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RESOURCE RECOVERY FROM WASTEWATER, SOLID WASTE AND WASTE GAS: ENGINEERING AND MANAGEMENT ASPECTS Cu/N-codoped TiO 2 prepared by the sol-gel method for phenanthrene removal under visible light irradiation

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Cu/N-codoped TiO 2 nanoparticles were prepared by the modified sol-gel method, to study its efficiency for the removing of polyaromatic hydrocarbon (phenanthrene) from an aqueous solution. Urea and copper sulfate pentahydrate were used as sources of
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  RESOURCE RECOVERY FROM WASTEWATER, SOLID WASTE AND WASTE GAS: ENGINEERING ANDMANAGEMENT ASPECTS Cu/N-codoped TiO 2  prepared by the sol-gel methodfor phenanthrene removal under visible light irradiation Zhenhua Zhao 1 &  Abduelrahman Adam Omer 1,2 &  Zhirui Qin 1 &  Salaheldein Osman 2,3 &  Liling Xia 4 & Rajendra Prasad Singh 5 Received: 21 February 2019 /Accepted: 18 June 2019 # The Author(s) 2019 Abstract Cu/N-codoped TiO 2  nanoparticles were prepared by the modified sol-gel method, to study its efficiency for the removing of  polyaromatic hydrocarbon (phenanthrene) from an aqueous solution. Urea and copper sulfate pentahydrate were used as sourcesof doping element for Cu/N-codoped TiO 2 , respectively. The characterizations of the nanoparticles were done by X-ray diffrac-tion (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectra. XRD revealed that all the nano- particles were indexed to the anatase phase structure, with crystallite size range from 11 to 30 nm, which decreased with thedopingofcopperand nitrogen. ThephotocatalyticactivitiesofCu/N-codopedTiO 2 showedthehighest activitiesthanother TiO 2 nanoparticles (TiO 2  and N-doped TiO 2 ). The photodegradation efficiency of Cu/N-codoped TiO 2  on phenanthrene under visiblelight irradiation was slightly higher (96%) comparing to UV light irradiation (94%). Cu/N-codoped TiO 2  was found to be veryefficient and economical for phenanthrene removal, because the smallest amount of Cu/N-codoped TiO 2  exhibited the best removal efficiency on phenanthrene. Keywords  Photocatalysts .CodopedTiO 2  .Cu/N-codopedTiO 2  .PAHremoval .Phenanthrene .Sol-gelmethod Introduction Due to the population growth which resulted in industrialdevelopmentandcivilization,atmosphereandwaterresourceswere polluted, therefore created severe environmental pollu-tion concern in the world (Patil et al. 2012). Polycyclic aro-matic hydrocarbons (PAHs) are organic compounds that aremostly colorless, white, or pale yellow solids. They are a ubiquitous group of several hundred chemically related com- pounds, environmentally persistent with various structuresand varied toxicity (Perrin 2005). Therefore, this pollutionshould be removed to maintain the safe environment and safewater resources. Phenanthrene, with 3-benzene-ring structure,is a typical member of PAHs although it lacks significant carcinogenicity (Patel et al. 2016). Phenanthrene was consid-ered a model compound for the PAH photodegradation study becauseitwas foundtobemostabundantinthePAH-pollutedenvironment (Agrawal et al. 2018). In addition, due to thehigher concentration of phenanthrene in PAH mixtures, phen-anthrene metabolites are more readily quantified. There aremany conventional treatment methods used for water pollu-tion control such as physical treatment, chemical treatment,and biological treatment (Heena Khan et al. 2015; Romão2015). However, these methods are not capable to completelyremove the pollution from water, so a new technology able toremove persistent pollutants has been required (Dung et al. Zhenhua Zhao and Abduelrahman Adam Omer contributed equally tothis work.Responsible editor: Suresh Pillai *  Abduelrahman Adam Omer abdu.8691@gmail.com 1 KeyLaboratoryofIntegratedRegulationandResourceDevelopment on Shallow Lake of Ministry of Education, College of Environment,Hohai University, Nanjing 210098, People ’ s Republic of China  2 Department of Civil Engineering, College of Engineering Science, Nyala University, Nyala, Sudan 3 Water Harvesting center, Nyala University, Nyala, Sudan 4  Nanjing Institute of Industry Technology, People ’ s Republic of, Nanjing 210016, China  5 School of Civil Engineering, Southeast University, Nanjing 210096,China  Environmental Science and Pollution Researchhttps://doi.org/10.1007/s11356-019-05787-7  2005). In this point of view, the semiconductor-mediated pho-tocatalytic decontamination of air and water is a promisingenvironmental remediation technology because fewer chemicals are required, and complete mineralization is possi- bleatroomtemperature(Chongetal.2010).Amongthesemi-conductor, TiO 2  is the most promising and attractive photocatalyst due to its high oxidative power, photostability,low cost, nontoxicity, chemical inertness, and commercialavailability (Kessler  2014). When TiO 2  is irradiating withUV light (wavelength  ≤ 387 nm), electrons will promoteacross the bandgap (3.2-eVanatase or 3.0-eVrutile crystalline phases) intotheconductionband, leavingelectronholes in thevalence band. These electron holes have strong oxidation power, allow them to react with adsorbed hydroxide ionsand water to produce hydroxyl radicals (the main oxidizingspecies), and are responsible for the photo-oxidationof organ-ic (Caratto et al. 2012; Taylor et al. 2013). Electron paramag- netic resonance (EPR) results indicated that photogeneratedelectrons and holes are trapped at different defect sites.Holes were trapped as oxygen-centered radicals covalentlylinked to surface titanium atoms and electrons were trappedas Ti(III) centers (Behnajady and Eskandarloo 2013).However, TiO 2  suffer from some limitations such as the ab-sorption wavelength of TiO 2  does not extend to the visiblelight region and the electron/hole pairs can recombine rapidly.To overcome the limitations of TiO 2 , numerous studies fo-cused on the surface modification of photocatalysts in manyways such as doping with metals and nonmetals (Taylor et al.2015; Maicu et al. 2006). Various transition metals were used asdopants,butthemono-dopingwithtransitionmetalssuffersfrom thermal instability and an increase of the charge carrier recombination (Bellardita et al. 2009), due to an increasingamount of the metal dopants (Jaiswal et al. 2015). In contrast,doping with nonmetal has features of narrowing bandgap,which allows the absorption shift toward the visible light and consequently enhance the photocatalytic activity.However, TiO 2  doped by a single element has not been foundto meet practical applications and codoping with different el-ements may lead to higher photoactivity (Behnajady andEskandarloo 2013; Jaiswal etal. 2015).Inthissituation,many studies about codoping with two dopants (metal/nonmetal)were suggested to overcome this issue, enhance the visiblelight absorption efficiency, and reduce the recombination of the photogenerated charges as well. Literature reviews report-ed that nonmetal doping with N − ions was introduced as themost promising dopant among other nonmetals by narrowingthe bandgap of TiO 2 , so that it extends the absorption spectra to the visible light, and subsequently enhances the photocata-lytic activity (Asahi et al. 2014). On the other hand, copper ions have been used extensively as dopant compared to other metals because of its ability to directly trap the electrons gen-erated from excited photocatalyst. These Cu 2+ ions could ex-tend the absorption spectrum and reduce chargerecombination by capturing electrons and proceed the chargecarrier  ’ s lifetime (Reza et al. 2017). Many studies were doneon monodoped TiO 2  using copper or nitrogen as dopants(Jagadale et al. 2008; Xin et al. 2008; Yoong et al. 2009; Asahi et al. 2014), but a few studies were done on Cu/N-codopedTiO 2 .Wangetal.(2014)examinedthephotocatalyticactivityof10mgCu/N-codopedTiO 2  bydegradationofmeth-ylene blue solution (10 mg, 30 ml). Their results showed animproved activity in the photodegradation of MB solution.Jaiswal et al. (2015) conducted photocatalysis experiment using 50 ml of 10 mg  p -nitrophenol solution containing20 mg of the photocatalyst to study the photoactivity of Cu/  N-codoped TiO 2 . They reported that Cu/N-codoped TiO 2  wasable to degrade  p -nitrophenol solution under light irradiationwithsignificantlybetterrateincomparisontomonodopedandundoped TiO 2 . However, none of the previous studies wasutilized to remove the polyaromatic hydrocarbon compoundsfrom aqueous solution, which is our novelty of this work.Therefore, this study aimed to (i) synthesize Cu/N-codopedTiO 2  nanoparticle by modified sol-gel method, (ii) study the photocatalytic activity of Cu/N-codoped TiO 2  and comparewith N-doped TiO 2  and pure TiO 2  under UV/visible light,and (iii) investigate the Cu/N-codoped TiO 2  efficiency to re-move polyaromatic hydrocarbon (phenanthrene as a model pollutant) from an aqueous solution under visible light. Materials and methods Materials and reagents Urea (CH 4  N 2 O) was obtained from Sinopharm ChemicalReagent Co. Ltd., Shanghai, China. Copper sulfate pentahydrate (CuSO 4 ·5H 2 O), acetylacetone (C 5 H 8 O 2 ), andethanol (C 2 H 5 OH) were obtained from Nanjing ChemicalReagent Co. Ltd., Nanjing, China. Tetrabutyl titanate(C 16 H 36 O 4 Ti) and nitric acid (HNO 3 ) were obtained fromChendu Kelong Chemical reagent Factory, Sichuan, China.Phenanthrene (C 14 H 10 ) and n-hexane (C 6 H 14 ) were obtainedfrom J&K Scientific, Shanghai, China. All chemicals werereagent grade and were used without further purification. Samples preparation Cu/N-codoped TiO 2  was prepared through the modified sol-gelmethod:ureaandcoppersulfatepentahydratewereusedassources of nitrogen and copper, respectively. The synthesis procedures were as follows: 4.3 g urea and 0.0325 g copper sulfate pentahydrate were dissolved in a solution of 12 mLtetrabutyl titanate, 6 mL ethanol, and 0.8 mL acetylacetone,and stirred for 30 min, marked as solution A. The concentra-tion ratio of C 16 H 36 O 4 Ti:H 4  N 2 O:CuSO 4 ·5H 2 O was 1:2:0.003for Cu/N-codoped TiO 2 , and 1:0:0.003 for N-doped TiO 2 , Environ Sci Pollut Res  10 mL ethanol, 1 mL distilled water, and nitric acid to adjust the pHbetween 2 and 3,labeled assolutionB.SolutionB wasadded dropwise into solution A and stirred for 2 h at roomtemperature using mechanical stirring. Afterwards, the mix-ture was left for 24 h at room temperature to form a gel andsubsequently dried at 80 °C for 48 h. Then the powder obtain-ed was calcined at 500 °C. The same procedures were used tosynthesize N-doped TiO 2  and pure TiO 2  just without the ad-dition of the designated dopants. Characterization The crystal structure of the powders was analyzed by X-raydiffraction instrument (XRD) using Cu K  α   radiation ( λ =1.54184 A°). The crystallite sizes of the nano-synthesized photocatalysts were estimated from the half-width of the fullmaximum (HWFM) of the maximum peaks of the doped and pureTiO 2 usingtheScherrerEq.(1)(Karunakaranetal. 2011)  D  ¼  K  λβ cos θ  ð 1 Þ where  D  is the mean crystallite size,  k   is a constant (  K  =0.90), λ  is the X-ray wavelength corresponding to Cu K  α   radiation( λ =1.54184 A°),  θ  is the diffraction angle, and  β   is the half-width of the full maximum (FWHM). The absorption spectra and the bandgaps of the parent TiO 2 , doped TiO 2 , andcodoped TiO 2  were determined by measuring the UV-vis ab-sorptions spectra (taken in diffuse reflectance mode) using900 UV-vis spectrophotometer, in the range of 200  –  900 nm.The surface composition and chemical states of the sampleswere examined by X-ray photoelectron spectroscopy (XPS)with a monochromatic Al K_ (1486.6 eV) X-ray source and a hemispherical analyzer. Photocatalytic experiment Basic experiment  The photocatalytic degradation experi-ments were carried out in a batch reactor (quartz tube,50 mL) under constant stirring at room temperature (Fig.1).Twentymillilitersof10ppmphenanthreneaqueoussolu-tionwastransferredintoaquartztubecontaining0.05gofthe photocatalystwhichwasaddedpreviously.Thesolutionwaskept in darkness for 30 min to reach the adsorption-desorption equilibrium point, then irradiated to the light of 300-Wxenonlamp(HSX-UV300NBeT,Beijing)withaUVcutoff filter to cut the entire UVregion (below 387 nm) and permit only the visible light. During the photodegradationreaction which continued on for 2 h, 3 mL of the samplewasfilteredoutevery60min,whichwasarrangedbysequen-tially placing the solutions in the dark for 30 min and in thelight for 30 min to control the temperature. Extraction of samples  The 3 mL of samples which wasfiltered out during the photodegradation reaction wastransferred into a glass extraction tube for extraction using2 mL  n -hexane and sodium sulfate powder (Na  2 SO 4 ) toremove the aqueous phase from the organic phase. After the aqueous phase was removed, the sample was kept inthe dark for 2 days to evaporate the organic phase, andthen the concentrated was re-dissolved in 3 mL of hexaneand filtered through a 0.22- μ  m syringe filter and thenmeasured. Calibration curve of phenanthrene solutionwas obtained by preparation of several concentrations of  phenanthrene solution (0.05, 0.1, 0.5, 1, 2, 4, 6 mg/L)(Karam et al. 2014). Samples measurement  The extracted samples were mea-sured using a UV spectrophotometer at   λ max  = 252 nmand the absorbance was determined. The phenanthreneconcentration corresponded to the absorbance obtainedfrom the calibration curve equation. The removal percent-age of phenanthrene was determined using the followingEq. (2) (Le et al. 2014): Removal % ð Þ ¼  Co − C   f   Co   100  ð 2 Þ Statistical analysis The obtained results underwent the analysis of variance (one-way ANOVA) with Tukey ’ s test (  p <0.05) according to theEcological Society of America (Society and Monographs2011) using the Origin Pro 9.0 (Trial vision). Results and discussion X-ray diffraction analysis XRD patterns of the three samples TiO 2 , N-doped TiO 2 , andCu/N-codoped TiO 2  calcined at 500 °C are shown in Fig. 2.All the diffraction peaks were indexed to anatase phase TiO 2 structure, which matched the standard patterns (JCPDS cardno. 21-1272). The peaks observed at 2 θ  values of 25.31,37.81, 47.91, 53.91, and 55.21 in all the powder samplescorrespond to 101, 004, 200, 105, and 211 planes of the ana-tase phase. From Fig. 2, no peak was detected due to Cuspecies, which suggests that Cu species were well dispersedin TiO 2  powder which agreed with Qian et al. (2019). The peaks indicated that the incorporation of Cu preserves theanatase at 500 °C and inhibit the crystallinity growth, whichwas seen from the decreased peak intensity of Cu/N-codopedTiO 2  compare to N-doped TiO 2 . The Cu 2+ ionic radii(0.87 A°) whichis bigger than Ti 4+ ionic radii (0.75 A°) could Environ Sci Pollut Res  allow the incorporation of the Cu dopant into the titania net-work. This result agrees with López et al. (2010) which re- ported that copper could be placed in the interstitial sites dueto  r  Ti < r  Cu , while integration of N dopant might be due tooverlap of N 2p with O 2p that allows N 2  ions to replace O 2 ions as reported by Asahi et al. (2001). The crystallite size of the doped and pure TiO 2  samples ranged from 11 to 30 nm.Doped samples showed decrease in crystallite size in compar-ison to parent samples (30 nm for TiO 2 , 14 nm for N-dopedTiO 2 , and 11 nm for Cu/N-codoped TiO 2 ). This result inter- prets that the Cu and N dopants might tend to retard thegrowth of TiO 2  lattice, which agreed with the resultsreported by Song et al. (2008) and Rajoriya et al. (2019). UV-vis absorption spectra analysis UV-vis diffuse reflectance spectra investigated the absorption properties of the pure, N-doped and Cu/N-codoped TiO 2  photocatalysts in the range of 200  –  900 nm, as shown in Fig.3a . It was observed that the N-doped TiO 2  photocatalyst shiftedtheedgeofabsorptionspectratothelongerwavelengthwith almost 472 nm. Meanwhile, Cu/N-codoped TiO 2  re-vealed the longest wavelength with a redshift of about 670nm(Songetal.2008).Thisabsorptionwavelengthshiftedconsented to that study reported by Karafas et al. (2019). The bandgap energies of pure TiO 2 , N-doped TiO 2 , and Cu/N-codoped TiO 2  samples were calculated through a Tauc plot of ( α h υ ) 2 vs.  h υ as shown in Fig. 3b (Jaiswal et al. 2015). The calculation bandgap energies are summarized in Table 1. PureTiO 2  bandgap was 3.17 eV which is very close to the titaniumanatase phase, and N-doped TiO 2  bandgap was 2.89 eV,which ensured the expected narrowed bandgap as reported by Asahi et al. (2001) due to the mixing of N 2p and O 2pstates. Cu/N-codoped TiO 2  showed much decrease in the bandgap (2.64 eV), which indicated much narrowing of the bandgap,whichmight beduetocreatinganintermediatebandin the TiO 2  bandgap as reported by Jaiswal et al. (2015). ThenarrowedbandgapagreedwiththeextendedspectrainFig.4a ,which indicated the possibility of Cu/N-codoped TiO 2  to ex-tend the absorption ranges to the visible light and might en-hance the photocatalytic activity. X-ray photoelectron spectroscopy analysis To investigate the chemical states of each element in Cu/N-codoped TiO 2  –  500 °C photocatalysts, XPS technique was Fig. 2  X-ray diffraction patterns of TiO 2  (black), N-doped TiO 2  (red),and Cu/N-codoped TiO 2  (blue) calcined at 500 °C Fig. 1  Scheme of batch photoreactor system.  a  Simulation of the inner components of the reactor system.  b  The real reactor system components Environ Sci Pollut Res  used to measure the binding energy of Ti 2p, O 1s, Cu 2p, and N 1s. Figure 3a  shows Ti 2p levels, in which two peaks wereobservedat458.85eVand464.50eV,whichcorrespondtothe binding energies of Ti 2p 3/2  and Ti 2p 1/2 . Both states are at-tributed to the role of Ti 4+ in the TiO 2  lattice (Wang et al.2014; Jaiswal et al. 2015). This consented to what Chen et al. (2019) reported. Figure 3b displays the O 1s peaks at  530.15 eVand 531.6 eV. The peak at 530.15 eV was assignedtotheTi  –  O bondinTiO 2  lattice, and the peakat531.6 eVwasrelated to the hydroxyl group, oxygen adsorbed, or water adsorbed on TiO 2  surface (Bokhimi et al. 1997; Jaiswalet al. 2015; Liu et al. 2017). Figure 3c represents N 1s core levelsatbindingenergiesof398.3eV,399.8eV,and405.7eV.The main peak at 398.3 eV is due to the incorporation of nitrogen in the TiO 2  lattice in the form of N  –  Ti  –  O environ-ment(Sathish etal. 2007; Jagadaleet al. 2008) and the peakat  399.7 eV might be due to interstitial atom of N in the TiO 2 lattice and/or oxidized N species (NO) to form Ti  –  O  –   N andTi  –  O  –   N  –  O linkage (Jaiswal et al. 2015; Rajoriya et al. 2019), while the peak at a binding energy of 405.7 eV is because of interstitial atom of oxidized nitrogen such as NO and NO 2  inTiO 2  lattice (Mrowetz et al. 2004; Chen et al. 2008; Ananpattarachai et al. 2009; Yang et al. 2010). Figure 3d depicts the Cu 2p peak spectrum of Cu/N-codoped TiO 2  –  500 °C; itwas showed that there are two peakslocated at 932.9 eV and 952.7 eV. The former peak corre-sponds to Cu 2p 3/2 , and the latter peak corresponds to Cu2p 1/2 , which suggests that Cu in the form of Cu 2 O was incor- porated in the TiO 2  lattice (Tseng et al. 2004; Liu et al. 2017). BET analysis The BET test analysis of three photocatalytic materials of TiO 2 , N-doped TiO 2  and Cu/N-codoped TiO 2 , is shown inTable 2. From this table, it can be clearly seen that thespecific surface area of pure TiO 2  is 42.51 m 2 /g, the porevolume is 0.094 mL/g, and the average pore diameter is5.87 nm; the specific surface area of N-doped TiO 2  is68.36 m 2 /g. The pore volume was 0.149 mL/g and theaverage pore diameter was 9.13 nm. Cu/N-codoped TiO 2 has a specific surface area of 94.70 m 2 /g, a pore volumeof 0.152 mL/g, and an average pore diameter of 7.15 nm.Longitudinal comparison of specific surface area revealedCu/N-codoped TiO 2  > N-doped TiO 2  > TiO 2 . It can also be found that the order of pore volume is also Cu/N-codoped TiO 2  > N-doped TiO 2  > TiO 2 . This agreed withthe BET result reported by Taylor et al. (2015). From a general point of view, the larger specific surface area and pore volume of the catalysts are beneficial to enhance the photodegradation of the pollutants (Tan et al. 2011). Thisis mainly due to the increase in specific surface area and pore volume, which facilitates the adsorption of reactant molecules and the oxidized species on the photocatalyst  ’ ssurface. Therefore, it is theoretically verified that Cu/N-codoped TiO 2  is better than N  –  TiO 2  and TiO 2  (Liu et al.2013). TEM analysis TEM is commonly used to detect the size and shape of nano- particles.Inthistest, Cu/N-codopedTiO 2 samplerangeinsizefrom 10 to 30 nm which confirm our XRD results. Fig. 3  a  UV-vis absorption spec-tra. ( b ) Tauc plot for pure TiO 2 , N-doped, and Cu/N-doped TiO 2  powders calcined at 500 °C Table 1  Crystallite size, absorption spectra, and energy bandgap of TiO 2 , N-doped TiO 2 , and Cu/N-codoped TiO 2  nanoparticlesSample name Crystallite size (nm) Wavelength (nm) Bandgap (eV)TiO 2  30 418 3.17 N  –  TiO 2  14 472 2.89Cu/N  –  TiO 2  11 673 2.64 Environ Sci Pollut Res

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