Reply #98: #3The earth naturally contains hundreds of billions of curies of K and Rb.

[NNadir]

Since we are now comparing cesium and potassium, it is probably a good point to compare the total radioactivity on earth associated with each of these elements, radioactivity that in the case of potassium (and rubidium) occurs naturally, and in the case of radiocesium, is a result of technology. Since the chemistry of cesium is closely tied to that of these two elements, if there is a lot of them, we would expect the biological and physical properties of group I elements that are radioactive to be of less concern than is advertised.

First let’s look at how much potassium is in the ocean, since it is the major source of radioactivity from among the group 1 elements (the elements in the first column of the periodic table). The volume of the ocean can be taken to be about 1.37 X 10^9 cubic kilometers.

http://www.geocities.com/ultrastupidneal/Knowledge-Geography-World-Ocean.html

http://xtide.ldeo.columbia.edu/mpa/Clim-Wat/Climate/lectures/ocean/

According to this website from the EPA, http://omp.gso.uri.edu/doee/science/physical/chsal1.htm, the average salinity of the ocean should be taken as 35 grams of salt for each kilogram of seawater. Of this salt, 1.13% of this is potassium.

According to this website, http://hypertextbook.com/facts/2002/EdwardLaValley.shtml, the average density of seawater is 1030 kg/m^3.

From this data it is straightforward to calculate how much potassium is found in the ocean: 6.7 X 10^(20) grams. Of this, 0.00117% is radioactive potassium-40, almost all of this radioactivity being left over from the supernova(e) from out of which the earth was created. This means that there is that there is 7.8 X 10^(16) grams of potassium-40 in the ocean, or in different units, 78 billion metric tons of this radioactive isotope. From the half-life of potassium-40, 1.28 billion years, it is also straightforward to calculate its specific activity, which is 259,000 Becquerel (Beq) per gram. Thus it is now possible to calculate the total radioactivity of the ocean, and when one does so, one finds that the ocean has 2.0 X 10^(22) Beq. Converting to other familiar units of radioactivity, Curies, by dividing this number by 3.7 X 10^10, we see that the ocean has about 550 billion curies of radiopotassium in it.

Similarly, this website http://www.stanford.edu/group/Urchin/mineral.html shows that the molar concentration of rubidium in seawater is 2.4 X 10^-6 M. Knowing this, and that 27.835% of rubidium is Rb-87, a radioactive isotope with a half life of 47.5 billion years, it is straightforward to show that the radioactivity attributable to rubidium in the ocean is about 6.9 billion curies. Thus the total radioactivity in the ocean from naturally occurring group I elements is about 560 billion curies over all.