Taicheng An

Photocatalytic degradation kinetics and mechanism of environmental pharmaceuticals in aqueous suspension of TiO2: A case of β-blockers

This study investigated the photocatalytic degradation of three beta-blockers in TiO(2) suspensions. The disappearance of the compounds followed pseudo-first-order kinetics according to the Langmuir-Hinshelwood model and the rate constants were 0.075, 0.072 and 0.182 min(-1) for atenolol, metoprolol and propranolol, respectively. After 240 min irradiation, the reaction intermediates were completely mineralized to CO(2) and the nitrogen was predominantly as NH(4)(+). The influence of initial pH and beta-blocker concentration on the kinetics was also studied. From adsorption studies it appears that the photocatalytic degradation occurred mainly on the surface of TiO(2). Further studies indicated that surface reaction with OH radical was principally responsible for the degradation of these three beta-blockers. The major degradation intermediates were identified by HPLC/MS analysis. Cleavage of the side chain and the addition of the hydroxyl group to the parent compounds were found to be the two main degradation pathways for all three beta-blockers.

Visible-Light-Driven Photocatalytic Inactivation of E. coli K-12 by Bismuth Vanadate Nanotubes: Bactericidal Performance and Mechanism

Bismuth vanadate nanotube (BV-NT), synthesized by a template-free solvothermal method, was used as an effective visible-light-driven (VLD) photocatalyst for inactivation of Escherichia coli K-12. The mechanism of photocatalytic bacterial inactivation was investigated by employing multiple scavengers combined with a simple partition system. The VLD photocatalytic bacterial inactivation by BV-NT did not allow any bacterial regrowth. The photogenerated h(+) and reactive oxidative species derived from h(+), such as OH(ads), H(2)O(2) and HO(2)/O(2)(-), were the major reactive species for bacterial inactivation. The inactivation by h(+) and OH(ads) required close contact between the BV-NT and bacterial cells, and only a limited amount of H(2)O(2) could diffuse into the solution to inactivate bacterial cells. The direct oxidation effect of h(+) to bacterial cells was confirmed by adopting F(-) surface modification and anaerobic experiments. The bacterial cells could trap e(-) in order to minimize e(-)-h(+) recombination, especially under anaerobic condition. Transmission electron microscopic study indicated the destruction process of bacterial cell began from the cell wall to other cellular components. The OH(ads) was postulated to be more important than OH(bulk) and was not supposed to be released very easily in the BV-NT bacterial inactivation system.

Naturally Occurring Sphalerite As a Novel Cost-Effective Photocatalyst for Bacterial Disinfection under Visible Light

The photocatalytic disinfection capability of the natural semiconducting mineral sphalerite is studied here for the first time. Natural sphalerite can completely inactivate 1.5 × 10(7) cfu/mL E. coli K-12 within 6 h under visible light irradiation. The photocatalytic disinfection mechanism of natural sphalerite is investigated using multiple scavengers. The critical role that electrons play in bactericidal actions is experimentally demonstrated. The involvement of H(2)O(2) in photocatalytic disinfection is also confirmed using a partition system combined with different scavengers. Moreover, the photocatalytic destruction of bacterial cells is observed through transmission electron microscopic analysis. A catalase activity study reveals that antioxidative enzyme activity is high in the initial stage of photocatalytic disinfection but decreases with time due to damage to enzymatic functioning. Natural sphalerite is abundant and easy to obtain and possesses excellent visible-light photocatalytic activity. These superior properties make it a promising solar-driven photocatalyst for large-scale cost-effective wastewater treatment.

Mechanistic Considerations for the Advanced Oxidation Treatment of Fluoroquinolone Pharmaceutical Compounds using TiO2 Heterogeneous Catalysis

Pharmaceutical compounds and metabolites are being found in surface and ground waters which is indicative of inefficient removal by conventional wastewater treatment technologies. Advanced oxidation processes (AOPs), which utilize free-radical reactions to degrade chemical contaminates, are an alternative to traditional water treatment. Three fluoroquinolone pharmaceutical compounds were studied and the absolute rate constants for hydroxyl radical (*OH) and hydrated electron (e(-)(aq)) are reported. For norfloxacin, levofloxacin, and lomefloxacin, the bimolecular reaction rate constants with *OH were determined as (6.18 +/- 0.18) x 10(9), (7.59 +/- 0.16) x 10(9) and (8.04 +/- 0.62) x 10(9) M(-1) s(-1), and with e(-)(aq) were (1.18 +/- 0.10) x 10(10), (2.46 +/- 0.05) x 10(10) and (2.79 +/- 0.05) x 10(10) M(-1) s(-1), respectively. To provide insights into the chemistry of destruction of these three target pharmaceuticals, transient spectra were obtained for the reaction of hydroxyl radicals with the three compounds. Photocatalysis was chosen as a representative advanced oxidation technology to degrade these three fluoroquinolones and their degradation pathways were proposed. Elimination of piperazynilic ring in fluoroquinolone molecules, addition of hydroxyl radical to quinolone ring, and ipso attack at the F atoms on the aromatic ring by hydroxyl radicals occurred. These results indicate that AOPs involving production of *OH radicals are efficiently alternative treatment technologies for degradation of fluoroquinolones in aqueous solution.

Photocatalytic degradation kinetics and mechanism of antivirus drug-lamivudine in TiO2 dispersion

Photocatalytic degradation kinetics of antivirus drug-lamivudine in aqueous TiO(2) dispersions was systematically optimized by both single-variable-at-a-time and central composite design based on the response surface methodology. Three variables, TiO(2) content, initial pH and lamivudine concentration, were selected to determine the dependence of degradation efficiencies of lamivudine on independent variables. Response surface methodology modeling results indicated that degradation efficiencies of lamivudine were highly affected by TiO(2) content and initial lamivudine concentration. The highest degradation efficiency was achieved at suitable amount of TiO(2) and with maintaining initial lamivudine concentration to a minimum. In addition, the contribution experiments of various primary reactive species produced during the photocatalysis were investigated with the addition of different scavengers and found that hydroxyl radicals was the major reactive species involved in lamivudine degradation in aqueous TiO(2). Six degradation intermediates were identified using HPLC/MS/MS, and photocatalytic degradation mechanism of lamivudine was proposed by utilizing collective information from both experimental results of HPLC/MS/MS, ion chromatography as well as total organic carbon and theoretical data of frontier electron densities and point charges.

Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli using g-C3N4/TiO2 hybrid photocatalyst synthesized using a hydrothermal-calcination approach

Biohazards are widely present in wastewater, and contaminated water can arouse various waterborne diseases. Therefore, effectively removing biohazards from water is a worldwide need. In this study, a novel visible-light-driven (VLD) graphitic carbon nitride (g-C3N4)/TiO2 hybrid photocatalyst with high photocatalytic bacterial inactivation activity was successfully synthesized using a facile hydrothermal-calcination approach. The optimum synthesized hybrid photocatalyst is composed of micron-sized TiO2 spheres (average diameter: ca. 2 μm) and wrapped with lamellar g-C3N4 (thickness: ca. 2 nm), with narrowing bandgap (ca. 2.48 eV), leading to a significant improvement of visible light (VL) absorption and effective separation of photo-generated electron-hole pairs. This greatly enhances VL photocatalytic inactivation activity towards bacteria in water. Using this hybrid photocatalyst, 10(7) cfu mL(-1) of Escherichia coli K-12 could be completely inactivated within 180 min under VL irradiation. SEM images indicate that bacterial cells were greatly damaged, leading to a severe leakage of intracellular components during photocatalytic inactivation processes. The study concludes that bacterial cell destruction and water disinfection can be achieved using this newly fabricated VLD hybrid photocatalyst.

Advanced Oxidation Kinetics and Mechanism of Preservative Propylparaben Degradation in Aqueous Suspension of TiO2 and Risk Assessment of Its Degradation Products

The absolute kinetic rate constants of propylparaben (PPB) in water with different free radicals were investigated, and it was found that both hydroxyl radicals (HO(•)) and hydrated electrons could rapidly react with PPB. The advanced oxidation kinetics and mechanisms of PPB were investigated using photocatalytic process as a model technology, and the degradation was found to be a pseudo-first-order model. Oxidative species, particularly HO(•), were the most important reactive oxygen species mediating photocatalytic degradation of PPB, and PPB degradation was found to be significantly affected by pH because it was controlled by the radical reaction mechanism and was postulated to occur primarily via HO(•)-addition or H-abstraction reactions on the basis of pulse radiolysis measurements and observed reaction products. To investigate potential risk of PPB to humans and aqueous organisms, the estrogenic assays and bioassays were performed using 100 μM PPB solution degraded by photocatalysis at specific intervals. The estrogenic activity decreased as PPB was degraded, while the acute toxicity at three trophic levels first increased slowly and then decreased rapidly as the total organic carbon decreased during photocatalytic degradation.

Synthesis and Characterization of Novel Plasmonic Ag/AgX-CNTs (X = Cl, Br, I) Nanocomposite Photocatalysts and Synergetic Degradation of Organic Pollutant under Visible Light

A series of novel well-defined Ag/AgX (X = Cl, Br, I) loaded carbon nanotubes (CNTs) composite photocatalysts (Ag/AgX-CNTs) were fabricated for the first time via a facile ultrasonic assistant deposition-precipitation method at the room temperature (25 ± 1 °C). X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption analysis, scanning electron microscopy, and ultraviolet-visible light absorption spectra analysis were used to characterize the structure, morphology, and optical properties of the as-prepared photocatalysts. Results confirmed the existence of the direct interfacial contact between Ag/AgX nanoparticles and CNTs, and Ag/AgX-CNTs nanocomposites exhibit superior absorbance in the visible light (VL) region owing to the surface plasmon resonance (SPR) of Ag nanoparticles. The fabricated composite photocatalysts were employed to remove 2,4,6-tribromophenol (TBP) in aqueous phase. A remarkably enhanced VL photocatalytic degradation efficiency of Ag/AgX-CNTs nanocomposites was observed when compared to that of pure AgX or CNTs. The photocatalytic activity enhancement of Ag/AgX-CNTs was due to the effective electron transfer from photoexcited AgX and plasmon-excited Ag(0) nanoparticles to CNTs. This can effectively decrease the recombination of electron-hole pairs, lead to a prolonged lifetime of the photoholes that promotes the degradation efficiency.

Synthesis of Carbon Nanotube Anatase TiO2 Sub-micrometer-sized Sphere Composite Photocatalyst for Synergistic Degradation of Gaseous Styrene

The carbon nanotube (CNT)-sub-micrometer-sized anatase TiO₂ sphere composite photocatalysts were synthesized by a facile one-step hydrothermal method using titanium tetrafluoride as titanium source and CNTs as structure regulator. Various technologies including X-ray diffraction, UV-visible absorption spectra, N₂ adsorption-desorption, scanning electron microscopy, and transmission electron microscopy were employed to characterize the structure properties of the prepared composite photocatalysts. The results indicated that the composite photocatalysts consisted of CNTs wrapping around the sub-micrometer-sized anatase TiO₂ spheres with controllable crystal facets and that the aggregated particles with average diameter ranged from 200 to 600 nm. The fabricated composite photocatalysts were used to degrade gaseous styrene in this work. As expected, a synergistic effect that remarkably enhancing the photocatalytic degradation efficiency of gaseous styrene by the prepared composite photocatalysts was observed in comparison with that the degradation efficiency using pure anatase TiO₂ and the adsorption of CNTs. Similar results were also confirmed in the decolorization of liquid methyl orange. Further investigation demonstrated that the synergistic effect in the photocatalytic activity was related to the structure of the sub-micrometer-sized anatase TiO₂ spheres and the significant roles of CNTs in the composite photocatalysts. By controlling the content of CNTs, the content of TiO₂ or the temperature during the hydrothermal synthesis process, anatase TiO₂ spheres with controllable crystallite size and dominant crystal facets such as {001}, {101}, or polycrystalline could be obtained, which was beneficial for the increase in the synergistic effect and further enhancement of the photocatalytic efficiencies.

Fabrication of Highly Ordered TiO2 Nanorod/Nanotube Adjacent Arrays for Photoelectrochemical Applications

This work reports a facile approach to fabricate a perpendicularly aligned and highly ordered TiO(2) nanorod/nanotube (NR/NT) adjacent film by directly anodizing a modified titanium foil. The titanium foil substrate was modified with a layer of crystalline TiO(2) film via a hydrothermal process in 0.05 M (NH(4))(2)S(2)O(8). The resultant NR/NT architecture consists of a highly ordered nanorod top layer that directly adjoins to a highly ordered nanotube array bottom layer. The thickness of the top nanorod layer was approximately 90 nm with average nanorod diameter of 22 nm after 20 min of anodization. The thickness of the bottom nanotube array layer was found to be ca. 250 nm after 20 min of anodization, having an average outer and inner tubular diameters of 120 and 80 nm, respectively. A broad implication of the method is that a simple modification to the substrate surface can lead to new forms of nanostructures. For as-anodized NR/NT samples, XRD analysis reveals that the nanorods are of anatase TiO(2) crystalline form while the nanotubes are amorphous. Anatase TiO(2) crystalline form of NR/NT film with high crystallinity can be obtained by thermally treating the as-anodized sample at 450 degrees C for 2 h in air. The resultant NR/NT film was used as a photoanode for photoactivity evaluation. Comparing with a nanotube array photoanode prepared by direct anodization of unmodified titanium foil, the NR/NT photoanode exhibits a unique feature of selective photocatalytic oxidation toward organics, which makes it very attractive to photocatalytic degradation of organic pollutants, sensing, and other applications.