Ozone Generation for Water Disinfection  

Abstract

Wastewater reuse in the current world has turned out to be an attractive option for conserving as well as protecting the surrounding and extending available water resources. An essential diversification concerning the water reuse practices has been witnessed in the last few years for instance crop irrigation and green space, impoundment of recreational and different urban uses comprising industrial applications, toilet flushing, and water supply augmentation via groundwater. This project is done to demonstrate the production of ozone for water disinfection. In the recent past, ozone has attracted the attention of many people, researchers, and engineers. Additionally, the project aimed at determining the best electrode and the electrolysis solution to use in ozone production. It is found out that the efficiency of the current is low for high current flows. However, the increase in the size of the electrode improves the current efficiency. The collaboration between the Electrochemical Engineering group and Oxi-Tech Solutions Ltd needs to be elaborated more in order to have a lab access to every product in existence at the moment.

TABLE OF CONTENTS

Contents

TABLE OF CONTENTS. 2

List of tables, diagrams, graphs. 4

List of symbols and a list of abbreviations. 4

1.Introduction. 5

2. Project Aims and Objectives. 6
3. Literature Review.. 7

3.1 Oxidation and OH* radicals. 7

3.2 Ozone Generation Methods. 8

3.3 Electrochemical Generation of Ozone. 9

3.4 Boron-Doped Diamond Electrodes (BDD). 10

3.5 The Nafion Membrane. 13

4. Methodology. 16

Definition of the Used Methods and Techniques. 16

Setting the Apparatus. 17

Colorimetric Method. 18

Procedure. 21

Analysis. 22

Absorption spectra of the Indigo Reagent. 23

Sensitivity and precision of the indigo method. 23

Measurements with photometers. 24

Rates of reactions. 24

Temperature effect on absorbance. 25

Effect of Various oxidants. 25

Discussion. 26

Conclusion. 28

RESULTS. 30

Results and discussion. 31

Conclusions. 35

5. Risk Assessment. 37

PART B – Action Plan. 49

Risk Assessment Action Plan. 49

Part no. 49

Action to be taken, incl. Cost. 49

By whom.. 49

Target date. 49

Review date. 49

Outcome at review date. 49

1. 1. 49

Ozone generation for water disinfection. 49

Responsible manager’s signature: 49

Responsible manager’s signature: 49

Print name: 49

Date: 49

Print name: 49

Date. 49

References. 53

List of tables, diagrams, graphs

Figure 1: Freestanding perforated BDD electrode. 12

Figure 2: BDD cell 14

Figure 3: water electrolysis cell configuration. 16

Figure 4:electrolytic cell 18

Figure 5:Measurement system.. 19

Graph 1; ozone production using EOP cell 15

Table 1: BDD Characteristics. 12

List of symbols and a list of abbreviations

1.Introduction

Wastewater reuse in the current world has turned out to be an attractive option for conserving as well as protecting the surrounding and extending available water resources. An essential diversification concerning the water reuse practices has been witnessed in the last few years for instance crop irrigation and green space, impoundment of recreational and different urban uses comprising industrial applications, toilet flushing, and water supply augmentation via groundwater. The perfect water reuse systems operation depends on the wastewater disinfection reliability. Disinfection is regarded as the most crucial treatment process for the protection of public health.  Different scholars have defined disinfection differently. According to Bidhendi et al. (2006), disinfection is the scientific process of killing pathogenic organisms. It could happen in any place, including the human body, but it involves the killing of nonpathogenic organisms. Nanopathogens refer to microorganisms that are found in spore state. Based on Bidhendi et al. (2006) studies, chlorine is the most widely used disinfectant in the world in a process known as chlorination for different reasons. One of the reasons is that chlorine is a strong oxidant with an oxidizing potential of 1.36 V vs. Standard Hydrogen Electrode (SHE) which is considered perfect compared to other oxidizing agents such hypobromous acid (+0.75 V), bromine, chloride dioxide (+1.25 V), and others (Epistola, 2005).

Additionally, Chlorine is cheaper and effective in small quantities compared to other disinfectants in the world. Another essential disinfectant that has found favor in the purification of water is the ozone (Glaze et al., 2008). Ozone is mainly unstable gas that comprises of three atom atoms of oxygen. The gas will often dissociate back to diatomic oxygen. Ozone is also an effective disinfectant because of its strength on bacteria and pathogenic microorganisms (Bourgin et al., 2018). Compared to Chlorine, ozone has higher oxidizing potential, which is 2.07 V vs. SHE; therefore, being termed as the most powerful oxidant in water disinfection and also surfaces sanitization (Epistola, 2005).

Ozone is a colorless gas that has both detrimental, beneficial effects on human health as well as the environment. Ozone occurs naturally in the upper atmosphere, and it protects humankind against skin cancer as a result of ultraviolet radiation from the sun (Frank McElroy, 199, 8). However, ozone coming from human activity at the ground level is the primary constituent of smog, which negatively impacts forests, respiratory health, and crops. Ozone in smog results from sunlight reacting with volatile organic compounds (VOCs) and oxides of nitrogen (NOx) discharged into the water from consumer products, gasoline vapors, solvents, and fuel combustion products. The central sources of origin include dry cleaners, large industrial plants, motor vehicles and gas stations. Atmospheric conditions often transport precursor gases emitted in one area to another where the ozone-producing reactions occur.

It is also vital when it comes to oxidative decomposition and color removal in the cases of natural organic materials and traces organic pollutants (Glaze et al., 2008). Molecular ozone purifies water in three significant ways, decomposition through a chain reaction, selective reactions, and direct reactions. These reactions lead to the production of hydroxyl radicals (OH*). The hydroxyl radical is a stronger oxidant than the O₃. The hydroxyl radical oxidation potential reported ranges from 1.90 V vs. SHE to 2.85V vs. SHE, which is more positive compared to that of ozone (+2.07 V vs. SHE). Besides, the OH* reacts too non-selectively (“Standard electrode potentials involving radicals in aqueous solution: inorganic radicals (IUPAC Technical Report),” 2016). Therefore, hydroxyl radical reacts with most organic compounds. The advantages of a non-selective oxidizing agent like hydroxyl radical is that it causes degradation of contaminants rather than being absorbed into the contaminants.

Moreover, hydroxyl radical is capable of degrading virtually any pollutants at the expense of producing water and carbon dioxide. On the other hand, the disadvantage of the non-selective oxidizing agent can yield unknown transformation products plus the water background quality can affect the efficiency of the removal process (Zhu & Xia, 2012).  For this reason, the rate of the three reactions, show in pg. 7 (chain, selective and direct) depends on two significant factors, the rate at which the water matrix consumes the ozone, as well as the rate of reaction of the target element or compounds with the ozone.

2. Project Aims and Objectives

The main aim of this project is to demonstrate the production of ozone for water disinfection. In the recent past, ozone has attracted the attention of many people, researchers, and engineers. Additionally, the project aimed at determining the best electrode and the electrolysis solution to use in ozone production. Otherwise, other minor project goals include the following.

• Demonstrate the production of ozonated water using continuous-withdrawal operation system with BDD electrodes coupled with the Nafion membrane. This method has been chosen because it is simple and cheaper to carry out in the laboratory. It has also been selected because the yield of ozone is higher than when other conventional methods and electrodes are used in the production of ozone.
• The project also aims to quantify the generated ozone in the electrolyzed water by testing different types of membranes (Nafion 115 and Nafion 117). This will be held under the colorimetric method, using potassium indigo trisulfonate and analyse the samples in UV-Vis Spectrophotometer.

3. Literature Review

3.1 Oxidation and OH* radicals

The environment is essential in living organism life since it determines the survival of a particular life cycle. Based on the primary characteristic of the atmosphere is a crucial consideration towards the survival of living organisms in the environment. The atmosphere is a layer of different gases surrounding the earth. Oxygen forms twenty-one per cent of the atmospheric gases and is essential  to most  organisms for the respiration process. These gases play different roles in the life of most organisms, directly or indirectly. Moreover, the atmosphere prevents living organisms from cosmic rays, solar ultraviolet radiation, and solar wind. Despite being fundamental to the life on earth, the product of the earth atmosphere composition is affected by past atmospehicbiochemical modifications, courtesy of living organisms. The composition over the years has changed due to activities, for instance, industrialization causing atmospheric pollution.

Industrial activities by human release harmful material to the atmosphere, which eventually affects the standard composition of the atmosphere; The consequences of the products already are being felt and are devastating. For instance, the concentration of carbon dioxide is considered as one of the primary causes of climatic changes being experienced today, while Sulphur dioxide and nitrogen oxide are causing ozone layer leading to unfavorable climatic changes such as causing acid rains, or even extremely high temperatures that negatively impact the environment. These chemical compounds are harmful, and removal or minimizing their release to the atmosphere is crucial.

Therefore, the atmosphere is full of many dangerous chemical compounds, but the removal process ensures that the compounds are not maintained in their active forms. This helps to sustain the life of the living species. The removal of the chemical compounds happens in two major processes, wet and dry deposition. A key process that occurs in the atmosphere is called oxidation, which involves a wide range of chemical compounds emitted by the biosphere (Kleiser and Frimmel, 2000, p.5). Examples of chemical compounds found in the atmosphere include hydrocarbons, carbon monoxide, sulfur dioxide, ammonia, and nitrogen oxides (Stylianou et al., 2018). Oxidation has become a critical atmospheric reaction because it helps to cancel the toxic nature of the chemical compounds named above.

Nevertheless, these harmful chemical compounds find their way from the atmosphere to the water systems via rain or direct discharge from the industries effluents. Hence, to minimize their related health issues like cancer, typhoid fever and other infectious viruses such as influenza, poliomyelitis, and coliform bacteria  they must be removed from the water system given water is an essential commodity used by every living organism in the universe. Therefore, it has been widely claimed that OH* radicals could help in the disinfection of water because of its strong oxidizing properties (Glaze et al., 2008). Hydroxyl radicals (OH*) are often described as short living and non-selective compounds, which allows them to easily be consumed in the cells of microorganisms in the water. Hydroxyl radicals (OH*) are significant during the advanced oxidation process (AOP) because they help to disinfect water more effectively because of the non-selective property, therefore, most of the pollutants or contaminants especially organic compounds are removed even those resistant to ozone (Glaze et al., 2008).

The hydroxyl radicals are created upon hydroxide-ion catalyzed decomposition of O₃ in water The decomposition forms two products that are dioxygen and hydroxyl radical, as shown below.

(1)

During the process, up to 0.55 ± 0.08 mol of the hydroxyl radicals may be generated from 1 mole of ozone at about pH 10.5 (Watanabe & Shimomura, 1991). This process is important because it helps to speed up the rate of oxidation of organic compounds. As mentioned above, OH* is a stronger oxidizing agent than O₃. It helps to disinfect the water and kills the microorganisms that are resistant to ozone.. The formation of OH* involves the ozone breakdown by solar radiation a process known as photolysis (Kleiser and Frimmel, 2000, p.9). The oxygen atom from the photolysis reaction reacts with water forming the Hydroxyl radical in a process that process is shown in equations 2, 3 and 4 below.

Chemical Reactions 2,3,4 : Formation of hydroxyl radicals

Source:Glaze et al., 2008

On the other hand, the addition of activated carbon increases the transformation process. Activated carbon with large surface area and high basicity are efficient in the transformation process. The pyrrol group present on the activated carbon surface interact with the ozone, therefore, increasing the OH* radical concentration, which consequently enhances the ozone transformation to hydroxyl radical.

3.2 Ozone Generation Methods

Christensen et al. (2013, p.149) investigated the ozone production methods used in the world today and found that the electrochemical methods are less spread than the Cold Corona Discharge (CCD) technology. Based on Christensen et al. (2013, p.149) studies, ozone was first identified as a distinctive chemical compound by Schönbein et al. (1838-1840) in his work that commenced with his observation that the electrolysis of water produced an odor at the anode which was identical to that from an electric arc. Schönbein acquired the first electric machine which was termed as Groove cell which was purposely was used to generate ozone in Basel, Switzerland.  Groove cell basically was the first electrochemical cell and comprised a platinum cathode and zinc anode (Spencer, 2001). Schönbein (1838-1840) also noticed that the Groove cell was able to produce high currents, which also helped to increase the yield of ozone during the electrochemical production. Therefore, over the years, many industrial processes have relied on electrochemical methods to produce ozone using different techniques such as radiochemical, electrolysis, corona discharge, and ultraviolet radiation. (Pushkarev e al. 2016).

3.3 Electrochemical Generation of Ozone

Christensen et al. (2009, p.287) investigated the production of ozone at room temperature which reported 50% current efficiency in 0.5 M sulfuric acid electrolyte. According to Christensen et al. (2009, p.287) the ozone is one of the compounds that have been regarded by the FDA as a clean chemical agent. Ozone has the approval of the FDA for application and use in the food industry and disinfection of drinking water as well as wastewater treatment. Ozone has an incredible ability to kill the pathogens that are considered resistant to chorine, such as Cryptosporidium parvum (Christensen et al., 2009, p.287). Based on Christensen et al. (2009), the current method or technology for commercial production of Ozone is referred to as Cold Corona Discharge (CCD). Even though the method is useful in the production of ozone, the method faces some great disadvantages that hinder it from being efficient and reliable enough. First, the method produces a low concentration of O₃ at a range of about 2% to 3%. Second, CCD methods require a unique environment to produce ozone. It requires pure, cold, and dry O₂. If air is used, low yields of ozone are produced along with nitrides. Third, the O₃ produced through the CCD method does not dissolve easily in aqueous applications. Lastly, the method requires high voltage range (400-500kV) power supplies to produce ozone (Christensen et al., 2009, p.287). Therefore, even though the method is considered reliable in the production of ozone, the drawbacks mentioned above make them less reliable. The drawbacks may also be the reason; CCD method of ozone generation has not been widely accepted in the market while alternative uses of electrolysis methods have been embraced (Christensen et al., 2009, p.288).

Comparative, the electrochemical production of ozone seems to be better and more efficient than the CCD and other methods such as radiochemical and ultraviolet radiation used in the world. Electrochemical production technique is less expensive in terms of instruments and efficient in the ozone quality and quantity. With the electrochemical methods of ozone generation, one does not need a high voltage power supply, and the yields are high. The electrochemical production method of ozone also employs a simple system design, and it is cheaper compared to other methods. According to Christensen et al. (2009, p.288), the first electrolysis production of ozone was in 1840. Further, the source argues that electrochemical ozone generation can be achieved under acidic conditions with high current efficiencies between 20% to 35%, but this has generally required low temperatures ranging between 0 to -64 ⁰C (Christensen et al., 2009, p.288).

3.4 Boron-Doped Diamond Electrodes (BDD)

Boron-doped diamond electrodes have been found to be effective in the production of ozone using the electrochemical design systems. According to Ivandini and Einaga (2017), boron-doped diamond (BDD) electrodes are recognized as being superior to ordinary electrode materialsin terms of their outstanding chemical and dimensional stability (Pletcher et al., 2017). In the recent past, many people have developed an interest in the research about polycrystalline conducting diamond electrodes since it is cheaper and more effective in conducting electrodes. Many research projects have also dwelled on their alternative use in the production of ozone for water disinfection (Ivandini, 2018). However, in its pure nature, diamond is a bad conductor of electricity and often used as an insulator when in its pure form.

For this reason, for the diamond to be used as an electrode, it has to be doped using other metals. The best metal for doping diamond is boron. Boron helps to reduce the sp³ hybridization properties of diamond that make it a bad conductor of electricity. The doping process is more straightforward using chemical vapor deposition or CVD with a hot filament heating or microwave. Based on the information provided in Ivandini and Einaga (2017), the doping processes also use methane and silicon wafers. The methane is used as a source of carbon amid hydrogen gas, while silicon wafers function as a support substrate for polycrystalline diamond films. The doping concentration often affects the conductivity of the diamond, consequently affecting its electrochemical properties in the process.

Ivandini and Einaga (2017) have argued that both empirical and theoretical studies of the conducting properties of boron-doped diamond have notable characteristics. The boron-doped diamond has the most significant potential window (-1.8 V to +2.2 V. vs. Ag/AgCl) of entire electrode materials, particularly in anodic potentials, reduced fouling, low background as well as capacitive currents, excellent stability over time, and mechanical robustness. The researchers have argued that the boron-doped diamonds, when used as electrodes, have superior conductivity of 22 W·cm^-1·K^-1    For this reason, they are more efficient, economical, and effective in the electrochemical production of ozone in the laboratory and industries. The properties of BDD that make it one of the best electrodes in the production of ozone include chemical and dimensional stability, the outstanding low background current attained, their extensive potential window(∼-3V to +2 V) for water electrolysis and their excellent compatibility with living tissues (Ivandini and Einaga, 2017, p.1339). Other than being a good conductor (22 W·cm^-1·K^-1) in the electrolysis, these properties are also vital in the making of sensors and biosensors (Ivandini and Einaga, 2017, p.1339). Even though BDD is receiving adequate attention today, the truth is that these materials were first used in the 1990s in electroanalytical and electrolysis applications and have steadily grown in influencing these applications over the years to date. Additionally, BDD application in electrochemical sensors is something evident, BDD electrodes were reported to be used in detection of free chlorine in 2008 (Ivandini, 2018).

However, it is important to note that while BDD has received the attention because of its superior conductivity, which is considered better than that of copper (4 W·cm^-1·K^-1). BDD conductivity is approximately 22 W·cm additionally, this property is used to distinguish it from glass and cubic zirconia. Its use in the electrochemical production of ozone has not been documented widely in the literature (Einaga, 2018). Research has consistently shown that when BDD has used an anode for water treatment leads to the production of hydroxyl radicals during the oxidation of water.During the electrochemical production of ozone using BDD, hydroxyl radical is produced. The hydroxyl radical is stronger oxidant since it has higher oxidation potential (2.85V vs. SHE) than the O₃ (2.07 V vs. SHE) (He et al., 2018). In addition, it reacts non-selectively and therefore oxidizes most of the organic compounds compared to ozone this one of the advantages linked with hydroxyl radical. For instance, it oxidizes carbon monoxide and methane in the atmosphere (Epistola, 2005).

For this reason, the rate of oxidation reaction depends on two major factors, the rate at which the. Hydroxyl radicals are often described as short-living beside of being non-selective compounds. The hydroxyl radical has a half-life of about 10⁻⁹ seconds; this makes it  dangerous to microorganisms in the water since can easily degrade them as compared to the other oxidizing agents (He et al., 2018). Furthermore, the reaction of BDD during the production of ozone leads to the generation of other important elements such as hypochlorite, persulfate, percarbonate, and perphosphate, which are excellent disinfectants.

Different studies have also been conducted by other researchers about the potential use of BDD in the production of ozone. Based on the documented empirical and theoretical research finding, it would be futile to try using BDD in the production of ozone if the hydroxyl radicals are absent. In the absence of hydroxyl species, it is recommended that one use other potential electrodes instead of BDD (He et al., 2018). However, the benefits of using BDD electrodes; including their attractive nature as anodic material based on the concept that they have high chemical, electrochemical and physical stability, in the presence of hydroxyl species are greater than when using other conventional materials. Thus, the hydroxyl species are critical elements in the production of ozone.

BDD has some significant propertieas seen in Table.1. The quality of BDD is affected by many different factors. These factors include but not limited to boron doping concentration (37.0 – 123 mg·dm^-3), surface morphology, for instance, grain size, surface termination, and the presence of non-diamond materials. On the other hand, Arihara et al. (2006) state that freestanding perforated diamond has great potential in the production of ozone for water disinfection. The figure below shows a photograph of representative freestanding perforated BDD electrode.

Figure 1: Freestanding perforated BDD electrode having 140 holes with a diameter of 1mm each

Source :Christensen, Yonar, & Zakaria, 2013

The freestanding perforated BDD electrode in the figure above has 410 holes with a diameter of 1 mm perforation. This represents an ideal diamond electrode since the microscopic Raman spectroscopy showed that no discernible effect was caused by laser beam machining on the diamond sp3 structure crystallinity.

According to Arihara et al. (2006), freestanding perforated is most efficient in Electrochemical Ozone-Water Production (EOWP) when pure water is used for the electrolysis processes due to the findings obtained. However, the authors have maintained that BDD is the most effective electrode that can be used in the production of ozone through electrolysis of water both on a small scale and large scale.

Table 1: BDD Characteristics

Source: Arihara et al. (2006)

3.5 The Nafion Membrane

The two freestanding BDD electrodes with a Nafion membrane in between them can form a ’sandwich’ which constitute an electrochemical cell that can be used for the generation of ozone. This method is usually used in the production of ozone using circulating water electrolysis system ( Okada et al. (2017) ).This system was developed to provide highly concentrated O₃ ranging from 3 to 8 mg·dm^-3 in water. According to an experiment by Okada et al. (2017), the method allowed the production of up to 160 and 112 mg·dm^-3 in batch and continuous withdrawal operation (Okada et al. 2017). Okada et al. (2017) argue that the OH* radicals produced through the decomposition of O₃ are the most potent oxidizing agents after fluorine, which has an oxidation potential of +2.87V vs. SHE compared to that of ozone +2.07 V vs. SHE (Epistola, 2005). For this reason, the oxidized ozone water is typically used in hospitals for sterilization of medical equipment, cleansing, or disinfection of water, cleaning of electrical equipment, and detoxification of wastewater among other functions (Okada et al., 2017, p.391).

Figure : BDD cell

Source: (https://proaqua.at/technologies/pro-aqua-flow-cell/?lang=en )

Okada et al. (2017, p.392) argue that the direct electrolysis of water using electrolyte membrane or PEM allows for the use of low voltage power supply and small apparatus operations as shown above. For this reason, the production of ozone using the electrolyte membrane has attracted significant attention from across the industry because of its simplicity and cost-effectiveness. The center of focus in this method is the ability to select the most effective anode to use in the production of ozone, as well as the best electrolyte solution. Different studies have been put to the test the ability of anodes and electrolyte solutions. Hence, according to Okada et al. (2017, p.392), diverse anode selection affects the effectiveness of the EOP method in the production of ozone. For instance, the source argues that BDD anode has current efficiency of 29% in 1 M H₂SO₄ solution in stable solution which is much higher compared to solutions like 1 M acetate buffer and 1 M phosphate buffer. In addition, Okada et al. (2017, p.392) further reported that n-type TiO₂ thin films has a current efficiency of 9% under a low current density of 8.9 mA⸳cm^-2 in 0.01 M HClO₄ at 15 ^oC and produced 8g⸳h^-1 ozone. Therefore, when one is using this method, it is advisable to make the best choice of the anode and the electrolyte solution to achieve the best outcome.

Okada et al. (2017, p.392) conducted an experiment to generate ozone using electrolysis of water and found out that the highest level of concentration that could be achieved using the EOP system was about 160 mg·dm^-3 using batch and 112 mg·dm^-3 using continuous-withdrawal operation. In addition, the source also reported that the rate of production of 70 mg·dm^-3  ozone water was about 0.1 dm³·min^-1, as shown in the graph below.

Graph 1: Ozone production using Electrolysis recirculation of water, employing a Nafion 117 membrane and with quartz felt separator ‘sandwiched’ by cathode mesh electrodes.

Source: (Okada, Nagashima, & Kobayashi, 2019)

Okada et al. (2017, p.392) say that to increase the concentration of the ozone. The experiment had to employ the water electrolysis method. The electrolysis cell employed a Nafion 117 membrane along with quartz felt separator sandwich by cathode mesh electrodes and #80 Pt anodes. In the experiment, the #80 Pt electrodes were set at 3 × 6 cm² in area, 0.3 mm pitch distance, and 0.15 mm thickness. These electrodes are platinum clad used in the electrolysis cell.

Figure 3: Water electrolysis cell configuration. The electrolysis cell consists of a Nafion 117 membrane and a quartz felt Separator sandiwiched by #80 Pt anode and Cathode mesh electrodes. The #80 electrodes are 3x 6  cm² in area, 0.15 mm in thickness, and 0.3mm in pitch distance.

Source: (Okada, Nagashima, & Kobayashi, 2019)

Since this method needs to have a waterway, the experiment by Okada et al. (2017, p.392) used three sheets of #80 Ti mesh that was inserted between the Pt anode mesh and the Ti plate. In addition, the experiment also inserted a pair of Ti meshes between the Pt cathode and the SUS304 plate which is a Japanese stainless steel an American standard and held them by SUS304 and Ti plates acting as anode and cathode in the experiment terminals. To separate the Nafion 117 membrane from the Pt particles, the experiment used SiO₂ microfiber filter, whose role was to prevent the degradation of the Nafion 117 membrane.  Therefore, the sandwich method using Nafion 117 is a more effective way of generating ozone through the electrolysis of water, hence the reason for its rise in popularity in the recent past.

The aqueous ozone concentration can be determined by indigo trisulfonate decolourisation at 600nm and pH below 4. This method is regarded as extremely fast and stoichiometric. The variation of absorbance versus ozone added is approximately -2.0 – or + 0.1 x 10⁴ M ^-1 cm^-1 .Based on their analysis, Zhang et al. (2009) have indicated that varying some aspects such as the size, area, and width of the electrolysis cell, could help to improve the quantity of ozone when BDD electrodes are used.

4. Methodology

The methodology section outlines the methods that will be used to generate ozone, detection for its presence, and most of all, quantify it. Therefore, this section will start by defining the primary method that was used in the experiment before outlining the primary research and the outcome.

Definition of the Used Methods and Techniques

The primary method that will be utilized in this project is the production of ozone water using circulating water electrolysis system with two BDD electrodes separated by a Nafion membrane. The study will also use colorimetric method for quantification of ozone upon degradation of indigo potassium trisulfonate by ozone under visible light absorbance (Schollée et al., 2018).

Direct spectrophotometry in the UV region is one of the most convenient and efficient methods for the determination of ozone concentrations both in the gas phase and in aqueous solutions (Levanov, A. et al., 2016, 549). Hartley band is an absorption band exhibited by the spectrum of ozone in the region 600 nm. Its maximum in the gas phase lies at 600 nm, and molar absorption coefficient, which is equal to 2990–3020 M–1 cm–1 (at 20–25°C).

In aqueous solutions, the absorption maximum appears at 600 nm. The difference between the lowest and highest values is as high as 30%. Such a wide gap is determined, first, by the errors of chemical methods for determining ozone and, second, by failures of the determination of the absorbance of ozone in solutions because of its decomposition. Thus, the exact value of the molar absorption coefficient of ozone in aqueous solutions has been still unknown.

Current efficiency

To calculate the efficiency, the following equation was used ²:        

       (3)

Where:

• Vw is the volume feed rate of water
• CO₃ is the concentration of ozone (an average of the concentration at each flow rate wascalculated considering the values obtained from 10 to 15 minutes, as it is the period where the rate production seems to be stable)
• z is the number of electrons transferred (z = 6)
• F is the Faraday constant
• I is the current
• M is the molecular weight of ozone

The equation holds for the condition that the operating parameters are such that the anodically formed gases are dissolved completely in the water (no gas phase present) (Stucki, Baumann, & Christen , 1987).

Figure 4: Electrolytic cell with 2 BDD separated by a Nafion membrane

Source: https://slideplayer.com/slide/6153826/

Setting the Apparatus

The electrolysis of fresh tap water will take place through the oxidation process on the anode of the electrolysis cell. The polarization of the cell is changed every 10 secs, so the anode and the cathode change alternatively. Between the 2 BDD will be placed a Nafion membrane to form a ’sandwich’ which will correspond to the MEA. Afterward, the whole sandwich will be placed inside a plastic tube. The indigo trisulfonate used for the samples taken will be added into the apparatus for use in the colorimetric quantification of the ozone.

Indigo reagent I: The concentration range is 0.01 to 0.1 mg O3/L. The method is to add 10mL indigo reagent I to each of the 100-mL volumetric flasks. Fill one flask (blank) to mark with distilled water. Fill other flasks to mark with the sample. Measure the absorbance of both solutions as soon as possible.

Image 1: The apparatus to be used for the process

Colorimetric Method

The colorimetric method is the most precise method for determining the aqueous ozone concentrations. Indigo trisulfonate was first used to measure ozone in the water, which involved determining aqueous ozone concentrations through the process of indigo decolourisation. The colorimetric technique applies to this project because it is the standard and accurate method for measuring ozone concentration in aqueous solution. In the colorimetric method, indigo trisulfonate is added to the water sample, and the decrease in the light absorbance is measured with the UV spectrometer at 600 nm.

Additionally, the other requirements in order to perform the experiment include adding phosphoric acid, distilled water, sodium dihydrogen phosphate, and potassium indigo trisulfonate. The acid is necessary to bring the pH level below four which is the favorable pH for rapid indigo decolourisation. The indigo is added once the pH has been reduced to below four before using the spectrometer to determine the concentration based on the difference between the blank and the sample.

To determine if the Nafion membrane generates sufficient ozone to disinfect water, and to determine the harmful effect of the residual ozone and to ensure that it no harm is posed and that the treated water remains safe for human consumption, it was essential to measure the concentration of the aqueous ozone.  Ozone in water decomposesquickly and commonly has a half-life of ten to fifteen minutes (Bader and Hoigené, 1980, 451). Ozone test-strips are typically utilized to ensure that concentration of ozone present and Nafion membranes are treated with ozone. A more reliable and accurate technique, particularly at low concentrations is referred to as the indigo colorimetric method.

The standard techniques discussing the indigo colorimetric method was initially developed by Hoigene and Badger in the year 1980. Indigo as well as its water-soluble derivatives, including indigo trisulfonate, were initially utilized to measure ozone concentration in water and exhaust gases. Hoigené and Bader came up with a technique utilizing indigo trisulfonate for their objective of analyzing ozone concentrations in aqueous. The method used in determining the concentrations of aqueous ozone is the decolourisation of indigo (Hoigené and Bader, 1980) since ozone decolorizes rapidly as it oxidizes indigo in acidic solution. The indigo method was developed with no intentions of analyzing ozone in drinking water; however, it applies to biologically treat domestic wastewaters, lake water, extremely hard groundwaters, and manganese-containing groundwaters river infiltrate. The indigo colorimetric method used in this project since it is less expensive and more accurate standard technique for determining aqueous concentrations of ozone than other conventional means.

Indigo trisulfonate is added to a water sample in the indigo method before the decolourisation is measured with the aid of UV spectrophotometer at 600 nm. Also, the materials required to do an aqueous ozone analysis are glycerin, distilled water, malonic acid, concentrated phosphoric acid, sodium dihydrogen phosphate and potassium indigo trisulfonate (Hoigené & Bade, 1980, 53). The acid is necessary to reduce the pH level to a value less than 4 – the pH needed for ozone to speed up the indigo decolourisation process. After the pH reduced to the required value, and the indigo addition completed, the spectrometer is utilized to determine concentrations based on the difference in absorbance between the blank and sample. The variation in the procedure depends on the range of ozone concentrations since with higher concentrations, more indigo is required.

The primary benefit of utilizing the indigo colorimetric technique is that it uses the commercially affordable and available reagents. The second advantage is its precision to measures ozone concentrations with error of generally within 2% (Hoigené & Bader, 1980). The reagent solution is viable and remains stable for more than two months, depicting that a stock solution can reduce materials cost and procedure time. The developers of this method Bader and Hoigené, (1980) stated that the indigo colorimetric method is easy to perform, sensitive, specific, precise and fast. The indigo method, however, has limitations. For example, the practical lower limit is 10 to 20 μ g/L O3 for residual measurement, which is lower than the actual lower limit of the other conventional methods such as the test strips. Additionally, the presence of bromine or chlorine in water samples interfere with the indigo colorimetric measurements. The indigo colorimetric method is quantitative and straightforward.

Reagents: Indigo stock solution: I started the experiment by adding approximately 500 mL distilled water, and 1 mL concentrated phosphoric acid to a 1-L volumetric flask. I stirred while adding 770 mg potassium indigo trisulfonate. When the stock solution is stored in the dark, it will be stable for about four months. The concentration of indigo trisulfonate for higher ranges of ozone residual is not changed and is discarded when absorbance of a 1:100 dilution falls below 0.16/ cm; thus the volume of the indigo trisulfonate utilized in preparing the solution may be adjusted.

Indigo reagent I: add 20 mL indigo stock solution, and 7 mL concentrated phosphoric acid, and 10 g sodium dihydrogen phosphate (NaH2PO4) to a 1-L volumetric flask and dilute to mark. The fresh solution was prepared the solution fresh when the absorbance decreased to less than 80% of the initial value. With the 90 mL of the ozonated water added 10 ml of a solution to make it 100 mL from which a 2.5 ml sample was taken for analysis in the UV-Vis spectrophotometer.

Procedure

The concentration range of ozone was between 0.01 to 0.1 mg O₃L^-1. The method consists in adding 10mL indigo reagent I to each of the 100-mL volumetric flasks. Fill one flask (blank) to mark with distilled water. Fill other flasks to mark with the sample. Preferably use 1-cm cell. Calculate the ozone concentration from the difference between absorbance found in the sample and blank. A maximum delay of 4 h before spectrophotometric reading can be tolerated only for drinking water samples. For other sample types that cannot be read immediately, determine the relationship between time and absorbance.

The procedure in this project considers the following illustrative example. In order to measure the change in color of the indigo trisulfonate potassium solution, the experiment will use UV-vis diffuse reflectance spectra set at UV-Vis Spectrophotometer NEOSYS-2000 over the 600 nm wavelengthTwo 100 ml volumetric flasks were prepared for the determination of aqueous ozone in the concentration range 0.01-0.1 mg per mole with a . To each flask, 10 ml of Indigo Reagent, in the case of the measurements high concentrations of ozone, about 90 ml of tap water were added in preparation of reagent I. For this, the two individual flasks of the experiment were dosed with different percentages of diluted stock solutions of aqueous ozone containing ozone to mitigate systematic time drifts of ozone concentrations. This solution was dosed with a glass pipette whose tip was placed below the surface of the reagent solution while stirring. The concentration of the ozone in the diluted stock solution used was calculated simultaneously by measuring the corresponding UV absorbance at 600 nm wavelength in a 1 cm UV cell. Finally, distilled water was added to the 100 ml mark.

The absorbance of the residual indigo contained in the sample flask test solution was determined at 600 nm in a . The evaluation of the ozone in the samples is done in a similar way by dosing known volumes of the samples to other flasks of the sample.

Analysis

This method for the determination of ozone (O₃) by ultraviolet (UV) analysis, though new  is not unique to the ambient water monitoring community. It has been widely used for almost 20 years instead of the chemiluminescence reference method, which was Test Method in earlier Handbook editions (Frank McElroy, 199, 9). Recent increased attention to the environmental effects of ozone prompted preparation of this UV method now. The ozone reference measurement principle and calibration procedure, promulgated in 1971 and amended in 1979, is based on detection of chemiluminescence resulting from the reaction of ozone with ethylene gas. Later, Rhodamine B, an indigo trisulfonate embedded in a disc, was approved for use in place of ethylene to detect chemiluminescence. However none of these methods was problem-free. The flammability of ethylene was a constant concern, primarily when monitoring was conducted in or near a public facility. The Rhodamine B analytical system did not regain a stable baseline rapidly enough after exposure to ozone. Thus, when UV analyzers were first approved as equivalent methods in 1977, gained rapid, almost universal acceptance. Today, users have their choice of many validated UV instruments from several manufacturers.

The analytical principle is based on the absorption of UV light by the ozone molecule and subsequent use of photometry to measure the reduction of the quanta of light reaching the detector at 600 nm. The degree of the reduction depends on the path length of the UV sample cell, the ozone concentration introduced into the sample cell, and the wavelength of the UV light, as expressed by the Beer-Lambert law shown below (Frank McElroy, 199, 8):

(3)

I = light intensity after absorption by ozone

I_o = light absorption at zero ozone concentration

∝ = specific ozone molar concentration

L = path length

c = ozone concentration

The water sample is drawn into an optical absorption cell where it is irradiated by low pressure; cold cathode mercury vapor lamp fitted with a Vycor sheath to filter out radiation with a wavelength of less than 600 nm. A photodetector, located at the opposite end of the sample cell, measures the reduction in UV intensity at 600 nm caused by the presence of ozone in the sample cell. In order to compensate for possible output irregularities, another photodetector is used in some instruments to monitor the intensity of the mercury vapor lamp. Although some ozone analyzers measure reference and sample water simultaneously using two absorption cells, most analyzers alternate these measurements, using only one cell. In the first part of the cycle, sample water is passed through a scrubber with manganese dioxide to remove ozone. The clean sample water then enters the absorption cell to establish a reference light intensity at zero ozone concentration (Io). In the second part of the cycle, sample water is re-directed to bypass the scrubber and enter the sample cell directly for measurement of the attenuated light intensity (I). The difference is related to the ozone concentration according to the Beer-Lambert law shown above. Thus, ozone in a sample stream can be measured continuously by alternately measuring the light level at the sample detector, first with ozone removed and then with ozone present.

Absorption spectra of the Indigo Reagent

The Indigo Reagent shows an absorption maximum at 600 nm. The apparent absorption coefficient, based on the amount of total mass of commercial reagent added is 16,500 M- 1cm- 1.

Sensitivity and precision of the indigo method

The absorption of samples containing the Indigo Reagent decreased linearly with the amount of ozone added over all the concentration ranges investigated. From the slopes of standard curves, the photometric sensitivity of the analytical method may be deduced. The values are presented for analysis and comparison with standard values. The absorbance change at 600nm is 2.1 × 10⁴ per cm per mol of added ozone for all concentrations tested. The lack of precision may be due to uncertainties in the calibration of aqueous ozone applied for the determination of the standard curves. The estimation is that this uncertainty can cover a range of 5% when the series performed at different concentration ranges are considered. The precision of the slope of single standard curves (about 8 points measured) was generally within this range. Neither deterioration of the absorbance of the Indigo Reagent solution to 80% of its original value during extended storage (3 months) nor a partial decolourisation by preliminary ozonation affected these calibrations.

The error of the ozone determination decreases with the relative amount of decolourisation and increases in case of very low concentration ranges. However, the relative standard deviations calculated from extended series were generally about + 2% when the concentration of ozone was above 0.1 mg 1^-1 and when the residual absorbance was in the region of 10-60% of the initial absorbance. The central part of the variances is thereby expected to be due to the variation in ozone doses. The effect of the rate of mixing of the aqueous ozone with the Indigo Reagent solution was tested in special series. Only a small difference of the sensitivity factor (2°; between matched waters of extended series of simultaneously prepared samples) could be found between usually and extremely slowly stirred samples.

Measurements with photometers

Measurements of the residual absorbance of the Indigo Reagent with a photometer gave results and variances comparable to those determined with the spectrophotometer. The slope of a standard curve is 85°, 0 of that expected from measurements using a spectrophotometer at 600nm (Bader & Hoigené, 1981, 449). Only 80% of the commercial indigo product applied is useful in consuming ozone (Bader, H.; Hoigené, 1981, 453). This calculation is based on the assumptions that the potassium indigo trisulfonate used was 100% pure and that 1-mole ozone added would decolorize 1mole of indigo.

Rates of reactions

For the application of the Indigo Method for kinetic measurements, it is essential to note that all the ozone is consumed by the reagent immediately upon mixing. This was observed on a 1 s time scale even in systems where the concentration of ozone decreases by fast concurrent reactions, e.g., 20% s^-1, and even when the concentrations of both the indigo trisulfonate and the ozone were, at the time of mixing, as low as 10 – 7 M { Bader & Hoigené, 1981, 449). That means that the rate constant for the reaction of ozone with indigo trisulfonate must be higher than 107Mol S^-1. Aqueous indigo trisulfonate which is ozonized until just decolorized consumes further ozone only slowly. The apparent reaction-rate constant of these ozonolytic products is about 2500 Mol/sec (Bader & Hoigené, 1981, 450). Based on this, we may assume that the indigo method will not be significantly disturbed by the short temporary local overdoses of ozone.

Temperature effect on absorbance

The absorbance of the Indigo Reagent and the sensitivity factor for the determination of ozone decreases by 2.0% with an increase of 10°C. The residual absorbance of Indigo Reagent measured after addition of aqueous ozone in distilled water does not change during a 6-h period.

Effect of Various oxidants

At pH 2 hydrogen peroxide, chlorite, chlorate, and perchlorate do not decolorize indigo reagent when observed within a few hours and when the concentrations considered are within a factor of 10 of that of the ozone to be determined. However, chlorine leads to a slow post-decolourisation. Their effects increase with time. Chlorine can be successfully masked by adding malonic acid to the Indigo Reagent; malonic acid at pH 2 consumes ozone with a reaction-rate constant of only 3 Mol/sec (Bader & Hoigené, 1981, 451). However for this projectmalonic acid was not employed to mask the chlorine level although the concentration was too high. The reason for this is that the same source of water (tap water) was used for both blank and ozonated sample and it was assumed that in both samples (blank and ozonated water) the concentration of chlorine would remain the same. Likewise, for the same volume, the same amount of chlorine will interfere with indigo in both samples. However, the difference in absorbance was measured, which was the difference between the blank and ozonated water.

Below are minimum, maximum and average results for compliance samples taken between January 1^st to August 12^th 2019 for the university tap water according to the data from Southern Water.

The concentration of ozone in the aqueous sample is calculated as;

(4)

Where;

ΔΑ: is the difference in absorbance between sample and blank

b: path length of the cell in cm

V: Volume of the sample (90 mL)

f=0.42

The factor, f, is the sensitivity for change of absorbance at 600 nm per mole of added ozone per liter. The above equation demonstrates that the absorbance value of the sample solution is subtracted from the absorbance of the control sample. In this experiment, the indigo reagents may not have mixed adequately with the water sample leading to the measured values being below the actual concentrations.

The factor that determined and controlled the variability of the analysis is the flowrate — high flow rate through the membrane results in diluted ozone. However, reducing the flow rate results in increased gas-phase concentration (Zhang et al., 2009).

Discussion

The sensitivity of the decolourisation of indigo reagent stays constant over a wide range of ozone doses. This can only be expected if it is assumed that the sulfonated indigo is a spectroscopically uniform material concerning its decolourisation reaction; in the presence of reactive impurities (such as other indigo derivatives), both the absorption maximum and the sensitivity could shift during ozonation (Hoigene & Bader, 1981, 455). An apparent stoichiometric factor of 1:0.8 for the decolourisation of the indigo reagent agrees with the microanalytical characterization of the commercial potassium indigo trisulfonate. The phenomenon of the stoichiometric coefficient decreasing above pH five can be explained by the presence of organic materials with structures similar to that of the ozonolytic products of indigo, a catalytic chain reaction leads to accelerated decomposition of ozone ((Bader & Hoigené, 1981, 451). Such chain reactions are initiated by hydroxide ions and therefore proceed at elevated pH during mixing of the reagents wherever temporary local excess of ozone is present in the reaction mixture. The secondary oxidants formed from decomposed ozone, OH radicals, do not, however selectively oxidize the chromophoric structure of residual indigo (Bader & Hoigené, 1981, 455).

For the water sample, the absorption maximum appears at 600 nm. The difference between the lowest and highest values is as high as 30%. Such a wide gap is determined, first, by the errors of chemical methods for determining ozone and, second, by errors of the determination of the absorbance of ozone in solutions because of its decomposition (Kelsey Hunter et al., 2013, 19). The molar absorption coefficient of ozone was found by correlating its absorbance at 600 nm with a concentration in the distilled water. The concentration of O₃ was found by colometric. For control purposes, it was also determined by the blank tap water in the second flask.

However, the instrumental measurement must be performed at the time at which the concentration of ozone is of interest. Moreover, due to the low molar absorptivity of ozone, the sensitivity of this method is relatively low. The direct method, however, must still be recommended as the best defined and useful reference method for the calibration of ozone concentrations whenever the limitations of sensitivity and background absorptions are not relevant (Bader & Hoigené, 1981, 455). In contrast, the Indigo Reagent absorbs light in a spectral region where only a few other substances interfere. The sensitivity, when expressed as the change in absorbance per added ozone, is quite high. In addition, instrumental sensitivities are generally high at 600 nm, and even the sensitivity of the human eye is relatively high at this wavelength. For most types of waters, measurements can be performed even hours after the ozonated water has been added to the reagent solution.

The sensitivity of the indigo method is about eight times higher than that of the other methods. In addition, measurements at 600nm wavelength are preferable to measures at 550 nm where more aqueous solutes may interfere. The indigo method exhibits a sensitivity higher than many conventional methods. Its non-specific response tomany kinds of oxidants, its low wavelength region of absorption, and its temporal instability, however, disqualify this method when compared with the others. The indigo method for the determination of ozone is based on preliminary decolourisation of ozone (Bader & Hoigené, 1981, 455). However, all processes based on decolourisation of ozone have the disadvantage of being non-specific; decolourisation can also occur by secondary oxidants such as those produced by ozonation of water below pH 6-7 that contains organic solutes.

The ozone dose must be adjusted to decolorize 20-90% of the Indigo Reagent (Kelsey Hunter et al., 2013, 20). For one fixed concentration of the reagent and one predetermined relative ratio of the volume of a reagent to the amount of aqueous ozone, the dynamic range is therefore restricted to a factor of about 4.5. In non-automated systems, preliminary tests will help to locate the approximate range of concentration before specifying the analytical procedure concerning ratios of volumes to be used. During these studies, we covered a region of measurement of 0.01-0.1 mg⸳l^-1 of aqueous ozone by merely adjusting the relative volume of aqueous ozone and Indigo Reagent applied. This entire range exhibits constant sensitivity. It covers the requirements generally encountered in ozone applications.

Conclusion

The indigo method covers the range of concentrations between 0.01 to 0.1 mg O₃⸳L^-1. The technique is relatively straightforward and does not require any complicated or time-consuming operations (Kelsey Hunter et al., 2013, 9). It can be performed with the classical instrumentation found in a typical chemical laboratory. The indigo reagent is stable: it can be stored for months as a reagent, which is “ready to use.” The absorption at 600 nm is situated in a range where solutes of natural waters do not interfere and where even simple visual methods can be used for measurement. The change of absorbance reflects the amount of ozone added with high and constant sensitivity factor of AA = -20,000cm^-1(mol^-l). . All standard curves are linear. The sensitivity does not vary with ozone concentration, with small changes of temperature of the reaction, or with the chemical composition of the water whenever the pH is below 4 (Kelsey Hunter et al., 2013, 10).

Chlorine is cheaper and effective in small quantities compared to other disinfectants in the world. Another essential disinfectant that has found favor in the purification of water is the ozone. Ozone is mainly unstable gas that comprises of three atom atoms of oxygen. (Kelsey Hunter et al., 2013, 12) The gas will often dissociate back to diatomic oxygen. Ozone is also an effective disinfectant because of its strength on bacteria and pathogenic microorganisms. Compared to chlorine, ozone has higher oxidizing potential, which is 2.07 V vs. SHE; therefore, being termed as the most powerful oxidant in water disinfection and also surfaces sanitization

For many types of waters, the instrumental or visual measurements can be performed even a few hours after the ozone has reacted with the indigo reagent (Kelsey Hunter et al., 2013, 20). The method is relatively selective. Secondary oxidation products generally do not interfere. The reaction of ozone with the Indigo Reagent is so fast that it can be applied to stop and measure even speedy ozonation reactions. The method has been well tested and used for the determination of ozone in many different types of waters (Bader & Hoigené, 1981, 455). Unexpected difficulties have not arisen. More practical experiences with this method are now being accumulated in many waterworks and laboratories.

RESULTS

This section describes the results obtained in this project. The subsections are made to provide details corresponding to the specific parameters tested. Generally, the subsections cover water flow rate, water, electrode length, time of cell operation and the length of electrodes.

Water Flow Rate

The project analyzed water from six taps. Moreover, two sizes of electrodes were evaluated by running water through them. Figure 1. Represents the experiment setup of apparatus.

During the experiment, several conditions were held constant: the water to be used was from taps, Temperature was 20 °C and the operating time kept at fifteen minutes. Also, the flow rate for the six taps was maintained at 2-7 liters per min while the electrodes were 0.5 and 1 square centimeters. OxiControl white box and Black box were used to supply power whereas the polarity was alternated every ten seconds. Finally, the 0.5 square centimeter electrode was supplied with 0.5 amperes of current while the 1 square centimeter electrode was supplied with one ampere.

Current and Membrane variation

The variation of current was done concurrently with the testing of tap water. The conditions were similar to those for measuring water flow rate with modifications being done on the membrane and current. Nafion 115, Nafion 117 and PVC mesh were used as the membrane. Also, the current was alternated between 0.1 to 1 ampere. The main assumption made during the experiment was that the current applied by Oxibox was faultless.

1

Associatin, A. P. (n.d.). Indigo Colorimetric Method 4500‐O3 B. Standard Methods for the examination of Water and Wastewater.

Figure 1. Water flow rate experiment setup.

Results and discussion

Figures 1 and 2 show the ozone production using 0.5 and 1 cm² area electrodes during 15 minutes operation for different flow rates using the WHITE Oxibox. The membrane that was used in the system (installed) was the Nafion 115. It can be observed that during the whole operation the production of ozone is not stable and alters constantly and this could be attributed to the statistical error due to quite low ozone concentration which is from its nature vulnerable to the statistical accuracy of the colorimetric method. It was also observed that for both electrodes the higher concentration of ozone was obtained by operating the cell at 6 and 7 L min^-1. The results obtained at operating the cell at 7 L min^-1 are not in good agreement between both electrode lengths performance, however it is important to highlight that the system used to feed the water was dependent on the change of pressure at the building where the experiment was carried out. Also, a rate aboveabove 6 L min^-1 was not always easy to maintain a stable flow rate.

Figure 1. Ozone production at different water flow rates – 0.5 cm2 electrodes

Figure 2. Ozone production at different water flow rates – 1 cm2 electrodes

According to Figure 3 below, the current efficiency is directly proportional to the water flow rate.A rate above7 L min^-1 flow rate may not have been under complete control throughout the 15 min of operation which could have caused the difference between both electrodes performance. However, the current efficiency is substantially low, due to the low ozone generation. Also, Figure 4 shows that the efficiency of the current is low for high current flows. However, the increase in the size of the electrode improves the current efficiency as seen in figure 5.

Figure 5Figure 3. Current efficiency at different flow rates for the two different applied currents (White box).

Figure 4. Current efficiency at different current for the two different membranes (Black box).

Figure 5. Ozone generation Vs Current for three different membranes (Black box).

Stucki, S., Baumann, H., & Christen , H. (1987). Performance of a pressurized electrochemical ozone generator. Journal of Applied Electrochemistry 17, 773.

Conclusions

The Oxi-Tech cell produced ozione concentrations of up to0.19 mg L^-1 although the concentration was found to be affected by the water flow rate. Also, the ozone concentration is found to be higher and lasting longer at higher current flows. Moreover, The proton exchange membrane was necessary to produce the concentration of ozone mentioned, but small strips of Nafion membrane separating the electrodes could also be used but in case that just a mesh was used to separate the two BDD electrodes the ozone generation was close to zero(within the statistical error of the method). The collaboration between the Electrochemical Engineering group and Oxi-Tech Solutions Ltd in this project should be elaborated more in order to have as a lab access to every product they are working at the moment.

5. Risk Assessment

In a highly technical project like this, the composition of a Risk Assessment is crucial for the careful conduction of the experiments. After the approval of the Risk Assessment there is the chance that new hazardous substances might need to get used in the experiment and/or dangers that have not been taken into account in the very beginning could emerge. In that case the submission and the approval of a new Risk Assessment is imposed.

Assessment Guidance

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APPENDIX

Gantt Chart