Mixed oxidant

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A mixed oxidant solution (MOS) is a type of disinfectant that has many uses including disinfecting, sterilizing, and eliminating pathogenic microorganisms in water. [1] An MOS may have advantages such as a higher disinfecting power, stable residual chlorine in water, elimination of biofilm, and safety. [2] The main components of an MOS are chlorine and its derivatives (ClO and HClO), which are produced by electrolysis of sodium chloride. [3] It may also contain high amounts of hydroxy radicals, chlorine dioxide, dissolved ozone, hydrogen peroxide and oxygen from which the name "mixed oxidant" is derived.

Contents

Performance

Reactions

A mixed oxidant solution (MOS) is produced by on-site electrolysis. The concentration of disinfectant output is proportional to the concentration of salt input, voltage, temperature, current, and electrolysis time. [4] An MOS production system contains corrosion-resistant electrodes or dimensionally-stable anodes (DSA) and is made so that different voltages for electrolysis are applied simultaneously to different parts. In this way, different reactions occur at the anode and cathode poles, and therefore, various oxidizing substances are produced. [5]

In this process, the chloride ions at the anode are converted to chlorine gas. After reducing the concentration of chloride ions in the presence of ClO and Cl2 compounds in the solution and applying the required conditions, ClO2 is produced and the final solution is stored. [6]

Half-reaction (V)
2Cl Cl2 + 2e

−1.36

0.5Cl2 + H2O HClO + H + + e

−1.61

Cl + H2O HClO + H+ + e

-1.48

Cl + 2OH ClO + H2O + 2e

-0.81

HClO + H2O → ClO2+ 3H++ 3e

-1.19

For generating ozone, the conditions for water electrolysis reactions must be provided. In this case, the following half reactions take place (which are given below). Hydrogen gas is produced at the cathode and oxygen gas at the anode. By increasing the voltage, the anode half reaction is changed and ozone is produced. [7]

Half-reaction (V)
2H2O + 2e H2 + 2OH

−0.8277

2H2O O2 + 4H+ + 4e

−1.229

3H2O O3+6H++6e

−1.53

The rate of ozone generation is influenced by the conditions of the electrolysis reactor. Sustained reactions are capable of generating a solution saturated in ozone, though the solubility of ozone depends on the concentration of other ions. [8]

In the next stage, with little change in reaction conditions, hydrogen peroxide is produced. Hydrogen peroxide and ozone are produced by different half reactions, but each of them may occur in practice. [6]

Half-reaction (V)
O2 + H2O O3 + 2H+ + 2e

−2.076

O2 + 2OH O3 + H2O + 2e

−1.24

3H2O O3 + 6H+ + 6e

−1.53

O2 + 2H+ + 2e H2O2

−0.7

2H2O H2O2 + 2H+ + 2e

−1.776

HO2 + H+ + e H2O2

−1.495

Various conditions, including changes in voltage, current, concentration, pH, temperature, flow, and pressure will change the standard reduction potential, and as a result, the rate of various reactions. However, the extent of the electrodes in the reactor, creating multiple layers of electrolyte and unequal conditions on the electrodes surfaces, will cause major changes in the standard modes of the half reactions. [7]

Production Cell

Operation principle of NaCl electrolysis cell Salt electrolysis.png
Operation principle of NaCl electrolysis cell

The basis of the mixed oxidant production cell is electrolysis of a water solution of sodium chloride. For producing a mixed oxidants solution, different types of electrolysis cells such as a membrane cell or a standard contact cell (both unipolar and bipolar) are used. [9]

Membrane cell

This cell consists of anode and cathode electrodes with an ion exchange membrane between them. This membrane allows cations pass through it and leads them to the cathode. [10] This cell has two inputs and two outputs for water. One pair of input and output is located at the cathode side and the other pair is located at the anode side. [11]

Certain cells feature various types of membranes. Some use ion exchange membranes capable of transporting cations and anions across sides. In these cells, a brine solution is introduced from one side, while water is fed from the opposite side. [12]

The half reaction in the cathode chamber is as follows:

2NaCl + 2H2O + 2e→ 2NaOH + 2Cl+ H2

On the anode side, part of the chloride ions are oxidized and dissolved in the passing water in the forms of Cl2, HOCl and small amounts of ClO2 due to the electrolysis of water. Small amounts of ozone and oxygen gas are produced at the anode side. The main half reaction at the anode side is:

2Cl→ 2e+ Cl2
Cl+ H2O → HClO + H++ 2e

When water flows through the anode chamber, it dissolves chlorine and its compounds. By adding the necessary quantity of this mixture to water, it can be purified. The solution exiting the anode chamber in membrane reactors is acidic, with a pH of about 2-3. Fixed titanium electrodes, which are resistant to corrosion at the anode, are suitable for this kind of electrolysis cell. [13]

Membraneless cell

Electrode arrangements in salt electrolysis reactors Bipolar vs unipolar.png
Electrode arrangements in salt electrolysis reactors

The structure of the cell without a membrane is similar to a membrane cell, with the exception that it has one brine solution input and one output for the products. In this case, the anode and cathode products are mixed and go to the cell output. Since the pH of the produced solution is around 8-9, using this solution for disinfection may be unsuitable for base-sensitive applications. Acidic solution is added to reduce pH in these situations. This type of cell can be unipolar or bipolar, as described below. [14]

Types of cell connections

Electrolysis cells with more than one anode and cathode pair have two types of arrangement: unipolar and bipolar.

Unipolar arrangement: Cells are arranged in parallel and therefore have the same potential difference between the anode-cathode pair. The total current flow is equal to the sum of each pair's current and the voltage is equal to one pair's voltage. In this case, the whole system voltage is low and its current is high. [14]

An example of a bipolar arrangement. Half of each electrode plate on both sides is an anode; the other half is the cathode Cell ordering.png
An example of a bipolar arrangement. Half of each electrode plate on both sides is an anode; the other half is the cathode

Bipolar arrangement: Cells are connected in series. [14] Bipolar ordering has multiple configurations. In one case, central electrodes on one side act as the anode and on the other side act as the cathode. In other cases, half of the electrode plate on both sides is the anode and the other half is the cathode.

Comparisons

Advantages of Mixed oxidant solution compared to other disinfecting methods

Application of mixed-oxidant solution for disinfecting water has several advantages compared to other methods, such as sodium hypochlorite bleach and calcium hypochlorite. The disinfecting effect of applying mixed oxidant is more efficacious and has fewer safety concerns than other methods, such as chlorination and ozonation. It is generally regarded as safer and with fewer risks. A summary of the comparison between the disinfection methods is provided in the table below. [15]

Comparing methods of water disinfection [15]
Mixed oxidant Bleach produced locally UV OzoneChlorine dioxide Chloramine Calcium hypochloriteBleachChlorine gas
Effective Disinfectionyesyesyesyesyesyesyesyesyes
safetyyesyesyesnononononono
Residual chlorineyesyesnononoyesyesyesyes
Less trihalomethanes productionyesnoyesyesyesyesnonono
Less chlorite and bromate productionyesyesyesyesnoyesyesyesyes
Biofilm removalyesnononoyesnononono
Algae removalyesnonoyesyesnononono
Virus removalyesnonoyesnonononono
Remove parasite eggsyesnononononononono
Usage in the pretreatmentyesnonoyesyesyesnonono
Removing taste and odor yesnonoyesnonononoyes
Easy maintenanceyesyesnononoyesnonoyes

In the next table, the effectiveness of mixed oxidant and bleach in terms of deactivating bacteria and viruses has been compared. In many cases a mixed oxidant is more effective against pathogens either by inactivating more pathogens, requiring less contact time, or less product than bleach. [16] An MOS is also effective against more bacteria and viruses than bleach. [16]

Comparison of Mixed oxidant and Bleach in terms of deactivating bacteria and viruses [16]
MicroorganismsInjection rate
(mg/L)
Contact time
(min)
Inactivation
(log)
Differentiating
Parameter
Mixed oxidantBleachMixed oxidantBleachMixed oxidantBleach
Bacteria
Vibrio cholerae 221.84.04time
Escherichia coli 223.85.04time
Pseudomonas aeruginosa 221010>4.82.2Efficacy
Legionella pneumophila 22101054.7Efficacy
Staphylococcus aureus 2260601.60.8Efficacy
4460603.72.3
Listeria monocytogenes 22606020.8
4460603.71.2
Bacteria spores
Bacillus stearothermophilus 223030>52.5Efficacy
Clostridium perfringens spore 2213182time
Bacillus globigii spores

Bacillus anthracis (Sterne) spores

2.52.515153.62.4Efficacy
Viruses
MS2 Coliphage22701684time
Vaccine (Smallpox surrogate) 5~70201043

Time, concentration, efficacy

Poliovirus vaccine strain 1>4NA30NA>5.5NA
Rotavirus SA-11>4NA30NA>5.5NA
Protozoa oocysts
Giardia lamblia >4NA30NA4NA
Cryptosporidium parvum 5524014403>noneTime and efficacy
Cryptosporidium parvum oocysts2525240240>10.25Efficacy, qRT-PCR and Tissue culture of infectivity.

Comparison of the membrane cell and membraneless cell

A mixed oxidant production cell generally works regardless of a membrane. Each of these structures has advantages and disadvantages that should be considered. The membraneless cell output contains hydroxide ions which increase the pH; therefore it affects the composition of the output products. To keep the pH in the neutral range, hydrochloric acid or sulfuric acid must be added to the disinfected water. In this kind of cell, the main product is sodium hypochlorite. On the other hand, in cells with 1 membrane, the anode output (anolyte) is acidic and the cathode output (catholyte) is basic. The anolyte (acidic solution) contains more than four types of oxidants, which can make disinfecting more effective. The output components of these two different cells are compared in the table below. [14]

Comparing output compounds of the membrane cell and the membraneless cell
Oxidizing substanceUnitsMembrane cellMembraneless cell
pH=2–3pH=8
ozoneppm20-
Chlorine dioxideppm26-
Hypochlorous acidppm1800-
Sodium hypochloriteppm-1400
Hydrogen Peroxideppm400
Oxygenppm115
ORP mV1140966

At pH higher than 5, most of the hypochlorous acid turns into hypochlorite ions, which is a weaker oxidant compared to hypochlorous acid. Moreover, in a membrane cell, other powerful oxidants such as ozone, chlorine dioxide and hydrogen peroxide can be produced, which are effective for killing bacteria and omitting[ clarification needed ] biofilms in water distribution systems and containers.

Comparing membraneless cell and membrane cell
PropertyUnitsBipolar cell without MembraneMembrane cell
Salt intakeGrams per grams of Chlorine55
Electricity consumptionWatt per grams of Mixed oxidant77
Acid consumption Hydrochloric acid Citric acid
Water consumptionLiter per grams of Mixed oxidant12
Maximum concentration of Mixed oxidantsGrams per liter1.61.8
Chlorine smellyesyes
Solution pH8–92.5–3

Today, membrane cell systems are some of the most promising and fast-developing techniques for producing Chloralkali (see chloralkali process) and it will undoubtedly replace other techniques. Since 1987, practically all new chloralkali plants worldwide apply the membrane system. However, due to their long lifetime and high replacement costs, the existing mercury and diaphragm cells are only very slowly being replaced with membrane cells. [14]

Applications

Mixed oxidant solutions for water treatment may improve safety, lower general corrosion rates, increase performance, and save money. MOS may be more effective than bleach and can be used for a variety of applications. Some of these applications are cited below.

Cooling water treatment: An MOS for industrial cooling water treatment and disinfection improves safety and thermal efficiency, lowers general corrosion rates, increases performance, and saves money, resulting in a reduction of downtime, maintenance, and expense. Additionally, it can improve workplace safety by eliminating the handling and storage of hazardous chemicals while maintaining steady microbiological control. [17]

Cooling tower water treatment: An MOS improves cooling tower efficiency, safety, and cost compared to conventional biocide treatment methods for legionella prevention, biofilm removal, and inactivation of other performance-inhibiting waterborne organisms. [18]

Industrial process water and wastewater treatment: As the lowest cost supplier of chlorine for disinfection and oxidation of process water and wastewater prior to discharge, an MOS is used in industrial wastewater treatment. MOS chemistry is more effective at biofilm control. Biochemical and Chemical oxygen demand removal, breakpoint chlorination of ammonia and hydrogen sulfide removal. [19]

Municipal wastewater: As one of the world's most precious natural resources, the reuse of water is becoming increasingly important. MOS is both the most cost-effective solution and the preferred technology for disinfection and oxidation of wastewater for reuse or reintroduction into the environment eliminating many of the negative problems associated with traditional chlorine disinfection. [19]

Drinking water & beverage facilities: An MOS is a proven disinfectant for improving the quality and safety of drinking water with significant economic savings. For providing clean, safe drinking water ranges from rural communities to large cities. Also providing clean, safe water at food and beverage facilities. It is ideally suited for carbonated soft drinks bottling, brewing, dairy farms and dairy and food processing applications. [20]

Aquatics and pools: An alternative to chlorine for pool cleaning is an MOS. It can reduce skin and eye irritation, and skin redness and dryness often associated with chlorine. An MOS can also reduce maintenance time and costs compared to chlorine as the need for "shocking" and draining the pool is minimized or unnecessary. [21]

Farm applications: There are many disinfecting needs an MOS is utilized for in farm application such as livestock watering, drinking water disinfection, dairy, milking operations, pre- and post-teat dip, CIP sanitizer, poultry cooling & humidification pad treatment, irrigation & drip line cleaning, and iron and manganese removal from the water supply. [19]

Crude oil & gas water management: Enhanced oil recovery almost always involves some kind of water treatment processes. Water treatment technology in the crude oil and gas industry includes disinfection treatment for produced water, frac-water, disposal well sites, enhanced oil recovery, and hydrogen sulfide removal. [22]

Related Research Articles

<span class="mw-page-title-main">Electrochemistry</span> Branch of chemistry

Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.

<span class="mw-page-title-main">Electrochemical cell</span> Electro-chemical device

An electrochemical cell is a device that generates electrical energy from chemical reactions. Electrical energy can also be applied to these cells to cause chemical reactions to occur. Electrochemical cells that generate an electric current are called voltaic or galvanic cells and those that generate chemical reactions, via electrolysis for example, are called electrolytic cells.

<span class="mw-page-title-main">Electrolysis</span> Technique in chemistry and manufacturing

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".

<span class="mw-page-title-main">Sodium hypochlorite</span> Chemical compound (known in solution as bleach)

Sodium hypochlorite, commonly known in a dilute solution as (chlorine) bleach, is an alkaline inorganic chemical compound with the formula NaOCl, consisting of a sodium cation and a hypochlorite anion. It may also be viewed as the sodium salt of hypochlorous acid. The anhydrous compound is unstable and may decompose explosively. It can be crystallized as a pentahydrate NaOCl·5H
2
O
, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.

<span class="mw-page-title-main">Water purification</span> Process of removing impurities from water

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Electrolytic cell</span> Cell that uses electrical energy to drive a non-spontaneous redox reaction

An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell's two electrodes; an anode and a cathode, which are immersed in an electrolyte solution. This is in contrast to a galvanic cell, which itself is a source of electrical energy and the foundation of a battery. The net reaction taking place in a galvanic cell is a spontaneous reaction, i.e., the Gibbs free energy remains -ve, while the net reaction taking place in an electrolytic cell is the reverse of this spontaneous reaction, i.e., the Gibbs free energy is +ve.

<span class="mw-page-title-main">Disinfectant</span> Antimicrobial agent that inactivates or destroys microbes

A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than sterilization, which is an extreme physical or chemical process that kills all types of life. Disinfectants are generally distinguished from other antimicrobial agents such as antibiotics, which destroy microorganisms within the body, and antiseptics, which destroy microorganisms on living tissue. Disinfectants are also different from biocides—the latter are intended to destroy all forms of life, not just microorganisms. Disinfectants work by destroying the cell wall of microbes or interfering with their metabolism. It is also a form of decontamination, and can be defined as the process whereby physical or chemical methods are used to reduce the amount of pathogenic microorganisms on a surface.

<span class="mw-page-title-main">Hypochlorous acid</span> Chemical compound

Hypochlorous acid is an acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO. HClO and ClO are oxidizers, and the primary disinfection agents of chlorine solutions. HClO cannot be isolated from these solutions due to rapid equilibration with its precursor, chlorine.

The chloralkali process is an industrial process for the electrolysis of sodium chloride (NaCl) solutions. It is the technology used to produce chlorine and sodium hydroxide, which are commodity chemicals required by industry. Thirty five million tons of chlorine were prepared by this process in 1987. The chlorine and sodium hydroxide produced in this process are widely used in the chemical industry.

<span class="mw-page-title-main">Sodium chlorate</span> Chemical compound

Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching pulp to produce high brightness paper.

A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton-exchange membrane fuel cell or of a proton-exchange membrane electrolyser: separation of reactants and transport of protons while blocking a direct electronic pathway through the membrane.

Electrocoagulation (EC) is a technique used for wastewater treatment, wash water treatment, industrially processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. There are many brands of electrocoagulation devices available, and they can range in complexity from a simple anode and cathode to much more complex devices with control over electrode potentials, passivation, anode consumption, cell REDOX potentials as well as the introduction of ultrasonic sound, ultraviolet light and a range of gases and reactants to achieve so-called Advanced Oxidation Processes for refractory or recalcitrant organic substances.

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.

In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically-determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential implies that the cell requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density is achieved.

In electrochemistry, electrosynthesis is the synthesis of chemical compounds in an electrochemical cell. Compared to ordinary redox reactions, electrosynthesis sometimes offers improved selectivity and yields. Electrosynthesis is actively studied as a science and also has industrial applications. Electrooxidation has potential for wastewater treatment as well.

<span class="mw-page-title-main">Electrolysed water</span> Chemical mixture in water solution

Electrolysed water is produced by the electrolysis of ordinary tap water containing dissolved sodium chloride. The electrolysis of such salt solutions produces a solution of hypochlorous acid and sodium hydroxide. The resulting water can be used as a disinfectant.

Chlorine gas can be produced by extracting from natural materials, including the electrolysis of a sodium chloride solution (brine) and other ways.

Electrodeionization (EDI) is a water treatment technology that utilizes DC power, ion exchange membranes, and ion exchange resin to deionize water. EDI is typically employed as a polishing treatment following reverse osmosis (RO), and is used in the production of ultrapure water. It differs from other RO polishing methods, like chemically regenerated mixed beds, by operating continuously without chemical regeneration.

Electrochlorination is the process of producing hypochlorite by passing electric current through salt water. This disinfects the water and makes it safe for human use, such as for drinking water or swimming pools.

Electro-oxidation(EO or EOx), also known as anodic oxidation or electrochemical oxidation (EC), is a technique used for wastewater treatment, mainly for industrial effluents, and is a type of advanced oxidation process (AOP). The most general layout comprises two electrodes, operating as anode and cathode, connected to a power source. When an energy input and sufficient supporting electrolyte are provided to the system, strong oxidizing species are formed, which interact with the contaminants and degrade them. The refractory compounds are thus converted into reaction intermediates and, ultimately, into water and CO2 by complete mineralization.

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