Papers by Author: Won Youl Choi

Abstract: The objective of this study is to evaluate the performance of photocatalysis/hydrogen peroxide/metal membrane system. Metal membrane for the separation of TiO2 particles was used in an experiment and nominal pore size is 0.5 μm. Hydrogen peroxide was tested as oxidants. The removal efficiency of CODCr and color for initial hydrogen peroxide concentration increases rapidly with an increase in the hydrogen peroxide concentration up to 50 mg/L. The removal efficiency of of CODCr and color for 50 mg/L of initial hydrogen peroxide concentration was about 95 % and 98 %, respectively. However, the addition of hydrogen peroxide over 50 mg/L inhibits the system efficiency. The addition of hydrogen peroxide in UV/TiO2 system enhanced removal efficiency of CODCr and color compared to no addition of hydrogen peroxide. This may be ascribed to capture the electrons ejected from TiO2 and to produce OH radicals. The application of metal membrane in UV/ TiO2/H2O2 system enhanced removal efficiency of CODCr and color due to the adsorption of metal membrane surface as well as the production of OH radical. In application of metal membrane with nominal pore size 0.5 μm, TiO2 particles were effectively separated from the treated water by metal membrane rejection. Photocatalytic metal membrane was much smaller resistance than these of humic acid, TiO2 and humic acid/ TiO2 due to humic acid degradation of photocatalytic reaction.

Abstract: The purpose of this study is to investigate the effect of the operational parameters of the UV intensity and TiO2 dosage for the removal of humic acid and heavy metals. It also evaluated the applicability of hollow fiber microfiltration for the separation of TiO2 particles in photocatalytic microfiltration systems. TiO2 powder P-25 Degussa and hollow fiber microfiltration with a 0.4 μm nominal pore size were used for experiments. Under the conditions of pH 7 and a TiO2 dosage 0.3 g/L, the reaction rate constant (k) for humic acid and heavy metals increased with an increase of the UV intensity in each process. For the UV/TiO2/MF process, the reaction rate constant (k) for humic acid and Cu, with the exception of Cr in a low range of UV intensity, was higher compared to that of UV/TiO2 due to the adsorption of the membrane surface. The reaction rate constant (k) increased as the TiO2 dosage increased in the range of 0.1~0.3 g/L. However it decreased for a concentration over 0.3 g/L of TiO2. For the UV/TiO2/MF process, TiO2 particles could be effectively separated from treated water via membrane rejection. The average removal efficiency for humic acid and heavy metals during the operational time was over 90 %. Therefore, photocatalysis with a membrane is believed to be a viable process for humic acid and heavy metals removal.

Abstract: Titanium of 99.7% purity was anodized in 1M potassium phosphate monobasic (KH2PO4) water solution with 0.15M NH4F. Titanium oxide nanotubes were fabricated at anodization potential of 20 V and 4.64 pH. To control the pH of the solution, we have added weak acid such as citric acid because it has three dissociation constants (pKa) of 3.09, 4.75, and 5.41. Citric acid was very useful to control the pH of the 1M KH2PO4 water electrolyte solution within 3 to 5. The diameter and length of the titanium oxide nanotubes were independent on anodization time. The diameter of 120 nm and length of 2.8 μm at anodization time of 5 hrs were observed by field emission scanning electron microscope (FESEM). Undesired thin oxide layer blocking the top of titanium oxide nanotubes was wiped out by increasing the anodization potential with the multi step voltage by 1 V reached to 25 V. The titanium oxide nanotubes having a very large surface area are very attractive for the battery, gas sensor, photocatalytic application, and biomaterials.

Abstract: This study was conducted to evaluate the applicability of photocatalytic metal membrane system for the treatment of humic acid and heavy metals in aqueous solution. The catalysts, TiO2 powder P25 Degussa and metal membrane with 0.5 μm nominal pore size were used for experiments. Removal efficiency of humic acid and heavy metals increased with the increase of TiO2 dosage, however decreased over 0.3 g/L of TiO2 dosage. The addition of H2O2 as an oxidation reagent had a positive effect for the removal rate of humic acid and heavy metals. Metal membrane with stainless steel seemed to be quite stable to UV light with oxidation reagent in long-term operational periods over 6 months. Moreover, TiO2 particles can be effectively separated from the treated water by membrane rejection and the permeation flux was also enhanced by the combination of photocatalytic reaction.

Abstract: This study focused on the treatment performance of membrane bioreactor (MBR) coupled with intermittent ozone bubbling for the effective recovery of dissolved organics from coagulated fresh sewage sludge. Intermittent ozone bubbling was effective in the prevention of permeation resistance increase caused by particle accumulation on membrane surface, which result in keeping high permeation flux. MBR with intermittent ozone bubbling is believed to be an effective system for the recovery of organic matter usefully utilized in biological denitrification as well as membrane fouling reduction.

Abstract: We studied the surface characterization of milled–silicon nitride nano-powders by XPS and TEM. The change of the chemical state and morphology of the oxide layer on the surface of Si3N4 nano-particles before and after a conventional wet–ball–milling process are investigated by X–ray photoelectron spectroscopy for measuring the chemical state of the oxide layer and transmission electron microscopy for observing surface morphology. The native oxide layers of as-received Si3N4 powders confirmed by HREM observation and their chemical states were different each other. As increasing ball–milling time, the chemical composition and the volume of oxide layer in Si3N4 powders were changed. The chemical state of as–received Si3N4 powder was near to SiO2 phase. After ball–milling process for long time, that of the milled Si3N4 powder shifted to Si2N2O phase. As increasing ball-milling time, the oxide layer of Si3N4 powder was also increased.

Abstract: This research investigated the water purification performance of thin film with anatase phase TiO2 powder having good photocatalytic property using an innovative aerosol deposition method at the normal room temperature. Fresh raw powder was dehydrated for good dispersion of TiO2 powder. To suppress the formation of second particle, the powder was dispersed for 90 minutes in alcohol bath with ultrasonic treatment and then desiccated. The average grain size of particle was identified to be 1µm in TiO2 thin film deposited on stainless steel mesh by scanning electron microscopy (SEM). The anatase phase of TiO2 thin film was investigated by X-ray diffraction (XRD). The anatase phase of fresh raw powder was favorably maintained after aerosol deposition treatment.

Abstract: We report on a novel method for the surface modification of indium tin oxide (ITO) in LCD glass by direct exposure to a dielectric barrier discharge (DBD) at atmospheric pressure and room temperature. To remove the organic contaminants from the surfaces of ITO film in LCD glass, the atmospheric pressure RF glow discharge plasma was used. Argon (Ar) and oxygen (O2) were used as the carrier gas and reactive gas, respectively. The addition of O2 gas to Ar decreased the contact angle of water and increased the surface cleaning rate due to the increase of oxygen radicals in the plasma. The chemical characteristics of ITO surface after the plama treatment were investigated using X-ray photoelectron spectroscopy (XPS), and new carboxyl group bond was produced. The contact angle of 64° before the plasma treatment was decreased to 7° in the processing condition with oxygen flow rate of 50 sccm, treatment speed of 100mm/sec, and input power of 300W. These hydrophilic effect will be very useful in the manufacturing processes of LCD glass.

Abstract: Metal leadframes (Alloy 42) were cleaned using atmospheric pressure glow discharge plasma. Atmospheric pressure glow plasma was generated by a RF source of 13.56MHz with a matching network (300W power) on to the leadframes free from streamers and arc. Argon (Ar) and oxygen (O2) were used as the carrier gas and reactive gas, respectively. The addition of O2 gas to Ar decreased the contact angle of water and increased the surface cleaning rate due to the ncrease of oxygen radicals in the plasma. The chemical characteristics and morphologies of leadframe surface after the plama treatment were analyzed using X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM), respectively. The contact angle of 82° before the plasma treatment was decreased to 5° in the processing condition with oxygen flow rate of 50sccm, treatment speed of 100mm/sec, and input power of 300W. These surface cleaning effect will be very useful in the replacement many steps of wet cleaning before electroplating.