ORIGINAL PAPER Photo-Fenton and photo-Fenton ...

Chemical Papers 64 (3) 378–385 (2010) DOI: 10.2478/s11696-010-0011-0

ORIGINAL PAPER

Photo-Fenton and photo-Fenton-like processes for the degradation of methyl orange in aqueous medium: Influence of oxidation states of iron L. Gomathi Devi*, S. Girish Kumar, K. S. Anantha Raju, K. Eraiah Rajashekhar Department of Post Graduate Studies in Chemistry, Central College Campus, Dr. B.R. Ambedkar Veedi, Bangalore University, Bangalore-560 001, India Received 22 May 2009; Revised 5 August 2009; Accepted 12 August 2009

Degradation of methyl orange (MO) was carried out by the photo-Fenton process (Fe2+ /H2 O2 /UV) 3+ /S2 O2− and photo-Fenton-like processes (Fe3+ /H2 O2 /UV, Fe2+ /S2 O2− 8 /UV, and Fe 8 /UV) at the acidic pH of 3 using hydrogen peroxide and ammonium persulfate (APS) as oxidants. Oxidation state of iron had a significant influence on the efficiency of photo-Fenton/photo-Fenton-like processes. It was found that a process with a source of Fe3+ ions as the catalyst showed higher efficiency compared to a process with the Fe2+ ion as the catalyst. H2 O2 served as a better oxidant for both oxidation states of iron compared to APS. The lower efficiency of APS is attributed to the generation of excess protons which scavenges the hydroxyl radicals necessary for degradation. Further, 2+ the sulfate ions produced from S2 O2− /Fe3+ ions thereby reducing the 8 form a complex with Fe concentration of free iron ions in the solution. This process can also reduce the concentration of hydroxyl radicals in the solution. Efficiency of the various MO degradation processes follows the order: Fe3+ /H2 O2 /UV, Fe3+ /APS/UV, Fe2+ /H2 O2 /UV, Fe2+ /APS/UV. c 2009 Institute of Chemistry, Slovak Academy of Sciences Keywords: photo-Fenton process, photo-Fenton-like process, ammonium persulfate, methyl orange, degradation pathway

Introduction The industrial waste water released from leather, paper and textile units usually contains various coloring substances causing a significant threat to the environment. Due to the large scale production and extensive application, large quantities of unused dyes are released damaging the aquatic environment. These dyes absorb sunlight and initiate various chemical reactions leading to the formation of intermediates which are much more harmful and carcinogenic than the parent dye molecule (Guetta¨ı & Ait Amar, 2005). The usual conventional techniques such as adsorption on activated carbon, flocculation, reverse osmosis, etc., transfer the contaminants from one phase to another. Advanced oxidation processes (AOPs) have drawn significant attention to the mineralization of these dyes.

The reason for the utilization of AOPs was mainly due to the inability of biological processes to treat highly contaminated toxic waste water. In this regard, Fenton and photo-Fenton processes are quite efficient methods for the degradation of dye molecules (Ghiselli et al., 2004; Ntampegliotis et al., 2006). This process is quite simple, inexpensive, effective and promising in the water treatment industry. The Fenton reaction involves the in situ production of hydroxyl radicals which are non selective in their attack on pollutant molecules. The main advantages of the photo-Fenton process can be summarized as follows: (i) depth of light penetration is high since the solution is transparent; (ii) contact between the pollutant and the oxidizing agent is high; (iii) no adsorption is needed since the process is homogeneous, (iv) cost effective and simple. Methyl orange (MO) is an acid dye be-

*Corresponding author, e-mail: gomatidevi [email protected]

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L. G. Devi et al./Chemical Papers 64 (3) 378–385 (2010)

(CH3)2N

N

N

SO3

Methyl orange (MO)

379

III, gold orange, tropaeoline D), 4-dimethylaminoazobenzene-4 -sulfonic acid sodium salt (Fig. 1), was supplied by BDH (Bangalore, India) and was used as received.

HO

Irradiation procedure OH

OH NH4

(CH3)2N

NO3

SO3H

OH

OH

OH

– H2O

O

CO2

H 2O O

CO2

The photocatalytic degradation experiment was carried out in a circular glass reactor (150 mm × 75 mm) whose surface area was 176 cm2 . Irradiation of the reactor was made by direct exposure of the solution with a 125 W medium pressure mercury vapor lamp (Philips) at a distance from 29 cm. Photon flux of the light source was found to be 7.75 mW cm−2 as determined by the ferrioxalate actinometry, the peak maximum was found at around 370 nm. The lamp was warmed for 10 min to reach a constant output. In a typical experiment, 200 mL of the 10 mg L−1 dye solution with calculated amounts of the Fe2+ /Fe3+ solution along with the oxidizing agent (H2 O2 /APS) were added. The pH value of the dye solution was 6.6 and it dropped to 2.2 on addition of the Fe2+ /Fe3+ ions. pH of the solution was adjusted by either adding dilute NaOH or H2 SO4 . Most of the experiments were carried out thrice under identical conditions and the error between the consecutive experiments was in the range of approximately 2.4 %. Analytical methods

H2O

Fig. 1. Probable degradation pathway for MO.

longing to the mono azo series used as a pH indicator and also as a silk and wool dye. Azo dyes are resistant to aerobic degradation (Pagga & Brown, 1986), but they can be decolorized by anaerobic treatment (Brown & Hamburger, 1987) leading to the production of toxic by-product such as aromatic amines (Baughman & Weber, 1994).Therefore their degradation process in aqueous media becomes very important. The present research work investigates the influence of oxidation states of iron, Fe2+ /Fe3+ , in the presence of symmetrical peroxides like H2 O2 and APS for the decolorization of MO under UV light. In particular, direct comparison of the influence of various processes like Fe2+ /H2 O2 /UV, Fe3+ /H2 O2 /UV, Fe2+ /APS/UV, and Fe3+ /APS/UV on the decolorization kinetics was made.

Experimental Hydrogen peroxide (30 mass %), ferrous ammonium sulfate hexahydrate, anhydrous ferric chloride, ammonium persulfate, and methanol were supplied from NICE chemicals (Bangalore, India). Methyl orange (MO) (synonymous names: helanthine B, orange

The samples (5 mL) were taken from the reactor at definite time intervals. They were analyzed by the UVVIS spectroscopic technique using a Shimadzu UV1700 Pharmaspec UV-VIS spectrophotometer. The samples were extracted into a non-aqueous ether medium and 1 µL of each was subjected to the GCMS analysis (using a GC-MS-QP-5000 Shimadzu) and a Thermo Electron Trace GC ultra coupled to a DSQ mass spectrometer equipped with an Alltech ECONOCAP-EC-5 capillary column (30 m × 0.25 mm i.d. × 0.25 mm film thickness) was used. Pure helium was used as the carrier gas at the flow rate of 1.2 mL min−1 . The injector, transfer line, and trap temperatures were 220 ◦C, 250 ◦C, and 200 ◦C, respectively. Electron impact ionization was carried out at 70 eV.

Results and discussion The various reactions between Fe2+ /Fe3+ with oxidants H2 O2 /S2 O2− along with their apparent rate 8 constants are shown in Table 1 (Kušić et al., 2006; Truong et al., 2004). Degradation in the Fenton process (dark) was ∼ 5 % for all the oxidation processes. This indicates that oxidants and oxidation states of iron have equivalent effect in dark. Complete decolorization for the above processes was achieved when the system was amended with UV light. The higher efficiency of photo-Fenton/photo-Fenton-like processes


380


Table 1. Reactions involved in photo-Fenton/photo-Fenton-like processes Reaction

Eq.

ka

Fe2+ + H2 O2 → Fe3+ + HO. + HO− Fe3+ + H2 O2 → Fe2+ + H+ + HOO. Fe3+ + HOO. → Fe2+ + O2 + H+ Fe3+ + H2 O → Fe2+ + HO. + H+ Fe2+ + HO. → Fe3+ + HO− −. Fe2+ + S2 O2− → Fe3+ + SO2− 8 4 + SO4 . −. 2− + + HO + H SO +H O → SO

(1) (2) (3) (4) (5) (6)

63–76 M−1 s−1 0.01–0.02 M−1 s−1 (0.1–3.1) × 105 M−1 s−1 3.33 × 10−6 M−1 s−1 (3.0–4.3) × 108 M−1 s−1 –

(7)

6.6 × 102 s−1

(8)

22.9 M−1

(9)

389 M−1

(10)

4470 M−2

(11) (12) (13) (14)

– – (4.2–5.3) × 109 M−1 s−1 –

4

2 Fe2+

4 + SO2− → FeSO4 4 Fe3+ + SO2− → FeSO+ 4 4 2− 3+ Fe + 2SO4 → Fe(SO4 )− 2 . Fe3+ + S2 O2− → Fe2+ + 2SO− 4 8 . + − H + HO + e → H2 O

HO. + HO. → H2 O2 . − + HO. → SO− 4 + HO

SO2− 4

a) Values of rate constants were taken as reported by Kušić et al. (2006) and Truong et al. (2004).

Effect of pH pH of the solution plays a significant role in the effective action of oxidants on the pollutants. Kang et al. (2002) reported that the photo-Fenton process can remove pollutants only in acidic condition since higher pH values are reported to be unsatisfactory. Degradation of the dye was carried out in the pH range of 1.2–9.5 by maintaining the concentration of Fe3+ ions and H2 O2 constant. At the lower pH, 1.2, the degradation rate is reduced due to the excess H+ ions present in the solution acting as hydroxyl radical scavengers according to Eq. (12) in Table 1 (Barbusi´ nski & Majewski, 2003). When pH of the medium was increased from 1.2 to 3.0, complete decolorization of the dye was achieved. At pH 3, half of the iron species exists as Fe3+ ions and half as complexes of [Fe (OH) (H2 O)5 ]2+ which are the dominant photo active species with highest light absorption coefficient and quantum yield for hydroxyl radicals production along with Fe2+ ions regeneration in the wavelength range of 280–370 nm. A change in this optimum pH leads to the decrease in the concentration of [Fe (OH)]2+ complexes and it can also result in precipitation of ferrous ions as oxyhydroxides. However, the exact ratio of these photoactive species is uncertain because the measurements generally suffer from uncertainty (Barnum, 1983). The various photoactive species of iron formed under different pH conditions 3+ 2+ are: [Fe (H2 O)6 ] (pH 1–2), [Fe (OH) (H2 O)5 ] (pH + 2–3), and [Fe (OH)2 (H2 O)4 ] (pH 3–4) (Neamtu et al., 2003). Beyond the optimum pH 3, degradation rate slightly decreased and in the alkaline medium, the dye

100

Decolorization/%

can be ascribed to the huge production of hydroxyl radicals under UV light. Oxidation and reduction of iron ions continuously take place in the photo-Fenton process whereas this cycle of reactions is rather slow in the dark Fenton process.

80 60 40 20 0

1

2

3

4

5

pH

6

7

8

9

10

Fig. 2. Influence of pH on the decolorization under experimental conditions: c[Fe3+ ] = 6 mg L−1 , c[H2 O2 ] = 10 mg L−1 , and c[MO] = 10 mg L−1 .

almost resisted degradation as shown in Fig. 2. This inefficiency at higher pH values may be caused by the instability of Fe2+ /Fe3+ ions since they precipitate as iron oxyhydroxide which reduces the concentration of hydroxyl radicals in the solution affecting thus the decolorization rate. Effect of oxidizing agents The influence of oxidant’s concentration on the degradation rate was monitored by maintaining other reaction parameters constant. The present study investigates the application of H2 O2 (hydrogen peroxide) and S2 O2− 8 (peroxydisulfate) which are symmetrical peroxides and can be used as potential oxidants in light induced reaction processes. Persulfate generates free radicals like sulfate and hydroxyl radicals which provide free radical mechanism similar to the hydroxyl radical pathways generated in the Fenton



5

3

-2

k · 10 /min

-1

4

2 1 0

0

10

20

30

40

-1

50

c /(mg L ) Fig. 3. Apparent rate constant k versus concentration of oxidants (H2 O2 /APS) on the decolorization kinetics: Fe2+ /H2 O2 /UV ( ), Fe3+ /H2 O2 /UV ( ), Fe2+ / APS/UV (), Fe3+ /APS/UV ( ); c[Fe3+ ] = 6 mg L−1 , c[Fe2+ ] = 10 mg L−1 for H2 O2 , c[Fe2+ /Fe3+ ] = 10 mg L−1 for APS. Standard deviations for all the experiments varied in the range of ± 0.009.

◦

•

5

3

-2

k · 10 /min

-1

4

2 1

0

3

6

9 2+

12 3+

15

18

21

-1

c [Fe /Fe ]/(mg L ) Fig. 4. Apparent rate constant k versus concentration of Fe2+ /Fe3+ ions on the decolorization kinetics: Fe2+ / H2 O2 /UV ( ), Fe3+ /H2 O2 /UV ( ), Fe2+ /APS/UV (), Fe3+ /APS/UV ( ); c[H2 O2 ] = 10 mg L−1 ; c[APS] = 20 mg L−1 . Standard deviations for all the experiments varied in the range of ± 0.009.

•

◦

process. The sulfate radical is one of the strongest oxidizing species in aqueous media with the redox potential of 2.6 V. It is next only to the hydroxyl free radical whose redox potential is 2.8 V. With an increase in the H2 O2 concentration from 2 mg L−1 to 10 mg L−1 , the decolorization efficiency of the Fe2+ /H2 O2 /UV and Fe3+ /H2 O2 /UV processes increased due to excess generation of hydroxyl radicals. The apparent rate constant of the photo-Fenton-like process is high compared to the photo-Fenton process. A further increase

381

in the concentration of oxidant leads to a decrease in the efficiency of the process. This can be due to the recombination process of the excess hydroxyl radicals generated as shown in Eq. (13). Similar experiments were carried out with APS and its optimum concentration was found to be 20 mg L−1 for both oxidation states of iron (Fig. 3). The decolorization time was longer when using APS for both oxidation states of iron compared to H2 O2 . This can be caused by the fact that APS generates protons along with hydroxyl radicals, which lowers pH of the reaction medium. The excess protons act as hydroxyl radical scavengers and lower the degradation efficiency. In addition, the sulfate anion forms a strong complex with Fe2+ /Fe3+ ions thereby preventing these ions to take part in the cyclic Fenton process (Eqs. (8)–(10)). Sulfate radicals can also scavenge the generated hydroxyl radicals thereby affecting the decolorization kinetics (Eq. (14)). Final pH of the solution remained at the level of 3.3–3.5 for H2 O2 . While for APS, final pH of the solution was in the range of 2.7–2.8. Effect of Fe2+ /Fe3+ ions Optimization of the catalyst is a necessary step in the photo-Fenton reaction mechanism. In the presence of oxidants alone, 47 % and 25 % of the dye was decolorized after three hours of UV illumination for H2 O2 and APS, respectively. Higher efficiency of H2 O2 is caused by the fact that direct photolysis of H2 O2 yields two hydroxyl radicals, while APS results in the formation of one sulfate radical. This sulfate radical, in a subsequent reaction with water, generates one hydroxyl radical (Eq. (7)). The production of hydroxyl radicals is a single step for H2 O2 as an oxidant while two steps are needed for APS. However, complete decolorization at a higher rate was achieved in the presence of Fe2+ /Fe3+ ions along with the oxidants. The Fe2+ /Fe3+ ions decompose the oxidants to the respective free radicals under UV light at a higher rate thus increasing the concentration of hydroxyl radicals in the solution, which enhances the degradation rate. The apparent rate constant calculated for the Fe3+ /H2 O2 /UV process is 4.6 fold higher than that of the Fe2+ /H2 O2 /UV process, despite the fact that the rate constant of the reaction between Fe3+ and H2 O2 is lower than that of the reaction between Fe2+ and H2 O2 (Fig. 4). Higher efficiency of Fe3+ ions can be ascribed to the formation of a hydroperoxyl radical which reduces the Fe3+ ion to the Fe2+ ion at a much higher rate (Eq. (3)). The Fe2+ ions so formed react with H2 O2 generating hydroxyl radicals (Eq. (1)). Fe3+ ions can also react with water forming hydroxo complexes which on UV irradiation generate Fe2+ ions and hydroxyl radicals (Eq. (4)). Though this sequence of reactions can also take place in the process with Fe2+ ions as the catalyst, it is still vital that these reactions depend mainly on pro-


382


Table 2. Decolorization at different initial dye concentrations for photo-Fenton and photo-Fenton-like processes c[MO]/(mg L−1 )

10 15 20

Decolorization/% Fe2+ /APS/UV

Fe3+ /APS/UV

Fe2+ /H2 O2 /UV

Fe3+ /H2 O2 /UV

100 54 31

100 66 40

100 60 40

100 76 60

Table 3. Decolorization, apparent rate constant (k), and process efficiency (Φ) in the absence (A) and in the presence (B) of methanol under optimized experimental conditions k · 10−2 /min−1

Decolorization/%

Φ · 10−12 /(mg L−1 Einstein−1 )

Advanced Fenton process Fe2+ /H2 O2 /UV Fe3+ /H2 O2 /UV Fe2+ /APS/UV Fe3+ /APS/UV

A

B

100 100 100 100

32 15 54 73

A 1.01 4.65 0.86 1.17

portional concentration of the Fe2+ /Fe3+ ions. The process with Fe3+ ions as the catalyst has higher concentration of these ions, which accounts for its higher efficiency. The concentration of Fe2+ was optimized to 10 mg L−1 and Fe3+ was optimized to 6 mg L−1 with H2 O2 , while both Fe2+ /Fe3+ were optimized to 10 mg L−1 with APS. The apparent rate constant calculated for the Fe3+ /APS/UV process was 1.36 fold higher than that of the Fe2+ /APS/UV. Fe2+ on reaction with APS generates one sulfate radical (Eq. (6)) which on reaction with water generates one hydroxyl radical. In contrast, Fe3+ generates two sulfate radicals which on reaction with water generate two hydroxyl radicals (Eq. (11)). Hence, a higher number of hydroxyl radicals is generated, which accounts for higher efficiency of the Fe3+ /APS/UV process. Higher concentration of the Fe2+ /Fe3+ ions reduces the apparent rate constant for both the oxidants due to the scavenging effect of hydroxyl radicals by the Fe2+ ions in the solution (Devi et al., 2009) (Eq. (5)). Effect of initial concentration of dye Efficiency of the dye degradation depends on the initial concentration of the dye. The degradation of the dye at various initial concentrations was investigated by maintaining the constant concentration of Fe2+ /Fe3+ ions and H2 O2 /APS. When the dye concentration was 10 mg L−1 , complete decolorization was achieved in the desired time by all oxidation processes studied. With the increase in the initial concentration to 15 mg L−1 , the decolorization efficiency decreased for all oxidation processes. Table 2 shows the percentage decolorization efficiency of all oxidation processes for different initial concentrations of the dye. Higher initial concentration lowers the efficiency due to insufficient availability of the oxidizing

± ± ± ±

B 006 008 005 007

0.18 0.23 0.24 0.56

± ± ± ±

006 004 003 002

A

B

16.2 36.5 10.8 13.9

5.82 5.4 5.8 10.1

agent; moreover, the dye solution can serve as an inner filter reducing the number of photons entering the solution (Dutta et al., 2001). This process decreases the concentration of hydroxyl radicals in the solution. It is therefore preferential to have a lower concentration of the pollutant to achieve higher rates in the photo-Fenton/photo-Fenton-like processes. The Fe3+ /H2 O2 /UV process retained its efficiency even at higher dye concentrations probably due to the excess generation of hydroxyl radicals. Effect of scavenger The role of hydroxyl radicals in the degradation of the MO dye was confirmed by carrying out the degradation in the presence of a free radical scavenger like methanol. Methanol is known to deactivate the hydroxyl radical and its derivatives (Devi et al., 2009). Methanol reacts with hydroxyl radicals and to a lower extent with hydrogen radicals whose second order rate constants are 9.7 × 108 mol−1 s−1 and 2.6 × 106 mol−1 s−1 , respectively (Eqs. (15) and (16)). . . CH3 OH + OH → CH3 O + H2 O . . CH3 OH + H → CH3 O + H2 c0 − c Φ= tIS

(15) (16) (17)

Though the initial step of the hydroxyl radicals quenching is the same for both the oxidants, the extent of quenching depends not only on the nature of the oxidants but also on the oxidation process. The percentage decolorization, apparent rate constant and the process efficiency calculated in the presence and in the absence of alcohol are shown in Table 3. Process efficiency can be defined as the concentration of the pollutant degraded divided by the amount of energy


383


1.2

5

A

Absorbance/a.u.

k · 10

-2

4 3 2 1 0

0.8

0.4

C

0.0 190 0.00

0.05

0.10

-1

0.15

0.20

c [MeOH] /(mol L ) Fig. 5. Effect of methanol concentration on the degradation ki-

•

netics of Fe3+/ H2 O2 /UV ( ) and Fe3+ /APS/UV ( ) processes. Standard deviations for all the experiments varied in the range of ± 0.009.

in terms of intensity and exposure surface area per time (Eq. (17), where c0 is the initial concentration of the dye substrate and c is the concentration at time t and (c0 − c) denotes the concentration of the dye degraded in mg L−1 , I is the irradiance [I/(Einstein m−2 s−1 ) = 8.36 × λ (nm) × power (W)], where λ is 370 nm and the power is 125 W, S denotes the solution irradiated plane surface area in cm2 and t represents the irradiation time in min). The decrease in the apparent rate constant is larger for the oxidant H2 O2 compared to APS (Fig. 5). This is due to the inability of methanol to quench sulfate radicals which mediate the degradation even in the absence of hydroxyl radicals. Possible reactions of sulfate radical anions with the organic pollutants are: (i) abstraction of a hydrogen atom from the saturated carbon; (ii) addition to unsaturated compounds; (iii) removal of an electron from anions and neutral molecules (Neta et al., 1977). The process with Fe3+ ions as the catalyst generates a higher number of hydroxyl radicals compared to Fe2+ ions. Hence, methanol quenches hydroxyl radicals to a greater extent and thus lower apparent rate constant is observed for the Fe3+ ion compared to Fe2+ ion mediated processes. The largest decrease in the apparent rate constant was observed for the Fe3+ /H2 O2 /UV process. This provides evidence for the importance of hydroxyl radicals in the photo-Fenton/photo-Fentonlike degradation mechanisms.

B

300

400 Wavelength/nm

500

Fig. 6. UV-VIS spectra of the MO dye before irradiation (A), at 30 min (B), and at 4 h (C) under experimental conditions: c[Fe3+ ] = 6 mg L−1 , c[H2 O2 ] = 10 mg L−1 ; pH 3.0.

Table 4. Major photoproducts of the degradation process identified by the GC-MS technique Degradation product 4-Dimethylaminophenol 4-Hydroxybenzenesulfonic acid Phenol 1,4-Dihydroxybenzene Hydroquinone Benzene

m/za 137 174 94 110 108 78

a) Values given for parent peaks [M]+ .

lution changes from orange yellow to red due to the formation of monoprotonated form of MO which exists as a resonance hybrid between its quinine diimine and azonium structures. In an acidic solution, more intense absorption band shift to higher wavelengths (at 506 nm). The visible region band can be attributed to azonium ions. Two new bands at 317 nm and 226 nm in the UV region appeared due to the modification of the π system delocalization. On irradiation, the band at 506 nm reduced its intensity and completely disappeared at 30 min and no new band appeared in the UV-VIS region. This confirms that photodegradation of the azo chromophore is responsible for the color of the MO dye molecule. The band at 229 nm decreased slowly indicating that the intermediates formed during the degradation contain a benzene ring. At 4 hours, the band completely disappeared confirming the complete degradation of the dye molecule as shown in Fig. 6.

UV-VIS spectroscopic analysis GC-MS analysis MO in the pH range of 4.1–9.5 exhibits a band at 465–485 nm in the visible region attributed to the azo form of the dye. The bands at 276 nm and 197 nm may be caused by the presence of benzene rings in MO. Under progressive protonation, color of the so-

Mass spectrum recorded at the time of decolorization (for the experiment using 10 mg L−1 of Fe3+ , H2 O2 , and MO, respectively, at pH 3) showed two peaks at m/z 137 and 174 corresponding to the


384


Fig. 7. Mass spectra of various intermediates formed during the reaction after the irradiation time of 30 min (A), 90 min (B), 2.5 h (C); c[Fe3+ ] = 10 mg L−1 , c[H2 O2 ] = 10 mg L−1 ; pH 3.0.

formation of 4-hydroxy-N,N-dimethylaniline and 4hydroxybenzenesulfonic acid, respectively (Table 4, Fig. 7). This shows that the degradation of azo dye by the homogeneous Fenton process proceeds through a rupture of azo bonds by the attack of hydroxyl radicals. Low intensity peaks were not accounted for. The solution after 90 min of irradiation showed intense peaks at m/z 94 and 110 and a low intensity peak at m/z 108, which may be attributed to the formation of phenol, 1,4-dihydroxybenzene, and hydroquinone, respectively. Formation of these intermediates can be explained as follows: (i) phenol is formed by the loss of the N,N-dimethyl substituent as a secondary amine; (ii) 4-hydroxybenzenesulfonic acid, on the loss of the sulfonate group followed by the subsequent hydroxylation, can give 1,4-dihydroxybenzene; (iii) 1,4-dihydroxybenzene can be oxidized to hydroquinone in the presence of atmospheric oxygen. The solution, after 2.5 h of irradiation, showed a peak at m/z 78 indicating the formation of benzene. The dehydroxylation of phenol, 1,4-dihydroxybenzene

and the loss of the sulfo group (SO3 H) in benzenesulfonic acid might result in the formation of benzene. Complete degradation of MO was confirmed by a peak at m/z 44 corresponding to the M+ ion of formed CO2 . Based on the intermediates analyzed by the UV-VIS spectroscopic analysis and the GC-MS technique, a probable degradation pathway was proposed (Fig. 1).

Conclusions Photo-Fenton and photo-Fenton-like processes were used as potential techniques for the degradation of MO. The oxidation state of iron ions had a significant effect on the decolorization kinetics. It was found that processes with Fe3+ ions as the catalyst show higher process efficiency compared to the processes with Fe2+ as the catalyst. H2 O2 served as a better oxidant for both oxidation states of iron compared to APS. A decrease in the apparent rate constant in the presence of methanol confirmed the role of hydroxyl radicals in the degradation process.



Efficiency of the studied processes for the degradation of MO follows the order: Fe3+ /H2 O2 /UV, Fe3+ /APS/UV, Fe2+ /H2 O2 /UV, Fe2+ /APS/UV. The degradation was monitored by the UV-VIS spectroscopy and GC-MS techniques. Based on the intermediates analyzed, a probable degradation mechanism has been proposed. Acknowledgements. Financial support from the UGC Major Research Project (2007–2010), India is acknowledged.

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