Is chlorine a compound

The average chlorine content in seawater is around 20,000 ppm (2%). But the element is also contained in fresh water. The concentration in river water is generally around 8 ppm. In a dissolved state, chlorine comes mainly in the form of Cl- before, but also Cl2 (in chlorinated water), HClO, ClO-, ClO2, ClO2-, ClO3- and ClO4- are possible manifestations.

How and in which compounds does chlorine react with water?

Chlorine is one of the most reactive elements and therefore also reacts with water. It forms hydrochloric acid and hypochlorite with this, which happens according to the following reaction equation:

Cl2(g) + H2O (l) <-> OCl-(aq) + 2H+(aq) + Cl-(aq)

The equilibrium of the reaction depends, among other things, on the pH of the solution, since H.+Ions are formed.
The chloride ion is much less reactive. The group of chloroalkyl ethers, for example, is also normally very stable in water.

Water solubility of chlorine and / or its compounds

The solubility of chlorine gas in water is about 6.3 g / L under standard conditions. Chlorine salts are also considered to be readily soluble in water. Some examples of the water solubility of chlorine compounds are chloral hydrate with 4740 g / L (at 17OC), chloramben with 700 mg / L (at 25OC), 4-chloroaniline with 2.9 g / L (at 20OC) and 1-chloroethene with 1.1 mg / L (at 25OC).

Solubility and how it can be influenced

How can chlorine get into water?

Chlorine is naturally found in some minerals such as halite, sylvine and carnallite. It does not occur naturally in free form, but only in compounds, mainly as sodium chloride (table salt). Table salt is obtained primarily from halite, but it can also be extracted from seawater. The use of de-icing salts on roads and human excretions in municipal wastewater quickly lead to an increased amount of chloride in the water and soil. The potash industry also amplifies this effect.
However, chlorine has numerous other commercial uses. Around 30% is used in the chemical industry, while around 25% is required for the production of PVC. Another 20% is used for water treatment, around 15% is used in solvents and around 10% in bleach. Not only cellulose for paper production, but also waste paper can be bleached with the help of chlorine. However, there are now more environmentally friendly methods of doing this.
The element is also required for the production of drugs, silicones and polymers, even if it is no longer contained in the end product. Chlorine is also a component of household bleach, paint removers, fire retardants and even pesticides. The insecticide and environmental toxin DDT is very well known, as are the herbicides chloramben or chloral hydrate. The same applies to chlordane. Some chlorinated hydrocarbon compounds also act as fungicides and insecticides. In some cases, because of their toxicity, there are significant restrictions on the use of chlorine compounds for this purpose.
Other well-known and notorious chlorine compounds are PCB, which is used, for example, for the production of capacitors and transformers, CKW and CFC, which were used as coolants, among other things. Other uses for chlorine compounds are the use for paints and glues, the production of ion exchangers or sleeping pills.
The elementary chlorine gas was used as a warfare agent. It is also created when acid is added to sanitary cleaners.
Chlorides and other chlorine compounds can escape from inadequately secured landfills for hazardous waste and building rubble. The combustion of substances containing chlorine compounds also releases them into the environment.
The number of chlorine compounds and their uses is so great that by far not all of them can be dealt with here. It is still interesting, however, that chlorine and its compounds are often released as waste products and are then lost or are mainly marketed as organochlorine compounds. Solvents in particular, but also PVC, are good options for this.
The radioactive one 36Cl is used in research.

What environmental problems can water pollution with chlorine cause?

Chlorine in the form of chloride is essential for numerous organisms. This also applies to higher plants. The ion is very mobile in the soil because it is not absorbed by minerals. Hence, it is easily washed out. The chloride concentration in normal soils varies from 50-2000 ppm. The highest salt concentrations can be found in arid and semi-arid areas as well as in areas near the sea.
Plants absorb chloride through their roots and also chlorine gas through the above-ground parts of the plant. In the plant there is often an accumulation of chloride in the cytoplasm. The chlorine content of plants is usually around 2000-20,000 ppm. The proportion of this in the chloroplasts is particularly high. Some trees, such as plane trees or oaks, can tolerate relatively high salt concentrations in the soil; if the concentrations in the soil are too low, the growth of plants can be inhibited. This doesn't happen very often, however, as it only matters at extremes of less than 2ppm.
In aquatic processes, there are often interactions between chloride and iron, which is the case, for example, in photochemical processes. It is also interesting that sodium chloride in sea salt particles can react with nitrogen oxides, forming chlorine atoms that can destroy ozone and greenhouse gases.
On the other hand, while chlorine is a very important element in the environment, it can also be very damaging very quickly if it occurs in forms other than chloride. It is assigned a great potential for spreading, and it is also considered a water-polluting substance. Larger quantities can also endanger the drinking water. Chlorine has a toxic effect on fish, the lethal concentration at which around 50% of a population die (LC50), is around 293 ppm. Chlorine compounds can also be toxic and highly hazardous to the environment. Some LD50- Levels of chlorine compounds known, which indicate the dose at which 50% of a population will die. This dose is 1.1 g / kg for oral ingestion of chloral hydrate by rats, 5.6 g / kg for chloramben and 370-420 mg / kg for chloraniline. Chloral hydrate is also referred to as a substance hazardous to water. The LC50-The value of chloraniline in rain trout is 14 mg / L within 96 hours. The toxicity of chlorine compounds can be based on various processes. Some substances inhibit methaemoglobin, while others influence the activity of enzymes.
The environmental hazard of some other chlorine-containing substances relates more to their reactions in the atmosphere, which in turn can have an impact on water and soil, as well as flora and fauna. For example, when substances containing chlorine are burned, hydrochloric acid can enter the atmosphere and contribute to acid rain, which can severely damage the soil and plants (see also nitrogen and water and sulfur and water).
Other problems arise from the use of CFCs and CFCs, which attack the ozone layer, increasing UV radiation on earth, which in turn affects life on earth. They also act as greenhouse gases and thus contribute to climate change, which also has an impact on aquatic ecosystems. CHCs are also one of the chlorine compounds that can accumulate in living things and have a carcinogenic effect.
The pesticides DDT and PCB are also known for their bioaccumulation and biomagnification. They accumulate not only within a living being, but also within an entire food chain.
Chlorine has two stable and eight unstable, i.e. radioactive, isotopes.

What health effects can chlorine cause in water?

Chlorine in the form of chloride is also essential for humans. The chlorine content in the human body is around 1200 ppm. We take in around 3-6.5 g daily, mainly in the form of table salt, of which only about 3 g are actually needed. About 17% of this comes from the food itself, while 40-50% is added during the production process and 30-40% by the consumer himself. About 3% of the amount comes from drinking water.
Chloride is the most important anion in the extracellular fluid, and it influences the water-electrolyte balance. As a component of hydrochloric acid, it is important for the formation of stomach acid. With increased intake, the chloride balance can usually be maintained, as the excess is excreted with the urine. The triggering of high blood pressure and heart disease through excessive consumption of table salt is attributed to its constituent sodium and less to chloride (see sodium and water).
While chloride is considered stable and relatively non-toxic, chlorine gas is very reactive and can quickly become dangerous. When preparing drinking water from surface water, there are often objections to the use of chlorine, as it can form trihalomethanes with the humic acids that are normally present, which are considered harmful.
Chlorine forms active oxygen and hydrochloric acid on the mucous membranes, which attack the tissue. When inhaled, the gas acts as a strong lung toxin that can even cause lung bleeding. A concentration of 500 ppm inhaled for five minutes can be fatal. Interestingly, however, white blood cells could use the element to protect the body from infectious diseases.
Toxic effects of chlorine compounds occur mainly in oxidizing forms such as chlorates. Chloral hydrate is also an example of a poisonous chlorine compound, with around 10-15 g being lethal. Like numerous other chlorine compounds, the substance also has a mutagenic and carcinogenic effect.
For example, DDT can also accumulate in the body. Its acute toxicity manifests itself in feelings of fear and dizziness, as well as headaches, confusion and imbalance, up to a coma.

What water purification technologies can be used to remove chlorine?

Chlorine or chloride can normally be found in all wastewater. While the concentration of the substance in drinking water is only around 30 mg / L, this can be as much as 120 mg / L in municipal wastewater. Wastewater from slaughterhouses contains around 1 g / L and that from fish processing even contains 24 g / l.
Ion exchangers and reverse osmosis, possibly in combination with electrodialysis, offer options for removing chlorine and its compounds from water. Activated carbon can also be used in some cases, or the use of sulfur dioxide or other reduced sulfur compounds.
The removal of chlorine by natural microorganisms takes a relatively long time and the result is not always complete. In 1997, however, a bacterium was discovered that is capable of this.
Chlorine itself can also be used for water treatment and, above all, disinfection. The effectiveness of chlorine is based on the following reaction equation:
Cl2 + H2O -> 2H+ + 2Cl- + O
The oxygen released from the hypochlorous acid can oxidize impurities. At pH values ​​below 6 the chlorine is in molecular form, while at pH values ​​above 10 the form of the hypochlorite ion predominates. This is significantly less effective. If the pH value is between 5 and 10, both forms are present (HOCl <-> H+ + OCl-).
This method is very inexpensive, but it can lead to an undesirable lowering of the pH value and an increase in the chloride content. Undesired by-products can also be formed. It is positive that the chlorine remains in the water and continues to have an effect for a while, in contrast to disinfection with ozone. The two methods can therefore also be combined.
In addition to chlorine gas, solid and liquid chlorine compounds can also be used for disinfection. These are often easier to use and less dangerous for the environment. However, the costs are usually higher.
A stronger disinfection with fewer by-products can be achieved by using chlorine dioxide. This is less reactive, which reduces the consumption of chlorine and also the risk of the formation of carcinogenic trihalomethanes. In addition, the typical chlorine odor is much less pronounced.
In addition to killing germs, chlorine can also oxidize organic compounds. For this reason, however, the chlorine concentration in the water initially decreases as the addition of chlorine increases. On the other hand, it increases proportionally with further addition. Effective chlorination should therefore go just beyond this turning point.

The drinking water standards of WHO and EU specify a maximum chloride content of 250 mg / l. This has also been adopted in German legislation.

Comparison of drinking water standards

References


To the periodic table of the elements

To the overview of the elements and water