Fixation of BiOI/BiOBr/MoS2 Powders on Fiber Cloths for Photocatalytic Degradation of Ammonia Nitrogen from Aqueous Solution

A novel photocatalyst powder, BiOI/BiOBr/MoS2, was synthesized by a simple solvothermal method. X-ray diffraction (XRD), specific surface area and pore size analyses, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray energy spectrometry (EDS) were utilized to characterize the prepared samples. After evaluating the photocatalytic performance of the catalyst, it was loaded on the glass fiber and carbon fiber by polyvinylidene fluoride (PVDF) and N-methylpyrrolidone, respectively. The photocatalytic activity of the composite was investigated by the degradation of ammonia nitrogen wastewater. The fiber cloth solved the problem of separation of powder from solution after reaction, and the presence of the binder reduces the agglomeration of the nanoparticles in the water. After four times repeated experiments, the degradation of simulate ammonia nitrogen wastewater by loaded glass fiber and loaded carbon fiber are 74.1% and 60.58%. Fixation of BiOI/BiOBr/MoS2 powders on fiber cloth solve the problem of difficult recovery of powder photocatalytic materials and it can be recycled, which has economic valuable.


Introduction
Ammonia is one of the major nitrogen-containing pollutants in wastewater (Lee, Park, & Choi, 2002), Ammonia mainly refers to the combined nitrogen in the form of ammonia ions and free nitrogen in water. The content of NH 3 molecules and NH4 + ions in water mainly depends on the pH value, temperature, salinity and other factors of water. When pH<7, NH4 + ions are the main form of ammonia in water; when pH>11, NH 3 molecules are the main form of ammonia in water (Hedstrom, 2001). NH 3 is a nutrient source that can promote eutrophication and algal growth in natural water (Xiao, Qu, Zhao, Liu, & Wan, 2009), the occurrence of red tide is because of the algae bloom. Excessive amounts of NH 3 in the environment can exert harmful on human health (Yuzawa, Mori, Itoh, & Yoshida, 2012). NH 3 attacks the human respiratory system, skin and eyes, and exposure to high concentration (>300 ppm) may cause death (Netting, 2000;Saha & Deng, 2010). To degrade ammonia nitrogen in wastewater, several chemical and physical methods have been developed and applied. Such as biological denitrification, stripping, breakpoint chlorination and ion exchange (Ahmed & Lan, 2012;Degermenci, Ata, Yildiz, & Chemistry, 2012;Eilbeck, 1984;Ricardo, Carvalho, Velizarov, Crespo, & Reis, 2012). However, most of these methods produced secondary pollution. In order to find an efficient denitrification technology without secondary pollution, researchers began to investigated advanced oxidation processes (AOPs) applicability to the removal of ammonia from water (Bonsen, Schroeter, Jacobs, & Broekaert, 1997;Schmelling & Gray, 1995). Advanced Oxidation Process (AOPs) refers to a series of redox reactions that treat wastewater by generating and utilizing free radicals, such as hydroxyl radicals (Diya'uddeen, Daud, & Aziz, 2011;Fakhru'l-Razi et al., 2009;Ribeiro, Nunes, Pereira, & Silva, 2015;Shahidi, Roy, & Azzouz, 2015;Yu, Han, & He, 2017). The advantage of using AOPs to treat ammonia nitrogen wastewater is that this method is safety and friendly component for the environment.
Photocatalysis is an advanced oxidation technology that coordinates the action of a catalyst under specific light sources to treat wastewater. Akira Fujishima and Kenichi Honda first discovered the phenomenon of light photocatalytic degradation of ammonia in water is that the redox reactions between the hydroxyl radicals (HO·), superoxide radicals (O ·) generated inorganic nitrogen ions during the photocatalytic process. Photocatalytic oxidation is achieved through hydroxyl substitution reactions, dehydrogenation reactions, and electron transfer processes. When the light intensity is greater than the semiconductor band gap, electrons are excited and electron-hole pairs generated on the catalyst surface. At the same time, the NOx molecules which adsorbed on the catalyst surface are seized by the electrons and oxidized (Zhao & Lou, 2008). Photocatalytic reduction refers to NO and NO in wastewater which are reduced to N 2 through a photocatalytic reaction. Degradation of ammonia nitrogen wastewater by photocatalytic can be expressed as the following formulas: 2NO 3 -+ 12H + + 10e ab -→ 2N 2 + 3H 2 O (5) Photocatalytic react through semiconductor materials. Semiconductors have a valence band (VB) and an empty conduction band (CB), with a forbidden band between them, so they can be used as a photosensitizer in the photooxidation process (Hoffmann, Martin, Choi, & Bahnemann, 1995). When a semiconductor material is illuminated at or above the bandgap width, the photogenerated holes on the valence band react with H 2 O to form hydroxyl radicals and the conduction band electrons react with O 2 in the solution to form superoxide radicals (Yang, Wang, Yang & Yang, 2017;Liang & Li, 2009), which can reduce the ammonia in wastewater. Ternary semiconductor compound bismuth oxyhalide is a new p-type semiconductor, generally expressed as BiOX, (X=F, Cl, Br, I). Bismuth is a ternary semiconductor compound which has a layered structure formed by overlapping [Bi2O2] 2+ layers and double X ion layers (Chang, Zhu, Fu, & Chu, 2013). Due to the relatively loose structure, the morphology of BiOX is usually shown as flakes or nano-flowers (Deng, Chen, Peng, & Tang). The valence band of Bi 3+ is formed by the hybridization of the 2p orbital of the O atom and the 6s orbital of the Bi atom. The polarization between orbitals weakens the symmetry of the electronic structure and forms a dipole moment that broadens the valence band of the semiconductor, in that case, semiconductor valence band becomes higher, and forbidden band width becomes narrower (Stoltzfus, Woodward, Seshadri, Klepeis, & Bursten, 2007). According to the calculation of density functional theory, the band gaps of BiOF, BiOCl, BiOBr, and BiOI are 3.34, 2.92, 2.65, and 1.75eV, respectively. Compared with TiO 2 , BiOX has a narrow band gap and a high usage of visible light, it is an efficient and economical semiconductor photocatalytic material.
Heterojunction is a crystalline interface formed by two contacting catalysts which have similar band structure. Lin and Lee synthesized a PbO 2 /BiOBr composite by hydrothermal method, and determined its catalytic performance by the degradation efficiency of the crystal violet (CV) under visible light irradiation (Lin et al., 2016). The experimental results show that the reaction rate of the composite material is three times higher than that of PbO 2 and two times higher than that of BiOBr, which indicates that the existence of heterojunctions inhibits photo-generated electron-hole recombination and improves the catalytic efficiency.
One of the major limitations in the application of the photocatalytic process in wastewater is the separation of powder from solution after reaction (Mohammadi, Sharifnia, & Shavisi, 2016). Immobilization of photocatalysts over a support, such as silica, zeolite and polymers have been used to overcome this disadvantage (Ali, Ismail, Najmy, Alhajry, & A-chemistry, 2014;Park, Park, Kim, Choi, & Reviews, 2013;Tseng et al., 2012). Shi and Cui studied photocatalytic activity of TiO 2 coated on an activities carbon fiber, the decoloration of methylene blue by TiO 2 /ACFs showed a high degradation efficiency (Shi et al., 2012).
In this paper, BiOI, BiOBr and MoS 2 composite particles were prepared by a solvothermal method and the morphology and structure of the samples were characterized. The simulate ammonia wastewater was degraded by composite materials, and the photocatalytic properties of the different samples were compared. To solve a problem where the photocatalytic material could not be recovered in practical applications, a photocatalytic material with a high degradation performance was selected to be coated on fiber cloth, and examined for its degradation efficiency after repeated use.

Materials
Bi(NO) 3 ·5H 2 O (2 mmol) was dissolved in anhydrous ethanol (30 mL) and stirred for 30 min as solution A. KI (1.2 mmol) and cetyltrimethylammonium bromide (1 mmol) (CTAB) were weighed and dispersed in deionized water (30 mL) and stirred for 30 min as solution B. Solution B was slowly added into A and stirred until the liquid turned orange-red. MoS 2 with a mass fraction of 0/0.5/1/2 wt% was added into the turbid liquid, mixed evenly and transferred to a 100-mL reaction kettle, which was then reacted in an oven at 160°C for 24h. After the reaction was completed and the product was cooled, the precipitation was cleaned and dried in an oven at 60°C for 12 h. BiOI/BiOBr/MoS 2 composites with different proportions were then obtained.
Glass fiber and carbon cloth were baked in a muffle furnace at 600°C for 3 h as pretreatment. The catalyst powder (50 mg) was uniformly mixed with polyvinylidene fluoride (5 mg) (PVDF), and an appropriate amount of N-methylpyrrolidone was added. The mixture was uniformly loaded on the fiber cloth with a brush after stirring and sonication.

Characterization
The crystal structure of the photocatalyst was examined by X-ray diffraction (XRD, D2-PHASER), with an X-ray diffractometer using CuKɑ (λ=1.5406Ȧ) radiation in the 2θ range of 5-40°. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific area (S BET ) based on the adsorption isotherm. The average pore diameter was obtained from the N 2 sorption/desorption isotherm, which was measured on a Quantachrome NOVA 4000e at -196°C. The morphology and surface characteristics of the sample were observed by scanning electron microscopy (SEM, HITACHI TM3030) and transmission electron microscopy (TEM, JEOL-2100F). The synthesis status of the composite was determined by energy dispersive spectroscopy (EDS, JOEL JPS-9030).

Photocatalysis Experiment
Ammonium chloride (NH 4 Cl) was used to simulate ammonia nitrogen wastewater, 30 mg/L, 50 mg/L, 70 mg/L, and 100 mg/L ammonium chloride solutions were configured. At room temperature, catalyst powder (30 mg) was dispersed in simulated wastewater (30 mL) under magnetic stirring. The light source was placed 40 cm above the vessels for the catalytic experiment. Before each light experiment, the catalytic system was stirred in the dark for 30 min to achieve a dynamic adsorption equilibrium. After the catalysis experiment, the supernatant was subjected to a Nessler colorimetric method.
The removal rate is calculated based on the measured nitrogen concentration of the simulated wastewater, and the calculation formula is: Where C represents the initial nitrogen concentration, C 0 represents the testing nitrogen concentration.

Loaded Fiber Repetitive Experiments
When performing repetitive experiments, a simple shallow pool reaction device was established as the reaction area, and the simulated ammonia nitrogen wastewater was smoothly and evenly pumped into the shallow tank reactor by peristaltic pump. The remainder of the simulated wastewater was stored in a beaker. A 500 W xenon lamp was placed 40 cm above the liquid level to simulate sunlight. Samples were taken from the beaker every 30 min for nitrogen content testing. Four replicate experiments were performed on each fiber, and each result was compared with the degradation results of the first experiment to investigate whether the material was reusable.  3. SEM image tos in Figure 4 2 . n wastewater is of NH4 + with onia wastewat on, in a stron hough the degr this paper, th ution. the degradati L, 50mg/L, 70 performed to adiation for 12 active sites on nitrogen wastew ency, the degra % after 60 min e degraded in higher than th concentration, e initial conce photocatalytic ewater (

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Conclusion
Composites were prepared by a solvothermal method. The photocatalytic performance of different catalytic materials was investigated by morphology, structure and their photocatalytic effects on ammonia wastewater. Compared to BiOBr monomer, BiOI/BiOBr/x wt% MoS 2 have higher photocatalytic activity because of the presence of heterojunctions. BiOI/BiOBr/1 wt% MoS 2 composite showed the highest photocatalytic efficiency of degradation of simulate ammonia wastewater. The addition of MoS 2 enhances the nonselective adsorption of materials. However, the excessive doping of MoS 2 may reduce the pore size and decrease the photocatalytic activity of the catalytic. The BiOI/BiOBr/1 wt% MoS 2 material retains a degradation efficiency around 80% when loaded onto a fiber cloth. After four repeated experiments, the recovery ratio of the loaed glass fiber and loaded carbon fiber are 74.1% and 60.58%. The fiber cloth provides more catalytically active sites, and the presence of the binder reduces the agglomeration of the nanoparticles in the water. The loaded fiber cloth prepared in this paper have high photocatalytic activity and can be repeatedly used, which is an economically valuable photocatalytic material.