Chemosphere 99 (2014) 199–206

Contents lists available at ScienceDirect

Chemosphere

journal homepage: www.elsevier .com/locate /chemosphere

An inhibitory effect of self-assembled soft systems on Fenton drivendegradation of xanthene dye Rhodamine B

0045-6535/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.10.074

⇑ Corresponding author. Tel.: +91 1942424900.E-mail address: aijaz_n5@yahoo.co.in (A.A. Dar).

Uzma Ashraf, Oyais Ahmad Chat, Aijaz Ahmad Dar ⇑Department of Chemistry, University of Kashmir, Hazratbal, Srinagar 190 006, J&K, India

h i g h l i g h t s

� Presence of surfactants in wastewaters influences the Fenton driven decay of RhB.� Cationic surfactants retard the decay of RhB more than non-ionic surfactants.� SDS inhibits the decay process completely.� Mixed micelles exhibit intermediate effect on decay kinetics of RhB.

a r t i c l e i n f o

Article history:Received 15 July 2013Received in revised form 9 October 2013Accepted 14 October 2013Available online 25 November 2013

Keywords:DyeRhodamine BSurfactantKinetics

a b s t r a c t

Rhodamine B (RhB) is known to be a common organic pollutant despite having various technical appli-cations. Treatment of effluents containing such compounds is important so as to minimize their effect onenvironment. Advanced Oxidation Processes (Fenton and Fenton like reactions) are such methods thatcan oxidize the contaminants powerfully and non-selectively. This work investigates the oxidation kinet-ics of dye RhB by hydroxyl radical (�OH) generated via Fenton reaction in presence of surfactant assem-blies of varying architectures using spectrophotometric, spectrofluorometic and tensiometric methods.The presence of surfactants viz. cationics, non-ionics and some binary mixtures in the pre-micellar andpost micellar concentration ranges were found to inhibit the degradation of RhB to a varying degree.However, the reaction was totally inhibited in anionic surfactant. The experimental data was fitted toa pseudo first order kinetic model and the kinetic parameters obtained thereof were explained on thebasis of the nature and type of interaction between the cationic form of RhB and the surfactants of vary-ing architectures. The work has a critical significance in view of the fact that degradation studied in pres-ence of surfactant assemblies is more representative than studied in aqueous solution because suchconditions compare well with the conditions prevailing in the environment.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Dyestuffs have been known to be serious organic pollutants inwastewaters for several years (Mehrdad et al., 2011). The annualproduction of dyes and pigments is more than 7 � 105 tons, andit has been estimated that approximately 5–15% are lost in indus-trial effluents. The treatment of effluents containing such com-pounds is, therefore, important so as to minimize their effect onthe environment and also to decolourize the water. A wide rangeof methods (Spadaro et al., 1994; Chen et al., 2011) have beendeveloped for the removal of synthetic dyes from the wastewatersbut all having one or the other disadvantage. Thus, there is a needfor developing treatment technologies that are more effective ineliminating dyes from wastewaters. Advanced Oxidation Processes

(AOP’S) are among such methods, which are based on the genera-tion of highly reactive �OH that can oxidize the contaminants pow-erfully and non-selectively (Moumeni and Hamdaoui, 2012).Among AOP’s, Fenton process is based on oxidation using Fentonreagent-an oxidative mixture of hydrogen peroxide and ferrousions (Fe2+) as catalyst. Although AOP’s have attracted considerableinterest in the degradation of dye components (Kusic et al., 2006;Fan et al., 2009; Xue et al., 2009; Hou et al., 2011), but no kineticinvestigation has been carried out on the effect of effluent compo-nents on the dye degradation by Fenton process.

Rhodamine B (RhB) is one of the important xanthene dye withmany different technical applications (Arbeloa and Ojeda, 1981;Liu et al., 2009; Sagoo and Jockush, 2011). The dye is known to ex-ist in three forms, optically active cationic and zwitterionic formsand a colourless lactone form. The dye is also known for its goodstability, hence has a comparatively high resistance to photo-and oxidative degradation (Xue et al., 2009). Owing to its stability

200 U. Ashraf et al. / Chemosphere 99 (2014) 199–206

and non-volatility it is considered as a common organic pollutantin wastewaters with carcinogenicity, mutagenicity, reproductiveand developmental toxicity as its major effects (Mirsalis et al.,1989; McGregor et al., 1991). In this direction, significant amountof research has been conducted with regard to the degradation ofRhB. For example, photo catalytic degradation (Yu et al., 2004;Asilturk et al., 2006; Li et al., 2006; Barka et al., 2008; Yang et al.,2008; Mehrdad et al., 2011), ultrasonic degradation (Sivakumarand Pandit, 2001; Behnajady et al., 2008; Wang et al., 2008a,b)and, sonocatalytic degradation (Wang et al., 2008a,b; Mehrdadand Hashemzadeh, 2010) of RhB has been investigated. All suchinvestigations have been done in pure solvent conditions withoutconsidering any effect arising due to presence of various natural/anthropogenic amphiphilic molecules.

Wastewaters containing such residual dyestuffs mostly containauxiliary chemicals such as surfactants, salts, greases, and oils. Sur-factants, the amphiphilic molecules having both hydrophilic headand a hydrophobic tail, self-assemble in aqueous solution formingcolloidal sized aggregates called micelles above critical micelleconcentration (Rosen, 2004). It has been vastly reported that pres-ence of surfactants in the reaction media can either have catalyticor inhibitory effect (Fendler and Fendler, 1975; Fendler, 1982;Rodriguez et al., 2003; Hassan et al., 2011; Singh et al., 2011). Sur-factant-dye interaction like interaction of surfactants with xan-thene dyes (Rhodamine B, Rhodamine 6G) (Tajalli et al., 2009),triphenylmethane dye (crystal violet) (Ghosh et al., 2012), benzi-dinedye (Congo red), Safranine T (Ray et al., 1997), methylene blue,and acridine orange (Park and Chung, 1986). has been a subject ofextensive investigation since they are important in a variety ofdyeing processes as well as in chemical research such as biochem-istry, analytical chemistry and photosensitization (Alavijeh et al.,2011). To the best of our knowledge there has been no report onthe effect of surfactants of varying architectures on the oxidationkinetics of RhB by �OH generated by Fenton reaction. Therefore,the focus of this work was to investigate the effect of surfactantsin single and mixed states on �OH induced oxidation of RhB. Thework has a critical significance in view of the fact that degradationbehavior observed in presence of surfactant assemblies is morerepresentative than studied in pure aqueous solution because theconditions of reaction media compares well with the conditionsprevailing in the environment. Also, this study investigates quanti-tatively the feasibility of Fenton reagent as oxidizing agent to de-grade dye compounds under such natural conditions.

2. Experimental

2.1. Materials

The non-ionic amphiphiles Polyoxyethylene(4) lauryl ether(Brij30), Polyoxyethylene(23) lauryl ether (Brij35), Polyoxyethyl-ene(20) cetyl ether (Brij58), cationic amphiphiles cetyltrimethyl-ammonium bromide (CTAB), dodecyltrimethylammoniumbromide (DTAB), and anionic amphiphile sodium dodecylsulfate(SDS) were all Aldrich products and were used without furtherpurification. The dye Rhodamine B (95%) was obtained from SigmaAldrich. FeSO4, H2O2, KBr, H2SO4 were of analytical grade. All thesolutions were prepared in triple distilled water. The structure ofthe surfactants and dye used are presented in Scheme 1.

2.2. Procedure

2.2.1. Optimization of experimental conditionsSelection of RhB Concentration: RhB absorption increases line-

arly in aqueous media with increase in concentration upto0.025 mM beyond which it shows deviation from Beer law

(Fig. S1). The deviation is attributed to the formation of aggre-gates of dye species at higher concentrations. These aggregatesare held together by dispersion forces which exist betweenp-systems of the dye and forces resulting from hydrophobiceffects (Tajalli et al., 2009). Thus, the concentration of 0.02 mMof RhB was selected for the kinetic experiments to overcomethe deviation effects.

Selection of [Fe2+]/[H2O2] ratio: The ratio of Fe2+ and H2O2 is a keyparameter in Fenton driven oxidation process. Many studies revealthat use of higher concentration of Fe2+ leads to self-scavenging of�OH by Fe2+ and hence reduces its efficiency to degrade pollutants.Also, high dose of H2O2 hinders the dye degradation as the highlyreactive �OH can be consumed by H2O2 leading to the formationof less reactive HO2 radicals (Lin and Gurol, 1998; Laat and Gallard,1999). Thus for kinetic measurements, [Fe2+]/[H2O2] ratio of 2:5(0.2 mM Fe2+, 0.5 mM H2O2) was selected as the optimum ratio.

Selection of reaction pH: For the effective oxidation of organiccontaminants by Fenton reagent, the solution pH must be in acidicrange. At higher pH values the rate of oxidative degradation isreduced due to the formation of Fe(OH)3 having lower catalyticactivity in the decomposition of H2O2 (Fan et al., 2009). Lu et al.also reported that the optimum pH for the oxidation of dichlorvosby Fenton’s reagent was between 3 and 4. They also found that therate of dichlorvos oxidation by Fenton reagent decreased when thesolution pH was 2.5 (Lu et al., 1999). The lower efficiency at pH 2.5is probably due to the formation of the complex [Fe(H2O)6]2+whichreacts more slowly with H2O2 than [Fe(OH)(H2O)5]+ and thereforeproduces fewer �OH (Gallard et al., 1999). Thus to ensure theeffective oxidation by Fenton reagent, FeSO4 stock solution wasprepared in 2 mM H2SO4 to have the pH of 4 in the reactionmedium. It is pertinent to mention that the dye exists in itscationic form under the prevailing experimental conditions.

2.2.2. Determination of cmcThe cmc values of all the surfactant solutions under prevailing

reaction conditions were determined from the plot of surface ten-sion (c) vs logarithm of surfactant concentration (Fig. 1). Surfacetension measurements were made by the Platinum ring detach-ment method with the Kruss K9 tensiometer equipped with a ther-mostable vessel holder. The surfactant concentration was varied byadding concentrated stock surfactant solution prepared in0.04 mM H2SO4 and 0.02 mM RhB of known concentration in smallinstallments using a Hamilton Syringe to 20 mL solution contain-ing 0.04 mM H2SO4 and 0.02 mM RhB in the sample vessel placedin a vessel holder. Readings were taken after thorough mixing andtemperature equilibration at 25 �C (±0.1 �C) by circulating waterfrom a Brook field TC-102 thermostat through the vessel holder.

2.2.3. Determination of kinetics of RhB degradation0.2 mM RhB, 10 mM FeSO4 in 2 mM H2SO4, 100 mM H2O2 were

prepared as stock solutions for the Fenton reaction. kmax of RhBwas obtained at 554 nm by recording its absorption spectra usingSchimadzu spectrophotometer (UV-1650PC) with matched pair ofquartz cuvettes (path length = 1 cm). Kinetic experiments for thedegradation of RhB by Fenton reagent were studied spectrophoto-metrically at the absorption maximum of RhB. 0.3 mL of 0.2 mMRhB, 0.06 mL of 10 mM FeSO4 in 2 mM H2SO4, 0.015 mL of100 mM H2O2 were transferred to a cuvette and final volumewas made upto 3 mL with triple distilled water alongwith the reac-tion initiator H2O2 which was added at the end. The decrease inabsorbance of RhB was recorded at 25 �C for a period of 1000 s.Since the kinetic experiments were performed under pseudo firstorder conditions with Fenton reagent in excess, the experimentalresults were fitted to a pseudo first order kinetic model (Alshamsiet al., 2007) to calculate rate constant (k) of RhB degradation.

Scheme 1. Structure of materials used in this study.

U. Ashraf et al. / Chemosphere 99 (2014) 199–206 201

The effect of single and binary surfactant mixtures on the deg-radation kinetics of RhB was studied by adding surfactants in thepre (below cmc) and post (above cmc) micellar concentration rangeto the above reaction mixture containing RhB, FeSO4, H2SO4, H2O2

at fixed concentrations. Some prototype degradation plots of RhBin various surfactant systems are presented in Fig. 2.

2.2.4. Determination of micelle-RhB interaction using fluorescencemeasurements

Fluorescence spectra of RhB were recorded in CTAB, SDS, Brij58surfactant media and their binary mixtures SDS-Brij30, CTAB-Brij58 under the reaction conditions using Shimadzu spectrofluo-rimeter (RF-5301PC) to find out the solubilization site of RhBwithin differently architectured micelles. The excitation wave-length used for RhB, placed in 1 cm quartz cuvette was 554 nmand the spectra were recorded from 565 to 650 nm at the excita-tion and emission slit width of 1.5 nm.

3. Results and discussion

3.1. Critical micelle concentration (cmc)

The cmc values of all the studied single and binary surfactantsystems obtained experimentally (cmcexp/mixcmcexp) and those re-ported in literature (cmclit) in aqueous media (Bhat et al., 2008;Chat et al., 2011) are presented in Table 1. The cmc values obtainedfor all the studied surfactant systems are lower under reaction con-ditions than their values in pure aqueous conditions. The decreasein cmc was found to be highest in presence of SDS, lowest in cat-ionic surfactants while as intermediate in presence of non-ionicsurfactants. It has been observed that even in presence of 8 MH3PO4, the decrease in cmc of different cationic, anionic and non-ionic surfactants is not very significant (Taha et al., 2013). Sincein our system the concentration of acid is only 0.04 mM, we expectan insignificant decrease in cmc for all the surfactants studiedwhich was confirmed by following a prototype experiment inpresence of 0.04 mM H2SO4 only. Therefore decrease in cmc forsurfactants could be attributed to the presence of cationic form

of RhB in the solution. Because of the strong electrostaticinteractions between the anionic surfactant and the positivelycharged RhB, mixed micellization between the RhB and SDS is ex-pected to occur resulting in almost 50% decrease in the cmc of SDSrelative to that in pure aqueous solution. This explanation is rea-sonable because literature report reveals that the cmc of SDS de-creases from 8.5 to 7.5 mM in presence of 1 mM acidconcentration (Dey and Ismail, 2012). In case of cationic surfac-tants CTAB and DTAB, a decrease of just approximately 20% is anindication that most of the RhB molecules would be present inthe bulk solution with small fraction participating in the mixedmicelle formation probably due to hydrophobic interaction andpossible interaction of positive charge on micelle with the p-elec-tron system of the xanthene moiety of RhB. In case of Brij surfac-tants, the decrease was observed to be nearly 40% which couldbe attributed to the interaction of positively charged RhB mole-cules with the lone pair of electrons of oxyethylene groups of suchsurfactants facilitating the stabilization within the palisade layerand hence early micellization.

The cmc values of binary surfactant systems were found to belower than ideal values (Table 1) calculated using Clint equation(Clint, 1975), indicating negative deviation from ideal behaviorfor mixed micelle formation. The estimate of negative deviationand hence non-ideality of binary surfactant systems has been ob-tained using Rubingh’s model (Rubingh, 1979). The interactionparameter b which accounts for deviation from ideality is an indi-cator of degree of interaction between two surfactants in mixedmicelles. b values along with the micellar mole fraction, XM

i andactivity coefficient fi, of the ith surfactant within mixed micellescalculated through Rubingh’s equation (Rubingh, 1979) are alsopresented in Table 1. A negative value of b indicates attractiveinteractions between two surfactants in mixed micelles. It is wellknown (Zhou and Rosen, 2003; Zhou and Zhu, 2004) that in io-nic + non-ionic mixed surfactant systems, the significant electro-static self-repulsion of ionics and weak steric self-repulsion ofnon-ionics (depending on the head group size) before mixing areweakened by dilution effects after mixing and that the electrostaticself-repulsion of the ionic surfactant is replaced by ion dipole inter-actions. Moreover, these mixed micelles are dominated by

Fig. 1. Plot of Surface tension (c) vs logarithm of surfactant concentration inaqueous solution containing 0.04 mM H2SO4 and 0.02 mM RhB for (A) single ionicssurfactant systems and, (B) single nonionic and binary nonionic/ionic surfactantsystems at 25 �C.

202 U. Ashraf et al. / Chemosphere 99 (2014) 199–206

non-ionic surfactants as indicated by XMi values in Table 1 which is

in conformity with analogous results in literature (Errico et al.,2002; Dar et al., 2006).

3.2. Solubilization of cationic RhB within micelles: fluorescence results

The fluorescence spectra of cationic RhB at different concentra-tions of SDS are shown in Fig. 3. The fluorescence of RhB is stronglyreduced in intensity with increase in concentration of SDS upto4.5 mM, beyond which there is large increase in fluorescenceintensity and a marked bathochromic shift. The decrease influorescence intensity can be explained in terms of the formation

of dye surfactant aggregates in the pre micellar region i.e, upto4.5 mM which is nearly cmc of SDS under prevailing reaction con-ditions as observed from tensiometric measurements. The electro-static and hydrophobic interactions between the anionic surfactantSDS and the cationic form of RhB result in the sandwich of dyemolecule between oppositely charged surfactant molecules(Macedo et al., 2011) and continues with the formation of dye sur-factant aggregates represented as (D+ S�)n (Tajalli et al., 2009). Thisprocess diminishes the electrostatic repulsion and thermal agita-tion between the dye monomers, resulting in the reduction of dis-tance between them, thereby inducing dimer formation (Park andChung, 1986; Zhang et al., 1998). Molecular aggregates of Rhoda-mine dyes in the form of dimers or higher aggregates are knownto quench fluorescence due to the transfer of excitation energy be-tween the monomers and aggregates which then decays non-radiatively (Tajalli et al., 2009). Also, dimers are known to lowerthe quantum yield by quenching process (Penzkofer and Leupach-er, 1987). However, the increase in fluorescence intensity with thebathochromic shift is due to the solubilization of dye monomers inthe relatively non-polar regions of surfactant micelles leading tocut- off of non-radiative decay mechanisms of fluorophores preva-lent in aqueous media. In the post micellar conditions, anionicsurfactant promotes the dissociation of aggregates, increasing theelectrostatic repulsion between dye molecules (Macedo et al.,2011), leading to the solubilization of monomeric form of thedye within the surfactant micelles which is favoured by both elec-trostatic and hydrophobic interactions. Therefore, these experi-mental results show that most of RhB molecules get solubilizedwithin nonpolar region of SDS micelles shielding it from aqueousenvironment.

The fluorescence spectra of cationic RhB in presence of differentconcentrations of CTAB (Fig. 3) indicate that there is not any signif-icant interaction between the cationic RhB and CTAB in both preand post micellar concentration regions as insignificant change influorescence intensity and emission maximum is observed. Thiscould be due to the electrostatic repulsion between the positivelycharged dye and the cationic heads of the surfactant. As a result,dye molecules are neither capable of forming the dye surfactantaggregates in the pre micellar region nor penetrate into the mi-celles in the post micellar concentration region as is observed incase of SDS. Since the fluorescence data indicate the averagebehavior of the fluorophores in the medium, we conclude thatmost of the RhB molecules lie in the bulk of the solution thougha slight partitioning of RhB molecules within such molecules can-not be overruled as indicated by a small decrease in value of cmc.

Prototype fluorescence spectra of RhB in presence of non-ionicsurfactant Brij58 shown in Fig. 3 again points to insignificant inter-action of non-ionic surfactant with the cationic RhB. However,some polar interactions are possible between the polyoxyethylenegroups of Brij58 and the positively charged dye as indicated earlierby significant decrease in cmc of such surfactants, neverthelessthey do not make any marked effect on the fluorescence spectraof the RhB. This could be explained by making a reasonable specu-lation that appreciable amount of RhB molecules are solubilizedwithin the palisade layer of such micelles. Since POE groups areheavily hydrated (Vringer et al., 1986), the solubilized RhB remainexposed to the aqueous medium leading to no effect on their fluo-rescence intensity after micellization. Deeper intercalation is ex-pected to increase their fluorescence intensity due to theirexposure to non-polar groups.

In presence of binary mixture of cationic and non-ionic surfac-tant system i.e, CTAB + Brij58, RhB does not show any decrease influorescence intensity in pre or post micellar regions as evidentfrom fluorescence spectra presented in Fig. 3 possibly because ofthe electrostatic repulsion with the cationic head of CTAB. It isbecause the mixed micelles of cationic + non-ionic surfactants are

0.0

0.2

0.4

0.6

0.8

1.0 0 0.01 0.02 0.03 0.05 0.07 0.1 0.3

Time/ sec.

[SDS+Brij30]/ mM

0.0

0.2

0.4

0.6

0.8

1.0

1.2 0 0.004 0.006 0.01 0.05 0.1 0.3 0.5 0.8

At/A

0

Time/ sec.

[Brij58]/ mM

0.0

0.2

0.4

0.6

0.8

1.0 0 0.1 0.5 1.0 2.0 5.0

Time/sec.

[CTAB]/ mM

0 200 400 600 800 1000

0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200

0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.001 0.005 0.05 0.1 0.8

Time/ sec.

[CTAB+Brij58]/ mM

At/A

0

At/A

0A

t/A0

Fig. 2. Variation of At/A0 with time of RhB degradation by hydroxyl radicals generated via Fenton reaction in Non-ionic (Brij58), Cationic (CTAB) and the mixed binary(CTAB + Brij58, SDS + Brij30) surfactant systems at 25 �C.

Table 1Experimental and literature critical micelle concentration values (cmcexp and cmclit) of single surfactant systems, experimental and ideal cmc values (mixcmcexp and cmcideal)calculated with clint equation for equimolar binary surfactant systems at 25 �C.

Single surfactant systems Binary surfactant systems

System cmcexp (cmclit)/(mM) System mixcmcexp (cmcideal)/(mM) b XMI =XM

2f1/f2

Brij30 0.022 (0.035) Brij30 + SDS 0.041 (0.043) �2.83 0.94/0.06 0.99/0.08Brij35 0.033 (0.05) CTAB + Brij 58 0.008 (0.009) �4.07 0.86/0.14 0.92/0.04Brij58 0.005 (0.008) DTAB + Brij 30 0.030 (0.043) �7.04 0.83/0.17 0.81/0.01SDS 4.3 (8.1)CTAB 0.61 (0.815)DTAB 11.8 (15.1)

The error in cmc values is ±5%.

U. Ashraf et al. / Chemosphere 99 (2014) 199–206 203

predominated by non-ionic surfactants (Table 1) and therefore isexpected to follow the solubilization within the aqueous mantleof mixed micelles similar to that in pure non-ionic surfactants.By comparing fluorescence spectra of RhB in SDS + Brij30 systemwith the fluorescence spectra of RhB in pure SDS and pure non-ionic (Brij58) surfactant systems (Fig. 3), the interaction of RhB isgreater with this binary system than pure non-ionic surfactant sys-tem but less than that of pure anionic system. There is slight de-crease in fluorescence intensity upto 0.049 mM of SDS + Brij30which corresponds to its critical micelle concentration beyondwhich there is slight bathochromic shift with further decrease inintensity followed by insignificant decrease at higher concentra-tions of SDS + Brij30. The pre micellar decrease in fluorescenceintensity is because of electrostatic interaction of the positively

charged RhB with anionic surfactant resulting in the formation ofdye-surfactant aggregates as in case of pure SDS system. However,the post micellar bathochromic shift with decrease in intensity isbecause of strong interaction of RhB with the polar aqueous mantleof SDS + Brij30 mixed micelles predominated by POE groups ofnon-ionic surfactant. The presence of anionic head groups in themixed micelles binds the positively charged dye strongly than incase of pure Brij micelles causing the bathochromic shift. However,since the mole fraction of non-ionic surfactant in the mixed mi-celles is more (Table 1), therefore, higher hydration of POE groupsin aqueous mantle keeps RhB exposed to aqueous environmentleading to slight decrease in fluorescence intensity and bathochro-mic shift rather than increase and large bathochromic shift as ob-served in pure SDS micelle.

20

30

40

50

60

70

80

90

Flou

resc

ence

Inte

nsity

Wavelength (nm)

Pure RhB RhB + 0.04 mM Acid

[CTAB+Brij58]= 0.008 mM 0.015 0.022 0.029 0.036 0.043 0.050 0.057 0.076 0.089 0.101 mM

CTAB+Brij58

20

30

40

50

60

70

80

Flou

resc

ence

Inte

nsity

Wavelength (nm)

Pure RhB RhB+0.04 mM Acid

[SDS+Brij30]= 0.010 mM 0.020 0.030 0.039 0.049 0.058 0.068 0.077 0.087 0.113 0.131 0.148 0.181 0.214 0.246 0.276 0.305 mM

SDS+Brij30

10

15

20

25

30

35

Flou

resc

ence

Inte

nsity

Wavelength (nm)

Pure RhB RhB + 0.04 mM Acid 0.0009 mM 0.0019 0.0029 0.0038 0.0047 0.0056 0.0065 0.0074 0.0082 0.0090 0.0099 0.0115 0.0130 mM

Brij 58

0

5

10

15

20

25

30

Flou

resc

ence

Inte

nsity

Wavelength (nm)

Pure RhB RhB+0.04 mM Acid

[SDS] = 1.2 mM 2.3 mM 3.4 mM 4.5 mM 5.5 mM 6.5 mM 7.4 mM 8.3 mM 9.1 mM 10.0 mM 10.8 mM 11.5 mM

SDS

570 580 590 600 610 570 580 590 600 610

570 580 590 600 610570 580 590 600 610 620 630 570 580 590 600 610

10

15

20

25

Flou

resc

ence

Inte

nsity

wavelength (nm)

Pure RhB RhB+0.04 mM Acid

[CTAB]= 0.13 mM 0.25 mM 0.37 mM 0.49 mM 0.61 mM 0.73 mM 0.85 mM 0.96 mM 1.07 mM

CTAB

Fig. 3. Fluorescence Spectra of Cationic RhB at different concentrations of anionic (SDS), cationic (CTAB), non-ionic (Brij58) and the binary surfactant mixtures (CTAB + Brij58and SDS + Brij30) in aqueous 0.04 mM acidic solution at 25 �C.

204 U. Ashraf et al. / Chemosphere 99 (2014) 199–206

3.3. Degradation kinetics of RhB in various surfactant systems

The spectral change in the RhB dye solution with reaction timeclarifies the degradation of RhB by Fenton reagent. The absorptionmaximum of RhB at 554 nm attributed to its xanthene chromo-phore (Ai et al., 2008) sharply diminishes with reaction time(Fig. S2). This indicates that the rapid degradation of RhB is dueto decomposition of its conjugated xanthene chromophore by�OH. The prototype degradation plots of RhB in different surfactantsystems presented in Fig. 2 indicate that RhB is effectively de-graded by �OH generated by Fenton reagent. The addition of surfac-tants viz cationic (CTAB, DTAB), non-ionic (Brij30, Brij35, Brij58),the binary mixtures (CTAB + Brij58), (DTAB + Brij30), and (SDS +Brij30) suppresses the degradation kinetics of RhB. However, thepresence of anionic surfactant (SDS) alone in the reaction mediumresults in the complete inhibition of the reaction as evident fromFig. S3. The rate constants (k) obtained by carrying out a linearregression on the plots of ln (At/A0) verses time (t) at various con-centrations of different surfactants are plotted in Fig. 4 and theaverage values of rate constant (kav) for each surfactant systemare presented in Table 2. From the data in the table it is clear thatthe kav follow the orderBrij30 > Brij58 > Brij35 > CTAB > DTAB >SDS in various surfactant assemblies and are found to be signifi-cantly lower than the value of 0.01641 s�1 in aqueous mediumindicating antagonistic effect of surfactants on the reaction of�OH with RhB. These kinetic results are explained in terms of thenature and type of interactions which exist between the RhB andthe differently charged surfactants.

In presence of SDS, the inhibition of oxidative degradation ofRhB by �OH in the post micellar conditions is explained in terms

of the micellar encapsulation of dye monomers within the micellarcore. This results in the occupation of reacting species i.e, �OH andRhB in different environments, aqueous and the micellar phaserespectively, which in turn decreases the chance of encounter oftwo reacting species and hence the inhibition of the subsequentreaction. The result also indicates the increased stability of dye to-wards �OH due to its encapsulation in the micellar core. However,the inhibition in the pre micellar region may be due to the sand-wich of dye molecule between the oppositely charged surfactantmolecules where they act as barrier to the �OH preventing the deg-radation of dye to occur. The presence of binary mixture of non-ionic surfactant Brij30 and SDS in the reaction media, however,leads to the degradation of dye to an appreciable extent. The pres-ence of non-ionic surfactant dilutes the effect of SDS to aggregatedye molecules in the pre micellar region and their micellar encap-sulation in the post micellar region, increasing the accessibility ofdye molecules to �OH in the bulk resulting in oxidation of RhB.Thus, Fenton process could remove RhB dye molecules more effi-ciently in binary mixture of anionic and non-ionic surfactant thanin presence of anionic surfactant alone.

In presence of CTAB, the oxidative degradation of RhB by �OH issuppressed to a marked extent. As mentioned earlier because ofthe electrostatic repulsion between the similarly charged dye andCTAB head group, it is reasonable to assume that the rate constantof the RhB degradation is not influenced by the amphiphilicity ofCTAB. The suppression in the degradation kinetics of RhB is attrib-uted to the presence of counter ions (Br�) and is explained on thebasis of the formation of dibromine radical anion which is formedfrom the reaction of �OH with Br� followed by the rapid complex-ation with another Br�. The dibromine radical anion is known to be

Fig. 4. Plots of rate constant (k) of degradation of RhB by hydroxyl radicalsgenerated by Fenton reaction versus concentration of different surfactants at 25 �C.

Table 2The average rate constant (kav) of degradation of RhB by hydroxyl radicals generatedby Fenton reaction in different surfactant systems at 25 �C.

Surfactant system kav (s�1)

Cationic surfactantCTAB 0.0051DTAB 0.0039

Nonionic surfactantBrij30 0.0113Brij35 0.0055Brij58 0.0091

Binary SurfactantSDS + Brij30 0.0082CTAB + Brij58 0.0074DTAB + Brij30 0.0092

The error in the k values is ±6%.

U. Ashraf et al. / Chemosphere 99 (2014) 199–206 205

less reactive than �OH and hence results in the decrease in magni-tude of kav (Neta et al., 1988). Similar results were obtained whenthe reaction was performed in presence of DTAB. The involvementof Br� in retarding the kinetics of RhB degradation was furthervalidated by studying the effect of varying concentrations of KBr

on the rate constant (k) of the reaction (Fig. S3 and Fig. 4). With in-crease in the concentration of KBr, decrease in the value of k wasobserved as in case of cationic surfactants (Fig. 4). However, slightpartitioning of cationic RhB within the cationic micelles as indi-cated by tensiometric results may also add to the suppression ef-fects by decreasing the chance of encounter between RhB and �OH.

The presence of non-ionic surfactants (Brij30, Brij35, Brij58) inthe reaction medium also decreases the value of k to a significantextent. This could be due to the localization of RhB in the polaraqueous mantle of the Brij micelles where it may undergo polarinteraction with the POE groups of non-ionic surfactants. Thiscould possibly decrease the accessibility of RhB to �OH in the bulkcausing the reduction in the reaction kinetics. Comparison of kav

values of the reaction in presence of Brij30 and Brij35 (Table 2), re-veals that its value is inhibited much more in Brij35 than Brij30.Since Brij30 and Brij35 possess identical hydrocarbon tail, the dif-ference in kav values can be attributed to the number of POEgroups. More the number of POE groups, more will be the solubili-zation of cationic RhB within the palisade layer of Brij micelles andhence more will be the suppression in reaction kinetics. Hence therate of reaction in presence of surfactant assemblies was inhibitedin the order Brij35 > Brij58 > Brij30 since the number of POEgroups in Brij35, Brij58 and Brij30 are 23, 20, and 4 respectivelyirrespective of hydrocarbon chain length.

In presence of equimolar binary surfactant mixtures of cationicand non-ionic surfactants (CTAB + Brij58, DTAB + Brij30), the rateof reaction was also inhibited. The kav of the reaction in presenceof mixed binary systems is inhibited at a rate intermediate to thatin single surfactant systems. The kav value decrease may be due tothe formation of less reactive dibromine radical anion in the reac-tion medium and decreased accessibility of RhB to �OH in the bulkdue to polar interactions with non-ionic surfactants as explained insingle surfactant systems.

4. Conclusion

The removal of RhB from wastewaters for environmental reme-diation process by Fenton driven oxidation method is highly sensi-tive to the presence of surfactants commonly found inwastewaters. It was found that anionic surfactant (SDS) completelyinhibits the oxidative degradation of RhB whereas cationic (CTAB,DTAB), non-ionic (Brij30, Brij35, Brij58) and the binary mixtures(CTAB + Brij58, DTAB + Brij30, SDS + Brij30) inhibit the reaction tovarying degrees. The reaction retards more in cationics than innon-ionic surfactant systems while their simultaneous presencein reaction medium retards the reaction to an intermediate value.Among the non-ionic surfactant systems, reaction was retarded inthe order Brij35 > Brij58 > Brij30 being function of number of oxy-ethylene groups in the head group of the surfactant irrespective ofhydrophobic chain length. It is quite advantageous to find thataddition of non-ionic surfactant to SDS in reaction medium leadsto significant degradation of RhB which otherwise is completelyinhibited in presence of SDS alone. This could be a remedial featurefor preventing the sustenance of RhB against �OH degradation inwastewaters which are rich in anionic surfactants.

Acknowledgments

UA acknowledges the financial support from the UniversityGrants Commision, India in the form of Junior Research Fellowship.AAD acknowledges Department of Science and Technology (DST),Govt. of India for providing funds under the FIST scheme to theDepartment of Chemistry, University of Kashmir for procuringvarious instruments.

206 U. Ashraf et al. / Chemosphere 99 (2014) 199–206

Appendix A. Supplementary material

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2013.10.074.

References

Ai, Z., Xiao, H., Mei, T., Liu, J., Zhang, L., Deng, K., Qiu, J., 2008. Electro-fentondegradation of Rhodamine B based on a composite cathode of Cu2O nanocubesand carbon nanotubes. J. Phys. Chem. C 112, 11929–11935.

Alavijeh, M.R., Javadian, S., Gharibi, H., Moradi, M., Bagha, A.R.T., Shahir, A.A., 2011.Intermolecular interactions between a dye and cationic surfactants: effects ofalkyl chain, head group, and counterion. Colloid Surface A 380, 119–127.

Alshamsi, F.A., Albadwawi, A.S., Alnauimi, M.M., Rauf, M.A., Ashraf, S.S., 2007.Comparative efficiencies of the degradation of Crystal violet using UV/hydrogenperoxide and Fenton’s reagent. Dyes Pigm. 74, 283–287.

Arbeloa, L., Ojeda, P.R., 1981. Molecular forms of Rhodamine B. Chem. Phys. Lett. 79,347–350.

Asilturk, M., Sayilkan, F., Erdemoglu, S., Akarsu, M., Sayilkan, H., Erdemoglu, M.,Arpac, E., 2006. Characterization of the hydrothermally synthesized nano-TiO2

crystal-lite and the photocatalytic degradation of Rhodamine B. J. Hazard.Mater. 129, 164–170.

Barka, N., Qourzal, S., Assabbane, A., Nounah, A., Ichou, Y.A., 2008. Factorsinfluencing the photocatalytic degradation of Rhodamine B by TiO2-coatednonwoven paper. J. Photoch. Photobio. A195, 346–351.

Behnajady, M.A., Modirshahla, N., Tabrizi, S.B., Molanee, S., 2008. Ultrasonicdegradation of Rhodamine B in aqueous solution: influence of operationalparameters. J. Hazard. Mater. 152, 381–386.

Bhat, P.A., Dar, A.A., Rather, G.M., 2008. Solubilization capabilities of some cationic,anionic, and nonionic surfactants toward the poorly water-soluble antibioticdrug erythromycin. J. Chem. Eng. Data 53, 1271–1277.

Chat, O.A., Najar, M.H., Mir, M.A., Rather, G.M., Dar, A.A., 2011. Effects of surfactantmicelles on solubilization and DPPH radical scavenging activity of Rutin. J.Colloid Interf. Sci. 355, 140–149.

Chen, C.C., Chen, W.C., Chiou, M.R., Chen, S.W., Chen, Y.Y., Fan, H.J., 2011.Degradation of crystal violet by anFeGAC/H2O2 process. J. Hazard. Mater. 196,420–425.

Clint, J.H., 1975. Micellization of mixed non-ionic surface active agents. J. Chem. Soc.Faraday Trans. 1 (71), 1327–1334.

Dar, A.A., Chatterjee, B., Das, A.R., Rather, G.M., 2006. Mixed micellization andinterfacial properties of dodecyltrimethylammonium bromide andtetraethyleneglycol mono-n-dodecyl ether in absence and presence of sodiumpropionate. J. Colloid Interf. Sci. 298, 395–405.

Dey, J., Ismail, K., 2012. Aggregation of sodium dodecylsulfate in aqueous nitric acidmedium. J. Colloid Interf. Sci. 378, 144–151.

Errico, G.D., Ortona, L., Paduano, L., Tedeschi, A., Vitagliano, V., 2002. Mixed micellaraggregates of cationic and nonionic surfactants with short hydrophobic tails. Anintradiffusion study. Phys. Chem. Chem. Phys. 4, 5317–5324.

Fan, H.J., Huang, S.T., Chung, W.H., Jan, J.L., Lin, W.Y., Chen, C.C., 2009. Degradationpathways of crystal violet by Fenton and Fenton-like systems: conditionoptimization and intermediate separation and identification. J. Hazard. Mater.171, 1032–1044.

Fendler, J.H., 1982. Membrane Mimetic Chemistry. John Wiley, New York.Fendler, J.H., Fendler, E.J., 1975. Catalysis in Micellar And Macromolecular Systems.

Academic Press, New York.Gallard, H., Laat, J.D., Legube, B., 1999. Spectrophotometric study of the formation of

iron(III)-hydroperoxy complexes in homogeneous aqueous solutions. WaterRes. 33, 2929–2936.

Ghosh, S., Mondal, S., Das, S., Biswas, R., 2012. Spectroscopic investigation ofinteraction between crystal violet and various surfactants (cationic, anionic,nonionic and gemini) in aqueous solution. Fluid Phase Equilibr. 332, 1–6.

Hassan, M., AlAhmadi, M.D., Mosaid, M., 2011. Micellar effect on the kinetics ofoxidation of methyl blue by Ce(IV) in sulfuric acid medium. Arab. J. Chem.http://dx.doi.org/10.1016/j.arabjc.2011.01.008.

Hou, M.F., Liao, L., Zhang, W.D., Tang, X.Y., Wan, H.F., Yin, G.C., 2011. Degradation ofRhodamine B by Fe (0)-based Fenton Process with H2O2. Chemosphere 83,1279–1283.

Kusic, H., Koprivanac, N., Srsan, L., 2006. Azo dye degradation using Fenton typeprocesses assisted by UV irradiation: a kinetic study. J. Photoch. Photobio. A181,195–202.

Laat, J.D., Gallard, H.E., 1999. Catalytic decomposition of hydrogen peroxide byFe(III) in homogeneous aqueous solution: mechanism and kinetic modeling.Environ. Sci. Technol. 33, 2726–2732.

Li, J., Li, L., Zheng, L., Xian, Y., Jin, L., 2006. Photoelectrocatalytic degradation ofRhodamine B by Ti/TiO2 electrode prepared by laser calcination method.Electrochim. Acta 51, 4942–4949.

Lin, S.S., Gurol, M.D., 1998. Catalytic decomposition of hydrogen peroxide on ironoxide: kinetics, mechanisms, and implications. Environ. Sci. Technol. 32, 1417–1423.

Liu, R., Li, X., Li, Y., Jin, P., Qin, W., Qi, J., 2009. Effective removal of Rhodamine B fromcontaminated water using noncovalent imprinted microspheres designed bycomputational approach. Biosens. Bioelectron. 25, 629–634.

Lu, M.C., Chen, J.N., Chang, C.P., 1999. Oxidation of dichlorvos with hydrogenperoxide using ferrous ion as catalyst. J. Hazard. Mater. 69, 277–288.

Macedo, E.R., Boni, L.D., Misoguti, L., Mendonca, C.R., Oliveria, H.P., 2011. Dyeaggregation and influence of pre-micelles on heterogeneous catalysis: aphotophysical approach. Colloid Surface A 392, 76–82.

McGregor, D.B., Brown, A.G., Howgate, S., McBride, D., Riach, C., Caspary, W.J., 1991.Responses of the L5178Y mouse lymphoma cell forward mutation assay. V: 27coded chemicals. Environ. Mol. Mutagen. 17, 196–219.

Mehrdad, A., Hashemzadeh, R., 2010. Ultrasonic degradation of Rhodamine B in thepresence of hydrogen peroxide and some metal oxide. Ultrason. Sonochem. 17,168–172.

Mehrdad, A., Massoumi, B., Hashemzadeh, R., 2011. Kinetic Study of degradation ofRhodamine B in the presence of hydrogen peroxide and some metal oxide.Chem. Eng. J. 168, 1073–1078.

Mirsalis, J.C., Tyson, C.K., Steinmetz, K.L., Loh, E.K., Hamilton, C.M., Baleke, J.P., Spald-ing, J.W., 1989. Measurement of unscheduled DNA synthesis and S-phasesynthesis in rodent hepatocytes following in vivo treatment: testing of 24compounds. Environ. Mol. Mutagen. 14, 155–164.

Moumeni, O., Hamdaoui, O., 2012. Intensification of sonochemical degradation ofmalachite green by bromide ions. Ultrason. Sonochem. 19, 404–409.

Neta, P., Huie, R.E., Ross, A.B., 1988. Rate constants for reactions of inorganic radicalsin aqueous solution. J. Phys. Chem. Ref. Data 17, 1027–1234.

Park, J.W., Chung, H., 1986. Aggregation and dissolution of cationic dyes with ananionic surfactant. Bull. Korean Chem. Soc. 7, 113–116.

Penzkofer, A., Leupacher, W., 1987. Fluorescence behavior of highly concentratedRhodamine 6G solutions. J. Lumin. 37, 61–72.

Ray, P., Bhattacharya, S.C., Moulik, S.P., 1997. Spectroscopic studies of theinteraction of the dye Safranine T with Brij micelles in aqueous medium. J.Photoch. Photobio. A108, 267–272.

Rodriguez, A., Graciani, M.M., Munoz, M., Moya, M.L., 2003. Water-ethylene glycolalkyltrimethylammonium bromide micellar solutions as reaction media:study of spontaneous hydrolysis of phenyl chloroformate. Langmuir 19,7206–7213.

Rosen, M.J., 2004. Surfactants and Interfacial Phenomenon. John Wiley, NewYork.

Rubingh, D.N., 1979. Mixed micelle solutions. In: Mittal, K.L. (Ed.), SolutionChemistry of Surfactants. Plenum Press, New York, pp. 337–354.

Sagoo, S.K., Jockush, R.A., 2011. The fluorescence properties of cationic Rhodamine Bin the gas phase. J. Photoch. Photobio. A220, 173–178.

Singh, T.R., Luwang, M.N., Srivastava, S.K., 2011. Kinetic studies on sodium dodecylsulfate micelle inhibited reactions of triphenylmethylcarbocations with cyanideion. Reac. Kinet. Mech. Cat. 104, 17–26.

Sivakumar, M., Pandit, A.B., 2001. Ultrasound enhanced degradation of RhodamineB: optimization with power density. Ultrason. Sonochem. 8, 233–240.

Spadaro, J.T., Isabelle, L., Renganathan, V., 1994. Hydroxyl radical mediateddegradation of Azo dyes: evidence for benzene generation. Environ. Sci.Technol. 28, 1389–1393.

Taha, A.A., Rahman, H.H.A., Abouzeid, F.M., 2013. Effect of surfactants on the rate ofdiffusion controlled anodic dissolution of copper in orthophosphoric acid. Int. J.Electrochem. Sci. 2013 (8), 6744–6762.

Tajalli, H., Gilani, A.G., Zakerhamidi, M.S., Moghadam, M., 2009. Effect of surfactantson the molecular aggregation of rhodamine dyes in aqueous solutions.Spectrochim. Acta A 72, 697–702.

Vringer, T.D., Joosten, J.G.H., Junginger, H.E., 1986. A study of the hydration ofpolyoxyehtylene at low temperatures by differential scanning calorimetry.Colloid Poly. Sci. 264, 623–630.

Wang, J., Jiang, Z., Zhang, Z., Xie, Y., Wang, X., Xing, Z., Xu, R., Zhang, X., 2008a.Sonocatalytic degradation of acid red B and Rhodamine B catalyzed by nano-sized ZnO powder under ultrasonic irradiation. Ultrason. Sonochem. 15, 768–774.

Wang, X., Wang, J., Guo, P., Guo, W., Li, G., 2008b. Chemical effect of swirling jet-induced cavitation: degradation of Rhodamine B in aqueous solution. Ultrason.Sonochem. 15, 357–363.

Xue, X., Hanna, K., Deng, N., 2009. Fenton like oxidation of Rhodamine B in thepresence of two types of iron (II, III) oxide. J. Hazard. Mater. 166, 407–414.

Yang, X., Xu, L., Yu, X., Guo, Y., 2008. One-step preparation of silver and indiumoxide co-doped TiO2photocatalyst for the degradation of Rhodamine B. Catal.Commun. 9, 1224–1229.

Yu, D., Cai, R., Liu, Z., 2004. Studies on the photodegradation of Rhodamine dyes onnanometer-sized zinc oxide, Spectrochim. Acta A 60, 1617–1624.

Zhang, H.M., Guo, X.Q., Zhao, Y.B., Wang, D.Y., Xu, J.G., 1998. Study on the dimer-monomer equilibrium of a fluorescent dye and its application in nucleic acidsdetermination. Anal. Chim. Acta 361, 9–17.

Zhou, Q., Rosen, M.J., 2003. Molecular interactions of surfactants in mixedmonolayers at the air/aqueous solution interface and in mixed micelles inaqueous media: the regular solution approach. Langmuir 19, 4555–4562.

Zhou, W., Zhu, L., 2004. Solubilization of pyrene by anionic–nonionic mixedsurfactants. J. Hazard. Mater. 109, 213–220.