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Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2018 DOI

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https: //doi. org/10.1007/s11595-018-1790-3

Different Heterogeneous Fenton Reaction Based on Foam Carrier Loaded with Photocatalysts QIU Shan, LI Guangming, DENG Fengxia, MA Fang

(State Key Laboratory of Urban Water Resources Center, School of Environment, Harbin Institute of Technology, Harbin 150090, China)

Abstract: The effect of heterogeneous Fenton reaction was studied on methylene blue (MB) and Nitrosomonas europaea (N. europaea) cells. Four Fenton systems were prepared and compared with each other, including Nickel Foam (NF)/TiO2, NF/Bi2WO6, Ceramic foam (CM)/TiO2, and CM/Bi2WO6. The order of effect of fenton reaction ranked as NF/TiO2>CM/TiO2>NF/Bi2WO6>CM/Bi2WO6. In acid or alkaline solution, the removal efficiency also decreased compared with neutral solution. With lower pH values, the nanoparticles were easier to break off from NF skeleton. Thus the synergetic effect of photocatalysis and fenton reaction can not take action. As for CM skeleton, the bond –Si-O- can bind with TiO2 or Bi2WO6. The membrane fluidity was used as an indicating parameter. After being treated by Fenton reaction, N. europaea surface was rougher than the native bacterium and the bulges on cell surface became irregular, which is attributed to change of lipopolysaccharide patches. Polarization of N. europaea cell membrane in acid medium increased more obvious than alkaline medium. Key words: fenton reaction; N europaea; polarization; degradation; membrane

1 Introduction Fenton reaction was an efficient technology for degradation of pollutants among advanced oxidation processes (AOP) [1-3]. Free radicals are produced in Fenton reaction which can damage and degrade the organic molecules. Hydroxyl radicals (OH•) is the main product among Fenton reaction with high reactivity. It converts organic molecules into CO2, H2O and other inorganic ions[4,5]. The other advantage of Fenton reaction is low cost. Fenton reaction can stimulate H2O2 without the extra energy, reaction, sophisticated equipment or pressurization device. The whole process is easy to operate and control. The oxidation in Fenton reaction can be used in all kinds of waste water treatment, included in the phenol wastewater, agricultural wastewater, coking wastewater, cyanide wastewater, dye wastewater, dye intermediate wastewater and landfill leachate. Fenton reaction can also be recognized as ‘‘green technology’’, which doesn’t produce harmful substances and © Wuhan University of Technology and Springer-Verlag GmbH Germany, Part of Springer Nature 2018 (Received: Dec. 9, 2016; Accepted: Oct. 23, 2017) QIU Shan(邱珊): Assoc. Prof.; E-mail: [email protected] Funded by National Natural Science Foundation of China, the State Key Laboratory of Urban Water Resource and Environment (No.51208141) and the National Key Research and Development Program of China (No.2016YFC0401102)

undergoes a process at room temperature and pressure. Therefore, Fenton reaction is a kind of AOP potentially used in environmental pollutant treatment areas[6-8]. The homogeneous Fenton system has some limitation, such as narrow range of pH (2-4) and the production of much sludge after Fenton process. At present, the heterogeneous Fenton reaction has become the focus of research. To overcome these drawbacks of homogeneous Fenton system, the heterogeneous Fenton system was performed through catalysts supported by carrier. The catalysts included Fe3O4, TiO2, Fe2O3, and so on. The carriers have been reported such as activated carbon Al2O3, mesoporous silica, zeolites, and clay mineral carbon aerogel[9,10]. In addition, it has also been reported that ultrasound is most commonly used for waste treatment which can degrade pollutants[11-14]. Some researchers have reported that Fenton reaction could inactivate bacteria. César Pulgarína reports combination of sonication and photo-Fenton for bacterial inactivation of secondary treated effluent[15]. C Pulgarin compared the effect of simulated solar light, UV, UV/H2O2 and photo-Fenton treatment in the Escherichia coli inactivation in artificial seawater[16]. C Pulgarín studied deleterious effect of homogeneous and heterogeneous near-neutral photo-Fenton system on Escherichia coli[17]. In the present work, several heterogeneous Fenton reactions were prepared and studied on their effect

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on pollutant degradation and bacteria inactivation. Nickel Foam and Ceramic foams were used as the carriers to support TiO2 and Bi2WO6, respectively. MB was used as a target to measure the pollutant degradation. The cell permeability and membrane fluidity were also used as an indication to compare with each other. Some factors like initial pH value were also studied.

2 Experimental 2.1 Cells and reagents Nickel foam (NF) sheet was purchased from Suzhou Gassider Foam Pioneer Metals Corporation. The structure and morphology of NF sheet were observed by scanning electron microscopy (SEM, Quanta FEG 450, FEI). Ceramic foam (CM) was supplied by Krosakiharima Co. MB, 30% H2O2, and other routine chemicals were purchased from Shenshi Chem. Bi(NO 3)3·5H2O and Na2WO4·2H2O were purchased from Sigma. Distilled water was used in all experiments. N. europaea was provided by Chinese Center for Type Culture Collections. The cells were cultured by a standard method. After the cell grew to a certain population, the cell suspensions were collected by centrifugation at 3 000 r/min for 5 minutes, and then the pellets were washed and resuspended in PBS buffer. 2.2 Preparation of ceramic foam coated by catalysts 2.2.1 Nickel foam supported TiO2 Nickel Foam was soaked into aqueous slurry of TiO2 sol-gel, prepared by adding tetrabutyl titanate into water dropwisely. Then it was dried at room-temperature overnight. The loading weight was calculated according to the weight of coated and uncoated foams. Then the sample was calcined at 400 ℃ for 2 h in N2 atmosphere. 2.2.2 Nickel foam supported Bi2WO6 Bi(NO3)3·5H2O and Na2WO4·2H2O were mixed in 70 mL deionized water. The pH value of the mixture was adjusted to 2 by HNO3. And then the mixture was stirred vigorously at room temperature. Finally, the mixture solution and Nickel Foam were put in Teflon-lined stainless steel autoclave and treated at 160 ℃ for 24 h. Then the sample was collected. 2.2.3 Ceramic foam supported TiO2 Ceramic foam was soaked into aqueous slurry of TiO2 sol-gel, prepared by adding tetrabutyl titanate into water dropwisely. Then it was dried at room-temperature overnight. The loading weight was calculated

according to the weight of coated and uncoated foams. Then the sample was calcined at 400 ℃ for 2 h in N2 atmosphere. 2.2.4 Ceramic foam supported Bi2WO6 Bi(NO3)3·5H2O and Na2WO4·2H2O were mixed in 70 mL deionized water. The pH value of the mixture was adjusted to 2 by HNO3. And then the mixture was stirred vigorously at room temperature. Finally, the mixture solution and ceramic foam were put in Teflon-lined stainless steel autoclave and treated at 160 ℃ for 24 h. Then the sample was collected. 2.3 Characterization of the samples The synthesized powders were characterized by X-ray diffractometer (XRD, PHILIPS P W3O4O/60X′PertPRO) with a Cu Kα ray source. The structure and morphology of sample were observed by environmental scanning electron microscopy (ESEM, Quanta FEG 450, FEI) together with a Backscattered electron (BSE). 2.4 Catalytic activity testing The four systems were denoted as NC/TiO2, NC/ Bi 2WO 6, CM/TiO 2, and CM/Bi 2WO 6. The solution contained 0.1 g/mL NF or CM. The initial MB concentration was 5.00 mg/L with pH value of 7. The initial concentration of H2O2 was 5.0 mmol/L, and ultrasonic power was 300 W. MB degradation was used to evaluate the efficiency of catalysis. The maximum absorption wavelength was 554 nm. A linear relationship was built between MB concentration and absorbance. The concentration of MB can be determined by the absorbance using a UV-vis spectrophotometer (724, shanghai 3rd analytical instrument Ltd.). The reaction temperature was maintained at 25±1 ℃. The pH values were adjusted to the required values by 0.1mg/L HCl and 0.1mol/ L NaOH. During the experiment, an ultrasonic processor (FS-600N, Shanghai Shengxi) was used with the ultrasonic probe at 1.5 cm below the water surface. For the UV light, an iodine tungsten lamp (150 W, FoShan lighting) was used as the light source and the distance between the lamp and the reaction liquid was 15cm. 2.5 N. europaea membrane fluidity alteration 1,6-diphenyl-1,3,5-hexatriene (DPH) was dissolved in tetrahydrofuran, then the solution was dripped into pH 7.2 phosphate buffer (PBS) with stirring. The final DPH concentration was 0.004 mmol/L. The prepared DPH was added into N. europaea suspension and incubated at 37 ℃ for 30 min. The samples were then washed by PBS to remove the free DPH molecules and measure fluorescence polarization of N. europaea cells by a fluorescence spectrometer (Shimadzu, RF-5301). The excitation and emission wavelengths were set as

Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2018

362 and 432 nm, respectively. The morphologies of the treated N. europaea cells were observed by SEM (Quanta FEG 450, FEI). The typical fixation of the cells by oxidization after reduction was performed. Then the dehydration process was carried out by ethanol with increasing concentration (50-100%).

3 Results and discussion 3.1 Morphology of four samples As shown in Fig.1, NF has the three-dimensional network structure, with high porosity, and large specific surface area. Especially, NF was characterized with high permeability and good mechanical properties. Therefore, NF can provide a more cavitation nuclei and strengthen the ultrasonic cavitation effect. Consequently NF increased the number of free radicals. Besides, due to adsorption-desorption in the gas-liquid boundary layer, the presence of NF increased the gas content in the solution, also strengthening the ultrasonic cavitation effect to strengthen[18-20]. Ceramic foams are cellular structures composed of a three-dimensional network of struts. These highly porous materials have a lot of applications as filters for molten metal, hot gas and diesel engine exhausts filters, catalyst carriers, biomaterials, thermal insulators for furnaces and aerospace applications, gas combustion burners and lightweight building materials. TiO2 particles spread uniformly without aggregation in Fig.1. The results proved that TiO2 was modified on the skeleton successfully. It is enough to perform the

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photocatalyctic activity. The application of nickel foam monoliths in catalytic reactors is in favor of the flow and heat and mass transfer. Especially, the permeability of the nickel foams to solution can be related to macroscopic properties such as the number of pores per unit length, the apparent density, or the void fraction. Bi2WO6 microspheres hold three levels of structure. The entire 3D hierarchical Bi2WO6 microspheres were built by 2D nanosheets and Bi2WO6 nanoplates were composed by rectangle nanoplates. The average sizes of Bi2WO6 microspheres, nanosheets and nanoplates were about 2.5 µm, 0.5 µm and 50 nm. After the photocatalysts were loaded on the carrier, the particle size of Bi2WO6 was bigger than that of TiO2. Under UV light, the photocatalyctic efficiency of TiO2 is larger than that of Bi2WO6. However, the photocatalyctic efficiency of Bi2WO6 exceeds that of TiO2 under UV daylight. 3.2 Removal of MB under different fenton reaction system For investigating the removal of MB under different Fenton catalysis, four AOP systems were compared with each other: NF/TiO2, NF/Bi2WO6, CM/TiO2, and CM/Bi2WO6 for the removal efficiency of MB. The solution contained 0.1 g/mL NF or CM. The initial MB concentration was 5.00 mg/L with pH value of 7. The initial concentration of H2O2 was 5.0 mmol/L, and ultrasonic power was 300 W. The removal efficiency of MB was calculated after 25 min reaction time.

Fig.1 Structure and morphologies of photocatalysts and carriers: (a) NF; (b) CM; (c) NF/TiO2; (d) NF/Bi2WO6; (e) CM/TiO2; (f) CM/Bi2WO6

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Fig.2 Removal efficiency of MB under different Fenton catalytic system

Fig.2 shows the removal efficiency of MB under different Fenton catalytic systems. The removal efficiencies of MB for NF/TiO2, NF/Bi2WO6, CM/TiO2, and CM/Bi2WO6 are 96.4%, 76.2%, 82.3% and 54.9%, respectively. As shown in Fig.1, NF and CM had the three-dimensional network structure, with high porosity, and large specific surface area. Especially, NF was characterized with high permeability and good mechanical properties. Therefore, NF can provide a more cavitation nuclei and strengthen the ultrasonic cavitation effect. Consequently NF increased the number of free radicals. Besides, due to adsorption-desorption in the gas-liquid boundary layer, the presence of NF increased the gas content in the solution, also strengthening the ultrasonic cavitation effect to strengthen (Holstvoogd et al 1998; Yusof et al 2016; Xu et al 2016). Under Fenton catalytic system, H2O2 could produce · OH as shown in Equations (1) and (2): (1) (2) In the presence of H2O2, free radicals in solution increased obviously. The order of MB removal efficiency ranked as NF/TiO 2>CM/TiO 2>NF/Bi 2WO 6>CM/ Bi2WO6. Both Fenton reaction can produce free radical and photocatalysis produce electrons (e -) and hole (h+). The free radical is in favor of the separation of e- and h+. The electron and hole can also produce •OH by oxidation of surface hydroxyl groups and adsorb water molecules. So Fenton reaction and photocatalysis have the synergetic effect. 3.3 Effect of initial pH value on the MB removal efficiency Fig.5 shows the initial pH value’s influence on the removal efficiency. Experiment was conducted under the conditions of 0.1 g/mL NF, initial 5.00 mg/L MB,

300 W ultrasonic power, and 5.00 mg/L H2O2. The initial pH value was adjusted to 4, 7, and 10, respectively. The result shows that at pH 7, MB removal effect is the highest. When pH decreases to 4, the removal rate of MB was significantly reduced. In acid or alkaline solution, the removal efficiency also decreased compared with neutral solution. With lower pH values, the nanoparticles are easier to break off from NF skeleton. Thus the synergetic effect of photocatalysis and fenton reaction can’t take action. As for CM skeleton, the bond –Si-O- can bind with TiO2 or Bi2WO6. As a result, the photocatalysts didn’t break off from CM skeleton and the synergetic effect can take action.

Fig.3 Effect of pH value on the polarization value of N. europaea cells

3.4 Effect of four Fenton systems on N. europaea cells DPH was used as a fluorescence indicator. After incubation for a period of time, DPH will be inserted into the hydrophobic region of cell membrane lipid. The fluorescence polarization of DPH can be measured and looked as an indication of membrane fluidity of cell membrane. The value of the fluorescence polarization degree is inversely proportional to the cell membrane fluidity, the greater the degree of polarization, the smaller the fluidity of cell membrane lipids. Membrane fluidity is very important for the transformation of nutrients, signal transfer and resisting various environmental stresses[21]. The initial concentration of H2O2 was 5.0 mmol/ L with pH value of 7, and ultrasonic power was 300 W. As shown in Fig.4, the initial polarization value was 0.29. With 300 W ultrasonic radiations alone, polarization value reached 0.43 at 30 min, indicating that ultrasonic energy can damage cells. In the presence of NF/Bi2WO6, the polarization value increased to 0.46. An increase in the fluorescence polarization of DPH was observed, indicating a significant decrease in membrane fluidity of N. europaea. Generally, the decrease

Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2018

in membrane fluidity will lead to the increase of permeability of cell membrane.

Fig.4 Polarization value for the control cells and treated by ultrasonic and Fenton system

The surface mophologies of N. europaea cells were observed by SEM in Fig.5. Small protrusions spread on the surface of native cell regularly. The protrusion is about 10 nm in size. Viewed in its entirety, the surface of the native N. europaea was smooth, with regular protrusions and raised grain at nano-level resolution. After treated by Fenton reaction, rough N. europaea surface was observed. Irregular bulges spread on the surface of treated cells. The bulges are about 100 nm in size.

Fig.5 SEM images of a native N. europaea (a) and the treated cell (b)

The results suggested that the surface molecules were damaged and the cell membrane structure was altered. The outer membrane of gram-negative bacteria is composed of Lipopolysaccharide (LPS), peptidoglycan and phospholipid layer. The mechanical strength of the cell membrane depends on peptidoglycan. LPS is responsible for the surface structure and morphology. LPS is formed by the amino sugars of N-acetyl glucose amine and N-acetyl acid two amino sugars by beta 1.4-

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link. Connecting four peptide side chains on the N-acetyl cell wall acid molecules, peptide ontacted by bridge or peptides by peptide, reticulate structure composed of a machinery is very strong. Therefore, the cell membrane plays a very important role in the maintenance of cell morphology. LPS is the outermost layer of the cell membrane, which is exposed to the environment. In the process of Fenton reaction, LPS of the outermost layer is subjected to attack of free radicals. The cell still maintained the original rod shape, suggesting that LPS was damaged while peptidoglycan was intact. As to the native N. europaea, LPS patches were arranged tightly on the cell surface, thus provided a protective layer and an effective permeability barrier for the gram-negative bacteria. After treated by Fenton reaction, however, LPS layer was damaged, resulting in decrease in membrane fluidity and the increase of permeability of cell membrane[20,21].

4 Conclusions In the present work, four systems were designed and compared with each other to investigate the effect of heterogeneous fenton reaction on MB and N. europaea cells. They are NC/TiO2, NC/Bi2WO6, CM/TiO2, and CM/Bi2WO6. The order of effect of fenton reaction ranked as NF/TiO 2>CM/TiO 2>NF/Bi 2WO 6>CM/Bi 2 WO6. In acid or alkaline solution, the removal efficiency also decreased compared with neutral solution. With lower pH values, the nanoparticles are easier to break off from NF skeleton. Thus the synergetic effect of photocatalysis and fenton reaction can not take action. As for CM skeleton, the bond –Si-O- can bind with TiO2 or Bi2WO6. The membrane fluidity was used as an indicating parameter. After treated by Fenton reaction, N. europaea surface exhibited higher roughness than that of the native bacterium. The size of bulges became irregular and they were not uniform on envelop of cells. The effect on N. europaea cells in acid medium was weakened in alkaline medium. References

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