Synthesis Of CdS Nanocrystals With TMTD As Sulphur Source And Photocatalytic Degradation Of Levofloxacin Lactate

Fluorinated quinolone-type antibiotics are widely used in human and veterinary systems for infectiousdisease prevention and growth promotion. However, most of them are only partially metabolized andexcreted into the environment with parent compounds and metabolites. Although there is the presenceof antibiotics at low concentration in the aquatic systems, their occurrence has become a concern asbiological impacts and potential risks to the environment and human health. With the development ofworld economy, environmental problems have become the primary problem for the sustainabledevelopment of mankind. In this dissertation, we want to seek the method of reducing antibioticpollution in environment by studying the degradation of the third generation fluoquinolones-levofloxacin lactate （LVFX） in aquatic environment.In the above purpose, LVFX was degradated applying semiconductor photocatalytic oxidationtechnique. Flower-like, ice-piton, wire-like and particle-like CdS nanocrystals were successfullysynthesized using solvothermal route in water-benzene, water, ethylenediamine and benzenerespectively, which took tetramethyl thiuram disulfide （TMTD） as a special structured organic sulphursource. The influence of reaction medium, temperatures and time on microstructure and opticalproperties of CdS nanocrystals was systematically investigated from thermodynamics and kinetics. Theoptical properties of CdS crystals were investigated by ultraviolet-visible spectroscopy （UV-Vis） andcathodoluminescence （CL）. The results indicated that the solvothermal temperature played a moreimportant role in the formation of crystal morphology, and the physical and chemical properties ofsolvent also affected the structure and formation of CdS nanocrystals from kinetics. Furthermore，It wasthus evident that different synthesis conditions and crystal morphologies led to different opticalproperties, and excellent optical property implied potential applications for the CdS nanocrystals asphotocatalysts. UV-Vis spectra of CdS nanowire and namoparticles were obtained. Quantumconfinement effects were revealed by blue shifts at the onset of absorption, compared with theabsorption of the bulk CdS, but UV-Vis spectra of flower-like and ice-piton CdS weren’t obtained. CLspectra of flower-like, ice-piton, wire-like CdS crystals were acquired in the visible region, whereasspectra of CdS nanoparticles weren’t obtained.In order to contrast the photocatalytic effect of CdS, Fe₃O₄nanocrystals were also synthesized withpolyethylene glycol （PEG） and polyvinyl pyrrolidone （PVP） as dispersant respectively bycoprecipitation, coprecipitation-gamma radiation and solvothermal process. Nanocrystals werecharacterized by transmission electron microscopy（TEM），X-ray diffraction（XRD）, fourier transform infrared spectroscopy（FT-IR） and vibrating sample magnetometer（VSM） respectively. The crystallinityand dispersity of nano-particles were improved by gamma radiation，compared with the particlesobtained by traditional coprecipitation. Globular and monodisperse Fe₃O₄nanocrystals synthesized bysolvothermal method had cubic phase no matter what dispersant was. However, the crystals had smallerdiameter, better dispersibility and crystallinity with PVP as dispersant.Photocatalytic degradation experiments were carried out with UV radiation as the energy source, and0.005g CdS or Fe₃O₄, which were catalysts, were put into a100mL LVFX aqueous solution with aconcentration of10mg/L. Compared with direct photolysis, the photocatalytic degradation effect ofLVFX is better. As the UV radiation time was short, the catalytic rate of particle-shape and wire-likeCdS was good to flower-like and ice-piton CdS crystals. As the UV radiation time was60min, thephotocatalytic degradation percentage of LVFX was about99%，99%，89%and97%respectively withflower-like, particle-shape, ice-piton, wire-like CdS crystals as photocatalyst. However, the directphotolysis percentage of LVFX was about85%. Thus, size effects and surface roughness of crystalsfavored increasing catalytic activity with submicron/nano grade CdS crystals as photocatalysts,nevertheless quantum size effect was more prominent. As the UV radiation time was60min, thephotocatalytic degradation percentage of LVFX with Fe₃O₄as catalyst, which were synthesized withPEG and PVP as dispersant respectively by solvothermal process, was about98%and95%respectively.When Fe₃O₄nanocrystals were synthesized by coprecipitation-γ with PVP as dispersant, thephotocatalytic degradation percentage of LVFX was89%. However, the direct photolysis percentage ofLVFX was about85%. It’s indicated that Fe₃O₄nanocrystals prepared by solvothermal process werebetter photocatalyst. Size effects don’t necessarily dominates AOPs of UV－magnetite, and moreover,crystal growth and the tendency of iron ions in solution to form complexes are also consideredimportant in mechanistic studies.