Comparison of dimethyl sulfoxide degradation by different Fenton processes

Chemical Engineering Journal 232 (2013) 418–424

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Comparison of dimethyl sulfoxide degradation by different Fenton processes Mark Daniel G. de Luna a, James I. Colades a, Chia-Chi Su b, Ming-Chun Lu b,⇑ a b

Department of Chemical Engineering, University of the Philippines – Diliman, Quezon City, Philippines Department of Environmental Resources Management, Chia-Nan University of Pharmacy and Science, Tainan, Taiwan

h i g h l i g h t s  The DMSO degradation was compared by different Fenton processes.  The effect of current density on the DMSO degradation was insignificant.  The DMSO degradation efficiency improved at chloride ion concentration was low. 3





 Inorganic ions inhibited the DMSO degradation in the following sequence: PO4 > F > Cl .

a r t i c l e

i n f o

Article history: Received 29 March 2013 Received in revised form 23 July 2013 Accepted 26 July 2013 Available online 7 August 2013 Keywords: Dimethyl sulfoxide Fenton process Advanced oxidation process Inorganic ions

a b s t r a c t Degradation of dimethyl sulfoxide (DMSO) was examined under varying parameters, including initial pH, initial Fe2+, initial H2O2, initial DMSO concentrations and current density. Efficiency in terms of degradation of DMSO and removal of total organic carbon (TOC) was also compared among Fenton, photo-Fenton and photoelectro-Fenton (PEF) processes. Effects of inorganic ions, namely Cl, F and PO3 4 , on DMSO degradation by the electro-Fenton (EF) process were also studied. The experimental results showed that the optimum pH was 2. The DMSO degradation in the double-cathode EF reactor reached 100% when current density and Fe2+ concentration exceeded 1.5 A and 2.0 mM, respectively. During DMSO degradation, application of electricity by EF process obtained a higher DMSO degradation rate compared to UVassisted Fenton process. The DMSO degradation rate after 20 min was 2.5 times higher in the PEF process than in the Fenton process. Between 20 min and 120 min, the DMSO degradation rates of the Fenton, photo-Fenton, single-cathode EF, double-cathode EF and PEF processes differed by 34%, 50%, 62%, 57% and 59%, respectively. The rate constant of double-cathode EF process was six times higher than the conventional Fenton process. The order of inhibiting effects of inorganic ions on DMSO degradation was   PO3 4 > F > Cl . Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Dimethyl sulfoxide (DMSO) is a colorless, organic liquid widely used as detergent or stripping agent in the manufacture of semiconductors and thin-film transistor liquid crystal display (TFTLCD) because of its excellent solvent properties [1]. DMSO along with other organic solvents, arise mainly from washing and rinsing processes, and account for nearly one-third of the estimated 200,000 m3/d wastewater generated by the TFT-LCD industry [2]. Ordinarily, semiconductor and TFT-LCD effluents contain 500– 800 mg DMSO per liter [3] and <1000 mg/L biodegradable nonDMSO organic compounds [4]. Dimethyl sulfoxide is toxic because it is easily absorbed by organisms, and it contributes to high osmo-

⇑ Corresponding author. Tel.: +886 6 2660489; fax: +886 6 2663411. E-mail addresses: [email protected], [email protected] (M.-C. Lu). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.07.107

tic pressure in biological systems [5]. DMSO is also thermally and chemically stable [6]. Several methods have been applied to treat DMSO-containing wastewater including biological treatment [2], photocatalysis by TiO2 [7] and ozone-based method [8]. However, biological treatment under aerobic conditions does not yield acceptable levels of DMSO decomposition and under anaerobic environment, DMSO biodegradation produces harmful intermediates [8]. Advanced oxidation processes (AOPs) such UV/H2O2 [9], Fenton [6] and Fentonlike [10] have also been used for DMSO removal since semiconductor and TFT-LCD industries employ hydrogen peroxide for various etching processes [11–13]. Typical etching solutions may contain chloride [13–15], fluoride [16–18]and phosphate [14,19]. AOPs have been widely used in wastewater treatment applications for the degradation of highly concentrated refractory and biologically toxic chemical pollutants [20–22]. One of the most popular AOP is the Fenton process which involves the catalysis of

M.D.G. de Luna et al. / Chemical Engineering Journal 232 (2013) 418–424

Fe3þ þ e ! Fe2þ

419

ð7Þ

The simultaneous application of ultraviolet (UV) irradiation and the Fenton reaction, known as the photo-Fenton process, enhances the oxidation rate of pollutant by photoreduction of ferric ions and ferric complexes. UV irradiation combined with the electro-Fenton process, on the other hand, is the so-called photoelectro-Fenton (PEF) process. For both photo-Fenton and PEF processes, ferrous ions are recycled continuously by UV irradiation and are not depleted during the oxidation reaction (Eq. (8)) [26].

FeðOHÞ2þ þ hm ! Fe2þ þ  OH

ð8Þ

Recently, a comparison between Fenton, electro-Fenton, sonoelectro-Fenton and photo-electro-Fenton processes on the degradation of phenol has been reported [27]. The results showed that degradation efficiency was observed in the order: photo-electroFenton > sono-electro-Fenton > electro-Fenton > Fenton. A high H2O2 to Fe2+ molar ratio was used to degrade 200 mg/L phenol solution. H2O2 concentration of 500 mg/L, Fe2+ dosage of 4 mg/L, initial pH of 3, current density of 12 mA/cm2 and optimum electrode distance of 5 cm resulted in high phenol degradation and COD removal. Given the high concentrations of DMSO in wastewaters generated by semiconductor and TFT-LCD industries, the electroFenton process was applied in this study for DMSO removal. Factors affecting DMSO degradation by the EF process such as pH, catalyst dosage, H2O2 loading, current density and initial DMSO concentration were investigated. In addition, conventional Fenton, photo-Fenton, electro-Fenton and photoelectro-Fenton processes were compared in terms of DMSO degradation and total organic carbon (TOC) removal. The effect of inorganic ions, i.e., Cl, F and PO3 4 , on DMSO degradation by the electro-Fenton process was also investigated. The presence of these ions is frequently reported in studies of semiconductor and TFT-LCD wastewaters [13–19]. 2. Materials and methods

Fig. 1. Schematic diagram of the photoelectron-Fenton reactor.

2.1. Chemicals hydrogen peroxide (H2O2) by ferrous ions (Fe2+) to produce hydroxyl radicals (OH) according to Eq. (1). The Fenton reaction takes place in acidic medium at pH 2–4 [23,24]. The oxidant produced is the hydroxyl radicals which is capable of complete mineralization of refractory pollutants. Fenton reaction might involve several sequential reactions

Fe2þ þ H2 O2 ! Fe3þ þ  OH þ OH Fe

3þ

2þ

þ H2 O2 $ FeOOH 2þ

FeOOH

! Fe

2þ

þ

þH



þ O2 H

3þ





OH þ Fe



OH þ H2 O2 ! H2 O þ  O2 H

! Fe

k ¼ 0:001  0:01 M

ð1Þ 1

s

1

ð2Þ ð3Þ

Fe3þ þ H2 O ! FeðOHÞ2þ þ Hþ 2þ

k ¼ 70 M1 s1

þ OH

All chemicals were of reagent-grade and were used without further purification. Stock solutions were prepared using deionized water (18.2 MO resistivity) from a Millipore system. Synthetic dimethyl sulfoxide wastewater (5–40 mM) was prepared by diluting DMSO (C2H6SO, 99.5%, Merck) with deionized water. Hydrogen peroxide (H2O2, 35%, Merck) and ferrous sulfate heptahydrate (FeSO47H2O, 99.5%, Merck) were used.

ð4Þ ð5Þ ð6Þ

The difference in rate constants of Eqs. (1) and (2) shows that ferrous ions are consumed faster than they are produced. Consequently, ferric hydroxide sludge is produced during the neutralization stage which requires additional treatment and disposal [25]. An improvement of the conventional Fenton process is the application of electricity for more effective and rapid regeneration of ferrous ions from the ferric ions produced by the Fenton reaction as shown in Eq. (7), the so-called electro-Fenton (EF) process. The EF process continuously generates hydroxyl radicals even at low ferrous ion dosage due to catalyst regeneration. Hence, minimal ferric hydroxide sludge is produced during the neutralization stage of wastewater treatment.

2.2. Analytical methods The concentration of DMSO was measured using a HighPerformance Liquid Chromatography (HPLC) instrument having a UV1000 detector set at 254 nm, a SpectraSYSTEM SN4000 system controller and an Asahipak ODP-50 6D column (150 mm  6 mm  5 mm). The HPLC mobile phase was 40% acetonitrile (>99.5%, Merck) and 60% deionized water. The Fe2+ concentration was measured with a UV–visible spectrophotometer (Thermo Spectronic Genesys 20) set to 510 nm. The TOC data were obtained with an Elementar-liqui TOC analyzer. 2.3. Experimental set-up The cylindrical vessel, used as a batch reactor in the study, had concentric electrodes and a working volume of 4L (Fig. 1). The cathodes were stainless steel (diameters, 2 and 13 cm) while the anode was a RuO2/IrO2-coated titanium net (diameter, 7 cm). The reactor

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was equipped with a probe sensor to monitor changes in solution pH and a centrifugal pump for fluid mixing and recirculation. For EF and PEF experiments, electric current was applied and maintained throughout the experiment. For photo-assisted Fenton and PEF experiments, 16 UVA lamps (Sunbeamtech.com), placed between the anode and the outer cathode (Fig. 1), supplied 48 W of photoionization energy.

3. Results and discussion 3.1. Effects of operating parameters on DMSO degradation Fig. 2 shows the effect of important parameters on DMSO degradation in a double-cathode EF reactor. The pH of the solution controls the production of hydroxyl radicals and the concentration of Fe2+ [28,29]. Fig. 2(a) shows the effect of pH on DMSO degradation. Increasing the pH of the solution from 1.0 to 2.0 improved the degradation efficiency of DMSO from 42% to 100% (Fig. 3(a)). However, DMSO degradation decreased from 100% to 50% when pH was further raised from 2.0 to 3.0. The optimum pH obtained was at 2. Masomboon et al. [23] reported that Fe3+ precipitates in the form of amorphous Fe(OH)3 at pH above 3. The formation of Fe(OH)3 decreases the dissolved Fe3+ concentration and inhibits Fe2+ regeneration by partially coating the electrode surface. Fig. 2(b) shows the effect of current density on DMSO degradation. DMSO degradation efficiency was invariant when the current density ranged from 0.5 to 2.5 A. After 60 min, DMSO degradation efficiency exceeded 60% at various current densities. At the end of 120 min, DMSO degradation efficiencies were 91% and 100%, when 0.5 A and 1.5 A were applied, respectively. These results can be explained by the faster electro-regeneration rate of ferrous ions from ferric ions with higher current. Ferrous ions catalyze the conversion of hydrogen peroxide into hydroxyl radicals, which in turn degrade toxic contaminants. Usually, refractory pollutant degradation efficiency by Fenton processes increases with Fe2+ concentration. The experimental results show that DMSO degradation improved when initial Fe2+ concentration increased from 1.0 to 3.0 mM (Fig. 2(c)). DMSO degradation efficiency reached 100% when using an initial Fe2+ concentration of 2.0–3.0 mM (Fig. 3(b)). Thus, the optimum initial Fe2+ concentration obtained from this experiment was 2.0 mM.

2.4. AOP experiments A known amount of FeSO4.7H2O (1.0–3.0 mM) was added to synthetic DMSO wastewater. The pH (1.0–3.0) of the wastewater was adjusted using 1 N HClO4 (70%, Fullin Chemical Co. Ltd.) and 1 N NaOH (>99%, Merck). Initial conditions of the system were determined. The desired amount of H2O2 (5–40 mM) was injected into the reactor to start the Fenton reaction. For electro-Fenton based processes, the introduction of constant electric current (0.5–2.5 A) was done simultaneously with the addition of hydrogen peroxide. Sampling was conducted at fixed time intervals (0, 5, 10, 20, 40, 60, 90 and 120 min). Each sample (4 ml) was combined with 15 ml deionized water and 1-ml 1 N NaOH to precipitate ferrous and ferric ions as hydroxides. The mixture was filtered using a 0.45 lm filter and the supernatant was separated for chemical analysis. All experiments were performed in duplicate at room temperature. The standard deviation of all measured values was less than 10%. Since electro-Fenton process is applicable for treating wastewater containing large amounts of organics [24], kinetic reactions were studied at DMSO concentration up to 40 mM. For the effect of inorganic ions on DMSO degradation by the EF process, known concentrations of chloride, fluoride and phosphate anions were incorporated to the simulated DMSO wastewater. A control experiment was conducted without addition of inorganic ions.

(b)

2.14 mM Fe 2+, 27 mM H2O2, 5 mM DMSO

1.0

DMSO remaining (C/C0)

0.8 0.6 0.4

pH 1.0 pH 1.5 pH 2.0 pH 2.5 pH 3.0

0.2

2.14 mM Fe2+, 27 mM H2O2, 5 mM DMSO

1.0

0.5 A 1.0 A 1.5 A 2.0 A 2.5 A

0.8 0.6 0.4 0.2 0.0

0.0 0

20

40

60

80

100

120

0

20

40

Time (min)

(d)

1.0 mM Fe 2+ 1.5 mM Fe 2+ 2.0 mM Fe 2+ 2.5 mM Fe 2+ 3.0 mM Fe 2+

1.0 0.8

DMSO remaining (C/C0)

DMSO remaining (C/C0)

(c)

0.6 0.4 0.2 0.0 27 mM H2O2, 5 mM DMSO, pH 2 0 20 40 60 80

Time (min)

120

80

100

120

(e)

1.0

5 mM H2O2 10 mM H2O2 20 mM H2O2 30 mM H2O2 40 mM H2O2

0.8 0.6 0.4 0.2

2.14 mM Fe 2+, 5 mM DMSO, pH 2

0.0 100

60

Time (min)

DMSO remaining (C/C0)

DMSO remaining (C/C0)

(a)

0

20

40

60

80

Time (min)

2.75 mM Fe 2+, 27 mM H2O2, pH 2

1.0 0.8 0.6 0.4

5 mM DMSO 10 mM DMSO 20 mM DMSO 30 mM DMSO 40 mM DMSO

0.2 0.0

100

120

0

20

40

60

80

100

120

Time (min)

Fig. 2. Effect of (a) pH, (b) current density, (c) Fe2+, (d) H2O2 and (e) DMSO concentrations on DMSO degradation in a double-cathode EF reactor (C and C0 were the concentrations of DMSO in the solution at any time t and time zero, respectively).

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100

(a)

DMSO degradation, %

DMSO degradation, %

100 80 60 40 20

60 40 20

2.14 mM Fe2+, 27 mM H 2O2, 5 mM DMSO

0

0.5

1.0

1.5

2.0

2.5

3.0

(b)

80

27 mM H2O2, 5 mM DMSO, pH 2

0 0.5

3.5

1.0

(c)

80 60 40 20 2.14 mM

0 0

5 mM DMSO, pH 2

10

20

2.0

2.5

3.0

100

DMSO degradation, %

DMSO degradation, %

100

Fe2+,

1.5

3.5

Fe2+ (mM)

pH

30

(d)

80 60 40 20 2.75 mM Fe2+, 27 mM H2O2, pH 2

0 40

50

0

10

H2O2 (mM)

20

30

40

50

DMSO (mM)

Fig. 3. The DMSO degradation efficiency at various (a) pH, (b) Fe2+, (c) H2O2 and (d) DMSO concentrations in a double-cathode EF reactor at 1.5 A and 120 min.

Pollutants are normally degraded by increasing the hydrogen peroxide concentration. Fig. 2(d) shows that DMSO degradation stopped at 40 min when the H2O2 concentration was low (5 mM). The reaction stopped because no more H2O2 was available in the solution to produce hydroxyl radicals. Fig. 3(c) shows that DMSO degradation decreased when the initial H2O2 concentration was higher than 20 mM. In the presence of excessive H2O2 concentration, degradation was inhibited by consumption of hydroxyl radicals by hydrogen peroxide (Eq. (6)). Fig. 2(e) shows that the increased DMSO concentrations significantly decreased DMSO degradation. DMSO degradation efficiency dropped from 100% to 20% when DMSO concentration increased from 5 to 40 mM (Fig. 3(d)). Similar results were presented by Muruganandham et al. [21], who reported that higher dye concentrations lowered removal efficiency and removal rate. The number of DMSO molecules in solution increased with DMSO concentration, but the amount of hydroxyl radicals generated remained the same resulting in the decline in DMSO degradation. 3.2. Comparison of different Fenton processes Fig. 4 compares DMSO degradation among various Fenton processes. Degradation efficiency was the lowest in the conventional

(b) 1.0 0.8

0.8

TOC (C/C0)

DMSO remaining (C/C0)

(a) 1.0

Fenton process and the highest in the photoelectro-Fenton process (Fig. 4(a)). The same trend was observed in TOC removal (Fig. 4(b)). These experimental results show that electricity and UV irradiation substantially improves the ability of the Fenton reaction to degrade DMSO and mineralize TOC. In terms of DMSO degradation efficiency, the electricity-assisted Fenton processes provided a better improvement compared to the UV-assisted Fenton process (Fig. 4(a)). Fig. 4(b) shows almost similar TOC removal efficiencies for both photo-Fenton and single-cathode EF processes while the double-cathode EF process exceeded both processes in the same respect. The improved efficiency of the EF process resulted from the efficient and rapid regeneration of ferrous iron (Eq. (7)), which limited the production of large amounts of ferric hydroxide sludge during DMSO degradation. The lines in Fig. 5 were fitted to the data by linear regression. The degradation of DMSO follows a first-order reaction, with all correlation coefficients r2 > 0.962. Generally, the degradation of organic wastewater such municipal and textile wastewater was fitted well for pseudo first-order reaction [30,31]. Double-cathode EF process had a higher rate constant at around 6 times than the conventional Fenton process. Fig. 6 shows the absorbance of the ferrous-phenanthroline complex with respect to time. Throughout the reaction, the double-cathode EF reactor maintained a higher Fe2+ concentration in solution, which

0.6 0.4 0.2

0.6 Fenton process Photo-Fenton Single-cathode EF Double-cathode EF Photoelectro-Fenton

0.4 0.2

0.0 0

20

40

60

Time (min)

80

100

120

0.0

0

20

40

60

80

100

120

Time (min)

Fig. 4. Comparison of (a) DMSO degradation and (b) TOC removal in various Fenton processes. Experimental conditions: 10 mM of DMSO, pH 2.0, 27.75 mM of H2O2, 2.76 mM of Fe2+ and 1.5 A.

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Fenton (k = 0.00858, r2 = 0.979) Photo-Fenton ((k = 0.01575, r2 = 0.991) Double-cathode EF (k = 0.05075, r2 = 0.962)

Degradation rate, mM min-1

0.25

(a) 0.0 ln (C/C0)

-0.5 -1.0 -1.5 -2.0

At 20 min At 120 min

0.20

0.15

Δ 62% Δ 50%

0.10

Δ 34% 0.05

0.0 Fenton 0

20

40

60

80

100

120

Time (min)

(b) 0.7

Process

0.2 0.0

20

40

60

Singlecathode EF

Doublecathode EF

PEF

Table 1 Efficiencies of H2O2 in DMSO degradation and TOC removal at various Fenton processes. Experimental conditions: 10 mM of DMSO, pH 2.0, 27.75 mM of H2O2 and 2.76 mM of Fe2+.

0.4

0

PhotoFenton

Fig. 7. DMSO Degradation rate in various Fenton processes. Experimental conditions: 10 mM of DMSO, pH 2.0, 27.75 mM of H2O2 and 2.76 mM of Fe2+.

Fenton (k = 0.00038, r2 = 0.328) Photo-Fenton (k = 0.00303, r2 = 0.833) Double-cathode (k = 0.00399, r2 = 0.700)

0.6

(1/C)-(1//C0)

Δ 59% Δ 57%

80

100

120

Fenton Photo-Fenton Single-cathode EF Double-cathode EF Photoelectro-Fenton

H2O2 efficiency EH, % DMSO

EH, % TOC

19 13 8 9 10

84 55 48 51 58

Time (min) Fig. 5. DMSO degradation data by various Fenton processes fitted into (a) pseudo first-order and (b) pseudo second-order kinetic models. Experimental conditions: 10 mM of DMSO, pH 2.0, 27.75 mM of H2O2, 2.76 mM of Fe2+ and 1.5 A.

Fe2+ absorbance

0.060

0.055

0.050

0.045 Single-cathode EF process Double-cathode EF process 0.040 0

20

40

60

80

100

120

Time (min)

DMSO degradation rates by the PEF process were higher than those by the Fenton process. Specifically, at 20 min, the DMSO degradation rate by the PEF process was 2.5 times greater than that by the Fenton process. Electricity and UV irradiation significantly improved DMSO degradation by the Fenton process. The DMSO degradation rates also decreased as reaction time increased (Fig. 7). For example, comparisons of DMSO degradation rates obtained by the Fenton, photo-Fenton, single-cathode EF, double-cathode EF and PEF processes after 20 min and after 120 min showed that the degradation rate differed by 34%, 50%, 62%, 57% and 59%, respectively. The smallest difference (34%) was observed in the Fenton process, which indicates that the hydroxyl radicals were efficiently generated during DMSO degradation. In the single-cathode EF, double-cathode EF and PEF processes, in which electricity was applied to the Fenton process, the difference in DMSO degradation rates exceeded 50%. Presumably, the electric current increased the rate at which Fe2+ regeneration on the cathode. Competitive reaction of Fe2+ then inhibited the DMSO degradation rate according to Eq. (5). The H2O2 efficiency of DMSO and TOC removal is based on the following equations:

Fig. 6. Dissolved Fe2+ in the single and double-cathode reactors.

represents another advantage of the double-cathode EF reactor over the single-cathode EF reactor. Similar results were obtained by Ting et al. [25], who reported that a cylinder-type double-cathode electrochemical reactor increased the work area for chemical reactions and increased current efficiency by 7%. The TOC removal increased rapidly during the first 20 min and then decreased until the reaction was complete (Fig. 4(b)). Hence, DMSO removals observed during the first 20 min and at the end of the reaction (120 min) were used to evaluate the degradation rates of DMSO. Fig. 7 shows the DMSO degradation rates in various Fenton processes at 20 and 120 min. The PEF process showed the highest DMSO degradation rate (0.2 mM min1). At 20 and 120 min,

DMSOi  DMSO  100% 0:47  ðC Hi  C H Þ TOC i  TOC For TOC : EH ; % ¼  100% 0:47  ðC Hi  C H Þ

For DMSO : EH ; % ¼

ð9Þ ð10Þ

where DMSOi and DMSO are the concentrations of DMSO in the solution at time zero and 120 min, respectively. TOCi and TOC are the concentrations of TOC in the solution at time zero and 120 min, respectively. The CHi and CH are the inlet H2O2 and final H2O2 concentrations, respectively. In Eqs. (9) and (10), the value 0.47 is used to indicate the disproportion in H2O2 by catalyst to give 0.5 mol O2 per mole H2O2 [32]. Low H2O2 efficiency causes inefficient scavenging reactions during treatment of DMSO and TOC

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(b) 0 mM Cl 2 mM Cl 20 mM Cl50 mM Cl-

0.8 0.6 0.4 0.2 0.0

(c)

1.0

0 mM F 2 mM F 20 mM F-

0.8

DMSO Removal (C/C 0 )

1.0

DMSO Removal (C/C 0 )

DMSO Removal (C/C 0 )

(a)

0.6 0.4 0.2 0.0

0

20

40

60

80

100

120

0 mM PO432 mM PO4320 mM PO43-

1.0 0.8 0.6 0.4

0.2 0.0

0

20

40

Time (min)

60

80

100

120

Time (min)

0

20

40

60

80

100

120

Time (min)

Fig. 8. Effect of (a) chloride ions, (b) fluoride ions and (c) phosphate ions on DMSO degradation in the double-cathode EF reactor. Experimental conditions: 5 mM of DMSO, 2.14 mM of Fe2+, 25.52 mM of H2O2, 1.5 A and pH 2.

[33]. Table 1 shows that the conventional Fenton process has the highest EH in terms of DMSO and TOC removal. Compared with the EF process, the EH value for DMSO degradation obtained by the Fenton process is almost twofold higher. The best explanation is the differing rates of H2O2 consumed by these processes. Equations (2) and (3) are applicable to the regeneration of ferrous ions in the conventional Fenton process, but the EF process can regenerate ferrous ions on the cathode surface. Equation (1) shows that, because of their very large rate constant, ferrous ions immediately react with H2O2 to produce hydroxyl radicals; nevertheless, not all of the produced hydroxyl radicals are used up in DMSO degradation. This reduces the EH value of the process. The EH values for the photoelectro-Fenton process are higher than those for the EF process due to the improvement obtained by using UV light for DMSO degradation. A possible explanation is the large amount of hydroxyl radicals generated according to Eq. (11) [9,25]. The two moles of hydroxyl radicals produced per mole of hydrogen peroxide might make the process more effective compared to production of only one mole hydroxyl radicals per mole hydrogen peroxide according to Eq. (1).

H2 O2 þ UV ! 2 OH

ð11Þ

In summary, all anions investigated in this study affected DMSO degradation. Among them, phosphate ions caused the greatest inhibition to DMSO degradation and the sequence of inhibition fol  lows PO3 4 > F > Cl . 4. Conclusion This study investigated the effect of pH, initial Fe2+, initial H2O2, initial DMSO concentrations and current density on DMSO degradation in a double-cathode EF reactor. In addition, the DMSO degradation was also compared among several Fenton processes, including Fenton, photo-Fenton, single-cathode EF, double-cathode EF and PEF. A degradation of 100% was achieved under experimental conditions of pH of 2, 5 mM of DMSO, 2.14 mM of Fe2+, 27 mM of H2O2 and 1.5 A current density after 120 min. In this study, the effect of current density on the DMSO degradation was insignificant. Increasing the current density from 0.5 to 1.5 increased DMSO degradation efficiency from 88% to 100% in 120 min. Of all the applied processes, PEF was the most effective for DMSO degradation and TOC mineralization. Inorganic ions inhibited DMSO   degradation in the following sequence: PO3 4 > F > Cl .

3.3. Effect of inorganic ions on DMSO degradation

Acknowledgments

Fig. 8 shows the effect of Cl, F and PO3 4 on DMSO degradation by the EF process. DMSO degradation efficiency of simulated wastewater containing 5 mM DMSO and 2 mM chloride ions was higher compared to the control experiment (Fig. 8(a)). Small concentrations of chloride ions improved the conductivity of the wastewater thereby increasing the efficiency of the Fenton treatment. At higher chloride ion concentration (>20 mM), Fenton reaction was inhibited by radical scavenging and complex formation [34,35]. Fe2+/Fe3+-chloride complexation reduced the amount of dissolved iron necessary for hydroxyl radical production [36]. Low fluoride concentration (2 mM) in simulated wastewater already caused a slight decline in DMSO degradation (Fig. 8(b)). The inhibitory effect of fluoride ions, however, is more pronounced at 20 mM F. Like chloride ions, fluoride ions, also form complexes and insoluble salts with iron in solution [37]. Compared to the control run, a much slower DMSO degradation was observed when phosphate of low concentration (2 mM) was present in the simulated wastewater (Fig. 8(c)). After 120 min, DMSO degradation efficiency dropped from 100% to 53%. Further increase in phosphate concentration from 2 to 20 mM, however, did not cause a significant reduction in DMSO degradation efficiency. At these conditions, the ferric ion concentration is already the limiting reactant in Eq. (12).

The authors would like to thank the National Science Council, Taiwan (Contract No. NSC 99-2221-E-041-012-MY3) and the Engineering Research and Development for Technology (ERDT) of the Department of Science and Technology, Philippines for providing financial support for this research undertaking.

Fe3þ þ PO3 4 ! FePO4

ð12Þ

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