Radium

Typical wastewaters

• oil and gas produced water and flowback (NORM) Ra²⁺ with RaCl⁺ and RaSO₄ ion pairs in brine above 100,000 mg/L TDS; Marcellus Shale total radium below detection to 18,000 pCi/L, median 2,460 pCi/L, radium-228 to radium-226 generally below 0.3 co-precipitates in barite when the brine meets sulfate, making NORM scale, filter cake and pond sludge
• uranium mine drainage and mill effluent dissolved Ra²⁺, limited in the US at 3 pCi/L monthly and 10 pCi/L daily dissolved radium-226, 10 and 30 pCi/L total limits, not measured concentrations
• drinking water treatment residuals (spent softening brine, sludge, manganese dioxide solids) Ra²⁺ concentrated in the sodium chloride regenerant brine of cation exchange softeners, about 600 pCi/L from a 10 pCi/L feed, in lime softening sludge and on hydrous manganese oxide at 21,000 pCi/g disposal to ponds, landfill, land spreading or sewer is state regulated as TENORM

1 · Identity

Symbol, number

Ra, 88

Oxidation states in water

+2 only, as Ra²⁺, a large, weakly complexing alkaline earth cation that follows barium into sulfate and calcium into bone; radium-226 (half life 1600 years, alpha, uranium-238 series) and radium-228 (5.75 years, beta, thorium-232 series) are the isotopes that matter, with radium-224 (3.64 days) as a short lived third (Clifford, Rowan).

Note

The element entry covers the salts and the decay chain. This chapter is about Ra²⁺ at picocurie levels: where it is, how to count it, and how to take it out without making a radioactive sludge problem.

2 · Occurrence in water

Natural sources

Decay of uranium-238 and thorium-232 in aquifer rock; radium enters water by recoil and desorption from grain surfaces and is highest where three geochemical conditions coincide: oxygen poor water, acidic conditions and high dissolved solids (USGS). About 3 percent of sampled wells in US principal aquifers exceeded the combined radium MCL, but more than 20 percent in the Mid Continent and Ozark Plateau Cambro Ordovician aquifer system and the Northern Atlantic Coastal Plain aquifer system; 98 percent of exceeding wells lie east of the High Plains (USGS Fact Sheet 2010-3113). Radium in formation brines rises with dissolved solids (Rowan 2011).

Anthropogenic sources

Oil and gas produced water and flowback (NORM), uranium mine drainage and mill effluent (regulated in the US at 3 pCi/L dissolved radium-226 monthly), phosphate rock processing and phosphogypsum, mine water and tailings in the ledger's mining chapter; water treatment residuals themselves become TENORM (EPA).

3 · Speciation

Radium is Ra²⁺ at every natural pH (Clifford), with chloride and sulfate complexes in brines (Rowan). It has no solid of its own at trace level: RaSO₄ is much less soluble than barite but radium never reaches its own solubility, so it is controlled by co-precipitation into barite and other alkaline earth sulfates, by sorption on manganese and iron oxides and clays, and by cation exchange. Radium stays in solution where sulfate is absent and competing cations are abundant, which is why saline, reducing, acidic groundwater and oilfield brines carry it.

Solubility

Radium sulfate is much less soluble than barite, anhydrite and other sulfate minerals (Rowan 2011), and RaSO₄ is the most insoluble sulfate known (element entry); radium chloride, bromide and nitrate are soluble. No solubility product was read.

Hydrolysis

Negligible; radium hydroxide is the strongest and most soluble alkaline earth hydroxide (element entry).

Complexation

Weak chloride and sulfate complexes in brines (Rowan); otherwise a free ion. Constants not read.

Precipitates

Ba(Ra)SO₄ solid solution in barite scale, filter cake and treatment sludge; radium sorbed on MnO₂ and Fe(OH)₃ sludges; radium in CaCO₃ and Mg(OH)₂ softening sludge; radium on spent cation resin.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify to pH below 2 at collection so radium does not sorb to the bottle or precipitate with barium sulfate; do not filter produced water samples before acidification or the radium on barite particles is lost. Count radium-228 promptly or correct for its 5.75 year decay; radium-224 decays in days. Gross alpha results must be reported with their counting error, and in high TDS water the thick source self absorbs and under reads (Rowan reports gross alpha and beta against the specific isotopes for exactly this reason).

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

A bone seeking alpha and beta emitter; the dose coefficients are 2.8 x 10⁻7 Sv/Bq for radium-226 and 6.9 x 10⁻7 Sv/Bq for radium-228 (WHO Table 9.2), so radium-228 carries the lower guidance level; the individual dose criterion of 0.1 mSv/year corresponds to an estimated annual cancer risk of about 5.5 x 10⁻6 (WHO).

Bioaccumulation

Follows calcium into bone (element entry); accumulates in treatment media and scale rather than in the food chain in the sources read.

Ecotoxicity

Not addressed in the sources read; no aquatic criterion exists for radium.

Flags

• The US occurrence distribution is the 1988 Longtin survey quoted by EPA; the USGS fact sheet gives only the percentages above the MCL, not concentrations.
• The produced water figures are one basin (northern Appalachian) and include historical data of variable quality, as Rowan notes.
• The co-precipitation and cation exchange equations are written from Clifford's slide notation; the removal percentages are his ranges and the residual activities his worked examples for a 10 pCi/L feed.
• EPA methods 903.1 and 904.0 are cited from their method numbers; only 903.0's scope was read. ISO 13165 and Standard Methods 7500-Ra were not read.
• The WHO Table 9.3 detection limits are for groundwater with TDS below 0.1 g/L; high TDS thick sources under read.
• GCC: only the Abu Dhabi prohibition on radioactive waste to sewer was read; no numeric radium discharge value was found.

Gaps

• No source read gives radium in surface water, seawater, municipal wastewater or phosphate industry effluent as numbers; the mining and fertiliser chapters of the ledger hold the tailings and phosphogypsum values.
• Solubility products of RaSO₄ and the Ba(Ra)SO₄ distribution coefficient are not printed in the sources read.
• Radium selective complexer run lengths and full scale HMO plant data were not read beyond Clifford's slides.
• Produced water treatment for radium (sulfate precipitation, zeolites) was not sourced.
• Detection limits for the EPA radium methods were taken from the rule's 1 pCi/L sensitivity requirement, not from the method texts.
• Other GCC standards and the oil and gas ELG (40 CFR 435) were not read.
• The radium carbonate equation stands for co-precipitation into calcium carbonate; radium does not reach its own carbonate saturation and no distribution coefficient was read.
• The hydrous manganese oxide preparation is the standard permanganate and manganous comproportionation, written here; Clifford names the preformed oxide without a recipe.

Sources

Identity

Name and symbol

Radium, Ra

Atomic number

88 protons

Position

group 2 · period 7 · s-block · alkaline earth metal

CAS number

7440-14-4

Atomic structure

Atomic mass

226 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s²
[Rn] 7s²

Electrons per shell

2, 8, 18, 32, 18, 8, 2

Valence electrons

2 outer shell

Physical properties

State at room temperature

Solid

Melting point

973 K (699.85 °C)

Boiling point

1,413 K (1,139.85 °C)

Density

5 g/cm3

Appearance

silvery white metallic

Thermal conductivity

18.6 W/(m·K)

Electrical resistivity

1 µΩ·m at 20 °C

Electrical conductivity

1 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+2

Electronegativity

0.9 (Pauling Scale)

Ionisation energy

5.279 eV
1st 509.3, 2nd 979 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 221, covalent 221, van der Waals 283 pm

Ionic radius

Ra²⁺ 148 (8-coordinate) pm

Reactivity

The heaviest alkaline earth metal, about as electropositive as barium and a little more volatile; it exists only as Ra2+ in solution, a large, weakly complexing ion whose salts mirror barium's (the sulfate is the most insoluble sulfate known), and the freshly made metal blackens in air and decomposes water.

with water

Decomposes water to the hydroxide and hydrogen: $\ce{Ra + 2 H2O -> Ra(OH)2 + H2}$, and radium hydroxide is the most soluble and most strongly basic of the alkaline earth hydroxides.

with oxygen, air

The brilliant white metal blackens quickly in air, forming the nitride rather than the oxide: $\ce{3 Ra + N2 -> Ra3N2}$, so the oxide RaO is poorly characterised.

with acids

Dissolves in acids as the colourless Ra2+ ion, evolving hydrogen: $\ce{Ra + 2 HCl -> RaCl2 + H2}$, giving the soluble chloride, bromide and nitrate; sulfuric acid instead precipitates the insoluble RaSO4.

with halogens

No direct reaction is described; the dihalides RaCl2 and RaBr2 (colourless, luminous, isomorphous with the barium salts) are made from solutions and crystallise as dihydrates.

Typical compounds

• RaCl₂ radium chloride the commercial form, luminous, yellows by self-radiolysis
• RaBr₂ radium bromide luminous, more soluble than the chloride, crystals can shatter
• RaSO₄ radium sulfate the most insoluble sulfate known
• Ra(NO₃)₂ radium nitrate less soluble in stronger nitric acid, used in purification
• Ra₃N₂ radium nitride black surface layer on the air-exposed metal
• Ra(OH)₂ radium hydroxide strongest and most soluble alkaline earth hydroxide

Occurrence, production and use

Crustal abundance

9×10-7 milligrams per kilogram

Oceanic abundance

8.9×10-11 milligrams per liter

Occurrence and sources

Originally, radium was obtained from the rich pitchblende ore found in Joachimsthal, Bohemia. The carnotite sands of Colorado furnish some radium, but richer ores are found in the Republic of Zaire and the Great Lake region of Canada. Radium is present in all uranium minerals, and could be extracted, if desired, from the extensive wastes of uranium processing. Large uranium deposits are located in Ontario, New Mexico, Utah, Australia, and elsewhere.

• radium-226 in uranium ores and uranium mine water present in all uranium ores (ores from DR Congo and Canada are richest); one MWEI operator reported radium-226 in extractive waste influenced water at 0.19 mg/l before and 0.06 mg/l after membrane treatment, and Canadian literature reports discharge below 0.025 mg/l (mwei-bref-2018, PDF p185, printed p157; co-precipitation PDF p466, printed p438; ion exchange PDF p473, printed p445); the mining chapter holds radium-226 as c-mn-ra-226
• radium-226 in phosphogypsum discharged to water from wet-process phosphoric acid Table 5.8 of the LVIC-AAF BREF reports radium-226 emissions of 1.4 and 2.3 mBq per tonne P2O5 (as printed) at two plants, 1996/97 data, plants since closed because of the discharge to sea, with polonium-210 and lead-210 at similar levels (PDF p253, printed p225); radioactivity is the main obstacle to using phosphogypsum as building gypsum, and stack leachate must be kept from soil and groundwater (PDF p271 and p275, printed p243 and p247)
• radium and radon in phosphogypsum phosphogypsum is stored indefinitely because of weak radioactivity from uranium and thorium and their daughters radium, radon and polonium; about five tonnes are generated per tonne of phosphoric acid and 100 to 280 million t per year worldwide
• radium in produced water produced water from oil and gas formations carries naturally occurring radioactive elements such as radium and uranium (MWEI glossary, PDF p651, printed p623)
• radon-222 as the decay product radon isotopes are the immediate decay products of radium isotopes; radon-222 is the only one long-lived enough to be released from the soil and rock where it forms

Extraction, production

Historically extracted as a by-product of uranium refining; today extracted from spent nuclear fuel rods

annual production fewer than 100 grams per year (rsc-element-88)

Radium metal by electrolysis of radium chloride onto a mercury cathode, then distilling off the mercury (Curie and Debierne, 1911)

no balanced equation printed by the source

Uses

The Curie, a unit used to describe the activity of a radioactive substance, is based on radium-226. It is equal to the number of atoms in a one gram sample of radium-226 that will decay in one second, or 37,000,000,000 decays per second.

Radium had been used to make self-luminous paints for watches, aircraft instrument dials and other instrumentation, but has largely been replaced by cobalt-60, a less dangerous radioactive source. A mixture of radium and beryllium will emit neutrons and is used as a neutron source. Radium is used to produce radon, a radioactive gas used to treat some types of cancer. A single gram of radium-226 will produce 0.000l milliliters of radon a day.

Radium is about one million times more active than uranium. The lab notebooks used by the Curies are too highly contaminated to be safely handled today.

One gram of radium produces about 0.0001 ml (stp) of emanation, or radon gas, per day. This is purged from the radium and sealed in minute tubes, which are used in the treatment of cancer and other diseases. Radium was used in the producing of self-luminous paints, neutron sources, and in medicine for the treatment of disease. Other radioisotopes, such as 60Co, are now being used in place of radium. Some of these sources are much more powerful, and others are safer to use. Radium loses about 1% of its activity in 25 years, being transformed into elements of lower atomic weight. Lead is a final product of disintegration. Stored radium and radium-containing products or minerals should be ventilated to prevent build-up of radon.

• Medicine: radium-223 alpha therapy for prostate cancer that has spread to bone, exploiting radium's calcium-like uptake in bone
• Uranium mining and oil and gas produced water: none as a product: radium-226 is the naturally occurring radionuclide of uranium mine water and tailings and of produced water, removed by co-precipitation with barium or strontium salts and by ion exchange co-precipitation with chloride or sulphate metal salts is a BAT conclusion for extractive waste water containing radium-226 (mwei-bref-2018, PDF p596, printed p568)
• Phosphoric acid and phosphate fertilisers: none as a product: radium-226 inherited from phosphate rock reports to phosphogypsum and to its discharge or stack leachate, listed for the ledger link radioactivity is the particular problem that has prevented most valorisation of phosphogypsum (aaf-bref-2007, PDF p275, printed p247)

Safety, toxicity

Inhalation, injection, or body exposure to radium can cause cancer and other body disorders. The maximum permissible border in the total body for 226Ra is 7400 becquerel.

Discovery and name

Discovered by

Pierre and Marie Curie

Discovered

1898

First isolated

Marie Curie

Named by

not in sources

Origin of the name

after Latin radius (ray), for emitting in the form of rays as it decays

Radium is obtained commercially as bromide and chloride; it is doubtful if any appreciable stock of the isolated element now exists. The pure metal is brilliant white when freshly prepared, but blackens on exposure to air, probably due to formation of the nitride. It exhibits luminescence, as do its slats; it decomposes in water and is somewhat more volatile than barium. It is a member of the alkaline-earth group of metals. Radium imparts a carmine red color to a flame. Radium emits alpha, beta, and gamma rays and when mixed with beryllium produce neutrons. One gram of 226Ra undergoes 3.7 x 1010 disintegrations per second. The curie is defined as that amount of radioactivity which has the same disintegration rate as 1 g of 226Ra. Twenty five isotopes are now known; radium 226, the common isotope, has a half-life of 1600 years.