Phsicochemical Treatment of Hazardous Wastes - Chapter 11 pot

11 Sonolysis 11.1 Introduction The sonochemical effects produced by sonolysis are due to the phenomenon of cavitation, which is the nucleation of bubbles in a liquid under the influence of ultrasound. Sonolysis is based on the fundamental concepts and theory involved in sonochemistry; the historical perspective of sonochemistry in Table 11.1 provides an insight into the discovery and understanding of fundamental processes in sonolysis. When a liquid of relatively high vapor pressure and dynamic tensile strength (such as water) is exposed to high- frequency ultrasonic waves (a few to several hundred kilohertz), acoustic cavitation in the liquid will occur. The cavitation process includes the formation, growth, and implosive collapse of small gas bubbles. Cavitation by ultrasound is accompanied by high temperature (2000 to 2500 K) and high pressure (hundreds of atmospheres), which are responsible for the degradation of organic pollutants. Therefore, sonolysis can degrade organic pollutants to CO 2 and H 2 O or convert them to compounds that are less harmful than the original compounds. The degradation of organic pollutants may result from combustion, supercritical water oxidation, and oxidation by radicals such as hydroxyl radicals and hydrogen radicals. Sonolysis was found to be efficient and economical to decontaminate industrial organics before they are discharged into aquatic ecosystems. Therefore, the applications of ultrasound to destroy organic pollutants have increased significantly in the past decade. 11.2 Fundamental Processes in Sonochemistry 11.2.1 Physical Processes Sonochemistry is defined as the chemical effects produced by ultrasonic waves. Ultrasound, with frequencies roughly between 15 kHz and 10 MHz, has a drastic effect on chemical reactions. It is the most important TX69272_C11.fm Page 423 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 424 Physicochemical Treatment of Hazardous Wastes phenomenon that produces a sonochemical effect on chemical reactions. This phenomenon proceeds as follows. A sound wave impinging on a solution is merely a cyclic succession of compression and expansion phases imparted by mechanical vibration. During the solution expansion phase, small vapor- filled bubbles are formed due to weak points in the solution, primarily at trapped gas pockets on particulate surfaces. These bubbles grow and contract in response to the expansion and compression phases of the cycle, respectively. Because the surface area of the bubble is greater during the expansion phase than during the compression phase, growth of the bubble is greater than the contraction, resulting in an increase in the average bubble size over many cycles. Over time, the bubble reaches a critical size depending on the ultrasonic frequency, whereupon the pressure of the vapor within the bubble cannot withstand the external pressure of the surrounding solution. The result is a violent collapse of the bubble, with high-velocity jets of solution TABLE 11.1 Historical Perspective of Sonochemistry Year Historical Event 1867 Early observations of cavitation by Tomlinson and Gernez 1880 Discovery of the piezoelectric effect 1883 Earliest ultrasonic transducer by Galton 1895 Cavitation as phenomenon recognized and investigated on propeller blades 1917 First mathematical model for cavitational collapse predicting enormous local temperatures and pressures by Rayleigh 1927 Publication of first paper on chemical effects of ultrasound (Richards and Loomis, 1927) 1933–1935 Observation of sonoluminescence effects 1933 Reports on reductions in viscosity of polymer solutions by ultrasound 1943 First patent on cleaning by ultrasound (German Patent No. 733.470) 1944 First patent on emulsification by ultrasound (Swiss Patent No. 394.390) 1950s Intensification of cavitation and ultrasound research; increasing number of applications using ultrasound 1950 Effect of ultrasound on chemical reactions involving metals (Renaud, 1950) 1950 Hot-spot model by Noltingk and Neppiras 1953 First review on the effects of ultrasound (Barnartt, 1953) 1963 Introduction of plastic welding 1970s Renaissance of sonochemistry research 1980s Growing research on sonochemical effects 1986 First international meeting on sonochemistry 1990 Foundation meeting of the European Society of Sonochemistry Source: Adapted from the European Society of Sonochemistry. TX69272_C11.fm Page 424 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC Sonolysis 425 shooting into the interior. As a result, extremely high temperatures and pressures are produced in and near the bubble. The use of ultrasound provides a unique means of integrating energy and matter and differs from traditional sources such as light, heat, or ionizing radiation in duration, pressure, and energy per molecule. Ultrasound provides a form of energy for the modification of chemical reactions; an ultrasound wave produces its effect via the generation and collapse of cavitation bubbles during the rarefaction and compression cycles of the wave when the liquid structure is torn apart. Ultrasonic enhancement has been used for some heterogeneous chemical reactions. The cavitation bubbles generated contain the vapor from the solvent and solute. When they collapse, these vapors are subjected to enormous changes in temperature and pressure; under such extreme conditions, the solvent and solute molecules suffer frag- mentation and generate small species, including reactive free radicals; they may further undergo some secondary reactions. Although ultrasound has a broad range of industrial applications, its potential as a water and wastewater treatment alternative has not been explored fully. It is estimated that 4 ¥ 10 8 bubbles/s/m 3 are produced. The bubbles are on the order of 10 to 200 m m in diameter, and they are short lived, with a lifetime near 10 m s; therefore, the bulk characteristics of the solution remain relatively unaffected, but the implosion of the bubble causes enormous local effects. For example, the temperature of the vapor within the bubble has been estimated to reach as high as 5000 K with a concomitant pressure near 1000 atm. Due to the extremely high temperatures created during the process, a cooling system generally needs to be included in the design of sonolysis reactors. The principal result of these conditions in an aqueous solution is the breakdown of water vapor in the bubble into hydrogen and hydroxyl radicals. This essentially transforms the bubble into a microreactor, where interesting chemistry can take place. If organic species are also present in the water subjected to ultrasonic waves, it is expected that degradation will occur, ultimately to complete mineralization. The extreme conditions created by acoustic cavitation initiate three distinct destruction pathways for organic contaminants: oxidation by hydroxyl radicals, supercritical water oxidation, and pyrolysis. It has been proposed that pyrolytic mechanisms dominate at high solute concentrations while hydroxyl radical attack dominates at low solute concentrations. From the view of pure physics, the effects of sonolysis on aqueous solutions can be described by three fundamental concepts in sonochemistry. The first phenomenon is compression and rarefaction, the second is cavitation, and the third is microstreaming. 11.2.1.1 Compression and Rarefaction A rapid movement of fluids caused by a variation of sonic pressure subjects the solvent to compression and rarefaction. This movement can be described as a motion that alternatively compresses and stretches the molecular structure within the cavitation process. This rapid movement of fluids is TX69272_C11.fm Page 425 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 426 Physicochemical Treatment of Hazardous Wastes caused by variation of the sonic pressure. Locally, the rarefaction phases of acoustic pressure wave generate gas and microbubbles (Sochard et al., 1998). 11.2.1.2 Cavitation Cavitation is a three-step process consisting of nucleation, growth, and collapse of gas- or vapor-filled bubbles in a body of liquid. The instantaneous pressure at the center of a collapsing bubble has been theoretically estimated to be about 75,000 psi. The temperature has been similarly estimated to reach a value as high as 13,000°F. Due to this local high temperature and pressure, it has been well recognized that cavitation can enhance the rate of a chemical reaction. Under such extreme conditions, the solvent and solute molecule fragmentizes to generate small pieces such as reactive free radicals. The free radicals may further precede some secondary chemical reactions. 11.2.1.3 Microstreaming Microstreaming is an event in which large amounts of vibrational energy are put into a small volume with little heating. Furthermore, microstreaming constitutes an unusual type of fluid flow associated with velocity, temperature, and pressure gradients (Laborde et al., 1998). 11.2.1.4 Cavitation Temperatures Probed by EPR High temperatures generated due to the diffuse energy produce hot spots in the liquid. When well-defined reactions due to ultrasound were studied, Suslick et al. (1986) determined the temperature of the imploding cavity to be 5500°C and the pressure to be around 500 atm according to thermodynamic principles. This short-lived hot spot, with heating and cooling rates greater than 10 9 K/s, is the source of sonochemistry. The reactions that take place at the gas–liquid interface of the bubbles are similar to combustion. The semiclassical model of the temperature dependence of the kinetic isotope effect for H and D atom formation was used to estimate the effective temperature of the hot cavitation regions in which H and D atoms are formed by ultrasound-induced pyrolysis of water molecules (Misik et al., 1995). The collapsing microbubbles, filled with dissolved gas (i.e., argon) and solvent vapor, are the reaction microchambers in which solvent vapor can be pyrolyzed, thus producing radicals that undergo further chemical reactions. The physical properties of the supercritical fluid differ from those of the bulk liquid. One of the most notable changes is the lower dielectric constant of polar solvents such as water which allows the accumulation of low- polarity solutes at this interface. This explains the crucial role of the hydro- phobicity of solutes during reactions in the solution. Thermolysis as well as radical abstraction reactions occur in this region. A temperature of approx- imately 800 K was determined for the interfacial region surrounding the TX69272_C11.fm Page 426 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC Sonolysis 427 cavitation bubbles by using the temperature dependence of C–N bond pyrolysis in oxygenated aqueous solutions. In general, the accuracy of estimating temperatures by measuring the kinetic isotope effect may be influenced by the following factors: (1) the accuracy of the semiclassical model, (2) the lesser sensitivity of the isotope effect at higher temperatures, and (3) the reactions of H (and D) atoms competing with spin trapping. Although OH radicals are produced by sonolysis of water, the correspond- ing spin adducts could not be detected using PBN due to the very short half- life of the PBN/OH adduct in aqueous solutions at neutral and slightly acidic pHs. According to the Rice–Herzfeld mechanism, the primary pyrolysis step is cleavage of the weakest bonds in the molecule, such as C–N (~85 kcal/ mol) or C–C bonds (~80 kcal/mol). In an isotope study by Misik et al. (1995), H 2 O, D 2 O, or a 1:1 mixture of both (1.7 mL) containing the spin trap was added to a Pyrex test tube, which was fixed in the center of a sonication bath with a frequency of 50 kHz. The temperature of the coupling water was 25°C. The sample was sealed with a rubber septum and bubbled with argon through a Teflon tube attached to a fine needle (argon flow rate was 50 mL/min) for 5 min before and during sonication. The time of sonolysis was kept at minimum (typically 45 s) to minimize decay of the spin adduct during sonolysis. After sonication, the electron paramagnetic resonance (EPR) spectrum of the sample was measured. After each experiment, the pHs in the samples were measured and found to be within a range of 6.7 ± 0.3 in all experiments. Immediately after sonication, the samples were transferred to EPR quarts cells, and acquisition of the spectrum typically started within 1 min after sonication. A Varian E9 X-band spectrometer with a 100-kHz modulation frequency and a micro- wave power of 20 mW was used to record the spectra. The temperature dependence of the kinetic deuterium isotope effect for the homolytic cleavage of the O–H and O–D bonds of the water molecule was used to estimate the temperature of the region in which this process occurs. The temperature dependence of the kinetic isotope effect has been used previously to study the temperatures of different sonochemical regions in organic liquids. In the semiclassical treatment, quantum mechanical tun- neling, which may contribute at lower temperatures, is not considered and the ratio of k H / k D for O–H or O–D bond homolytic cleavage is determined internally by the zero-point energy difference of the initial states, and the difference of the zero-point energies of the transition states is neglected. The zero-point energy difference of the ground states (1.24 kcal/mol) was determined from the infrared frequencies of H 2 O and D 2 O vapor: k H /k D = exp{1.24 kcal mol –1 /RT} (11.1) The value of k H /k D calculated from Equation (11.1) is 8.09, 1.87, and 1.23 at 298 K, 1000 K, and 3000 K, respectively. The intramolecular isotope effect (the k H / k D ratio from HOD) is equal to the intermolecular isotope effect TX69272_C11.fm Page 427 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 428 Physicochemical Treatment of Hazardous Wastes within the semiclassical approximation. The equations below show the reactions of radical formation by O–H and O–D bond cleavage and H and D trapping in 1:1 molar mixtures of H 2 O and D 2 O exposed to ultrasound: XO–H Æ H + O (11.2) H+ ST Æ ST/H (11.3) XO–D Æ D + OX (11.4) D + ST Æ ST/D (11.5) OX + ST Æ ST/OX (11.6) where ST is the spin trap used (PBN, POBN, PYBN, or DMPO) and X is either H or D (in a 1:1 mixture of H 2 O and D 2 O, 50% of the water molecules are present as HOD). The spin trap method has also been used by Yanagida et al. (1999) to develop a reaction kinetic model of water sonolysis. 11.2.2 Chemical Processes Although it is generally agreed that the origin of sonochemical effects lies in the bubble collapses and subsequent radical formation, the actual way these collapses achieve a sonochemical effect has been explained by two different main theories. One concept is the electrical theory, which assumes that the extreme conditions associated with collapse are due to an intense electrical field where the collapse is fragmentative. On the other hand, the thermal theory or “hot-spot” theory considers that the collapses are quite adiabatic. Here, the resulting internal pressures and temperatures are so high that vapor molecules dissociate, giving rise to free radical which, when released in the liquid, can react with other species (Sochard et al., 1998). Water vapor is pyrolyzed to OH radicals and hydrogen atoms, and gas- phase pyrolysis and/or combustion reactions of volatile substances dissolved in water occur. As a result, interfacial regions exist between the cavitation bubbles and the bulk solution. Since the temperature in these regions is lower than in the bubbles, a temperature gradient is present in this region. Locally condensed • OH radicals in this region have been reported. Bulk solutions at ambient temperature might undergo reactions of OH radicals or hydrogen atoms that survive migration from the interface. At the same time, the role of supercritical water during cavitation may play an important role in this region. Supercritical water is a phase of water that exists above its critical temperature and pressure (647 K and 221 atm, respectively). This unique state of water has different density, viscosity, and ionic strength properties than water under ambient conditions. Because the solubility of organic TX69272_C11.fm Page 428 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC Sonolysis 429 contaminants increases significantly in supercritical water, these organic species are brought into close proximity with the oxidant, usually oxygen from dissolved air. Oxidation rate is therefore several magnitudes higher than wet air oxidation. During sonolysis, it has been proposed that supercritical water is present in a small, thin shell around the bubble. This mode of destruction is expected to be secondary in importance because the fraction of water in the supercritical state is estimated to be on the order of 0.0015 parts out of 100 parts of water. Alternatively, the volume of the gaseous bubble is estimated to be 2 ¥ 10 4 times greater than the volume of the thin supercritical water shell surrounding the bubble. The value of supercritical water may be limited to increasing the solubility of the organic contaminant near the bubble interface for radical attack. The possible occurrence of supercritical water oxidation in the sonochemical reactor, however, may be one reason to justify fast degradation of organic compounds without O 2 . Pyrolysis is defined as the thermal destruction of a compound in the absence of oxygen. The high temperatures attained within the bubbles are well above the temperatures required to destroy organic materials. This mechanism, however, requires the compound to be present in the vapor phase within the bubble. Compounds with higher vapor pressures will have a higher vapor concentration inside the bubble. It is expected then that pyrolysis will be more prevalent as the vapor pressure of the contaminant increases. During collapse of the bubble, organic species present within the bubble interior would clearly degrade, but because bubble implosion occurs due to the influx of a jet stream of the surrounding liquid it may not be necessary for the organic contaminants to be initially present inside the bubble for degradation to occur. This implosion scenario is analogous to the injection of contaminated liquid directly into the hot reaction zone. Several parameters such as frequency applied have been found to influence the cavitation process. Following are the most important parameters that influence cavitation: • Frequency • Intensity • Solvent • Bubbled gas • External temperature and pressure Strong oxidation as well as reduction reactions have been observed due to generation of H and OH radicals. The main primary chemical process in the sonolysis of water is the thermal dissociation of water to hydrogen atoms and hydroxyl radicals: (11.7) HO H OH 2 •• Æ+ TX69272_C11.fm Page 429 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 430 Physicochemical Treatment of Hazardous Wastes In the sonolysis of pure water under argon, the formation rates of hydrogen radical and hydrogen peroxide are estimated to be 10.7 and 10.0 m M min –1 , respectively. In an oxygen or air atmosphere, hydrogen radicals will react with oxygen as follows: (11.8) (11.9) 11.2.2.1 H 2 –O 2 Combustion in Cavitation Bubbles Hart and Henglein (1986) discovered that typical flame reactions occur when ultrasonic waves at intensities sufficient to produce cavitation are propagated through water containing a gas or a mixture of gases. These reactions are brought about by temperatures of several 1000 K that exist in the compression phase of oscillating or collapsing gas bubbles. The yields of such gas-phase reactions in many cases are substantially higher than the yields of reactions occurring in the liquid phase. In early studies on the formation of hydrogen peroxide by ultrasound in water under various mixtures of oxygen and hydrogen, it has been found that the yield depends on the composition of the mixture in the complex manner. The intermediates during the formation of hydrogen peroxide are free radicals and free atoms, and the question arises whether the radicals can escape from the cavitation bubbles into the bulk solution. The gas consumption determinations were carried out during sonolysis by direct addition of the H 2 O 2 mixture from a syringe to the gas phase to keep gas pressure constant. The rate of H 2 O 2 formation as a function of the composition of the gas atmosphere under which the water has been insonated can be observed. Under pure oxygen, H 2 O 2 was formed at a rate of 17 m M /min. No H 2 O 2 was formed upon insonation under pure hydrogen. The rate of gas consumption is much higher than that of H 2 O 2 formation. Therefore, the rate of gas consumption is a function of the composition of the gas atmosphere. The H 2 O 2 combustion into flames is a branched chain reaction, • H and • O atoms and • OH and • HO 2 radicals are the intermediates, and generally the chains are very long. The combustion in the cavitation bubbles occurs via short chains, at a maximum rate of 220 m M /min. Ozone in oxygen bubbles decomposes at a rate of about 1 m M /min, and nitrous oxide in argon bubbles decomposes at a rate of about 300 m M /min. A rough estimate of the chain length can be obtained using the ratio of the yields of gas consumption and hydrogen peroxide formation. H 2 O 2 is formed according to the following reactions: HO HO • 22 +Æ ∑ ∑∑ +Æ+HO HO O H O 22222 TX69272_C11.fm Page 430 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC Sonolysis 431 (11.10) (11.11) It must also be taken into consideration that part of the H 2 O 2 that reaches the bulk solution is decomposed, as • OH and HO 2 • radicals may escape the hot spots and react with H 2 O 2 in the bulk solution according to the well- known mechanism: (11.12) (11.13) In addition to these destructive reactions, the radicals may also form H 2 O 2 molecules in the bulk solution. The conditions under which the H 2 –O 2 combustion occurs in the cavitation bubbles are quite different from those exist- ing in flames. The yields of H 2 O 2 and HO 2 ∑ first increase with increasing H 2 concentration as more • H atoms are formed and fewer OH radicals are available that could destroy HO 2 ∑ radicals via Equation (11.14): (11.14) This may be due to the fact that the temperature in the compressed cavity bubbles is lower in H 2 -saturated water than in O 2 , H 2 O 2 , or air due to the high thermal conductivity of hydrogen. From the solubilities of hydrogen and oxygen, the concentrations of these two gases in the liquid are calculated to be in the molar ratio of 2:1 at an 80:20 composition of the gas atmosphere. The tiny cavitation bubbles are not in thermodynamic equilibrium with the surrounding solution; that is, the gaseous composition does not correspond to that of the gas atmosphere above the insonated liquid. The second maximum of the yield occurs when the cavitation bubbles contain the H 2 –O 2 mixture in a 2:1 ratio. In the presence of Fe 2+ or Cu 2+ as scavengers for • OH and HO 2 ∑ , the H 2 O 2 yield of 16 m M /min is practically the same as in the absence of these solutes. Although appreciable amounts of • OH and HO 2 ∑ radicals are found in the solution, there seems to be no change in the H 2 O 2 yield. It thus seems that as much H 2 O 2 is destroyed by the radicals in the bulk solution as is formed there. In the second maximum of the yields, the H 2 O 2 yield in the absence of radical scavengers is even greater than in the presence of the scavengers. In this solution, no destruction of H 2 O 2 occurs, and this result is understood in terms of the absence of • OH radicals. Roughly 50% of the H 2 O 2 is formed in the hot spots; the other 50% of H 2 O 2 is formed by HO 2 ∑ radicals in the bulk solution. ∑∑ +ÆOH OH H O 22 HO HO H O O 22222 ∑∑ +Æ + ∑ +Æ+OH H O H O HO 22 2 2 HO HO HO O OH 22 2 22 ∑ +Æ++ ∑∑ +Æ+OH HO H O O 222 TX69272_C11.fm Page 431 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 432 Physicochemical Treatment of Hazardous Wastes 11.3 Degradation of Organic Pollutants in Aqueous Solutions Sonolysis of organic pollutants in water involves research on how and what different factors influence the efficiency of sonolysis. For example, it has been proven that the frequency applied is a very important factor influ- encing the degradation rate: the higher the frequency applied, the higher the resulting removal efficiency. Moreover, current research is concentrated on optimizing the effects of sonolysis. These optimization techniques can be useful for compliance with environmental laws, pollution prevention, and remediation of aqueous wastes. The degradation of different organic compounds by ultrasound and the combination of sonolysis and other advanced oxidation processes such as combining ozonolysis and sonolysis are also very effective. In the treatment of hazardous wastewater, ultrasonic radiation can decompose water vapor molecules in the bubbles into free radicals such as hydroxyl (OH), hydrogen (H), and hydroperoxyl (HO 2 ). Evidence for the formation of free radicals by ultrasound in aqueous solution has recently been demonstrated. The hydroxyl radical is particularly reactive with carbon–chlorine and carbon–carbon double bonds and is capable of cleaving the aromatic ring. The primary mechanism is hydroxyl radical oxidation. The severe conditions are enough to break down water vapor within the bubble into hydrogen and hydroxyl radicals, but the highly reactive nature of these radicals prevents a long travel path-length into the solution; therefore, only organic molecules present within the bubble or very near the bubble surface will be destroyed in this fashion. The simultaneous production of the hydrogen radical indicates that reductive pathways may also be available for the destruction of organic pollutants. Hydrogen peroxide will also be produced by radical combination of two hydroxyl radicals, even though the amount may be too small to be signifi- cant. The addition of hydrogen peroxide can increase free-radical concentration in the solution. Local ultrasound intensities can be affected by several factors, such as water level in the ultrasonic tank, position of reaction vessel in the tank, shape of reaction vessel, and solvent level in the reaction vessel. This variation in ultrasound intensity can lead to differences in the progress of the chemical reaction. Hydrogen peroxide and hydrogen gas are now considered to be the principal products formed when the intensity of ultrasonic waves is strong enough to create cavitation propagated through water. The liquid must contain a monoatomic gas such as argon or a diatomic gas such as oxygen for cavitation to occur. Hydrogen atoms, oxygen atoms, hydroxyl radicals, and perhydroxyl radicals are believed to be the intermediates in the production of hydrogen peroxide. The products obtained when water is sonicated have been found to be dependent on the acoustical power, the insonation cell, temperature, external pressure, and dissolved gas present. TX69272_C11.fm Page 432 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC [...]... amount of Fe(II) addition will increase the efficiency of decomposition of HBAs by ultrasound © 2004 by CRC Press LLC TX69272_C11.fm Page 446 Tuesday, November 11, 2003 12:24 PM 446 Physicochemical Treatment of Hazardous Wastes TABLE 11. 3 Pseudo First-Order Rate Constants for Decomposition of Hydroxybenzoic Acids Argon Compound 2-Hydroxybenzoic acid 3-Hydroxybenzoic acid 4-Hydroxybenzoic acid 3,4-Dihydroxybenzoic... temporal course of the sonochemical processes was monitored by HPLC Ultrasonic irradiation (~50 W/cm2) of a 100-mL air-equilibrated aqueous solution of 4-chlorophenol resulted in the first-order disappearance of the phenol, accompanied after a 1-hr delay by the first-order growth of Cl– The pH of the isonated solution dropped gradually from the initial value of 5.1 to 3.5 after 11 hr Sonolysis of the aqueous... LLC TX69272_C11.fm Page 442 Tuesday, November 11, 2003 12:24 PM 442 Physicochemical Treatment of Hazardous Wastes sonication time, while PNP attacked by •OH in homogenous solution follows zero-order kinetics Extensive investigations of high-temperature reactions of •OH with aromatic compounds have demonstrated that at T > 400 K the main reaction pathway is hydrogen atom abstraction instead of hydroxyl... the gases and vapor present, leading to the detection of H2O2: H + •OH Æ H2O (11. 17) OH + •OH Æ O– + H2O (11. 18) OH + •OH Æ H2O2 (11. 19) • • • © 2004 by CRC Press LLC TX69272_C11.fm Page 434 Tuesday, November 11, 2003 12:24 PM 434 Physicochemical Treatment of Hazardous Wastes • OOH + •OOH Æ H2O2 + O2 (11. 20) The main fraction of the H2O2 formed during water sonolysis seems to come from the OH and OOH... two chlorine atoms: CCl4 Æ •CCl2 + Cl2 © 2004 by CRC Press LLC (11. 32) TX69272_C11.fm Page 448 Tuesday, November 11, 2003 12:24 PM 448 Physicochemical Treatment of Hazardous Wastes In the presence of oxidizing species, the trichloromethyl radical can act as a scavenger of hydroxyl radicals: • CCl3 + •OH Æ HOCCl3 (11. 33) Sufficient quantities of trichloromethyl radical are formed Therefore, recombination... hydrocarbons Runs at 10°C for both CFC -1 1 and CFC -1 13 were conducted in the circulating reactor system to examine temperature effects In both cases, the apparent first-order rate constant declined slightly The decline was somewhat lower for CFC -1 13 than for CFC -1 1, which may even have remained unchanged A sonochemical approach has the advantage of not requiring any transference of the target molecule from an... Tuesday, November 11, 2003 12:24 PM 452 Physicochemical Treatment of Hazardous Wastes CH 3 OH + 1.5O 2 Æ CO 2 + 2H 2 O (11. 36) Incomplete combustion of methanol can be described by the following overall processes: CH 3 OH + O 2 Æ CH 2 O + H 2 O 2 (11. 37) CH 3 OH + 2O 2 Æ CO + 2H 2 O 2 (11. 38) CH 3 OH + 1.5O 2 Æ HCOOH + H 2 O 2 (11. 39) The maximal hydrogen peroxide yield occurs in a 40-vol% methanol solution... • • Decrease of reaction time and/or increase of yield Use of less forcing conditions (e.g., lower reaction temperature ) Possible switching of reaction pathway Use of less or no phase-transfer catalysts Forced reactions with gaseous products by degassing Use of crude or technical reagents Activation of metals and solids Reduction of any induction periods Enhancement of the reactivity of reagents or... of CCl4 during sonication of p-NP in an Ar-saturated aqueous solution enhanced the rate constant of p-NP degradation by a factor of 4.5 compared to sonication without CCl4 Degradation during sonolysis appears to be as follows: CCl4 Æ •CCl3 + •Cl (11. 30) CCl3 Æ •CCl2 + •Cl (11. 31) • Formation of dichlorocarbene (•CCl2) is also thought to occur by the simultaneous formation of two chlorine atoms: CCl4... competitive technology for water treatment 11. 3.5 Pentachlorophenate The ultrasonic wave effect on the degradation of pentachlorophenate (PCP) at 530 kHz was studied by Petrier et al (1992) PCP was chosen as a model © 2004 by CRC Press LLC TX69272_C11.fm Page 440 Tuesday, November 11, 2003 12:24 PM 440 Physicochemical Treatment of Hazardous Wastes because it belongs to the class of aromatic halides that have . movement of fluids is TX69272_C11.fm Page 425 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 426 Physicochemical Treatment of Hazardous Wastes caused by variation of the sonic. H O O 222 TX69272_C11.fm Page 431 Tuesday, November 11, 2003 12:24 PM © 2004 by CRC Press LLC 432 Physicochemical Treatment of Hazardous Wastes 11. 3 Degradation of Organic Pollutants. course of the sonochemical processes was monitored by HPLC. Ultrasonic irradiation (~50 W/cm 2 ) of a 100-mL air-equilibrated aqueous solution of 4-chlorophenol resulted in the first-order disappearance