Showing papers by "Wonyong Choi published in 2005"

Different Inactivation Behaviors of MS-2 Phage and Escherichia coli in TiO2 Photocatalytic Disinfection

Abstract: Despite a wealth of experimental evidence concerning the efficacy of the biocidal action associated with the TiO2 photocatalytic reaction, our understanding of the photochemical mechanism of this particular biocidal action remains largely unclear. It is generally accepted that the hydroxyl radical (·OH), which is generated on the surface of UV-illuminated TiO2, plays the main role. However, our understanding of the exact mode of action of the hydroxyl radical in killing microorganisms is far from complete, and some studies report that other reactive oxygen species (ROS) (H2O2 and O2·−, etc.) also play significant roles. In particular, whether hydroxyl radicals remain bound to the surface or diffuse into the solution bulk is under active debate. In order to examine the exact mode of action of ROS in inactivating the microorganism, we tested and compared the levels of photocatalytic inactivation of MS-2 phage and Escherichia coli as representative species of viruses and bacteria, respectively. To compare photocatalytic microbial inactivation with the photocatalytic chemical degradation reaction, para-chlorobenzoic acid, which rapidly reacts with a hydroxyl radical with a diffusion-limited rate, was used as a probe compound. Two different hydroxyl radical scavengers, tert-butanol and methanol, and an activator of the bulk phase hydroxyl radical generation, Fe2+, were used to investigate their effects on the photocatalytic mode of action of the hydroxyl radical in inactivating the microorganism. The results show that the biocidal modes of action of ROS are very different depending on the specific microorganism involved, although the reason for this is not clear. It seems that MS-2 phage is inactivated mainly by the free hydroxyl radical in the solution bulk but that E. coli is inactivated by both the free and the surface-bound hydroxyl radicals. E. coli might also be inactivated by other ROS, such as O2·− and H2O2, according to the present results.

Visible light active platinum-ion-doped TiO2 photocatalyst.

Abstract: Platinum-ion-doped TiO2 (Ption-TiO2) was synthesized by a sol-gel method, and its visible light photocatalytic activities were successfully demonstrated for the oxidative and reductive degradation of chlorinated organic compounds. Ption-TiO2 exhibited a yellow-brown color, and its band gap was lower than that of undoped TiO2 by about 0.2 eV. The flat band potential of Ption-TiO2 was positively shifted by 50 mV compared with that of undoped TiO2. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy analyses showed that the Pt ions substituted in the TiO 2 lattice were present mainly in the Pt(IV) state with some Pt(II) on the sample surface. Ption-TiO2 exhibited higher photocatalytic activities than undoped TiO 2 under UV irradiation as well. The visible light activity of Pt ion-TiO2 was strongly affected by the calcination temperature and the concentration of Pt ion dopant, which were optimal at 673 K and 0.5 atom %, respectively. Under visible irradiation, Pt ion-TiO2 degraded dichloroacetate and 4-chlorophenol through an oxidative path and trichloroacetate via a reductive path. The activity of Pt ion-TiO2 was not reduced when used repeatedly under visible light. However, visible-light-illuminated Pt ion-TiO2 could not degrade substrates such as tetramethylammonium and trichloroethylene, which are degraded with UV-illuminated TiO2. The characteristics and reactivities of Ption-TiO2 as a new visible light photocatalyst were investigated in various ways and discussed in detail.

Simultaneous and synergistic conversion of dyes and heavy metal ions in aqueous TiO2 suspensions under visible-light illumination.

Abstract: This study reports synergistic effects in the simultaneous conversion of dyes and heavy metal ions in aqueous TiO2/dye/metal ion systems (ternary components) under visible light (λ > 420 nm). TiO2/Cr(VI)/Acid Orange 7 (AO7), TiO2/Cr(VI)/Rhodamine B (RhB), TiO2/Ag+/AO7, and TiO2/Ag+/RhB were chosen as test systems. Although dyes can be degraded in TiO2 suspensions under visible light, their removal rates were markedly enhanced in the presence of metal ions. Similarly, the reduction rates of metal ions in visible-light-illuminated TiO2 suspensions were negligible, but they were highly accelerated with dyes present. In particular, the synergistic effect in the ternary system of TiO2/Cr(VI)/AO7 was outstanding. The presence of dissolved oxygen increased the photoreduction rate of Cr(VI) despite the fact that Cr(VI) and O2 are competing electron acceptors. This is ascribed to in-situ photogenerated H2O2 from O2, which acts as a reductant of Cr(VI). RhB and Ag+ ions could be also converted simultaneously under ...

Visible-light-induced photocatalytic degradation of 4-chlorophenol and phenolic compounds in aqueous suspension of pure titania: demonstrating the existence of a surface-complex-mediated path.

Abstract: The visible-light-induced degradation reaction of 4-chlorophenol (4-CP) was investigated in aqueous suspension of pure TiO2. Contrary to common expectations, 4-CP could be degraded under visible illumination (λ > 420 nm), generating chlorides and CO2 concomitantly. The observed visible reactivity was not due to the presence of trace UV light since the visible-light-induced reactions exhibited behaviors distinguished from those of UV-induced reactions. Dichloroacetate could not be degraded under visible light, whereas it degraded with a much faster rate than 4-CP under UV irradiation. The addition of tert-butyl alcohol, a common OH radical scavenger, did not affect the visible reactivity of 4-CP, which indicates that OH radicals are not involved. Other phenolic compounds such as phenol and 2,4-dichlorophenol were similarly degraded under visible light. The surface complexation between phenolic compounds and TiO2 appears to be responsible for the visible light reactivity. Diffuse reflectance UV−vis spectra ...

Photocatalytic reactivity of surface platinized TiO2: substrate specificity and the effect of Pt oxidation state.

Abstract: Surface platinized TiO2 (Pt/TiO2) has been frequently studied, but its photocatalytic reactivities reported in the literature are not consistent in some cases. To understand the discrepancies, the effects of Pt speciation on TiO2 on the photocatalytic degradation (PCD) of a few chlorinated organic compounds (trichloroethylene (TCE), perchloroethylene (PCE), dichloroacetate, etc.) were investigated with several Pt/TiO2 samples that were prepared differently. The oxidation state of Pt deposits was analyzed by X-ray photoelectron spectroscopy and was found to be the most important factor in determining the initial PCD rates of chlorinated organic compounds. TiO2 with oxidized Pt species (Ptox/TiO2) was less reactive than TiO2 with metallic Pt (Pt0/TiO2) for all substrates tested. In particular, Ptox/TiO2 strongly inhibited the PCD of TCE and PCE whereas it was more reactive than pure TiO2 for the PCD of other compounds. The photocurrents obtained with the Ptox/TiO2 electrode were lower than those with the Pt...

Photocatalytic reactivities of Nafion-coated TiO2 for the degradation of charged organic compounds under UV or visible light.

Abstract: Nafion (perfluorinated polymer with sulfonate groups)-coated TiO2 particles (Nf/TiO2) were prepared and their reactivities for the photocatalytic degradation (PCD) of charged organic substrates were investigated. The presence of Nafion adlayers drastically changed the positive TiO2 surface charge to a negative one over the entire pH range and significantly influenced the PCD kinetics and mechanisms. The UV-induced PCD of tetramethylammonium (TMA; cationic substrate) was greatly enhanced in the presence of Nafion adlayers on TiO2 because the ion-exchange sites within the Nafion can hold cationic substrates. On the other hand, despite the unfavorable electrostatic interaction between the Nf/TiO2 and anionic substrates, the PCD of dichloroacetate (DCA) and acid orange 7 (AO7) with Nf/TiO2 was not significantly inhibited. The visible-light-sensitized degradation of dyes was enhanced with Nf/TiO2 not only for cationic dyes (methylene blue (MB) and rhodamine B (RhB)) whose uptake on Nf/TiO2 is enhanced, but als...

Photocatalytic Degradation of N-Nitrosodimethylamine: Mechanism, Product Distribution, and TiO2 Surface Modification

Abstract: The photocatalytic degradation (PCD) reaction of N-nitrosodimethylamine (NDMA) in water was investigated using pure and surface-modified TiO2. The PCD products of NDMA were methylamine (MA), dimethylamine (DMA), nitrite, nitrate, and ammonium, and their distribution could be changed by modifying the surface of TiO2. The PCD reaction of NDMA seems to be initiated mostly by OH radicals, not valence band holes, because the addition of excess oxalates (hole scavengers) only moderately retarded the PCD rate. The presence of oxalate, however, enabled a new reductive transformation path in which the CO2-• radicals generated from the oxalate converted NDMA into DMA. In acidic suspensions of pure TiO2, the formation of MA was highly favored over DMA and NH3, whereas all degradation products (MA, DMA, and NH3) were generated at comparable concentrations at basic pH. It is suggested that there are three parallel paths depending on the position of the initial attack of OH radical on NDMA and the product distribution ...

UV photolytic mechanism of N-nitrosodimethylamine in water: dual pathways to methylamine versus dimethylamine.

Abstract: The direct ultraviolet (UV) photolysis of N-nitrosodimethyl-amine (NDMA), a well-known potential carcinogen, was investigated in aqueous solution with its degradation products analyzed quantitatively. NDMA is known to be photolyzed either to dimethylamine (DMA) or to methylamine (MA) by two distinct pathways. However, the mechanism through which NDMA is photolyzed to DMA is still not clearly understood. This study reveals a new mechanistic pathway of NDMA photolysis to DMA by identifying the factors influencing the photolysis pathway. The two pathways of NDMA photolysis were found to be strongly dependent on the initial NDMA concentration and solution pH. Increasing the initial NDMA concentration clearly favored the DMA formation path. DMA production was optimized in the region of pH 4-5. The nitrite ion (NO2-) produced from the NDMA photolysis was identified as a key reagent in directing the NDMA photolysis toward DMA production. The observed photolytic behaviors of NDMA photolysis could be successfully explained in terms of the new mechanism involving the role of NO2-.

UV photolytic mechanism of N-nitrosodimethylamine in water: roles of dissolved oxygen and solution pH.

Abstract: The photolytic mechanism of N-nitrosodimethylamine (NDMA) in aqueous solution was investigated under ultraviolet (UV) irradiation at 253.7 nm, by determining the quantum yield of NDMA photolysis and analyzing the photolysis products in the pH range of 2−11 under O2 or N2 saturation. Several important photolytic behaviors of NDMA photolysis regarding the roles of dissolved oxygen and solution pH were newly observed, and novel photolytic mechanisms were proposed based on these observations. First, the presence of dissolved oxygen clearly increased the quantum yield of the NDMA photolysis, and promoted the photolytic pathway leading to the formation of methylamine and nitrate. The direct oxidation of photoexcited NDMA by dissolved oxygen (photooxidation) was suggested as a novel photolytic pathway to explain the observed effect of dissolved oxygen. This photooxidation mechanism was supported by the detection of superoxide radicals during the photolysis of NDMA under O2 saturation. Second, the dependence of t...

Synthesis of new visible light active photocatalysts of Ba(In(1/3)Pb(1/3)M'(1/3))O3 (M' = Nb, Ta): a band gap engineering strategy based on electronegativity of a metal component.

Abstract: We have synthesized new, efficient, visible light active photocatalysts through the incorporation of highly electronegative non-transition metal Pb or Sn ions into the perovskite lattice of Ba(In(1/3)Pb(1/3)M'(1/3))O3 (M = Sn, Pb; M' = Nb, Ta). X-ray diffraction, X-ray absorption spectroscopic, and energy dispersive spectroscopic microprobe analyses reveal that tetravalent Pb or Sn ions exist in the B-site of the perovskite lattice, along with In and Nb/Ta ions. According to diffuse UV-vis spectroscopic analysis, the Pb-containing quaternary metal oxides Ba(In(1/3)Pb(1/3)M'(1/3))O3 possess a much narrower band gap (E(g) approximately 1.48-1.50 eV) when compared to the ternary oxides Ba(In(1/2)M'(1/2))O3 (E(g) approximately 2.97-3.30 eV) and the Sn-containing Ba(In(1/3)Sn(1/3)M'(1/3))O3 derivatives (E(g) approximately 2.85-3.00 eV). Such a variation of band gap energy upon the substitution is attributable to the broadening of the conduction band caused by the dissimilar electronegativities of the B-site cations. In contrast to the ternary or the Sn-substituted quaternary compounds showing photocatalytic activity under UV-vis irradiation, the Ba(In(1/3)Pb(1/3)M'(1/3))O3 compounds induce an efficient photodegradation of 4-chlorophenol under visible light irradiation (lambda > 420 nm). The present results highlight that the substitution of electronegative non-transition metal cations can provide a very powerful way of developing efficient visible light harvesting photocatalysts through tuning of the band structure of a semiconductive metal oxide.

Remote Photocatalytic Oxidation Mediated by Active Oxygen Species Penetrating and Diffusing through Polymer Membrane over Surface Fluorinated TiO2

Abstract: Active oxygen species desorbed from the illuminated TiO2 surface can penetrate through organic polymer membrane with degrading dye substrates loaded within the pores, and their permeating action in...

Enhanced photocatalytic degradation of tetramethylammonium on silica-loaded titania

Abstract: Photocatalytic degradation (PCD) of tetramethylammonium (TMA) in water was studied using both pure TiO2 and silica-loaded TiO2 (Si–TiO2). Use of Si–TiO2 catalyst prepared from commercial TiO2 powder by a simple method developed in this work enhanced the PCD rate of TMA considerably. The Si/Ti atomic ratio of 18% was found to be an optimum in photoactivity and the calcined sample was more efficient than the uncalcined one. Several factors were noted to be responsible for the higher photoefficiency of Si–TiO2 catalyst. Si–TiO2 calcined at 700 °C did not show any sign of change in the crystalline structure from that of uncalcined pure TiO2. The increased thermal stability of Si–TiO2 enabled the bulk defects to be removed at high temperatures without forming the inactive rutile phase, which may partly contribute to the higher photoactivity. The most outstanding characteristics of Si–TiO2 is its surface charge modification. Loading silica on to a titania surface made the surface charge highly negative, which was confirmed by zeta potential measurements. The enhanced electrostatic attraction of cationic TMA onto the negatively charged Si–TiO2 surface seems to be the main reason for the enhanced photoactivity of Si–TiO2. As a result of this surface charge change, the TMA PCD rate with Si–TiO2 exhibited a maximum around pH 7 whereas the PCD with pure TiO2 was minimized at pH 7. The X-ray photoelectron spectroscopic analysis showed the formation of SiOx on the TiO2 surface but the diffuse reflectance UV spectra indicated no significant difference in the band gap transition between pure TiO2 and Si–TiO2. In addition, the diffuse reflectance IR spectra showed the presence of more surface OH groups on Si–TiO2 than on pure TiO2, which may also contribute to the higher photoactivity of Si–TiO2 through generating more OH radicals upon UV illumination.

A Highly Sensitive Spectrophotometric Method for the Determination of Cr(VI) Concentration

Abstract: A highly sensitive spectrophotometric method, which is based on the oxidation of leuco methylene blue (colorless) to methylene blue (colored) by Cr(VI), was developed for the determination of the d...