Chemosphere 84 (2011) 1542–1547

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Removal of Rhodamine B under visible irradiation in the presence of Fe0,H2O2, citrate and aeration at circumneutral pH

Jun Hong a,b,⇑, Sijia Lu a, Caixiang Zhang b, Shihua Qi a,b, Yanxin Wang a,b

a School of Environmental Studies, China University of Geosciences, Wuhan 430074, PR Chinab Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 January 2011Received in revised form 25 May 2011Accepted 27 May 2011Available online 22 June 2011

Keywords:RhBDegradationFe0

H2O2

CitrateCircumneutral

0045-6535/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2011.05.056

⇑ Corresponding author at: School of EnvironmentaGeosciences, Wuhan 430074, PR China. Tel.: +8687436235.

E-mail address: hongjun18888@163.com (J. Hong)

A new Vis–Fe0–H2O2–citrate–O2 system comprising zero-valent iron, hydrogen peroxide, citrate anionand aeration at circumneutral pH under visible irradiation was studied. 21 lmol L�1 of Rhodamine B(RhB) was chosen as the substrate to be tested. Experiments were conducted under conditions of2.9 mmol L�1 of H2O2, 12.6 g of Fe0 and 1.0 mmol L�1 of citrate at pH 7.5. Results showed that, in 1 h reac-tion, 54% of RhB was removed with corresponding 26% of COD reduced. Meanwhile, the amount ofreleased dissolved irons from Fe0 surface was found to be at a very low level as <5.4 lmol L�1. Extinguish-ing tests with isopropanol suggested that RhB oxidation by hydroxyl radicals was the main process takenplace in Vis–Fe0–H2O2–citrate–O2 system, which accounted for 75% of substrate removal in 3 h reaction.Control and factor influencing experiments showed that the prohibitive extents of individual factorimportance on RhB removal followed a decreasing order of Fe0 > H2O2 > citrate > Vis > O2. This studyshowed an excellent system that could remove refractory organic compounds from water in laboratoryresearches, and also provided a good idea to reduce secondary contamination by dissolved irons in futureinvestigations.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction and Peres, 2007; Dong et al., 2008; Ay et al., 2009). However, a dis-

Dyestuff polluted wastewater may become visual eyesores, andis found to be toxic and carcinogenic to aquatic environments(Pierce, 1994). In China, more than 1.6 � 109 m3 per year of dyecontaining wastewater are drained into environmental water sys-tems without treatment (Liu et al., 1999). Vis-Fenton (The Fentonreaction under visible irradiation) is one of the advanced technol-ogies to treat these wastewater, and has intrigued scientists world-wide because of its low cost when visible irradiation was usedwhich occupied 95% of solar energy (Sun et al., 2005). MeanwhileVis-Fenton, like traditional Fenton reaction, also has good perfor-mance on dyes oxidation due to �OH (+2.8 eV) that is able todecompose most organic compounds and even mineralize theminto CO2 and H2O (Bozzi et al., 2002). In that process, Fe(II) is trans-formed into Fe(III) (reaction 1). While regeneration of Fe(II),though can proceed slowly as reaction 2, mainly occurs throughphoto-transformation of Vis-sensitive complex, such as polycar-boxylate (citrate, oxalate or malonate) with Fe(III) (reaction 3)(Wu and Deng, 2000). To obtain a high efficiency for Vis-Fentonreaction, most studies necessitated a dissolved iron level of>0.2 mmol L�1 (Selvam et al., 2005; Katsumata et al., 2006; Lucas

ll rights reserved.

l Studies, China University of27 67883153; fax: +86 27

.

solved iron level at 5.4 lmol L�1 (i.e. 0.3 mg L�1), 37 times lowerthan above Vis-Fenton studies, was required by EnvironmentalQuality Standards for Surface Water of China (GB 3838-2002),and by Drinking Water Quality of China (GB 5749-2006). This ironstandard was also suggested by National Secondary DrinkingWater Regulations from EPA, US, which had listed iron as one ofthe contaminants that might cause cosmetic or aesthetic effects(http://water.epa.gov/drink/contaminants/index.cfm). If a numberof dissolved irons were used in Vis-Fenton treatment, it wouldneed more steps to get rid of them, for example, filtration by sand,adsorption by active carbon, but these cost money. Otherwise itwould be hazardous to the eco-system when they are dischargeddirectly into environments at such a high level. Unfortunately, lit-tle attention had been paid on it when Vis-Fenton technology wasinvestigated.

H2O2þFeðIIÞ ! FeðIIIÞ þHO�þHO�k1 ¼ 40� 60 L ðmol sÞ�1 ð1Þ

H2O2þFeðIIIÞ ! FeðIIÞ þHþþHO�2K2 ¼ 2� 10�3 L ðmol sÞ�1 ð2Þ

FeðIIIÞ � organic acid complex!VisFeðIIÞ þ organic radicals ð3Þ

As an environmental remediation technology, Fe0 had madegreat progress in both fundamental understanding andpractical applications in the past decades (Reynolds et al., 1990).

J. Hong et al. / Chemosphere 84 (2011) 1542–1547 1543

Many Fe0 induced transformations are reductive (reaction 4)(Matheson and Tratnyek, 1994; Klausen et al., 2003; Song andCarraway, 2005). On the other hand, Fe0 can take advantage of oxi-dative reactions in the presence of H2O2, which generates �OHthrough Fenton reaction (Bremner et al., 2006), or via reductionof O2 on Fe0 surface leading to OH radical-induced oxidation inthe absence of H2O2 addition (reaction 5) (Lee et al., 2007), andaccompanying continuous accumulation of Fe(III). To maintainthe oxidation ability of Fe0 system, a consecutive release of Fe(II)from Fe0 surface is needed. As a result, the accumulation of dis-solved irons is enhanced, thus facing a problem of getting rid ofthe irons. Alternatively, if combining visible irradiation and organicacid into Fe0 system, regeneration of Fe(II) will occur throughphoto-transformation of Fe(III)-organic acid complex under visibleirradiation (Abrahamson et al., 1994; Hug et al., 1997; Nad-tochenko and Kiwi, 1998). That closes a loop mechanism to giverise to additional �OH, enhancing the oxidative ability of Fe0 systemin the presence of H2O2 or O2. At the same time, the complex can beused as a pH buffer, extending solution pH from acidity to neutralone (Sun and Pignatello, 1992), therefore decreasing iron corrosionrate at the neutral pH (Katsoyiannis et al., 2008). Due to this rea-son, it was possible to suppress the concentration of dissolved ironat a low level in a neutral Vis–Fe0–H2O2–citrate system. To ourknowledge, however, most previous studies on this aspect wereconducted by using Fe(III) or Fe(II), other than Fe0, under Vis orUV irradiation. For example, Katsumata et al. (2006) built a UV/Fe(II)–citrate/H2O2 degradation system with 0.1 mmol L�1 Fe(II)at pH up to 8.0. Silva et al. (2007) investigated a degradation sys-tem of UV-Fenton and ferric citrate complex (1.0 mmol L�1) at neu-tral pH. Lucas and Peres (2007) used UVC/Fenton and ferrioxalate/H2O2/solar light processes with 0.15 mmol L�1 dissolved iron todegrade Reactive Black 5, and so on (Selvam et al., 2005; Donget al., 2008). For the above researches, little attention was paidon the overdose of dissolved iron, and few attempts had been doneon integrating Fe0 with visible irradiation.

Fe0 þ SOX ! FeðIIÞ þ Sred ð4Þ

Fe0 þ O2 þ 2Hþ ! FeðIIÞ þH2O2 ð5Þ

Our group was trying to develop a new Vis–Fe0–H2O2–citrate–O2 system at circumneutral pH. Iron nails, instead of iron powders,were used as Fe0 to avoid difficulties in removing fine particles. Acommercial energy-saving lamp, rather than expensive deuteriumor xenon lamp, was used throughout experiments. Rhodamine B(RhB), a representative dye pollutant of industrial wastewater,was chosen as the substrate. The purpose of our research was toevaluate the degradation efficiency and Fe0 corrosion in Vis–Fe0–H2O2–citrate–O2 system, also the degradation mechanism andthe influencing parameters were what we were trying to exploreand investigate. We hope this work may provide a new idea for fu-ture Vis-Fenton’s researches and applications.

2. Materials and methods

2.1. Materials

RhB (CAS No. 81-88-9), H2O2 (30%), sodium citrate, H2SO4, NaOH,1, 10-phenanthroline monohydrate, ferrous ammonium sulfate, gla-cial acetic acid, ammonium acetate, hydroxylamine hydrochloride,hydrochloride, and isopropanol were of analytic grade. All solutionswere prepared using double distilled water with an automatic dou-ble-distillation pure water device (Shanghai Yarong BiochemicalInstruments Co., Ltd. China). Iron nails (each nail was in the samespecification: 2.3 mm for outer diameter, 42.0 mm for length, and1.4 g for weight) were purchased from a grocery store in the campus

of China University of Geosciences (Wuhan). They were pretreatedwith 1 M HCl for 5 min to remove surface impurities, then werewashed with double distilled water for three times before using.

2.2. Experimental procedure

A desired concentration of RhB solution was dumped slowlyinto a glass beaker followed by additions of iron nails and sodiumcitric. An aerator (RS-8801, Risheng electric equipment Co. inZhongshan city, China) was connected to a diffuser via a plasticpipe (0.3 cm for inner diameter) to supply the solution with oxy-gen. An 85 W energy-saving lamp (ZSZ-10, Kexing IlluminantEquipments Co., Ltd. in Changsha city, China) with a shade wasset vertically 10 cm high over the up surface of the solution. Theilluminance at 10 cm beneath the lamp was measured to be7.22 � 102 l� with a luminometer (ST-80C, Peking Normal Instru-ments Co., Ltd.). Reaction was timed when H2O2 was added intothe solution. Sample aliquots of 2.5 mL were withdrawn at a regu-lar time interval, and were measured immediately. Triple experi-ments were carried out at given conditions. Blank experimentswithout RhB were conducted to eliminate effects of other factorsin the system.

2.3. Analysis

Fe(II) concentration was determined spectrophotometrically(absorbance measurement at 510 nm) by 1, 10-phenanthrolinemethod (Fortune and Mellon, 1938). Total dissolved irons(Fe(II) + Fe(III)) were analyzed when ferric irons were reduced toferrous by hydroxylamine hydrochloride. Considering possibleinterference in phenanthroline method by complexation of sodiumcitric with dissolved iron, experimental verification was conductedusing an atomic absorption spectrophotometer (AAS) (TAS-990,PERSEE in China) to calibrate the concentration of total dissolvediron. Results show that dissolved irons from AAS measurement in-creased linearly with those from phenanthroline, and regression onthese data gave

[AAS measured dissolved iron] = 0.9673 � [phenanthrolinemeasured dissolved iron] – 0.1139. Where r2 = 0.9925 for n = 6,experimental conditions were 21 lmol L�1 of RhB, 2.9 mmol L�1

of H2O2, 12.6 g of Fe0, 1.0 mmol L�1 of citrate, pH 7.5, with aeration.This provided evidence for the conclusion that it was accurate toanalyze dissolved iron by phenanthroline method in this work.

RhB concentration was determined with a 7200 visible spectro-photometer (Unico (Shanghai) Instruments Co., Ltd.) at a wave-length of 550 nm. UV–Vis spectrums for certain samples werescanned by a UV-2800 UV–Vis spectrophotometer (Unico (Shang-hai) Instruments Co., Ltd.). The pH was measured using a pH meter(PHS-3C, Shanghai Precision Scientific Instruments Co., Ltd.). CODCr

was determined by titration of potassium dichromate. Dissolvedoxygen (DO) was measured with a DO instrument (HQ 30d, HACH).

3. Results and discussion

3.1. Decomposition of RhB by Vis–Fe0–H2O2–citrate–O2 system atneutral pH

RhB was selected as the target substrate because it was one ofthe refractory organic compounds and its degradation was welldocumented in literature (Horikoshi et al., 2002; Song et al.,2006). In Vis–Fe0–H2O2–citrate–O2 system, the conditions as12.6 g of Fe0, 1.0 mmol L�1 of citrate, and 2.9 mmol L�1 of H2O2 atoriginal pH of 7.5 under aeration, were made to evaluate the deg-radation of 21 lmol L�1 of RhB. The solution pH was measured tobe constant at 7.5 throughout reactions. While color fading of the

1544 J. Hong et al. / Chemosphere 84 (2011) 1542–1547

solution was found to turn from initial pink–red to nearly colorless.Also Fig. 1A displays the changes of temporal absorption spectrumsof RhB in Vis–Fe0–H2O2–citrate–O2 system. RhB absorbance at550 nm dropped rapidly, finally disappeared completely in 3 h irra-diation. Meanwhile, a blue-shift was observed, with its shift num-bers as Dk = 18 nm (554–536 nm in 2.5 h reaction). Usually for RhBdegradation, two competitive processes occur (Ai et al., 2007). Oneis the destruction of conjugated xanthene structure (i. e.

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Fig. 1. (A) UV–Vis absorbance spectrums of RhB at different reaction times, insetwas absorption spectrum enlarged (480–600 nm), (B) Concentration of dissolvediron, (C) RhB and COD removal, in Vis–Fe0–H2O2–citrate–O2 system, conditionswere 21 lmol L�1 of RhB, 2.9 mmol L�1 of H2O2, 12.6 g of Fe0, 1.0 mmol L�1 ofcitrate, pH 7.5, and aeration.

(i.e. chromophores). Decrease of RhB absorption band at 550 nmcorresponds to the destruction of conjugated xanthene ring. Theother one is N-de-ethylation. The absorbance spectra at 550 nmshifting toward blue region suggests de-ethylated RhB moleculeformed (Chen et al., 2002). It is clear in Fig. 1A that the intensityof RhB absorption spectra at 550 nm is reduced, also the absor-bance spectra at 550 nm shifts toward blue region. Therefore itwas inferred that both the cleavage of RhB chromophore ringstructure and the de-ethylation steps proceeded in our Vis–Fe0–H2O2–citrate–O2 system.

Another issue we concerned about was the corrosive dissolu-tion of Fe0 in this neutral Vis–Fe0–H2O2–citrate–O2 system. Ifplenty of dissolved irons were released into aqueous solution, itwould need more steps to remove them and increase the overallcosts. Basically, Fe0 erosion should be slowed down at neutral oralkaline pH, which is different from that in acidic environment(Kusic et al., 2006; Katsoyiannis et al., 2008; Keenan and Sedlak,2008). Experimental data in Fig. 1B shows that total dissolved iron(Fe(II) + Fe(III)) is increased linearly, from initial zero to nearly5.4 lmol L�1 at 1 h, which is below the Environmental QualityStandards for Surface Water of China (GB 3838–2002), but turnshigher afterwards and achieves 17.7 lmol L�1 at 3 h. For Fe(II), itis concentration increases to 10.8 lmol L�1 quickly in 3 h reaction,reflecting a more and more strong oxidative ability to produce �OHwhen sufficient H2O2 exists.

Moreover, for 1 h reaction in Vis–Fe0–H2O2–citrate–O2 system,54% of RhB decomposition and 26% of COD removal were obtained(Fig. 1C), which was a little more competitive against related re-search works (Silva et al., 2007; Fernández et al., 2010). Furtherstudy on removing Fe0 from the system at the first hour was inves-tigated (Fig. 1C). The solution was still able to degrade RhB withoutincrease of any dissolved irons. An overall 62% of RhB degradationwas observed at the end of the following 2 h, though the degrada-tion tendency increased very slowly. This is very meaningful forVis–Fe0–H2O2–citrate–O2 system when it is put into practicalapplications, since it could degrade substrate to a far extent at avery low level of dissolved irons (5.4 lmol L�1).

To investigate whether �OH was involved in Vis–Fe0–H2O2–cit-rate–O2 system, extinguishing tests with isopropanol were per-formed. Isopropanol, containing an a-hydrogen, is highly reactivewith �OH and poorly reactive with O�2 (Hwang et al., 2010). Thesecond-order rate constant of isopropanol with OH-radicals is6 � 109 M�1 s�1 (Buxton et al., 1988). So isopropanol can be usedto investigate the extent and mechanism of �OH-mediated transfor-mation reactions. Results are shown in Fig. 1C. In the presence of30 mmol L�1 isopropanol that should scavenge �OH quantitatively(compared to 2.9 mmol L�1 H2O2), RhB degradation was sharplyinhibited (75% dropped in 3 h), which implied that the degradationin Vis–Fe0–H2O2–citrate–O2 system was mainly ascribed to theoxidation of �OH generated. Still 23% of RhB removal was observedin this situation, indicating that another one or more processes wereinvolved in the reaction. At pH 7, the particles of ferric hydrousoxides (Fe(OH)3) with much larger specific surface areas(typically 300–700 m2 g�1) will be formed in solution (Eq. (6)), andthat surface reactions on newly forming ferric hydrous oxides aremost likely very important for substrate removal (Katsoyianniset al., 2008, 2009a, b). In order to check the effect of RhB removalby dissolved irons, experiments with little influence of �OH were de-signed and conducted. For no addition of H2O2 in bothVis–Fe0–citrate-aeration and Vis–Fe0–citrate treatments (We foundno obvious variations of RhB removal before and after isopropanolwas added to these two treatments, separately. Also citrate couldinfluence the erosion of Fe0 in the aqueous solution from our exper-iments. These were partly the reasons why the two treatments weredesigned and investigated), they showed nearly 20% of RhB removalfor 3 h reaction. This removal percentage was very close to that by

J. Hong et al. / Chemosphere 84 (2011) 1542–1547 1545

�OH quenching test in Vis–Fe0–H2O2–citrate–O2 system (23% of RhBremoval as described above). Since RhB is visible photo-stable andcitrate ferric complexes will not influence RhB’s absorbance spec-trum at 550 nm, the 20% of RhB removal should be ascribed to reac-tions on the surfaces of ferric hydrous oxides. At pH 7, the dominantaqueous Fe(III) species are Fe(OH)3, which are transformed from Fe0

by oxygen oxidation. It was possible that these parts of RhB were ad-sorbed on the surfaces of Fe(OH)3 because of their large specific sur-face areas.

4Fe0 þ 3O2 þ 6H2O! 4FeðOHÞ3 ð6Þ

3.2. Control experiments

In order to investigate the role of individual factor in Vis–Fe0–H2O2–citrate–O2 system, and to further explore the mechanism,five treatments as Vis–Fe0–citrate–O2, Vis–citrate–H2O2–O2, Vis–Fe0–H2O2–O2, Fe0–citrate–H2O2–O2 and Vis–Fe0–citrate–H2O2

were investigated, respectively (Fig. 2A). Only 15% RhB was decom-posed in 4 h for Vis–citrate–H2O2–O2 treatment without Fe0, whichimplied that no Fenton reaction could be taken place without iron,though a few substrates were possibly degraded by �OH from H2O2

decomposition.In Vis–Fe0–citrate–O2 treatment without H2O2, the loss of RhB

reached 26%, showing that this treatment was able to removesubstrate in the presence of iron and absence of H2O2. Further

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A

Fig. 2. (A) Control experiments, (B) DO measurement, conditions if involved:21 lmol L�1 of RhB, 2.9 mmol L�1 of H2O2, 12.6 g of Fe0, 1.0 mmol L�1 of sodiumcitric, and pH 7.5.

�OH quenching test by addition of isopropanol into this treatmentwas performed. No degradation decrease was found, indicatingthat the hydroxyl radicals were not the main species for oxidationin this Vis–Fe0–citrate–O2 treatment. It was pointed out that H2O2

could be produced from Fe0 corrosion, thus producing hydroxylradicals (Katsoyiannis et al., 2008). But this seemed as in a smallertransient concentration and producing less oxidation from ourexperiments.

In Vis–Fe0–H2O2–O2 treatment without citrate, removal of RhBachieved 31% in 2.5 h, afterwards went into a steady state, suggest-ing that Fenton reaction was initially taking into effect because �OHplayed an important role in substrate degradation as shown above,but decreased slowly when H2O2 decaying.

For Fe0–citrate–H2O2–O2 treatment without irradiation, a muchhigher removal rate was observed, which reached 49% in 4 h, butstill lower than that in Vis–Fe0–citrate–H2O2–O2 system. Fentonreactions could produce �OH, while transforming Fe(II) into Fe(III).As a result, its oxidation ability would weaken since less Fe(II) wasleft in solution. The Fe(II) measurement for Fe0–citrate–H2O2–O2

treatment confirmed that its concentration was only 48–87% ofthat for Vis-Fe0–citrate–H2O2–O2 system.

For Vis–Fe0–citrate–H2O2 treatment without aeration, 58% ofRhB removal was observed, the highest among control experi-ments. As oxygen can possibly reduce the quenching of �OH in Fen-ton’s reaction (Zhao et al., 2004), and also aeration can enhance theinteraction between �OH and substrate molecular, aeration plays apartial role in the degradation of RhB. The oxygen concentrationwas also measured. Fig. 2B depicts that, although initially less oxy-gen participates in Vis–Fe0–citrate–H2O2 treatment, more andmore oxygen is supplied afterwards from ambient atmosphere,and turns to be identical with that in Vis–Fe0–citrate–H2O2–O2 sys-tem at 2.5 h.

In general, from the viewpoint of individual factor importanceon RhB removal in Vis–Fe0–H2O2–citrate–O2 system, achievedprohibitive extents followed the decreasing order Fe0 > H2O2 >citrate > Vis > O2.

3.3. Factors affecting the Vis–Fe0–H2O2–citrate–O2 system

Since Fe0, H2O2 and citrate were the three most importantparameters enormously influencing RhB removal in Vis–Fe0–H2O2–citrate–O2 system, it was necessary to investigate to whatextent these parameters would affect the substrate removal.Experiments were carried out individually.

Effect of initial H2O2 concentrations in a wide range from 1.0 to195.9 mmol L�1 on RhB removal rate constants in Vis–Fe0–H2O2–citrate–O2 system was investigated (Fig. 3A). The experimentaldata were found to be well fitted the first-order kinetics(R2 > 94%). The loss of RhB was enhanced when initial H2O2 con-centration was increased from 1.0 to 2.9 mmol L�1, and the fastestrate constant as k = 0.024 min�1 was seen at 2.9 mmol L�1 of H2O2.However, further addition of H2O2 to high concentrations drasti-cally decreased the efficiency. For the highest level of H2O2 as195.9 mmol L�1 we studied, which was 67 times higher than theinitial addition of 2.9 mmol L�1, k was decreased to 0.002 min�1,12 times lower than that for 2.9 mmol L�1 of H2O2. This phenome-non might be ascribed to �OH extinguishing, and self-decomposi-tion of H2O2 when too much H2O2 was added (Katsumata et al.,2006).

Fe0 doses on RhB removal were also carried out, as shown inFig. 3B. The weights of iron nails were varied from 9.8 to 84.0 g.Increasing Fe0 from 9.8 g to 12.6 g accelerated RhB removal. Ahighest pseudo first-order reaction rate constant ask = 0.014 min�1 was obtained for 12.6 g of Fe0. While much moreFe0 (14.0–84.0 g) intensely decreased the degradation. Theconstants turned to be 0.001 min�1 for both 56.0 g and 84.0 g of

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Fig. 3. (A) Influence of H2O2 concentrations (1.0 mmol L�1 citrate, 12.6 g Fe0), (B)Influence of Fe0 amounts (3.9 mmol L�1 H2O2, 1.0 mmol L�1 citrate), and (C)Influence of citrate concentrations (3.9 mmol L�1 H2O2, 12.6 g of Fe0), on RhBremoval rate constants in conditions of 21 lmol L�1 RhB, pH = 7.5.

1546 J. Hong et al. / Chemosphere 84 (2011) 1542–1547

Fe0. Since 4/3 mol of Fe0 could be oxidized per mole of O2 at pH 7(Eq. (6)), dissolved iron (Fe(II) or Fe(III)) released from Fe0 wouldconsume H2O2 and extinguish �OH (Eq. (7)) in a large scale (Duranet al., 2006; Monteagudo et al., 2008).

FeðIIÞ þHO� ! FeðIIIÞ þHO� ð7Þ

Citrate is one of the buffer agents, and also a ligand to complexFe(III). This complex compound as ferric citrate can exhibit absor-bance activity at visible region, and turn Fe(III) into Fe(II)

(Hug et al., 2001). However, citrate could compete �OH with sub-strate in certain conditions (Katsumata et al., 2006). In our exper-iments, 0.6–12.0 mmol L�1 of citrate were chosen to check theirtreatment efficiencies (Fig. 3C). Results show that the reaction rateconstant peaked at 1.0 mmol L�1 of citrate, and dropped whenevercitrate was less or more than this value. For high level of citrate,the reason why the removal was weakened was possibly that, cit-rate ions could react with �OH to produce 3-hydroxo-glutarate rad-ical and3-oxo-glutarate, as shown in Eqs. (8)–(10), thus suppressing theoxidation of �OH toward the substrate.

Citrate3� þHO� ! 3-hydroxo-glutarate�2� ð8Þ

3-hydroxo-glutarate�2� þ FeðIIIÞCitrate-OH�

! 3� oxo-glutarate2� þ FeðIIÞCitrate ð9Þ

3-oxo-glutarate2� ! acetoneþ CO2 ð10Þ

3.4. Possible mechanism in Vis–Fe0–H2O2–citrate–O2 system

Based on the above results and discussion, possible mechanismin Vis–Fe0–H2O2–citrate–O2 system was deduced. At circumneutralpH as 7.5 in Vis–Fe0–H2O2–citrate–O2 system, the aerobic corrosiveof Fe0 firstly occurred with yielding dissolved irons (Fe(II) andFe(III)). Fenton reaction was then initiated when ferrous was re-acted with added H2O2, producing �OH. Meanwhile, Fe(III) wastransformed into Fe(II) when Fe(III)citrate OH� complex absorbedvisible photons. This newly formed Fe(II) could also participate inFenton reaction. The produced hydroxyl radicals oxidized RhBthrough destruction of chromophores and N-de-ethylation, whichaccounted for 75% of RhB removal. Except the oxidation by �OH,reactions on the surfaces of Fe(OH)3 were expected in the system.For example, the adsorption of RhB and co-precipitation withFe(OH)3. Still further researches should be directed at exploringthe mechanism.

4. Conclusion

A new Vis–Fe0–H2O2–citrate–O2 system was built at circum-neutral pH. It could obtain a relatively high RhB removal efficiency,and a finite release of dissolved iron from Fe0 surface. Upon controltests and factors experiments, the system was concluded to under-go a main process of �OH oxidation. From the experiments we con-ducted, several merits for this system were summarized. (1) Ironnails other than iron powders were used, which were much easyto be removed from treated solution. (2) A neutral working pH at7.5, which was different from ordinary Fenton reaction that re-quired acidic pH, and expanded the application of Fe0 system toa broad pH range. (3) Very slow iron corrosion rate with compara-tively high substrate removal efficiency. (4) Cost was to be reducedsince a cheap energy-saving lamp was applied.

Acknowledgements

The authors wish to thank NSFC of China (50808167) and ChinaScholarship Council for financial supports for this work. Also wethank Prof. Oliver Zafiriou for his kind advice and suggestions onthis manuscript. We are grateful to editor M. Oehme and two anon-ymous referees for their constructive comments on themanuscript.

J. Hong et al. / Chemosphere 84 (2011) 1542–1547 1547

References

Abrahamson, H.B., Rezvani, A.B., Brushmiller, J.G., 1994. Photochemical andspectroscopic studies of complexes, of iron(III) with citric acid and othercarboxylic acids. Inorg. Chim. Acta 226, 117–127.

Ai, Z., Lu, L., Li, J., Zhang, L., Qiu, J., Wu, M., 2007. Fe@Fe2O3 core-shell nanowires as

iron reagent. 1. Efficient degradation of rhodamine by a novel sono-fentonprocess.. J. Phys. Chem. C 111, 4087–4093.

Ay, F., Catalkaya, E.C., Kargi, F., 2009. A statistical experiment design approach foradvanced oxidation of direct red azo-dye by photo-Fenton treatment. J. Hazard.Mater. 162, 230–236.

Bozzi, A., Yuranova, T., Mielczarski, J., Lopez, A., Kiwi, J., 2002. Abatement of oxalatescatalyzed by Fe-silica structured surfaces via cyclic carboxylate intermediatesin photo-Fenton reactions. Chem. Commun., 2202–2203.

Bremner, D.H., Burgess, A.E., Houllemare, D., Namkung, K.C., 2006. Phenoldegradation using hydroxyl radicals generated from zero-valent iron andhydrogen peroxide. Appl. Catal. B 63, 15–19.

Buxton, G.V., Greenstock, C., Hellman, W.P., Ross, A.B., 1988. Critical review of rateconstants for reactions of hydrated electrons, hydrogen atoms and hydroxylradicals (OH/O�) in aqueous solution. J. Phys. Chem. Ref. Data 17, 513–886.

Chen, C., Li, X., Ma, W., Zhao, J., Hidaka, H., Serpone, N., 2002. Effect of transitionmetal ions on the TiO2-assisted photodegradation of dyes under visibleirradiation: a probe for the interfacial electron transfer process and reactionmechanism. J. Phys. Chem. B 106, 318–324.

Dong, Y., He, L., Yang, M., 2008. Solar degradation of two azo dyes by photocatalysisusing Fe(III)-oxalate complexes/H2O2 under different weather conditions. DyesPigments 77, 343–350.

Duran, A., Monteagudo, J.M., Mohedano, M., 2006. Neural networks simulation ofphoto-Fenton degradation of reactive blue 4. Appl. Catal., B 65, 127–134.

Fernández, C., Larrechi, M.S., Callao, M.P., 2010. An analytical overview of processesfor removing organic dyes from wastewater effluents. TrAC, Trends Anal. Chem.29, 1202–1211.

Fortune, W.B., Mellon, M.G., 1938. Determination of iron with o-phenanthroline: aspectrophotometric study. Ind. Eng. Chem. 10, 60–64.

Horikoshi, S., Hidaka, H., Serpone, N., 2002. Environmental remediation by anintegrated microwave/UV-illumination method. 1. Microwave-assisteddegradation of rhodamine-B dye in aqueous TiO2 dispersions.. Environ. Sci.Technol 36, 1357–1366.

Hug, S.J., Canonica, L., Wegelin, M., Gechter, D., Von Gunten, U., 2001. Solaroxidation and removal of arsenic at circumneutral pH in iron containing waters.Environ. Sci. Technol. 35, 2114–2121.

Hug, S.J., Laubscher, H.U., James, B.R., 1997. Iron(III) catalyzed photochemicalreduction of chromium(VI) by oxalate and citrate in aqueous solutions. Environ.Sci. Technol. 31, 160–170.

Hwang, S., Huling, S.G., Ko, S., 2010. Fenton-like degradation of MTBE: effects of ironcounter anion and radical scavengers. Chemosphere 78, 563–568.

Katsoyiannis, I.A., Ruettimann, T., Hug, S.J., 2008. PH dependence of fenton reagentgeneration and As(III) oxidation and removal by corrosion of zero valent iron inaerated water. Environ. Sci. Technol. 42, 7424–7430.

Katsoyiannis, I.A., Ruettimann, T., Hug, S.J., 2009a. Response to Comment on ‘pHdependence of fenton reagent generation and As(lll) oxidation and removal bycorrosion of zerovalent iron in aerated water. Environ. Sci. Technol. 43, 234.

Katsoyiannis, I.A., Ruettimann, T., Hug, S.J., 2009b. Response to comment on pHdependence of fenton reagent generation and As(III) oxidation and removal bycorrosion of zero valent iron in aerated water. Environ. Sci. Technol. 43, 3980–3981.

Katsumata, H., Kaneco, S., Suzuki, T., Ohta, K., Yobiko, Y., 2006. Photo-Fentondegradation of alachlor in the presence of citrate solution. J. Photochem.Photobiol., A 180, 38–45.

Keenan, C.R., Sedlak, D.L., 2008. Factors affecting the yield of oxidants from thereaction of nanoparticulate zero-valent iron and oxygen. ACS National MeetingBook of Abstracts, Philadelpia, PA.

Klausen, J., Vikesland, P.J., Kohn, T., Burris, D.R., Ball, W.P., Roberts, A.L., 2003.Longevity of granular iron in groundwater treatment processes: solutioncomposition effects on reduction of organohalides and nitroaromaticcompounds. Environ. Sci. Technol. 37, 1208–1218.

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

Lee, J., Kim, J., Choi, W., 2007. Oxidation on zerovalent iron promoted bypolyoxometalate as an electron shuttle. Environ. Sci. Technol. 41, 3335–3340.

Liu, G., Wu, T., Zhao, J., Hidaka, H., Serpone, N., 1999. Photoassisted degradation ofdye pollutants. 8. Irreversible degradation of alizarin red under visible lightradiation in air-equilibrated aqueous TiO2 dispersions.. Environ. Sci. Technol 33,2081–2087.

Lucas, M.S., Peres, J.A., 2007. Degradation of reactive black 5 by Fenton/UV-C andferrioxalate/H2O2/solar light processes. Dyes Pigments 74, 622–629.

Matheson, L.J., Tratnyek, P.G., 1994. Reductive dehalogenation of chlorinatedmethanes by iron metal. Environ. Sci. Technol. 28, 2045–2053.

Monteagudo, J.M., Duran, A., Lopez-Almodovar, C., 2008. Homogeneus ferrioxalate-assisted solar photo-Fenton degradation of Orange II aqueous solutions. Appl.Catal., B 83, 46–55.

Nadtochenko, V.A., Kiwi, J., 1998. Photolysis of FeOHþ2 and FeClþ2 in aqueoussolution. Photodissociation kinetics and quantum yields. Inorg. Chem. 37,5233–5238.

Pierce, J., 1994. Colour in textile effluents-the origins of the problem. J. Soc. DyersColour. 110, 131–133.

Reynolds, G.W., Hoff, J.T., Gillham, R.W., 1990. Sampling bias caused by materialsused to monitor halocarbons in groundwater. Environ. Sci. Technol. 24, 135–142.

Selvam, K., Muruganandham, M., Swaminathan, M., 2005. Enhanced heterogeneousferrioxalate photo-fenton degradation of reactive orange 4 by solar light. Sol.Energy Mater. Sol. Cells 89, 61–74.

Silva, M.R.A., Trovó, A.G., Nogueira, R.F.P., 2007. Degradation of the herbicidetebuthiuron using solar photo-Fenton process and ferric citrate complex atcircumneutral pH. J. Photochem. Photobiol., A 191, 187–192.

Song, H., Carraway, E.R., 2005. Reduction of chlorinated ethanes by nanosized zero-valent iron: kinetics, pathways, and effects of reaction conditions. Environ. Sci.Technol. 39, 6237–6245.

Song, W., Cheng, M., Ma, J., Ma, W., Chen, C., Zhao, J., 2006. Decomposition ofhydrogen peroxide driven by photochemical cycling of iron species in clay.Environ. Sci. Technol. 40, 4782–4787.

Sun, B., Reddy, E.P., Smirniotis, P.G., 2005. Visible light Cr(VI) reduction and organicchemical oxidation by TiO2 photocatalysis. Environ. Sci. Technol. 39, 6251–6259.

Sun, Y., Pignatello, J.J., 1992. Chemical treatment of pesticide wastes. Evaluation ofFe(III) chelates for catalytic hydrogen peroxide oxidation of 2,4-D atcircumneutral pH.. J. Agric. Food Chem. 40, 322–327.

Wu, F., Deng, N., 2000. Photochemistry of hydrolytic iron(III) species andphotoinduced degradation of organic compounds. A mini review.Chemosphere 41, 1137–1147.

Zhao, X.K., Yang, G.P., Wang, Y.J., Gao, X.C., 2004. Photochemical degradation ofdimethyl phthalate by Fenton reagent. J. Photochem. Photobiol., A 161, 215–220.