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!!