Neptunium

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

full Neptunium has no part in ordinary water treatment. Neptunium-237, whose half life is 2.14 million years, reaches water only from weapons fallout, reactor effluent cooling water, reprocessing and high level waste. Where it does matter, at legacy sites and around repositories, it matters more than the actinides beside it, and for one reason in its chemistry: across the pH and Eh of most groundwater neptunium travels as the neptunyl cation NpO₂⁺, a singly charged ion that resists hydrolysis until about pH 8.5, does not compete with Ca²⁺ for exchange sites and is therefore barely held by soil, with Kd values as low as 0.2 mL/g. Soluble and weakly sorbed, it outruns plutonium and americium. What stops it is a change of oxidation state rather than a filter: Fe(II) minerals, zero valent iron or bacteria reduce Np(V) to Np(IV), which hydrolyses at once to a sparingly soluble hydrous oxide, and reoxidation reverses that. The WHO guidance level is 1 Bq/L, and because neptunium-237 is an alpha emitter it is caught by gross alpha screening.

Typical wastewaters

nuclear reactor effluent cooling water
the neptunyl ion NpO₂⁺, soluble and weakly sorbed
nuclear fuel reprocessing and high level waste (repository leachate)
NpO₂⁺ over most of the pH and Eh range, with NpO₂(OH) (aq) and the carbonate complexes above about pH 8.5; neptunium-237 grows in from americium-241 and becomes a major part of the repository inventory after about 100,000 years
immobilised only where Fe(II) minerals, zero valent iron or bacteria reduce it to Np(IV)
industrial processing of reactor produced neptunium-237
NpO₂⁺ in process effluent

1 · Identity

Symbol, number: Np, 93
Oxidation states in water: +5 as the neptunyl ion NpO₂⁺, dominant over most environmental pH and Eh, with NpO₂(OH) (aq) and NpO₂CO₃⁻ above about pH 8.5; +4 as sparingly soluble NpO₂ hydrous oxide and hydroxide in reducing groundwater; +6 only in highly oxidising solution and not important in the environment ¹. Neptunium-237 is the isotope of concern; neptunium-239 is a short lived reactor product.
Note: The element chapter covers the discovery, the oxides and the fluorides. This chapter is about why NpO₂⁺ moves and what stops it.

2 · Occurrence in water

Natural sources: Trace neptunium-237 in uranium ores from neutron capture (element chapter); not a natural water constituent at measurable levels.
Anthropogenic sources: Fallout from nuclear weapons, effluent cooling water from fission reactors, industrial processing of reactor produced neptunium-237, and high level waste, in which neptunium-237 grows in from americium-241 and becomes a major part of the repository inventory after about 100,000 years ¹. The account is qualitative: no concentration in any water is quoted here.

3 · Speciation

Over the pH range of most natural waters Np(V) is the free neptunyl ion NpO₂⁺; above about pH 8.5 the hydroxide NpO₂(OH) (aq) and the carbonate NpO₂CO₃⁻ take over, and in high carbonate alkaline water NpO₂(CO₃)₂³⁻, NpO₂(CO₃)₃⁵⁻ and mixed hydroxo carbonate complexes form; phosphate complexes NpO₂HPO₄⁻ and NpO₂PO₄²⁻ and humate complexes are known, and humic acid also reduces Np(V) to Np(IV) under anaerobic conditions ¹. Np(V) solids are quite soluble: the solubility limits are Np₂O₅ hydrate or NpO₂OH in carbonate free water and NaNpO₂CO₃ (below pH 8.5) or Na₃NpO₂(CO₃)₂ (above pH 9) in carbonate water. Np(IV) forms sparingly soluble NpO₂ hydrous oxide and hydroxide, and Np(IV) humic colloids can form that stay mobile in porous aquifers. Sorption of Np(V) is negligible below pH 5, rises between pH 5 and 7 on iron oxides and falls again above pH 7 to 9 as carbonate complexes form; NpO₂⁺ does not compete with Ca²⁺ for sites, so soil Kd values are low, 0.2 to 3.5 mL/g in the Routson data and 2 to 13 mL/g on Hanford sediments ¹.

condition	dominant species	note
oxic to mildly reducing groundwater, pH 4 to 8.5	NpO₂⁺	the mobile form; Kd minimum 0.2 mL/g suggested for screening
pH above 8.5, carbonate bearing	NpO₂(OH) (aq), NpO₂CO₃⁻, NpO₂(CO₃)₂³⁻, NpO₂(CO₃)₃⁵⁻	sorption on iron oxides falls again as the carbonate complexes form
reducing groundwater, Fe(II) minerals, zero valent iron barriers, dithionite reduced soils, Shewanella	Np(IV) as NpO₂ hydrous oxide, colloids and humic bound Np(IV)	immobilised by heterogeneous reduction; bioreduction alone did not remove neptunium from solution without phosphate release by a second organism

Solubility: Np(V): Np₂O₅ hydrate or NpO₂OH in carbonate free water; NaNpO₂CO₃, Na₃NpO₂(CO₃)₂ and KNpO₂CO₃ in carbonate water. Np(IV): NpO₂ hydrous oxide, amorphous NpO₂, far lower ¹. No numeric solubility is quoted here.
Hydrolysis: NpO₂⁺ resists hydrolysis except in concentrated solution (element chapter); NpO₂(OH) (aq) above about pH 8.5.
Complexation: Carbonate strong at high pH, phosphate, sulfate weak, chloride and fluoride weak, humate; no stability constants are quoted.
Precipitates: Sodium neptunyl carbonates in alkaline carbonate waste; NpO₂ hydrous oxide after reduction.

\ce{NpO2^+ + H2O -> NpO2(OH) (aq) + H+}

the only hydrolysis step neptunyl takes in natural water, and it waits until about pH 8.5; below that the bare NpO2^+ cation travels, which is the whole reason neptunium outruns the other actinides; EPA names the species without a constant and none is quoted

\ce{NpO2^+ + CO3^2- -> NpO2CO3^-}

carbonate complexation above about pH 8.5 in river water composition ²

\ce{NpO2CO3^- + CO3^2- -> NpO2(CO3)2^3-}

second carbonate step in alkaline carbonate water; each step makes the complex more negative, which is why sorption on iron oxides falls away again above pH 7 to 9

\ce{NpO2(CO3)2^3- + CO3^2- -> NpO2(CO3)3^5-}

third carbonate step, reached in high carbonate alkaline water and in alkaline waste; constants from Lemire 2001 were not read

\ce{NpO2^+ + HPO4^2- -> NpO2HPO4^-}

phosphate complex named by EPA alongside NpO2PO4^2-; weaker than carbonate at natural pH but the reason a phosphate releasing organism precipitates neptunium in the bioprecipitation work

\ce{Na^+ + NpO2^+ + CO3^2- -> NaNpO2CO3 (s)}

the solid that caps dissolved Np(V) below about pH 8.5 in sodium carbonate water, with Na3NpO2(CO3)2 above pH 9; written from the phases EPA names, no solubility product was read

\ce{NpO2^+ + Fe^2+ + 4 H+ -> Np^4+ + Fe^3+ + 2 H2O}

surface mediated reduction on Fe(II) bearing oxide and silicate minerals, the Np(IV) then hydrolysing to NpO2 hydrous oxide; balanced here from the source's word description, not printed by it

\ce{Np^4+ + 2 H2O -> NpO2 (s) + 4 H+}

what happens the moment Np(V) is reduced: Np(IV) hydrolyses straight to the sparingly soluble hydrous dioxide, so the reduction step above and this one together are the immobilisation; the hydration water of the solid is omitted and no solubility product was read

4 · Role in treatment

as a problem

the mobile actinide at nuclear sites

NpO₂⁺ is soluble, weakly sorbed and stable across normal groundwater Eh, so it outruns plutonium and americium

the basis of the long term repository concern ¹

5 · Removal and control

reductive immobilisation (zero valent iron permeable barriers, dithionite reduced soil, Fe(II) minerals)

Np(V) reduced to Np(IV) at the mineral surface with Fe(II) oxidised to Fe(III); Np(IV) precipitates as hydrous oxide or sorbs

\ce{NpO2^+ + Fe^2+ + 4 H+ -> Np^4+ + Fe^3+ + 2 H2O}

being tested for groundwater contaminants at the time of the EPA review; reoxidation reverses it

Efficiency: not recorded
Interferences: carbonate complexation, oxygen ingress, humic colloids that carry Np(IV)

bioprecipitation

Shewanella putrefaciens reduces Np(V) and a Citrobacter releases phosphate from glycerol 2-phosphate, precipitating neptunium and its protactinium-233 daughter

laboratory study ³; reduction alone did not remove neptunium from solution

Efficiency: not recorded

6 · Analytics

method	standard	detection limit	note
gross alpha screening	ISO 9696, ISO 10704; WHO Table 9.3	0.02 to 0.1 Bq/L; Euratom 0.04 Bq/L	neptunium-237 is an alpha emitter and counts in gross alpha; WHO screening level 0.5 Bq/L, Euratom 0.1 Bq/L, US 15 pCi/L
alpha spectrometry or ICP-MS after actinide separation	method number not read	not recorded	mass 237 is free of natural isobars, so ICP-MS is the sensitive route at legacy sites

Sampling pitfalls: Preserve the oxidation state or fix it: Np(V) is stable in acidified oxic samples, but reducing groundwater samples reoxidise on contact with air and the Np(IV) colloid fraction changes; ultrafiltration in the field separates colloidal Np(IV) from dissolved NpO₂⁺.

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.

drinking water

body	limit	note
WHO GDWQ Annex 6, Table A₆.1	1 (neptunium-237); 100 (neptunium-239) Bq/L	guidance levels; not in the chapter 9 short table; the chapter 9 footnote for artificial radionuclides says they may not occur in drinking water in normal situations and are lower priority after a screening exceedance
EU Directive 2013/51/Euratom, Annex III	not set	no neptunium isotope in the derived concentration table; caught by the 0.1 Bq/L gross alpha screening level
US EPA NPDWR	15 pCi/L	gross alpha MCL excluding radon and uranium; no neptunium specific MCL

8 · Health and environmental effects

Toxicity: Toxic through its radioactivity (element chapter); an alpha emitter.

Read these with care

The Fe(II) reduction equation is balanced from the EPA text, which describes the reaction in words.
Kd values are the EPA compilation's screening minima and a few soils; site specific values are essential, as EPA itself says.
No measured neptunium concentration in any water was read; the occurrence section is qualitative.
The Euratom table was read from the retained UK copy on legislation.gov.uk.

What this chapter does not cover

No measured neptunium-237 concentration in groundwater, surface water, seawater or effluent was read (Hanford, Sellafield and fallout data exist but were not sourced).
No numeric solubility or stability constants were read; the Lemire 2001 NEA review is cited by EPA but was not opened.
No dose coefficient or toxicity value for neptunium-237 was read.
No removal efficiency was read; WHO Table 9.4 has no neptunium row.
The CWW BAT conclusions have no radioactivity parameter; no discharge row is written.
The neptunium phosphate solid formed in the bioprecipitation study is not identified by the source, so the bioprecipitation row carries no equation.
Bioaccumulation: Not addressed in the sources read.
Ecotoxicity: Not addressed in the sources read.

Notes

EPA Kd volume III
EPA Kd volume III, Figure 5.8
Lloyd 2000 via EPA

Sources

US EPA, Understanding Variation in Partition Coefficient, Kd, Values, Volume III: Review of Geochemistry and Available Kd Values for Americium, Arsenic, Curium, Iodine, Neptunium, Radium and Technetium, EPA 402-R-04-002C (July 2004), section 5.6 neptunium
WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), Annex 6 Supporting information on radionuclides, Table A6.1 (NCBI Bookshelf)
WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first addendum, chapter 9 Radiological aspects 4
Council Directive 2013/51/Euratom, Annex III (screening levels, derived concentrations, limits of detection), read in the retained UK copy on legislation.gov.uk
US EPA, National Primary Drinking Water Regulations (table of MCLs and MCLGs, radionuclides)
The Element Book, element chapter and reference text for Np

Identity

Name and symbol: Neptunium, Np
Atomic number: 93 protons
Position: no group (f-block) · period 7 · f-block · actinide
CAS number: 7439-99-8

Atomic structure

Atomic mass: 237 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹
[Rn] 7s²⁵f⁴⁶d¹
Electrons per shell: 2, 8, 18, 32, 22, 9, 2
Valence electrons: 7 ns, (n-1)d and (n-2)f

nuclide	half-life	decay
²³⁷Np	2.144 My	α=100%; SF<2e-10%; 30Mg<4e-12%
²³⁶Np	153 ky	ε=86.3±0.8%; β-=13.5±0.8%; α=0.16±0.4%
²³⁵Np	396.1 d	ε=99.99740±1.3%; ; α=0.00260±1.3%
²³⁴Np	4.4 d	β+=100%

no stable isotope; the longest-lived nuclides

Physical properties

State at room temperature: Solid
Melting point: 917 K (643.85 °C)
Boiling point: 4,175 K (3,901.85 °C)
Density: 20.25 g/cm3
Appearance: silvery metallic
Thermal conductivity: 6.3 W/(m·K)
Electrical resistivity: 1.220 µΩ·m
Electrical conductivity: 819,672.131 S/m
Crystal structure: orthorhombic
Molar heat capacity: 29.46 J/(mol·K)

Chemical properties

Oxidation states: +6, +5, +4, +3
Electronegativity: 1.36 (Pauling Scale)
Ionisation energy: 6.266 eV
1st 604.5 kJ/mol
Electron affinity: not in sources
Atomic radius: empirical 190, covalent 190, van der Waals 221 pm
Ionic radius: Np²⁺ 110; Np³⁺ 101; Np⁴⁺ 87; Np⁵⁺ 75; Np⁶⁺ 72; Np⁷⁺ 71 pm

Reactivity: A silvery, very reactive and pyrophoric actinide, the first transuranic element; it shows five oxidation states from +3 to +7, with +5 the most stable in solution and +4 preferred in solids, and its ions hydrolyze and complex readily.
with water: The metal is attacked by water vapor and tarnishes in moist air; in solution Np(VII) is reduced by water, Np(IV) hydrolyzes above about pH 1, and the stable NpO2+ ion resists hydrolysis except in concentrated solution.
with oxygen, air: Tarnishes in air and is pyrophoric when finely divided; the stable oxide is greenish-brown NpO2, formed simply by burning neptunium oxyacid salts in air, while black-brown Np2O5 decomposes back to NpO2 and oxygen on heating.
with acids: Dissolves in acids to give the ions Np3+, Np4+, NpO2+ and NpO2^2+ depending on the oxidant present; Np3+ is oxidized to Np(IV) by air and Np(VI) is easily reduced to the stable Np(V) ion.
with halogens: Fluorides NpF3, NpF4, NpF5 and the volatile NpF6 are known, the hexafluoride from NpF4 and fluorine gas; chlorides NpCl3 and NpCl4, bromides NpBr3 and NpBr4 and the iodide NpI3 are made from the oxides, and no pentachloride has been obtained.

Typical compounds

NpO₂ neptunium dioxide
greenish-brown, the stable oxide over a wide range
Np₂O₅ neptunium pentoxide
black-brown, loses oxygen to NpO2 on heating
NpF₄ neptunium tetrafluoride
from NpO2 with hydrogen fluoride, precursor to NpF6
NpF₆ neptunium hexafluoride
volatile like UF6, studied for fuel reprocessing
NpCl₄ neptunium tetrachloride
from the oxide and carbon tetrachloride near 500 C

Occurrence, production and use

Crustal abundance: Not Applicable
Oceanic abundance: Not Applicable

Occurrence and sources

trace neptunium in uranium ores; by-product in spent fuel
extracted from spent uranium fuel rods

Extraction, production: not in sources

Uses

Neptunium's most stable isotope, neptunium-237, has a half-life of about 2,144,000 years. It decays into protactinium-233 through alpha decay. Neptunium-237, which is produced in gram quantities as a by-product of the production of plutonium in nuclear reactors, is used in neutron detectors.

Once considered to be completely artificial, extremely small amounts of neptunium are produced naturally in uranium ores through the interaction of atoms of uranium in the ore with neutrons produced by the decay of other atoms of uranium in the ore.

Safety, toxicity: not in sources

Discovery and name

Discovered by: Edwin McMillan and Philip H. Abelson
Discovered: 1940
First isolated: not in sources
Named by: not in sources
Origin of the name: after planet Neptune, itself named after Roman god of the sea Neptune

Neptunium metal buttons (photo courtesy Lawrence Berkeley National Laboratory)

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.