Phosphogypsum (PG) is the

calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...

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 consid ...

of the production of

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) 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 ...

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 pento ...

. It is mainly composed of

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate 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, blackboard or sidewalk chalk, and drywa ...

(CaSO₄·2H₂O). Although gypsum is a widely used material in the

construction industry Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and co ...

, phosphogypsum is usually not used, but is stored indefinitely because of its weak radioactivity caused by the presence of naturally occurring

uranium Uranium is a chemical element with the 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. Uranium is weak ...

(U) and

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...

(Th), and their daughter isotopes

radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rat ...

(Ra),

radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains th ...

(Rn) and

polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...

(Po). The long-range 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

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, w ...

by treating phosphate ore (

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...

) with

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

according to the following reaction: :Ca₅(PO₄)₃X + 5 H₂SO₄ + 10 H₂O → 3 H₃PO₄ + 5 (CaSO₄ · 2 H₂O) + HX :where X may include OH, F, Cl, or Br Phosphogypsum 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 (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

phosphate deposits, because uranium is present in

seawater Seawater, or salt 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 appro ...

at about 3 ppb (roughly 85 ppb of

total dissolved solids Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often report ...

). 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 ocean ...

deposits as dissolved solids precipitate in order of

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

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 , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

(5–10%),

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ty ...

(F, ~1%),

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

(P, ~0.5%),

iron Iron () is a chemical element with symbol Fe (from la, ferrum) 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, right in ...

(Fe, ~0.1%),

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

(Al, ~0.1%),

barium Barium is a chemical element with the 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 with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

(Pb, ~5 ppm),

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...

(Cr, ~3 ppm),

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

(Se, ~1 ppm),

cadmium Cadmium is a chemical element with the Symbol (chemistry), 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. Li ...

(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 is a human-created underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, or improve surfing. Many ...

s and oyster beds *Cover for

landfill A landfill site, also known as a tip, dump, rubbish dump, garbage dump, or dumping ground, is a site for the disposal of waste materials. Landfill is the oldest and most common form of waste disposal, although the systematic burial of the wast ...

s * Road pavement *

Roof tile A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temper ...

s *

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, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...

". The rest remains in stack.

In the United States

The

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

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

pico Pico may refer to: Places The Moon * Mons Pico, a lunar mountain in the northern part of the Mare Imbrium basin Portugal * Pico, a civil parish in the municipality of Vila Verde * Pico da Pedra, a civil parish in the municipality of Ribe ...

curie In computing, a CURIE (or ''Compact URI'') defines a generic, abbreviated syntax for expressing Uniform Resource Identifiers (URIs). It is an abbreviated URI expressed in a compact syntax, and may be found in both XML and non-XML grammars. A 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 is the activity per unit mass of a radionuclide and is a physical property of that radionuclide. Activity is a quantity (for which the SI unit is the becquerel) related to radioactivity, and is defined as the number of radi ...

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 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. Which - if any - health risks are associated with the low doses of radiation that may result from the use of phosphogypsum in construction is hard to quantify as natural

background radiation Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources. Background radiation originates from a variety of source ...

usually imparts a higher dose than those additional sources. Policy is based on the

linear no threshold model The linear no-threshold model (LNT) is a dose-response model used in radiation protection to estimate stochastic health effects such as radiation-induced cancer, genetic mutations and teratogenic effects on the human body due to exposure to io ...

and thus usually mandates to keep any avoidable dose as low as reasonably achievable. Given that the involved radionuclides are alpha emitters, whose main danger comes from incorporation (inhalation, ingestion etc.), an unsuitably stacked deposit which emits dust to the environment may actually impart a higher

absorbed dose Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection (reduction of har ...

than durable construction materials that do not release dust to the environment.

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 Asia, the third-longest in the world, and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains (Tibetan Plateau) and flows ...

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 the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...

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 freshwater 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 Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods 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 ...

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, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thic ...

. It is also used as a chemical feedstock for producing sulfates, 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. 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 metal A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury and lead, all of which appear in the World ...

s 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

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 also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...

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 (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissues of the lung. Lung carcinomas derive from transformed, mali ...

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 p ...

. 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, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...

may be used to clean up already contaminated water and soil. Microbials can remove heavy metals and radioactive material itation needed regarding radioactivity removal by plants 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 wasn't 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 ...

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

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and ...

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