Protactinium

Typical wastewaters

• uranium ore refining raffinate (pitchblende, St. Louis airport site and Weldon Spring raffinate pits) Pa-231 following Th-230 into the raffinate solids and slurry left after ether extraction of uranium, with Ac-227 growing in from it source term for ore in equilibrium: Pa-231 at 0.02 of the uranium activity, 0.01 of total alpha; NIOSH found dose reconstruction for raffinate workers exposed to Th-230, Pa-231 and Ac-227 not feasible; a residue slurry rather than a discharged liquid, and no activity per litre was read

1 · Identity

Symbol, number

Pa, 91

Oxidation states in water

+5 as hydrolysed hydroxo oxo species that adsorb to vessel walls and particles at any pH above strong acid; +4 under strongly reducing conditions (element entry). Protactinium-231 (half life 32,500 years, alpha, uranium-235 chain) and the short lived protactinium-234 and -233 are the isotopes; the element has no stable form.

Note

The element entry covers the discovery and the fluoro complexes. This chapter says only that Pa(V) is among the most particle reactive actinides and how it is regulated.

2 · Occurrence in water

Natural sources

Decay of uranium-235 in uranium ores and in seawater, where protactinium-231 is present at tens of attograms per litre in filtered water and is detectable in about 2 L of surface seawater for the particulate fraction (Shen 2003); it is removed onto sinking particles and is used together with thorium-230 as a tracer of ocean circulation.

Anthropogenic sources

Uranium mill tailings and spent fuel; neptunium-237 in waste decays to protactinium-233, which was removed from solution together with neptunium by bioprecipitation in one laboratory study (EPA Kd volume III). No effluent figure was read.

3 · Speciation

Pa(V) hydrolyses at once to hydroxy oxide species that stick to vessel walls, so aqueous protactinium is kept only in strong acid or as fluoride complexes such as PaF₈³⁻ (element entry). In natural water it is therefore almost entirely on particles and is removed from the water column by scavenging; the dissolved remainder is a hydrolysed Pa(V) species whose formula was not given by any source read.

Solubility

Pa₂O₅ and the hydrous oxide are insoluble; the dioxide resists nitric, hydrochloric and sulfuric acid and dissolves in hydrofluoric acid (element entry).

Hydrolysis

Immediate and complete in dilute acid and above (element entry).

Complexation

Fluoride; nothing sourced for carbonate or organics.

Precipitates

Hydrous protactinium oxide on any surface or particle.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

Not relevant or not given for this element.

6 · Analytics

Sampling pitfalls

Protactinium adsorbs to every surface it meets; samples must be acidified hard at collection and spiked with tracer before any handling, and filtration separates a particulate fraction that holds most of it.

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

Toxic through its radioactivity (element entry); an alpha emitter with no chemical toxicity relevant at these masses. No dose coefficient was read.

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The seawater level is quoted from an analytical method paper, not a survey.
• Speciation is qualitative from the element entry; no hydrolysis constants or a formula for the dissolved species were read.
• The Euratom table was read from the retained UK copy on legislation.gov.uk.

Gaps

• No source read gives protactinium in fresh water, groundwater, drinking water, wastewater or uranium mill effluent.
• No removal study exists in the sources read; the removal list is empty on purpose.
• No dose coefficient or toxicity value for protactinium-231 was read.
• The CWW BAT conclusions have no radioactivity parameter; no discharge row is written.
• The relative particle reactivity of protactinium and thorium, the basis of the protactinium-231 to thorium-230 circulation tracer, was not read this session and is not stated.
• No balanced equation is written: protactinium hydrolysis is described qualitatively in the sources read; no constant or stoichiometry was printed.

Sources

Identity

Name and symbol

Protactinium, Pa

Atomic number

91 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-13-3

Atomic structure

Atomic mass

231.035 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹
[Rn] 7s²⁵f²⁶d¹

Electrons per shell

2, 8, 18, 32, 20, 9, 2

Valence electrons

5 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,845 K (1,571.85 °C)

Boiling point

4,027 K (3,753.85 °C)

Density

15.37 g/cm3

Appearance

bright, silvery metallic luster

Thermal conductivity

47 W/(m·K)

Electrical resistivity

177 nΩ·m at 0 °C

Electrical conductivity

5.65 MS/m

Crystal structure

body-centered tetragonal

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+5, +4

Electronegativity

1.5 (Pauling Scale)

Ionisation energy

5.89 eV
1st 568 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 200, covalent 200, van der Waals 243 pm

Ionic radius

Pa³⁺ 104; Pa⁴⁺ 90; Pa⁵⁺ 78 pm

Reactivity

A dense, silvery-gray actinide that keeps its luster in air for a while but is chemically reactive; the +5 state dominates its solid compounds (5f2 6d1 7s2 configuration), with +4, +3 and +2 also known.

with water

Attacked by water vapor; in solution Pa(V) hydrolyzes at once to hydroxy-oxide species that stick to vessel walls, so aqueous protactinium is kept in strong acid or as fluoride complexes.

with oxygen, air

Reacts readily with oxygen; the stable oxide is the white pentoxide Pa2O5, the black dioxide PaO2 is made from it by hydrogen reduction, and the monoxide PaO exists only as a surface film on the metal.

with acids

Dissolves in inorganic acids to Pa(V); the dioxide resists dilute and concentrated nitric, hydrochloric and sulfuric acid but dissolves easily in hydrofluoric acid, which forms fluoro complexes such as PaF8^3-.

with halogens

Combines with iodine at about 600 C to give iodides, and pentahalides are known for all four halogens (white PaF5, yellow PaCl5, brown PaBr5, PaI5); PaCl5 hydrolyzes readily in water.

Typical compounds

• Pa₂O₅ protactinium(V) oxide white, the most stable oxide, from igniting the hydroxide in air
• PaO₂ protactinium(IV) oxide black, from hydrogen reduction of the pentoxide
• PaF₅ protactinium(V) fluoride white solid, made from the oxide with bromine fluorides
• PaCl₅ protactinium(V) chloride yellow, volatile, hydrolyzes in water
• Na₃PaF₈ sodium octafluoroprotactinate cubic PaF8 unit, from hydrofluoric acid solutions

Occurrence, production and use

Crustal abundance

1.4×10-6 milligrams per kilogram

Oceanic abundance

5×10-11 milligrams per liter

Occurrence and sources

Protactinium is one of the rarest and most expensive naturally occurring elements. The average concentrations of protactinium in the Earth's crust is typically on the order of a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. The element occurs in pitchblende to the extent of about 1 part 231Pa to 10 million parts of ore. Ores from Zaire have about 3 ppm. In 1959 and 1961, it was announced that the Great Britain Atomic Energy Authority extracted by a 12-stage process 125 g of 99.9% protactinium, the world's only stock of the metal for many years following. The extraction was made from 60 tons of waste material at a cost of about $500,000.

• protactinium-231 in uranium ores small amounts, from the decay of uranium-235; also in spent fuel rods

Extraction, production

not in sources

Uses

Due to its scarcity, high radioactivity and toxicity, there are currently no uses for protactinium outside of basic scientific research.

Because of its scarcity, high radioactivity and high toxicity, there are currently no practical uses for protactinium other than that of basic scientific research, and for this purpose, protactinium is generally extracted from spent nuclear fuel.

Safety, toxicity

not in sources

Discovery and name

Discovered by

Kasimir Fajans and Oswald Helmuth Göhring

Discovered

1913

First isolated

Aristid von Grosse

Named by

Otto Hahn and Lise Meitner

Origin of the name

means "(nuclear) precursor of actinium," since some of its isotopes decay into actinium

Protactinium metal is a dense, silvery-gray material with a bright metallic luster which it retains for some time in air but it does readily react with oxygen, water vapor and inorganic acids to form various compounds. In solid compounds protactinium is most stable in the oxidation state +5, but it also exists in the +4, +3 and +2 oxidation states. In solution the +5 state rapidly hydrolyzes by combining with hydroxide ions to form soluble or insoluble hydroxy-oxide solids which have a tendency to stick to the surfaces of vessels in which it is contained. A number of protactinium compounds are known, some of which are colored. The element is superconductive below 1.4K.