Enhancement of ultrasonic and ultraviolet irradiation with chemical oxidants

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The combination of ultrasound and ozone was used to study the degradation of nitrobenzene, 4-nitrophenol, 4-chlorophenol, cyclohexene, and pentachlorophenol in aqueous solutions. Using ultrasonic frequencies of 20 and 500 kHz revealed apparent enhancement at 20 kHz and antagonism at 500 kHz for the combined system although the first-order degradation rate constants in the absence of 03 (ozone) was typically a factor of 10 larger at 500 kHz. A comparison of the first-order degradation rate constants for nitrobenzene, 4-nitrophenol, and 4-chlorophenol by sonication, ozonation, and the combination of sonication and ozonation revealed that the observed enhancement upon the combination of ultrasound and ozone is mainly the result of thermolytic decomposition of ozone in a collapsing cavitation bubble. A continuous flow reactor closed to the atmosphere, open to the atmosphere, and open to the atmosphere with gas bubbling was used to probe the effects of ultrasound on [...] mass transfer. Enhanced mass transfer of [...] appeared to be the result of rapid decomposition of [...] in a cavitation bubble. The degradation of pentachlorophenol and observation of intermediates showed OH attack of the aromatic ring. A chemical kinetic model was developed to gain insight into the formation of radicals in various bubbles at 20 and 500 kHz with and without [...] present. The model revealed [...] pyrolysis slightly before the end of collapse followed rapid free- radical formation in the last nanoseconds due to [...] and [...] decomposition. In addition, a new advanced oxidation process, photoactivated periodate, was developed to investigate the decomposition of triethanolamine, its associated chemical oxygen demand, and the oxidation of an industrial wastewater. The optimal [...] for COD degradation was determined to be pH 7.6 due to the combined effects of pH on the speciation of TEA and [...]. Increasing the [...] ratio increased the degradation rate up to an apparent saturation value. Irradiation with a 1000 W Hg(Xe) lamp increased the pseudo first-order degradation rate constant of COD by a factor of 5.5 for synthetic TEA solutions and 2.3 for the industrial wastewater as compared to irradiation with a 1000 W Xe lamp

Sonolytic Decomposition of Ozone in Aqueous Solution: Mass Transfer Effects

The sonolytic degradation of ozone (O_3) was investigated in both closed and open continuous-flow systems to examine effects of mass transfer on chemical reactivity in the presence of ultrasound. Degradation of O_3 followed apparent first-order kinetics at frequencies of both 20 and 500 kHz in all the systems. Degassing of O_3 was observed at 20 kHz due to the effects of rectified diffusion and larger resonant radii of the cavitation bubbles than at 500 kHz. Increased mass transfer of O_3 diffusing into solution due to ultrasound as measured by the mass transfer coefficient, k_La_2, was observed at both frequencies. At 20 kHz, an increase in mass transfer rates in the presence of ultrasound may be partially attributed to turbulence induced by acoustic streaming. However, the main process of increased gas−liquid mass transfer in the presence of ultrasonic waves appears to be due to the sonolytic degradation of O_3 creating a larger driving force for gaseous O3 to dissolve into solution. From first-order cyclohexene degradation kinetics obtained by sonolysis, ozonolysis, sonolytic ozonolysis, and comparing the large diameter of an O_3 diffusing gas bubble to the size of an active cavitation bubble, it appears that diffusing gas bubbles containing O_3 are not directly influenced by ultrasonic fields

Degradation of triethanolamine and chemical oxygen demand reduction in wastewater by photoactivated periodate

The rapid reduction of chemical oxygen demand (COD) of industrial wastewater is achieved using a novel oxidant, periodate (IO_4^−), coupled with ultraviolet (UV) irradiation. The wastewater is characterized by a high COD, low total suspended solids, variable triethanolamine (TEA) concentrations, and low concentrations of iron and zinc. The use of periodate and UV irradiation with either aqueous TEA solutions or real wastewater is shown to be effective in reducing the COD to acceptable levels. The optimal pH for COD degradation is determined to be 7.6 because of the combined effects of pH on the speciation of TEA and IO_4^− Increasing the ratio of the initial concentrations of period ate to TEA, [IO_4^−]_0/[TEA]_0, increased the degradation rate up to an apparent saturation value. Irradiation with a 1 000-W mercury-xenon lamp increased the COD pseudo-first-order degradation rate constant by a factor of 5.5 for synthetic TEA solutions and 2.3 for industrial wastewater, compared to irradiation with a 1 000-W xenon lamp

Aromatic Compound Degradation in Water Using a Combination of Sonolysis and Ozonolysis

The combination of sonolysis and ozonolysis as an advanced oxidation process was investigated to gain insight into factors affecting enhancement of the combined system. Sonolysis, ozonolysis, and a combination of the two were used to facilitate the degradation of three known organic contaminants, nitrobenzene (NB), 4-nitrophenol (4-NP), and 4-chlorophenol (4-CP), in water. Experiments were performed at frequencies of 20 and 500 kHz. At 20 kHz, there appeared to be an enhancement due to sonolytic ozonation, while at 500 kHz, an apparent retardation was seen. The catalytic effects of NB, 4-NP, and 4-CP degradation at 20 kHz increased with decreasing k_(O_3) of the compounds, whereas retardation at 500 kHz was correlated with increasing k_(O_3). The correlation of apparent rate enhancement at 20 kHz and retardation at 500 kHz with k_(O_3) is consistent with a pathway involving the thermolytic destruction of ozone to form atomic oxygen. Atomic oxygen then reacts with water vapor in cavitation bubbles, yielding gas-phase hydroxyl radical. Enhancement in loss of total organic carbon (TOC) by sonolytic ozonation was considerable at both 20 and 500 kHz with all three compounds. In addition, intermediate product formation was observed

Chemical bubble dynamics and quantitative sonochemistry

We model the collapse of a bubble by taking into account all the energy forms involved (i.e., mechanical, thermal, chemical, and radiative) and compare the calculated radical yields with sonochemical data in H2O. Water decomposition plays a critical role in the energy balance, but trails equilibrium even in bubbles collapsing at subsonic speeds. Integration of the equation of bubble motion coupled with a full chemical mechanism reveals that (1) terminal gas temperatures and Mach numbers ML increase in cooler water, (2) ΓOH, the number of OH-radicals produced per unit applied work at maximum MLswhen bubbles become unstable and disperse into the liquidsdecreases at small and very large sound intensities. We show that available data on the sonochemical decomposition of volatile solutesssuch as CCl4, which is pyrolyzed within collapsing bubbless are compatible with the efficient conversion of ultrasonic energy into transient cavitation. On this basis we calculate ΓOH) (1 ( 0.5) × 1017 molecules/J for R0) 2 µm bubbles optimally sonicated at 300 kHz and 2.3 W/cm2 by assuming mass and energy accommodation coefficients of R e 7 × 10-3 and ɛ e 0.04, respectively, in gas-liquid collisions, and values about 3-fold smaller after averaging over the nuclei size distribution. Since there is negligible radical recombination during dispersal, these ΓOH values represent available oxidant yields, that agree with experimental data on iodide sonochemical oxidation. Bubbles emit little radiation, suggesting that only radial shock waves may heat small regions to the 104-105 K range required by some sonoluminescence experiments. The contribution of this sonoluminescent core to sonochemical action is, however, insignificant. We show that much larger accommodation coefficients would lead to higher temperatures, but also to O atoms rather than OH radicals and ultimately to excess O2, at variance with experimental evidence