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.     

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.     

Obtaining Real-Time Kinetic Parameters on Ozone Decay from a Full-Scale Ozone Contactor Using the Axial Dispersion Reactor Model

Ozone decay kinetic parameters, including fast ozone demand ([D]₀), ozone decay rate constant (k_D), and rate constant for ozone reaction with ozone demand (k_R), are required for a numerical simulation targeting the design and operational optimization of an ozone contactor. The kinetic parameters of ozone decay and dispersion number were obtained from a full-scale ozone contactor for the axial dispersion reactor model simulation. The sensitivity analysis showed that the influence of k_R was minor and the constant 13 L mg^?1 min^?1 for k_R was suitable for carrying out simulations for sand-filtered raw water without measuring it. Curve fitting with on-site ozone concentrations and the ADR simulation results using a trial-and-error method could successfully provide kinetic parameters on ozone decay (i.e., k_D and [D]₀). Using these real-time kinetic parameters, we successfully predicted the CT, residual ozone, C. parvum log inactivation, and bromate formation. Compared to a method based on the CSTR in series, this method could provide more accurate CT and residual ozone for an ozone contactor with horizontal meandering flow and low dispersion number.     

The Use of para-Chlorobenzoic Acid (pCBA) as an Ozone/Hydroxyl Radical Probe Compound

Since OH^· radicals cannot be measured directly during an ozonation process, para-chlorobenzoic acid (pCBA) has been used recently as an OH^· radical probe compound during ozonation based on its very slow direct reaction with ozone and fast reaction with OH^· radicals. However, in experiments of this study it was shown that pCBA accelerated ozone decay. Furthermore, the formation of hydrogen peroxide was observed during this process. The formed H₂O₂ increases the decomposition of aqueous ozone and leads to enhanced formation of OH^· radicals. The chain reaction therefore changes to HO₂ ^? ion initiated decay of ozone instead of hydroxide ion, OH^?. Thus, an error in applying pCBA as a probe compound in low scavenger containing waters is likely to occur if the scavenging rate of pCBA makes up more than 5% of the total scavenging rate.     

Promoting Hydroxyl Radical Production during Ozonation of Municipal Wastewater

Hydroxyl radical (HO^?) production during ozonation of municipal wastewater was investigated with and without liquid or solid-phase promoters. For liquid-phase promoters, an “ozone dose threshold” was observed, below which addition of H₂O₂ yielded no discernible increase in the rate of HO^? production. This threshold occurs because ozonation of bulk organics in wastewater promotes HO^? due to the presence of ambient promoters. Although solid-phase catalysts are reported to promote oxidation of contaminants, ozonation of effluent over TiO₂ or GAC was no more effective on trace organic removal than over inert surfaces.     

Kinetics of MIB and Geosmin Oxidation during Ozonation

Ozonation is an effective means for oxidation of two common earthy/musty odorants (MIB and geosmin) in drinking waters. Second order constants were experimentally determined between the two odorants with ozone and hydroxyl radicals (HO^?). Geosmin was oxidized faster than MIB. Under most surface water treatment conditions, hydroxyl radical mediated reactions dominate over ozone reactions during MIB or geosmin oxidation. MIB and geosmin oxidation increases with greater ozone dose, higher pH, higher temperature or addition of H₂O₂.     

Effects of Ozone-Gas Bubble Size and pH on Ozone/UV Treatment

A series of ozone/UV treatment under injection of ozone with different ozone-gas bubble sizes was performed at pH 1.7 and 7.4. The increase in the bubble size and the decrease in pH enhanced the ozone utilization efficiency. The enhancement of ozone utilization efficiency was caused by the shift of the production pathway of hydroxyl radical (OH) from the OH production via O₃ ^– to the UV photolysis of H₂O₂. The lower pH caused this shift through the chemical equilibrium of H₂O₂ and HO₂ ^–, and the large bubbles caused this shift through the augmentation of H₂O₂ transport from the bubble surface to the bulk solution.     

Systematic characterization of petroleum residua based on SFEF

Supercritical fluid extraction and fractionation (SFEF) has been used to separate a variety of petroleum residua and other heavy oils into narrow-cut fractions with total yields up to 75-90%. Any insoluble material, or end-cut, corresponds to the asphaltene fraction in the parent oil. The narrow-cut fractions were analyzed comprehensively and separated into the solubility classes of saturate, aromatic, resin, and asphaltene fractions. The boiling points were measured up to 700 °C and correlations were established with the key factors such as density and molecular weight. This allows extrapolation of boiling points of residue fractions up to 1000 °C. Unlike bulk property measurements, the narrow-cut characterization data show increasing concentrations of key contaminants as the fractions become heavier. The solubility parameter for each narrow-cut fraction was measured using high-pressure fluid phase equilibrium with propane. The corresponding values for the end-cuts were obtained by the conventional precipitation method. The distribution and reactivity of sulfur species were determined by XPS in the bitumen pitch fractions and the corresponding residua produced during thermocracking and hydrocracking. The average structures for the narrow-cuts were constructed from molecular weight and elemental analyses together with FTIR, ¹H-NMR and ¹³C-NMR data. The results were used to develop a generalized feedstock characteristic index, K_R. This index shows good correlation with feedstock hydrocarbon constituents and can be used to assess feedstock reactivity and processability. Downstream refiners can use the narrow-cut data and K_R values for process optimization by either cutting deeper into residua bottoms to increase yield or by selecting the most appropriate process units for the various residue fractions. This information can also be used by upstream operators to determine the economic feasibility of utilizing the end-cut onsite.     

Influence of presence of tannic acid on removal of sodium dodecylbenzenesulphonate by O3 and advanced oxidation processes

BACKGROUND: The objective of the present investigation was to determine the role of the tannic acid (TAN) component of organic matter dissolved in water, in the removal of sodium dodecylbenzenesulphonate (SDBS) by ozone and by O₃/H₂O₂, O₃/granular activated carbon (GAC) and O₃/powdered activated carbon (PAC) advanced oxidation processes. RESULTS: Low doses of TAN (1 mg L^?1) during SDBS ozonation cause (i) an increased ozone decomposition rate and (ii) an increased SDBS removal rate. The SDBS removal rate with ozone in the presence of TAN was reduced when HCO₃^? ions were added. A rise in TAN concentration increased the SDBS removal rate, with a linear relationship between added TAN and the removal rate. SDBS was removed more effectively by O₃/GAC, O₃/PAC and O₃/H₂O₂ systems in the presence of TAN. CONCLUSIONS: Results obtained indicated two mechanisms involved in the generation of HO^· radicals by the O₃/TAN interaction: (i) direct generation of HO^· radicals from the reaction between ozone and TAN, and (ii) increased generation of O₂^?· radicals in the medium, enhancing the transformation of ozone into HO^· radicals by different radical reactions. In O₃/GAC and O₃/PAC systems, HO^· radicals are mainly generated in the O₃/TAN interaction, which is a homogeneous reaction with fast kinetics, whereas the O₃/GAC and O₃/PAC interactions are in a heterogeneous phase with much slower kinetics, and are therefore not competitive in the generation of HO^· radicals. Copyright © 2008 Society of Chemical Industry     

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. II. The Effect of Temperature,pH, Alkalinity,and DOM Properties

The influence of temperature, pH, alkalinity, and type and concentration of the dissolved organic matter (DOM) on the rate of ozone (O₃) decomposition, O₃-exposure, ?OH-exposure and the ratio R_ct of the concentrations of ?OH and O₃ has been studied. For a standardized single ozone dose of 1 mg/L in all experiments, considerable variations in O₃-exposure and ?OH-exposure were found. This has important implications for water treatment plants regarding the efficiency of oxidation and disinfection by O₃. In oligotrophic surface waters and groundwaters, minimal calibration experiments are needed to model and control the ozonation process, whereas in eutrophic surface waters more frequent measurements of O₃ kinetics and R_ct values are required to evaluate seasonal variations.     

Bicarbonate Effect in the Ozone-UV Process in the Presence of Nitrate

Ozonation and advanced oxidation processes (AOP) are very efficient methods for the destruction of refractory organic matters. These virtues have always been related to the production of hydroxyl radicals HO^?, which are extremely powerful and non-selective oxidants. In this study, the O₃-UV process is used as an AOP, where hydroxyl radicals are generated from the photodecomposition of ozone by short wavelength ultraviolet radiation. The obtained results indicated a weak scavenging effect of tert-butanol proving that hydroxyl radicals and ozone are not the only oxidants existing in the medium. Moreover, bicarbonate, known for a long time as effective HO^? radical scavengers, does not slow down the oxidation of benzoic acid, but surprisingly increases it. Chlorides significantly decrease the degradation of organic compounds through their reaction with HO^? radicals to produce chlorine. Carbonate radicals, nitrate and nitrogenated species as peroxynitrite/?peroxynitrous acid are involved in the oxidative mechanisms.     

Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Electrochemical reduction of oxygen (O₂) in air electrode with manganese oxides (MnOx) as electrocatalysts was studied with MnOx/Nafion-modified gold (Au) electrodes using cyclic voltammetry, potential-controlled amperometry and rotating ring-disk electrode (RRDE) voltammetry in alkaline aqueous solution. At Nafion-modified (MnOx free) Au electrode, O₂ reduction undergoes two successive two-electron processes with HO₂⁻ as intermediate. The presence of MnOx, including Mn₂O₃, Mn₃O₄, Mn₅O₈ and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O₂ to hydrogen peroxide (HO₂⁻ in this case) and decreases the second one of HO₂⁻ to OH⁻, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO₂⁻ to O₂ and OH⁻ and thus, the O₂ reduction in air electrode was considered to include an initial two-electron reduction of O₂ to HO₂⁻ followed by a disproportionation reaction of HO₂⁻ into O₂ and OH⁻ catalyzed by MnOx. The excellent activity of MnOx for the follow-up disproportionation reaction substantially results in an overall four-electron reduction of O₂ at MnOx/Nafion-modified Au electrodes in the first reduction step, depending on potential scan rate and the kind of MnOx. The present work provides a scientific significance of the mechanism of O₂ reduction in air electrode using MnOx as electrocatalysts to effect a four-electron reduction of O₂ to OH⁻.     

Electrochemical study on orthorhombic LiMnO2 as cathode in rechargeablelithium batteries

Orthorhombic LiMnO₂ was synthesized via a solid-state reaction. Its electrochemical properties as cathode in lithium batteries were examined. It was found that initially, a few cycles are necessary to activate the electrochemical reactivity of o-LiMnO₂, which is related to the transformation from the orthorhombic phase to a spinel-like phase. A maximum discharge capacity of 180–190 mA h g^- for o-LiMnO₂ electrodes was achieved. An electrochemical impedance spectroscopy (EIS) study showed that the charge-transfer resistance (R _CT) for the initial o-LiMnO₂ electrode is much larger than that for the o-LiMnO₂ electrode in the charged state. The o-LiMnO₂ electrode demonstrated a better cyclability than that of the spinel LiMn₂O₄ directly synthesized by solid-state reaction.     

Predicting Hydroxyl Radical Activity and Trace Contaminants Removal in Ozonated Water

There is a renewed interest in predicting R_CT following growing evidence that AOP is effective against many emerging contaminants. Five surface waters were investigated to evaluate the OH-radical activity using the R_CT concept, predict R_CT using traditional water quality characteristics and predict contaminants removal by ozonation and peroxone. It was shown that R_CT was dependent on water quality characteristics and could be modeled (R² = 0.92), using water characteristics and treatment conditions. Predictions of MIB oxidation closely matched the published data of Kawamura (2000 Kawamura, S. 2000. Integrated Design and Operation of Water Treatment Facilities, 2nd, New York: John Wiley & Sons, Inc.  [Google Scholar]) and bench-scale assays performed on one of the water under investigation.     

Ozone Application during Coagulation of Wastewater: Effect on Dissolved Organic Matter

The aim of this work was to determine the effect of ozone on dissolved organic matter (DOM) during wastewater coagulation using alum. Adding ozone to the coagulation treatment (O₃ECT) enhanced the quality of the final effluent in comparison to conventional coagulation treatment (CT). Final effluents were analyzed by Fourier transform infrared Spectroscopy (FTIR) and UV-Vis spectroscopy. The hydrophilic polar fraction was separated from the hydrophobic fraction using fractionation resins XAD-8 and XAD-4. Wastewater hydrophilic DOM was characterized by molecular weight distribution (MWD), and, BOD₅, TOC and DOC parameters. Results show that FTIR spectra for both effluents were very similar in the frequency range 7000 to 1400 cm^?1; however, some differences occurred in the O₃ECT effluent in the range 1100 to 700 cm^?1, which indicated that amines and aromatics groups were affected. In the same way, the UV-Vis spectra showed that there was an increase in absorbance within the wavelength range 285 to 300 nm for the O₃ECT effluent (when compared with CT) while the absorbance decreased in the range 210 to 455 nm, particularly at 380 nm. With regards to the hydrophilic polar compounds of DOM, the most abundant fraction in raw wastewater was found in the 3–10 kDa range. After CT, the effluents fraction concentrations decreased in all cases, but for O₃ECT the 10–30 kDa range exhibited a marked increase while the smaller (<3 kDa, 3–10 kDa) and larger fractions (>30 kDa) decreased. The ozone application during coagulation slightly increased TOC and DOC percentage removals, 84.4% to 89.1% and 80% to 82.5%, respectively. This suggests that the small dose of ozone transferred (1.47 mg/L) caused only minor changes in DOM. Finally, the O₃ECT effluent exhibits 10 mg/L more of biological oxygen demand (BOD₅) than a single CT.     

Evaluation of Fe-Functionalized 4A Zeolite as Ozone Catalyst for an Enhancement of Hydroxyl Radical Pathway in a Multiphase Reactor

Iron doping 4A zeolite (Fe-4A) was prepared and demonstrated as ozone catalyst to enhance hydroxyl radical (HO·) pathway. Fe-4A composites with different iron load were characterized, and the catalytic effects were discussed dealing with para-chlorobenzoic acid (p-CBA), atrazine (ATZ), and oxalic acid (OA) as model compounds. Results showed that Fe-4A/ozone system promoted the production of HO·, which was in-line with the strength of Lewis acidity that might have accelerated ozone-to-HO· transformation. Modeled R_CT value validated the enhanced HO· exposure during catalysis that was more evident for 4%-Fe-4A. The Fe-4A exhibited structural stability and allows repeated use after facile regeneration.     

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. I. The Rct Concept

The ozonation of model systems and several natural waters was examined in bench-scale batch experiments. In addition to measuring the concentration of ozone (O₃), the rate of depletion of an in situ hydroxyl radical probe compound was monitored, thus providing information on the transient steady-state concentration of hydroxyl radicals (√OH). A new parameter, R_ct , representing the ratio of the √OH-exposure to the O₃-exposure was calculated as a function of reaction time. For most waters tested, including pH-buffered model systems and natural waters, R_ct was a constant value for the majority of the reaction. Therefore, R_ct corresponds to the ratio of the √OH concentration to the O₃ concentration in a given water (i.e. R_ct = [√OH]/[O₃]). For a given water source, the degradation of a micropollutant (e.g. atrazine) via O₃ and √OH reaction pathways can be predicted by the O₃ reaction kinetics and R_ct .     

A comparison of charge transport behavior in functionalized and non-functionalized poly 3,4-(ethylenedioxythiophene) films

Poly 3,4-(ethylenedioxythiophene) (PEDOT) films electropolymerized from an aqueous micellar solution containing sodium dioctyl sulfosuccinate and the monomer were functionalized with 1-fluoro-2-nitro-4-azidobenzene (FNAB) molecules by a photochemical nitrene insertion reaction. The variation in redox activity and the changes in the charge transfer and diffusion (through bulk) behavior of the functionalized and the non-functionalized PEDOT films have been followed by electrochemical impedance spectroscopy and cyclic voltammetry. While the functionalized film allows a reversible insertion and extraction of guest cations and anions, the non-functionalized film is capable of exchanging only anions. The higher level of oxidation attained in the functionalized film is also reflected in the longer diffusion length (l_D) observed for the ions in this film. In both films the barrier to charge transfer is resistive rather than capacitive. Both charge transfer and diffusion resistance (R_CT and R_D) are lower for the functionalized film, a consequence of a higher surface roughness and a more nodular morphology and therefore higher optical contrast and faster color-bleach kinetics are achieved in this film. For the functionalized and the non-functionalized films, both R_CT and R_D are greatly enhanced during reduction than for oxidation. In particular, in the low frequency regime, the hindered diffusion-controlled extraction of anions from the bulk of the film is also evident from the larger R_D as compared to R_CT and the difference in their magnitudes is more pronounced for the functionalized film thus confirming that functionalization is a useful method for controlling the redox response of conducting polymer films.     

The influence of various factors on aqueous ozone decomposition by granular activated carbons and the development of a mechanistic approach

The decomposition of aqueous ozone in the presence of various granular activated carbons (GAC) was studied. The variables investigated were GAC dose, presence of tert-butyl alcohol (TBA), aqueous pH as well as textural and chemistry surface properties of GAC. All the GAC tested enhanced the rate of ozone decomposition to some extent. From the analysis of experimental results it was deduced that ozone transformation into HO radicals mainly occurred in the liquid bulk through a radical chain reaction initiated by OH⁻ and ions. Hydroperoxide ions arise from the formation of H₂O₂ on surface active sites of GAC and its further dissociation. No direct relationship between textural properties of GAC and the rate of ozone decomposition was found. However, a multiple regression analysis of data revealed that basic and hydroxyl surface oxygen groups (SOG) of GAC favor the kinetics of the ozone decomposition process. It is thought that these groups are the active sites for ozone transformation into H₂O₂. Repeated used of GAC in ozonation experiments resulted in loss of basic and hydroxyl SOG with formation of carboxyl, carbonyl and lactone-type groups. Then, pre-ozonation of GAC reduces its ability to enhance the aqueous ozone transformation into hydroxyl radicals.     

Ozone and chlorine dioxide: Similar chemistry and measurement issues

The chemical reactions associated with ozone and chlorine dioxide can be complicated and involve numerous intermediates. When ozone is applied, the presence of reactive intermediate species (O₂ ^‐, O₃ ^‐, OH, HO₂, HO₂ ^‐, and H₂O₂) influence the extent of oxidation that takes place and determines the amount and types of by‐products formed. Similarly, when chlorine dioxide is applied the amount of intermediate (Cl₂O₂) formed determines whether chlorine dioxide producing reactions or chlorate ion forming reactions occur. Ozone and chlorine dioxide are excellent agents for inactivating Cryptosporidium and Giardia. Microbiologically, each of the agents are very reactive. In the case of ozone, typically each molecule undergoes a one‐electron change. The mechanism of chlorine dioxide inactivation involves a recycling process whereby chlorine dioxide is reduced to chlorite ion followed by the “regeneration” of chlorine dioxide that continues to react within the cell over and over again. Chlorite ion also has oxidizing power and in some cases, is a biocide. When ozone and chlorine dioxide are used in combination, it is important that the chlorine dioxide application follow the ozone treatment to prevent the formation of unwanted by‐products such as ClO₃ ^‐.