View source for Phosphogypsum

{{Short description|Manmade calcium sulfate hydrate by-product}}
[[File:Kėdainių fosfogipso kalnai.JPG|thumb|Phosphogypsum stack located near [[Kėdainiai]], [[Lithuania]] {{Coord|55|14|47|N|24|01|44|E|source:kolossus-ltwiki}}.]]

'''Phosphogypsum''' (PG) is the [[calcium sulfate]] hydrate formed as a [[by-product]] of the production of [[fertilizer]], particularly [[phosphoric acid]], from [[phosphate rock]]. It is mainly composed of [[gypsum]] ({{chem2|CaSO4 · 2 H2O}}). 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 sulfate|calcium sulphates]] and  elements such as [[silicon]], [[iron]], [[titanium]], [[magnesium]], [[Aluminium|aluminum]], and [[manganese]].<ref>{{cite journal |author=Chernysh, Yelizaveta |author2=Yakhnenko, Olena |author3=Chubur, Viktoriia |author4=Roubík, Hynek |title=Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects |journal=Applied Sciences |date=2021 |volume=11 |issue=4 |page=1575 |doi=10.3390/app11041575 |doi-access=free}}</ref> However, the long-term storage of phosphogypsum is controversial.<ref name=Ayr>Ayres, R. U., Holmberg, J., Andersson, B., "Materials and the Global environment: Waste Mining in the 21st Century", MRS Bull. 2001, 26, 477. {{doi|10.1557/mrs2001.119}}</ref> 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.<ref name=Taylor>{{cite journal|doi=10.1016/j.jenvman.2009.03.007|pmid=19406560|title=Environmental Impact and Management of Phosphogypsum|journal=Journal of Environmental Management|volume=90|pages=2377–2386|year=2009|last1=Tayibi|first1= Hanan|last2=Choura|first2=Mohamed|last3=López|first3=Félix A.|last4=Alguacil|first4=Francisco J.|last5=López-Delgado|first5=Aurora|issue=8|bibcode=2009JEnvM..90.2377T |hdl=10261/45241|s2cid=24111765 |hdl-access=free}}</ref>

[[File:Sfaz ISS satfoto 2015.jpg|thumb|A 2015 astronaut photo of the [[Medina of Sfax]] with part of the port and the distinctive circular earth works of the 420 ha [[Taparura]] redevelopment project of which 260 ha have been [[Reclaimed land|reclaimed]] from the sea by depositing phosphogypsum.<ref name="Tribune">[http://www.latribune.fr/actualites/economie/international/20130729trib000777970/tunisie-comment-sfax-veut-recuperer-sa-mer-.html Stéphanie Wenger, « Tunisie : comment Sfax veut récupérer « sa » mer », ''La Tribune'', 29 juillet 2013]</ref>]]

==Production and properties ==
Phosphogypsum is a by-product from the production of [[phosphoric acid]] by treating phosphate ore ([[apatite]]) with [[sulfuric acid]] according to the following reaction:
:{{chem2|Ca5(PO4)3X + 5 H2SO4 + 10 H2O -> 3 H3PO4 + 5 (CaSO4 · 2 H2O) + HX}}
:where X may include OH, F, Cl, or Br

It is radioactive due to the presence of naturally occurring [[uranium]] (5&ndash;10 [[parts per million|ppm]]) and [[thorium]], and their [[daughter nuclide]]s radium, radon, polonium, etc. Marine-deposited phosphate typically has a higher level of radioactivity than [[igneous]] phosphate deposits, because uranium is present in [[seawater]] at about 3 [[parts per billion|ppb]] (roughly 85 ppb of [[total dissolved solids]]). Uranium is concentrated during the formation of [[evaporite]] deposits as dissolved solids precipitate in order of [[solubility]] 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]] (5&ndash;10%), [[fluoride]] (F, ~1%), [[phosphorus]] (P, ~0.5%), [[iron]] (Fe, ~0.1%), [[aluminum]] (Al, ~0.1%), [[barium]] (Ba, 50 ppm), [[lead]] (Pb, ~5 ppm), [[chromium]] (Cr, ~3 ppm), [[selenium]] (Se, ~1 ppm), and [[cadmium]] (Cd, ~0.3 ppm).<ref name=Taylor/><!-- Figures are missing in the free PDF archive. Data taken and converted into rough elemental mass fractions from following ref, which is likely identical as it comes from the same authors as the citation of Taylor. Ranges are simplified into approximate geometric averages due to laziness. --><ref>{{cite periodical |title=Use of By-Product Phosphogypsum In Road Construction |author=Ramzi Taha |author2=Roger K. Seals |author3=Marty E. Tittlebaum |author4=Willis Thornsberry Jr |author5=James T. Houston|periodical=Transportation Research Record |number=1345|url=https://onlinepubs.trb.org/Onlinepubs/trr/1992/1345/1345-004.pdf}}</ref> About 90% of Po and Ra from raw ore is retained into Phosphogypsum.<ref name=Taylor/> 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:<ref name=Ayr/>
*[[Artificial reef]]s and [[oyster bed]]s
*Cover for [[landfill]]s
*[[Pavement (material)|Road pavement]]
*[[Roof tile]]s
*[[Soil conditioner]]

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]]". The rest remains in stack.<ref name=Taylor/>

===In the United States===
[[File:GypStack.JPG|thumb|right|A phosphogypsum stack or "gyp stack",<ref name=BioDiv>{{cite web |title=Imminent Failure of Phosphogypsum Stack in Tampa Bay Exposes Phosphate Industry Risks |url=https://biologicaldiversity.org/w/news/press-releases/imminent-failure-of-phosphogypsum-stack-in-tampa-bay-exposes-phosphate-industry-risks-2021-04-03/ |publisher=Center for Biological Diversity |location=Tucson, AZ |date=April 3, 2021}}</ref> located near [[Fort Meade, Florida]]. These contain the waste byproducts of the phosphate fertilizer industry.]]
The [[United States Environmental Protection Agency]] (EPA) has banned most applications of phosphogypsum having a [[Radium-226|<sup>226</sup>Ra]] concentration of greater than 10 [[Pico-|pico]][[Curie (unit)|curie]]/gram (0.4&nbsp;[[Becquerel|Bq]]/g)<ref>United States Environmental Protection Agency (EPA) (1992). "Subpart R - National Emission Standards for Radon Emissions From Phosphogypsum Stacks." ''Code of Federal Regulations,'' {{uscfr|40|61}}</ref> in 1990.<ref name=Taylor/> 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 {{citation needed|date=November 2022}}. Given the traditional definition of the Curie via the [[specific activity]] of {{chem|226|Ra}}, this limit is equivalent to {{convert|0.01|mg}} of radium per metric ton or a concentration of 10 parts per trillion. (See {{section link||Gyp stacks}} below.)

EPA approved the use of phosphogypsum for road construction during the [[First Trump Administration|Trump Administration]] in 2020, saying that the approval came at the request of The Fertilizer Institute, which advocates for the fertilizer industry.<ref>{{cite press release |title=EPA Approves Use of Phosphogypsum in Road Construction |date=2020-10-14 |publisher=EPA |url=https://www.epa.gov/newsreleases/epa-approves-use-phosphogypsum-road-construction |archive-url=https://web.archive.org/web/20210318000232/https://www.epa.gov/newsreleases/epa-approves-use-phosphogypsum-road-construction |archive-date=2021-03-18}}</ref> Environmentalists opposed the decision, saying that using the radioactive material in this way can pose health risks.<ref>{{cite news |url=https://thehill.com/policy/energy-environment/521307-epa-allows-use-of-radioactive-material-in-some-road-construction |title=EPA allows use of radioactive material in some road construction |first=Rachel |last=Frazin |work=The Hill |date=2020-10-15}}</ref> In 2021, the EPA withdrew the rule authorizing the use of phosphogypsum in road construction.<ref>{{Cite news |last=Budryk |first=Zack |date=July 2, 2021 |title=EPA withdraws rule allowing use of radioactive material in road construction |work=The Hill |url=https://thehill.com/policy/energy-environment/561377-epa-withdraws-rule-allowing-use-of-radioactive-material-in-road |url-status=live |access-date=July 4, 2021|archive-url=https://web.archive.org/web/20210703022618/https://thehill.com/policy/energy-environment/561377-epa-withdraws-rule-allowing-use-of-radioactive-material-in-road|archive-date=July 3, 2021}}</ref>

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]] to study the use of phosphogypsum in road construction, including demonstration projects, though this would require federal approval.<ref>{{cite news |url=https://www.npr.org/2023/05/09/1174789570/florida-roads-radioactive-paving-phosphogypsum |title=Florida lawmakers want to use radioactive material to pave roads |website=[[NPR]] |date=May 9, 2023 |author=Bill Chappell}}</ref> 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]] on June 29, 2023.<ref>{{Cite web |last=Chappell |first=Bill |date=June 30, 2023 |title=Florida moves forward on radioactive road paving plan as Gov. DeSantis signs new law |url=https://en.wikipedia.org/w/index.php?title=Phosphogypsum&veaction=edit&section=3 |access-date=July 1, 2023 |website=NPR}}</ref>

=== 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]] watershed, with phosphorus accounting for 56% of all breaches of water quality standards. Phosphorus, which still remains in phosphogypsum, can lead to [[eutrophication]] of bodies of water and hence [[algal bloom]]s or even [[anoxic event]]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.<ref name=XinHua>{{cite web |author=经济日报 (Economy Daily) |script-title=zh:长江边的"渣山"是固废还是璞玉——磷石膏堆存污染及综合利用调查|trans-title=Are the "slag mountains" unmovable waste or uncut jade? A survey on the storage, pollution, and use of phosphogypsum |url=http://www.xinhuanet.com/politics/2020-12/21/c_1126885088.htm |website=XinhuaNet}}</ref>

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]].<ref name=XinHua/> It is also used as a chemical feedstock for producing [[sulfates]], and as a [[soil conditioner]] similar to regular gypsum.<ref name=Hebei>{{cite web |author=Hebei DONR, Science & Tech External Affairs Office |script-title=zh:磷石膏的综合利用探讨 |trans-title=On the uses of phosphogypsum |url=http://zrzy.hebei.gov.cn/heb/gongk/gkml/kjxx/kjfz/10640896508485853184.html |website=Ocean Administration, Department of Natural Resources, Hebei Province |access-date=10 March 2022}}</ref> The total consumption in 2020 was 31 Mt, much lower than the rate of accumulation.<ref name=XinHua/> 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 of China|five-year plans]].<ref name=Hebei/>

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]]s and radioactive elements {{why|date=November 2022}} {{citation needed|date=November 2022}}, 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.<ref name=Hebei/>

== Pollution and cleanup ==
{{cleanup section|reason=Ideally we explain open-air gyp stacks *before* we talk about how it causes pollution.|date=June 2022}}
Phosphogypsum may pollute the environment by its phosphorus content causing [[eutrophication]], by its [[toxic heavy metal]] content, and by its radioactivity. PG releases [[radon]], which can accumulate indoors if used as a construction material. Open-air stores also release radon at a level potentially hazardous to workers.<ref name=Taylor/> Radon is a [[noble gas]] 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]] after smoking.<ref>{{Cite journal|pmid = 29883874|year = 2018|last1 = Vogeltanz-Holm|first1 = N.|last2 = Schwartz|first2 = G. G.|title = Radon and lung cancer: What does the public really know?|journal = Journal of Environmental Radioactivity|volume = 192|pages = 26–31|doi = 10.1016/j.jenvrad.2018.05.017|s2cid = 47009598|doi-access = free| bibcode=2018JEnvR.192...26V }}</ref> 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]].<ref name=Taylor/> Accumulation of water inside of gypstacks can lead to weakening of the stack structure, a cause of several alarms in the United States.<ref name=BioDiv/>

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)<ref name=Taylor/> 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.<ref>{{cite journal |last1=Liu |first1=Y |last2=Chen |first2=Q |last3=Wang |first3=Y |last4=Zhang |first4=Q |last5=Li |first5=H |last6=Jiang |first6=C |last7=Qi |first7=C |title=In Situ Remediation of Phosphogypsum with Water-Washing Pre-Treatment Using Cemented Paste Backfill: Rheology Behavior and Damage Evolution. |journal=Materials |date=18 November 2021 |volume=14 |issue=22 |page=6993 |doi=10.3390/ma14226993 |pmid=34832394 |pmc=8618653|bibcode=2021Mate...14.6993L |doi-access=free }}</ref>

[[Bioremediation]] may be used to clean up already contaminated water and soil. Microbials can remove heavy metals, radioactive material{{Citation needed|date=October 2024}}, and any organic pollutants within, and reduce the sulfate material.<ref>{{cite journal |last1=Trifi |first1=Houda |last2=Najjari |first2=Afef |last3=Achouak |first3=Wafa |last4=Barakat |first4=Mohamed |last5=Ghedira |first5=Kais |last6=Mrad |first6=Faten |last7=Saidi |first7=Mouldi |last8=Sghaier |first8=Haïtham |title=Metataxonomics of Tunisian phosphogypsum based on five bioinformatics pipelines: Insights for bioremediation |journal=Genomics |date=January 2020 |volume=112 |issue=1 |pages=981–989 |doi=10.1016/j.ygeno.2019.06.014 |pmid=31220587 |doi-access=free}}</ref> With suitable soil amendments and additives, PG can also support the growth of hardy plants, hopefully preventing further erosion.<ref>{{cite journal |last1=Komnitsas |first1=K. |last2=Paspaliaris |first2=I. |last3=Lazar |first3=I. |last4=Petrisor |first4=I.G. |title=Remediation of phosphogypsum stacks. Field pilot scale application |journal=Process Metallurgy |date=1999 |volume=9 |pages=645–654 |doi=10.1016/S1572-4409(99)80154-0|isbn=9780444501936 }}</ref>

=== Gyp stacks ===
Often phosphogypsum reuse is uneconomical due to impurities{{Explain|date=November 2022}}, mining companies commonly dump the waste into man-made hills called "phosphogypsum stacks"  or waste ponds near the mine. Waste ponds<ref>{{Cite web |title=Risks of Contamination from Toxic Wastewater Ponds {{!}} Hydroviv |url=https://www.hydroviv.com/blogs/water-smarts/drinking-water-contaminated-by-toxic-waste-ponds |access-date=2022-04-22 |website=www.hydroviv.com |language=en}}</ref> 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).<ref>{{Cite web |last=US EPA |first=OAR |date=2018-11-28 |title=Radioactive Material From Fertilizer Production |url=https://www.epa.gov/radtown/radioactive-material-fertilizer-production |access-date=2022-04-22 |website=www.epa.gov |language=en}}</ref> A leaking phosphogypsum waste pond that nearly collapsed, if waste was not allowed to flow into [[Tampa Bay]] in Florida in 2021, highlights the dangers and near-disasters associated with wastewater ponds throughout the country.<ref>{{Cite news |last=Tabuchi |author-link=Hiroko Tabuchi |first=Hiroko |date=2021-04-06 |title=Florida Crisis Highlights a Nationwide Risk From Toxic Ponds |language=en-US |work=The New York Times |url=https://www.nytimes.com/2021/04/06/climate/florida-ponds-toxic-waste.html |access-date=2022-04-22 |issn=0362-4331}}</ref>

Central [[Florida]] 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.<ref name="FIPR">Florida Institute of Phosphate Research. [https://web.archive.org/web/20150219224641/http://www1.fipr.state.fl.us/PhosphatePrimer/0/684AE64864D115FE85256F88007AC781 "Phosphogypsum and the EPA Ban"] Archived February 19, 2015.</ref>

== See also ==
* {{section link|Gypsum|Refinery waste}}
* [[Red mud]] - highly alkaline waste product from aluminum processing
* [[Mine tailings]] - general issue of waste products left after mining
* [[Acid mine drainage]] - highly acidic waters produced from interactions between water oxygen and sulfur compounds deposited under reducing conditions

==References==
<references/>

==Further reading==
* {{cite journal |last1=Chernysh |first1=Yelizaveta |last2=Yakhnenko |first2=Olena |last3=Chubur |first3=Viktoriia |last4=Roubík |first4=Hynek |title=Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects |journal=Applied Sciences |date=9 February 2021 |volume=11 |issue=4 |pages=1575 |doi=10.3390/app11041575 |doi-access=free}}

[[Category:Radioactive waste]]
[[Category:Sulfates]]