Radon

Typical wastewaters

• oil and gas produced water (and the gas and gas liquid streams it separates from) dissolved Rn-222 travelling with the brine from the reservoir and degassing at the separators Algerian onshore fields: 0.98 to 18.50 Bq/L in produced water, 40 to 1000 Bq/m₃ in natural gas and 300 to 2500 Bq/m₃ in natural gas liquids (Lucas cell and electret ion chamber); IAEA Table III gives 5 to 200,000 Bq/m₃ in natural gas and lists produced water as the largest NORM waste stream by volume (IAEA SRS 34)
• underground uranium mine water dissolved Rn-222 released to the mine air as the water flows, the release rate correlating with water flow rate Indian underground uranium mines: radon dose from mine water 2.3 to 9.6 µSv/y (geometric means, two mines) and 4.9 to 27.1 µSv/y (arithmetic means, three mines), below the 100 µSv/y value cited; mine water ranks after backfilled tailings and ore as a radon source (Sahu 2023); no dissolved activity in Bq/L was read
• drinking water treatment (aeration off-gas and granular activated carbon beds) radon stripped to air from the top of packed towers and diffused bubble units; on GAC, adsorbed radon in a steady state with its short lived progeny while lead-210 accumulates on the carbon, which is discarded rather than regenerated venturi aeration removed 78 to 95 percent on two US supplies; the gamma dose from a GAC unit modelled for 185 kBq/m₃ in and 25 kBq/m₃ out is 0.148 to 0.173 µSv/h for a point of entry unit (NRC 1999 Appendix E); medium and large communities need a removal route that avoids discharging radon to the atmosphere via aeration off-gas

1 · Identity

Symbol, number

Rn, 86

Oxidation states in water

0 only: a dissolved noble gas that reacts with nothing in water, forms a clathrate hydrate (element entry), and leaves the water by degassing or by decay. Radon-222 (half life 3.8 days, alpha, from radium-226) is the isotope of water supplies; radon-220 (thoron, 56 seconds) is too short lived to matter.

Note

The element entry gives the physics and the fluorides. In water radon has no chemistry, only solubility, half life and progeny.

2 · Occurrence in water

Natural sources

Continuous production from radium-226 in aquifer rock; groundwater from wells and boreholes usually contains higher radon concentrations than surface water, and in some extreme circumstances very high concentrations occur in supplies from crystalline bedrock; radon in surface water is readily released to outdoor air by agitation, so high concentrations are seldom found in surface supplies (WHO chapter 9). Solubility falls rapidly with rising temperature (WHO).

Anthropogenic sources

None as a discharge; uranium mine water and radium bearing treatment residuals release radon, and granular activated carbon beds that strip radon become radioactive as the progeny lead-210 and polonium-210 accumulate.

3 · Speciation

Radon is a dissolved monatomic gas. Its concentration in water is a balance between production from radium in the rock, decay with a 3.8 day half life, and loss to the atmosphere wherever the water meets air; stirring, transfer between containers, standing and above all boiling release it (WHO 9.7.4). Nothing in water treatment changes its chemical state; treatment is degassing or decay.

Solubility

Radon dissolves in water and forms a clathrate hydrate about as stable as the chlorine hydrate (element entry); solubility decreases rapidly with increasing temperature (WHO). No Henry constant was read.

Hydrolysis

None.

Complexation

None.

Precipitates

None; the progeny lead-210 and polonium-210 are the solids that appear in treatment media and scale.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Radon is lost by stirring, by transfer between containers, on standing and completely on boiling (WHO 9.7.4). Collect from a running tap with no aeration, fill the vial from the bottom without headspace, cap at once, record the time and count within a day or two, correcting to sampling time for the 3.8 day half life. Never sample after a storage tank or aerator if the well is the question.

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

Long term exposure to high radon in indoor air increases lung cancer risk (WHO 2009 via chapter 9); radon ingested in drinking water gives a dose to the lining of the stomach, but on average 90 percent of the dose from radon in water is by inhalation of the released gas and its progeny (UNSCEAR 2000 via WHO).

Bioaccumulation

None; a gas with a 3.8 day half life.

Ecotoxicity

Not relevant; radon degasses from surface water.

Flags

• WHO sets no numeric radon in water level; the 100 Bq/L figure is Euratom, not WHO, and the brief's 'WHO screening' is read as WHO's advice to set national screening levels from the air reference.
• The US 300 and 4000 pCi/L values are a 1999 proposal never promulgated; they are quoted from EPA's consumer fact sheet, not the Federal Register.
• The Norwegian concentrations are from a two well time series paper; Finnish and Swedish national survey figures were not read and are not quoted.
• The 1000 Bq/L to 100 Bq/m₃ transfer figure is WHO's average; the unit in the extracted text was cut and is restored as Bq/m₃.
• The degassing equation is a phase transfer written here so that the chapter carries at least one equation; radon has no aqueous reaction.
• ISO 13164 and Standard Methods 7500-Rn are cited from their numbers, not read; the 10 Bq/L detection limit is the Euratom performance characteristic.

Gaps

• No national survey of radon in groundwater (Finland, Sweden, USA) was read; the Norwegian two well study and WHO's qualitative statements stand in.
• Henry's law constant and the temperature dependence of radon solubility are not quantified in the sources read.
• The EPA 1999 proposal's monitoring and multimedia mitigation provisions were not read beyond the consumer fact sheet.
• Radon-220 (thoron) in water and radon in seawater are not covered.
• No GCC document mentions radon; none was read.
• Radon has no reaction chemistry in water, so no further equation is written: the phase transfer already given is the only one, and radioactive decay is not a chemical equation.

Sources

Identity

Name and symbol

Radon, Rn

Atomic number

86 protons

Position

group 18 · period 6 · p-block · noble gas

CAS number

10043-92-2

Atomic structure

Atomic mass

222 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶
[Xe] 6s²⁴f¹⁴⁵d¹⁰⁶p⁶

Electrons per shell

2, 8, 18, 32, 18, 8

Valence electrons

8 outer shell

Physical properties

State at room temperature

Gas

Melting point

202 K (-71.15 °C)

Boiling point

211.45 K (-61.7 °C)

Density

0.0097 g/cm3 (gas at STP, so 9.73 g/L)

Appearance

colorless gas

Thermal conductivity

3.61-3 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

face-centered cubic |crystal structure comment=(predicted)

Molar heat capacity

not in sources

Chemical properties

Oxidation states

0

Electronegativity

2.6 (Allen Scale)

Ionisation energy

10.745 eV
1st 1,037 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 150, covalent 150, van der Waals 220 pm

Ionic radius

not in sources

Reactivity

A radioactive noble gas with a closed 6p shell, chemically inert in ordinary conditions though slightly less so than xenon; its only established compounds are fluorides, plus oxide species claimed at trace scale, and its 3.8 day half-life and cost keep its chemistry almost unstudied.

with water

Does not react with water; it dissolves in it and forms a clathrate hydrate about as stable as the chlorine hydrate.

with oxygen, air

No reaction with oxygen; a trioxide RnO3 has been claimed only by hydrolysing a higher fluoride made at trace scale.

with acids

Does not react with acids; radon is pumped unchanged out of acid solutions of its parents (radium sources, or uranium ores in 1 percent hydrochloric or hydrobromic acid).

with halogens

Fluorine oxidises it to the difluoride: $\ce{Rn + F2 -> RnF2}$, which decomposes above 250 C and is reduced back to radon by water or hydrogen; the halogen fluorides ClF3, BrF5 and IF7 do the same.

Typical compounds

• RnF₂ radon difluoride the one confirmed compound, from radon and fluorine
• [RnF][Sb₂F₁₁] fluororadon hexafluoroantimonate salt of the RnF+ cation made in antimony pentafluoride

Occurrence, production and use

Crustal abundance

4×10-13 milligrams per kilogram

Oceanic abundance

6×10-16 milligrams per liter

Occurrence and sources

• decay product of radium-226 in rocks; trace in the atmosphere everywhere, accumulating indoors over uranium-bearing ground and in granite buildings
• crustal and oceanic abundance 4 x 10^-13 mg/kg crust and 6 x 10^-16 mg/L seawater (PubChem); 0.0000000000004 ppm (BGS via RSC)

Extraction, production

Natural decay of radium-226

radon was first collected from radium as it decayed; no industrial production

Uses

Small amounts of radon are sometimes used by hospitals to treat some forms of cancer. Radon fluoride (RnF) is the only confirmed compound of radon.

Radon is still produced for therapeutic use by a few hospitals by pumping it from a radium source and sealing it in minute tubes, called seeds or needles, for application to patient. This practice has been largely discontinued as hospitals can get the seeds directly from suppliers, who make up the seeds with the desired activity for the day of use.

Safety, toxicity

Care must be taken in handling radon, as with other radioactive materials. The main hazard is from inhalation of the element and its solid daughters which are collected on dust in the air. Good ventilation should be provided where radium, thorium, or actinium is stored to prevent build-up of the element. Radon build-up is a health consideration in uranium mines. Recently radon build-up in homes has been a concern. Many deaths from lung cancer are caused by radon exposure. In the U.S. it is recommended that remedial action be taken if the air in homes exceeds 4 pCi/l.

Discovery and name

Discovered by

Ernest Rutherford and Robert B. Owens

Discovered

1899

First isolated

William Ramsay and Robert Whytlaw-Gray

Named by

not in sources

Origin of the name

shortened from "radium emanation", since it is released as radium decays

Radon is present in the atomosphere at very low concentrations. See Wikipedia for discussion of concentration. At ordinary temperatures radon is a colorless gas; when cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. It has been reported that fluorine reacts with radon, forming a fluoride. Radon clathrates have also been reported.