Editing Neptunium (section)

==Characteristics==

===Physical===
Neptunium is a [[hardness (materials science)|hard]], silvery, [[ductility|ductile]], [[radioactivity|radioactive]] actinide [[metal]]. In the [[periodic table]], it is located to the right of the actinide [[uranium]], to the left of the actinide [[plutonium]] and below the [[lanthanide]] [[promethium]].<ref name="Yoshida718" /> Neptunium is a hard metal, having a bulk modulus of 118&nbsp;[[pascal (unit)|GPa]], comparable to that of [[manganese]].<ref>{{cite journal |last1=Dabos |first1=S. |last2=Dufour |first2=C. |last3=Benedict |first3=U. |last4=Pagès |first4=M. |date=1987 |title=Bulk modulus and P–V relationship up to 52&nbsp;GPa of neptunium metal at room temperature |journal=Journal of Magnetism and Magnetic Materials |volume=63–64 |pages=661–3 |doi=10.1016/0304-8853(87)90697-4|bibcode = 1987JMMM...63..661D }}</ref> Neptunium metal is similar to uranium in terms of physical workability. When exposed to air at normal temperatures, it forms a thin oxide layer. This reaction proceeds more rapidly as the temperature increases.<ref name="Yoshida718" /> Neptunium melts at {{Nowrap|639 ± 3 °C}}: this low [[melting point]], a property the metal shares with the neighboring element plutonium (which has melting point 639.4&nbsp;°C), is due to the [[orbital hybridization|hybridization]] of the 5f and 6d orbitals and the formation of directional bonds in the metal.<ref name="Yu. D. Tretyakov" /> The [[boiling point]] of neptunium is not empirically known and the usually given value of 4174&nbsp;°C is extrapolated from the [[vapor pressure]] of the element. If accurate, this would give neptunium the largest liquid range of any element (3535&nbsp;K passes between its [[Melting point|melting]] and [[boiling point]]s).<ref name="Yoshida718" /><ref name="Gray" />

Neptunium is found in at least three [[allotrope]]s.<ref name="CRC" /> Some claims of a fourth allotrope have been made, but they are so far not proven.<ref name="Yoshida718">Yoshida et al., p. 718.</ref> This multiplicity of allotropes is common among the [[actinide]]s. The [[crystal structure]]s of neptunium, [[protactinium]], uranium, and plutonium do not have clear analogs among the [[lanthanide]]s and are more similar to those of the 3d&nbsp;[[transition metal]]s.<ref name="Yu. D. Tretyakov">{{cite book|editor=Yu. D. Tretyakov|title = Non-organic chemistry in three volumes| place =Moscow|publisher = Academy|date = 2007|volume = 3|series = Chemistry of transition elements|isbn = 978-5-7695-2533-9}}</ref>

{| class="wikitable" style="margin:auto; text-align:center;"
|+ Allotropes of neptunium<ref name="Arblaster 2018" /><ref name="Yoshida718" /><ref name = "alo">{{cite journal | last1 = Lee | first1 = J. | last2 = Mardon | first2 = P. | last3 = Pearce | first3 = J. | last4 = Hall | first4 = R. | title = Some physical properties of neptunium metal II: A study of the allotropic transformations in neptunium | journal = Journal of Physics and Chemistry of Solids | volume = 11 | pages = 177–181 | date = 1959 | doi = 10.1016/0022-3697(59)90211-2 | issue = 3–4|bibcode = 1959JPCS...11..177L }}</ref>
|-
!Allotrope
!α (measured at 20&nbsp;°C)
!β (measured at 313&nbsp;°C)
!γ (measured at 600&nbsp;°C)
|-
!Transition temperature<ref name="Arblaster 2018" />
|(α→β) 280&nbsp;°C
|(β→γ) 576&nbsp;°C
|(γ→liquid) 639&nbsp;°C
|-
!Crystal structure
|[[Orthorhombic]]
|[[Tetragonal]]
|[[Body-centered cubic]]
|-
![[Pearson symbol]]
|oP8
|tP4
|cI2
|-
![[Space group]]
|''Pnma''
|''P42<sub>1</sub>2''
|''Im{{overline|3}}m''
|-
!Density (g/cm<sup>3</sup>, <sup>237</sup>Np)<ref name="Arblaster 2018" />
|20.45
|19.36
|18.08
|-
![[Lattice parameter]]s ([[picometer|pm]])<ref name="Arblaster 2018" />
|''a'' = 472.3<br/>''b'' = 488.7<br/>''c'' = 666.3
|''a'' = 489.5<br/>''c'' = 338.9
|''a'' = 351.8
|}

[[File:Phase diagram of neptunium (1975).png|thumb|upright=1.1|left|Phase diagram of neptunium]]
α-neptunium takes on an [[orthorhombic]] structure, resembling a highly distorted body-centered cubic structure.<ref name="Lemire">Lemire, R. J. et al.,''Chemical Thermodynamics of Neptunium and Plutonium'', Elsevier, Amsterdam, 2001.</ref><ref>{{cite web|url=http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|title=Crystal Lattice Structures: The αNp (Ac) Structure|publisher=United States Naval Research Laboratory Center for Computational Materials Science|access-date=2013-10-16|url-status=dead|archive-url=https://web.archive.org/web/20121002050018/http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|archive-date=2012-10-02}}</ref> Each neptunium atom is coordinated to four others and the Np–Np bond lengths are 260&nbsp;pm.<ref name="Yoshida719">Yoshida et al., p. 719.</ref> It is the densest of all the actinides and the fifth-densest of all naturally occurring elements, behind only [[rhenium]], [[platinum]], [[iridium]], and [[osmium]].<ref name="Gray">Theodore Gray. ''The Elements''. Page 215.</ref> α-neptunium has [[semimetal]]lic properties, such as strong [[covalent bond]]ing and a high [[electrical resistivity and conductivity|electrical resistivity]], and its metallic physical properties are closer to those of the [[metalloid]]s than the true metals. Some allotropes of the other actinides also exhibit similar behaviour, though to a lesser degree.<ref>Hindman J. C. 1968, "Neptunium", in C. A. Hampel (ed.), ''The encyclopedia of the chemical elements'', Reinhold, New York, pp. 434.</ref><ref>{{cite journal | last1 = Dunlap | first1 = B. D. | last2 = Brodsky | first2 = M. B. | last3 = Shenoy | first3 = G. K. | last4 = Kalvius | first4 = G. M. | date = 1970 | title = Hyperfine interactions and anisotropic lattice vibrations of <sup>237</sup>Np in α-Np metal | journal = Physical Review B | volume = 1 | issue = 1| pages = 44–46 | doi = 10.1103/PhysRevB.1.44 | bibcode = 1970PhRvB...1...44D }}</ref> The densities of different isotopes of neptunium in the alpha phase are expected to be observably different: α-<sup>235</sup>Np should have density 20.303&nbsp;g/cm<sup>3</sup>; α-<sup>236</sup>Np, density 20.389&nbsp;g/cm<sup>3</sup>; α-<sup>237</sup>Np, density 20.476&nbsp;g/cm<sup>3</sup>.<ref name="critical">{{cite web|publisher = Institut de Radioprotection et de Sûreté Nucléaire|title = Evaluation of nuclear criticality safety data and limits for actinides in transport|page = 15|url = http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|access-date=2010-12-20 }}</ref>

β-neptunium takes on a distorted tetragonal close-packed structure. Four atoms of neptunium make up a unit cell, and the Np–Np bond lengths are 276&nbsp;pm.<ref name="Yoshida719" /> γ-neptunium has a [[body-centered cubic]] structure and has Np–Np bond length of 297&nbsp;pm. The γ form becomes less stable with increased pressure, though the melting point of neptunium also increases with pressure.<ref name="Yoshida719" /> The β-Np/γ-Np/liquid [[triple point]] occurs at 725&nbsp;°C and 3200&nbsp;[[pascal (unit)|MPa]].<ref name="Yoshida719" /><ref>{{cite journal |last1=Stephens |first1=D. R. |date=1966 |title=Phase diagram and compressibility of neptunium |journal=Journal of Physics |volume=27 |issue=8 |pages=1201–4 |doi=10.1016/0022-3697(66)90002-3|bibcode = 1966JPCS...27.1201S }}</ref>

====Alloys====
Due to the presence of valence 5f electrons, neptunium and its alloys exhibit a very interesting magnetic behavior, like many other actinides. These can range from the itinerant band-like character characteristic of the [[transition metal]]s to the local moment behavior typical of [[scandium]], [[yttrium]], and the [[lanthanide]]s. This stems from 5f-orbital hybridization with the orbitals of the metal [[ligand]]s, and the fact that the 5f orbital is [[relativistic effects|relativistically]] destabilized and extends outwards.<ref name="Yoshida720">Yoshida et al., pp. 719–20.</ref> For example, pure neptunium is [[paramagnetic]], Np[[aluminium|Al]]<sub>3</sub> is [[ferromagnetic]], Np[[germanium|Ge]]<sub>3</sub> has no magnetic ordering, and Np[[tin|Sn]]<sub>3</sub> may be a [[heavy fermion material]].<ref name="Yoshida720" /> Investigations are underway regarding alloys of neptunium with uranium, [[americium]], [[plutonium]], [[zirconium]], and [[iron]], so as to recycle long-lived waste isotopes such as neptunium-237 into shorter-lived isotopes more useful as nuclear fuel.<ref name="Yoshida720" />

One neptunium-based [[superconductivity|superconductor]] alloy has been discovered with formula Np[[palladium|Pd]]<sub>5</sub>Al<sub>2</sub>. This occurrence in neptunium compounds is somewhat surprising because they often exhibit strong magnetism, which usually destroys superconductivity. The alloy has a tetragonal structure with a superconductivity transition temperature of −268.3&nbsp;°C (4.9&nbsp;K).<ref name="lanl" /><ref>{{cite journal |author=T. D. Matsuda |author2= Y. Hagal |author3= D. Aoki |author4= H. Sakai |author5= Y. Homma |author6= N. Tateiwa|author7=E. Yamamoto |author8=Y. Onuki |date=2009 |title=Transport properties of neptunium superconductor NpPd<sub>5</sub>Al<sub>2</sub> |journal=Journal of Physics: Conference Series |volume=150 |issue=4 | pages=042119 |doi=10.1088/1742-6596/150/4/042119|bibcode = 2009JPhCS.150d2119M |doi-access=free }}</ref>

===Chemical===
Neptunium has five ionic [[oxidation state]]s ranging from +3 to +7 when forming chemical compounds, which can be simultaneously observed in solutions. It is the heaviest actinide that can lose all its valence electrons in a stable compound. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of [[coordination compound]]s.<ref name="Himiya neptuniya">{{cite book|title = Analytical chemistry of neptunium|editor=V. A. Mikhailov|place =Moscow |publisher = [[Nauka (publisher)|Nauka]]|date = 1971}}</ref>

===Atomic===
A neptunium atom has 93 electrons, arranged in the [[electron configuration|configuration]] <nowiki>[</nowiki>[[Radon|Rn]]<nowiki>]</nowiki>&nbsp;5f<sup>4</sup>&nbsp;6d<sup>1</sup>&nbsp;7s<sup>2</sup>. This differs from the configuration expected by the [[Aufbau principle]] in that one electron is in the 6d [[Electron shell#Subshells|subshell]] instead of being as expected in the 5f subshell. This is because of the similarity of the electron energies of the 5f, 6d, and 7s subshells. In forming compounds and ions, all the valence electrons may be lost, leaving behind an inert core of inner electrons with the electron configuration of the [[noble gas]] radon;<ref>{{cite book|author = Golub, A. M. |title = Общая и неорганическая химия (General and Inorganic Chemistry)|date = 1971|volume = 2|pages=222–7}}</ref> more commonly, only some of the valence electrons will be lost. The electron configuration for the tripositive ion Np<sup>3+</sup> is [Rn]&nbsp;5f<sup>4</sup>, with the outermost 7s and 6d electrons lost first: this is exactly analogous to neptunium's lanthanide homolog promethium, and conforms to the trend set by the other actinides with their [Rn]&nbsp;5f<sup>''n''</sup> electron configurations in the tripositive state. The first [[ionization potential]] of neptunium was measured to be at most {{val|6.19|0.12|u=[[electronvolt|eV]]}} in 1974, based on the assumption that the 7s electrons would ionize before 5f and 6d;<ref name="NIST">{{cite journal |first1=W. C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugan |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |url=https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |journal=J. Phys. Chem. Ref. Data |volume=3 |issue=3 |pages=771–9 |access-date=2013-10-19 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M |archive-date=2014-02-11 |archive-url=https://web.archive.org/web/20140211144635/https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |url-status=dead }}</ref> more recent measurements have refined this to 6.2657&nbsp;eV.<ref>David R. Lide (ed), ''CRC Handbook of Chemistry and Physics, 84th Edition''. CRC Press. Boca Raton, Florida, 2003; Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions.</ref>

===Isotopes===
{{Main|Isotopes of neptunium}}
[[Image:Decay Chain(4n+1, Neptunium Series).svg|thumb|The 4''n'' + 1 [[decay chain]] of neptunium-237, commonly called the "neptunium series"]]
Twenty-four neptunium [[radioisotope]]s have been characterized, with the most stable being <sup>237</sup>Np with a [[half-life]] of 2.14 million years, <sup>236</sup>Np with a half-life of 154,000 years, and <sup>235</sup>Np with a half-life of 396.1 days. All of the remaining [[radioactive]] isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has at least four [[meta state]]s, with the most stable being <sup>236m</sup>Np with a half-life of 22.5 hours.<ref name="unc">{{cite web |url=http://www.nucleonica.net/unc.aspx |title=Universal Nuclide Chart |author=Nucleonica |date=2007–2013 |website=Nucleonica: Web Driven Nuclear Science |access-date=2013-10-15}} {{registration required}}.</ref>

The isotopes of neptunium range in [[atomic weight]] from 219.032 [[atomic mass unit|u]] (<sup>219</sup>Np) to 244.068 u (<sup>244</sup>Np), though <sup>221</sup>Np has not yet been reported.{{NUBASE2020|ref}} Most of the isotopes that are lighter than the most stable one, <sup>237</sup>Np, [[radioactive decay|decay]] primarily by [[electron capture]] although a sizable number, most notably <sup>229</sup>Np and <sup>230</sup>Np, also exhibit various levels of decay via [[alpha emission]] to become [[protactinium]]. <sup>237</sup>Np itself, being the [[beta-decay stable isobars|beta-stable isobar]] of mass number 237, decays almost exclusively by alpha emission into <sup>233</sup>[[Isotopes of protactinium|Pa]], with very rare (occurring only about once in trillions of decays) [[spontaneous fission]] and [[cluster decay]] (emission of <sup>30</sup>Mg to form <sup>207</sup>Tl). All of the known isotopes except one that are heavier than this decay exclusively via [[beta emission]].<ref name="unc" /><ref name="Yoshida702" /> The lone exception, <sup>240m</sup>Np, exhibits a rare (>0.12%) decay by [[isomeric transition]] in addition to beta emission.<ref name="unc" /> <sup>237</sup>Np eventually decays to form [[bismuth]]-209 and [[thallium]]-205, unlike most other common heavy nuclei which decay into [[isotopes of lead]]. This [[decay chain]] is known as the [[neptunium series]].<ref name="lanl">{{cite web| url=http://periodic.lanl.gov/93.shtml| title=Periodic Table Of Elements: LANL - Neptunium| publisher=Los Alamos National Laboratory| access-date=2013-10-13}}</ref><ref>{{cite book|author=C. M. Lederer|author2=J. M. Hollander|author3=I. Perlman|date=1968|title=Table of Isotopes|edition=6th|location=New York|publisher=[[John Wiley & Sons]]}}</ref> This decay chain had long been extinct on Earth due to the short half-lives of all of its isotopes above bismuth-209, but is now being resurrected thanks to artificial production of neptunium on the tonne scale.<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167|chapter=Transuranium Elements|isbn=978-3527306732}}</ref>

[[Image:Np sphere.jpg|thumb|This nickel-clad neptunium sphere was used to experimentally determine the critical mass of Np at Los Alamos National Lab.]]
The isotopes neptunium-235, -236, and -237 are predicted to be [[fissile]];<ref name="critical" /> only neptunium-237's fissionability has been experimentally shown, with the [[critical mass]] being about 60&nbsp;kg, only about 10&nbsp;kg more than that of the commonly used [[uranium-235]].<ref name="Weiss">{{cite journal |last=Weiss |first=Peter |date=2 July 2009 |title=Neptunium nukes?: Little-studied metal goes critical |journal=Science News |volume=162 |issue=17 |pages=259 |doi=10.2307/4014034 |jstor=4014034 }}</ref> Calculated values of the critical masses of neptunium-235, -236, and -237 respectively are 66.2&nbsp;kg, 6.79&nbsp;kg, and 63.6&nbsp;kg: the neptunium-236 value is even lower than that of [[plutonium-239]]. In particular, <sup>236</sup>Np also has a low neutron [[cross section (physics)|cross section]].<ref name="critical" /> Despite this, a neptunium [[atomic bomb]] has never been built:<ref name="Weiss" /> uranium and plutonium have lower critical masses than <sup>235</sup>Np and <sup>237</sup>Np, and <sup>236</sup>Np is difficult to purify as it is not found in quantity in [[spent nuclear fuel]]<ref name="Yoshida702" /> and is nearly impossible to separate in any significant quantities from <sup>237</sup>Np.<ref name="Jukka">{{cite book |author=Jukka Lehto |author2=Xiaolin Hou |date=2011|chapter=15.15: Neptunium |title=Chemistry and Analysis of Radionuclides |page=231 |no-pp=yes |edition=1st |publisher=[[John Wiley & Sons]] |isbn=978-3527633029}}</ref>

===Occurrence===
The longest-lived isotope of neptunium, <sup>237</sup>Np, has a half-life of 2.14 million years, which is more than 2,000 times shorter than the [[age of the Earth]]. Therefore, any [[primordial nuclide|primordial]] neptunium would have decayed in the distant past. After only about 80 million years, the concentration of even the longest-lived isotope, <sup>237</sup>Np, would have been reduced to less than one-trillionth (10<sup>−12</sup>) of its original amount.<ref name="Yoshida704">Yoshida et al., pp. 703–4.</ref> Thus neptunium is present in nature only in negligible amounts produced as intermediate decay products of other isotopes.<ref name="Himiya neptuniya" />

[[Trace radioisotope|Trace]] amounts of the neptunium isotopes neptunium-237 and -239 are found naturally as [[decay product]]s from [[Nuclear transmutation|transmutation]] reactions in [[uranium ore]]s.<ref name="CRC" /><ref name="emsley345347">Emsley, pp. 345–347.</ref> <sup>239</sup>Np and <sup>237</sup>Np are the most common of these isotopes; they are directly formed from [[neutron capture]] by uranium-238 atoms. These neutrons come from the [[spontaneous fission]] of uranium-238, naturally neutron-induced fission of uranium-235, [[cosmic ray spallation]] of nuclei, and light elements absorbing [[alpha particle]]s and emitting a neutron.<ref name="Yoshida704" /> The half-life of <sup>239</sup>Np is very short, although the detection of its much longer-lived [[daughter product|daughter]] <sup>239</sup>Pu in nature in 1951 definitively established its natural occurrence.<ref name="Yoshida704" /> In 1952, <sup>237</sup>Np was identified and isolated from concentrates of uranium ore from the [[Belgian Congo]]: in these minerals, the ratio of neptunium-237 to uranium is less than or equal to about 10<sup>−12</sup> to 1.<ref name="Yoshida704" /><ref name="thompson1to4">
{{cite journal |last=Thompson |first=Roy C. |date=1982 |title=Neptunium: The Neglected Actinide: A Review of the Biological and Environmental Literature |journal=Radiation Research |volume=90 |issue=1 |pages=1–32 |doi=10.2307/3575792 |pmid=7038752 |jstor=3575792 |bibcode=1982RadR...90....1T }}</ref><ref name="NUBASE">{{NUBASE 2003}}</ref> Additionally, <sup>240</sup>Np must also occur as an intermediate decay product of [[plutonium-244|<sup>244</sup>Pu]], which has been detected in meteorite dust in marine sediments on Earth.<ref name="WallnerFaestermann2015">{{cite journal|last1=Wallner|first1=A.|last2=Faestermann|first2=T.|last3=Feige|first3=J.|last4=Feldstein|first4=C.|last5=Knie|first5=K.|last6=Korschinek|first6=G.|last7=Kutschera|first7=W.|last8=Ofan|first8=A.|last9=Paul|first9=M.|last10=Quinto|first10=F.|last11=Rugel|first11=G.|last12=Steier|first12=P.|title=Abundance of live <sup>244</sup>Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis|journal=Nature Communications|volume=6|year=2015|pages=5956|issn=2041-1723|doi=10.1038/ncomms6956|pmid=25601158 |pmc=4309418 |arxiv=1509.08054|bibcode=2015NatCo...6.5956W}}</ref>

Most neptunium (and plutonium) now encountered in the environment is due to atmospheric nuclear explosions that took place between the detonation of the [[Trinity test|first atomic bomb]] in 1945 and the ratification of the [[Partial Nuclear Test Ban Treaty]] in 1963. The total amount of neptunium released by these explosions and the few atmospheric tests that have been carried out since 1963 is estimated to be around 2500&nbsp;kg. The overwhelming majority of this is composed of the long-lived isotopes <sup>236</sup>Np and <sup>237</sup>Np since even the moderately long-lived <sup>235</sup>Np (half-life 396&nbsp;days) would have decayed to less than one-billionth (10<sup>−9</sup>) its original concentration over the intervening decades. An additional very small amount of neptunium, produced by neutron irradiation of natural uranium in nuclear reactor cooling water, is released when the water is discharged into rivers or lakes.<ref name="Yoshida704" /><ref name="thompson1to4" /><ref>{{cite book |last=Foster |first=R. F. |title=Environmental behavior of chromium and neptunium ''in'' Radioecology |date=1963 |publisher=Reinhold |location=New York |pages=569–576}}</ref> The concentration of <sup>237</sup>Np in seawater is approximately 6.5&nbsp;×&nbsp;10<sup>−5</sup>&nbsp;[[becquerel (unit)|millibecquerels]] per [[liter]]: this concentration is between 0.1% and 1% that of plutonium.<ref name="Yoshida704" />

Once released in the surface environment, in contact with atmospheric [[oxygen]], neptunium generally [[oxidation|oxidizes]] fairly quickly, usually to the +4 or +5 state. Regardless of its [[oxidation state]], the element exhibits much greater mobility than the other actinides, largely due to its ability to readily form aqueous solutions with various other elements. In one study comparing the diffusion rates of neptunium(V), plutonium(IV), and americium(III) in sandstone and limestone, neptunium penetrated more than ten times as well as the other elements. Np(V) will also react efficiently in pH levels greater than 5.5 if there are no [[carbonate]]s present and in these conditions it has also been observed to readily bond with [[quartz]]. It has also been observed to bond well with [[goethite]], [[ferric oxide]] colloids, and several clays including [[kaolinite]] and [[smectite]]. Np(V) does not bond as readily to soil particles in mildly acidic conditions as its fellow actinides americium and curium by nearly an order of magnitude. This behavior enables it to migrate rapidly through the soil while in solution without becoming fixed in place, contributing further to its mobility.<ref name="thompson1to4" /><ref name="atwood4">Atwood, section 4.</ref> Np(V) is also readily absorbed by [[concrete]], which because of the element's radioactivity is a consideration that must be addressed when building [[nuclear waste]] storage facilities. When absorbed in concrete, it is [[Redox|reduced]] to Np(IV) in a relatively short period of time. Np(V) is also reduced by [[humic acid]]s if they are present on the surface of goethite, [[hematite]], and [[magnetite]]. Np(IV) is less mobile and efficiently [[Sorption|adsorbed]] by [[tuff]], [[granodiorite]], and [[bentonite]]; although uptake by the latter is most pronounced in mildly acidic conditions. It also exhibits a strong tendency to bind to [[colloid|colloidal particulates]], an effect that is enhanced when in surface [[soil]] with high [[clay]] content. The behavior provides an additional aid in the element's observed high mobility.<ref name="thompson1to4" /><ref name="atwood4" /><ref name="atwood1">Atwood, section 1.</ref><ref>{{cite web| url=http://hpschapters.org/northcarolina/NSDS/neptunium.pdf| title=Human Health Fact Sheet - Neptunium| publisher=Health Physics Society| date=2001| access-date=2013-10-15}}</ref>