Jaekyung Yoon

Removal of toxic ions (chromate, arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration membranes

Rejection characteristics of chromate, arsenate, and perchlorate were examined for one reverse osmosis (RO, LFC-1), two nanofiltration (NF, ESNA, and MX07), and one ultrafiltration (UF and GM) membranes that are commercially available. A bench-scale cross-flow flat-sheet filtration system was employed to determine the toxic ion rejection and the membrane flux. Both model and natural waters were used to prepare chromate, arsenate, and perchlorate solutions (approximately 100microgL(-1) for each anion) in mixtures in the presence of other salts (KCl, K(2)SO(4), and CaCl(2)); and at varying pH conditions (4, 6, 8, and 10) and solution conductivities (30, 60, and 115mSm(-1)). The rejection of target ions by the membranes increases with increasing solution pH due to the increasingly negative membrane charge with synthetic model waters. Cr(VI), As(V), and ClO(4)(-) rejection follows the order LFC-1 (>90%) > MX07 (25-95%) congruent withESNA (30-90%)>GM (3-47%) at all pH conditions. In contrast, the rejection of target ions by the membranes decreases with increasing solution conductivity due to the decreasingly negative membrane charge. Cr(VI), As(V), and ClO(4)(-) rejection follows the order CaCl(2)<KCl congruent withK(2)SO(4) at constant pH and conductivity conditions for the NF and UF membranes tested. For natural waters the LFC-1 RO membrane with a small pore size (0.34nm) had a significantly greater rejection for those target anions (>90%) excluding NO(3)(-) (71-74%) than the ESNA NF membrane (11-56%) with a relatively large pore size (0.44nm), indicating that size exclusion is at least partially responsible for the rejection. The ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare ion rejection. For all of the ions, the rejection is higher than 70% when the r(i,s)/r(p) ratio is greater than 0.4 for the LFC-1 membrane, while for di-valent ions (CrO(4)(2-), SO(4)(2-), and HAsSO(4)(2-)) the rejection (38-56%) is fairly proportional to the r(i,s)/r(p) ratio (0.32-0.62) for the ESNA membrane.

Application of immobilized nanotubular TiO2 electrode for photocatalytic hydrogen evolution: Reduction of hexavalent chromium (Cr(VI)) in water

In this study, immobilized TiO(2) electrode is applied to reduce toxic Cr(VI) to non-toxic Cr(III) in aqueous solution under UV irradiation. To overcome the limitation of powder TiO(2), a novel technique of immobilization based on anodization was applied and investigated under various experimental conditions. The anodization was performed at 20V-5 degrees C for 45min with 0.5% hydrofluoric acid, and then the anodized samples were annealed under oxygen stream in the range 450-850 degrees C. Based on the results of the experiments, the photocatalytic Cr(VI) reduction was favorable in acidic conditions, with approximately 98% of the Cr(VI) being reduced within 2h at pH 3. Among the samples tested under same anodizing condition, the nanotubular TiO(2) annealed at 450 and 550 degrees C showed highest reduction efficiencies of Cr(VI). In addition, the surface characterizations (zeta potential, XRD, and SEM) of these samples proved that the Cr(VI) reduction efficiency was higher under acidic conditions and at a lower annealing temperature. From this research, it was concluded that the anodized TiO(2) has the potential to be a useful technology for environmental remediation as well as photocatalytic hydrogen production from water.

Use of surfactant modified ultrafiltration for perchlorate (ClO4−) removal

Determinations of perchlorate anion (ClO(4)(-)) transport and rejection were performed using a surfactant modified ultrafiltration (UF) membrane. Perchlorate anion (at a concentration of 100 microg/L of ClO(4)(-), spiked with KClO(4)) was introduced to the membrane as a pure component, in binary mixtures with other salts, cationic and anionic surfactants, and at various ionic strength conditions (conductivity). Also, a natural source water was spiked with perchlorate in the presence of cationic and anionic surfactants and used to determine the effects of a complex mixture (including natural organic matter (NOM)) on the observed rejection. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. The objective of this study was to modify a negatively charged UF membrane in terms of the fundamental mechanisms, steric/size exclusion and electrostatic exclusion and to enhance perchlorate rejection, with synthetic water and a blend of Colorado River water and State Project water (CRW/SPW). Previous work suggested that perchlorate was dominantly rejected by electrostatic exclusion for charged nanofiltration (NF) and UF membranes (Rejection of perchlorate by reverse osmosis, nanofiltration and ultrafiltration (UF) membranes: mechanism and modeling. Ph.D. dissertation, University of Colorado, Boulder, USA, 2001). In that research, perchlorate rejection capability was quickly lost in the presence of a sufficient amount of other ions. However, this study showed that ClO(4)(-) was excluded from a (negatively) charged UF membrane with pores large with respect to the size of the ion. Although perchlorate rejection capability due to apparent electrostatic force was reduced in the presence of a cationic surfactant, a desired amount of the ClO(4)(-) was excluded by steric exclusion. The steric exclusion was due to decreasing membrane pore size caused by the adsorption of the cationic surfactant.

Ultrasonic degradation of acetaminophen and naproxen in the presence of single-walled carbon nanotubes

Ultrasonic (US) and single-walled carbon nanotube (SWNT)-catalyzed ultrasonic (US/SWNT) degradation of a pharmaceutical (PhAC) mixture of acetaminophen (AAP) and naproxen (NPX) used as analgesics was carried out in water. In the absence of SWNTs, maximum degradations of AAP and NPX occurred at a high frequency (1000 kHz) and under acidic conditions (pH 3) and different solution temperatures (25 °C at 28 kHz and 35 °C at 1000 kHz) during US reactions. Rapid degradation of PhACs occurred within 10 min at 28 kHz (44.5% for AAP; 90.3% for NPX) and 1000 kHz (39.2% for AAP; 74.8% for NPX) at a SWNT concentration of 45 mgL(-1) under US/SWNT process, compared with 28 kHz (5.2% for AAP; 10.6% for NPX) and 1000 kHz (29.1% for AAP; 46.2% for NPX) under US process. Degradation was associated with the dispersion of SWNTs; small particles acted as nuclei during US reactions, enhancing the H2O2 production yield. NPX removal was greater than AAP removal under all US-induced reaction and SWNT adsorption conditions, which is governed by the chemical properties of PhACs. Based on the results, the optimal treatment performance was observed at 28 kHz with 45 mgL(-1) SWNTs (US/SWNT) within 10 min.

Simultaneously photocatalytic treatment of hexavalent chromium (Cr(VI)) and endocrine disrupting compounds (EDCs) using rotating reactor under solar irradiation.

In this study, simultaneous treatments, reduction of hexavalent chromium (Cr(VI)) and oxidation of endocrine disrupting compounds (EDCs), such as bisphenol A (BPA), 17α-ethinyl estradiol (EE2) and 17β-estradiol (E2), were investigated with a rotating photocatalytic reactor including TiO₂ nanotubes formed on titanium mesh substrates under solar UV irradiation. In the laboratory tests with a rotating type I reactor, synergy effects of the simultaneous photocatalytic reduction and oxidation of inorganic (Cr(VI)) and organic (BPA) pollutants were achieved. Particularly, the concurrent photocatalytic reduction of Cr(VI) and oxidation of BPA was higher under acidic conditions. The enhanced reaction efficiency of both pollutants was attributed to a stronger charge interaction between TiO₂ nanotubes (positive charge) and the anionic form of Cr(VI) (negative charge), which are prevented recombination (electron-hole pair) by the hole scavenging effect of BPA. In the extended outdoor tests with a rotating type II reactor under solar irradiation, the experiment was extended to examine the simultaneous reduction of Cr(VI) in the presence of additional EDCs, such as EE2 and E2 as well as BPA. The findings showed that synergic effect of both photocatalytic reduction and oxidation was confirmed with single-component (Cr(VI) only), two-components (Cr(VI)/BPA, Cr(VI)/EE2, and Cr(VI)/E2), and four-components (Cr(VI)/BPA/EE2/E2) under various solar irradiation conditions.

Mass- and heat-transfer-enhanced catalyst system for Fischer-Tropsch synthesis in fixed-bed reactors

Fischer-Tropsch synthesis (FTS) was carried out using Al2O3-supported Co catalyst coated on metallic monolith. Considering the liberation of a large amont of heat from the highly exothermic FTS reaction, catalytic activity of Co catalyst coated on metallic monolith was tested and compared with that of pellet-type catalysts. The reaction was carried out in a conventional tubular fixed-bed reactor and simulated distillation (SIMDIS) analysis method was used to determine the liquid products distribution. Proper control of degree of reaction, as well as the reaction temperature gave rise to a shift of products selectivity toward higher hydrocarbons, especially C13−C18 diesel range hydrocarbons.

Evaluation of performance with small and scale-up rotating and flat reactors; photocatalytic degradation of bisphenol A, 17β–estradiol, and 17α–ethynyl estradiol under solar irradiation

In this study, the performances of photocatalytic reactors of the small and scale-up rotating and flat types were evaluated to investigate the treatment of new emerging contaminants such as bisphenol A (BPA), 17α-ethynyl estradiol (EE2), and 17β-estradiol (E2) that are known as endocrine disrupting compounds (EDCs). In the laboratory tests with the small-scale rotating and flat reactors, the degradation efficiencies of the mixed EDCs were significantly influenced by the change of the hydraulic retention time (HRT). In particular, considering the effective two-dimensional reaction area with light and nanotubular TiO2 (NTT) on a Ti substrate, the rotating reactors showed the more effective performance than the flat reactor because the degradation efficiencies are similar in the small effective area. In addition, the major parameters affecting the photocatalytic activities of the NTT were evaluated for the rotating reactors according to the effects of single and mixed EDCs, the initial concentrations of the EDCs, the UV intensity, and dissolved oxygen. In the extended outdoor tests with the scale-up photocatalytic reactors and NTT, it was confirmed from the four representative demonstrations that an excellent rotating-reactor performance is consistently shown in terms of the degradation of the target pollutants under solar irradiation.

Autothermal reforming of methane to syngas for Fischer-Tropsch synthesis with promoted palladium and a fast start-up device

In this study, a Pd catalyst was prepared with promoters such as CeO2, BaO and SrO in a washcoated form on a metallic monolith for autothermal reforming of methane to syngas for the Fischer-Tropsch synthesis. A reactor was installed with an electric heater in the form of the metallic monolith as a start-up device instead of a burner with which stable and fast start-ups (within 4 min) were achieved. Gas hourly space velocity and O2/CH4 governed, methane conversion, while H2O/CH4 controlled H2/CO ratio. A methane conversion of approx. 96%, H2+CO selectivity of approx. 85%, and H2/CO of approx. 2.6 were obtained under the conditions of gas hourly space velocity (GHSV) at 103000 h−1, O2/CH4=0.7 and H2O/CH4=0.35.

Performance evaluation of rotating photoelectrocatalytic reactor for enhanced degradation of methylene blue

Enhanced oxidation of organic pollutant, methylene blue (MB) was conducted using a newly designed rotating photoelectrocatalytic process (PECP), compared with photocatalysis. A significant synergy of photoelectrocatalytic reaction was observed such that the degradation of methylene blue (MB) by the photoelectrocatalytic mode was 80% higher than that (61.6%) of photocatalytic mode. To confirm the potentials in the application of water treatment, the effects of various parameters affecting reaction performance were studied with the newly designed rotating photoelectrocatalytic reactor consisting of TiO2 nanotubes and Ti lath as the photoanode and cathode, respectively, for applying electrical potential under UV irradiation. As the result of parameter studies, such as applied electrical potential (voltage), UV light intensity, rotating speeds, the highest degradation efficiencies of MB were achieved at 2.5 V or less (electrical potential), 90 rpm (rotating speed), and higher UV intensity. In addition, the stability and activity of TiO2 nanotubes electrode were studied through repeated experiments and showed a good performance, excellent stability, and reliability in the rotating photoelectrocatalytic process (PECP). This study provides an basis for the development of a rotating PECP to water treatment.