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Iodine cycle

Iodine cycle is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Iodine cycle rather than just read about it. In short: The iodine cycle is a biogeochemical cycle that primarily consists of natural and biological processes that exchange iodine through the lithosphere, hydrosphere, and atmosphere. Iodine exists in many forms, but in the environment, it generally has an oxidation state of −1, 0, or +5.

Iodine cycle — main illustration
Iodine cycle — illustration

Key takeaways

  • Iodine cycle belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Iodine cycle to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Iodine cycle from memory before moving on to harder problems.

Reference excerpt

The iodine cycle is a biogeochemical cycle that primarily consists of natural and biological processes that exchange iodine through the lithosphere, hydrosphere, and atmosphere. Iodine exists in many forms, but in the environment, it generally has an oxidation state of −1, 0, or +5.

Oceanic cycling

Iodine in the ocean exists mostly in oceanic sediments and seawater. During subduction of oceanic crust and seawater, most of the iodine cycles into seawater through brine, while a minor amount is cycled into the mantle. Marine biota, including seaweed and fish, accumulate iodine from the seawater and return it during decomposition. Sedimentation of oceanic iodine replenishes the ocean sediment sink. The losses of iodine from the oceanic sink are to the atmospheric sink. Sea spray aerosolization accounts for a portion of this loss. However, the majority of the iodine cycled into the atmosphere occurs through biological conversion of iodide and iodate to methyl forms, primarily methyl iodide. Algae, phytoplankton, and bacteria are involved in reducing the stable Iodate ion to iodide, and different species produce volatile methyl iodide which leaves the oceans and forms aerosols in the atmosphere.

Terrestrial cycling Iodine rarely occurs naturally in mineral form, so it comprises a very small portion of rocks by mass. Sedimentary rocks have higher concentrations of iodine compared to metamorphic and igneous rocks. Due to the low concentration of iodine in rocks, weathering is a minor flux of iodine to soils and the freshwater hydrosphere. Soils contain a much higher concentration of iodine compared to their parent rock, though most of it is bound to organic and inorganic matter, potentially due to microbial activity. The major source of iodine to soils is through dry and wet deposition of aerosolized iodine in the atmosphere. Due to the high production of atmospheric iodine from the oceans, both the concentration of iodine and the flux of iodine to soils is greatest near coastal regions. Plants uptake iodine from the soil through their roots and return the iodine when they decompose. Fauna that consume plants may uptake this iodine but similarly return it to soils upon decomposition. Some iodine may also be cycled into the freshwater hydrosphere through leaching and runoff, where it may return to the oceans. Similar to oceanic iodine, the majority of iodine cycled out of soil is volatilized through conversion to methyl forms of iodine by bacteria. Unlike ocean volatilization, however, bacteria are thought to be the only organisms responsible for volatilization in soils.

Anthropogenic influences Iodine is a necessary trace nutrient for human health and is used as a product for various industries. Iodine intended for human use and consumption is taken from brines, which accounts for a minor perturbation to the global iodine cycle. A much larger anthropogenic impact is through the burning of fossil fuels, which releases iodine into the atmosphere. Iodine-129, a radioisotope of iodine, is a waste product of nuclear power generation and weapons testing. Unless present in high concentrations, I-129 likely does not present danger to human health. Early research has attempted to use the I-129/I-127 ratio as a tracer for the iodine cycle.

References

Illustrations

Iodine cycle: Biogeochemical iodine cycle: Inventories are in Tg iodine per year. Labeled flux arrows are in Gg iodine per year. Unlabeled inventories (sinks) and fluxes are of unknown quantities. Iodine cycles through the lithosphere, atmosphere, hydrosphere, and biosphere. [1][2][3][4] Freshwater iodine is calculated by subtracting oceanic iodine[4] from total iodine in the hydrosphere.[1] In oceans sediments and crust, iodine is replenished by sedimentation[1] and is cycled into seawater through release as brine during subduction.[4] Marine biota uptake iodine from seawater[1] where it may be volatilized by transformation to methyl iodide.[3] Sea spray aerosolization, volcanic activity, and fossil fuel burning cycles iodine from the hydrosphere and lithosphere into the atmosphere as well,[1] while wet[2] and dry deposition remove iodine from the atmosphere.[1] In soil, small quantities of iodine are cycled through weathering of parent rock.[1] Terrestrial biota uptake and remove iodine from soil, and bacteria volatilize iodine by methylizing it.[1]
Biogeochemical iodine cycle: Inventories are in Tg iodine per year. Labeled flux arrows are in Gg iodine per year. Unlabeled inventories (sinks) and fluxes are of unknown quantities. Iodine cycles through the lithosphere, atmosphere, hydrosphere, and biosphere. [1][2][3][4] Freshwater iodine is calculated by subtracting oceanic iodine[4] from total iodine in the hydrosphere.[1] In oceans sediments and crust, iodine is replenished by sedimentation[1] and is cycled into seawater through release as brine during subduction.[4] Marine biota uptake iodine from seawater[1] where it may be volatilized by transformation to methyl iodide.[3] Sea spray aerosolization, volcanic activity, and fossil fuel burning cycles iodine from the hydrosphere and lithosphere into the atmosphere as well,[1] while wet[2] and dry deposition remove iodine from the atmosphere.[1] In soil, small quantities of iodine are cycled through weathering of parent rock.[1] Terrestrial biota uptake and remove iodine from soil, and bacteria volatilize iodine by methylizing it.[1]
Iodine cycle illustration

Worked examples

Example 1 — a first encounter with Iodine cycle

Start with the simplest possible case. Write down what Iodine cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Iodine cycle before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Iodine cycle ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Iodine cycle

In research
Iodine cycle appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Iodine cycle in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Iodine cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biogeochemical cycle, Iodine, so understanding it makes those chapters shorter.
In everyday life
Look for Iodine cycle outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Iodine cycle in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Iodine cycle means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Iodine cycle out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Iodine cycle in simple terms?

The iodine cycle is a biogeochemical cycle that primarily consists of natural and biological processes that exchange iodine through the lithosphere, hydrosphere, and atmosphere. Iodine exists in many forms, but in the environment, it generally has an oxidation state of −1, 0, or +5.

Why does Iodine cycle matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Iodine cycle?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Iodine cycle.

Tags

  • Biogeochemical cycle
  • Iodine

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