficient enough to degrade all the interme-diates formed. This result shows that the studied system couldbe applied for higher concentration of RB dye solution also.

Effects of gas bubbling

The trend in color removal of RB with the bubbling gasesis shown in Fig. 8. The experimental conditions were asfollows: 60-min sonication time, 2.08 · 10 - 5 mol/L RB, so-lution pH (6.7), and 100-mL reaction volume. To avoid theair (which is present over the surface) dissolving into thereaction volume during sonication, respective gas is passedover the head space of the reactor. The percentage degradationof RB with varying gas contents follows the order Ar > air >O2 > N2. The rate constant values were 11.17 · 10- 3 min - 1,6.37 · 10- 3 min - 1, 6.7 · 10- 3 min - 1, and 5.73 · 10- 3 min - 1.Since RB is characterized by higher solubility (*15 g/L at20�C) and lower vapor pressure (1.89E-19 mmHg at 25�C),the degradation of RB should occur in the bulk liquid mediumby hydroxylation reaction and that is attained through thereaction of RB molecules with HOc radicals produced fromrespective nuclei. The simulation results of bubble dynamicswith varying gas contents (Sivasankar and Moholkar, 2009)showed that maximum number of HOc radicals is producedfor Ar (due to higher specific heat ratio) containing bubblefollowed by air, O2, and N2. The inert nature of Ar gas and thehigher final caviation collapse temperature (i.e., two times ofthe diatomic or triatomic gases) of monoatomic-Ar-bubbledsolution than other gases lead to higher radial production.Although the final collapse temperatures attained were in thesame range for air-, O2-, and N2-bubbled aqueous solutions,there exists scavenging of hydrogen atom and hydroperoxylradical by O2 to form H2O2, O2, O, and H2 and, with N2, thescavenging action leads to formation of oxides of N2. Also, inthe case of N2, the fact that lower HOc radical production andinability to conserve the HOc radical contribute to its lower

degradation of RB than O2 and air. Air-bubbled aqueoussolution exhibits both the effects of O2 and N2. These ex-periments were done to represent the location of degradationof RB during the cavitation process. As per the experimentalresults the percentage color removal has increased withproduction of HOc radicals, which indicates that the degra-dation of RB might occur in the bulk liquid medium.

Effect of NaCl

This experimental parameter was applied in order to makea further justification on the location of degradation of RB. Asit could be seen from Fig. 9, the color removal of RB in-creases with increase in NaCl addition under the experi-mental conditions of 60-min sonication time, 2.08 · 10 - 5

mol/L RB, solution pH (6.7), and 100-mL reaction volume.Since RB is highly hydrophilic in nature, the interactionbetween the radicals formed out of cavitation bubble and RBmolecule would have lesser possibility. Once NaCl is addedto the RB aqueous solution, the RB molecules are driventoward the bubble–bulk interface. This leads to effective

FIG. 8. Effect of gas bubbling (RB: 2.08 · 10 - 5 mol/L,solution pH 6.7, and temperature 25�C).

FIG. 9. Effect of NaCl addition (RB: 2.08 · 10 - 5 mol/L,solution pH 6.7, and temperature 25�C).

Table 1. Total Organic Carbon of 2.08 · 10 - 5mol/L

Rhodamine B Aqueous Solution

at Various Experimental Conditions

S. No.Experimental

conditions

USirradiationtime (min)

TOC(ppm)

% TOCremoval

1 RB — 7.771 —2 US + pH 6.7 60 6.424 17.333 US + pH 3 + H2O2 60 6.271 19.304 US + pH 3 + PDS 60 2.368 69.535 US + pH 3 + PMS 60 1.273 83.626 US + pH 3 +

H2O2 + Fe2 +12 2.489 67.97

7 US + pH 3 +PDS + Fe2 +

12 0.557 92.83

8 US + pH 3 +PMS + Fe2 +

12 Belowdetectablelimit

—

H2O2/PDS/PMS = 2.64 · 10 - 2 mol/L; Fe2 + = 1 mmol/L.PDS, peroxydisulfate; PMS, peroxymonosulfate; RB, Rhodamine

B; TOC, total organic carbon; US, ultrasound.

ULTRASOUND-ASSISTED NOVEL FENTON TREATMENT OF RHODAMINE B 7

interaction of radicals and RB molecule that resulted inhigher color removal with an increase in NaCl addition asshown in Fig. 9. This theory is well presented with the resultsof researchers already available in the literature (Seymourand Gupta, 1997; Gogate et al., 2004; Mahamuni and Pandit,2006; Bapat et al., 2007; He et al., 2009). Higher probability

of interaction between radicals and RB gives enhanced hy-droxylation reaction and hence higher color removal. Thecolor removal is 32% with 0.5 g NaCl (rate constant is6.01 · 10 - 3 min - 1) and 35% with 1 g NaCl (rate constant is6.35 · 10 - 3 min - 1) against 27% with simple ultrasoundtreatment (rate constant is 5.13 · 10 - 3 min - 1).

FIG. 10. ESI mass spec-trum of intermediates (RB:2.08 · 10 - 5 mol/L, H2O2:2.64 · 10 - 2 mol/L, Fe2 + :1 mmol/L, initial pH 3, andtemperature 25�C). (a) TotalIon Current for specific massand (b) ionic spectrum.

8 KURUKUTLA ET AL.

Mineralization study using TOC analysis

As color removal would only show the parent compoundgetting degraded, it is mandatory to study the mineralizationof RB in order to validate its degradation. For this, the ir-radiated solution (2.08 · 10 - 5 mol/L RB) is subjected toTOC analysis. Table 1 gives the values of TOC at variousexperimental conditions studied. The observed mineraliza-tion shows that there is no complete mineralization of RB inall the cases. Of all the cases studied, the mineralization rateor TOC removal rate for Fe2 + + PMS + pH 3 solution ishigher or complete (below detectable limit) when comparedwith Fe2 + + PDS + pH 3 (*93%) or Fe2 + + H2O2 + pH 3(*68%) solution for 12 min of ultrasound irradiation. Thistrend is similar to the color-removal rate of RB with theseconditions and the mechanisms were explained in earliersections.

LC/GC/MS study

LC, GC, and MS studies were done to identify the inter-mediates formed out of the sonochemical degradation pro-cess. The analysis results were shown in Figs. 10a and b, 11,and 12. Figure 10a and b shows the Total Ion Current (TIC) ofspecific mass spectra for untreated RB (2.08 · 10 - 5 mol/LRB, solution pH) and sonochemically treated RB (2.08 · 10 -

5 mol/L RB, initial pH 3, 2.64 · 10 - 2 mol/L H2O2, 1 mmol/LFe2 + ). Figures 10 and 11 show the GC and mass spectra ofsonochemically treated RB (2.08 · 10 - 5 mol/L RB, initial pH3, 2.64 · 10 - 2 mol/L PDS and PMS, 1 mmol/L Fe2 + ) solu-tion. The analysis shows that there were a number of addi-tional products formed in the case of sonochemically treatedRB aqueous solution when compared with an untreated RBaqueous solution. From Fig. 10a, the mass spectrum of un-treated RB solution exhibits a single peak at m/z 443.

FIG. 11. GCMS spectrumof intermediates (RB:2.08 · 10 - 5 mol/L, PDS:2.64 · 10 - 2 mol/L, Fe2 + :1 mmol/L, initial pH 3, andtemperature 25�C).

ULTRASOUND-ASSISTED NOVEL FENTON TREATMENT OF RHODAMINE B 9

Whereas, sonochemically treated RB solution exhibits anew peak at m/z 460 (Fig. 10b), validating that the generatedhydroxyl radicals attacked the RB molecule to form a newintermediate compound (see the structure that follows) fol-lowed by removal of CO. Such degradation pathway is shownhere indicating the generation of more hydroxyl radicals viasonochemical process that degrades RB molecules rapidly. In

addition, N-ethyl group cleavage (peaks at m/z 386 and 298),chromophore cleavage (peak at m/z 294), and open-ringcleavage (peak at m/z 166) also take place as reported bymany researchers (Chen et al., 2003; Lei et al., 2005). Similartrend of degradation pathway (AlHamedi et al., 2009; Zhonget al., 2009; Gazi et al., 2010; Mehrdad et al., 2011) is fol-lowed in the case of PDS- and PMS-treated RB solution as

FIG. 12. GCMS spectrumof intermediates (RB:2.08 · 10 - 5 mol/L, PMS:2.64 · 10 - 2 mol/L, Fe2 + :1 mmol/L, initial pH 3, andtemperature 25�C).

FIG. 13. Proposed pathway of degradation of RB.

10 KURUKUTLA ET AL.

many simpler compounds were observed from the massspectra (Fig. 13).

Conclusions

Color removal, mineralization (TOC), and the degradationmechanisms of RB dye using cavitation as the promisingAOPs were reported. When experimental conditions, such asinitial pH, H2O2, type of oxidants (PDS and PMS), Fenton’sreagent (Fe2 + + H2O2), and novel Fenton-like reagents(Fe2 + + PDS and Fe2 + + PMS), were used, the color removal/degradation rate had significantly improved. Complete colorremoval was achieved for novel Fenton-like reagent (1 mmol/L Fe2 + + 2.64 · 10 - 2 mol/L PMS, initial pH 3) for RB dyeconcentrations of 2.08 · 10 - 5, 1.04 · 10 - 4, and 2.08 · 10 - 4

mol/L. Mineralization study showed that novel Fenton-likereagent (Fe2 + + PMS, initial pH 3) not only removes color butcould remove TOC content within 12 min of ultrasound ir-radiation. Gas bubbling and NaCl-addition experiments pro-vided an indication that the degradation of RB would occuronly in the bulk liquid medium through hydroxylation reactionand the strategy followed to improve the radical productionthrough novel Fenton-like reagent proved impeccable.

Acknowledgment

The author T. Sivasankar would like to thank the Depart-ment of Science and Technology (DST), Government of In-dia, for financially supporting this work under Fast TrackScheme for Young Scientists.

Disclosure Statement

No competing financial interests exist.

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