Beomguk Park

Uniform core–shell structured magnetic mesoporous TiO2 nanospheres as a highly efficient and stable sonocatalyst for the degradation of bisphenol-A

Uniform core–shell structured magnetic mesoporous TiO2 (Fe3O4@SiO2@mTiO2) nanospheres were fabricated via a kinetically controlled Stober method. A silica interlayer with a thickness of ∼25 nm was introduced as a passivation barrier to prevent photodissociation, as well as increase the thermal stability of the core–shell materials. After crystallizing at 600 °C under nitrogen, the resultant nanospheres (Fe3O4@SiO2@mTiO2-600) possessed well-defined core–shell structures with a high magnetic susceptibility (∼17.0 emu g−1) and exhibited uniform mesopores (∼5.2 nm), large BET surface area (∼216 m2 g−1) and large pore volume (∼0.20 cm3 g−1). More importantly, the magnetic mesoporous TiO2 was demonstrated for the first time as a highly efficient and stable sonocatalyst for the degradation of bisphenol-A. The pseudo first-order-reaction constant of the magnetic mesoporous TiO2 was measured to be 0.164 min−1, which is 1.49 and 2.27 times higher than that of P25 and ultrasound alone, respectively. The remarkable performance is attributed to the fast mass diffusion, large adsorption rate and enhanced hydroxyl-radical-production rate of the nanospheres. More importantly, the catalyst can be easily recycled within 2 minutes using an external magnetic field, and a constant catalytic activity is retained even after eight cycles. This study paves a promising way for the design and synthesis of magnetically separable sonocatalysts for the degradation of organic pollutants, which is of significant importance for practical applications from both environmental and industrial points of view.

Application of Box-Behnken design with response surface methodology for modeling and optimizing ultrasonic oxidation of arsenite with H2O2

AbstractA combined ultrasound (US)/H2O2 process was used to oxidize arsenite to arsenate, yielding a synergistic effect value of 1.26. This showed that the combined process could be an effective method of oxidizing arsenite, instead of using either ultrasonic or H2O2 oxidation processes. This combined process was successfully modeled and optimized using a Box-Behnken design with response surface methodology (RSM). The effects of the US power density, the initial concentration of arsenite, and the H2O2 concentration on the sonochemical oxidation efficiency of arsenite were investigated. Analysis of variance indicated that the proposed quadratic model successfully interpreted the experimental data with coefficients of determination of R2 = 0.95 and adjusted R2 = 0.91. Through this model, we can predict and control the oxidation efficiency under different conditions. Furthermore, the optimal conditions for the oxidation of arsenite were found to be a US power density of 233.26 W L−1, an initial arsenite concentration of 0.5 mg L−1, and an H2O2 concentration of 74.29 mg L−1. The predicted oxidation efficiency obtained from the RSM under the optimal conditions was 88.95%. A confirmation test of the optimal conditions verified the validity of the model, yielding an oxidation efficiency of 90.1%.

Ordered mesoporous C/TiO2 composites as advanced sonocatalysts

Ordered mesoporous C/TiO2 composites have been fabricated via an evaporation induced co-assembly method, and demonstrated as a highly efficient sonocatalyst. The effects of the carbon content in the composites and calcination temperature have been investigated thoroughly in this work and optimized for the production of well-defined mesoporous C/TiO2 materials. The resultant composites possess superior “brick–mortar” frameworks with uniform TiO2 nanocrystals glued by a carbon matrix, and exhibit highly ordered mesostructures with high surface area (∼200 m2 g−1). More importantly, the mesoporous C/TiO2 composites show a high sonocatalytic degradation rate of Rhodamine B. The maximum pseudo-first-order reaction rate constant obtained with the composites 15C–85TiO2-450 (C: 15.2 wt%, TiO2: 84.8 wt%, calcined at 450 °C) is 0.178 min−1, which is 2.7 and 4.8 times higher than that of P25 (0.062 min−1) and ultrasound (0.037 min−1) alone, respectively. The excellent sonocatalytic performance is a result of fast mass diffusion, enhanced nucleation rate and rapid surface hydroxyl radical oxidation. In addition, the recycling test shows that the sonocatalytic degradation rate with 15C–85TiO2-450 is retained even after five cycles, which is related to the well-retained mesostructure with superior mechanical stability. We believe that the present results provide important insights into the design and synthesis of advanced sonocatalysts.

Effect of ultrasonic frequency and power density for degradation of dichloroacetonitrile by sonolytic ozonation

The degradation of dichloroacetonitrile (DCAN) by means of the processes of sonolysis, ozonolysis and sonolytic ozonolysis was studied, and degradation rate constants were evaluated at various frequencies and power densities of ultrasound. The ultrasonic frequencies used were 35, 170, 283, 450, and 935 kHz. The power densities were in the range of 9.5 to 20 W/L. The degradation rate constants for the sonolytic ozonolysis were (3.1–4.4)×10-3 min-1 with the power density of 9.5 W/L and the ozone dose of 3.7 g/h. And the synergistic effect in sonolytic ozonolysis was significant at 35 and 283 kHz among the five frequencies. The sonolytic ozonolysis provided an extra oxidation mechanism by generating additional hydroxyl radicals, giving significant enhancement on the process. The calculated values of synergistic effect were 2.56 and 2.15 at 35 and 283 kHz, respectively.

Ultrasonic and mechanical soil washing processes for the removal of heavy metals from soils.

In order to determine the optimal operating conditions of full-scale soil washing processes for the removal of heavy metals, the effect of high-power ultrasound on the conventional mechanical soil washing process was investigated in a large lab-scale 28kHz sonoreactor. The soil samples were obtained from an abandoned railway station site in Seoul, Korea, which was contaminated with Cu (242.7±40.0mg/kg), Pb (441.3±49.8mg/kg), and Zn (358.0±35.7mg/kg). The treated concentrations of three heavy metal species in each process were compared with the regulation levels. It was found that higher performance, satisfying the regulation levels, was obtained in the ultrasonic/mechanical process due to the combined effects of macroscale mixing and microscale sonophysical effects. Moreover ultrasound played a more important role in less favorable conditions for the mechanical washing process (less acidic or less washing liquid conditions). Considering the application of the ultrasonic/mechanical soil washing process in real contaminated sites, the optimal conditions for the reactor with the bottom area of 15×15cm2 and the input ultrasound power of 250W were determined as follows: (1) the amount of soil per an operation was a 300g; (2) the ratio of soil and liquid was 1:3; (3) the concentration of acidic washing liquid was 0.5M HCl.

Sonophotocatalytic Destruction of Chloroform: Comparison of Processes and Synergistic Effects

This study compared ultrasound, ultraviolet, and catalyst processes and evaluated their respective synergistic effects. The ultrasonic frequencies in this study used 35, 283, 450, and 935 kHz, whereas short wavelength ultraviolet lamp (UVC) was used. The dose of TiO2 was 0.3 g/L. The degradation rate constants for the sonophotolytic processes were (4.2–8.7)×10-3 min-1, nearly the same for the sonolytic processes. The value of the synergistic effect was 1.07. The main mechanism of this process was pyrolysis by ultrasound. The ultraviolet provided another mechanism as using oxidation by hydroxyl radicals, but the enhancement was not significant. The rate constants of sonophotocatalytic processes were (48.1–64.6)×10-3 min-1. The calculated value of synergistic effect was 1.54. In this process, the main mechanism for degradation was oxidation by hydroxyl radicals on the surface of TiO2. The roles of the ultrasound were the dispersion of catalyst and mass transport of pollutant to the surface of the catalyst.

Distribution of electrical energy consumption for the efficient degradation control of THMs mixture in sonophotolytic process

Sonophotolytic degradation of THMs mixture with different electrical energy ratio was carried out for efficient design of process. The total consumed electrical energy was fixed around 50W, and five different energy conditions were applied. The maximum degradation rate showed in conditions of US:UV=1:3 and US:UV=0:4. This is because the photolytic degradation of bromate compounds is dominant degradation mechanism for THMs removal. However, the fastest degradation of total organic carbon was observed in a condition of US:UV=1:3. Because hydrogen peroxide generated by sonication was effectively dissociated to hydroxyl radicals by ultraviolet, the concentration of hydroxyl radical was maintained high. This mechanism provided additional degradation of organics. This result was supported by comparison between the concentration of hydrogen peroxide sole and combined process. Consequently, the optimal energy ratio was US:UV=1:3 for degradation of THMs in sonophotolytic process.

Investigation of sonochemical activities at a frequency of 334 kHz: the effect of geometric parameters of sonoreactor

In this study, the effect of the dimensions of the bottom plate and liquid height was investigated for high-frequency sonoreactors under a vertically irradiated system. The dimensions of the bottom plate did not significantly influence sonochemical activity considering power density. However, as the bottom plate was increased in size, the hydroxyl radical generation rate decreased because of a decrease in power density. It is therefore recommended that sonoreactors with bottom-plate dimensions close to those of the ultrasonic transducer module be used. Liquid height had a significant effect on sonochemical activity, but the trend of the activity considering power density changed as the initial pollutant concentration changed. In the case of low initial concentration of As(III) (1 mg/L), the maximum cavitation yield for As(III) oxidation was observed at liquid heights of 150 mm.

Arsenite removal using a pilot system of ultrasound and ultraviolet followed by microfiltration.

Batch and continuous-flow pilot tests using ultrasound (US), ultraviolet (UV) and a combination of US and UV were conducted to determine the oxidation rates of arsenite [As(III)]. Compared to the single processes of US or UV, the combined US/UV system was more effective for As(III) oxidation with a synergy index of more than 1.5. A high rate constant of As(III) removal was obtained as ferrous [Fe(II)] ions existed. Like the pseudo-Fenton reaction, Fe(II) species can participate in the production of additional ·OH by reacting with H2O2 produced by US, before being oxidized to Fe(III). From the results of batch tests, the optimum molar ratio of Fe(II)/As(III) and pH were found to be 83 and 6-9.5, respectively. Similarly, the continuous-flow pilot tests showed that US/UV system could remove As(III) below the regulation [10 μg L(-1) as total As (Astot)] at 91 of molar ratio [Fe(II)/As(III)] and 3-h HRT. The continuous-stirred-tank-reactor (CSTR) modeling showed that the scavenging effect of anionic species (Cl(-) and CO3(2-)) for ·OH might prevail in the single processes, whereas it is insignificant in the combined process. Without using chemicals, microfiltration (MF) was adopted to treat sludge produced in oxidation step. In terms of an engineering aspect, the operational critical flux (CF) and cycle time were also optimized through the continuous-flow tests of MF system. As an energy-utilizing oxidation technique that does not require a catalyst, the combined energy system employing US/UV followed by MF could be a promising alternative for treating As(III) and Fe(II) simultaneously.

Effects of Power Density and TiO2 Dose in the Sonocatalytic Degradation of Diethyl Phthalate Using High Frequency

Few studies using high frequencies have been carried out on the sono-TiO2 process, and consistent results based on the specific experimental conditions have not been reported thus far. Therefore, in the present work the effects of power density and dose on the kinetic constant of diethyl phthalate at 500 and 35 kHz using TiO2 have been evaluated. The slopes of kinetic constants depending on the power density regardless of TiO2 were increased and they were shown to be linear. However, the enhancement percentage according to the frequencies at 500 kHz was lower than that at 35 kHz, though clear discussions on the enhancement in the presence of TiO2 have not yet been produced. Also, the optimal dose was 1 g/L, which was not changed according to the frequency.