Showing papers by "Wonyong Choi published in 2002"

Photocatalytic Oxidation of Arsenite in TiO2 Suspension: Kinetics and Mechanisms

Abstract: Arsenite [As(III)] and arsenate [As(V)] are highly toxic aquatic contaminants. Since arsenite is more mobile in natural waters and less efficiently removed in adsorption/coagulation processes than arsenate, the oxidation of arsenite to arsenate is desirable in water treatment. We performed the photocatalytic oxidation of arsenite in aqueous TiO2 suspension and investigated the effects of pH, dissolved oxygen, humic acid (HA), and ferric ions on the kinetics and mechanisms of arsenite oxidation. Arsenite oxidation in UV-illuminated TiO2 suspension was highly efficient in the presence of dissolved oxygen. Homogeneous photooxidation of arsenite in the absence of TiO2 was negligibly slow. Since the addition of excess tert-butyl alcohol (OH radical scavenger) did not reduce the rate of arsenite oxidation, the OH radicals should not be responsible for As(III) oxidation. The addition of HA increased both arsenite oxidation and H2O2 production at pH 3 under illumination, which could be ascribed to the enhanced superoxide generation through sensitization. We propose that the superoxide is the main oxidant of arsenite in the TiO2/UV process. The addition of ferric ions also significantly enhanced the arsenite photooxidation. In this case, the addition of tert-butyl alcohol reduced the arsenite oxidation rate, which implied thatthe OH radical-mediated oxidation path was operative in the presence of ferric ions. Since both Fe3+ and HA that were often found with the arsenic in groundwater were beneficial to the photocatalytic oxidation of arsenite, the TiO2/UV process could be a viable pretreatment method. This can be as simple as exposing the arsenic-polluted water in a TiO2-coated trough to sunlight.

Kinetics and Mechanisms of Photocatalytic Degradation of (CH3)nNH4-n+ (0 ≤ n ≤ 4) in TiO2 Suspension: The Role of OH Radicals

Abstract: The photocatalytic degradation of a series of (CH3)nNH4-n+ (0 ≤ n ≤ 4) was systematically studied in the UV-illuminated TiO2 aqueous suspensions at pH ranges of 3−11. By investigating the pH-dependent kinetics and analyzing intermediates and products, we elucidated the mechanistic pathways and the role of OH radicals in the photocatalytic oxidation. The deprotonated neutral species more rapidly degraded than their protonated counterparts for these homologous compounds because the OH radicals favorably reacted with the lone-pair electron on the nitrogen atom. Therefore, the photocatalytic degradation was highly enhanced at alkaline solutions for all substances except (CH3)4N+. The H-atom abstraction (from (CH3)4N+) by OH radicals initiated successive demethylation processes to generate tri-, di-, and monomethylammonium/amine as an intermediate and NH3/NH4+ as a final product. On the other hand, the OH-addition to the N-atom with the lone-pair electron led to NO2-/NO3- whose production was highly favored at...

Selective photocatalytic oxidation of NH3 to N2 on platinized TiO2 in water

Abstract: Selective photocatalytic oxidation of NH3 to N2 is proposed as a new treatment method for controlling the levels of ammonia in water. The photocatalytic oxidation of ammonia on naked and metallized TiO2 in water saturated with air, nitrogen, or N2O gas was investigated. While the slow photocatalytic oxidation of NH3 to NO2-/NO3- is the only pathway for decomposition of NH3 on naked TiO2 and Au/TiO2, a new pathway, that of selective oxidation of ammonia to dinitrogen, opens up on Pt/TiO2. The formation of dinitrogen from the oxidation of 15NH3 was confirmed by mass spectrometric detection of 15N2. The photocatalytic conversion of NH3 to N2 greatly increases when the Pt/TiO2 suspension is saturated with N2O gas, whereas N2O itself shows little reactivity with naked TiO2 and Au/TiO2. Over 80% of the total nitrogen available in ammonia (0.1 mM) is converted into N2 within 40 min illumination of the N2O-saturated Pt/TiO2 suspension. The ability of N2O to accept the conduction band electrons of Pt/TiO2 was veri...

Photoelectrochemical Approach for Metal Corrosion Prevention Using a Semiconductor Photoanode

Abstract: Metal corrosion was successfully prevented using a TiO2-based photoelectrochemical system. Under UV illumination, a TiO2 electrode in a hole scavenging medium supplied photogenerated conduction band electrons to an electrically connected steel electrode with the generation of photocurrent and shifted the coupled potential to much more negative values. In this galvanic pair, the steel and the TiO2 electrode acted as a cathode and a photoanode, respectively, which is essentially a variation of cathodic protection. The performance of the photocathodic protection depended little on the light intensity as long as there were enough photons to compensate for the dark corrosion current of the steel. The shiny surface of the steel electrode remained intact in a corrosive electrolyte solution as long as it was connected to the UV-illuminated TiO2 photoanode, but it quickly corroded and was covered by red-brown rust in the absence of light. An outdoor test under solar light also showed a similar effect of corrosion ...

Dual Photocatalytic Pathways of Trichloroacetate Degradation on TiO2: Effects of Nanosized Platinum Deposits on Kinetics and Mechanism

Abstract: The effects of nanosized platinum deposits, dioxygen, and electron donors on the kinetics and mechanism of trichloroacetate (TCA) degradation in UV-illuminated TiO2 suspensions are described. Although platinization has been a routine method of TiO2 modification, its effects on photocatalytic reaction mechanisms in pollutant degradation need to be fully understood. By selecting TCA as a model compound, we demonstrated that the presence of Pt deposits (typical diameters of 1−2 nm) on TiO2 markedly changed its mechanism of photocatalytic degradation. TCA degraded photocatalytically in the presence or absence of dioxygen, which proposed that two mechanistic pathways (oxic vs anoxic) were operative. While the anoxic path was not important on bare TiO2, it became dominant on the platinized TiO2 surface. It is suggested that the platinum surface stabilized the reactive intermediates (e.g., dichlorocarbene) and subsequently changed the mechanistic pathway of TCA degradation. Therefore, the introduction of dioxyge...

Solid Phase Photocatalytic Reaction on the Soot/TiO2 Interface: The Role of Migrating OH Radicals

Abstract: The photocatalytic degradation of flame-deposited hexane soot on TiO2 films was carried out under UV illumination (λ > 300 nm) in air in order to investigate the behavior of photooxidants at the so...

Visible light-induced reactions of humic acids on TiO2

Abstract: We investigated the sensitized degradation of humic acids on TiO 2 under visible light ( λ >420 nm) irradiation. The photolysis rates were strongly dependent on pH due to the pH-dependent adsorption of humic acids on TiO 2 with the maximum rate observed at pH∼3. Reduction in the UV–visible absorbance and fluorescence emission ( λ ex =350 nm) of humic acids was observed during the irradiation. Humic acids acted as a sensitizer for injecting electrons from their excited state to the conduction band of TiO 2 , which were subsequently transformed and decolorized through a series of electron transfer reactions. However, the dissolved organic carbon (DOC) of humic acid solutions remained almost unchanged under visible light while the UV-irradiation was able to remove part of the total DOC. The trihalomethane formation potential (THMFP) on the contrary increased with visible light irradiation. The macromolecular humic acids on visible light-illuminated TiO 2 degraded with the reduction of aromatic character but not to the complete mineralization. Humic acids that can both donate and accept electrons to and from TiO 2 conduction band seem to withstand many electron transfers without undergoing mineralization. Humic acids also successfully acted as a sensitizer for the reductive degradation of CCl 4 on platinized TiO 2 under visible light irradiation, while no dechlorination was observed in the absence of either humic acids or TiO 2 .

Understanding the congener-specific toxicity in polychlorinated dibenzo-p-dioxins: chlorination pattern and molecular quadrupole moment.

Abstract: It is well known that the biological activities and toxicities of planar polychlorinated aromatic compounds are extremely sensitive to chlorination pattern. Although their toxic responses have been correlated with the relative affinity for the receptor, the origin of this congener specificity is not well understood. We present a general interpretation of the congener-specific activity in polychlorinated dibenzo-p-dioxins, which concludes that molecular electrostatics is the principal factor determining the structure−activity relationship in this highly controversial environmental pollutant even though this electrostatic interaction represents only a part of the total interaction energy. Through calculations of the molecular charge distribution in the complete set of 76 dioxin congeners, we show that all active congeners share a unique charge distribution pattern, which is quantitatively described in terms of the molecular quadrupole moment (QM). The QM of dioxins changes sensitively and systematically wit...

Efficient destruction of CF4 through in situ generation of alkali metals from heated alkali halide reducing mixtures

Abstract: Perfluorocarbons (PFCs) are the most potent green house gases that are very recalcitrant at destruction. An effective way of converting PFCs using hot solid reagents into safe products has been recently introduced. By investigating the thermal reductive destruction of tetrafluoromethane (CF4) we provided new insight and more physicochemical consideration on this novel process. The complete destruction of CF4was successfully achieved by flowing the gas through a heated reagent bed (400-950 degrees C) that contained powder mixtures of alkali halides, CaO, and Si. The silicon acted as a reducing agent of alkali halides for the in-situ production of alkali metals, and the calcium oxide played the role of a halide ion acceptor. The absence of any single component in this ternary mixture drastically reduced the destruction efficiency of CF4. The CF4 destruction efficiencies with the solid reagent containing the alkali halide, MX, increased in the order of Li approximately Na < K < Cs for alkali cations and I < Br < Cl < F for halide anions. This trend agreed with the endothermicity of the alkali metal generation reaction: the higher the endothermicity, the lower the destruction efficiency. Alkali metal generation was indirectly detected by monitoring H2 production from its reaction with water. The production of alkali metals increased with NaF, KF, and CsF in this order. The CsF/CaO/Si system exhibited the complete destruction of CF4 at as low as 600 degrees C. The solid product analysis by X-ray diffraction (XRD) showed the formation of CaF2 and the depletion of Si with black carbon particles formed in the solid reagent residue. No CO/CO2 and toxic HF and SiF4 formation were detected in the exhaust gas.

UV Light Induced Photocatalytic Degradation of Cyanides in Aqueous Solution over Modified TiO2.

Abstract: Metal doping was adopted to modify TiO 2 (P-25) and enhance the photocatalytic degradation of harmful cyanides in aqueous solution. Ni, Cu, Co, and Ag doped TiO 2 were found to be active photocatalysts for UV light induced degradation of aqueous cyanides generating cyanate, nitrate and ammonia as main nitrogen-containing products. The photoactivity of Ni doped TiO 2 was greatly affected by the state of Ni, that is, the crystal size and the degree of reduction of Ni. The modification effects of some mixed oxides, that is, Ni-Cu/ TiO 2 were also studied. The activity of Ni-Cu/TiO 2 for any ratio of Cu/Ni was higher than that of Ni- or Cu-doped TiO 2 , and the catalyst at the Cu/Ni ratio of 0.3 showed the highest activity for cyanide conversion.