This study used Na2S2O8, NaBrO8 and H2O2 to degrade sulfadiazine (SDZ), sulfamethizole (SFZ), sulfamethoxazole (SMX) and sulfathiazole (STZ) under ultraviolet (UV) irradiation. The initial concentration of sulfonamide and oxidant in all experiments was 20 mg/L and 5 mM, respectively. The degradation rate for sulfonamides satisfies pseudo-first-order kinetics in all UV/oxidant systems. The highest degradation rate for SDZ, SFZ, SMX and STZ was in the UV/Na2S2O8, UV/NaBrO3, UV/Na2S2O8 and UV/H2O2 system, respectively. In the UV/Na2S2O8 system, the photodegradation rate of SDZ, SFZ, SMX and STZ was 0.0245 min−1, 0.0096 min−1, 0.0283 min−1 and 0.0141 min−1, respectively; moreover, for the total organic carbon removal rate for SDZ, SFZ, SMX and STZ it was 0.0057 min−1, 0.0081 min−1, 0.0130 min−1 and 0.0106 min−1, respectively. Experimental results indicate that the ability of oxidants to degrade sulfonamide varied with pollutant type. Moreover, UV/Na2S2O8 had the highest mineralization rate for all tested sulfonamides.

INTRODUCTION

Antibiotics have been detected worldwide in environmental matrices, due to the excessive amounts used by humans and administered to animals, indicating that their removal from water and wastewater using conventional methods is ineffective. The wide distribution of antibiotics has received increased attention due to their potential risk to aquatic ecosystems and human health. In tropical Asian waters, the most abundant antibiotic was sulfamethoxazole (SMX), followed by lincomycin and sulfathiazole (STZ) (Shimizu et al. 2013). Sulfonamide is one of the largest classes of antibiotics used worldwide. Eight common sulfonamides are currently used: sulfamethizole (SFZ), SMX, sulfadiazine (SDZ), sulfacetamide, sulfadoxine, sulfanilamide, sulfasalazine and sulfisoxazole (Garoma et al. 2010). An effective method to remove these antibiotics from wastewater is needed. Hence, SDZ, SFZ, SMX and STZ were the target compounds in this study.

Advanced oxidation processes are technologies based on the intermediacy of hydroxyl and other radicals to oxidize recalcitrant, toxic and non-biodegradable compounds into various by-products and eventually to inert end-products. These processes effectively degrade and reduce recalcitrant wastewater loads from industry. For one of the most widely prescribed sulfonamide antimicrobial compounds, SMX, ultraviolet (UV) radiation (Nasuhoglu et al. 2011; Kim et al. 2012; Lekkerkerker-Teunissen et al. 2012; Liu et al. 2012), (Abellan et al. 2007; Hu et al. 2007; Baran et al. 2009; Nasuhoglu et al. 2011; Dias et al. 2014), (Baeza & Knappe 2011; Lekkerkerker-Teunissen et al. 2012), ozonation (Garoma et al. 2010; Kim et al. 2012; Liu et al. 2012; Goncalves et al. 2013), (Ji et al. 2014), (Ghauch et al. 2013; Ayoub & Ghauch 2014), (Ayoub & Ghauch 2014) and photo-Fenton (Trovo et al. 2009; Dias et al. 2014) processes have been used to degrade it. These methods were also applied to degrade SDZ (Baran et al. 2009; Garoma et al. 2010; Baeza & Knappe 2011; Batista & Nogueira 2012; Batista et al. 2014), STZ (Hu et al. 2007; Baran et al. 2009; Garoma et al. 2010; Batista & Nogueira 2012) and SFZ (Hu et al. 2007; Garoma et al. 2010). Gao et al. (2014), who utilized chlorine, ozone and permanganate to oxidize SMX, found that chlorine and ozone were more effective than permanganate in removing SMX. Dias et al. (2014) demonstrated that is an effective method to degrade SMX; however, the mineralization rate is substantially lower than the photo-Fenton reaction. No study had compared the efficiency of UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 in degrading sulfonamides. This study used the UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 systems to degrade SDZ, SFZ, SMX and STZ. The study objectives were to: (i) determine the effectiveness of UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 when photodegrading SDZ, SFZ, SMX and STZ; and (ii) evaluate the mineralization efficiency of SDZ, SFZ, SMX and STZ in the UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 systems.

MATERIALS AND METHODS

Materials

The sulfonamides SDZ, SMX and STZ were obtained from Sigma–Aldrich (Shanghai, China) and SFZ was purchased from Alfa Aesar (New Delhi, India). The H2O2, Na2S2O8 and NaBrO3 were applied as oxidants to evaluate photodegradation efficiency. The oxidants Na2S2O8 NaBrO3 and H2O2 were obtained from Sigma–Aldrich, Katayama (Osaka, Japan) and Merck, respectively. Solution pH was adjusted using 0.1 M and 0.1 M . All solutions were prepared using deionized water (Milli-Q, Molsheim, France) and reagent-grade chemicals. All chemicals were used as received.

Experimental methods

The initial concentration of sulfonamide and oxidant in all experiments was 20 mg/L and 5 mM, respectively. The molar concentration of 20 mg/L SDZ, SFZ, SMX and STZ was 0.080, 0.074, 0.079 and 0.078 mM, respectively. The molar ratio of oxidant/SDZ, oxidant/SFZ, oxidant/SMX and oxidant/STZ was 62.5, 67.5, 63.3 and 63.8, respectively. Photodegradation experiments were conducted in a 3-L hollow cylindrical glass reactor. An 8-W UV lamp (254 nm, 1.12 W/m2, Philips, Tokyo, Japan) was placed inside a quartz tube as the light source. All systems were stirred continuously at 300 rpm and temperature was controlled at 25 °C. A 20-mL aliquot was withdrawn from the photoreactor at pre-specified intervals. The concentration of SDZ, SFZ, SMX and STZ was measured by a spectrophotometer (U-5100, Hitachi, Tokyo, Japan) at 262, 260, 265 and 285 nm, respectively. Photodegradation efficiency was calculated as the difference between sulfonamide concentrations before and after each experiment. The decrease in total organic carbon (TOC), measured using an O.I. 1010 TOC analyzer (O.I., College Station, TX, USA), indicated sulfonamide mineralization. Data reported are the average of triplicate samples. Final experimental results are given as mean (), where t is the student value, s is the standard deviation and n is the number of replicates. For n = 3 at 95% confidence interval, the t value is 4.303. Error bars in figures represent uncertainty at the 95% confidence level.

RESULTS AND DISCUSSION

Table 1 lists the physicochemical properties of the four sulfonamides. These four sulfonamides are of the same pharmacophore group. Only SDZ connected the pharmacophore group with a six-member heterocyclic ring, others were the pharmacophore group with a five-member heterocyclic ring. Moreover, these four sulfonamides have two pKa values, resulting in different concentrations of protonated, non-protonated and deprotonated forms at various pH values. When at experimental condition (pH 4), the non-protonated form is the predominant form for these four sulfonamides. Figure 1 shows the UV–visible spectra of the four sulfonamides in aqueous solution. The maximum absorption wavelength of SDZ, SFZ, SMX and STZ was 262, 260, 265 and 285 nm, respectively.

Table 1

The physicochemical properties of four sulfonamides

Compounds SDZ SFZ SMX STZ 
CAS No. 68-35-9 144-82-1 723-46-6 72-14-0 
Molecular structure     
Molecular formula     
Molecular weight 250 g/mol 270 g/mol 253 g/mol 255 g/mol 
Dissociation constantsa pKa1 = 2.0; pKa2 = 6.4 pKa1 = 1.9; pKa2 = 5.3 pKa1 = 1.7; pKa2 = 5.6 pKa1 = 2.0; pKa2 = 7.1 
λmax 262 nm at pH 4 260 nm at pH 4 265 nm at pH 4 285 nm at pH 4 
Compounds SDZ SFZ SMX STZ 
CAS No. 68-35-9 144-82-1 723-46-6 72-14-0 
Molecular structure     
Molecular formula     
Molecular weight 250 g/mol 270 g/mol 253 g/mol 255 g/mol 
Dissociation constantsa pKa1 = 2.0; pKa2 = 6.4 pKa1 = 1.9; pKa2 = 5.3 pKa1 = 1.7; pKa2 = 5.6 pKa1 = 2.0; pKa2 = 7.1 
λmax 262 nm at pH 4 260 nm at pH 4 265 nm at pH 4 285 nm at pH 4 
Figure 1

UV–visible spectra of sulfonamides in aqueous solution (pH = 4 and [sulfonamide] = 20 mg/L).

Figure 1

UV–visible spectra of sulfonamides in aqueous solution (pH = 4 and [sulfonamide] = 20 mg/L).

Direct photolysis by UV irradiation and direct oxidation by an added oxidant were employed as control experiments. After reaction for 180 min, 34, 51, 70 and 67% of SDZ, SFZ, SMX and STZ was removed by direct photolysis, respectively, and 5, 8, 7 and 8% of SDZ, SFZ, SMX and STZ was mineralized, respectively. The removal of sulfonamides and TOC by direct oxidation via the added oxidant was <5%. Direct photolysis by UV irradiation and direct oxidation by the added oxidant did not mineralize the sulfonamides. Figure 2(a)(d) shows the photodegradation rates for SDZ, SFZ, SMX and STZ, respectively. After 180 min reaction, 93, 97, 98 and 99% of SDZ, SFZ, SMX and STZ was removed in the system, respectively, 85, 89, 93 and 96% in the system, respectively, and 93, 92, 97 and 99% in the system, respectively. The degradation of these four sulfonamides approximately followed pseudo-first-order kinetics, expressed as ln(Ct/C0) = −kt, where t is reaction time, k is the pseudo-first-order rate constant, and C0 and Ct are the concentrations of sulfonamide at time of t = 0 and t = t, respectively. Table 2 summarizes the pseudo-first-order reaction rate constants and correlation coefficients of four sulfonamides in various UV/oxidant systems. The degradation rate for the sulfonamides satisfies pseudo-first-order kinetics; various studies have demonstrated that degradation rates for antibiotics can be approximated using pseudo-first-order kinetics (Baran et al. 2009; Baeza & Knappe 2011; Batista & Nogueira 2012; Liu et al. 2012).

Table 2

The pseudo-first-order removal rate constant (k, min−1) and linear coefficient (R2) of sulfonamides in various UV/oxidant systems

Systems UV/Na2S2O8 UV/NaBrO3 UV/H2O2 
Sulfonamides k R2 K R2 k R2 
SDZ 0.0245 (0.0057) 0.9450 (0.9528) 0.0097 (0.0028) 0.9999 (0.8812) 0.0133 (0.0034) 0.9945 (0.8888) 
SFZ 0.0096 (0.0081) 0.9916 (0.9196) 0.0145 (0.0075) 0.9832 (0.9783) 0.0095 (0.0040) 0.9834 (0.9326) 
SMX 0.0283 (0.0130) 0.9838 (0.9201) 0.0261 (0.0052) 0.9727 (0.9689) 0.0271 (0.0037) 0.9805 (0.9870) 
STZ 0.0141 (0.0106) 0.9776 (0.9387) 0.0228 (0.0044) 0.9913 (0.8782) 0.0322 (0.0047) 0.9843 (0.9249) 
Systems UV/Na2S2O8 UV/NaBrO3 UV/H2O2 
Sulfonamides k R2 K R2 k R2 
SDZ 0.0245 (0.0057) 0.9450 (0.9528) 0.0097 (0.0028) 0.9999 (0.8812) 0.0133 (0.0034) 0.9945 (0.8888) 
SFZ 0.0096 (0.0081) 0.9916 (0.9196) 0.0145 (0.0075) 0.9832 (0.9783) 0.0095 (0.0040) 0.9834 (0.9326) 
SMX 0.0283 (0.0130) 0.9838 (0.9201) 0.0261 (0.0052) 0.9727 (0.9689) 0.0271 (0.0037) 0.9805 (0.9870) 
STZ 0.0141 (0.0106) 0.9776 (0.9387) 0.0228 (0.0044) 0.9913 (0.8782) 0.0322 (0.0047) 0.9843 (0.9249) 

Values for TOC removal in parentheses.

Figure 2

Photodegradation of four sulfonamides in various UV/oxidant systems (a) SDZ, (b) SFZ, (c) SMX, and (d) STZ.

Figure 2

Photodegradation of four sulfonamides in various UV/oxidant systems (a) SDZ, (b) SFZ, (c) SMX, and (d) STZ.

Ivanov et al. (2000) suggested that persulfate ions undergo photolysis under irradiation with light, generating sulfate free radicals (Equation (1)). These sulfate free radicals then react with water molecules to produce hydroxyl radicals (Equation (2)). Zuo & Katsumura (1998) investigated the mechanisms of the system, as given by Equations (3)–(8). Equation (9) describes the key reaction of the system. Selvam et al. (2007) found that the photodegradation efficiencies of 4-fluorophenol followed the order . Conversely, Ravichandran et al. (2007) found the photodefluoridation of pentafluorobenzoic acid followed the order . The highest k value for STZ and SFZ was in the systems, respectively. Moreover, for both SDZ and SMX, the system had the highest k value (Table 2). Experimental results suggest that the ability of oxidants to degrade a pollutant varies with contaminant molecule structure: 
formula
1
 
formula
2
 
formula
3
 
formula
4
 
formula
5
 
formula
6
 
formula
7
 
formula
8
 
formula
9
The theoretical TOC value of 20 mg/L SDZ, SFZ, SMX and STZ is 9.6, 8.0, 9.5 and 8.5 mg/L, respectively, and the experimental TOC value generated by the TOC analyzer was 9.2, 7.8, 9.9 and 8.5 mg/L, respectively. Figure 3(a)(d) shows TOC removal rates during photodegradation of SDZ, SFZ, SMX and STZ, respectively. After 180 min reaction, TOC removal during photodegradation of SDZ, SFZ, SMX and STZ was as follows: 86, 94, 96 and 92% in the system; 65, 75, 57 and 68% in the system; and 56, 60, 47 and 53% in the system. For the TOC removal percentage and rate, the system performed best for all tested compounds. Ayoub & Ghauch (2014) suggested that a low concentration (e.g., 1.0 mM) was more advantageous than a high concentration in terms of SMX mineralization, especially in oxic medium for the system. However, this study demonstrated that TOC removal of SMX under oxic and anoxic (the solution was purged with nitrogen for 20 min before anoxic reaction) conditions was somehow close to that of the system (e.g., 96% compared to 95%). Since the system did not oxidize/reduce , the oxygen concentration in the solution did not significantly influence the extent of SMX mineralization. A comparison of these four sulfonamides shows that SDZ was the most difficult to mineralize in all tested UV/oxidant systems. This experimental result was most likely because SDZ is connected to the pharmacophore group and has a six-member heterocyclic ring, which is more resistant to mineralization than the five-member heterocyclic ring (SFZ, SMX and STZ). Baran et al. (2009), who degraded SMX, SDZ and STZ in the system, also found that the degradation rate for SDZ was lowest. It is well known that sulfate radicals are very selective and like molecules are rich in electrons, especially, when a haloatom or atoms with nonbonding electrons are present. In the system, the mineralization rate was SMX > STZ > SFZ. The different atomic composition of the five-member rings of SFZ, SMX and STZ most likely accounts for this rate. In the case of SFZ, 1S and 2N atoms exist in the cycle, compared to 1N and 1O for SMX and 1S and 1N for STZ.

Abellan et al. (2007) obtained a 23% TOC reduction with SMX (0.395 mM at pH 5) degradation with the system after 360 min. In the same system but only after 300 min, Baran et al. (2009) obtained a TOC reduction of 63% with SMX (0.1 mM at pH 5.63) degradation, a TOC reduction of 22% with SDZ (0.1 mM at pH 6.60) degradation, and a TOC reduction of 64% with STZ (0.1 mM at pH 7.10) degradation. Ozonation removed 34% of TOC with SMX (0.198 mM at pH 4.8) degradation after 180 min (Goncalves et al. 2013). The TOC removal efficiency of tested sulfonamides in the system was higher than that in the system and by ozonation in the literature. In addition, the TOC removal rate was lower than the sulfonamide photodegradation rate (Table 2). The mineralization rate was lower than that of degradation (Wang et al. 2014). This study suggested that was effective for mineralization of SDZ, SFZ, SMX and STZ.

CONCLUSIONS

This study investigated the effectiveness of photodegradation of SDZ, SFZ, SMX and STZ by the UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 systems. Among the four sulfonamides, SDZ was the most difficult to mineralize. Moreover, the system had a higher mineralization rate than the systems for all tested sulfonamides. Further investigation of the intermediates of sulfonamides in the UV/Na2S2O8, UV/NaBrO3 and UV/H2O2 systems should be conducted before applying UV/oxidant systems in wastewater treatment.

Figure 3

TOC removal for four sulfonamides in various UV/oxidant systems (a) SDZ, (b) SFZ, (c) SMX, and (d) STZ.

Figure 3

TOC removal for four sulfonamides in various UV/oxidant systems (a) SDZ, (b) SFZ, (c) SMX, and (d) STZ.

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. NSC 101-2221-E-151-038-MY3.

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