The water disinfectant of chlorine

What is chlorine?
Chlorine is one of the most commonly used disinfectants for water disinfection. Chlorine can be applied for the deactivation of most microorganisms and it is relatively cheap.

At standard temperature and pressure, two chlorine atoms form the diatomic molecule Cl2. This is a pale yellow-green gas that has its distinctive strong smell, the smell of bleach. The bonding between the two atoms is relatively weak (only of 242.580 ±0.004 kJ/mol) which makes the Cl2 molecule highly reactive.

Along with fluorine, bromine, iodine and astatine, chlorine is a member of the halogen series that forms the group 17 of the periodic table—the most reactive group of elements. It combines readily with nearly all elements.

Compounds with oxygen, nitrogen, xenon, and krypton are known, but do not form by direct reaction of the elements.Chlorine, though very reactive, is not as extremely reactive as fluorine. Pure chlorine gas does, however, support combustion of organic compounds such as hydrocarbons, although the carbon component tends to burn incompletely, with much of it remaining as soot. At 10 °C and atmospheric pressure, one liter of water dissolves 3.10 L of gaseous chlorine, and at 30 °C (86 °F), 1 L of water dissolves only 1.77 liters of chlorine.

Chlorine is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. With metals, it forms salts called chlorides. As the chloride ion, Cl?, it is also the most abundant dissolved ion in ocean water.

How is chlorine produced?
Chlorine is produced from chlorine bonds by means of electrolytic or chemical oxidation. This is often attained by electrolysis of seawater or rock salt. The salts are dissolved in water, forming brine. Brine can conduct a powerful direct current in an electolytic cell. Because of this current chlorine ions (which originate from salt dissolving in water) are transformed to chlorine atoms. Salt and water are divided up in sodium hydroxide (NaOH) and hydrogen gas (H2) on the cathode and chlorine gas on the anode. These cathode and anode products should be separated, because hydrogen gas reacts with chlorine gas very agressively.

How can chlorine be stored?
Watery chlorine should be protected from sunlight. Chlorine is broken down under the influence of sunlight. UV radiation in sunlight provides energy which aids the break-down of underchloric acid (HOCl) molecules. First, the water molecule (H2O) is broken down, causing electrons to be released which reduce the chlorine atom of underchloric acid to chloride (Cl-). During this reaction an oxygen atom is released, which will be converted into an oxygen molecule: 2HOCl -> 2H+ + 2Cl- + O2

How is chlorine transported?
Chlorine is a very reactive and corrosive gas. When it is transported, stored or used, safety precautions must be taken. In Holland for example, chlorine is transported in separate chlorine trains.

Chlorine applications and uses
Chlorine is applied on a massive scale. Chlorine is a very reactive element, causing it to quickly form compounds with other substances. Chlorine also has the ability to develop a bond between two substances that do not normally react with one another. When chlorine bonds to a substance that contains carbon atoms, organic substances are formed. Examples are plastic, solvents and oils, but also several human body fluids. When chlorine chemically binds to other elements, it often replaces a hydrogen atom during a so-called substitution reaction. Multiple hydrogen atoms in the same molecule can be replaced by chlorine atoms, causing new substances to form one after another.

Chlorine plays an important role in medical science. It is not only used as a disinfectant, but it is also a constituent of various medicines. The majority of our medicines contain chlorine or are developed using chlorine-containing byproducts. Medical herbs also contain chlorine. The first anaesthetic used during surgery was chloroform (CHCl3).

The chemical industry creates ten thousands of chlorine products using a small number of chlorine containing chemicals. Emaples of products which contain chlorine are glue, paints, solvents, foam rubbers, car bumpers, food additives, pesticides and antifreeze. One of the most commonly used chlorine-containing substances is PVC (poly vinyl chloride). PVC is widely used, for example in drainpipes, insulation wires, floors, windows, bottles and waterproof clothes.

Chlorine-based bleach is applied as a disinfectant on a large scale. The substances are also used to bleach paper. Bleaching occurs as a result of chlorine or hypochlorite oxidation.

About 65% of industrialized chlorine is used to produce organic chemicals, such as plastics. About 20% is used to produce bleach and disinfectants. The remaining chlorine is used to produce inorganic compounds from chlorine and several different elements, such as zinc (Zn), iron (Fe) and titanium (Ti).

Chlorine as a disinfectant
Chlorine is one of the most widely used disinfectants. It is very applicable and very effective for the deactivation of pathogenic microorganisms. Chlorine can be easily applied, measures and controlled. Is is fairly persistent and relatively cheap.

Chlorine has been used for applications, such as the deactivation of pathogens in drinking water, swimming pool water and wastewater, for the disinfection of household areas and for textile bleaching, for more than two hundred years. When chlorine was discovered we did not now that disease was caused by microorganisms. In the nineteenth century doctors and scientists discovered that many diseases are contagious and that the spread of disease can be prevented by the disinfection of hospital areas. Very soon afterward, we started experimenting with chlorine as a disinfectant. In 1835 doctor and writer Oliver Wendel Holmes advised midwifes to wash their hands in calcium hypochlorite (Ca(ClO)2-4H2O) to prevent a spread of midwifes fever.

However, we only started using disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases. Chlorine has played an important role in lenghthening the life-expectancy of humans.

Chlorine as a bleach
Surfaces can be disinfected by bleaching. Bleach consists of chlorine gas dissolved in an alkali-solution, such as sodium hydroxide (NaOH). When chlorine is dissolved in an alkalic solution, hypochlorite ions (OCl-) are formed during an autoredox reaction. Chlorine reacts with sodium hydroxide to sodium hypochlorite (NaOCl). This is a very good disinfectant with a stable effect.

Bleach cannot be combined with acids. When bleach comes in contact with acids the hypochlorite becomes instable, causing poisonous chlorine gas to escape. The accompanying underchloric acid is not very stable.

Bleaching powder (CaOCl2) can also be used. This is produced by directing chlorine through calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid. This makes it easier to apply as a disinfectant in medical areas, next to its use as a bleach. When bleaching powder dissolves, it reacts with water to underchloric acid (HOCl) and hypochlorite ions (OCl-).

What is free and bound active chlorine?
When chlorine is added to water for disinfection purposes, it usually starts reacting with dissolved organic and inorganic compounds in the water. Chlorine can no longer be used for disinfection after that, because is has formed other products. The amount of chlorine that is used during this process is referred to as the ‘chlorine enquiry’ of the water.

Chlorine can react with ammonia (NH3) to chloramines, chemical compounds which contain chlorine, nitrogen (N) and hydrogen (H). These compounds are referred to as ‘active chlorine compounds’ (contrary to underchloric acid and hypochlorite, which are referred to as ‘free active chlorine’) and are responsible for water disinfection. However, these compounds react much more slowly than free active chlorine.

What doses of chlorine does one apply?
When dosing chlorine one has to take into acount that chlorine reacts with compounds in the water. The dose has to be high enough for a significant amount of chlorine to remain in the water for disinfection. Chlorine enquiry is determined by the amount of organic matter in the water, the pH of the water, contact time and temperature. Chlorine reacts with organic matter to disinfection byporducts, such as trihalomethanes (THM) and halogenated acetic acids (HAA). Chlorine can be added for disinfection in several different ways. When ordinary chlorination is apllied, the chlorine is simply added to the water and no prior treatment is necessary. Pre- and postchlorination means adding chlorine to water prior to and after other treatment steps. Rechlorination means the addition of chlorine to treated water in one or more points of the distribution system in order to preserve disinfection.

What is breakpoint chlorination?
Breakpoint chlorination consists of a continual addition of chlorine to the water upto the point where the chlorine enquiry is met and all present ammonia is oxidized, so that only free chlorine remains. This is usually applied for disinfection, but it also has other benefits, such as smell and taste control. In order to reach the breakpoint, a superchlorination is applied. To achieve this, one uses chlorine concentrations which largely exceed the 1 mg/L concentration required for disinfection.

Which chlorine concentration is applied?
Chlorine gas can be obtained as fluid gas in 10 bar pressure vessels. It is highly water soluble (3 L chlorine/ 1 L water). To kill bacteria little chlorine is required; about 0,2-0,4 mg/L. the concentrations of chlorine added to the water are usually higher, because of the chlorine enquiry of the water. Nowadays chlorine gas is only used for large municipal and industrial water purification installations. For smaller applications one usually ads calcium or sodium hypochlorite.

What are the health effects of chlorine?
The reaction of the human body to chlorine depends on the concentration of chlorine present in air, and on the duration and frequency of exposure. Effects also depend on the health of an individual and the environmental conditions during exposure.

When small amounts of chlorine are breathed in during short time periods, this can affect the respirational system. Effects vary from coughing and chest pains, to fluid accumulation in the lungs. Chlorine can also cause skin and eye irritations. These effects do not take place under natural conditions. When chlorine enters the body it is not very persistent, because of its reactivity.

Pure chlorine is very toxic, even small amounts can be deadly. During World War I chlorine gas was used on a large scale to hurt or kill enemy soldiers. The Germans were the first to use chlorine gas against their enemies.

Chlorine is much denser than air, causing it to form a toxic fume above the soil. Chlorine gas affects the mucous membrane (nose, throat, eyes). Chlorine is toxic to mucous membranes because it dissolves them, causing the chlorine gas to end up in the blood vessels. When chlorine gas is breathed in the lungs fill up with fluid, causing a person to sort of drown.

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