In this investigation, UV/H2O2, UV/H2O2/Fe2+ (photo-Fenton) and UV/H2O2/Fe3+ (photo-Fenton-like) systems were used to mineralize sulfamethizole (SFZ). The optimal doses of H2O2 (1–20 mM) in UV/H2O2 and iron (0.1–1 mM) in photo-Fenton and photo-Fenton-like systems were determined. Direct photolysis by UV irradiation and direct oxidation by added H2O2, Fe2+ and Fe3+ did not mineralize SFZ. The optimal dose of H2O2 was 10 mM in UV/H2O2 and that of iron (Fe2+ or Fe3+) was 0.2 mM in both UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems. Under the best experimental conditions and after 60 min of reaction, the SFZ mineralization percentages in UV/H2O2, UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems were 16, 90 and 88%, respectively. The UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems effectively mineralized SFZ.

INTRODUCTION

Advanced oxidation processes (AOPs) are treatment processes that based on the formation of radicals, which are highly reactive and non-selective oxidants of a wide range of organic compounds in wastewater. Among AOPs, Fenton-type processes have achieved pollutant removal efficiencies of over 90% (Neamtu et al. 2003; Kusic et al. 2006; Lucas & Peres 2006; Tokumura et al. 2006; Orozco et al. 2008). The Fenton-type process combines iron (Fe2+ or Fe3+) with hydrogen peroxide to generate hydroxyl radicals. The general mechanism involves Fenton reagents that use Fe2+ (Fenton) or Fe3+ (Fenton-like) ions as a catalyst to decompose hydrogen peroxide. Several studies have demonstrated that photo-Fenton (Neamtu et al. 2003; Muruganandham & Swaminathan 2004; Kusic et al. 2006; Saritha et al. 2007) and photo-Fenton-like (Neamtu et al. 2003; Kusic et al. 2006) systems effectively degrade various compounds.

The extensive use of antibiotics is attracting increasing attention owing to their potential risks to aquatic ecosystems and human health. Sulfonamides are one of the largest classes of antibiotics that are used globally. Holm et al. (1995) detected a high concentration (330 μg/L) of sulfamethizole (SFZ) in the groundwater downgradient of a landfill that accepts both household and pharmaceutical manufacturing waste. Since exposure to SFZ increases the risk of miscarriage in the subsequent week before miscarriage (Ratanajamit et al. 2003), SFZ must be carefully removed from every effluent. Sulfonamides that contain five-membered heterocyclic groups (such as SFZ, sulfamethoxazole (SMX), and sulfathiazole (STZ)) and six-membered heterocyclic groups (such as sulfadiazine, sulfamethazine, sulfamerazine, sulfadimethoxine, and sulfachloropyridazine) have been observed to undergo direct photolysis slowly (Boreen et al. 2004, 2005; Guerard et al. 2009). Hence, other powerful methods have been used to degrade sulfonamides. Hu et al. (2007) utilized a UV/TiO2 system to oxidize SMX, SFZ, and STZ and found that SFZ exhibited the lowest degradation rate. Dias et al. (2014) demonstrated that UV/TiO2 effectively degrades SMX, but that its mineralization rate is considerably lower than that achieved by the photo-Fenton reaction. Garoma et al. (2010) indicated that ozonation effectively removed SFZ from aqueous solution. Wu et al. (2015) showed that the mineralization rate of SFZ is less than the degradation rate of SFZ in a UV/H2O2 system. In the UV/Na2S2O8 system, the mineralization rates follow the order SMX > STZ > SFZ. Since SFZ is difficult to mineralize and must be carefully removed from wastewater, SFZ was utilized as the parent compound herein. No study has compared the efficiencies of photo-Fenton and photo-Fenton-like processes in mineralizing SFZ. Therefore, in this study, UV/H2O2, UV/H2O2/Fe2+ (photo-Fenton) and UV/H2O2/Fe3+ (photo-Fenton-like) were used to mineralize SFZ. The objectives of this study were: (i) to determine the optimal dose of H2O2 for SFZ mineralization in a UV/H2O2 system; (ii) to evaluate the effects of iron dosage on SFZ mineralization in UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems; and (iii) to compare the mineralization efficiencies of SFZ in UV/H2O2, UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems.

MATERIALS AND METHODS

Materials

The parent compound SFZ was purchased from Alfa Aesar. Hydrogen peroxide (H2O2, 30% w/w) was used as an oxidant. The sources of Fe2+ and Fe3+ were ferrous sulfate (FeSO4) and ferric sulfate (Fe2(SO4)3), respectively. Sodium persulfate (Na2S2O8) and phosphoric acid (H3PO4, 85%) were used in the total organic carbon (TOC) analyzer. All chemicals other than SFZ were purchased from Merck. All chemicals were used as received. The pH of the solution was controlled by adding HNO3 or NaOH using an automatic titrator. All solutions were prepared using deionized water (Milli-Q) and reagent-grade chemicals.

Experimental methods

The mineralization of SFZ in the UV, H2O2 (10 mM), Fe2+ (0.2 mM) and Fe3+ (0.2 mM) systems was tested as control experiments. SFZ at 20 mg/L was used in all experiments and the theoretical TOC value was 8.0 mg/L. Several investigations have demonstrated that pH 3 is optimal for Fenton and photo-Fenton systems (Neamtu et al. 2003; Muruganandham & Swaminathan 2004; Saritha et al. 2007) and so pH 3 was used in all mineralization experiments herein. H2O2 doses of 1, 2, 5, 10 and 20 mM were used to obtain the optimal H2O2 dosage in the UV/H2O2 system. The obtained optimal H2O2 dose and iron doses of 0.1, 0.2, 0.5, and 1 mM were used to evaluate the effects of the iron dosage on the photo-Fenton and photo-Fenton-like systems. These experiments were conducted in a 3 L hollow cylindrical glass reactor. A UV lamp (8 W, 254 nm, 1.12 W/m2, Philips) was placed inside the quartz tube as an irradiation source. All experiments were conducted with stirring at 300 rpm and continuous aeration. The temperature was maintained at 25 °C using a water circulation system. Aliquots (20 mL) were withdrawn from the photoreactor at predetermined intervals. The solution was filtered through a 0.22 μm filter (Millipore). Sodium persulfate and phosphoric acid were utilized as the oxidant and the acidifier in the TOC analyzer, respectively. The TOC values were measured using the thermal persulfate oxidation method. The decrease in TOC, measured using an O.I. 1010 TOC analyzer, indicated the degree of SFZ mineralization.

RESULTS AND DISCUSSION

Table 1 presents the physicochemical properties of SFZ. SFZ consists of a pharmacophore group that is connected to a five-member heterocyclic ring, and it has two known pKa values, associated with different concentrations of protonated, non-protonated and deprotonated forms at various pH values (Wu et al. 2015). Under the experimental conditions herein (pH 3), the non-protonated SFZ was the dominant form. Direct photolysis by UV irradiation and direct oxidation by the added H2O2, Fe2+ and Fe3+ did not mineralize SFZ (<1%). The SFZ mineralization percentages that were obtained with only SO42− addition (0.2 mM) and in the UV/SO42− (0.2 mM) system were 0.5 and 1.2%, respectively. Sulfate ions had no effect on the SFZ mineralization process. Figure 1 shows the effect of H2O2 dosage on SFZ mineralization in the UV/H2O2 system with 1, 2, 5, 10 and 20 mM of H2O2 addition. Although H2O2 did not mineralize SFZ, it markedly increased the efficiency of mineralization when combined with UV irradiation. After 60 min of reaction, the SFZ mineralization percentages in the UV/H2O2 system with 1, 2, 5, 10 and 20 mM of H2O2 were 1, 3, 5, 16 and 11%, respectively. Equation (1) specifies the reaction in the UV/H2O2 system. The mineralization efficiency of SFZ increased with the H2O2 concentration up to a critical H2O2 concentration, because more hydroxyl radicals were formed as the H2O2 concentration increased (Equation (1)). At a high H2O2 concentration, competition occurred between the substrate and H2O2, and H2O2 scavenges hydroxyl radicals to form hydroperoxide radicals, which have much lower oxidation capacities than hydroxyl radicals (Equation (2)) (Kusic et al. 2006; Tokumura et al. 2006; Alnuaimi et al. 2007). The recombination of hydroxyl radicals also reduced the mineralization efficiency (Equation (3)). Based on the pKa of the hydroperoxide radicals, superoxide radical anions may have participated in the process (Equation (4)). As the H2O2 concentration in the UV/H2O2 system increased from 1 to 10 mM, the mineralization efficiency increased, but above this range, no further improvement was achieved. Therefore, experiments herein on the effects of the iron dose in UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems were conducted at a H2O2 concentration of 10 mM. 
formula
1
 
formula
2
 
formula
3
 
formula
4
Table 1

The physicochemical properties of SFZ

CAS no. 144-82-1 
Molecular structure  
Molecular formula C9H10N4O2S2 
Molecular weight 270 g/mol 
Dissociation constants* pKa1 = 1.9; pKa2 = 5.3 
CAS no. 144-82-1 
Molecular structure  
Molecular formula C9H10N4O2S2 
Molecular weight 270 g/mol 
Dissociation constants* pKa1 = 1.9; pKa2 = 5.3 
Figure 1

Effects of H2O2 dose on SFZ mineralization in UV/H2O2 system.

Figure 1

Effects of H2O2 dose on SFZ mineralization in UV/H2O2 system.

Figures 2 and 3 plot the effects of Fe2+ and Fe3+ doses on SFZ mineralization in UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems, respectively. In the UV/H2O2/Fe2+ system, SFZ was rapidly mineralized in the first 60 min, and then mineralization proceeded slowly (Figure 2). In the first 60 min, SFZ mineralization reached approximately 84, 90, 82 and 81% at Fe2+ doses of 0.1, 0.2, 0.5 and 1 mM, respectively, in the UV/H2O2/Fe2+ system, and 80, 88, 90 and 64% at Fe3+ doses of 0.1, 0.2, 0.5 and 1 mM, respectively, in the UV/H2O2/Fe3+ system. The effects of iron dose on SFZ mineralization were similar to those of the H2O2 dose: the reaction rate initially increased to a critical value and then fell (Alnuaimi et al. 2007). This investigation suggests that the optimal dose of both Fe2+ and Fe3+ in UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems was 0.2 mM.
Figure 2

Effects of Fe2+ dosage on SFZ mineralization in UV/H2O2/Fe2+ system.

Figure 2

Effects of Fe2+ dosage on SFZ mineralization in UV/H2O2/Fe2+ system.

Figure 3

Effects of Fe3+ dosage on SFZ mineralization in UV/H2O2/Fe3+ system.

Figure 3

Effects of Fe3+ dosage on SFZ mineralization in UV/H2O2/Fe3+ system.

After 60 min, with the addition of 10 mM H2O2 and 0.2 mM iron, the degrees of SFZ mineralization in the UV/H2O2, UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems were 16, 90, and 88%, respectively. The UV/H2O2/Fe2+ system was more effective in SFZ mineralization than the UV/H2O2/Fe3+ system, as revealed by the greater extent of mineralization for a given contact time. Neamtu et al. (2003), Kusic et al. (2006), and Orozco et al. (2008) reported that the degradation rate achieved using UV/H2O2/Fe2+ exceeded that using the UV/H2O2/Fe3+ system. The typical Fenton reaction involves several cyclical reactions. Based on Equations (5)–(8), Fe2+ and H2O2 are formed in their original states at the end of the cyclical reactions (Dominguez et al. 2005; Kusic et al. 2006; Lucas & Peres 2006). When a Fenton-like reagent is utilized, the reaction sequence begins as described by Equation (6). Equations (1)–(9) explain the reaction mechanisms in the UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems. 
formula
5
 
formula
6
 
formula
7
 
formula
8
 
formula
9

In the photo-Fenton system, hydroxyl and hydroperoxyl radicals are formed by Fe2+ and Fe3+, according to Equations (5) and (6), respectively. However, H2O2 and Fe2+ scavenge hydroxyl radicals, as described by Equations (2) and (9), and Fe2+ and Fe3+ scavenge hydroperoxyl radicals, as described by Equations (7) and (8), respectively. Attention must be paid to the molar Fe2+:H2O2 and Fe3+:H2O2 ratios to prevent undesired radical scavenging reactions in the photo-Fenton and photo-Fenton-like systems. Experimental results demonstrate that the mineralization efficiency of SFZ in the UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems exceeded that in the UV/H2O2 system, which is consistent with findings by previous studies, which show that the reaction rates in photo-Fenton and photo-Fenton-like systems exceed that in the UV/H2O2 system (Dominguez et al. 2005; Muruganandham & Swaminathan 2006).

CONCLUSIONS

In this work, photo-Fenton and photo-Fenton-like systems were utilized successfully to mineralize SFZ. UV/H2O2, UV/H2O2/Fe2+ and UV/H2O2/Fe3+ systems were used to mineralize SFZ and the optimal dosages of H2O2 and iron were obtained. The effects of the doses of H2O2 and iron on SFZ mineralization were similar across all tested systems. The addition of excess H2O2, Fe2+ and Fe3+ caused the consumption of reactive radicals that would have otherwise participated in SFZ mineralization. As H2O2 and iron dosages increased, the SFZ mineralization efficiency initially increased to a critical value and then declined. The SFZ mineralization efficiencies followed the order UV/H2O2/Fe2+ ≧ UV/H2O2/Fe3+ > UV/H2O2 at the optimal H2O2 and iron dosages.

ACKNOWLEDGEMENT

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 104-2221-E-151-002.

REFERENCES

REFERENCES
Alnuaimi
M. M.
Rauf
M. A.
Ashraf
S. S.
2007
Comparative decoloration study of neutral red by different oxidative processes
.
Dyes Pigm.
72
(
3
),
367
371
.
Guerard
J. J.
Chin
Y. P.
Mash
H.
Hadad
C. M.
2009
Photochemical fate of sulfadimethoxine in aquaculture waters
.
Environ. Sci. Technol.
43
(
22
),
8587
8592
.
Hu
L.
Flanders
P. M.
Miller
P. L.
Strathmann
T. J.
2007
Oxidation of sulfamethoxazole and related antimicrobial agents by TiO2 photocatalysis
.
Water Res.
41
(
12
),
2612
2626
.
Kusic
H.
Koprivanac
N.
Bozic
A. L.
Selanec
I.
2006
Photo-assisted Fenton type processes for the degradation of phenol: a kinetic study
.
J. Hazard. Mater.
136
(
3
),
632
644
.
Neamtu
M.
Yediler
A.
Siminiceanu
I.
Kettrup
A.
2003
Oxidation of commercial reactive azo dye aqueous solutions by the photo-Fenton and Fenton-like processes
.
J. Photochem. Photobiol. A: Chem.
161
(
1
),
87
93
.
Orozco
S. L.
Bandala
E. R.
Arancibia-Bulnes
C. A.
Serrano
B.
Suarez-Parra
R.
Hernadez-Perez
I.
2008
Effect of iron salt on the color removal of water containing the azo-dye reactive blue 69 using photo-assisted Fe(II)/H2O2 and Fe(III)/H2O2 systems
.
J. Photochem. Photobiol. A: Chem.
198
(
2–3
),
144
149
.
Ratanajamit
C.
Skriver
M.
Nørgaard
M.
Jepsen
P.
Schønheyder
H.
Sørensen
H.
2003
Adverse pregnancy outcome in users of sulfamethizole during pregnancy: a population-based observational study
.
J. Antimicrob. Chemother.
52
(
5
),
837
841
.
Saritha
P.
Aparna
C.
Himabindu
V.
Anjaneyulu
Y.
2007
Comparison of various advanced oxidation processes for the degradation of 4-chloro-2 nitrophenol
.
J. Hazard. Mater.
149
(
3
),
609
614
.