The chlorine cycle (Cl) is the biogeochemical cycling of

chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...

through the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...

biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...

, and

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

. Chlorine is most commonly found as inorganic

chloride ions The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

, or a number of

chlorinated In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...

organic forms. Over 5,000 biologically-produced chlorinated organics have been identified. The cycling of chlorine into the atmosphere and creation of chlorine compounds by anthropogenic sources has major impacts on climate change and depletion of the ozone layer. Chlorine plays essential roles in many biological processes, including numerous roles in the human body. It also acts as an essential co-factor in enzymes involved in plant

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

Troposphere

Chlorine plays a large role in atmospheric cycling and climate, including, but not limited to

chlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and prop ...

s (CFCs). The major flux of chlorine into the troposphere comes from sea salt aerosol spray. Both organic and inorganic chlorine is transferred into the troposphere from the oceans. Biomass combustion is another source of both organic and inorganic forms of chlorine to the troposphere from the terrestrial reservoir. Typically, organic chlorine forms are highly un-reactive and will be transferred to the stratosphere from the troposphere. The major flux of chlorine from the troposphere is via surface deposition into water systems.

Hydrosphere

Oceans are the largest source of chlorine in the Earth's hydrosphere. In the hydrosphere, chlorine exists primarily as chloride due to the high solubility of the Cl⁻ ion. The majority of chlorine fluxes are within the hydrosphere due to chloride ions' solubility and reactivity within water systems. The cryosphere is able to retain some chlorine deposited by rainfall and snow, but the majority is

eluted In analytical and organic chemistry, elution is the process of extracting one material from another by washing with a solvent; as in washing of loaded ion-exchange resins to remove captured ions. In a liquid chromatography experiment, for exa ...

into oceans.

Lithosphere

The largest reservoir of chlorine resides in the lithosphere, where of global chlorine is found in Earth's

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

. Volcanic eruptions will sporadically release high levels of chlorine as

HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a spe ...

into the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

, but the majority of the terrestrial chlorine flux comes from seawater sources mixing with the mantle. Organically bound chlorine is as abundant as chloride ions in terrestrial soil systems, or the

pedosphere The pedosphere (from Greek ''pedon'' "ground" or "earth" and ''sphaira'' "sphere") is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, h ...

. Discovery of multiple Cl-mediating genes in microorganisms and plants indicate that numerous biotic processes use chloride and produce organic chlorinated compounds, as well as many abiotic processes. These chlorinated compounds can then be

volatilized In chemistry, volatility is a material quality which describes how readily a substance Vaporization, vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapour, while a substance with low ...

or leached out of soils, which makes the overall soil environment a global sink of chlorine. Multiple anaerobic

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

s have been found to contain genes and show activity for chlorinated organic volatilization

Biological processes

Chlorine's ability to completely dissociate in water is also why it is an essential electrolyte in many biological processes. Chlorine, along with

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

, is the sixth most common element in organic matter. Cells utilize chloride to balance pH and maintain

turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibri ...

at equilibrium. The high electrical conductivity of Cl⁻ ions are essential for neuron signalling in the brain and regulate many other essential functions in biology

Anthropogenic chlorinated compounds

The depleting effects of

s (CFCs) on

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

over

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

has been studied extensively since the 1980s. The low reactivity of CFCs allow it to reach the upper

stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...

, where it interacts with UV-C radiation and forms highly reactive chloride ions that interact with methane. These highly reactive chlorine ions will also interact with volatile organic compounds to form other ozone depleting acids.

Chlorine-36 Chlorine-36 (36Cl) is an isotope of chlorine. Chlorine has two stable isotopes and one naturally occurring radioactive isotope, the cosmogenic isotope 36Cl. Its half-life is 301,300 ± 1,500 years. 36Cl decays primarily (98%) by beta-minus decay ...

is the radioactive isotope produced in many nuclear facilities as byproduct waste. Its half-life of , mobility in the

, and ability to be taken up by organisms has made it an isotope of high concern among researchers. The high solubility and low reactiveness of is also has also made it a useful application for research of biogeochemical cycling of chlorine, as most research uses it as an isotope tracer

References

{{Biogeochemical cycle Biogeochemical cycle Chlorine