Catalysis of advanced oxidation reactions by ultrasound: a case study with phenol.

The present study is about the enhancement in ozone-mediated degradation and UV (254nm) photolysis of phenol in aqueous solutions by 300kHz acustic cavitation and the selection of operating parameters for optimum phenol removal efficiency. The method was based on monitoring of the concentration of phenol during 90min exposure to ozonation, sonication, UV photolysis, O(3)/ultrasound, UV/ultrasound and O(3)/UV/ultrasound operations. It was found that ozonation at alkaline pH was an effective method of phenol destruction, but it was considerably more effective when applied simultaneously with ultrasonic irradiation. The observed synergy particularly at alkaline pH was attributed to combined effects of: (i) increased ozone mass transfer (upon hydrodynamic shear forces created by ultrasonic bubbles) and (ii) excess hydroxyl radical formation (upon thermal decomposition of ozone in the gaseous cavity bubbles). Degradation of phenol by photolysis alone was negligible, while combination of UV-irradiation and ultrasound rendered considerable degrees of decay. The synergy here was explained by excess hydroxyl radicals that are formed by photolysis of ultrasound-generated H(2)O(2). Maximum rate of phenol degradation was observed in case of combined application of ozone, UV and ultrasound at basic pH.

Effects of operating parameters on sonochemical decomposition of phenol.

Ultrasonic removal of phenol under irradiation at 20, 300 and 520 kHz was investigated to assess the impacts of operating parameters on the efficiency of the systems. It was found under our experimental conditions that 20 kHz was the least effective frequency for ultrasonic decomposition of phenol, owing to the low volatility of phenol and the slow rate of OH radical ejection to the bulk solution at this frequency. Assessment of relative rates of destruction and ultrasonic yields showed that maximum efficiency was accomplished with 300 kHz employed in a reactor enclosed with an ultrasonic energy of 14.7 W. The same reactor and frequency was found to provide maximum ejection of hydroxyl radicals to the solution. Impacts of pH and initial concentration on the efficiency of phenol removal were such that acidic pH and high concentrations accelerated the process as related to the increased likelihood of phenol at these conditions to approach the cavity sheath. Separate injection of equivalent volumes of air and argon into the reactors showed that the decomposition was enhanced in the presence of air by virtue of the production of additional reactive species via the reaction of nitrogen with molecular oxygen.

Degradation of aryl-azo-naphthol dyes by ultrasound, ozone and their combination: effect of alpha-substituents.

Lab-scale degradation of azo dyes with ultrasound (300 kHz), ozone and both was investigated using an aryl-azo-naphthol dye-C.I. Acid Orange 8. It was found that in all schemes color decay was faster than UV absorbance, and the rates followed pseudo-first-order kinetics except for the decay of UV-254 band by ozone. Sonication alone was sufficient for decolorization, but not for UV absorption abatement or mineralization. Ozonation was more effective than ultrasound in bleaching, but not as much for the mineralization of the dye. Combined operation of ultrasound and ozone improved the rate of bleaching and UV absorption decay and remarkably enhanced the mineralization of the dye. This was attributed to increased mass transfer of ozone in solution and its decomposition in the gas phase to yield hydroxyl radicals and other oxidative species. The effect of alpha-methyl substituent at the aryl carbon of the dye was found to decelerate the rate of degradation as a result of weakened intramolecular hydrogen bonding.

Individual and combined effects of ultrasound, ozone and UV irradiation: a case study with textile dyes.

Comparative degradation of azo dyes by 520 kHz ultrasonic irradiation and its combinations with ozone and/or ultraviolet light (UV) was investigated using a probe dye C.I. Acid Orange 7. Operation parameters such as ultrasonic power density, ozone flow, UV intensity, and type and injection mode of the bubbling gas were optimized based on the rate of absorption decay in the visible and UV bands as estimated by regression analysis of absorption-time data. At equivalent initial dye concentrations and contact times, individual effects of UV irradiation, ultrasound and ozone were "no effect", "bleaching", and "bleaching/organic carbon degradation", respectively. UV irradiation, however, was found to induce a catalytic effect when applied in combination with either ultrasound or ozone schemes; and the overall degradation process was most rapid under simultaneous operation of the three in the presence of a continuous flow of a gas mixture made of argon and oxygen. The synergy observed in combined schemes was attributed to enhanced ozone diffusion by mechanical effects of ultrasound, and the photolysis of ultrasound-generated H(2)O(2) to produce hydroxyl radicals.

Degradation and toxicity reduction of textile dyestuff by ultrasound.

Degradability of four textile dyes was investigated in deionized water solutions during 520 kHz ultrasonic irradiation. It was found that for all dyes, the rate of color decay was first order in the visible absorption of the dye, and related to the type of functional groups that characterized the chromophore in the dye molecules. The destruction of aromatic/olefinic carbons in azo dyes was slower than that of color--to be attributed to the priority of hydroxyl radical attack on the N=N bonds, and to the formation of numerous oxidation intermediates of organic character during the course of dye degradation. Toxicity analysis of the dye solutions prior to sonolysis revealed that "reactive" dyes were non-toxic, but "basic" ones were toxic at the test concentrations employed in the study. Significant degrees of toxicity reduction were accomplished along with color and aromatic carbon degradation.

Combinative dyebath treatment with activated carbon and UV/H2O2: a case study on Everzol Black-GSP.

Treatability of textile dyebath effluents by two simultaneously operated processes comprising adsorption and advanced oxidation was investigated using a reactive dyestuff, Everzol Black-GSP (EBG). The method was comprised of contacting aqueous solutions of the dye with hydrogen peroxide and granules of activated carbon (GAC) during irradiation of the reactor with ultraviolet light (UV). Control experiments were run separately with each individual process (advanced oxidation with UV/H2O2 and adsorption on GAC) to select the operating parameters on the basis of maximum color removal. The effectiveness of the combined scheme was tested by monitoring the rate of decolorization and the degree of carbon mineralization in effluent samples. It was found that in a combined medium of advanced oxidation and adsorption, color was principally removed by oxidative degradation, while adsorption contributed to the longer process of dye mineralization. Economic evaluation of the system based on total color removal and 50% mineralization showed that in the case of Everzol Black-GSP, which adsorbs relatively poorly on GAC, the proposed combination provides 25% and 35% reduction in hydrogen peroxide and energy consumption relative to the UV/H2O2 system. Higher cost reductions are expected in cases with well adsorbing dyes and/or with less costly adsorbents.

Aqueous phase disinfection with power ultrasound: process kinetics and effect of solid catalysts.

The effectiveness of power ultrasound as a viable alternative for destroying pathogenic organisms in homogeneous and heterogeneous mixtures of aqueous solutions was investigated. The method involved monitoring of total coliform bacteria during sonication of E. coli suspensions in the absence and presence of equivalent mass concentrations of ceramic, metallic zinc, and activated carbon. It was found that disinfection by ultrasound is accelerated with solids in the order activated carbon > ceramic > metallic zinc. Process kinetics for each test system were assessed by nonlinear regression analysis of bacterial density vs time data, and the predicted model was found to resemble a well-known expression describing chlorination kinetics. The model denoted that in the presence of activated carbon the process rate was pseudo first order, and the required contact time to accomplish 50% kill was 2.8, 2.4, and 4 times shorter than it was in the zinc-catalyzed, ceramic-catalyzed, and noncatalyzed systems, respectively. It was further found that catalytic effects faded away with increased sonication time and/or reduced number of bacteria, denoting (i) decreased probability of bacterial contact with the solid-liquid interface; (ii) erosion of solid surfaces by vibrational effects; and (iii) reduced cavity formation due to degassing effects of ultrasound.