Polonium

Typical wastewaters

• drinking water treatment residuals from radium bearing groundwater (scale, filter media, sludge) Po-210 grown in from Pb-210 on the solids, which is why WHO ties polonium-210 monitoring to high radon supplies on the solids rather than in a liquid effluent; no produced water or phosphate industry polonium figure was read
• oil and gas produced water, with the NORM scale and sludge it deposits Po-210 grown in from Pb-210 and also emanating directly from the reservoir (IAEA notes Po-210 above its grandparent Pb-210 in gas condensates); partitions to sludge and scale rather than staying in the brine IAEA Table III: produced water 0.002 to 0.08 Bq/L, hard scale 0.02 to 1.5 Bq/g, sludge 0.004 to 160 Bq/g, crude oil 0 to 0.01 Bq/g; produced water is the largest NORM waste by volume at low activity; EPA lists polonium-210 with radium-226, radium-228, lead-210 and radon among the radionuclides of produced water, scale and sludge (EPA TENORM page)
• shale gas produced fluids (Marcellus) particle reactive polonium: ultrafiltration puts polonium with uranium and thorium on particles while radium and lead stay dissolved in the high ionic strength brine partitioning study with a Pb-203 tracer; no polonium activity is given in the abstract read
• phosphoric acid production (phosphogypsum slurry discharged to rivers and sea) Po-210 carried with the phosphogypsum into receiving water, sediment and biota IAEA Annex II: discharge of phosphogypsum into surface water bodies has produced significantly elevated Po-210 in water, sediments and biota, with doses of 2 to 10 µSv/a and up to 150 µSv/a where local fish and shellfish are eaten
• phosphogypsum stack leachate to a river (Wislinka, Martwa Wisla, Poland) dissolved Po-210 and Pb-210 with uranium in river water beside the stack maxima of 2.0 mBq/L Po-210, 3.2 mBq/L Pb-210 and 11.7 mBq/L U-238, Po-210 to Pb-210 activity ratio up to 0.69; the authors judge the impact on the river insignificant
• uranium mill tailings facility effluent (Jaduguda, India) Po-210 in the tailings pond residual water and the treated effluent, taken up by filamentous algae and sediment rooted plants effluent, surface water and bottom sediment were analysed; the highest plant activity, 4884 Bq/kg fresh weight, was in filamentous algae from residual water of the tailings pond; the effluent activity itself is not given in the abstract read

1 · Identity

Symbol, number

Po, 84

Oxidation states in water

+2 as Po²⁺ in acid to neutral water, where it is sorbed; +4 as the hydrolysed dihydrogen polonate H₂PoO₃ (aq) at high pH, poorly sorbed and mobile; and, under strongly reducing conditions, the hydrogen polonide anion HPo⁻ (Szabo 2020). Polonium-210 (half life 138 days) is the isotope that matters, supported by lead-210 (22 years) and radon-222 in the aquifer.

Note

The element entry covers the discovery, the alpha heat and the neutron and antistatic sources. This chapter is about polonium-210 at millibecquerel to becquerel levels in well water.

2 · Occurrence in water

Natural sources

Decay of radon-222 and lead-210 in aquifer minerals, then release to water where the chemistry allows. In 1263 US public supply wells across 19 principal aquifers, polonium-210 was above the 0.7 pCi/L risk threshold in 1.5 percent of samples, almost only in confined Coastal Plain aquifers with old, reducing, high pH (above 7.5), high sodium to chloride water from cation exchange; polonium is poorly sorbed at high pH, and iron, manganese and sulfate reduction and cation exchange mobilise it from mineral surfaces (Szabo 2020). In Lahontan Valley, Nevada, 63 wells ranged from 0.01 to 178 pCi/L, median 2.88 pCi/L, the high wells having dissolved oxygen below 0.1 mg/L and commonly pH above 9, with the polonium unsupported by lead-210 and therefore mobilised from the sediments (Seiler 2011). WHO notes that where radon is high, polonium-210 can be a major contributor to dose.

Anthropogenic sources

NORM: polonium-210 grows in from lead-210 in radium bearing scale, filter media and sludge, which is why WHO ties polonium-210 monitoring to high radon supplies (WHO chapter 9; see the Ra chapter). Commercial polonium is a Russian reactor product used in sealed sources (element entry) and is not a discharge.

3 · Speciation

Polonium follows tellurium: in acid to neutral water it is the cation Po²⁺ (its own alpha radiation oxidises solutions toward Po(IV), element entry), which sorbs to iron and manganese oxyhydroxides and clays and so stays put; at high pH it is the hydrolysed, poorly sorbed H₂PoO₃ (aq), and under strongly reducing conditions the hydrogen polonide anion HPo⁻ (Szabo 2020). Reductive dissolution of the iron and manganese oxides that hold it, sulfate reduction and cation exchange release it, which is why the problem water is old, anoxic and alkaline. Volatile polonium has been reported in one Florida well and not in Nevada (Seiler 2011).

Solubility

Not solubility controlled at these activities; sorption controls it. No constants read.

Hydrolysis

Polonium ions hydrolyse above about pH 1 (element entry); the high pH species is H₂PoO₃ (aq).

Complexation

Chloride complexes in acid (the tetrachloride dissolves in hydrochloric acid, element entry); not relevant at groundwater pH.

Precipitates

None of its own; carried in Fe and Mn oxide, sulfide and radium bearing scale and sludge.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify at collection and count promptly: polonium-210 decays with a 138 day half life and grows in from lead-210, so an unsupported sample falls and a supported one rises between sampling and counting. Evaporating a planchet can lose volatile polonium (Seiler 2011). Do not filter reducing groundwater before acidification if total polonium is wanted, because it is on the particles.

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 pure alpha emitter with a dose coefficient of 1.2 x 10⁻6 Sv/Bq by ingestion (WHO Table 9.2), about four times that of radium-226; Szabo puts the lifetime cancer risk threshold at 0.7 pCi/L and Seiler the 1 in 10,000 level at 1.1 pCi/L. Chemical toxicity is irrelevant at these masses.

Bioaccumulation

Not addressed in the sources read for water; polonium-210 is known to concentrate in seafood but no figure was read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The US figures are from abstracts: the national survey gives a percentage above a threshold, not a range, and the Nevada range is one alkaline basin.
• The 0.7 and 1.1 pCi/L risk thresholds are the authors' values, not regulatory limits.
• Speciation species are as named in the Szabo abstract; no thermodynamic data were read.
• The Euratom values were read from the retained UK copy of the directive on legislation.gov.uk, not from EUR-Lex.
• ISO 13161 is cited from the WHO Annex 6 list; the method text was not read.

Gaps

• No polonium specific removal efficiency was read; WHO Table 9.4 has no polonium row.
• The Euratom detection limit for polonium-210 was not returned by the page read; only the gross alpha and beta limits are written.
• EU DWD 2020/2184 defers radioactivity to the Euratom directive and was not read this session.
• The polonium-210 (138 days) and lead-210 (22 years) half lives are standard values, not read this session.
• No balanced equation is written: the sources read describe polonium speciation and sorption in words only, without stoichiometry.

Sources

Identity

Name and symbol

Polonium, Po

Atomic number

84 protons

Position

group 16 · period 6 · p-block · post-transition metal

CAS number

7440-08-6

Atomic structure

Atomic mass

209 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁴
[Xe] 6s²⁴f¹⁴⁵d¹⁰⁶p⁴

Electrons per shell

2, 8, 18, 32, 18, 6

Valence electrons

6 outer shell

Physical properties

State at room temperature

Solid

Melting point

527 K (253.85 °C)

Boiling point

1,235 K (961.85 °C)

Density

9.32 g/cm3

Appearance

silvery

Thermal conductivity

20 W/(m·K)

Electrical resistivity

α-Po: 0.40 µΩ·m at 0 °C

Electrical conductivity

2.5 MS/m

Crystal structure

cubic

Molar heat capacity

26.4 J/(mol·K)

Chemical properties

Oxidation states

+4, +2

Electronegativity

2 (Pauling Scale)

Ionisation energy

8.417 eV
1st 812.1 kJ/mol

Electron affinity

1.9 eV

Atomic radius

empirical 140, covalent 140, van der Waals 197 pm

Ionic radius

Po⁴⁺ 94; Po⁶⁺ 67 pm

Reactivity

A rare, intensely radioactive chalcogen that behaves chemically like tellurium with the metallic character of its neighbours thallium, lead and bismuth; it shows the +2 and +4 states (and +6 only in PoF6), and because its alpha activity breaks bonds and heats samples, nearly all its chemistry has been done at trace scale.

with water

Not known: the sources describe only the hydrolysis of polonium ions in water above about pH 1, not a reaction of the metal, and the hydride PoH2 is a thermally unstable liquid.

with oxygen, air

The three oxides PoO, PoO2 and PoO3 are the products of oxidising polonium; heated in air the volatile metal gives the dioxide: $\ce{Po + O2 -> PoO2}$, the usual oxide.

with acids

Readily dissolved by dilute acids to pink Po2+ solutions that turn yellow as its own alpha radiation oxidises them to Po4+; only slightly soluble in alkali.

with halogens

The dihalides form by direct reaction of the elements: $\ce{Po + Cl2 -> PoCl2}$, the tetrahalides PoX4 come from the dioxide and the hydrogen halides, and fluorine gives PoF6; all dissolve in the matching hydrogen halide.

Typical compounds

• PoO₂ polonium dioxide the usual oxide, yellow, from heating polonium in air
• PoCl₄ polonium tetrachloride yellow tetrahalide, soluble in hydrochloric acid
• PoCl₂ polonium dichloride from the elements or by reducing PoCl4
• PoH₂ polonium hydride volatile, unstable liquid at room temperature
• Na₂Po sodium polonide antifluorite polonide, the most stable compound class
• PbPo lead polonide one of the few natural polonium compounds

Occurrence, production and use

Crustal abundance

2×10-10 milligrams per kilogram

Oceanic abundance

1.5×10-14 milligrams per liter

Occurrence and sources

Polonium is a very rare natural element. Uranium ores contain only about 100 micrograms of the element per ton. Its abundance is only about 0.2% of that of radium.

In 1934, scientists discovered that when they bombarded natural bismuth (209Bi) with neutrons, 210Bi, the parent of polonium, was obtained. Milligram amounts of polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors.

• trace member of the uranium decay chain in uranium ores parts per billion in pitchblende; not economic to extract
• crustal and oceanic abundance 2 x 10^-10 mg/kg crust and 1.5 x 10^-14 mg/L seawater (PubChem); 0.0000000002 ppm (BGS via RSC)

Extraction, production

Neutron irradiation of bismuth-209

209Bi + n -> $\ce{210Bi -> 210Po + e-}$ (beta decay)

RSC states bismuth-209 is bombarded with neutrons to give bismuth-210, which decays to polonium; all commercial production is in Russia

Uses

Polonium can be used to eliminate static electricity in machinery that is caused by processes such as the rolling of paper, wire or sheet metal, although other materials which emit beta particles are more commonly used for this purpose. Polonium is also used in brushes for removing dust from photographic films, although the polonium must be carefully sealed to protect the user from contamination. Polonium is also combined with beryllium to form neutron sources.

Because almost all alpha radiation is stopped within the solid source and its container, giving up its energy, polonium has attracted attention for uses as a lightweight heat source for thermoelectric power in space satellites.

Polonium can be mixed or alloyed with beryllium to provide a source of neutrons. The element has been used in devices for eliminating static charges in textile mills, etc.; however, beta sources are both more commonly used and less dangerous. It is also used on brushes for removing dust from photographic films. The polonium for these is carefully sealed and controlled, minimizing hazards to the user.

• Antistatic devices and research: alpha-particle sources on stainless steel discs
• Space power and neutron sources: radioisotope heat source for space equipment; polonium-beryllium neutron sources

Safety, toxicity

Polonium-210 is very dangerous to handle in even milligram or microgram amounts, and special equipment and strict control is necessary. Damage arises from the complete absorption of the energy of the alpha particle into tissue.

The maximum permissible body burden for ingested polonium is only 0.03 microcuries, which represents a particle weighing only 6.8 x 10-12 g. Weight for weight it is about 2.5 x 1011 times as toxic as hydrocyanic acid. The maximum allowable concentration for soluble polonium compounds in air is about 2 x 10-11 microcuries/cm3.

Discovery and name

Discovered by

Pierre and Marie Curie

Discovered

1898

First isolated

William H. Beamer and Charles R. Maxwell

Named by

not in sources

Origin of the name

after Polonia, Latin for Poland, homeland of Marie Curie

Polonium-210 is a low-melting, fairly volatile metal, 50% of which is vaporized in air in 45 hours at 55°C. It is an alpha emitter with a half-life of 138.39 days. A milligram emits as many alpha particles as 5 g of radium.

The energy released by its decay is so large (140W/g) that a capsule containing about half a gram reaches a temperature above 500C. The capsule also presents a contact gamma-ray dose rate of 0.012 Gy/h. A few curies (1 curie = 3.7 x 1010Bq) of polonium exhibit a blue glow, caused by excitation of the surrounding gas.

Polonium is readily dissolved in dilute acids, but is only slightly soluble in alkali. Polonium salts of organic acids char rapidly; halide amines are reduced to the metal.