Phosphogypsum (PG) is the

calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...

hydrate formed as a

by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

of the production of

fertilizer A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Man ...

, particularly

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

, from

phosphate rock Phosphorite, phosphate rock or rock phosphate is a non- detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentox ...

. It is mainly composed of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

(). Although

is a widely used material in the

construction industry Construction are processes involved in delivering buildings, infrastructure, industrial facilities, and associated activities through to the end of their life. It typically starts with planning, financing, and design that continues until the ...

, phosphogypsum is usually not used, but is stored indefinitely because of its weak

radioactivity Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

caused by the presence of naturally occurring

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

(U) and

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

(Th), and their daughter isotopes

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

(Ra),

radon Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to b ...

(Rn) and

polonium Polonium is a chemical element; it has symbol Po and atomic number 84. A rare and highly radioactive metal (although sometimes classified as a metalloid) with no stable isotopes, polonium is a chalcogen and chemically similar to selenium and tel ...

(Po). On the other hand, it includes several valuable components— calcium sulphates and elements such as

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

, and

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

. However, the long-term storage of phosphogypsum is controversial.Ayres, R. U., Holmberg, J., Andersson, B., "Materials and the Global environment: Waste Mining in the 21st Century", MRS Bull. 2001, 26, 477. About five tons of phosphogypsum are generated per ton of phosphoric acid production. Annually, the estimated generation of phosphogypsum worldwide is 100 to 280 million metric tons. Sfaz ISS satfoto 2015

Production and properties

Phosphogypsum is a by-product from the production of

by treating phosphate ore (

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

) with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

according to the following reaction: : :where X may include OH, F, Cl, or Br It is radioactive due to the presence of naturally occurring

(5–10 ppm) and

, and their daughter nuclides radium, radon, polonium, etc. Marine-deposited phosphate typically has a higher level of radioactivity than

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

phosphate deposits, because uranium is present in

seawater Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...

at about 3 ppb (roughly 85 ppb of

total dissolved solids Total dissolved solids (TDS) is a measure of the dissolved solids, dissolved combined content of all inorganic compound, inorganic and organic compound, organic substances present in a liquid in molecule, molecular, ionized, or micro-granular (so ...

). Uranium is concentrated during the formation of

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...

deposits as dissolved solids precipitate in order of

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

with easily dissolved materials such as sodium chloride remaining in solution longer than less soluble materials like uranium or sulfates. Other components of phosphogypsum include

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

(5–10%),

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...

(F, ~1%),

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

(P, ~0.5%),

(Fe, ~0.1%),

(Al, ~0.1%),

barium Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

(Ba, 50 ppm),

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

(Pb, ~5 ppm),

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

(Cr, ~3 ppm),

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

(Se, ~1 ppm), and

cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

(Cd, ~0.3 ppm). About 90% of Po and Ra from raw ore is retained into Phosphogypsum. Thus it can be considered technologically enhanced naturally occurring radioactive material ( TENORM).

Use

Various applications have been proposed for using phosphogypsum, including using it as material for: *

Artificial reef An artificial reef (AR) is a human-created freshwater or marine benthic structure. Typically built in areas with a generally featureless bottom to promote Marine biology#Reefs, marine life, it may be intended to control #Erosion prevention, erosio ...

s and

oyster bed Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...

s *Cover for

landfill A landfill is a site for the disposal of waste materials. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate and final covers only began in the 1940s. In the past, waste was ...

s * Road pavement * Roof tiles *

Soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...

According to Taylor (2009), "up to 15% of world PG production is used to make building materials, as a soil amendment and as a set controller in the manufacture of

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

". The rest remains in stack.

In the United States

The

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...

(EPA) has banned most applications of phosphogypsum having a ²²⁶Ra concentration of greater than 10 pico

curie Curie may refer to: *Curie family, a family of distinguished scientists: :* Jacques Curie (1856–1941), French physicist, Pierre's brother :* Pierre Curie (1859–1906), French physicist and Nobel Prize winner, Marie's husband :* Marie Curi ...

/gram (0.4 Bq/g) in 1990. As a result, phosphogypsum which exceeds this limit is stored in large stacks since extracting such low concentrations of radium is either not possible or not economical with current technology for either the use of the gypsum or the radium . Given the traditional definition of the Curie via the

specific activity Specific activity (symbol ''a'') is the activity per unit mass of a radionuclide and is a physical property of that radionuclide. It is usually given in units of becquerel per kilogram (Bq/kg), but another commonly used unit of specific activi ...

of , this limit is equivalent to of radium per metric ton or a concentration of 10 parts per trillion. (See below.) EPA approved the use of phosphogypsum for road construction during the Trump Administration in 2020, saying that the approval came at the request of The Fertilizer Institute, which advocates for the fertilizer industry. Environmentalists opposed the decision, saying that using the radioactive material in this way can pose health risks. In 2021, the EPA withdrew the rule authorizing the use of phosphogypsum in road construction. The state of Florida has approximately 80% of the world's phosphogypsum production capacity. In May 2023, the Florida legislature passed a bill requiring the

Florida Department of Transportation The Florida Department of Transportation (FDOT) is a decentralized agency charged with the establishment, maintenance, and regulation of public transportation in the U.S. state of Florida. The department was formed in 1969. It absorbed the power ...

to study the use of phosphogypsum in road construction, including demonstration projects, though this would require federal approval. The law, which requires the department to complete a study and make a recommendation by April 1, 2024, was signed into law by Governor

Ron DeSantis Ronald Dion DeSantis (; born September 14, 1978) is an American politician, attorney, and former United States Navy, naval officer serving as the 46th List of governors of Florida, governor of Florida since 2019. A member of the Republican Pa ...

on June 29, 2023.

In China

China's phosphate fertilizer production exceeded that of the US in 2005, and with it came the problem of excess phosphogypsum. By 2018, inappropriate storage has become a major problem in the

Yangtze River The Yangtze or Yangzi ( or ) is the longest river in Eurasia and the third-longest in the world. It rises at Jari Hill in the Tanggula Mountains of the Tibetan Plateau and flows including Dam Qu River the longest source of the Yangtze, i ...

watershed, with phosphorus accounting for 56% of all breaches of water quality standards. Phosphorus, which still remains in phosphogypsum, can lead to

eutrophication Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth of organisms that may deplete the oxygen in the water; ie. the process of too many plants growing on the s ...

of bodies of water and hence

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...

s or even

anoxic event An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic ( anoxic and sulfidic) waters. Although anoxic events have not happened for millions of years, the geol ...

s ("dead zones") in the lower layers of a body of water. The total amount of phosphogypsum in storage by 2020 exceeds 600 Mt, with 75 Mt produced each year. The construction industry is the number one user of phosphogypsum in 2020, with 10.5 Mt used as concrete set retarder and 3.5 Mt used in

drywall Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gib board, gypsum board, buster board, turtles board, slap board, custard board, gypsum panel and gyprock) is a panel made of calcium sulfate dihydrate (gypsum), with or with ...

. It is also used as a chemical feedstock for producing

sulfates The sulfate or sulphate ion is a Polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

, and as a

soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...

similar to regular gypsum. The total consumption in 2020 was 31 Mt, much lower than the rate of accumulation. There has been a significant push to expand the use of phosphogypsum on the national level since 2016, being part of two consecutive

five-year plans Five-year plan may refer to: Nation plans * Five-year plans of the Soviet Union, a series of nationwide centralized economic plans in the Soviet Union * Five-Year Plans of Argentina, under Peron (1946–1955) * Five-Year Plans of Bhutan, a series ...

. Phosphogypsum may require pre-processing to remove contaminants before use. Phosphorus (P) significantly retards curing and reduces the strength of the material, an important concern in construction. Fluorine (F) may accumulate in crops. Although Chinese phosphogypsum generally contain less toxic heavy metals and radioactive elements , 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). Barriers to further use include cost of heavy metal removal and considerable variation among sources of phosphogypsum.

Pollution and cleanup

Phosphogypsum may pollute the environment by its phosphorus content causing

, by its toxic heavy metal content, and by its radioactivity. PG releases

, which can accumulate indoors if used as a construction material. Open-air stores also release radon at a level potentially hazardous to workers. Radon is a

noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...

that is heavier than air and thus tends to accumulate in poorly ventilated underground spaces like mines or cellars. Naturally occurring radon is considered the second most common cause of

lung cancer Lung cancer, also known as lung carcinoma, is a malignant tumor that begins in the lung. Lung cancer is caused by genetic damage to the DNA of cells in the airways, often caused by cigarette smoking or inhaling damaging chemicals. Damaged ...

after smoking. 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 A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal pu ...

. Accumulation of water inside of gypstacks can lead to weakening of the stack structure, a cause of several alarms in the United States. The main approach to reducing PG pollution is to act before it leaches into the environment. This can mean recycling purified materials from PG in a variety of applications (see above) or converting it into a more stable form for storage. Cement paste backfill converts hazardous mining waste, such as PG, into a cement paste, and then uses the paste to fill in voids created by mining the rocks.

Bioremediation Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi in mycoremediation, and plants in phytoremediation), living or dead, is employed for removing environmental pollutants from air, wate ...

may be used to clean up already contaminated water and soil. Microbials can remove heavy metals, radioactive material, and any organic pollutants within, and reduce the sulfate material. With suitable soil amendments and additives, PG can also support the growth of hardy plants, hopefully preventing further erosion.

Gyp stacks

Often phosphogypsum reuse is uneconomical due to impurities, mining companies commonly dump the waste into man-made hills called "phosphogypsum stacks" or waste ponds near the mine. Waste ponds are open-air reservoirs that contain a variety of different types of industrial and agricultural waste. including at least 70 phosphogypsum stacks (from phosphate mines used for fertilizer production). A leaking phosphogypsum waste pond that nearly collapsed, if waste was not allowed to flow into

Tampa Bay Tampa Bay is a large natural harbor and shallow estuary connected to the Gulf of Mexico on the west-central coast of Florida, comprising Hillsborough Bay, McKay Bay, Old Tampa Bay, Middle Tampa Bay, and Lower Tampa Bay. The largest freshwater i ...

in Florida in 2021, highlights the dangers and near-disasters associated with wastewater ponds throughout the country. Central

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

has a large quantity of phosphate deposits, particularly in the Bone Valley region. The marine-deposited phosphate ore from central Florida is weakly radioactive, and as such, the phosphogypsum by-product (in which the radionuclides are somewhat concentrated) is too radioactive to be used for most applications. As a result, there are about a billion tons of phosphogypsum stacked in 25 stacks in Florida (22 are in central Florida) and about 30 million additional tons are generated each year.Florida Institute of Phosphate Research
"Phosphogypsum and the EPA Ban"
Archived February 19, 2015.

Production and properties

Use

In the United States

In China

Pollution and cleanup

Gyp stacks

See also

References

Further reading