Phosphogypsum

Phosphogypsum (PG) is the calcium sulfate hydrate formed as a by-product of the production of fertilizer, particularly phosphoric acid, from phosphate rock.

Although gypsum is a widely used material in the construction industry, phosphogypsum is usually not used, but is stored indefinitely because of its weak radioactivity caused by the presence of naturally occurring uranium (U) and thorium (Th), and their daughter isotopes radium (Ra), radon (Rn) and polonium (Po).

On the other hand, it includes several valuable components—calcium sulphates and elements such as silicon, iron, titanium, magnesium, aluminum, and manganese.

[3] Phosphogypsum is a by-product from the production of phosphoric acid by treating phosphate ore (apatite) with sulfuric acid according to the following reaction: It is radioactive due to the presence of naturally occurring uranium (5–10 ppm) and thorium, and their daughter nuclides radium, radon, polonium, etc.

[3] The United States Environmental Protection Agency (EPA) has banned most applications of phosphogypsum having a 226Ra concentration of greater than 10 picocurie/gram (0.4 Bq/g)[7] in 1990.

Given the traditional definition of the Curie via the specific activity of 226Ra, this limit is equivalent to 0.01 milligrams (0.00015 gr) of radium per metric ton or a concentration of 10 parts per trillion.

By 2018, inappropriate storage has become a major problem in the Yangtze River watershed, with phosphorus accounting for 56% of all breaches of water quality standards.

[citation needed], some nevertheless exceed acceptable radioactivity limits for building material, or produce crops with unacceptable amounts of arsenic (As), lead (Pb), cadmium (Cd), or mercury (Hg).

[3] Radon is a noble gas that is heavier than air and thus tends to accumulate in poorly ventilated underground spaces like mines or cellars.

[15] More substantial however is the leaching of the contents of phosphogypsum into the water table and consequently soil, exacerbated by the fact that PG is often transported as a slurry.

[3] Accumulation of water inside of gypstacks can lead to weakening of the stack structure, a cause of several alarms in the United States.

[17] With suitable soil amendments and additives, PG can also support the growth of hardy plants, hopefully preventing further erosion.