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.
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
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.
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
|Molecular weight||270 g/mol|
|Dissociation constants*||pKa1 = 1.9; pKa2 = 5.3|
|Molecular weight||270 g/mol|
|Dissociation constants*||pKa1 = 1.9; pKa2 = 5.3|
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).
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.
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.