Iodine

Typical wastewaters

• hospital effluent and municipal sewage (radiopharmaceuticals) iodine-131 excreted by thyroid patients, passing activated sludge and leaving mostly in the effluent at 1.8 to 227 Bq/L one small plant with a thyroid cancer treatment centre in its catchment; earlier studies of larger plants up to 32.2 Bq/L
• nuclear reactor releases (raw water supplies after an accident) iodine-131; speciation not stated by the source; not detected by gross activity screening removal 10 to 40 percent by coagulation or sand filtration, 40 to 70 by activated carbon or ion exchange, above 70 by reverse osmosis (WHO Table 9.4)
• oil and gas produced water and hydraulic fracturing flowback iodide up to 56 mg/L in Appalachian brines; brine treatment plant effluents up to 28 mg/L promotes iodinated disinfection by-products at downstream chlorination
• hospital wastewater (iodinated X-ray contrast media) iodinated contrast media making up 95 percent of the micropollutant mass in biologically treated hospital wastewater; diatrizoate recalcitrant 50 to 65 percent removal by ozone (1.08 g O₃ per g DOC), 23 mg/L PAC or UV 2400 J/m₂
• municipal wastewater effluent (stable iodine) iodide and iodate; 2 to 25 percent captured in sludge, the rest discharged effluent average 4.0 µg/L (1.0 to 16 µg/L); some effluent streams up to 1,910 µg/L

1 · Identity

Symbol, number

I, 53

Oxidation states in water

-1 iodide (I⁻, the natural form in almost all water); 0 molecular iodine (I₂, only when dosed as a disinfectant or formed transiently by oxidants); +1 hypoiodous acid (HOI) and hypoiodite (OI⁻), the reactive intermediates formed by chlorine, chloramine, ozone and permanganate; +5 iodate (IO₃⁻), the stable end product of strong oxidation and the harmless sink. Triiodide I₃⁻ forms when I₂ meets excess iodide (element entry).

Note

The element entry covers the hydrolysis to iodide and iodate in alkali and the triiodide equilibrium. This chapter follows iodide into the treatment plant, where the oxidant decides whether it ends as iodate or as an iodinated organic.

2 · Occurrence in water

Natural sources

Iodine occurs naturally in water as iodide (WHO fact sheet). The ocean is the reservoir (element entry), so iodide is highest in seawater, saline groundwater, brines and coastal aquifers with marine intrusion; rivers and lakes carry a few micrograms per litre. Traces of iodine are produced by oxidation of iodide during water treatment (WHO).

Anthropogenic sources

Iodine dosed deliberately as a field or emergency disinfectant (WHO 2018: 2.5 to 7 mg/L historically, tablets since the 1940s, about 2 mg/L residual aboard spacecraft); iodine-131 excreted by thyroid patients and reaching sewage (Rose 2012); iodine chemicals manufacture and iodinated X-ray contrast media are in the ledger, not sourced here.

3 · Speciation

In untreated water iodine is iodide, a conservative, weakly sorbed anion. Added I₂ hydrolyses to HOI and iodide; between pH 5 and 9 the active disinfectant shifts from I₂ (99 percent at pH 5) towards HOI, and at pH 8 and above HOI slowly disproportionates to iodate and iodide (WHO 2018). Chlorine and ozone push iodide all the way to iodate; chloramine stops at HOI, which is why chloraminated coastal supplies form iodinated by-products (WHO fact sheet). Iodate is inert and non toxic; HOI is the species that reacts with natural organic matter.

Solubility

Elemental iodine is only sparingly soluble: 0.34 g/L at 25 C in the WHO background document (about 1 g in 3.5 L in the element entry); a saturated solution made by passing water through an iodine crystal column holds about 200 mg/L at 10 C and 400 mg/L at 30 C and is diluted to the dose (WHO 2018). Iodide and iodate salts are freely soluble; no iodine solid limits iodine in natural water.

Hydrolysis

I₂ + H₂O gives HOI, iodide and a proton; the overall stoichiometry holds between pH 2 and 7 (WHO 2018, after Lengyel, Epstein and Kustin 1993). The hydrolysis is less pH sensitive than that of chlorine, which is why iodine keeps its efficacy from pH 4.5 to 8.1 and declines only slowly to pH 10 (WHO 2018).

Complexation

Iodide forms the triiodide ion with I₂ (element entry) and is otherwise a free anion; iodine binds covalently to natural organic matter through HOI substitution, which is the by-product route, not a complexation equilibrium. No stability constants are printed in the sources read.

Precipitates

None relevant in water treatment. Silver iodide is the analytical and photographic precipitate (element entry) and the reason iodide interferes with silver based disinfection; no iodine solid forms in ordinary treatment.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Iodide is stable in a clean bottle but oxidises to I₂ and volatilises if the sample is chlorinated or acidified in air; quench disinfectant residuals and keep samples cool and dark. Iodine residual is measured on site. Iodine-131 samples decay by half every 8 days, so the count must be corrected to sampling time.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Essential for thyroid hormones; WHO/FAO recommended intakes 90 to 200 µg/day, JECFA PMTDI 1000 µg/day, upper intake levels 500 to 1100 µg/day. Excess causes hypothyroidism with or without goitre, hyperthyroidism and changes in thyroid malignancies; no threshold for thyrotoxicosis could be identified, and iodine and iodide differ in their thyroid effects, which is why no guideline exists (WHO). Iodinated DBPs are the concern in treated water; iodine-131 is a thyroid dose.

Bioaccumulation

Iodine concentrates in seaweed (0.1 to 0.2 percent iodide by weight) and seafood (200 to 1000 µg/kg), the natural dietary sources (WHO background document); iodine-131 concentrates in thyroid tissue.

Ecotoxicity

Not addressed in the sources read; no EPA aquatic life criterion or EU EQS exists for iodine.

Flags

• The 0.5 to 20 µg/L river and lake figure is a WHO compilation of averages; coastal and saline groundwaters run far higher and were not sourced.
• The seawater figure is the book's own element entry, not a water chemistry source.
• The iodine-131 effluent range is one plant with a thyroid cancer centre in its catchment (Rose 2012).
• The WHO 2018 disinfectant document prints an iodine solubility of 0.03 mg/L at 20 C, which contradicts the background document (0.34 g/L at 25 C) and the element entry; the g/L figure is used and the mg/L figure treated as a misprint.
• The iodate formation, chloramine and iodometric equations are electron balances written here; WHO describes the pathways without printing equations.
• The Standard Methods 4500-I method numbers are cited from memory of the method index, not re-read; no DOI was obtained.
• Iodine detection limits by ICP-MS and ion chromatography were not read.

Gaps

• No source read gives iodide in groundwater, seawater as a range, municipal wastewater or industrial wastewater; only the WHO averages and one iodine-131 effluent study are quoted.
• Iodinated DBP occurrence data (iodoacetic acid, iodinated trihalomethanes in µg/L) were not read; the WHO fact sheet only states that they occur in coastal plants.
• Rate constants for iodide oxidation by chlorine, chloramine and ozone are not in the sources read.
• The GCC tables list no iodine; other GCC states not read.
• Iodine resin disinfection is not covered; iodide removal by anion exchange is written as the standard halide exchange behind the WHO radioiodine table, no resin capacity or selectivity figure was read, and silver impregnated carbon is not covered.
• The HOI dissociation constant, the iodide ozonation rate and the thiosulfate titration factors are not in the sources read; the triiodide and silver iodide equations come from the book's own iodine entry.

Sources

Identity

Name and symbol

Iodine, I

Atomic number

53 protons

Position

group 17 · period 5 · p-block · diatomic nonmetal

CAS number

7553-56-2

Atomic structure

Atomic mass

126.904 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵
[Kr] 5s²⁴d¹⁰⁵p⁵

Electrons per shell

2, 8, 18, 18, 7

Valence electrons

7 outer shell

Physical properties

State at room temperature

Solid

Melting point

386.85 K (113.7 °C)

Boiling point

457.55 K (184.4 °C)

Density

4.93 g/cm3

Appearance

lustrous metallic gray solid, black/violet liquid, violet gas

Thermal conductivity

0.449 W/(m·K)

Electrical resistivity

1.3×10⁷ Ω·m at 0 °C

Electrical conductivity

7.69e-8 S/m

Crystal structure

base-centered orthorhombic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+7, +5, +1, -1

Electronegativity

2.66 (Pauling Scale)

Ionisation energy

10.451 eV
1st 1,008.4, 2nd 1,845.9, 3rd 3,180 kJ/mol

Electron affinity

3.059 eV

Atomic radius

empirical 139, covalent 139, van der Waals 198 pm

Ionic radius

I⁻ 220; I⁵⁺ 95; I⁷⁺ 53 pm

Reactivity

The least reactive of the stable halogens and the weakest oxidising agent among them, a volatile blue-black solid; it combines with many elements but is displaced from iodides by the lighter halogens and shows some metallic character.

with water

Only slightly soluble in water (about 1 g in 3.5 L at 20 C), dissolving far better as triiodide when iodide is present: $\ce{I2 + I- -> I3^-}$, and it disproportionates only slightly in neutral water.

with oxygen, air

Does not react with oxygen directly; the pentoxide I2O5 is made by dehydrating iodic acid.

with acids

No reaction with acids; in alkali it disproportionates to iodide and iodate: $\ce{3 I2 + 6 OH- -> 5 I- + IO3^- + 3 H2O}$.

with halogens

Combines with the lighter halogens to interhalogens such as iodine monochloride: $\ce{I2 + Cl2 -> 2 ICl}$, and with fluorine to IF5 and the heptafluoride IF7.

Typical compounds

• HI hydrogen iodide hydroiodic acid, strong acid used in acetic acid manufacture
• KI potassium iodide iodised salt; dissolves iodine as triiodide
• AgI silver iodide yellow photographic halide
• I₂O₅ iodine pentoxide oxidises carbon monoxide; CO detection reagent
• KIO₃ potassium iodate stable iodate salt; iodate minerals are an ore
• CHI₃ iodoform organoiodine from the iodoform test

Occurrence, production and use

Crustal abundance

4.5×10-1 milligrams per kilogram

Oceanic abundance

6×10-2 milligrams per liter

Occurrence and sources

Ultrapure iodine can be obtained from the reaction of potassium iodide with copper sulfate. Several other methods of isolating the element are known.

• iodate in nitrate (caliche) ores Atacama desert, Chile; the leading world source
• iodide in oilfield, gasfield and salt-well brines Chiba, Japan; north-western Oklahoma; Turkmenistan, Azerbaijan, Iran
• seawater (0.05 to 0.06 ppm) and Laminaria seaweeds (up to 0.45 percent dry) oceans hold about 90 billion t; seaweed was the source before 1959
• crustal and oceanic abundance about 0.71 ppm (BGS figure via RSC); 0.45 mg/kg crust and 0.06 mg/L seawater (PubChem)

Extraction, production

Release of iodine from iodate recovered from nitrate ores

RSC states iodine is liberated from the iodate of nitrate ores but names no reductant, so no equation is written

Extraction of iodine vapour from processed brine

RSC states iodine is stripped as vapour from brine after processing; the oxidant that converts iodide to iodine is not named, so no equation is written

Courtois, 1811 (historical)

sulfuric acid added to seaweed ash released purple iodine vapour; stoichiometry not stated

Uses

Iodine is used as a test for starch and turns a deep blue when it comes in contact with it. Potassium iodide (KI) is used to make photographic film and, when mixed with iodine in alcohol, as an antiseptic for external wounds. A radioactive isotope of iodine, iodine-131, is used to treat some diseases of the thyroid gland.

Care should be taken in handling and using iodine. It can burn the skin and damage the eyes and mucous membranes. Pure iodine is poisonous if ingested.

Iodine compounds are important in organic chemistry and very useful in medicine. Iodides, and thyroxine which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds. Potassium iodide finds use in photography. The deep blue color with starch solution is characteristic of the free element.

• Pharmaceuticals (contrast media and drugs): iodinated X-ray contrast media, the leading world use of iodine; iodide salts and alkyl iodides in drug synthesis; iodine recovered from mother liquors; iodine-131 for thyroid therapy X-ray contrast media and pharmaceuticals rank first and second worldwide by quantity (usgs-mcs2025-iodine)
• Food and beverage (sanitisers, feed, supplements): iodophor and polyvinylpyrrolidone-iodine disinfectants in cleaning-in-place; animal feed iodine supplements; iodised salt and food supplements
• Electronics (LCD polarisers): iodine-doped polarising films for liquid crystal displays
• Chemicals: fluorochemical intermediates; catalysts; nylon; printing inks and dyes; photographic silver iodide inorganic iodine compounds about 80 percent and organic about 20 percent of US consumption in 2024 (usgs-mcs2025-iodine)
• Medicine and biocides: disinfectants and biocides; antibiotics, bromine or chlorine are the only partial substitutes
• Mining (brines and nitrate ores): by-product of Chilean nitrate mining and of oil and gas brine production

Safety, toxicity

Care should be taken in handling and using iodine, as contact with the skin can cause lesions; iodine vapor is intensely irritating to the eyes and mucus membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m3 (8-hour time-weighted average - 40-hour).

Discovery and name

Discovered by

Bernard Courtois

Discovered

1811

First isolated

not in sources

Named by

not in sources

Origin of the name

from the Ancient Greek ιώδης, "violet", for the color of its vapor

Iodine is a bluish-black, lustrous solid, volatizing at ordinary temperatures into a blue-violet gas with an irritating odor; it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions. It is only slightly soluble in water.