Effect of Chemical Reaction and Mass Transfer on Ozonation of the Azo Dyes Reactive Black 5 and Reactive Orange 96

Ozonation of wastewater containing azo dye has been studied to evaluate the enhancement of ozone mass transfer from O₂O₃ gas into water with the presence of chemical reactions in a bubble column reactor. Experiments were performed at different initial dye concentrations and at various gas flow rates. C.I. Reactive Black 5 (RB 5) and C.I. Reactive Orange 96 (RO 96) have been chosen as representative model substances being found in wastewater from textile-finishing wastewater. Results show that the rate of ozone mass transfer increases with increasing initial dye concentration and gas flow rate. Consequently, an enhancement factor E for ozone mass transfer with chemical reaction could be calculated which increases with dye concentration. The chemical reaction between ozone and dye enhanced the mass transfer within the liquid film of the gas liquid boundary. The greatest enhancement factor for wastewater containing RO 96 of 2050 mgL^?1 is E = 15.4 compared with E = 9.1 for RB 5 of 3800 mgL^?1, both for gas flow rates of 19 Lh^?1. For lower gas flow rates, higher enhancement factors were observed, particularly for RO 96.     

Impact of ozonation on subsequent treatment of azo dye solutions

BACKGROUND: Aqueous solutions of four azo‐dyes, Acid Red 14 (AR14), Congo Red (CR), Reactive Black 5 (RB5), and Reactive Violet 5 (RV5) were treated with ozone, and the impact of ozonation on their subsequent treatability by aerobic biodegradation processes was investigated. RESULTS: In all cases, ozonation at the highest ozone doses investigated could remove more than 96% of the original dye, and the corresponding residual colour of the azo dye solutions declined to less than 20 on the Pt‐Co scale. Ozonation also resulted in reduction of chemical oxygen demand (COD), total organic carbon (TOC) the COD/TOC ratio and pH, while in all cases electrical conductivity of the dye solutions increased. Activity of the microbial colonies present in domestic wastewater was not inhibited when un‐ozonized solutions of these dyes were mixed in a 1:1 volumetric ratio with domestic wastewater, although the dyes themselves were not degraded. Also, no significant inhibition of microbial activity was observed in 1:1 mixtures of ozonized dye solutions and domestic wastewater, especially when the initial dye concentration was low and the applied ozone dose was high. In almost all cases, progressively enhanced BOD exertion was observed in mixtures containing dye solutions ozonized with progressively higher doses. This indicated that some ozonation by‐products of the above dyes could be degraded by microorganisms present in domestic wastewater. CONCLUSIONS: It was concluded that the above dye solutions, after ozonation for partial or complete colour removal, could be mixed with domestic wastewater for subsequent treatment by aerobic biological processes, with no adverse impact on the activity of the microbial colonies present in domestic wastewater. Copyright © 2007 Society of Chemical Industry     

Ozone,Hydrogen Peroxide/Ozone And UV/Ozone Treatment Of Chromium- And Copper-Complex Dyes: Decolorization And Metal Release

Metal-complex azo dyes constitute a significant fraction of the dyes used in the textile industry and exhibit properties such as superior light- and wash-fastness. While effluent color is not always regulated, the textile finishing industry often decolorizes wastewater using processes including chemical oxidation. In this study, the use of ozone, hydrogen peroxide/ozone and UV/ozone oxidant systems was examined for treatment of two common metal-complex (premetalized) dyes, Acid Black 52 (chromium) and Direct Blue 80 (copper). Oxidant dosages required for decolorization of these dyes were determined. The effect of bicarbonate alkalinity on the ozonation and the hydrogen peroxide/ozone processes also was examined.     

Ozonation of Humic Substances: Effects on Molecular Weight Distributions of Organic Carbon and Trihalomethane Formation Potential

Four different sources of humic substances were studied to determine the effects of ozonation on molecular weight-distributions, based on dissolved organic carbon (DOC) and trihalomethane formation potential (THMFP). Solutions of two soil-derived fulvic acids and a one soil-derived humic acid, as well as dissolved organic matter (DOM) associated with a natural water source were studied. Both gel permeation chromatography (GPC) and ultrafiltration (UF) were employed to define apparent molecular weight (AMW). Applied ozone doses ranged from 2.0 to 2.5 mg O₃/mg DOC. Overall samples of untreated and ozonated waters, as well as individual molecular weight fractions, were characterized according to DOC, UV absorbance, and THMFP. Ozonation resulted in a significant disappearance of higher AMW material with a corresponding increase in lower AMW material. Although little overall reduction in DOC concentration was observed, significant overall reductions in UV absorbance and THMFP levels were observed.     

Unstationary Film Model for the Determination of Absolute Gas-Liquid Kinetic Rate Constants: Ozonation of Acid Red 27, Acid Orange 7, and Acid Blue 129

A method for the determination of absolute kinetic rate constants is proposed using an unstationary film model. This methodology avoids the experimental determination of parameters like the enhancement factor or the Hatta number which are usually model-dependent. The mathematical model is general for gas-liquid systems with irreversible second order reactions. An optimization procedure based on artificial neural networks is used to estimate the initial guess of the parameters and the subsequent application of Gauss-Newton algorithm for the final nonlinear parameter estimation. The model is tested with the ozonation reaction of Acid Red 27, Acid Orange 7 and Acid Blue 129. The second-order kinetic rate constants for the direct reaction with O₃ are 1615 ± 93, 609 ± 83, and 49 ± 2 M^?1s^?1, respectively.     

Ozonation and Advanced Oxidation of Wastewater: Effect of O3 Dose,pH, DOM and HO•-Scavengers on Ozone Decomposition and HO• Generation

The decomposition of ozone in wastewater is observed starting 350 milliseconds after ozone addition. It seems not to be controlled by the autocatalytic chain reaction, but rather by direct reactions with reactive moieties of the dissolved organic matter (DOM). A larger ozone dose increases ozone consumption prior to 350 milliseconds but decreases the rate of ozone decomposition later on; this effect is predicted by a second-order kinetic model. Transferred Ozone Dose (TOD) is poorly correlated with ozone exposure (= ∫[O₃]dt) indicating that TOD is not a suitable parameter for the prediction of disinfection or oxidation in wastewater. HO^? concentrations (> 10^?10 M) and R_ct (=∫[HO^?]dt/∫[O₃]dt > 10^?6) are larger than in most advanced oxidation processes (AOP) in natural waters, but rapidly decrease over time. R_ct also decreases with increasing pre-ozonation doses. An increase in pH accelerates ozone decomposition and HO^? generation; this effect is predicted by a kinetic model taking into account deprotonation of reactive moieties of the DOM. DOC emerges as a crucial water quality parameter that might be of use to normalize ozone doses when comparing ozonation in different wastewaters. A rapid drop of absorbance in the water matrix—with a maximum between 255–285 nm—is noticeable in the first 350 milliseconds and is directly proportional to ozone consumption. The rate of absorbance decrease at 285 nm is first order with respect to ozone concentration. A kinetic model is introduced to explore ozone decomposition induced by distributions of reactive moieties at sub-stoichiometric ozone concentrations. The model helps visualize and comprehend the operationally-defined “instantaneous ozone demand” observed during ozone batch experiments with DOM-containing waters.