UV Based Advanced Oxidation Process for Degradation of Ciprooxacin: Reaction Kinetics, Effects of Interfering Substances and Degradation Product Identication

10 In the photochemical UV-H 2 O 2 advanced oxidation process, H 2 O 2 absorbs UV light and is decomposed to form 11 hydroxyl radicals (OH · ), which are highly excited and reactive for electron-rich organic compounds and hence 12 can degrade organic compounds. In the present work, the UV-H 2 O 2 process was investigated to degrade 13 ciprofloxacin (CIP), one of India's widely used antibiotics, from aqueous solutions using a batch type UV reactor 14 having photon flux = 1.9 (± 0.1) ×10 -4 Einstein L -1 min -1 . The effects of UV irradiation time on CIP degradation 15 were investigated for both UV and UV-H 2 O 2 processes. It was found that about 75% degradation of CIP was 16 achieved within 60 s with initial CIP concentration and peroxide concentration of 10 mg L -1 and 1 mol H 2 O 2 / mol 17 CIP, respectively, at pH of 7(±0.1) and fluence dose of 113 mJ cm -2 . The experimental data were analyzed by the 18 first-order kinetics model to find out the time- and fluence-based degradation rate constants. Under optimized 19 experimental conditions (initial CIP concentration, pH and H 2 O 2 dose of 10 mg L -1 , 7(±0.1) and 1.0 mol H 2 O 2 / 20 mol CIP, respectively), the fluence-based pseudo-first-order rate constant for the UV and UV-H 2 O 2 processes 21 were determined to be 1.28(±0.0) ×10 -4 and 1.20(±0.04) ×10 -2 cm 2 mJ -1 respectively. The quantum yields at 22 various pH under direct UV were calculated. The impacts of different process parameters such as H 2 O 2 23 concentration, solution pH, initial CIP concentration, and wastewater matrix on CIP degradation were also 24 investigated in detail. CIP degradation was favorable in acidic conditions. Six degradation products of CIP were 25 identified. Results clearly showed the potentiality of the UV-H 2 O 2 process for the degradation of antibiotics in 26 wastewater.


Introduction 30
Consumption of pharmaceuticals (PhCs) like antibiotics is increasing day by day throughout the world because   corresponds to the 50% mortality (LC50) value for these antibiotics on these organisms to be two orders of 52 magnitude higher than environmentally significant concentrations.

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In the present study, CIP was taken as a representative antibiotic as this compound is used widely in the 54 Indian subcontinent. It is commonly used to kill the infection-causing bacteria in humans and animals by 55 interposing with the enzymes, which helps to stop the synthesis of DNA and protein (Khaki et al. 2015). CIP has 56 a higher solubility in aqueous solution, soil, and wastewater system at different pH (Zhuang et al. 2015).

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Therefore, the removal of CIP from the aquatic environment is essential. However, the present literature indicates 58 that conventional wastewater treatment plants are not capable of removing the high load of antibiotics, including fitted inside a metal enclosure with a highly polished stainless-steel reflector. The photon flux and corresponding 104 fluence rate are the most important parameters of the UV reactor. These parameters were determined by 105 ferrioxalate actinometry (Hatchard et al. 1956). The photon flux and corresponding fluence rate were 1.81×10-4 106 Einstein L -1 -min -1 and 112.92 mJ min -1 cm 2 , respectively, at UV 253.7.

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Analytical Methods

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The concentration of CIP was determined by the Thermo Fisher Scientific ultra-high-performance liquid 109 chromatography system (UHPLC) (Dionex Ultimate 3000). A 20 µL CIP sample was inoculated directly into a 110 C18 (250 mm length, 4.6 mm diameter) UHPLC column at the temperature of 35°C. Acetonitrile (25%) and 2% 111 acetic acid (75%) at the flow rate of 1 mL min -1 were used as the mobile phase. Ortho-phosphoric acid was used 112 to maintain a pH of 3.0 of the mobile phase. The absorbance of CIP was measured at 263 nm using a photo diode 113 array (PDA) detector.

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Experimental procedure

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The molar absorptivity of CIP was mapped across a wide range of pH of 2-11 at wavelength 253.7 nm by UV-

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VIS Spectrophotometer (Model: 117, Make: Systronics India Ltd., Ahmedabad, India). The pH of the CIP solution 117 (10 mg L -1 ) was adjusted using dilute HCl and NaOH. The pH was measured using a digital pH meter (PB-11;

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Apart from these, a control experiment was also performed in the dark condition with an H2O2 dose of 1 144 mol H2O2 / mol of CIP and CIP concentration of 10 mg L -1 . The samples were kept for 15 days in the dark, and

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CIP concentration was measured on alternate days.

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The effect of H2O2 dose on UV-H2O2 degradation of CIP was studied to determine the optimum quantity 147 of the chemical. Different H2O2 amounts in the range of 0.125 to 2 mol H2O2 / mol of CIP were considered. The

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initial CIP concentration and pH were 10 mg L -1 and 7.0(±0.1), respectively. Impacts of several other process 149 parameters such as initial CIP concentration (1-20 mg L -1 ), pH (3 -11), and ionic strength (0-1000 mg of NaCl L -150 1 ) were investigated. In all these cases, the H2O2 dose was taken as 0.5 mol H2O2 / mol CIP. The pH of experimental 151 solutions was adjusted using concentrated aqueous HCl or NaOH. Experiments were performed to identify the 152 degradation products of CIP using HPLC-MS/MS for 50 and 75% degraded samples. Besides, CIP degradation 153 was studied in real wastewater. The wastewater was collected from the municipal sewer (IIEST, Shibpur, India),

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and it was filtered using Whatman 42 filter paper before use. The filtered wastewater was analyzed for its 155 composition, and it was used to prepare a CIP spiked solution, which was subjected to the UV-H2O2 process. In 156 all cases, the initial CIP and H2O2 concentrations were 10 mg L -1 and 0.5 mol H2O2 / mol CIP, respectively. Also,

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The degradation of CIP under direct UV was studied at different pH since the molar absorptivity of CIP

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The rate constant obtained in the UV-H2O2 process is almost two order magnitude greater than the direct 214 photolysis case. It is to note that the determination of the true rate constant in UV-based processes is difficult as

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Effect of initial CIP concentration on UV-H2O2 process 235 The effect of initial CIP concentration on UV-H2O2 degradation was studied at different initial concentrations (Co 236 = 1, 5, 10, and 20 mg L -1 ) with an H2O2 dose of 0.5 mol H2O2/mol CIP and irradiated for 1 min at pH 7(±0.1).

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The fluence based pseudo-first-order reaction rate constants were determined to be 1.50(±0.06) ×10 -2 , 1.18(±0.03) observed that the CIP degradation decreased with an increase of pH from 3 to 11 (Fig. 8). The degradation of CIP 254 was observed ~ 76 % at pH 3 and 38 % at pH 11. The apparent fluence-based first-order rate constant was also 255 found to decrease with increased pH. In alkaline solutions, H2O2 (pKa =11.8), dissociates to hydroperoxide (HO2 -
that the direct photolysis was not effective at hihly alkaline conditions.

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Effect of ionic strength on CIP degradation

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Degradation product of CIP

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The molecular structure of CIP has been shown in Fig. 1. Quinolone moiety is present in the center, and carboxyl  (Table 1).

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The CIP degradation might take place in three way: (1) reaction take place at the piperazinyl ring, (2)

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Effect of wastewater matrix on CIP degradation 283 The applicability of the UV-H2O2 process for treating pharmaceutical compounds was studied using municipal 284 wastewater spiked samples with an initial CIP concentration of 10 mg L -1 . The molar ratio of H2O2 to CIP was 285 0.5, and pH was 7(±0.1). Degradation of CIP was reasonably affected in the case of wastewater spiked samples.

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The composition of wastewater was determined as: pH -7.1; total alkalinity -221 mg L -1 as CaCO3; nitrate -13 287 mg L -1 ; five-day biochemical oxygen demand (BOD5) -180 mg L -1 ; chemical oxygen demand (COD) -420 mg 288 L -1 and UV absorbance at 253.7 nm (UV254) -0.496 cm -1 . The percentage of degradation was 85 and 64% in 289 distilled water and wastewater, respectively, within 2 min (Fig. 9). The fluence-based rate constants of CIP 290 degradation were determined to be 0.85(±0.03)×10 -2 cm 2 mJ -1 and 0.48(±0.06)×10 -2 cm 2 mJ -1 for distilled water 291 and wastewater, respectively, which present that rate constant for the distilled water spiked sample was 2 times higher than the wastewater-spiked sample. The slower kinetics in wastewater might be due to hydroxyl radicals' 293 scavenging by the dissolved organic matter (DOM) present in wastewater. As the COD and UV254 of wastewater 294 are very high, it is expected that DOM will also be high. DOM presents in the wastewater also absorbs UV photons 295 and reduces the quantity of hydroxyl radicals (Xu et al. 2013;Walse et al. 2004

Fig. 3
Variation of apparent molar absorptivity of CIP at various pH at 253.7 nm.

Fig. 5
Effect of pH on first-order reaction rate constant (a) and quantum yield (b) on degradation of CIP under direct UV light (initial CIP concentration =10 mg L -1 ).