# Why does phosphate rock contain uranium?

I have been doing some research on the issue of phosphogypsum pollution. Phosphogypsum is a waste by-product formed with the "wet process" method of extraction of phosphoric acid out of phosphate rock, which uses sulfuric acid and water to break it:

$$\ce{Ca10(PO4)6F2 + 10H2SO4 + 20H2O -> 10CaSO4.2H2O + 6H3PO4 + 2HF}$$

Returning the phosphoric acid and gypsum (calcium sulphate, or CaSO4). The problem here is that the uranium in the phosphate rock produces 226Ra (among other radionuclides) during its decay, and since 226Ra is an "Alkalin Earth Metal" (it's on the 2nd column of the periodic table), it can form radium sulphate and mimic calcium, leading to radioactive gypsum.

The point here is, why is the phosphate rock enrichened in uranium that can produce radium? Is there a trend for phosphate and uranium to accumulate together, and if so, what is the geochemical reason??

I have already found out that:

Sedimentary rock phosphates contain much higher concentrations of potentially hazardous elements (As, Cd, Cr,Pb, Se and U)than igneous rock phosphates. (1)

Phosphate rock varies considerably in content of U, Ra, and Th, depending on the geographical area from which it was mined. (2)

1 - Mamdoh Sattouf, Identifying the Origin of Rock Phosphates and Phosphorous Fertilisers Using Isotope Ratio Techniques and Heavy Metal Patterns.

2 - John J. Mortvedt and James D. Beaton, Heavy Metal And Radionuclide Contaminants In Phosphate Fertilizers.

• very good researched question. – gansub Apr 2 '15 at 12:12
• @antortjim: You've made me aware of something I didn't know. – Fred Apr 2 '15 at 12:23

To understand why sedimentary phosphate rocks (hereafter referred to as phosphorites) have elevated uranium contents we first need to understand what are they made of and why do they form in the first place.

Phosphorites are rocks that are made of apatite, a mineral with the formula $\ce{Ca5(PO4)3(F,Cl,OH)}$. This mineral (especially the OH variant) is one of the main constituents of skeletons of living organisms. The hard mineralised part of your teeth and bones are made of apatite, for instance.

Skeletons of dead marine animals (e.g. fish) are deposited on the ocean floor as apatite. The source of apatite need not be biogenic: it can also be sourced from igneous or hydrothermal activity occurring on the ocean floor, or derived as clastic material from the continents. The end result, is the deposition of apatite as phosphorites.

An important property of apatite is that it can accommodate some uranium. The mechanism is not yet completely understood, but it's most likely the incorporation of $\ce{U^4+}$ in the mineral structure of apatite, replacing calcium. Now here's the interesting part - uranium in sea water is mostly present as $\ce{U^6+}$ (in the form of the uranyl ion). The uranyl ion is more soluble than $\ce{U^4+}$ so it mostly remains in solution in the sea water. But, as we mentioned earlier, apatite deposits form by the decay of dead marine animals. This accumulation of decaying organic material reduces the $\ce{U^6+}$ to $\ce{U^4+}$, facilitating its incorporation into the apatite structure.

That's only one side of the coin. Apparently $\ce{U^6+}$ can also be adsorbed on apatite grains. This is even used in some cases to stop uranium contamination from migrating through soil - by adding apatite to it in order to immobilise it.

Whatever the exact process (or combination of processes) that make uranium such a good friend of apatite, the end result is that apatite (and rocks containing it) is enriched in uranium relative to its environment.

• Thank you for this great answer. I understood everything except for the part of the uranyl ion is less soluble than U4+ so it mostly remains in solution in the sea water . I thought that in order to make it easier for the uranium to incorporate into the apatite, it should evolve to a less soluble species. If organic material makes it evolve to U4+ and U4+ enters more easily the structure of apatite, it should be less soluble that U6+. Correct me if I am wrong! – antortjim Apr 2 '15 at 14:23
• "The source of apatite must not be biogenic" Do you mean "... is not necessarily biogenic"? "Must not be" means that it's forbidden, whereas I suspect you mean "It's not the case that it must be", i.e., that it's not mandatory. – David Richerby Apr 2 '15 at 17:17
• @antortjim and David, yes. Forgive me for not proof-reading it :) Should be ok now. – Gimelist Apr 3 '15 at 4:08
• Could you please add a reference? Thank you! – antortjim Apr 29 '15 at 16:12
• I had no idea about the above explanation. My thought was that since phosphoric salts tend to be less soluble than any other mixed salt with the same anion (consult any solubility table and see) then a mixed phosphoric salt grain would be less likely to form a crack due to dissolution of a part of it, so it should physically trap radioactive elements inside. Perhaps the key factor, perhaps simply a contributing factor, but 10 years ago I thought it was the main factor. – marathon16 May 27 '15 at 19:58