The history of a bag of phosphate fertilizer

Phosphorus, along with nitrogen and potassium, is one of the three key nutrients that plants need and the main ingredient in phosphate fertilizers. Maintaining proper levels, phosphorus helps a plant acquire and store energy, as well as transfers it throughout the plant. It promotes the development of roots, flowers and fruit, and helps to promote early maturity⁽¹⁾.

Phosphate fertilizer typically comes from phosphate rock, phosphorite, a mineral mined in large open pit mines. Millions of tons are mined from locations around the world; notable deposits are in Morocco, China, Florida and South America. The raw ore could be used as a fertilizer but the relatively low amount of phosphorous, along with higher transportation costs make it more expensive than refined phosphate fertilizer.

The phosphate rock is dissolved by sulfuric acid creating a solid/liquid mixture of phosphoric acid and calcium sulfate called phosphogypsum (PG). The desired phosphoric acid component is

separated from the mixture by filtration, creating the so called “super phosphate”, leaving PG as the waste product. The water soluble super phosphate fertilizer is then mixed with water for application to fields and crops. Phosphate fertilizers can be combined with other primary fertilizers to create complete fertilizers.
But where is the problem? Phosphate ores contain high concentrations of natural occurring radionuclides: Uranium, and its decay products, such as Radium-226. Uranium concentrations in phosphate deposits vary worldwide, with a range of 25–200 mg/kg. This is considerably higher than the average uranium concentration in soil, which is 3 mg/kg. U can have activity concentrations up to 3.7 Bq/g, which is 3.7 times more than the exemption limit for radwaste. During the wet process, radionuclides present in the phosphate ore are selectively separated and concentrated. Around 80 percent of the radium-226 becomes concentrated in the PG. A small fraction of the radium accompanies the product, phosphoric acid, and ends up in commercial fertilizer⁽²⁾. As the Ra-226 decays to noble gas Rn-222, PG deposits can represent significant source of exposure to Rn-222 through inhalation.

The amount of PG generated worldwide is estimated to be around 100-280 Mt per year. Only 15% of world’s PG production is recycled as building materials, agricultural fertilizers or soil stabilization amendments and as a set controller in the manufacture of Portland cement. The remaining 85% is disposed of without any

treatment. This byproduct is usually dumped in large stockpiles exposed to weathering processes, occupying considerable land areas and causing serious environmental damage (chemical and radioactive contamination), particularly in coastal regions⁽³⁾.
Radiation levels in PG vary considerably, from stack to stack and from different locations in a single stack. This is due to a number of factors: porosity and moisture content, the amount of radionuclides originally present in the ore, and variations within different ores. Radionuclides of importance include U-238, Ra-226, Rn-222, Pb-210 and Po-210. Radiochemists can help to characterize such variations, which supports decision making on how to remediate and manage sites. Mobility and solubility of radionuclides in PG can also vary significantly and radiochemists apply sophisticated methods to determine migration behavior and speciation in conditions at which radionuclides are present in these complex environments. Due to large variability in PG radiation levels, each disposal site has to be evaluated separately and the specifics of each site may govern its future management.
PG has been used in agriculture as a source of calcium and sulfur for soils that are deficient in these elements, but now this is limited to PG with activity of less than 0.37 Bq/g.
The Phosphate rock can be converted to elemental phosphorus by a thermal process, the byproduct of this is phosphate slag. Its physical properties and its high carbonate content make slag less susceptible to radionuclide leaching than PG. However, concentrations of uranium, thorium and radium in phosphate slag have been measured up to 1.85 Bq/g. The radon emanating from slag is on average lower than native soil samples (0.0185 Bq/m² per second compared to 0.06 - 0.629 Bq/m² per second).
Slag byproduct has been reused for a variety of applications: highway construction aggregate, Portland cement and concrete, railroad ballast and general construction, but in some cases it has been banned in habitable buildings.