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Journal of Hazardous Materials 186 (2011) 2083–2088

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

hotodegradation mechanism and kinetics of methyl orange catalyzed by Fe(III)nd citric acid

ing Guoa, Yanyan Dua, Yeqing Lana,∗, Jingdong Maob,∗∗

College of Sciences, Nanjing Agricultural University, Nanjing 210095, PR ChinaDepartment of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529, USA

r t i c l e i n f o

rticle history:eceived 6 July 2010eceived in revised form 31 October 2010ccepted 23 December 2010

a b s t r a c t

In this study, the photodegradation process of methyl orange (MO) catalyzed by Fe(III) and citric acid andthe reaction kinetics were investigated in detail at pHs from 2 to 8. The results show that the photodegra-dation of MO is slow in the presence of Fe(III) or citric acid alone. However, it is markedly enhanced whenFe(III) and citric acid coexist. High initial citric acid or initial Fe(III) concentrations lead to increased pho-

vailable online 5 January 2011

eywords:ethyl orange

e(III)itric acidatalysis

todegradation of MO. And Fe(III) citrate mediated photodegradation of MO is optimized at pH 6. Thephotoproduction of hydroxyl radicals (·OH) in different catalytic systems was determined by HPLC. Andthe concentrations of Fe(II) and citric acid concentration in the process of the reaction were analyzed.The photodegradation of MO obeys to pseudo-zero order kinetics with respect to MO and the degrada-tion reaction occurs in two phases. At the initial initiation stage, degradation rate is relatively slow, andsignificantly increases at a later acceleration stage.

hotodegradation

. Introduction

Azo dyes, which contain one or more nitrogen to nitrogen dou-le bonds (–N N–), have been widely used in industries such asextiles, foodstuffs and leather. Azo dyes are released as indus-rial wastewater into the environment. The release of these coloredastewaters poses a serious threat to the environment [1]. There-

ore, the degradation of azo dyes for environmental treatment haseceived increasing attention [2–4].

Due to the aromatic character of most dye molecules and thetability of modern dyes, biological, physical and traditional chem-cal techniques for removal of dyes are not efficient enough [5,6].hus, the development of new technologies of wastewater purifi-ation leading to complete destruction of the contaminants hasmportant theoretical meaning and practical value. Photocatalyticegradation is generally favored as a cleaner and greener technol-gy for the removal of toxic organic pollutants from wastewater7–9]. Over the past two decades, the photochemical activity of

he complexes of Fe(III) and carboxylate anions has received con-iderable attention. Silva et al. [10] and Kuma et al. [11] reportedhat Fe(III) and citric acid or tartaric acid could form stable com-lexes, which were of photocatalytic activity, so could EDTA and

∗ Corresponding author. Tel.: +86 25 84396697; fax: +86 25 84395255.∗∗ Corresponding author. Tel.: +1 757 683 6874; fax: +1 757 683 4628.

E-mail addresses: lanyq102@njau.edu.cn (Y. Lan), Jmao@odu.edu (J. Mao).

304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2010.12.112

© 2011 Elsevier B.V. All rights reserved.

some other organic ligands [12,13]. Deng et al. [14] studied thephotooxidation of dye solutions including C. I. reactive red 2, C. I.reactive blue 4, and others by Fe(III)–citrate complexes, finding thatit followed pseudo-first order kinetics. Ou et al. [15] observed thephotodegradation of atrazine by Fe(III)–citrate complex in aqueoussolution. Quici et al. [16] reported the self-catalyzed Fenton withcitric acid.

Many studies have recognized that the photoexcitation ofFe(III)–carboxylate complexes can result in the formation of Fe(II)and hydroxyl radical (·OH) through a ligand-to-metal chargetransfer (LMCT) path. The generation of ·OH is the critical stepbecause ·OH can oxidize most organic molecules quickly and non-selectively and mineralize them to carbon dioxide and water owingto its high oxidation potential (E0 = +2.80 V versus NHE) [17]. Inthe presence of dissolved oxygen, H2O2 is generated from the pho-todegradation of organic carboxylic acid. Then, the reaction of H2O2and Fe(II) can generate Fe(III) and ·OH, which causes decomposi-tion of organic compounds as well as reoxidation of Fe(II) to Fe(III),respectively [18,19]. Therefore, Fe(III) and organic carboxylic acid,which coexist in natural environments, can set up a photo-Fentonsystem with H2O2 produced in situ [14,20].

In this study, the photodegradation process of MO catalyzed by

Fe(III) and citric acid and the reaction kinetics were investigatedunder irradiation with different light sources. MO was selected asthe model pollutant in this paper since it is a typical azo dye widelyused in the textile industry. Fe(III) and citric acid were applied sincethey often exist in natural waters and they do not pose a threat to

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084 J. Guo et al. / Journal of Hazard

he environment. The effects of the Fe(III), citric acid, and MO ini-ial concentrations, as well as pH values were examined. A possible

echanism is suggested based on measurements of the concentra-ion of ·OH, Fe(II), and citric acid with reaction time and the speciesistribution in these systems obtained from computer simulationoftware, which has never been reported.

. Materials and methods

.1. Materials

Methyl orange was obtained from Beijing Chemical Reagentsompany. The stock solution (1000 mg/L) of methyl orange wasrepared in deionized water. Fe(III) (0.01 M) was prepared by dis-olving Fe2(SO4)3·9H2O (s) (Guangdong Taishan Chemical Factory)n dilute H2SO4 solution, and kept at pH < 2 in order to preventydrolysis. The stock solution of citric acid (0.5 M) was obtained byissolving reagent-grade citric acid monohydrate (Shantou Xilonghemical Factory) in deionized water. Methyl alcohol of chromato-raphic grade was purchased from Tedia Company, United States ofmerica. Benzene (Shanghai Chemical Reagents Limited Company)hich was used as a probe to determine the photoproduction of

OH in aqueous solution was of analytical grade. The chromogeniceagent, o-phenanthroline, was supplied from Sigma Aldrich Com-any, and its stock solution was prepared by dissolving 0.3568 g-phenanthroline in 1000 mL deionized water. All the stock solu-ions were stored in a refrigerator at 4 ◦C in the dark prior to use.he other chemicals used in this study were at least of analyticalrade.

All glassware used in the experiments was cleaned by soakingn 1 M HCl for 12 h and thoroughly rinsed with tap water and theneionized water.

.2. Photochemical experiments

The photodegradation of MO was carried out in a XPA-7 photo-hemical reactor (Xujiang electromechanical plant, Nanjing, China),nd the schematic diagram was shown in our previous work21]. The temperatures of the reaction solutions were maintainedt 25 ± 2 ◦C by cooling water circulation. Light sources included00, 300 and 500 W medium pressure Hg lamps and a 500 Wenon lamp, and the light extensities at the positions of quartz

ubes are 12.7, 16.8, 20.1 mW cm−2 (measured using UV-A irra-iation meter, Beijing Normal University, China) and 26 500 Luxmeasured using ST-80C illumination meter, Beijing Normal Uni-ersity, China), respectively. The initial pH values of reactionolutions were adjusted with dilute sulfuric acid solution andodium hydroxide solution, and the final volume of the solutionas adjusted to 40 mL with deionized water. Then, the reaction

ubes with 40 mL solution were put into the photochemical reac-or and stirred with magnetic stirrers. At given irradiation timentervals, 1 mL aliquot of sample was removed with a pipette andiluted to 10 mL in colorimetric tube to determine MO concen-ration. Samples were analyzed immediately in order to avoidurther reaction. Once the dye solution in the quartz tube becameompletely colorless, which means that the detected Abs valuef the solution was zero, the reaction was thought to be com-leted.

All the experiments in this section were performed in triplicate.

.3. Determination of hydroxyl radicals (·OH)

Aromatic hydroxylation is considered to be one of the typicaleactions of ·OH and is used for the detection of ·OH in the case of

aterials 186 (2011) 2083–2088

the photo-Fenton reactions at lower pH (Eq. (1)) [22].

+ ·OH hv OH(1)

In this study, 0.8 mmol benzene was added into different cat-alytic systems under an irradiation of 500 W medium pressure Hglamp at pH 6 to indirectly determine the quantum yield of ·OHby HPLC. Excessive benzene was used as a probe to support thereaction mechanism. Dark controls, direct photolysis of the cor-responding amount of benzene and phenol were also carried outseparately under the same conditions.

2.4. Analytical methods

A CyberScan pH2100 Bench Meter (Eutech Instruments), afterthree-point calibration, was used to measure the pH of the reac-tion solution. The concentrations of MO were determined at thecharacteristic �max of 460 nm at pH 4–8 and 506 nm at pH 2 on theUV–VIS spectrometer (Beijing Ruili Corp, UV-9100). The concentra-tion of Fe(II) was quantitatively analyzed by a colorimetric methodusing o-phenanthroline. Because of the coexistence of MO which ischromogenic reagent itself, the concentration of photo generatedFe(II) was calculated from the absorption intensity at 510 nm on aUV-9100 Spectrophotometer, that is,

Abs(Fe(II) + o-phenanthroline) = Abs(Fe(II)) + o-phenanthroline

+ MO + Abs(MO) (2)

The variation of citric acid concentration with reaction timewas detected by HPLC (Waters 2489 with Arcus Ep-C18 (5 �m,4.6 mm × 250 mm)) at 210 nm. A 10% (v/v) methyl alcohol in 25 mMNaH2PO4 at pH 2.5 (H3PO4) was used as a mobile phase with aflow rate of 0.4 mL/min. The formation of phenol from the reactionof benzene and ·OH was detected at 268 nm by HPLC. The mobilephase consisted of 60% (v/v) methyl alcohol. Standard solutions ofphenol were prepared to calibrate the HPLC determination of phe-nol concentrations produced from the oxidation of benzene addedto different catalytic systems.

3. Results and discussion

3.1. Preliminary studies

The photodegradation of MO was conducted under differentconditions. The results presented in Fig. 1 showed no noticeablechange in MO concentration under dark condition, indicating thatthe loss of MO resulted from the volatilization and the adsorptiononto the reactor could be ignored.

In the single system of MO under UV irradiation, the photodegra-dation efficiency of MO in one hour is 19.33%. Therefore, directUV irradiation is insufficient to decompose this dye. While in thetwo-component system with MO and citric acid, the photodegra-dation efficiency at the same time is 22.50%. A small increase in thedegradation efficiency of MO is attributed to the possible oxidants(e.g. H2O2) that are produced by photolysis of citric acid ([15] andreferences therein):

H3Cit + O2 + h� → H3Cit·+ + O2·− (3)

H+ + O2· � HO2· (4)

2HO2· → H2O2 + O2 (5)

The produced H2O2 is induced by UV irradiation to generate ·OH[23] and then MO is oxidized by ·OH.

H2O2 + hv → 2·OH(� < 300 nm) (6)

MO + ·OH → products(unknown) (7)

J. Guo et al. / Journal of Hazardous Materials 186 (2011) 2083–2088 2085

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Fig. 1. Photodegradation of 50 mg/L MO in different reaction systems with an initialconcentration of 0.2 mM Fe(III), 5 mM citric acid under full light of a 500 W mediumpoIs

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Fig. 2. Effect of initial concentrations of Fe(III) (a) and citric acid (b) on photodegra-dation of 50 mg/L MO by Fe(III)–citrate under full light of a 500 W medium pressureHg lamp at pH 6 and 25 ◦C. Effect of initial concentrations of Fe(III) with an initial

ressure Hg lamp at pH 6 and 25 ◦C. � dark control; ♦ methyl orange; � methylrange + citric acid; � methyl orange + Fe(III);� methyl orange + citric acid + Fe(III).nsert displays the photoproduction of hydroxyl radicals (·OH) in different reactionystems with excessive benzene.

In the catalytic system containing MO and Fe(III), the pho-odegradation efficiency is increased to 24.10%. This may be dueo the formation of ·OH radicals from photoreduction of Fe(III) [24].nd MO is decomposed according to Eq. (7).

Fe(OH)]2+ + hv → Fe(II) + ·OH (8)

As is reported by Wu and Deng [20], in the acidic condition, therere at least four different species of Fe(III) ions in aqueous solution:e(III); [Fe(OH)]2+; [Fe(OH)2]+ and the dimer [Fe2(OH)2]4+, and theuantum yield of ·OH produced from the photolysis of [Fe(OH)]2+

s much higher than that of the other species.In the MO/Fe(III)/citric acid ternary system, the color removal is

uch more efficient, and the photodegradation efficiency reaches8.26% in 18 min. The reason may be attributed to the formation ofe(III)–citrate complex, which is of high photocatalytic activity toroduce hydroxyl radicals through a photo-Fenton reaction system.his assumption is further supported by the determination of ·OHy HPLC using benzene as a probe (see insert in Fig. 1), and theeaction mechanism and kinetics of this catalytic system will beiscussed in detail in the following paragraphs.

.2. The photoproduction of hydroxyl radicals (·OH) in differenteaction systems

As shown in the insert of Fig. 1, photoproduction of ·OH coulde induced and accumulated in different catalytic systems underedium pressure Hg lamp with an increasing exposure time

lthough dark controls, direct photolysis of the correspondingmount of benzene and phenol adjusted to desired pH separatelyhowed negligible loss of benzene or formation of phenol after8 min of illumination under the 500 W Hg lamp (data not shown).he results indicated that the photoproduction of ·OH in the singleystem of MO was smaller than that in binary and ternary system,he photoproduction of ·OH in the binary system containing MOnd Fe(III) was greater than that in the system with MO and cit-

ic acid. The photoproduction of ·OH in the MO/Fe(III)/citric acidernary system was significantly increased. The photoproductionf ·OH are in agreement with the photodegradation of MO in theifferent catalytic systems, which proves that ·OH produced fromhe photocatalysis is the key to leading the degradation of MO.

concentration of 5 mM citric acid (a): ♦ 0 mM; � 0.1 mM; � 0.2 mM; � 0.5 mM. Effectof initial concentrations of citric acid with an initial concentration of 0.2 mM Fe(III)(b): ♦ 0 mM; � 1 mM; � 2 mM; � 5 mM; � 10 mM.

3.3. Effect of the initial concentrations of Fe(III) and citric acid onthe photodegradation of MO

The initial concentrations of Fe(III) and citric acid play an impor-tant role in the degradation of MO. As shown in Fig. 2, the increaseof Fe(III) or citric acid in the ternary system of MO/Fe(III)/citric acidgreatly shortened the photodegradation time. Similar results werereported by Balmer and Sulzberger [25], who also noted that higheroxalate concentration led to higher degradation of atrazine. Theenhancement of degradation by the addition of Fe(III) and citricacid is due to the increased production of ·OH radicals as follows:

FeIII(Cit) + h� → FeII–Cit· → Fe(II) + Cit2−· (9)

Cit2−· + O2 → 3-oxoglutarate + CO2 + O2·− (10)

H+ + O2−· � HO2· (11)

HO2·/O2− + Fe(II) → Fe(III) + H2O2 (12)

H2O2 + Fe(II) → Fe(III) + ·OH + ·OH− (13)

2086 J. Guo et al. / Journal of Hazardous Materials 186 (2011) 2083–2088

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mainly result from the distribution of Fe(III) and citric acid species.The distribution of Fe(III) and citric acid species against pH wassimulated by Visual MINTEQ with a span of pH from 2 to 8 at a reg-ular interval of 1. Based on the results, Fe(III) ions exist mostly as

60

ig. 3. Effect of initial concentration of MO on photodegradation of MO bye(III)–citrate with an initial concentration of 0.2 mM Fe(III), 5 mM citric acid underull light of a 500 W medium pressure Hg lamp at pH 6 and 25 ◦C. * 25 mg/L; �0 mg/L; � 100 mg/L.

.4. Effect of the initial MO concentration on thehotodegradation of MO

In this section, a 1:25 molar ratio of Fe(III) (0.2 mM) to citric acid5 mM) was adopted in order to keep Fe(III) completely coordinated14–16]. The effect of initial concentrations of MO with a range of5–100 mg/L on the photodegradation rate was investigated at pHand 25 ◦C and the results are illustrated in Fig. 3. The same degra-ation tendency can be observed in all three curves from Fig. 3.he plotting of MO concentration to reaction time can be describedy two phases. At the initial initiation stage (a), the degradationate is relatively slow, and significantly increases during an accel-ration stage (b). A similar process was reported by Lan et al. [26]n the study of sulfide reducting Cr(VI), and in the investigation ofhe catalysis of manganese(II) on Cr(VI) reduction by citrate [27]. Inddition, the two reaction stages in all three curves are linear withne linear dependence (R2 > 0.98) (see Table 1), suggesting that thehotodegadation of MO obeys to pseudo-zero reaction.

The concentration of Fe(II) which can control ·OH formation byhotocatalytic reactions of Fe(III)–citrate complex according to Eqs.9)–(13) was monitored, and the results are given in Fig. 4. There isn excellent correlation between the generation rate of Fe(II) andhe degradation rate of MO. Similarly, two stages are observed, andhe turning point of Fe(II) generation at each selected concentrationf MO is the same as that of the MO degradation. At the initiallicitation stage, Fe(II) was rapidly generated and the degradationf MO was slow. However, at the later acceleration stage, when theoncentration of Fe(II) kept constant in the catalytic system, MOegan to degrade at a high speed.

It has been suggested in the above discussion that ·OH, one of the

ost powerful oxidation agents, would be responsible for the fast

egradation of MO catalyzed by Fe(III) and citric acid. However, it isoncluded from the results shown in Figs. 3 and 4 that the transfor-ation of Fe(III) to Fe(II) is a precondition of ·OH formation, that is to

ay, ·OH is generated in the process of Fe(II) transforming to Fe(III).

able 1he fitted equation and correlation coefficients of the zero-order reaction foremoval of MO by Fe(III)–citrate under 500 W medium pressure Hg lamp.

Concentrationof MO (mg/L)

Reactionstage

The fittedequation

Thecorrelationcoefficient

25 a y = −1.3x + 24.933 R2 = 0.998b y = −3.27x + 33.173 R2 = 0.9869

50 a y = −1.3333x + 49.933 R2 = 0.9967b y = −3.3867x + 61.053 R2 = 0.9958

100 a y = −1.32x + 101.5 R2 = 0.9982b y = −3.341x + 128.34 R2 = 0.9826

Fig. 4. The concentration of Fe(II) during the degradation of MO with different ini-tial concentrations of MO by Fe(III)–citrate with an initial concentration of 0.2 mMFe(III), 5 mM citric acid under full light of a 500 W medium pressure Hg lamp at pH6 and 25 ◦C. � 25 mg/L; � 50 mg/L; ♦ 100 mg/L.

And the recreated Fe(III) can be reduced again to Fe(II) by super-oxide or hydroperoxyl radicals, initiating a continuous Fe(III)/Fe(II)cycle, which enhances substrate degradation [20]. Fe(III) (added orgenerated from Fe(II)) gives rise to the radical chain mechanismdescribed above, and the Fenton reaction (Eq. (13)) is improved bythe participation of photogenerated Fe(II).

3.5. Effect of pH on the photodegradation of MO

Dye wastewater is discharged at different pH. Therefore, it isimportant to study the role of pH in the photodegradation of MO.Experiments were conducted at a pH range of from 2 to 8 with theinitial concentrations of MO (50 mg/L), Fe(III) (0.2 mM), and citricacid (5 mM) at 25 ◦C and the results are illustrated in Fig. 5. It isobserved that the photodegradation of MO was significantly influ-enced by pH and the degradation rates of MO are pH 6 > pH 4 > pH8 > pH 2. MO photodegradation’s dependence on pH is considered to

0

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MO

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Fig. 5. Effect of pH on the photodegradation of 50 mg/L MO by Fe(III)–citrate withan initial concentration of 0.2 mM Fe(III), 5 mM citric acid under full light of a 500 Wmedium pressure Hg lamp at 25 ◦C. ♦ pH 2; � pH 4; � pH 6; � pH 8.

J. Guo et al. / Journal of Hazardous Materials 186 (2011) 2083–2088 2087

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Fig. 7. Effect of light intensity on the photodegradation of 50 mg/L MO by

ig. 6. The variation of citric acid concentration with reaction time followed byPLC. The initial conditions are: 5 mM citric acid, 0.2 mM Fe(III), 50 mg/L MO, full

ight of a 500 W medium pressure Hg lamp at pH 6 and 25 ◦C.

olecular species FeIII(Cit) (>98%) at pH 2–7, as FeIII(Cit) (42.73%)nd [Fe(OH)2]+ (49.32%) at pH 7–8. The differences in pH betweenhe pre- and post-reaction were also investigated in this study (dataot shown). At the initial pH of 2.0, 4.0 and 6.0, all of the correspond-

ng values after reaction rose to some degree, which is consistentith Eq. (11), indicating that H+ was consumed in the process of

OH formation. However, when the initial pH was 8.0, the corre-ponding pH after reaction decreased more. The possible reason ishat there are some other photoreactive substances participating ineactions. It has been reported that FeIII(Cit) displays fairly stronghotochemical activity [14,15,20,28]. But, some of the photoreac-ive substances should be [Fe(OH)2]+ instead of part of FeIII(Cit) atH 8. Wu and Deng [18] held that [Fe(OH)2]+ may also photolyze toenerate ·OH radicals, owing to the fact that its precursors, Fe(III)nd [Fe(OH)2]+, and the further hydrolytic dimerization product,Fe2(OH)2]4+, are all photoreactive. The possible mechanism at pHis predicted as the following equations:

e(III) + 2H2O → [Fe(OH)2]+ + 2H+ (14)

Fe(OH)2]+ + h� → ·OH + [Fe(OH)]+ (15)

Fe(III) is hydrolyzed to [Fe(OH)2]+ and H+, then the system pHrops. And [Fe(OH)2]+ is photolyzed to form ·OH, together witheIII(Cit) to degrade MO.

The research on C. I. reactive red 2 photodegradation in aunlight/Fe(III)-hydroxy system by Deng et al. [29] showed that theost rapid reaction rate was reached under acidic condition with

he 1:1 Fe/citric acid molar ratio. Instead, in the present study, theegradation rate of MO was first fast then followed by a slow pro-ess at pH 2. And repeated experiments showed the good stabilitynd reliability of the results. The possible causes are the distribu-ion of citric acid species and the quinoid structure of MO [30].

ore than 90% of citric acid exists as molecular species at pH 2btained by Visual MINTEQ. As an organic compound, the molec-lar species of citric acid could be easily degraded under UV light16], resulting in the competition for ·OH with MO, which inhibitshe photodegradation of MO. On the other hand, in acid medium,he structure of MO is quinoid (Eq. (16)), which is more stablehan azo form. Therefore, the strong acidic condition blocks MOegradation.

CH3)2N+ NHN SO3

- OH-

H+(CH3)2N

pKa=3.4red (quinoid)

The variation of citric acid concentration with reaction time waslso measured by HPLC, the results in Fig. 6 demonstrated that cit-

N N SO3-

Fe(III)–citrate with an initial concentration of 0.2 mM Fe(III), 5 mM citric acid underfull light of a 500 W medium pressure Hg lamp at pH 6 and 25 ◦C. � 500 W Xenonlamp; ♦ 100 W Hg lamp; � 300 W Hg lamp; � 500 W Hg lamp.

ric acid was degraded according to Eq. (3) as well as being ligandaccording to (9) in the MO/Fe(III)/citric acid ternary system.

3.6. Effect of light intensity on the photodegradation of MO

The photodegradation of MO under mimicked solar light of a500 W Xenon lamp and full light of 100–500 W medium pressureHg lamps with an initial concentration of 50 mg/L MO, 0.2 mMFe(III) and 5 mM citric acid was investigated at pH 6 and 25 ◦C.The results in Fig. 7 show that MO degradation under the irra-diation of mimicking solar light was relatively slow and MOconcentration decreased by 40% for 300 min. But intensive irra-diation can significantly enhance the rate of MO degradation.The times required for complete MO degradation were 130, 40and 18 min, respectively under irradiations of 100, 300, 500 Wmedium pressure Hg lamps. It is very obvious that the pho-todegradation of MO strongly depends on light intensity in thissystem.

4. Conclusion

While high citric acid or Fe(III) concentrations led to increasedphotodegradation of methyl orange, the photodegradation rate ofmethyl orange can be markedly improved when Fe(III) and cit-ric acid coexist. The photodegradation of methyl orange is alsostrongly influenced by solution pH. The optimal pH for degrada-tion of MO is 6, and higher or lower pH will result in decrease in MOremoval. It can be inferred that in natural environments with sun-light, this transformation cycle of Fe(III) to Fe(II) occurs when Fe(III)and organic carboxylic acid coexist together, although with rela-tively minor efficiency as compared with an intensive irradiation

yellow (azo) (16)

induction. The results from this study are of mechanistic andapplied use for pollution control.

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cknowledgments

This study was supported by the National Natural Science Foun-ation of China (Grant Nos. 40671089 and 40930738). Partialupport by the National Science Foundation (EAR-0843996 andBET-0853950) is acknowledged.

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