Jun 6, 2009 - acid and indigo carmine solution will be used to determine the variation in the yields and ..... Lowering pH reduces formation of bromate as it shifts the reaction .... TNSA model with the addition of the first excited state of molecular oxygen. ..... 91.1. 0.51. 0.324. 0.20. 61.8. 0.50. 0.328. 0.40. 0.397. 0.30. 0.508 ...
Effect of oxygen volumetric flowrate on decontamination efficiency of a microplasma ozonation system Tom Butterworth Supervisor: Prof. W Zimmerman
Summary Disinfection and decontamination of water are an essential process to provide clean and safe drinking water. Ozone and advanced oxidation processes (AOPs) are well-‐established, highly effective methods of disinfecting water. However chlorine based disinfection treatments are more popular because of their significantly lower operating cost, despite the negative effects that chlorination can have on drinking water quality. Hence there is a need to be able to provide economically viable methods of disinfecting and decontaminating water using ozone or AOPs. A radial microplasma reactor developed by Zimmerman et al at the University of Sheffield has shown a potentially economical method of disinfecting water via the production of ozone, ultraviolet light and possibly the powerful oxidants, hydoxyl radicals. This research project used computational modeling techniques using the Comsol Multiphysics software, to research the effect of the inlet velocity of oxygen into the microplasma reactor on the reactor residence time. Using the data collected from the computational modeling, an empirical relationship was derived relating electrode radius (r), residence time (τ), inlet velocity (ui), inlet Reynolds number (Re) and a reference radial position (r0), with two additional coefficients, n and γ, for a laminar flow radial reactor: r 2 − r0 2 n τ =γ (Re) 2r0 ui The derived equation was rearranged into various alternate forms to calculate, amongst other things, ideal reactor radius for a range of possible inlet velocities € theoretical reactor residence time of 0.1s. It was found to achieve a maximum that a reduction in electrode width might be possible, dependant on optimum oxygen inlet velocity, leading to potentially large increases in reactor efficiency by reducing the power requirement of the reactor. An experiment was devised and constructed to determine the optimum oxygen inlet velocity for the microplasma reactor. Two chemical indicators, terephthalic acid and indigo carmine solution will be used to determine the variation in the yields and production rates of hydroxyl radicals and oxidizing species (such as ozone, atomic oxygen and hydroxyl radicals) with varying oxygen inlet velocities to the plasma reactor. It is envisaged that this information regarding the optimum inlet velocity will be used in conjunction with the derived equations in order to assist in a redesign of the microplasma reactor to improve the operating efficiency of the device.
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Acknowledgments I would like to thank Professor Will Zimmerman and Dr Jaime Lozano-‐Parada for their patience, assistance, guidance and most of all, expertise, during my research project, without which it would not have been possible. I would also like to thank Tom Holmes and Dr Hemaka Bandulasena who have also provided assitance along the way.
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Table of Contents Summary ............................................................................................................... 2 Acknowledgments ................................................................................................. 3 1.0 Introduction ..................................................................................................... 5 2.0 Literature Review............................................................................................. 7 2.1 Dielectric Barrier Discharge...................................................................................................................7 2.2 Formation of Bromate...............................................................................................................................9 2.3 Effects of pressure and temperature on ozone formation ........................................................9 2.4 Microbubble Ozonation......................................................................................................................... 10 2.5 Oxygen Radicals for water treatment ............................................................................................. 10 2.6 Kinetic models of reactions of aqueous ozone ............................................................................ 12 2.7 Ozone and Ultraviolet light .................................................................................................................. 13 2.8 Methods of detecting ozone and hydroxyl radicals................................................................... 15 3.0 Hypothesis ..................................................................................................... 19 4.0 Computational Modelling............................................................................... 20 4.1 Developing an Empirical Relationship............................................................................................ 24 5.0 Experimental Method .................................................................................... 30 5.1 Equipment Description ......................................................................................................................... 30 5.2 Description of Experimental Setup .................................................................................................. 30 5.3 Description of Microplasma Reactor ............................................................................................... 32 5.6 Preparation of Indigo Carmine Solution ........................................................................................ 34 5.7 Preparation of buffered water............................................................................................................ 35 5.8 Calibration of the spectrophotometer ............................................................................................ 35 5.11 Calibration of the Power Supply ..................................................................................................... 36 5.12 Experimental procedure .................................................................................................................... 36 6.0 Experimental Analysis .................................................................................... 38 7.0 Conclusions .................................................................................................... 39 8.0 Suggestions for future work ........................................................................... 40 9.0 Appendix........................................................................................................ 41 Mathematica Syntax ....................................................................................................................................... 41 9.1 Empirical Equations -‐ Derivation...................................................................................................... 43 9.2 Emprical Equations -‐ Dimensional Analysis................................................................................. 46 9.3 Empirical Equation – Error Analysis ............................................................................................... 49 10.0 References ................................................................................................... 56
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1.0 Introduction Water is a very valuable resource; as global population increases, the effects of climate change are felt and more of the global population adopt “western lifestyles” there is an increasing pressure to secure clean and safe water supplies. For water to be considered clean and safe it should be free from all biological and chemical components that have the potential to cause harm or impart an unpleasant odour, taste or appearance. To remove harmful biological components, drinking water is treated by a process called disinfection. Removal of chemical components is more complex usually requiring a number of treatments due to the wide range of potentially harmful chemicals which can be found in drinking water, including heavy metals, toxic inorganics, organic chemicals and radio nuclides. Ozone was first documented as having disinfectant properties in Germany in 1891 when Frohlich demonstrated it’s bactericidal properties (Haley & Watts, 1986). Since then further research into the ozone has shown it is also capable of destroying or removing metals (particularly iron and manganese), minerals, inorganics, dyes and organic contaminants in drinking water, particularly when used as part of an advanced oxidation process (AOP) (Eagleton, 1999). Currently the most commonly used disinfectants globally are chlorine based treatments such as chlorine dioxide, hypochlorite solutions and chlorine gas. However chlorine treatments have a number of disadvantages: • Chlorine has a broad spectrum of action against microorganisms, however it is incapable of destroying cryptosporidium oocytes and giardia cysts (WHO, 1996) • Chlorine gas, hypochlorite solutions, chlorine dioxide and the precursor chemicals used to produce it are toxic. In order to treat water they need to be stored and transported in large quantities. This is inherently hazardous, so it would be preferable to avoid excessive handling • Chlorine reacts with naturally occurring organic compounds in the water to form trihalomethanes, some of which are possible carcinogens such as chloroform. Chlorine can also react with other organic contaminants in the water to form a wide range of chlorinated chemicals, some of unkown toxicity, such as chlorinated acetic acids, chloral hydrate, chloroacetones, cyanogen chloride and chlorophenols which impart a bad taste and odour on the water (WHO, 1996) It is well documented that ozone is a much more effective disinfectant than chlorine, requiring shorter contact times and lower concentrations to destroy the majority of microorganisms. It is also effective against cryptosporidium oocysts and giardia cysts, which as stated previously chlorine is not (WHO, 2002). Ozone is also capable of destroying organic contaminants, so unlike chlorine that can cause formation of toxic or carcinogenic chemicals, ozone can reduce the concentration of hazardous chemicals in water. (However it can also lead to formation of the possible carcinogen Bromate)
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Despite the apparent benefits of ozone over chlorine-‐based treatments it is not nearly as popular in industry, this is because ozone has a number of unresolved disadvantages: • Ozone is usually produced using plasma technology. A simple explanation of an ozone producing plasma is that it is the result of an electrical discharge between two electrodes over an air (or oxygen) filled gap (Soloman et al, 1998). The atoms and molecules in the gas are excited and ionized which starts off a chain of reactions ultimately leading to the formation of ozone. Currently the production of ozone using plasma requires a large amount of power. Despite the obvious benefits of using ozone as a disinfectant, due to the high cost of electricity, disinfection using ozone is not economically viable when compared with chlorine-‐ based treatments. As such large volumes of water have to be treated, operating cost is a very important factor when choosing a disinfection process. This is currently the greatest barrier to the widespread usage of ozone disinfection. • The formation of bromate – Ozone (or rather hydroxyl radicals) oxidize bromine to form the possible carciogen bromate. A number of possibilites for avoiding formation of bromate are discussed in the literature review. • The formation of NOx – When using an air feed for a plasma reactor some NOx is formed. NOx emissions are subject to strict regulation, so they should be avoided. Using an oxygen feed eliminates this problem, however such a solution requires the transport and storage of oxygen potentially resulting in higher operating costs. Similarly ozone treatment is a commonly used method in removal of chemical contaminants from water. However AOPs which utilize ozone are more effective still, as they tend to promote the formation of hydroxyl radicals. Table 1: Oxidation Potential of various species
Species F2 OH Radical O(1D) Radical O3 H2O2 HO2 Cl2
Oxidation Potential (V) 3.03 2.80 2.42 2.07 1.78 1.70 1.36
Table 1 shows the oxidation potential of chlorine, fluorine and reactive species which may be present in an ozone or AOP system, clearly hydroxyl radicals have the highest oxidation potential. Hydroxyl radicals are the most reactive and least selective oxidizing species used in oxidative pollution abatement in wastewater and drinking water treatment, these same properties could also be of great benefit for disinfection processes. The purpose of this research is to identify a method of improving efficiency and rate of disinfection of water using an experimental microplasma reactor producing ozone and it’s reactive
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intermediates from oxygen. This is achieved by drawing on current research into AOPs, plasmas and reaction kinetics of aqueous ozone and other oxygen containing reactive species.
2.0 Literature Review The purpose of this literature review is to: 1. Identify possible methods of improving the yield of hydroxyl radicals produced by the microplasma reactor. 2. Find ways in which the efficiency of the microplasma reactor can be improved with regards to disinfection per unit of power. 3. Research potential methods of determining hydroxyl radical formation, as well as a method of differentiating between the potential range of products formed by the micro plasma reactor. 4. Consider the interactions of ozone and hydroxyl radicals with common chemicals found in water, and how this might effect the rate of disinfection or formation of potentially harmful chemicals.
2.1 Dielectric Barrier Discharge Commercially ozone is produced on a large scale by dielectric barrier discharge (DBD). The DBD is characterized by 2 electrodes separated typically by a gap of 1mm for ozone generators (Kogelschatz et al, 1999). A dielectric layer divides this gap. The DBD is charecterised by filaments of plasma of nanosecond duration (microdischarges). These microdischarges are caused by build up of charge on the dielectric surface, within afew nanoseconds of breakdown the electric field at the site of the discharge is reduced to the extent that current flow at that point is interrupted. As a result of this short duration, limited current flow and energy dissipation there is relatively little heating of the gas through which the plasma travels. This is beneficial as the majority of the electron energy can be used for exciting molecules or atoms in the gas, hence initiating chemical reactions and emission of radiation. (Kogelschatz et al, 1999) DBD ozone generators typically use cylindrical discharge tubes of 20-‐50mm diameter and 1-‐ 3m in length. In traditional ozone generators pyrex tubes with a conductive internal coating (Usually aluminium) are mounted inside stainless steel tubes forming a discharge gap of 0.5-‐1mm. With the outer tube serving as the ground electrode. Increasingly a dielectric coating, usually ceramic, is applied to a stainless steel tube. High performance ozone generators use non-‐glass dielectrics and smaller discharge gaps. Ozone production on a large scale could use several hundred discharge tubes. Modern high power ozone generators use square waves of between 0.5-‐5kHz (as opposed to mains frequency of 50Hz). Higher operating frequencies deliver greater power densities requiring lower operating voltages. Using modern DBD methods ozone can be generated at a cost of 2US$/kg (Kogelschatz et al, 1999). Baroch et al (2008) investigated a DBD system for direct disinfection of water. They attached a porous ceramic layer to the metallic electrode that acted as a guide for flowing water. The hydrophilic properties of the ceramic layer allowed
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the flow of water to remain undisturbed by the electrical discharge. They report that this allowed them to reduce the discharge gap, which increased the intensity and stability of the plasma. It also transitioned the plasma from filamentous to semi homogenous. They used an organic die to test the efficacy of the disinfection process, and found that using the ceramic layer was 35% more effective at discoloration of the organic dye than without the ceramic. Significant to this research project is an experiment carried out by Williamson et al (2006) comparing the effects of high voltage AC and pulsed power supply to a DBD ozone generator. The AC power supply had a frequency ranging from 0.3 – 2 kHz with a voltage of up to 10kV, whilst the pulsed power supply had a pulse frequency of 50-‐600Hz and a voltage of up to 30kV. They found that the pulsed power supply was capable of increasing ozone concentration much more rapidly as a function of average power input than the AC supply. With the maximum ozone density achieved by the AC power supply being 3×10-‐15 cm-‐3 at 25W compared with the pulsed power supply achieving densities 8.5×10-‐15 cm-‐3 at 20W, which they note as being 4 times greater than that achieved by the AC at the same power level. The increase in ozone density compared with input power for an AC and pulsed power supply is shown in figure 1:
Figure 1: Ozone number density as a function of average deposited power into the DBD for ac and short-pulse excitation. (Williamson et al, 2006)
Similar results have been found in other studies comparing AC and pulsed power supplies for DBD discharges, with much higher ozone densities consistently being found for pulsed power supplies per watt than AC. (Pemen et al, 2009; Dhainaut et al, 2002) These results suggest that simply switching the power source to the microplasma reactor to a pulsed supply could significantly increase efficiency of ozone generation, however further research into the most appropriate supply would have to be carried out.
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2.2 Formation of Bromate
One of the major disadvantages of using ozone for the disinfection of water is that it causes the formation of the possible carcinogen bromate. There a number of methods of both minimizing formation and also removing bromate during treatment of drinking water. The various methods are briefly discussed by Von Gunten (2003) and are summarized here: • The addition of ammonia does not eafect ozone stability and hence does not effect the disinfection process. Ammonia interferes with a key intermediate step in the bromate formation mechanism. • Lowering pH reduces formation of bromate as it shifts the reaction equilibrium of HOBr-‐/OBr-‐ towards HOBr. This is because hydroxyl radicals lead to the dominant mechanism of oxidation of HOBr-‐/OBr-‐. However this method could possibly lead to a reduced efficiency of disinfection. • Bromate can be reduced by the addition Iron (II) however it is unlikely to be a feasible option as dissolved oxygen competes with bromate as an oxidant for iron(II). • Most relevant to this piece of research is that UV irradiation at a wavelength of 255nm leads to the reduction of bromate to hydrobromous acid and eventually bromide. However the required exposure to UV is higher than is typically required by disinfection processes by up to a factor of 100 times which in most situations is likely to restrict it’s usage for economical reasons. • The addition of granular activated carbon (GAC) is able to reduce bromate to bromide. However the presence of natural organic matter decreases the ability of GAC to reduce bromate to bromide. This limits the usage of GAC to reducing bromate as a post ozonaton treatment.
2.3 Effects of pressure and temperature on ozone formation
Yasuoka et al produced ozone using an ozone generator that they describe as a micro hollow cathode discharge plasma reactor. The claimed benefits of the reactor are high current density, high electron density and stability with high pressure gasses. It was found that by decreasing the residence time by increasing gas velocity of the reagents inside their reactor that more ozone is formed as a result. They conclude that this is caused by reduced oxygen temperature and less electron impacts which promote the decomposition mechanisms of ozone. These findings are supported by computational models produced by Lozano-‐ Parada of a plasma producing ozone from oxygen, it was found that by lowering the plasma temperature and increasing gas pressure that significantly higher yields of ozone could be produced. This supports the findings of Yasuoka et al and suggests that increasing flow rate of oxygen through the reactor will yield more ozone as there would be fewer electron impacts, higher pressure and a lower gas temperature.
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2.4 Microbubble Ozonation
As a fluidic oscillator to produce microbubbles will be used with the microplasma reactor in future experiments, it is worth considering the effects that this will have on disinfection processes. Previous work carried out by Zimmerman et al have extensively documented the benefits of using microbubbles for significantly improving rates of mass transfer. Chu et al tested the effect of ozonation of simulated dyestuff wastewater using microbubbles. Similarly to the work carried out by Zimmerman et al they found higher rates of mass transfer and higher rates of reaction using the microbubble contactor over a standard bubble contactor (Mass transfer coefficient was 1.8 times higher and pseudo first order rate constant was 3.2 – 3.6 times higher). More interestingly however was, even after taking into account the effects of enhanced mass transfer, that 1.3 times more organic carbon was destroyed per gram of ozone using the microbubble system than the standard bubble system. They used terepthalic acid as a chemical probe to test the concentration of hydroxyl radicals being formed and found a greater concentration when using the microbubble system. They support their claim by stating that according to the Young-‐Laplace equation, microbubbles have a higher internal pressure which enhanced the formation of hydroxyl radicals.
2.5 Oxygen Radicals for water treatment
Excited atomic oxygen radicals, O(1D), are highly reactive species with a very short lifetime (~10-‐30ns) in atmospheric oxygen. Upon reaction with water they generate hydroxyl radicals by equation 1 (Yamatake et al, 2006): O( 1D) + H 2O →•OH + •OH Equation 1
Potentially, excited atomic oxygen could be generated to react with water to form hydroxyl radicals. However their short lifetime is problematic and hence € research generally focuses around direct generation and injection of hydroxyl radicals into water. Yamatake et al (2006) tested a DC driven microhollow cathode discharge to produce a direct “injection” of oxygen radicals into water. This was achieved by having the cathode in direct contact with the water, indeed the cathode formed the base of the flask in which the experiment was being carried out. The anode was positioned 500 micrometers beneath the discharge hole (diameter of 200-‐ 300 micrometers) in the centre of the cathode separated by an aluminium oxide dielectric spacer, a comparison was made to a similar reactor where the electrodes were separated by an undisclosed but significant distance from the water so that no oxygen radicals would be injected into the water. Flow rates of oxygen tested through the reactor were 100, 500 and 1,000 standard cubic centimeters per minute. The effect of direct oxygen radical injection was tested by using acetic acid as a chemical probe and measuring the change in
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concentration of total organic carbon (TOC) using a TOC analyzer. They found that using their radical injection system that acetic acid was observed to decompose, compared with the ozone system where it did not. They also observed that with increasing gas flow rate that the efficiency of acetic acid decomposition also increased. They conclude that in order for O radical injection to be effective, a high flow rate of oxygen into the reactor needs to be used so that there is an effective reaction at the gas water interface. This is explained by the very short lifetime of the oxygen radicals, which Yamatake suggests would not last over a distance of several hundred micrometers. This finding suggests that increasing volumetric flow rate of oxygen may not result in direct injection of oxygen radicals into the water for the microplasma reactor being researched, and hence may affect the rate of disinfection observed. Similarly to Yamatake et al, Reuter et al (2009) tested an alternative method of direct atomic oxygen injection into an effluent using a low temperature atmospheric pressure plasma jet (APPJ) using a gas mix of 99.5mol% helium and 0.5mol% oxygen discharging into air. They report high densities of atomic oxygen near to the nozzle (the jet outlet) of approximately ~1016 cm-‐3, while several cm away the atomic oxygen concentration is found to be 1% of that at the nozzle. They also report finding excited atomic oxygen 10cm away from the jet nozzle, a surprising result considering the lifetime of these species is about ~30ns. Due to the extremely high improbability of an excited oxygen atom reaching such distances at relatively low gas velocities (~6m/s), they conclude that some energy is transferred from the plasma and into the effluent. After some investigation they dismiss the possibility of electrons, ions or metastable helium atoms as a source of energy as their concentrations rapidly decrease with distance into the effluent. The energy transport is attributed to vacuum ultraviolet radiation (VUV) produced by the plasma, which reaches far into the effluent. The main spectral bands of VUV produced by the plasma are in the Schumann-‐Runge bands of O2 at about 181nm and the atomic oxygen line at 130nm. Hence the formation of atomic oxygen at a distance from the jet nozzle is in fact caused by photo-‐dissociation of ozone and molecular oxygen. Previous work carried out by Dr Lozano-‐Parada for a PhD thesis involved the computer simulation of a plasma reactor for ozone synthesis. A number of models were produced of the temporal evolution of different oxygen species inside a plasma reactor. 131 different reactions were considered, however 5 different models of increasing complexity were produced. The models were produced on the assumption that kinetic processes are significantly faster than diffusion, convective or electrical drift processes so that the reactions do not need to be spatially resolved. The “three neutral species approximation” (TNSA) model considers only the ground states of atomic oxygen, molecular oxygen and ozone. The “four neutral species approximation” considers those species in the TNSA model with the addition of the first excited state of molecular oxygen. The “nine neutral species approximation” (NNSA) model includes an additional 8 oxygen species, including O(1D), which compromises of 62 kinetic equations. The formation of O(1D) is observed to increase and plateau at about 10-‐5s. The most complex model incorporates the roles of the ions of atomic and molecular oxygen as participating species. There are two models which are discussed, that
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of Lieberman et al and Soria et al. Both of which found that show that atomic oxygen is formed and decays between 0.1 and 10 microseconds. Suggesting that in order for oxygen radicals to be formed inside the plasma reactor, rather than ozone, the residence time inside the plasma should be 0.1 to 10 microseconds. These models also generally show that steady state is reached after 0.1 seconds, although ozone generation reaches a maximum after 10-‐5 seconds. These theoretical required residence times give an approximate guideline for the target residence times inside the plasma of the plasma reactor. As stated previously however, the lifetime of O(1D) is in the order of 10s of nanoseconds, the significance of this being that the oxygen radicals need to be transported from the plasma into the water within this time scale which is likely to be why Yamatake et al and Reuter et al tested whether direct oxygen radical injection would be successful as even with very high oxygen inlet velocities to the plasma reactor, such short time scales are unlikely to be achievable.
2.6 Kinetic models of reactions of aqueous ozone
The first kinetic model for the decomposition of ozone was developed by Weiss in 1935 (Beltran, 2005; Weiss, 1935), it has become a focal point for research with a number of different models being developed. The two most widely accepted models are the Tomiyasu, Fukutomi, and Gordon (TFG) model developed in 1985, and the Staehelin, Bühler, and Hoigné (SBH) model in 1982 (Beltran, 2005). The discrepancies between the SBH and TFG models were based on the initiation steps and the significance of certain chain carriers. However Hoigné later suggested that the SBH model does in fact have the same initation step as the TFG model (Hrubec, 1998). Hoigné’s alteration of the initation step implies that other mechanisms outside of the model also take place (Beltran, 2005). The SBH model is generally the most widely accepted, except at high pH where the TFG model is considered more representative (Beltran, 2005). More recently however challenges have been made to the results obtained in the TFG and SBH models because of, “experimental complications and oversimplified evaluation methods”. (Fábián, 2006) Fábián has reviewed the experiments’ methods used in the development of the SBH and TFG models. He argues that due to the instability of aqueous ozone and the limitations of earlier methods that calculated values for some rate constants are incorrect, particularly for the initiation step. The reactions of aqueous ozone are further complicated by impurities that can either stabilise or promote decomposition of the ozone in water. These substances can also contribute to the appearance or inhibition of free radicals. (Staehelin & Hoigné, 1985). There have been countless publications identifying mechanisms by which ozone, occasionally with UV or titanium dioxide, react with other species in aqueous systems. However a broad, flexible model for the disinfection of water has not been developed. This is not surprising given the infinite quantity of variables to be considered.
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Nöthe et al studied the effect of UV ozonation on micropollutants (Pharmaceuticals, human care and technical products), they found that at low ozone concentrations of 5mg/L that only the most reactive micropollutants (k> 3x10-‐3 M-‐1 s-‐1) are destroyed, where as at concentrations of 10mg/L the much less reactive micropollutants (k = 300 M-‐1 s-‐1) are oxidized. They add that usually the very first reactive step suppresses the biological activity of a pollutant. In addition they state that electron rich compounds such as phenols, amines and alkoxylated aromatics enhance the formation of hydroxyl radical, which is less selective than ozone so enhances disinfection (Beltran, 2005; Nöthe et al, 2009). It is well established that at a higher pH the rate of ozone decomposition is accelerated due to the presence of hydroxyl ions (Yershov et al, 2009; Ignat ́ev et al, 2009). Yershov et al tested the rate of decompostion of ozone over a range of pH, temperature and with the presence of addtives. They found that over the range of pH tested the order of the reaction did not change, suggesting that the mechanisms of decomposition do not change from pH 4 -‐ 8. The addition of carbonate ions, phosphate ions or hydrogen peroxide is also an area that has been extensively studied, as dissolved CO2 or phosphate buffers used to maintain pH can inhibit ozone decopmposition. Ignat ́ev et al used mathematical modeling techniques to find the apparent rate constants of ozone decomposition and hydroxyl concentration in the presence of compounds (Hydrogen peroxide, phosphate and carbonate) that can inhibit or accelerate the reactions of ozone and water.
2.7 Ozone and Ultraviolet light
Ultraviolet light results in photolytic reactions of ozone and causes the formation of hydrogen peroxide and hydroxyl radicals, for which the mechanism was first discussed in 1988 by Peyton and Glaze. Similarly to the SBH model O3-‐ was identified as the main charge carrier (Peyton & Glaze, 1988) although clearly the initiation steps were different because of the addition of UV light. Peyton and Glaze’s experiments used light with a wavelength of “primarily” 254nm, this will differ from this research which will use the plasma reactor itself as the UV source. Previous research on the experimental reactor by Dr Lozano-‐Parada produced a UV-‐vis emission spectrum for an air plasma at atmospheric pressure shown in figure 2.
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Figure 2: Emission Sectrum from the microplasma reator using air (Source: J. Lozano-Parada)
It is immediately clear from figure 2 that the UV light is emitted over a range of wavelengths at varying intensities. It is also worth noting that this emission spectrum is from a plasma reactor using air rather than oxygen, and additionally that any emission of ultraviolet light in the VUV range was not considered. This is likely to be because of the inherent difficulty (and probably expense) of detecting light in the VUV range, as VUV is strongly absorbed by both UV and water. However if light in this range is being emitted, it would likely be of benefit for a water disinfection process as VUV light is known to produce atomic oxygen radicals, and as a result also produce hydroxyl radicals. Oppenlander et al (2004) used a xenon excimer lamp to emit ultraviolet light in the range λ=160-‐200nm into different samples of water containing organic chemicals including: 1-‐heptanol, benzoic acid, potassium hydrogen phthalte and cyclohexanol. Additionally they tested whether rate of TOC mineralization was dependant upon dissolved oxygen concentration by bubbling oxygen through the test solutions. They found that the VUV lamp did cause the oxidation and mineralization of the organic contaminants, significantly they also found that rate of mineralization and oxiation was strongly dependant upon dissolved oxygen concentration. This was attributed to fast scavenging of dissolved oxygen by hydrogen radicals and carbon centered radicals, causing a permanant oxygen deficit within the irradiated area, hence the injection of molecular oxygen removed this deficit. The work of Oppenlander et al is notable because the bubbling of oxygen was found to significantly increase the rate of destruction of organic contaminants in the water in the presence of VUV light. However the VUV light produce by the plasma reactor may have a greater range than that tested by Oppenlander et al. As noted previously Reuter et al used an atmospheric pressure plasma jet, and VUV light down to a wavelength of about λ=130nm was observed, corresponding to the emission spectrum of atomic oxygen. As shorter wavelength photons have higher energies, they can cause the photolytic dissociation of a greater range of molecules as the energy of the
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photon becomes greater than that of molecular bond energies. Clearly the difficulties in direct detection of VUV, particularly in water, would make it difficult to ascertain whether or not the plasma reactor does emit VUV light. However there may be a method of inferring VUV presence by the addition of a chemical that is photoluminescent from excitation in the VUV range. Although this would be a matter for further research if the formation of hydroxyl radicals was found to occur inside the reactor.
2.8 Methods of detecting ozone and hydroxyl radicals
As both hydroxyl radicals and ozone are being investigated in this research, a suitable method of detection capable of differentiating between both species needs to be selected. Locke and Sahni (2006) discuss methods of detecting hydroxyl radicals, two of which are discussed at length; using either dimethyl sulphoxide (DMSO) or terephthalic acid as the indicator. The former method uses the reaction between DMSO and the hydroxyl radical to yield methane sulfinic acid and a methyl radical. The authors state that several DMSO methods are unsuitable as they calculate hydroxyl radical concentration by quantifying either methane sulfinic acid, or the subsequent reaction products methane sulfonic acid or sulphate anions, however this is problematic because of the multiple by-‐products. They suggest that quantification of either the methyl radical or it’s subsequent byproduct formaldehyde is more appropriate. The detection of formaldehyde can either be achieved by fluorescence detection using the Hantzch reaction or derivatization using 2,4-‐dinitrophenylhydrazine and subsequently using HPLC/UV-‐vis detection of the hydrozone, a method developed by the authors themselves. The terephthalic acid method reacts terephthalic acid (TA) in a 0.5mM solution with hydroxyl radicals to form a solution of 2-‐hydroxyterephthalic acid which fluoresces (at a wavelength of 315nm) under light at a wavelength of 425nm. This method of detection is used in the microbubble ozonation experiment carried out by Chu et al described previously. Locke and Sahni used the disodium salt of terephthalic acid (NaTA) as TA is less soluble and as such is usually dissolved in sodium hydroxide prior to use. They reported that the pH of the resultant 2-‐hydroxyterephthalic acid solution should be between pH 6-‐11, as the fluorescence intensity drops outside of this range. Hence sodium hydroxide or sodium bicarbonate was added to adjust the pH as required. Terephthalic acid reacts with hydroxyl radicals in a 1:1 stochiometric ratio to form 2-‐ hydroxyterephthalic acid. Matthews (1980) and Sahni et al (2006) reports that in the presence of oxygen the yield of 2-‐hydroxyterephthalic acid is only 35% of the hydroxyl radical yield. This is caused by the variety of possible reaction mechanisms that occur. An addition reaction of the hydroxyl radical either takes place at the ipso position (15%) of the carboxylate group or the ortho position (85%) as illustrated in figure 3.
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Figure 3: Possible mechanism of hydroxyl radical addition to TA illustrating the ispo and ortho positions
It has also been proposed that molecular oxygen adds reversibly to the hydrocyclohexadienyl radical to form the hydroxycyclohexadienylperoxyl radical. This is followed by removal of the peroxy radical to form 2-‐ hydroxyterephthalic acid in competition with other ring fragmentation reactions as shown in figure 4. These alternative mechanisms account for the 35% yield of HTA in the presence of oxygen.
Figure 4: Oxidation mechanism of NaTA in the presence of oxygen to form HTA and other compounds. Taken from Locke & Sahni, 2006.
Mason et al (1994) describe a method of preparation of both a terephthalic acid solution and a 2-‐hydroxyterephthalic acid solution, they describe using a phosphate buffer to maintain a pH of 7.4, however as Ignat’ev et al found the addition of a phosphate buffer “noticeably affects the decomposition rate of ozone”. So rather than using a phosphate buffer, the pH could be corrected by the addition of sodium bicarbonate or sodium hydroxide to between pH 6 -‐11 after a sample is removed from the experiment. The most commonly used method of detection of aqueous ozone uses indigo carmine and as such it is well documented, a chemical dye that is widely used for the production of clothes. Bader and Hoigne (1981) determined that the reaction between indigo and ozone was stochiometric and very fast (k>107M-‐1s-‐1). They also found that the absorbance of indigo at 600nm against ozone concentration is -‐2.0±0.1×104M-‐1cm-‐1, and the apparent absorption coeffecient of indigo is
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16,500M-‐1cm-‐1 at the absorption peak of 600nm. The reaction scheme is shown in figure 5.
Figure 5: Indigo trisulfonate reaction scheme with ozone and it’s reaction product. Taken from Bader and Hoigne, 1981.
However indigo carmine does not exclusively react with ozone, it also reacts with hydroxyl radicals (Flox et al, 2006) and the superoxide (Kettle et al, 2004), both of which can be produced by microplasma ozone generation, meaning that it cannot be used to detect ozone concentration exclusively. Rather it can be used to test the overall oxidizing rate of the species produced by the microplasma reactor. Alternately an OH radical scavenger could be introduced, which if added in high concentrations will lead to lower hydroxyl radical concentrations. A radical scavenger can outcompete other solutes for oxidation by hydroxyl radicals, (Amy, 1997) hence a radical scavenger can eliminate any indirect oxidation reactions. This would allow the indigo carmine solution to be used to determine the concentration of ozone being formed. Amy states that tertiary butanol is a suitable radical scavenger for such purposes as it is effective at separating indirect from direct oxidation reactions; as opposed to his other suggestion, inorganic carbon as the radical scavenger, which forms secondary radical oxidants. However addition of tertiary butanol was found to affect mass transfer of ozone from the gaseous to the aqueous phase (Tizaoui et al, 2009). T-‐butanol reduced surface tension and viscosity by up to 4% and 30% respectively. Additional effects from the addition of t-‐butanol include increased bubble hold up and significantly decreased bubble diameter. These affected properties were correlated to the observed enhancement of mass transfer. The findings of Tizaoui et al would have to be compensated for in attempting to infer ozone
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concentration from discoloration of a chemical dye if t-‐butanol was used as a radical scavenger. As previously mentioned (see Oxygen Radicals for Water treatment and Yamatake et al, 2006), acetic acid can be used as a chemical probe to measure concentration of O and OH radicals being formed from oxygen. Whilst this method does not measure OH radicals exclusively, it does measure species, such as hydroxyl and oxygen radicals, with an oxidation potential greater than 2.40V (Oxidation potential of ozone is 2.07V). The acetic acid is broken down into carbon dioxide and hence TOC concentration decreases, which is measured using a TOC analyzer. Despite the simplicity of this method, the very high price of a TOC analyzer makes it an inappropriate testing method for preliminary experimentation.
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3.0 Hypothesis Changing the volumetric flowrate of oxygen into the plasma reactor will increase the rate of disinfection. There are a number of reasons why this is likely to occur: 1. If the conversion rate of oxygen to ozone (or hydroxyl radicals) is either maintained or increased at increasing flowrate, then there will also be an increased volumetric flowrate of the reaction products out of the reactor. So reactive species will be produced at a higher rate, and hence rate of disinfection will also increase. 2. Ozone formation is favoured at low temperatures. Higher volumetric flowrates will increase the rate of heat transfer out of the reactor, which will maintain lower gas and plasma temperatures. 3. Very short residence times in the plasma reactor will form unstable oxygen species such as O(1D) that react with water to produce hydroxyl radicals which are widely acknowledged to be more powerful disinfectants than ozone. 4. Once the gas is ionized, the reactions proceed for a short period of time without further ionization, i.e. a burnout time. If the volumetric flowrate of oxygen into the reactor can increase the outlet velocity of the gasses sufficiently for the burnout time to exceed the time it takes for the gasses to reach the water interface, then potentially higher yields of hydroxyl radicals will be formed. 5. As changing flowrate through the reactor will influence the reaction products formed, the plasma reactor will emit light at different wavelengths for each flowrate. Whilst the wavelengths of UV light emitted are likely to be similar, there will be variation in the intensities of light emitted at specific wavelengths. There is also some possibility that light in the Vacuum UV range (λ=100-‐200nm) may cause photolysis of ozone and oxygen to atomic oxygen radicals, leading to formation of hydroxyl radicals.
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4.0 Computational Modelling In order to verify the hypothesis, an experiment testing the effect of volumetric flowrate of oxygen through the plasma reactor on the concentration of aqueous hydroxyl radicals and aqueous ozone should be carried out. The reaction mechanism of oxygen to ozone and other reactive species that takes place inside the plasma reactor is believed to be known. This reaction mechanism gives an approximate idea of the residence time of the gaseous mixture inside the reactor to form specific products. By using computational software to calculate the approximate residence time of the gasses flowing through both the electrodes of the plasma reactor and the area between the electrodes and the water interface and comparing the results to the lifetime of species formed the mechanism of their formation, it is possible to estimate which inlet flowrates will be most effective for disinfection. Using this information, an initial idea of which oxygen volumetric flow rates should be experimentally investigated can be estimated. It is important to stress that due to limitations of this model, it has been used only to find an approximate idea of residence time corresponding to each volumetric flowrate. Using the computational modelling software, Comsol Multiphysics 4.1, a geometry was produced which replicates the important parameters of the micro plasma reactor being evaluated. This geometry is shown in figure 6 with the dimensions and boundary conditions given, note that any unlabelled boundary in the diagram is a wall boundary.
Figure 6: Comsol geometry showing dimensions and boundary conditions. Any boundary that has not been labeled is a "Wall boundary"
Figure 7 shows a comparison between the Comsol geometry and a CAD drawing of the microplasma reactor:
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Figure 7: Comparison between Comsol geometry and CAD drawings of the microplasma reactor
The model uses an axisymmetric geometry consisting of 4 connected rectangles, corresponding to the inlet space, the plasma space and an additional space which connects the inlet and plasma spaces. The plasma space is kept separate from the connecting space so that additional physics models can be applied here as required. As low Reynolds number are being used (Re
