Why does phosphate rock contain uranium?

Question

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.

Thank you in advance for your help!!

Answer 1

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 uranium mostly remains as utanyl ion 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.

Answer 2

Please read this article. It explains the heavy metal separation from the wet process.

http://dx.doi.org/10.1016/j.jrras.2014.01.001

I believe the decay rate of thorium to radium in the uranium decay series is 77k years. Therefore, however much radium/thorium/uranium you measure in a sample, it won't really change during your lifetime.

According to the United Nations Scientific Committee on the Effects of Atomic Radiation the normal concentration of uranium in soil is 300 μg/kg to 11.7 mg/kg. It is found in most soils and rock naturally at highly varying concentrations.

Phosphate rock does not become enriched with uranium/thorium/radium until the extraction process separates and concentrates these metals. When the phosphate rock is treated with sulfuric acid, the uranium and thorium become concentrated in the aqueous phase while radium concentrates into the gypsum. A lot of other heavy metals also get incorporated into the phosphoric acid phase since most are soluble in dilute acid solutions. I suppose these contaminants could be marketed as micronutrients in the phosphate fertilizer products.