Fe-modified local clay as effective and reusable heterogeneous photo-Fenton catalyst for the decolorization of Acid Green 25

Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

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JTICE-863; No. of Pages 9

Fe-modified local clay as effective and reusable heterogeneousphoto-Fenton catalyst for the decolorization of Acid Green 25

N.H.M. Azmi a, O.B. Ayodele b, V.M. Vadivelu a, M. Asif c, B.H. Hameed a,*a School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysiab Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiac Chemical Engineering Department, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia

A R T I C L E I N F O

Article history:

Received 27 September 2013

Received in revised form 21 January 2014

Accepted 2 March 2014

Available online xxx

Keywords:

Banda Baru clay

Photo-Fenton

Catalyst

Dye pollutant

Decolorization

A B S T R A C T

Locally sourced Bandar Baru clay (BBC) from Kedah in Malaysia was modified with NaCO3 and applied as

catalyst support. Six different Fe supported BBC (Fe-BBC) catalysts were prepared with different

calcination temperature (300–500 8C) and time (4–6 h) and their activities were tested on the

decolorization of Acid Green 25 (AG25) pollutant dye. The best Fe-BBC was that calcined at 300 8C for 4 h,

and it was characterized. The BET result showed increase in surface area and pore volume which was

supported by the SEM result. The presence of the incorporated Fe in the catalyst was verified with EDX,

XRD and FTIR techniques. The best experimental condition decolorized 99% of 50 ppm (AG25) in 20 min

with 1.25 g/L Fe-BBC300/4 and 6.7 mM of hydrogen peroxide at pH 3. Its reusability after three cycles

showed 97.5% decolorization of 50 ppm of AG25. The versatility of Fe-BBC300/4 was also verified on the

decolorization of Reactive Blue 4 and Direct Blue 71.

� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

Currently the world technological development ranging fromindustrial, pharmaceutical and other allied areas such as militaryweapon development and advance research into newer products areon the highest ever increasing rate. Consequently, the proliferationof different conventional and alien pollutants is also on the alarmingrate of increase. Current researches toward curtailing and minimiz-ing the effects of these deleterious pollutants on the environment isalso gathering momentum especially in the fields of appliedchemistry and environmental engineering bearing in mind boththe economics of the researched processes and their sustainability.Different methods of pollutant degradation and mineralization suchas biological, physical and chemical methods have been widelystudied and reported [1–5]. These methods can be used separately orcombined to enhance the overall treatment efficiency [1]. Theadvent of Fenton process which is an advance oxidation process(AOP) where ferrous ion (Fe2+) is employed to generate hydroxylradical (HO�) with sufficiently high oxidation potential of 2.8 eVfrom hydrogen peroxide (HP) (Eq. (1)) had made easy thedegradation, decolorization and even mineralization of virtuallyall known pollutants at ambient conditions [2–4]. After each cycle of

* Corresponding author. Tel.: +60 45996422; fax: +60 45941013.

E-mail addresses: [email protected], [email protected] (B.H. Hameed).

Please cite this article in press as: Azmi NHM, et al. Fe-modified local cfor the decolorization of Acid Green 25. J Taiwan Inst Chem Eng (20

http://dx.doi.org/10.1016/j.jtice.2014.03.002

1876-1070/� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V.

HO� generation (Eq. (1)) and subsequent usage for pollutantsdepuration (Eq. (2)), the Fe2+ is oxidized to Fe3+ (Eq. (1)) which mustbe reduced to Fe2+ (Eq. (3)) for another cycle of reaction. A moreeconomical and efficient reduction process for Fe3+ known as photo-reduction process is depicted by Eq. (4), which has been reported tobe superior to the route shown in Eq. (3) [2,5], the overall process isreferred to as photo-Fenton process [1–8]. In addition, the requiredHO� can also be generated from HP according to Eq. (5), a procedurereferred to as photolysis of HP [5].

Fe2þ þ HP ! Fe3þ þ OH� þ HO� (1)

Pollutant þ HO� ! Intermediate ! CO2þ H2O

þ other benign products (2)

Fe3þ þ HP ! Fe2þ þ HO2� þ Hþ (3)

Fe3þðOH�Þ þ hv ! Fe2þ þ HO� (4)

HP þ hv ! HO� (5)

Conventionally, photo-Fenton process had been practiced in thehomogeneous phase but the overall efficiency of the process isconsidered to be generally poor due to the slow rate of Fe2+

regeneration from Fe3+ [9] and inability to easily recover theFe catalyst due to the formation of ferric sludge which makes theprocess require additional processing steps like coagulation,sedimentation and filtration [6]. In addition, the process can only

lay as effective and reusable heterogeneous photo-Fenton catalyst14), http://dx.doi.org/10.1016/j.jtice.2014.03.002

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N.H.M. Azmi et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx2

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be operated within a narrow pH range [5,10]. In view of the above,the replacement of the homogeneous photo-Fenton process withheterogeneous where the active metal can be convenientlyintercalated into solid supports becomes an essential alternative.Several attempts have been made to develop supported catalystsusing rice husks, graphite and activated carbon [11–13]. Recently,we also reported the application of industrial processed clay suchas kaolin, montmorillonite and bentonite in the degradation ofdifferent pollutants in photo-Fenton [1,2,13,14] and Fenton-likeprocess [5]. All these clay supports and several others reported insome critically reviewed articles [4,8,15] were purchased asindustrial processed clay minerals which increases the cost ofcatalyst development and in turn challenged the industrialapplication of heterogeneous photo-Fenton process. In order tomake this process economically attractive, the need to substitutethese industrially refined supports with low cost and locallysourced materials such as raw clay becomes expedient.

In this report, raw clay from the paddy field at Bandar Baru,Kedah, Malaysia was modified with NaCO3 and studied asheterogeneous catalysts support for photo-Fenton process. Thecatalysts were screened for the best calcination temperature andtime on the decolorization of Acid Green 25. The best catalyst wascharacterized for physical and chemical properties, tested atdifferent operational conditions and its versatility was validatedon the decolorization of Reactive Blue 4, Acid Green 25 and DirectBlue 71.

2. Material and methods

2.1. Catalyst support preparation

The catalyst support is Bandar Baru clay (BBC) obtained fromthe paddy field at Bandar Baru in Kedar, Malaysia. Prior to its use,the gray colored BBC was grinded to powder using laboratorymortar and washed several times with double distilled water toremove impurities. The samples were dried in an oven at 100 8C for12 h followed by sieving to obtain particle size range of 63–75 mm.

2.2. Catalyst preparation

The BBC supported Fe (Fe-BBC) Fenton catalyst was prepared byimpregnation method where 0.2 M of Fe(NO3)3�9H2O obtainedfrom Merck Co. was dissolved in a beaker containing distilledwater and vigorously stirred. Na2CO3 was added to obtain a molarratio of 1:1 (Na+/Fe3+) in order to enhance the possibility ofsuccessful intercalation of the Fe3+ into the BBC support via simpleion exchange reaction where the Na+ can be replaced by Fe3+

[16,17]. In addition, it had been reported that the addition ofNa2CO3 at the catalyst synthesis stage is capable of increasing boththe surface area and pore volume [16–18]. 2.0 g of the powderedBBC was added to the Na+/Fe3+ solution and was continuouslystirred for 2 h followed by drying in an oven at 80 8C for 24 h. Thedried solid was washed several times with double distilled water toremove excess Na+ ions and re-dried in an oven at 80 8C for 12 h[16,19]. Finally, the obtained sample was calcined at temperatureand time range of 300–500 8C and 4–6 h, respectively in a mufflefurnace and labeled for instance as Fe-BBC300/4, for Fe-BBC samplecalcined at 300 8C for 4 h.

2.3. Catalyst characterization

The catalysts were characterized by nitrogen adsorption/desorption isotherm using Brunauer–Emmett–Teller (BET) meth-od, energy dispersive X-ray (EDX), Fourier transformed infraredspectroscopy (FTIR), X-ray diffraction (XRD) and scanning electronmicroscope (SEM).


Nitrogen adsorption–desorption measurements with BET meth-od were performed at liquid nitrogen temperature (�196 8C) withan autosorb BET apparatus, Micromeritics ASAP 2020, surface areaand porosity analyzer. The analysis procedure is automated andoperates with the static volumetric technique. Before eachmeasurement, the samples were first degassed at 200 8C for 2 h.EDX was performed to determine the chemical composition in thesamples using the same instrument with the SEM. FTIR analyseswere performed on the samples, using a Perkin-Elmer Spectrum GXInfrared Spectrometer with resolution of 4 cm�1, in the range of4000–400 cm�1. The sample and analytical grade KBr were dried at100 8C over-night prior to the FTIR analysis and were prepared withthe disc technique using a finely ground mixture of 0.25 mg ofsample and 100 mg of KBr. The XRD patterns of the samples weremeasured with Siemens XRD D5000 equipped with Cu Ka radiationand recorded in the range of 5–908 with a scanning rate of 28 min�1.SEM was used to study their surface morphology, the analysis wascarried out using a scanning electron microscope (Model EMJEOL-JSM6301-F) with an Oxford INCA/ENERGY-350 microanalysissystem.

2.4. Experimental procedure

All experiments were carried out in 1 L beaker filled with200 mL of AG25 dye solutions with initial concentration of 50 ppm(except for studies on the effect of initial dye concentration). Thebeaker was placed on a hot thermostated variable speed magneticstirrer. The temperature and pH was monitored using EcoScanSC11-4115 (EXATECH ENTERPRISES) probe inserted inside thebeaker. For each experimental run, the solution pH was adjustedwith the addition of 1.0 M H2SO4 or 1.0 M NaOH followed by theaddition of catalyst. The reaction was initiated immediately afterthe UV irradiation lamp (254 nm, 0.75 W/m2, model GPX9 9W,Jiangyin Juguang Photoelectric Instrument Co., Ltd., China) wasswitched on and the desired amount of HP added. The extent ofdecolorization was monitored by taken filtered samples from thereaction mixture at selected time intervals and measured usingUV–vis spectrophotometer. Similar procedure was repeated for thestudy on the versatility of the catalyst to decolorize other pollutantdyes namely Reactive Blue 4 (RB4) and Direct Blue 71 (DB71).

2.5. Analytical methods

The maximum absorbance wavelength (lmax) of AG25, RB4 andDB17 recorded from 700 to 200 nm using a spectrophotometricquartz cell in a UV–vis spectrophotometer (Shimadzu, model UV1700 PharmaSpec, Japan) was found at 642, 598 and 584 nm,respectively. The withdrawn filtered samples were quicklyanalyzed at the established wavelength to minimize experimentalerrors since the reaction could still continue after withdrawal. Theanalyzed samples were again quickly re-analyzed three times forconsistency and the average values were recorded. The decolori-zation efficiency of the dye pollutants was evaluated as follows:

Decolorization efficiency ðDcEÞ ¼ C0 � Ct

Ct

� �� 100% (6)

where C0 and Ct are the initial concentration and the measuredconcentration of the pollutants at the time of withdrawal,respectively.

2.6. Catalyst reusability test

For stability test, the catalysts were tested in three differentconsecutive experiments using fresh dye solution at the bestobserved experimental conditions. After each experimental runs,the used catalyst was filtered, washed with distilled water, and



0

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0 10 20 30 40 50 60 80 10 0 12 0 150

Dec

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f A

G25, %

Time, min

300 °C/4 h

300 °C/6 h

400 °C/4 h

400 °C/6 h

500 °C/4 h

500 °C/6 h

Fig. 1. Effect of Fe-BBC calcinations temperature and duration of time on AG25

decolorization, experimental conditions: [AG25]0 = 50 mg/L; catalyst

loading = 10 g/L; [H2O2]0 = 6 mM; pH 3; T = 30 8C.

N.H.M. Azmi et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx 3

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dried in the oven at 100 8C for 12 h followed by calcination at300 8C for 4 h to remove any occluded material before reuse.

3. Results and discussions

3.1. Effect of calcination temperature and time

Since to the best of our knowledge the BBC has not beencharacterized for any physical and chemical properties, especiallyTGA that can reveal the appropriate thermal treatment to removedifferent kinds of water such as free surface, physisorbed andstructural water in a clay sample and XRD/FTIR that can reveal theclay metal oxide phases, formation and chemistry of bondingbetween BBC and incorporated metals, there is need to firstcarryout thermal analysis on the samples at different temperatureand time. In addition, it has been reported that calcination playsvital role on the activity of heterogeneous catalyst as it affects thedegree of active metal dispersion and transformation fromcrystallinity to amorphous and vice versa [2,5,20,21]. Fig. 1 showsthe result of calcination of the catalyst at the condition ascribed fordistinctive nomenclature. It can be observed that as both thecalcination temperature and time progresses the catalystsdecolorization efficiency decreases. This could be ascribed to anumber of factors which include (1) increase in the crystal sizewhich is always occasioned with reduction in surface area and porevolume, Liu et al. [20] reported similar observation that ascalcination temperature increases both surface area and porevolume showed an obvious decrease. Therefore, it can be inferredthat this was responsible for the reduction in the catalytic activitysince the amount and structure of the active metal exposed on the

Table 1Fe ions leach out from the catalysts calcined at different calcination temperature and

Catalyst samples

300 8C

4 h 6 h

Fe-BBC 1.284 1.096

Table 2Textural property of BBC and Fe-BBC300/4.

Sample Surface area (m2/g) Pore size (nm) Pore volume (cm3

BBC 6.96 11.6 0.021

Fe-BBC300/4 35.44 5.23 0.027


catalyst support surface was significantly reduced due to theoccurrence of crystallization or phase transformation. (2) Accord-ing to Oh et al. [22] as the calcination temperature increases,sharps peaks also emerges and increases, an indication of increasein the degree of crystallization which resulted from increase in theaverage crystallite size (i.e. crystal growth) due to a propensity forminimization of the interfacial surface energy. These observationsin conjunction with the recent report of Jia et al. [23] whichreported a linear relationship between calcination temperatureand crystal growth (crystallinity) but an inverse correlation withcatalytic activity explains and justify why higher calcinationtemperature in this study resulted into comparably lowerdecolorization efficiency. The deductions made for calcinationtemperature is also logical for the calcination time studies since thecalcined samples will receive more thermal treatment withincreased time and hence more crystal growth, for example, thecatalyst sample at 500 8C for 6 h showed a clear departure fromthat of 4 h sample in Fig. 1.

3.2. Effect of calcination temperature and time on Fe ion leaching

The effect of calcination temperature and time on the Fe2+ ionleaching from the BBC support is shown in Table 1. It is obviousthat as calcination temperature and time increases, the amount ofFe2+ leached reduces. This can be explain from two standpoints, asearlier elucidated that higher calcination temperature reducessurface area which in turn reduces the amount of Fe2+ that isavailable and exposed (active metal occlusion) at the surface of theBBC support for reaction [20], therefore leaching is more imminentat lower calcination temperature since large portion of the activemetal are present at the surface. Secondly, a highly reactivecatalyst is liable to suffer more active metal leach out due torelatively higher interaction between the catalyst active site usedfor molecular cleavage and the reacting molecules, which seldomleads to several attempts of active metal exclusion from thesupport under the reaction hydrodynamics as the products formedare detached and diffused into the bulk of the reaction [2,4,5].

3.3. Catalyst characterization

From the decolorization efficiency and Fe leaching studies, Fe-BBC calcined at 300 8C for 4 h appeared to be the best catalyst,hence further study such as characterization and testing atdifferent experimental conditions were based on it.

3.3.1. Nitrogen adsorption–desorption isotherms (BET)

The textural properties of the BBC and Fe-BBC300/4 samples areshown in Table 2 and Fig. 2. The surface area and the pore volumeof the Fe-BBC300/4 can be seen to have increased relative to theBBC. This could be ascribed to the addition of Na2CO3 to the catalyst

time.

Fe concentration (ppm)

400 8C 500 8C

4 h 6 h 4 h 6 h

1.190 0.274 0.569 0.276

/g) Elemental composition (wt%)

Si Al K S O Fe Mg

34.91 14.58 1.82 1.93 42.58 2.82 1.35

33.33 13.07 5.02 0.66 40.25 6.56 1.11



0

5

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15

20

25

10.80.60.40.20

Quan

tity

adso

rpti

on (

cm3/g

ST

P)

Relative pressure (P/Po)

BBC Adsorption

BBC Desorption

Fe-BBC Adsorption

Fe-BBC Desorption

Fig. 2. Nitrogen adsorption/desorption isotherms of BBC and Fe-BBC300/4 sample.

4006001000 1400180024003200 4000

Wavenumber (cm-1)

Fe –BBC 300 °C / 4 h

BBC

3699

3621

3421

1637

1031

912

796778

693

534

470431

3698

3621 3401

1637

1031913

796778

693

534470427

Tra

nsm

itta

nce

(%

T)

Fig. 3. FTIR Spectra of BBC and Fe-BBC300/4 samples.

0

3000

6000

9000

12000

15000

18000

21000

1009080706050403020100

Inte

nsi

ty (

a.u

)

2θ°

36°54.9°

26.8°

21.8°BBC

Fe-BBC300 °C/ 4 h50.1° 60° 68.1°38.5°

43°

35.5°

Fig. 4. XRD Patterns of BBC and Fe-BBC300/4 samples.


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precursor at the synthesis stage which permit the intercalation ofNa+ into the intergallery layer of the clay mineral [4,5,7,18] andprobably the effect of calcination which is capable of burning offvolatile and organic matter [2]. This observation is at variance withwhat most researchers [6–11,19] in heterogeneous catalyst havereported, in those reports, reduction in surface area is alwaysobserved due to partial occlusion of the catalyst and othermaterials that cannot be completely removed at the washing andcalcination stages. We had also reported [5] a similar observationin a study where copper pillared bentonite was applied as photo-Fenton catalyst in the degradation of 4-nitrophenol. The observa-tion was based on the novel catalyst synthesis method whichpermits Fe bonding to Cu to form cuprospinel Q (CuFe2O4) thatwidens the bentonite basal spacing. From Fig. 2, the nitrogenadsorption and desorption isotherms of the BBC and Fe-BBCsamples corresponds to the type IV in the Brunauer, Deming,Deming and Teller (BDDT) classification [5,13]. The hysteresis aresimilar to type H4 which suggests that the slit shape layeredstructure was not destroyed during catalyst synthesis, but wasdisordered in form and size as revealed by the filling of amonolayer at relatively low pressures followed by buildup ofmultilayers until capillary condensation sets in [13]. The incre-ment in the surface area and pore volume observed in Table 2 canalso be confirmed in Fig. 2 as the hysteresis loop of the Fe-BBC300/4 showed more quantity of adsorbed/desorbed N2.

3.3.2. Elemental dispersive X-ray (EDX)

The result of analysis of elemental composition shown inTable 2 revealed that the BBC has a Si/Al ratio of �2.4 whichsuggested that the clay mineral belongs to the smectite groupwhich includes dioctahedral smectites like montmorillonite andnontronite, and trioctahedral smectites such as saponite [24].Smectites are commoly referred to as a 2:1 clay, i.e. oneoctahedral sheet sandwiched in between two tetrahedral sheets[4,8,24]. The composition of Fe is seen to have increased from2.82% which is the average characteristic of smectites to 6.56%after successful incorporation of Fe and calcination. Althoughthis value is comparably lower compared to the calculated valueof 7.2%, this observation is similar to what we had earlierreported [1], and it can be ascribed to the high hydration degreeof solid at the stages of the preparation procedure. The effect ofcalcination could have led to the reduction in the value ofsulphur probably due to the evolution of SO2 and also thereduction in the value of oxygen confirms the reduction in boththe loosely bonded water and bending vibration mode ofphysisorbed water on the surface of free silica as seen in theFTIR plot.


3.3.3. Fourier transform infrared spectroscopy (FTIR)

The result of FTIR analysis is presented in Fig. 3, and it shows thevariation in the clay mineral chemistry after successful incorpo-ration of Fe and calcination. The octahedral structural hydroxylgroup bands at 3699, 3621 and 3401 cm�1 which are seen in the O–Hstretching region can be assigned to the Al–OHsrt, with absorption at3621 cm�1 in the inner OH groups sandwiched between theoctahedral and tetrahedral sheets of smectite clay minerals [5,24].The band at 3401 cm�1 in BBC shifted to 3421 cm�1 in Fe-BBC300/4after incorporation of the catalyst and reduced in intensity due tocalcination which minimizes the amount of the BBC structuralwater. The peaks at 1637 cm�1 in BBC is due to the presence ofloosely bonded water molecules that are present in the firstcoordination sphere of the interlayer space [5,25], after calcinationat 300 8C the peak reduced considerably in Fe-BBC300/4. Thedecreases of the peaks in the range of 1090–400 cm�1, are detectedfor metal intercalated smectite clays, which indicates that the Fepolyoxocations could link with Al–O in the alumina octahedral sheetand Si–O in the silica tetrahedron plates [1].

3.3.4. X-ray diffraction (XRD)

The XRD patterns of the BBC and Fe-BBC300/4 samples areshown in Fig. 4. The main mineral found in the BBC is quartz (00-033-1161) with sharp and small diffraction peaks at 2u of 21.88,26.88, 36.58, 50.28, 558, 608 and 688. Quartz is a continuousframework of SiO4 silicon–oxygen tetrahedral sharing each oxygenbetween two tetrahedrals and gave the overall formula of SiO2.This observation supports the EDX findings which showed that Siand O elements are dominant in BBC sample. After the calcinationprocess, XRD results obtained for Fe-BBC300/4 did not showsignificant change on its crystalline structure compared to startingmaterial. This according to Unuabonah et al. [26] suggested thatthe incorporation of Fe ions was effective only on the surface of theclay and not on the crystal of the clay mineral. This confirmed why



Fig. 5. SEM image of (a) BBC and (b) Fe-BBC300/4 samples (Mag X 5000).


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there is no serious variation in the spectra of BBC and Fe-BBC300/4,this is also in accordance with the FTIR plot that does not showconsiderable change around band 900–950 cm�1 that has beenpopularly reported for the presence of Fe in Fe–O–Fe and Si–O–Febond structure formation [1,2,5,27]. This could be due to thecomparable lower amount of Al (EDX result) in the BBC which isrequired for the substitution of Fe ions into the BBC matrix, andprobably the effect of hydration during catalyst developmentwhich caused a reduced variation in the amount of Fe calculatedand the amount shown in the EDX result. However, even thoughboth BBC and Fe-BBC300/4 showed almost coincided peaks, theXRD pattern of Fe-BBC300/4 revealed baseline increment after Feincorporation and calcination, which is a validation of the presenceof Fe. The peaks of iron oxide related to 2u = �358 and 438 can beassigned to Fe3O4, while �368 corresponds to FeO, and those of38.58 and 41.38 correspond to a-Fe2O3 [1,27–29].

3.3.5. Scanning electron microscope (SEM)

The morphology of BBC and Fe-BBC300/4 samples examinedusing SEM is shown in Fig. 5. The incorporation of Fe into the BBCand calcination can be seen to have developed some large voids inthe Fe-BBC300/4. In addition, the coalesced particle-like appear-ance of BBC has also transformed into choppy layered structurecharacterized with increased pores and curved flake-like layerswhich confirmed that the addition of Na2CO3 and calcinationactually increase the pore volume and surface area as earlier seenin the BET result.

3.4. Effect of UV light/catalyst support/catalyst/hydrogen peroxide

The result in Fig. 6 shows the combination of various photo-Fenton process parameters to understudy the possible contribution

0

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80

100

1501209060300

Dec

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f A

G25, %

Time, min

UV alone

HP + UV

BBC alone

Fe-BBC alone

BBC + HP + UV

Fe-BBC + HP (DarkFenton)

Fe-BBC + HP + UV(Photo-Fenton)

Fig. 6. Profile of preliminary experiments on AG25 decolorization efficiency using

BBC and Fe-BBC300/4, experimental conditions: [AG25]0 = 50 ppm; Fe-BBC300/4

loading = 1.0 g/L; BBC loading = 1.0 g/L; [H2O2]0 = 6 mM; pH = 3; T = 30 8C.


of each parameter. The application of only UV does not yield anyresult within 120 min of study since both the HP that is required togenerate the HO� and the Fe catalyst were not present. However,when HP was added to the system in the presence of UV, the DcErecorded was 32% in 120 min. This suggests that the generation ofthe HO� was achieved via photolysis of HP as shown in Eq. (5). In astudy where only the BBC was tested, the DcE was about 10% in120 min, even when BCC was replaced by Fe-BBC, there was still noimprovement in the DcE because both UV and especially the HPrequired to generate the reactive HO� are absent, thus the observedDcE can be ascribed to their adsorptive capacities. However, whenboth HP and UV were added to the BBC, there was appreciableincrease to about 61% in DcE within 120 min. This increase is due tothe combined contributory role of BBC adsorptive capacity,photolysis of HP and probably partial generation of HO� from HPbased on the little in situ Fe present in the BBC as seen in the EDXresult. Decolorization of AG25 in dark Fenton process where only Fe-BBC300/4 and HP were employed showed more improved DcE ofabout 92% in 120 min, this signify that Fe-BBC300/4 has highercatalytic activity at generating the required HO� in an alternativeroute as depicted in Eq. (3). The consequence of this is that the overallprocess HP requirement will increase. Finally, the combination of allthe variables achieved over 96% DcE in 60 min, this incredibleobservation is due to the presence of UV assistance in the photoreduction of Fe3+ to Fe2+ (Eq. (4)) and the process is also additionalsource of HO� generation. This confirmed earlier study [14] usingartificial neural network (ANN) to model degradation efficiency ofamoxicillin in photo-Fenton process that catalyst and HP dosage aswell as treatment time and pollutant concentration plays highsignificant roles.

3.5. Effect of initial pH on the decolorization of AG25

The contributory role of pH in a photo-Fenton process is veryfundamental as it affects Fe activity and stability, HP stability andthe hydrolyzation of the HO� and its activities. Fig. 7 shows that pH3 is the best for the decolorization of AG25, which is in accordancewith established trends in classical photo-Fenton process [4]. At pH4 and 5 the DcE decreased sharply signaling that the HO� formationis being retarded due to the hydrolysis of Fe(III) and possibleprecipitation of FeOOH from the solution. Similar observationswere obtained by several researchers as recently reviewed byHerney-Ramirez et al. [4].

3.6. Effect of hydrogen peroxide dosage on the decolorization of AG25

The result of the effect of HP concentration on the decoloriza-tion of AG25 in Fig. 8 shows that HP concentration has significanteffect on the efficiency of photo-Fenton process as earlier seen in



0

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6040200

Dec

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f A

G25, %

Time, min

3.3 mM of HP

6.7 mM of HP

10 mM of HP

13.3 mM of HP

Fig. 8. Effect of hydrogen peroxide concentration on AG25 decolorization efficiency,

experimental conditions: [AG25]0 = 50 ppm; Fe-BBC300/4 loading = 1.25 g/L; pH 3;

T = 30 8C.

0

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6050403020100

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G25, %

Time, min

0.75 g/L Fe-BBC300/4

1.25 g/L Fe-BBC300/4

1.75 g/L Fe-BBC300/4

Fig. 9. Effect of Fe-BBC300/4 loading on AG25 decolorization efficiency,

0

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6050403020100

Dec

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f A

G25, %

Time, min

pH 3

pH 4

pH 5

Fig. 7. Effect of pH on AG25 decolorization efficiency, experimental conditions:

[AG25]0 = 50 ppm; Fe-BBC300/4 loading= 1.25 g/L; [H2O2]0 = 67 mM; T = 30 8C.


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Fig. 6. The best HP dosage that decolorized AG25 efficiently is6.7 mM HP. When the concentration is lower, the DcE reducedbecause of insufficient HP to generate the required HO�. The DcEalso reduced when the HP concentration was increased to 10 and13.3 mM, this can be explained from the possibility of scavengingof HO� at higher concentrations of HP [5–9] as illustrated by Eq. (7).Similarly, the undesirable hydroperoxyl radical (HO2

�) with farlower comparable oxidation potential that was generated in Eq. (7)can have additional scavenging effect on the HO� (Eq. (8)). Therecould also be possibility of HO� dimerization when produced inexcess of requirements [5] as shown in Eq. (9) since the HPconcentration is directly related to the number of HO� generated[4]. The culmination effect of all these has a diminishing return onthe overall process efficiency as clearly seen in Fig. 8., however, thescavenging effect is more pronounced at shorter reaction times,since initial rate of reaction is generally higher at high initialreactants concentration, this trend is also similar to other results[13,16–18].

HP þ HO� ! HO2� þ H2O (7)

HO2� þ HO� ! H2O þ O2 (8)

HO� þ HO� ! HP (9)


3.7. Effect of catalyst loading on the decolorization of AG25

Fig. 9 shows the result of Fe-BBC300/4 loading in thedecolorization of AG25 at the earlier established best pH and HPdosage. The result followed well known trend in photo-Fentonprocess, i.e. increase in Fe-BBC300/4 loading from 0.75 to 1.25 g/Lresulted in enhanced DcE from �78 to 91.2% in 30 min becausemore catalyst active sites were available for hydroxylation of theHO�. However, as the catalyst loading was increased from 1.25 to1.75 g/L, the DcE decreased to 82.5% after 30 min. This decrease inDcE could be ascribed to the excessive loading of Fe which also hasscavenging effect on the HO� [1–5] as shown in Eq. (10). In addition,this deleterious effect could also be partly attributed to theturbidity of the suspension that would cause a relevant fraction ofthe incident radiation to be lost via scattering and not beingabsorbed, thereby minimizing the contributory effect of the photoreduction process [4,11].

Fe2þ þ HO� ! Fe3þ þ OH� (10)

3.8. Effect of initial dye concentration on the decolorization of AG25

The result of effect of initial concentration of AG25 is shown inFig. 10. At lower initial concentration, the DcE is comparably higherthan at higher concentration. When initial concentration is 10 ppmover 96% DcE was recorded in the first 20 min compared to the79.8% recorded for 100 ppm within the same reaction time. Thisreduced efficiency could be ascribed to a number of factors such asinsufficient amount of HO� required for the decolorization sincethe concentration of both the HP and Fe-BBC300/4 are notincreased when the AG25 concentration was increased. Similarly,based on Langmuir–Hinshelwood-type mechanism, as the AG25concentration is increased, the number of active sites on the Fe-BBC300/4 that is available for hydroxylation process is decreaseddue to the presence of adsorbed AG25 molecules. Consequently,there would be competitive adsorption on the Fe-BBC300/4 activesurface between AG25 and HP molecules [4,10,16]. Since thepollutant under consideration is a colored dye, higher initialconcentration will have a deeper color that could affect the UVlight penetration depth thus causing a slower decolorization. Thisphenomenon is ascribable to the characteristics of the UV visibleabsorption spectrum of the dye, which is still significant at 254 nm,hence the solution with an initial higher dye concentration willabsorb significant fraction of the emitted UV light at 254 nm thanthat with a lower initial concentration. Therefore the number of

experimental conditions: [AG25]0 = 50 ppm; [H2O2]0 = 67 mM; pH = 3; T = 30 8C.



0

20

40

60

80

100

6050403020100

Dec

olo

riza

tion e

ffic

iency

, %

Time, min

RB4

DB71

Fig. 12. Decolorization efficiency of RB4 and DB71, experimental conditions:

[dye]0 = 50 ppm; Fe-BBC300/4 loading = 1.25 g/L; [H2O2]0 = 67 mM; pH 3; T = 30 8C.

0

20

40

60

80

100

6050403020100

Dec

olo

riza

tion o

f A

G25, %

Time, min

10 ppm of AG25

20 ppm of AG25

50 ppm of AG25

100 ppm of AG25

Fig. 10. Effect of initial concentration of AG25 on decolorization efficiency,

experimental conditions: Fe-BBC300/4 loading = 1.25 g/L; [H2O2]0 = 67 mM;

pH = 3; T = 30 8C.


G Model


available photons decreases which in turn reduces the photoreduction stage shown in Eq. (4). However, the initial rate of reactionwas higher when the initial concentration was higher, for exampleafter 10 min of reaction time, over 64 ppm had been decolorizedfrom the 100 ppm, while in the same reaction time only about9.2 ppm was decolorized from the 10 ppm experimental run, thissuggests that the process is strongly concentration controlled.

3.9. Effect of reaction temperature on the decolorization of AG25

The contributory effect of temperature on the DcE of AG25 isshown in Fig. 11. The increase in reaction temperature from 30 to50 8C significantly enhanced the DcE from 81.4 to 98.2% within20 min reaction time. This enhancement is due to the increase inthe rate of hydroxylation (i.e. Arrhenius dependence of rateconstants), because both the collision frequency of molecules atthe surface of the catalyst and the fraction of molecules thatpossesses energy in excess of the activation were increased [18].Similarly, since temperature is a thermodynamic state functionthat enhances the feasibility of a chemical process, it could alsohave a significant role at enhancing the extent of decolorization ofAG25 with the generated HO�.

0

20

40

60

80

100

6050403020100

Dec

olo

riza

tion o

f A

G25, %

Time, min

30 °C

40 °C

50 °C

Fig. 11. Effect of reaction temperature on AG25 decolorization efficiency,

experimental conditions: [AG25]0 = 50 ppm; Fe-BBC300/4 loading = 1.25 g/L;

[H2O2]0 = 67 mM; pH 3.


3.10. Effect of Fe-BBC300/4 catalytic activity on other dye pollutant

Based on the brilliant catalytic activity of the Fe-BBC300/4 onthe decolorization of AG25, its versatility was also verified on thedecolorization of Reactive Blue 4 (RB4) and Direct Blue 71 (DB71)at the best experimental conditions ([Dye]0 = 50 ppm; Fe-BBC300/4 loading = 1.25 g/L; [H2O2]0 = 6.7 mM; pH 3) earlier establishedfor AG25. The results in Fig. 12 show that DB71 is more easilydecolorized compared to RB 4, especially at the initial reactiontimes, for example, DB71 and RB4 achieved 94 and 81.3% DcE,respectively in 20 min. The higher DcE and faster decolorizationrate observed in RB4 could be due its simpler chemical structuresand relatively lower molecular mass as compared to the DB71which has 17, 14, 1, 4 and 2 numbers of C, H, N, O and S atoms,respectively higher than RB4, in addition, it contains one Na atom(Appendix A). Hence, DB71 will require more decolorization timeas well as higher Fe-BBC300/4 loading and HP dosage. Similarly,the comparably larger number of atoms will increase the tendencyof formation of larger quantity of intermediate compounds [5]which will require more decolorization time.

3.11. Catalyst reusability and leaching test

Fig. 13 shows the result of three reusability studies on the Fe-BBC300/4 for the decolorization of AG 25 with 98.6, 98 and 97.5%DcE after 1st, 2nd and 3rd reuse, respectively, during a 60 minstudy. The higher reusability of the catalyst can be ascribed to its

0

20

40

60

80

100

1st run 2nd run 3th run

Dec

olo

riza

tion o

f A

G25, %

Experimental runs

Fig. 13. Plot of catalysts reusability on AG25 decolorization process at the best

operating conditions, experimental condition: [AG25]0 = 50 ppm; Fe-BBC300/4

loading = 1.25 g/L; [H2O2]0 = 67 mM; pH 3; T = 30 8C; time = 60 min.




G Model


preparation protocols, especially the addition of Na2CO3 and thecareful selection of calcination conditions. In addition, the ability ofthe BBC to provide sufficient anchor for the Fe ion on the extendedsurface area provided by the addition of Na2CO3 made BBC apromising heterogeneous catalyst support for industrial applica-tion. Proper calcination has also been reported to have the capacityto transform a metal from crystalline form into amorphous therebyminimizing its deleterious removal tendencies [5,30]. To corrobo-rate this further, inductive plasma coupling was used to detect theamount of Fe leached into the solution during the process and theamount measured was 1.096, 0.68 and 0.47 ppm during the 1st,2nd and 3rd reuse, respectively. These values showed superiorityto about 3 ppm reported in the literature [16] using industriallymodified support.

4. Conclusion

The locally sourced Bandar Baru clay (BBC) modified withNa2CO3 has demonstrated excellent result in the decolorization of

Appendix A. Properties of Acid Green 25, Reactive Blue 4 and Dire

Synthetic dye Acid Green 25 (AG25) React

Empirical formula C28H20N2Na2O8S2 C23H1

Molecular weight (g/mol) 622.58 637.4

lmax 642 598

Molecular structure


AG25 pollutant dye in batch photo-Fenton process. The addition ofthe Na2CO3 to the catalyst at the synthesis stage enhances both thesurface area and the pore volume thereby providing sufficientsurface area for the incorporation of the Fe ions. The catalystsscreening process via calcination temperature and time providedadditional information on their effect on the decolorizationefficiency. The best calcination condition is 300 8C and 4 h. Thebest experimental condition to decolorize 99% of 50 ppm of AG25was 1.25 g/L Fe-BBC300/4 loading, 6.7 mM [H2O2]0 and pH 3 in20 min. The catalyst reusability study confirmed that Fe leaching isminimal compared to some industrially modified support in theliterature. These results were satisfactory to recommend BBC as anovel catalyst support to be explored.

Acknowledgment

Support of Deanship of Scientific Research grant for ResearchGroups RGP-VPP-292 at King Saud University is appreciated by M.Asif.

ct Blue 71

ive Blue 4 (RB4) Direct Blue 71 (DB71)

4Cl2N6O8S2 C40H28N7NaO13S4

3 965.94

584

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Fe-modified local clay as effective and reusable heterogeneous photo-Fenton catalyst for the decolorization of Acid Green 25