Enhanced UV Light-Assisted Degradation Of Tetracycline Using Metal Oxide Semiconductor Photocatalysts

The literature of this study has concisely described the different advanced oxidative processes(AOPs)and their effectiveness in the degradation of toxic organic pollutants,highlighting the important role of hydroxyl radicals(·OH).The(·OH),are the second most powerful oxidant next to fluorine,and are produced from oxidizing agents like hydrogen peroxide(H2O2).They react with most organic toxic pollutants oxidizing them to less-toxic forms or even mineralizing them to CO2 and H2 O.The major limitations of some AOPs such as the traditional Fenton process include narrow p H,slow reaction speed,high iron consumption and sludge generation which requires further treatment and disposal.To overcome these limitations,heterogeneous Fentonlike processes are developed.In the heterogeneous Fenton/Fenton-like process,sludge formation is regulated due to less consumption of iron species,moreover unlike the homogenous Fenton which depends only on the chemical reaction between the reagents and contaminates,the heterogeneous Fenton/Fenton-like depends on the chemical reaction between the reagents and contaminates in conjunction with the physical processes such as surface reaction and desorption on the catalyst surface.Semiconductor nanostructured materials have large surface area,which gives more active sites for the generation of ·OH and increases the adsorption of the pollutants.The catalytic process occurs at shorter time,therefore,semiconductors photocatalyst have low cost and low secondary pollution.Although,different processes have been used for the synthesis of metal oxides catalysts,most of these methods requires the use of toxic chemicals,needs sophisticated equipment,requires high energy,time-consuming procedures making the process difficult,expensive,and unsafe.Researchers are therefore tasked with coming up with safer,easier,simple,and costeffective ways of developing the catalysts.In this study,semiconductors Zinc oxide(Zn O),Titanium Oxide(Ti O2),and Hematite(α-Fe2O3)were synthesized using plant extracts as soft bio templates.The extracts used for this study are Azadirachta indica(Common name: Neem),Cymbopogan citratus(Common name: Lemongrass),and Mangifera indica(Common name: Mango).They are known medicinal plants abundant in Nigeria and were chosen based on the reported presence of phytochemicals.The biosynthesized nanostructured materials were well characterized by different characterization techniques such as Xray Diffraction(XRD),Fourier Transform Infrared(FT-IR)spectroscopy,Scanning Electron Microscopy(SEM),Transmission Electron Microscopy(TEM),Higher Resolution TEM(HRTEM),Energy Dispersive X-ray(EDX),UV-Visible spectrophotometer,Brunauer-Emmett-Teller(BET)and Barret-Joyner-Halenda(BJH)analysis,X-ray Photoelectron Spectroscopy(XPS),Electrochemical Impedance Spectroscopy(EIS),Differential Thermal Analysis and Thermo Gravimetric Analysis(DTA/TGA)and Photoluminescence(PL)study.In addition,a composite catalyst Ti O2/Fe2O3 is synthesized and supported on Al2 Si O5 fiber board.The Ti O2/Fe2O3@Al2Si O5 was applied to a well-constructed plate fixed bed reactor to test and investigate the possibility to treat TC wastewater via the UV/Fenton process for practical application.The Ti O2/Fe2O3@Al2Si O5 was characterized by XRD,XPS,SEM and EDX.In this study,antibiotics tetracycline(TC)is chosen as the model pollutant due to its increasing presence in environmental matrices such as water.TC are very stable and poorly biodegradable and can therefore remain in water/wastewater for sometimes,causing harmful effects to both aquatic organism and humans.The soft bio-templated catalysts synthesized in this work were investigated for the degradation of TC(50 mg/L)in aqueous solution using different processes such as adsorption,oxidation(Fenton),Photocatalysis,and Photo-Fenton reaction in the presence of UV-light irradiation.The following results were obtained in this thesis:1.In the first study,Zinc oxide nanoparticles(Zn O NPs)were fabricated using Zinc Nitrate(Zn(NO3)2·6H2O)as precursor salt and extracts from Azadirachta,Cymbopogan citratus and Mangifera indica for the synthesis of Zn O NPs.A chemically synthesized CS-Zn O was used to compare the morphological structure of the biosynthesized Zn O NPs.XRD pattern revealed hexagonal Wurtzite phase of Zn O,with no other impurity peaks present in all the Zn O NPs.XRD crystalline sizes calculated using Scherrer’s equation ranged from 13.94-16.37 nm for the bio synthesized,and 32 nm for the chemically synthesized CS-Zn O.SEM images revealed spherical shapes for all Zn O with sizes almost in agreement with the XRD.The PL spectrum of the Zn O NPs showed a strong emission peak around 425 nm due to the recombination of electrons in the conduction band and holes in the valence band.There is also a weak emission peak at around 470 nm due to ionized oxygen vacancies for the biosynthesized Zn O.The PL of Zn O-AI showed reduced peak intensity,indicating high separation of electron/hole pair recombination.Furthermore,the as-synthesized Zn O NPs were evaluated for their catalytic and photocatalytic performance in the degradation of aqueous TC(50 mg/L).Results indicate that the Zn O NPs exhibit photodegradation efficiency of TC in varying degrees with the order Zn O-AI >Zn OMI >Zn O-Cyc>CS-Zn O.The results for the Zn O catalysts alone without light irradiation are Zn O-AI(37.2 %),Zn O-MI(32.0 %),Zn O-Cyc(22.4 %)and CS-Zn O(20.8 %).This is improved upon UV light irradiation where the degradation efficiency for the UV/Zn O system are Zn O-AI(63.9 %),Zn O-MI(52.7 %),Zn O-Cyc(46.5 %)and CS-Zn O(39.1 %)respectively at the initial p H of the solutions without adjustment,indicating higher efficiency for Zn O-AI.The influence of operational parameters(such as catalyst dosage,TC concentration and p H)were studied for the UV/Zn O-AI catalyzing system and results showed maximum degradation of TC at neutral p H 7(81.4 %)with low catalyst dosage requirement.The catalyst has high chemical stability which is suitable for economic gains as there is no apparent difference in the degradation efficiency after five consecutive cycles(>75.9 %).Moreover,TC degradation kinetics well fitted into a pseudo first-order kinetic model.The quenching experiment were conducted to determine the main active species in the photocatalytic process and the mechanism of TC degradation was proposed.2.In the second study,an efficient hybrid UV/Fenton TC degradation process using semiconductor Ti O2 photocatalyst was investigated.Titanium dioxide(Ti O2)was synthesized using Azadirachta indica(AI)extract as bio template and Titanium(IV)isopropoxide(C12H28O4Ti)as precursor.Morphological properties revealed well crystalline anatase phase with XRD crystalline size of 19.8 nm for the biotemplated Ti O2(Ti O2-AI)compared to 24.2 nm of the non-templated Ti O2(N-Ti O2).Spherical shape structure from SEM and a nano sheet-like structure from TEM were confirmed for the biotemplated Ti O2-AI.The nano sheet-like structure might be due to the replication of the structure from the chloroplast of the bio template.The Ti3+ ions are possibly trapped within stack structure via ion-exchange with the hydrogen of the chlorophyll resulting in the Ti O2 nanosheet.The Ti O2-AI exhibited larger surface area(35.45 m2/g)and average pore diameter(5.74 nm)compared to the N-Ti O2(19.72 m2/g)and(2.028 nm)respectively.XPS results showed carbon peaks indicating small biocarbon doping on the Ti O2 lattice,which can promote charge transfer upon light excitation during photocatalyst reactions.The reduced PL intensity of the Ti O2-AI compared to the N-Ti O2 indicates the suppression of recombination of charge carriers or electron-hole pairs in the biosynthesized Ti O2,which promotes the photocatalytic reactions typical of C doped Ti O2.Results show that the TiO2-AI catalyst without UV light slightly removed TC(20.2 %),compared to N-Ti O2(11.8 %)due to limited active species.The photocatalytic activity for the degradation of TC by the UV/Fenton/Ti O2 hybrid system increases significantly due to the synergetic effect of the UV/Fenton system and the Ti O2 photocatalyst with the UV/Fenton/Ti O2-AI showing higher catalytic performance(87.8 %),compared to UV/Fenton/N-Ti O2(79.1 %).The higher performance is due to the synergetic effect of the UV/Fenton/Ti O2 hybrid system using the Ti O2-AI that further improves the catalytic process by preventing the recombination of photogenerated e-/h+ via efficient reduction of Fe3+ to Fe2+ by the photoinduced electrons,overcoming the limitation of fast recombination likelihood of e-/h+ pairs typical of Ti O2.Moreover,the nanosheet structure of the bio templated Ti O2 exposes the surface and allows better utilization of all surface-active sites.The catalyst is stable(> 80 %)after five reuse cycle,and influential parameters such as Ti O2-AI dosage,TC concentration,p H,Fe2+ to H2O2 molar ratio were studied and optimized.Meanwhile,the degradation efficiency of the bio templated Ti O2 obeyed first-order reaction kinetics with degradation rates for UV/Fenton/Ti O2-AI process higher than other as-compared processes.3.In the third study,the hybrid UV/Fenton degradation of TC is further enhanced using a solid iron photocatalyst as source of iron,minimizing the presence of dissolved Fe2+ in the reaction medium.Nanostructured α-Fe2O3(hematite)were further synthesized using biotemplate AI and ferric chloride(Fe Cl3.6H2O)as the precursor.Findings revealed XRD patterns of the samples to be of rhombohedral(hexagonal)structure with crystallite sizes of 18.9 nm for the bio synthesized α-Fe2O3-AI compared to 30.1 nm of the normal α-Fe2O3.SEM results revealed that the α-Fe2O3-AI retains the irregular leaflike morphology of AI bio template and TEM images showed the α-Fe2O3-AI with hollow microstructure due to small bio-C doping to be of perfect crystallinity compared to the normal α-Fe2O3,which displayed some aggregation.XPS results gives the α-Fe2O3-AI more oxygen vacancies leading to more available reactive sites than the normal α-Fe2O3.The α-Fe2O3-AI with small bio-C doping possess large surface area and exhibited high performance(92.8 %)as a heterogeneous catalyst for the degradation of TC using the enhanced UV/α-Fe2O3-AI/H2O2 hybrid system.This is due to the synergistic effect of the UV-Fenton(UV/α-Fe2O3/H2O2)compared to the Fenton-like oxidation(α-Fe2O3/H2O2)(55.3 %)and Photocatalysis(UV/α-Fe2O3)(31.3 %).The biotemplated hollow micro structured α-Fe2O3 Al with small bio-C doping stimulates the synergistic effect of the UV/Fenton system and improves its performance compared to the normal α-Fe2O3.The use of visible light irradiation is introduced;however,TC degradation efficiency is higher for UV/α-Fe2O3-AI/H2O2(92.8 %)compared with Vis/α-Fe2O3-AI/H2O2(74.12 %).Results obtained for the Total Organic Carbon(TOC)after 120 mins are 22.6 % for the Fenton oxidation,25.2 % for the photocatalysis,and 42.1 % for the photo-Fenton processes.The catalyst demonstrated excellent stability as no tremendous loss of photocatalytic performance was observed even after five consecutive cycles,with the fifth cycle still achieving >86 % TC degradation.Additionally,the possible mechanism of TC degradation was investigated by electron spin resonance technique and free radicals trapping experiments.The ESR results indicated that hydroxyl(·OH)and superoxide(˙O2-)radicals were generated in the small bio-C doped α-Fe2O3-AI catalyzing UV/Fenton system for the degradation of pollutant TC,while the radical trapping test indicates that the ·OH and the superoxide radical(˙O2-)are the main active species that predominantly contributed to the degradation of TC in the UV/α-Fe2O3-AI/H2O2 system.4.In the fourth study,a plate fixed bed photocatalytic reactor is constructed for practical application of UV/Fenton TC wastewater treatment.Composite Ti O2/Fe2O3 coated on Al2 Si O5 fiber board(Ti O2/Fe2O3@Al2Si O5)were fabricated using the calcination method.The resulting supported catalyst(Ti O2/Fe2O3@Al2Si O5)was applied to the plate fixed bed continuous flow reactor with TC as the target pollutant to establish the UV/Fenton treatment process.The influence of loading ratio of Fe2O3 to Ti O2 catalysts was first investigated and the n Fe2O3:n Ti O2 of 5:1 with 94.0 % TC degradation was chosen for subsequent experiment.The success of the as-prepared supported catalyst was confirmed by XRD,XPS,SEM and EDX.The effect of the dosage of the Ti O2/Fe2O3@Al2Si O5 on the treatment of the 50 mg/L simulated TC wastewater was also investigated and maintained at 0.5 g/L for the process.Other operational conditions were optimized and the optimum p H,H2O2 concentration,UV light intensity,hydraulic retention time(HRT),influent TC concentration were found to be p H 5.0(93.8 %),10 m M(93.8 %),15W(95.2 %),120 min(93 %),and 25-50 mg/L(94.5-94.3 %)respectively.In addition,the stability of the TC degradation by continuous operation of the reactor was investigated by means of monitoring the continuous inlet and outlet and the plate fixed bed reactor has demonstrated excellent stability for economic gain and hence suitable for practical application.