Visible light and UV-assisted photodegradation of phenylephrine by bulk NiO and NiO immobilized on magnetic polypyrrole support

The nanophotocatalysts; NiO and NiO immobilized on the surface of magnetized polypyrrole (NiO-PPYFe3O4) were synthesized and used for photodegradation of phenylephrine. The synthesized photocatalysts were characterized by FTIR, TG, XRD, BET, VSM, TEM and SEM techniques. Diffuse refl ectance spectroscopy was used to measure the band gap nergy of the synthesized photocatalyst. Photoluminescence analysis was employed to estimate the electron-hole recombination. The results indicated that immoblization of NiO on the surface of the polypyrrole support increased its photocatalytic activity. Under visible light, the degradation effi ciency obtained by bulk NiO was 38% which was increased to 71% after immobilization on the catalyst support. The increase was attributed to the shift of band gap energy to higher wavelenght as indicated by DRS tests, and to the lowering of electron–hole recombination as revealed by PL analysis. The catalyst support improved the regeneration ability of the photocatalyst. After fi ve regeneration cycles, 90% of NiO-PPY-Fe3O4 activity was remained while for bulk NiO, the value was about 15%. The paramagnetic nature of NiO-PPY-Fe3O4 photocalyst facilitated its separation from the degradation solution without tedious fi lteration or centrifugation steps. In low concentrations, H2O2 showed an enhancing effect on the degradation but the effect was reversed in higher concentrations. Research Article Visible light and UV-assisted photodegradation of phenylephrine by bulk NiO and NiO immobilized on magnetic polypyrrole support Hossein Faghihian and Firoozeh Torki* Department of Chemistry, Naghshejahan Higher Education Institute, Isfahan, Iran Received: 23 September, 2019 Accepted: 30 December, 2019 Published: 31 December, 2019 *Corresponding author: Firoozeh Torki, Department of Chemistry, Naghshejahan Higher Education Institute, Isfahan, Iran, E-mail: ;


Introduction
Many pharmaceutical compounds are considerred as environmental pollutants and their presence even at very low concentration (ng.L -1 ) may adversely infl uence the biological system [1]. These compounds are frequently detected in wastewaters, surface and ground waters and even in the treated wastewaters because the waste treatment facilities are not equipped to remove them. As a result, many streams and rivers are exposed to entrance of pharmaceutical compounds from stimulants and antibiotics to analgesics and antihistamines compounds.
Phenylephrine hydrochloride ( Figure 1S) used for nasal congestion is considered as one of the most important pharmaceutical pollutant [2]. Several methods have been used for elimination of the pollutants from aqueous solution including sono degradation [3], Fenton oxidation [4], nanofi ltration [5], adsorption [6] and advance oxidation processes [7,8]. Advanced oxidation process have also been intensively used for effective degradation of many organic pollutants [9][10][11][12][13]. The essential requirement for the AOP process is an adequate heterogeneous photocatalyst, which is irradiated by UV, visible or sunlight radiations to produce electron-hole pairs (e -/h + ). The electronhole pairs subsequently generate highly active species such as OH radicals. The radicals then react with the pollutant molecules adsorbed on the surface of the photocatalyst [14,15].
The method is capable to destroy environmentally persistent pollutants by using photocatalysts activated by photons with energy higher or equal to the band gap energy of the photocatalyst. When the photon is absorbed by photocatalyst particle, an Electron (e -) from the Valence Band The nano-sized photocatalyst possesses many advantages over their micro-sized counterparts, including higher degradation effi ciency, proper mineralization potential, and higher effi ciency with visible light. However, separation of the nano-sized photocatalysts from degradation solution is usually imperfect and time consuming. The nano-sized particles also tend to aggregate to large clusters, leading to lower specifi c surface area and lower degradation effi ciency [16]. To eliminate these drawbacks, the particles can be magnetized by the aid of Fe 3 O 4 . The magnetized photocatalyst can be readily removed by use of a magnetic bar put outside of degradation vessel [17].
In photocatalytic degradation, recombination of electron/ hole is a common problem, which adversely infl uence the effi ciency of the photocatalyst. The process can be quenched or   The pH of the solution was adjusted to 11 by drop wise addition of NaOH solution. The solid product was then separated by a magnet bar put outside of the vessel. It was thoroughly washed with distilled water until the fi ltrate become neutral. The product was dried at 60˚C for 8h [19].  Table 1S. The black-coloured product (Fe 3 O 4 -PPY) was magnetically separated and thoroughly washed with deionized water and methanol and dried at 60˚C for 24h (Figure 2Sa), [21].

Preparation of NiO and Fe3O4-PPY-NiO nanophotocatalysts
The NiO nanoparticles were fi rst prepared as follows 12.0g of NiCl 2 .6H 2 O was dissolved in 30mL of deionized water, the pH was adjust to 11.0 by addition of 0.1M NaOH solution. The precipitate; Ni(OH) 2 was separated by fi ltration, dried at 100˚C and calcined at 550˚C for 3h [22].
Concisely, 1.0g of Fe 3 O 4 @PPY and 1.0g of NiO nanoparticle were dispersed in 20mL of m-cresol solution and the mixture was refl uxed for 15h at room temperature under N 2 atmosphere.
The magnetized photocatalyst was collected by use of a magnet bar put outside of the vessel. It was thoroughly washed with deionized water and methanol and dried at 70˚C for 6h. In the same manner, several photocatalysts with different NiO content (10%-80%) was prepared [23]. amount of photocatalyst (0.2g) was added into 25mL of phenylephrine solution. The suspension was shaken for 30min at dark and then was irradiated for 240min. The photocatalyst was magnetically separated and the PHE concentration in the remaining solution was measured by HPLC-UV at  215 nm. The degradation effi ciency was calculate by the following equation:

Photodegradation of the pollutant
Where C o and C are respectively the PHE concentration before and after irradiation.

Characterization of the synthesized photocatalyst
In the SEM image of The surface area of the samples measured by N 2 adsorptiondesorption isotherms indicated that after immobilization of NiO on the surface of magnetized support, the surface area was reduced from 62.2 to 34.5m 2 g -1 (Table 1). With lower surface area, it was expected to obtain lower degradation effi ciency because lower amount of the pollutant was adsorbed on the photocatalyst surface, but immobilization of NiO on the surface of the magnetized support increased the effi ciency of the photocatalysts by lowering the electron-hole recombination.
In the FT-IR spectrum of Fe 3 O 4 (Figure 7Sa), the adsorption band appeared at 470cm -1 was attributed to the characteristic Fe-O band and the peak at 1400cm -1 belonged to OH in-plane vibration of water molecules. The peaks appeared at 1600 and 3400cm -1 were related to water molecules adsorbed on the surface. The  (Figure 7Sc), the characteristic absorption bands of polypyrrole were observed. The bands at 1080 and 900cm -1 were arisen respectively from CH in plane and CH out-of-plane vibration, indicating the successful polymerization of pyrrole [27]. The bands at 1500 and 1474cm -1 were respectively attributed to C-N and C-C asymmetric and symmetric ring stretching of PPY [26]. The peak at 1400cm -1 was attributed to the stretching band related to C=C of PPY ring. The weak bands of NH and CH stretching vibration of polypyrrole appeared respectively at 3404 and 2358cm -1 and the band at 3404cm -1 related to symmetrical stretch vibration of NH group has been buried under board OH peak of water molecules at 3400cm -1 [28] . From the results, it was concluded that the polymerization was successfully performed. In Fe 3 O 4 -PPY-NiO sample, the NiO stretching and vibrating band at 626cm -1 was observed. The main absorption band at 1600cm -1 belonged to water molecules became more intense due to the adsorption of water molecules by NiO nanoparticles. The absorption band at 3400cm -1 related to Fe 3 O 4 was disappeared because of adsorption of water molecules by NiO particles.
In the thermal curves (TG-DTG) of Fe 3 O 4 -PPY-NiO represented in Figures 8Sa,b, a continuous weight loss peak from 25-800˚C with two distinct steps was observed. The weight loss peak appeared between 80-15˚C was attributed to the elimination of water molecules and unreacted monomer [23]. The second weight loss started from 500˚C and ended at 800˚C was attributed to the decomposition of PPY molecules. It was reported that pure PPY decomposed around 150˚C, and after combination with nanoparticles, its thermal decomposition shifted to higher temperature (600-800˚C) [29,30].  However, the magnetic property of the synthesized photocatalyst was suffi cient for removal of the used photocatalyst by putting a magnet bar outside of the vessel.
The magnetized particles were collected on the internal surface of the vessel within two second and immediately redistributed when the magnet bar was removed (Figure 1d).

Optical properties of the photocatalysts
The absorption edges of Fe 3 O 4 -PPY-NiO and NiO were obtained by DRS spectra (Figures 2a,b). The optical band gap (E gopt ) was obtained by equation 3.

Photocatalyst performance
The initail degradtion experiment was performed by   buffer (pH 4.7) and methanol (70:40 v/v). The fl ow rate was adjusted at 1.0mLmin -1 . The retention time for PHE was found to be at 2.5 min [2]. From the chromatograms of the samples taken before and after degradation it was concluded that the intensity of PHE peak (RT 2.5min) was signifi cantly decreased after degradation indicating the good performance of the photocatalyst for degradation of the pollutant (Figures 9Sa,b).
The statistical data of the method is given in Table 2S. the R 2 value of 0.9972 for calibration curve, and the acceptable RSD value indicated the the applicability of the method.
To optimize the degradation effi ciency, the effect of different infl uencing parameters on the degradation effi ciency were studied.  [36]. In this work, the effect of pH on the degradation effi ciency was studied on pH range of 3-9 ( Figure 6). The maximal degradation was obtained at pH 7. To evaluate the effect of pH on the surface charge of the photocatalyst, its pH pzc was determined. As indicated in (Figure 6), the PH pzc of the photocatalysts was at pH 7. At this pH, the surface of the S1 S1 S1 S1 S1 S1 S1 S1 S1 S2  S2  S2  S2  S2  S2  S2  S2  S2  S3  S3  S3  S3  S3  S3  S3  S3  S3   S4  S4  S4  S4  S4  S4   S4  S4  S4   40  was obtained with the catalyst containing 13% of NiO [39].

Optimizing of degradation effi ciency
The effect of temperature on the degradation effi ciency: In this work, the degradation effi ciency was measured at different temperatures of 25, 35, 45 and 55˚C (Figure 8). The results indicated that at higher temperature, lower degradation effi ciency was obtained. It was suggested that by increasing of temperature, the amount of dissolved oxygen was decreased.
Tseng, et al., studied the effect of hydrogen peroxide on the photocatalytic degradation of monochlorobenzene in the presence of TiO 2 and reported that degradation effi ciency in the presence of hydrogen peroxide was enhanced [41]. Barba  who studied the photodegradation of some drugs reported that addition of H 2 O 2 increased the of degradation effi ciency [42,43].

Regeneration of the photocatalyst
One of the most important advantages of magnetized photocatalyst is the ease of separation of catalysts from the reaction solution which facilities the regeneration of the photocatalyst. In this work, the used photocatalyst was separated from the solution by a magnet bar put outside of the degradation cell. The photocatalyst was then regenerated by heat treatment at 250˚C for 6h to eliminate the degradation products deposited on the surface of the photocatalyst. The regenerated photocatalyst was reused for degradation of PHE at optimized conditions. The process was repeated for fi ve successive regeneration steps ( Figure 11S). It was concluded that the activity of the catalysts, in terms of the PHE degradation effi ciency was slowly decreased in each cycle, and the decrease was more pronounced with increasing the number of reusing cycles.
However, after fi fth regeneration cycle, 80% of the initial effi ciency was observed for S1 sample. The decrease was explained by partial occupation of adsorption sites by degradation products, or dislocation and aggregation of NiO particles during heat treatment.

GC-MS Analysis and identifi cation of degradaton products
The degradation products of PHE were analyzed by full scan mode of GC-MS. Aliquots of PHE solutions after degradation and a sample containing standard PHE were prepared. The extration of degradation products from aqueous solution was done by chloroform. It note that PHE is unsoluble in chlorofom but degradation products extracted. The Mass spectrum of PHE was similar to that of reference sample ( Figure 12S). The GC spectra of PHE before and after degradation is displyed in Figure 13S. To identify the degradation fagments, they were separated from the solution by extraction with chloroform. and the mass spectra of the identifi ed degradation products is represented in Figure 14S.
The MS spectrum of degradation products were concluded that the peaks appeared at retention time of 3.8, 3.9, 4.7, 4.8, 7.7 and 7.5mins beonged to the degradation products and the peak appeared at RT 11.0 was related to the remaining PHE. The identifi ed compounds are listed in Table 3S. The main degradation products are di-sec-buthyl ether, disec-buthyl acetal, benzyl ether, 4-oxo-pentanoic acid and 4-chlorobenzaldehyde.
The aim of AOPs process is to mineralize the organic pollutant and to convert them into H 2 O and CO 2 . To evalute the mineralization degree of the pollutant, the Total Organic Carbon (TOC) of the degradtion solution was measured before and after irradiations. It was concluded that 85% of degraded polutant was mineralized. Therefore, the concentration of the identifi ed degradtion products was very small.

Conclusions
In this work, a magnetized support was prepared by in-situ oxidative polymerization of polypyrrole in the presence of Fe 3 O 4 . The support was then employed as the catalyst support for NiO photocatalyst. The synthesized catalyst was characterized by different techniques. The SEM images showed that the particle size of Fe 3 O 4 -PPY and Fe 3 O 4 -PPY-NiO were respectively 45 and 65nm. The DRS analysis indicated that band gap energy of NiO after immobilization on thes surface of the support shifted to lower energy which was benfi cial for visible light degradation of the pollutant. PL analysis of the photocatalysts showed that after embeding of NiO on the suport, the electron-hole recmbination was signifi cantly quenched leading to higher degradtion efi iciency. The synthesized photocatalyst, and the bulk NiO, were used for degradtion of PHE under UV and visible light irradiaions. The degradation process was kinetically fast and the equilibrium was achieved within 4h of irradiaton. The used magnetized nanophotocatalyst was removed from the degradation solution by applying external magnetic fi eld and was regenerated by heat treatment at 250˚C. Most of its initial activity was remained after fi ve regeneration cycles.