Treatment of wastewater contaminated with sulfamethoxazole drug using advanced oxidation processes

Reagents

All solutions were prepared using distilled water, H₂O₂ (30% w/w), and FeSO₄.7H₂O (Fischer Scientific), H₂SO₄ and NaOH from BHD were used as received. The antibiotic SMX [4-amino-N-(5-methylisoxazol-3-yl)-benzene sulfonamide, C₁₀H₁₁N₃O₃S] was obtained from SDI, high-performance liquid chromatography (HPLC) mobile phase acetonitrile, and acetic acid from BHD.

Chemical Analysis

The concentration of antibiotic SMX in aqueous solution was analyzed by HPLC at a maximum absorption wavelength of 272 nm, with a YL 9100 Instrument Co. Ltd., HPLC with a UV detector and column ODS-3, 10 µm and the elution was carried out using gradient mode. Mobile phases were 50% acetonitrile and 50% acetic acid (0.5%) (v/v). Antibiotic was detected using UV absorbance at 272 nm.[8] pH was adjusted by pH m WTW, inoLab® pH 720/7200, Germany. The experiments were performed on laboratory scale in 250 ml glass reactor under complete mixing at 25 ± 2°C. The reaction solution was prepared by concentration of antibiotic SMX (250 ppm) and subjected to Fenton treatment. Degradation of antibiotic during Fenton oxidation was considered under experimental conditions include pH (3, 4, 5, 6 and 7), molar ratio of (H₂O₂)/(Fe+2) (0.3–5.25), H₂O₂ (3−10−4 M up to 5.25 × 10−3 M), Fe+2 (1 × 10−5 up to 1 × 10−3 M), and reaction time (1, 3, 5, 8, 10, 15, 30, and 60) min. To initiate experiments, the samples were withdrawn at the reaction times and analyzed by HPLC.[8]

Experimental Procedure

All experiments were performed in an open batch glass system with a stirring bar; 250 ml of SMX sample in 500 ml conical flasks with initial SMX concentration 250 mg/l (9.869 × 104 M) was used. The initial pH of the reaction solutions was adjusted with NaOH (0.1 M) or (0.1 M) H₂SO₄ solution for Fenton’s treatment. The required amount of FeSO₄.7H₂O (0.00001–0.001 M) and H₂O₂ (3 × 10−4 M–5.25 × 10−3 M) was added, mixed by stirring continuously and kept at a required temperature for different reaction time. After each reaction time, the samples were allowed to stand for 30 min. The pH of the mixture was adjusted at 8.0 to precipitate Fe+3 and Fe+2 compounds, then filtered for analysis by HPLC before and after treatment.

AOPs

AOPs rely on the generation of radicals such as hydroxyl radicals, which are very reactive with many organic and inorganic compounds. These radicals are very efficient in degradation process of the contaminant. The general process for AOPs was happening in the following order: [9,10]

All AOPs are designed to produce hydroxyl radicals, which act as high efficiency to destroy organic compounds.

Fenton Oxidation

Fenton’s reagent, demonstrated that a mixture of H₂O₂ and Fe+2 in acidic medium, has been proposed as a very effective oxidizing agent for organic compounds.[11] Mechanism of Fenton process proposes that HO* is formed according to the reaction (1), then the catalyst Fe2+ was regenerated through reaction (2).[12]

Hydroxyl free radical can oxidize organic compounds (RH or R) by hydrogen abstraction (R*) or by hydroxyl addition (*ROH). The highly reactive molecules (R* and *ROH) can be oxidized, then the highly reactive molecules (R* and *ROH) oxidized, as shown in reactions (3) and (4).

Effect of H₂O₂ Concentration

Hydrogen peroxide is a determining factor in Fenton oxidation of wastewater. Excessive H₂O₂ consumes hydroxyl radicals without the degradation of the target organic matter. As a result, the oxidation efficiency of pollutant by the Fenton process would be reduced.[13] Result of wastewater degradation was 5.2, which calculated by the molar ratio of H₂O₂/Fe2+ through constant Fe2+ at 1 × 10−3 M and variable value of H₂O₂.

Degradation of wastewater was increased after Fenton oxidation with raising the concentration of H₂O₂ (3 × 10−4 M) up to 5.25 × 10−3 M for 60 min due to the higher yield of hydroxyl radical. Therefore, H₂O₂ (5.25 × 10−3 M) was chosen as the optimal concentration to use in the next experiments and evaluate the effects of Fe2+ concentration on the SMX wastewater degradation [Figure 2].

Figure 2: Effect of variable value of H₂O₂ dosages, experimental condition (pH=3, sulfamethoxazole=250 mg/L [0.986×10−3 M], and FeSO₄.7H₂O=0.001 M) to removal by Fenton’s process

Effect of Fe2+ Concentration

Fe2+ concentration is an important parameter in Fenton’s reactions because it directly influences the yield of hydroxyl radical (•OH) by catalytic decomposing of H₂O₂ as shown in reaction (1), also acts as scavengers of (•OH) radicals if it was overdosed.[14] Therefore, the influence of Fe2+ concentration on the SMX wastewater degradation was evaluated by fixing the initial H₂O₂ concentration at 5.25 × 10−3 M and pH 3. Results showed that wastewater was not degraded with the absence of Fe2+ and the presence of H₂O₂ (5.25 × 10−3 M), which demonstrated the important role of the Fe2+ in the Fenton process. However, when concentration of H₂O₂ was constant at 5.25 × 10−3 M, the wastewater degradation was increasing with the raising of Fe2+ concentration from 1 × 10−6 to 1 × 10−3 M. Reducing of degradation with concentration than (5 × 10−5 M) may be attributed to low concentration of Fe2+ and produced low amount of hydroxyl radical in solution. Therefore, (1 × 10−3 M) was chosen as the optimum concentration of Fe2+ as shown in [Figure 3].

Figure 3: Effect of varying FeSO₄ dosages, experimental condition (pH=3, sulfamethoxazole=250 mg/L [0.986×10−3 M], and H₂O₂=5.25×10−3 M) to removal by Fenton’s process

Effect of pH

The pH was strongly affected the degradation efficiency in Fenton process since a change in pH solution involves a variation of Fe2+ concentration, and consequently, the production rate of·•OH radicals.[15] Parallel experiments were conducted at four initial pH values (3, 4, 5, and 7). Results showed that the antibiotic SMX was completely degraded (100%) in pH of 3. The Fenton process can operate well under acidic condition,[16] but its function reduces in low pH because of slower FeOOH+2 formation and decreases production rate of Fe+2 and· •OH.[17] The Fenton reactions have a maximum catalytic activity and greater degradation at pH 3 with higher generation of· •OH radicals. The reasons for hydroxyl radical (•OH) as a reduction factor belong to the formation of ferric hydroxo complexes, which subsequently form Fe(OH)₃ at higher pH [Figure 4].

Figure 4: Effect of pH, experimental condition H₂O₂=5.25×10−3 M and FeSO4=1×10−3 M to removal sulfamethoxazole=250 mg/L (0.986×10−3 M) by Fenton’s process

The optimal conditions for 100% degradation of SMX in aqueous solution were achieved after 60 min of reaction were determined and found to be H₂O₂ = 5.25 × 10−3 M, Fe+2 = 1 × 10−3 M, pH = 3, molar ratio (H₂O₂)/(Fe+2) = 5.2, and for SMX = 9.869 × 10−4 M [Figure 5] show the chromatogram of SMX by HPLC instrument before and after treatment by Fenton’s process at the optimum conditions.

Figure 5: Measured concentration of sulfamethoxazole before and after treatment by Fenton’s process at the optimum conditions