Editing Neptunium (section)

===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>