Perchlorate

Perchlorate
Names
Systematic IUPAC name
Perchlorate[1]
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.152.366
2136
MeSH 180053
Properties
ClO−
4
Molar mass 99.451 g mol−1
Conjugate acid Perchloric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is ☑Y☒N ?)
Infobox references

A perchlorate is the name for a chemical compound containing the perchlorate ion, ClO−
4. The majority of perchlorates are produced commercially. Perchlorate salts are mainly used for propellants, exploiting properties as powerful oxidizing agents and to control static electricity in food packaging.[2] Perchlorate contamination in food, water and other parts of the environment has been studied in the U.S. because of its harmful effects on human health. Perchlorate reduces thyroid hormone production in the thyroid gland.

Most perchlorates are colorless solids that are soluble in water. Four perchlorates are of primary commercial interest: ammonium perchlorate (NH4ClO4), perchloric acid (HClO4), potassium perchlorate (KClO4), and sodium perchlorate (NaClO4). Perchlorate is the anion resulting from the dissociation of perchloric acid and its salts upon their dissolution in water. Many perchlorate salts are soluble in non-aqueous solutions.[3]

Production

Perchlorate salts are produced industrially by the oxidation of solutions of sodium chlorate by electrolysis. This method is used to prepare sodium perchlorate. The main application is for rocket fuel.[4] The reaction of perchloric acid with bases, such as ammonium hydroxide, give salts. The highly valued ammonium perchlorate can be produced electrochemically.[5]

Curiously, perchlorate can be produced by lightning discharges in the presence of chloride. Perchlorate has been detected in rain and snow samples from Florida and Lubbock, Texas.[6]

Uses

  • The dominant use of perchlorates is as oxidizers in propellants for rockets and fireworks. Of particular value is ammonium perchlorate composite propellant as a component of solid rocket fuel. In a related but smaller application, perchlorates are used extensively within the pyrotechnics industry and in certain munitions and for the manufacture of matches.[4]
  • Perchlorate is used to control static electricity in food packaging. Sprayed onto containers it stops statically charged food from clinging to plastic or paper/cardboard surface.[7]
  • Niche uses include lithium perchlorate, which decomposes exothermically to produce oxygen, useful in oxygen "candles" on spacecraft, submarines, and in other situations where a reliable backup oxygen supply is needed. For example, oxygen "candles" are used in commercial aircraft during emergency situations to compensate for oxygen insufficiency.
  • Potassium perchlorate has, in the past, been used therapeutically to treat hyperthyroidism resulting from Graves' disease. It impedes the accumulation of iodide in the thyroid, which blocks thyroid hormone production.[8]

Chemical properties

The perchlorate ion is the least reactive oxidizer of the generalized chlorates. Perchlorate contains chlorine in its highest oxidation number. A table of reduction potentials of the four chlorates shows that, contrary to expectation, perchlorate is the weakest oxidant among the four in water.[9]

IonAcidic reactionE° (V)Neutral/basic reactionE° (V)
HypochloriteH+ + HOCl + e− → ​1⁄2 Cl2(g) + H2O1.63ClO− + H2O + 2 e− → Cl− + 2OH−0.89
Chlorite3 H+ + HOClO + 3 e− → ​1⁄2 Cl2(g) + 2 H2O1.64ClO−
2 + 2 H2O + 4 e− → Cl− + 4 OH−		0.78
Chlorate6 H+ + ClO−
3 + 5 e− → ​1⁄2 Cl2(g) + 3 H2O		1.47ClO−
3 + 3 H2O + 6 e− → Cl− + 6 OH−		0.63
Perchlorate8 H+ + ClO−
4 + 7 e− → ​1⁄2 Cl2(g) + 4 H2O		1.42ClO−
4 + 4 H2O + 8 e− → Cl− + 8 OH−		0.56

These data show that the perchlorate and chlorate are stronger oxidizers in acidic conditions than in basic conditions.

Gas phase measurements of heats of reaction (which allow computation of ΔHf°) of various chlorine oxides do follow the expected trend wherein Cl2O7 exhibits the largest endothermic value of ΔHf° (238.1 kJ/mol) while Cl2O exhibits the lowest endothermic value of ΔHf° (80.3 kJ/mol).[10]

The chlorine in the perchlorate anion is a closed shell atom and is well protected by the four oxygens. Most perchlorate compounds, especially salts of electropositive metals such as sodium perchlorate or potassium perchlorate, do not oxidize organic compounds until the mixture is heated. This property is useful in many applications, such as flares, where ignition is required to initiate a reaction. Ammonium perchlorate is stable when pure but can form potentially explosive mixtures with reactive metals or organic compounds. The PEPCON disaster destroyed a production plant for ammonium perchlorate when a fire caused the ammonium perchlorate stored on site to react with the aluminum that the storage tanks were constructed with and explode.

Potassium perchlorate has the lowest solubility of any alkali metal perchlorate (1.5 g in 100 ml of water at 25 °C).

Biology

Over 40 phylogenetically and metabolically diverse microorganisms capable of growth via perchlorate reduction have been isolated since 1996. Most originate from the Proteobacteria but others include the Firmicutes, Moorella perchloratireducens and Sporomusa sp., and the archaeon Archaeoglobus fulgidus.[11][12] With the exception of A. fulgidus, all known microbes that grow via perchlorate reduction utilize the enzymes perchlorate reductase and chlorite dismutase, which collectively take perchlorate to innocuous chloride.[11] In the process, free oxygen (O2) is generated.[11]

Oxyanions of chlorine

Chlorine can assume oxidation states of −1, +1, +3, +5, or +7, an additional oxidation state of +4 is seen in the neutral compound chlorine dioxide ClO2, which has a similar structure. Several other chlorine oxides are also known.

Chlorine oxidation state −1 +1 +3 +5 +7
Name chloride hypochlorite chlorite chlorate perchlorate
Formula Cl− ClO− ClO−
2		 ClO−
3		 ClO−
4
Structure

Natural abundance

Naturally occurring perchlorate at its most abundant can be found comingled with deposits of sodium nitrate in the Atacama Desert of northern Chile. These deposits have been heavily mined as sources for nitrate-based fertilizers. Chilean nitrate is in fact estimated to be the source of around 81,000 tonnes (89,000 tons) of perchlorate imported to the U.S. (1909–1997). Results from surveys of ground water, ice, and relatively unperturbed deserts have been used to estimate a 100,000 to 3,000,000 tonnes (110,000 to 3,310,000 tons) "global inventory" of natural perchlorate presently on Earth.[13]

On Mars

In May 2008, the Wet Chemistry Laboratory (WCL) on board the 2007 Phoenix Mars lander performed the first wet chemical analysis of Martian soil. The analyses on three samples, two from the surface and one from a depth of 5 cm (2.0 in), revealed a slightly alkaline soil and low levels of salts typically found on Earth. Unexpected though was the presence of ~0.6% by weight perchlorate (ClO−
4), most likely as a mixture of 60% Ca(ClO4)2 and 40% Mg(ClO4)2.[14][15][16] These salts, formed from perchlorates, act as antifreeze and substantially lower the freezing point of water. Based on the temperature and pressure conditions on present-day Mars at the Phoenix lander site, conditions would allow a perchlorate salt solution to be stable in liquid form for a few hours each day during the summer.[17]

The possibility that the perchlorate was a contaminant brought from Earth has been eliminated by several lines of evidence. The Phoenix retro-rockets used ultra pure hydrazine and launch propellants consisting of ammonium perchlorate. Sensors on board Phoenix found no traces of ammonium, and thus the perchlorate in the quantities present in all three soil samples is indigenous to the Martian soil.

In 2006, a mechanism was proposed for the formation of perchlorates that is particularly relevant to the discovery of perchlorate at the Phoenix lander site. It was shown that soils with high concentrations of chloride converted to perchlorate in the presence of titanium dioxide and sunlight/ultraviolet light. The conversion was reproduced in the lab using chloride-rich soils from Death Valley.[18] Other experiments have demonstrated that the formation of perchlorate is associated with wide band gap semiconducting oxides.[19] In 2014, it was shown that perchlorate and chlorate can be produced from chloride minerals under Martian conditions via UV using only NaCl and silicate.[20]

Further findings of perchlorate and chlorate in the Martian meteorite EETA79001 [21] and by the Mars Curiosity rover in 2012-2013 support the notion that perchlorates are globally distributed throughout the Martian surface.[22][23][24] With concentrations approaching 0.5% and exceeding toxic levels on Martian soil, Martian perchlorates would present a serious challenge to human settlement,[25] as well as microorganisms.[26]

On September 28, 2015, NASA announced that analyses of spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars instrument (CRISM) on board the Mars Reconnaissance Orbiter from four different locations where recurring slope lineae (RSL) are present found evidence for hydrated salts. The hydrated salts most consistent with the spectral absorption features are magnesium perchlorate, magnesium chlorate and sodium perchlorate. The findings strongly support the hypothesis that RSL form as a result of contemporary water activity on Mars.[27][28][29][30][31]

Contamination in environment

Perchlorate is of concern because of uncertainties about toxicity and health effects at low levels in drinking water, impact on ecosystems, and indirect exposure pathways for humans due to accumulation in vegetables.[8] Perchlorate is water-soluble, exceedingly mobile in aqueous systems, and can persist for many decades under typical groundwater and surface water conditions.[32] Detected perchlorate originates from disinfectants, bleaching agents, herbicides, and mostly from rocket propellants. Perchlorate is a byproduct of the production of a rocket fuel and fireworks.[3] The removal and recovery of the perchlorate compounds in explosives and rocket propellants include high-pressure water washout, which generate aqueous ammonium perchlorate.

In U.S. drinking water

Low levels of perchlorate have been detected in both drinking water and groundwater in 26 states in the U.S., according to the Environmental Protection Agency (EPA).[33] In 2004, the chemical was also found in cow's milk in California with an average level of 1.3 parts per billion (ppb, or µg/L), which may have entered the cows through feeding on crops that had exposure to water containing perchlorates.[34] The chemical has been detected at levels as high as 5 µg/L at Joint Base Cape Cod (formerly Massachusetts Military Reservation), well over the Massachusetts state regulation of 2 µg/L.[35][36] Fireworks are also a source of perchlorate in lakes.[37]

At the Olin Flare Facility, Morgan Hill, California perchlorate contamination beneath the former flare manufacturing plant was first discovered in 2000, several years after the plant had closed. The plant had used potassium perchlorate as one of the ingredients during its 40 years of operation. By late 2003, the State of California and the Santa Clara Valley Water District had confirmed a groundwater plume currently extending over nine miles through residential and agricultural communities. The California Regional Water Quality Control Board and the Santa Clara Valley Water District have engaged in a major outreach effort, a water well testing program has been underway for about 1,200 residential, municipal, and agricultural wells. Large ion exchange treatment units are operating in three public water supply systems which include seven municipal wells with perchlorate detection. The potentially responsible parties, Olin Corporation and Standard Fuse Incorporated, have been supplying bottled water to nearly 800 households with private wells, and the Regional Water Quality Control Board has been overseeing cleanup efforts.[38]

The source of perchlorate in California was mainly attributed to two manufacturers in the southeast portion of the Las Vegas Valley in Nevada, where perchlorate has been produced for industrial use.[39] This led to perchlorate release into Lake Mead in Nevada and the Colorado River which affected regions of Nevada, California and Arizona, where water from this reservoir is used for consumption, irrigation and recreation for approximate half the population of these states.[3] Lake Mead has been attributed as the source of 90% of the perchlorate in Southern Nevada's drinking water. Based on sampling, perchlorate has been affecting 20 million people, with highest detection in Texas, southern California, New Jersey, and Massachusetts, but intensive sampling of the Great Plains and other middle state regions may lead to revised estimates with additional affected regions.[3] An action level of 18 μg/L has been adopted by several affected states.[32]

In food

In 2004, the chemical was found in cow's milk in California at an average level of 1.3 parts per billion (ppb, or µg/L), which may have entered the cows through feeding on crops exposed to water containing perchlorates.[34] A 2005 study suggested human breast milk had an average of 10.5 µg/L of perchlorate.[40]

In minerals and other natural occurrences

In some places, there is no clear source of perchlorate, and it may be naturally occurring. Natural perchlorate on earth was first identified in terrestrial nitrate deposits of the Atacama Desert in Chile as early as the 1880s[41] and for a long time considered a unique perchlorate source. The perchlorate released from historic use of Chilean nitrate based fertilizer which the U.S.imported by the hundreds of tons in the early 19th century can still be found in some groundwater sources of the United States.[42] Recent improvements in analytical sensitivity using ion chromatography based techniques have revealed a more widespread presence of natural perchlorate, particularly in subsoils of Southwest USA,[43] salt evaporites in California and Nevada,[44] Pleistocene groundwater in New Mexico,[45] and even present in extremely remote places such as Antarctica.[46] The data from these studies and others indicate that natural perchlorate is globally deposited on Earth with the subsequent accumulation and transport governed by the local hydrologic conditions.

Despite its importance to environmental contamination, the specific source and processes involved in natural perchlorate production remain poorly understood. Laboratory experiments in conjunction with isotopic studies[47] have implied that perchlorate may be produced on earth by oxidation of chlorine species through pathways involving ozone or its photochemical products.[48] Other studies have suggested that perchlorate can also be created by lightning activated oxidation of chloride aerosols (e.g., chloride in sea salt sprays),[49] and ultraviolet or thermal oxidation of chlorine (e.g., bleach solutions used in swimming pools) in water.[50][51][52]

From fertilizers

Although perchlorate as an environmental contaminant is usually associated with the storage, manufacture, and testing of solid rocket motors,[53] contamination of perchlorate has been focused in the use of fertilizer and its perchlorate release into ground water. Fertilizer leaves perchlorate anions to leak into the ground water and threaten the water supplies of many regions in the US.[53] One of the main sources of perchlorate contamination from fertilizer use was found to come from the fertilizer derived from Chilean calich (calcium carbonate), because Chile has rich source of naturally occurring perchlorate anion.[54] Perchlorate in the solid fertilizer ranged from 0.7 to 2.0 mg g−1, variation of less than a factor of 3 and it is estimated that sodium nitrate fertilizers derived from Chilean caliche contain approximately 0.5–2 mg g−1 of perchlorate anion.[54] The direct ecological effect of perchlorate is not well known; its impact can be influenced by factors including rainfall and irrigation, dilution, natural attenuation, soil adsorption, and bioavailability.[54] Quantification of perchlorate concentrations in fertilizer components via ion chromatography revealed that in horticultural fertilizer components contained perchlorate ranging between 0.1 and 0.46%.[32] Perchlorate concentration was the highest in Chilean nitrate, ranging from 3.3 to 3.98%.[32]

Cleanup

There have been many attempts to eliminate perchlorate contamination. Current remediation technologies for perchlorate have downsides of high costs and difficulty in operation.[55] Thus, there have been interests in developing systems that would offer economic and green alternatives.[55]

Treatment ex situ and in situ

Several technologies can remove perchlorate, via treatments ex situ and in situ.

Ex situ treatments include ion exchange using perchlorate-selective or nitrite-specific resins, bioremediation using packed-bed or fluidized-bed bioreactors, and membrane technologies via electrodialysis and reverse osmosis.[56] In ex situ treatment via ion exchange, contaminants are attracted and adhere to the ion exchange resin because such resins and ions of contaminants have opposite charge.[57] As the ion of the contaminant adheres to the resin, another charged ion is expelled into the water being treated, in which then ion is exchanged for the contaminant.[57] Ion exchange technology has advantages of being well-suitable for perchlorate treatment and high volume throughput but has a downside that it does not treat chlorinated solvents. In addition, ex situ technology of liquid phase carbon adsorption is employed, where granular activated carbon (GAC) is used to eliminate low levels of perchlorate and pretreatment may be required in arranging GAC for perchlorate elimination.[56]

In situ treatments, such as bioremediation via perchlorate-selective microbes and permeable reactive barrier, are also being used to treat perchlorate.[56] In situ bioremediation has advantages of minimal above-ground infrastructure and its ability to treat chlorinated solvents, perchlorate, nitrate, and RDX simultaneously. However, it has a downside that it may negatively affect secondary water quality. In situ technology of phytoremediation could also be utilized, even though perchlorate phytoremediation mechanism is not fully founded yet.[56]

Health effects

Thyroid inhibition

Perchlorate is a potent competitive inhibitor of the thyroid sodium-iodide symporter.[58] Thus, it has been used to treat hyperthyroidism since the 1950s.[59] At very high doses (70,000–300,000 ppb) the administration of potassium perchlorate was considered the standard of care in the United States, and remains the approved pharmacologic intervention for many countries.

In large amounts perchlorate interferes with iodine uptake into the thyroid gland. In adults, the thyroid gland helps regulate the metabolism by releasing hormones, while in children, the thyroid helps in proper development. The NAS, in its 2005 report, Health Implications of Perchlorate Ingestion, emphasized that this effect, also known as Iodide Uptake Inhibition (IUI) is not an adverse health effect. However, in January 2008, California's Department of Toxic Substances Control stated that perchlorate is becoming a serious threat to human health and water resources.[60] In 2010, the EPA's Office of the Inspector General determined that the agency's own perchlorate reference dose of 24.5 parts per billion protects against all human biological effects from exposure. This finding was due to a significant shift in policy at the EPA in basing its risk assessment on non-adverse effects such as IUI instead of adverse effects. The Office of the Inspector General also found that because the EPA's perchlorate reference dose is conservative and protective of human health further reducing perchlorate exposure below the reference dose does not effectively lower risk.[61]

Perchlorate affects only thyroid hormone. Because it is neither stored nor metabolized, effects of perchlorate on the thyroid gland are reversible, though effects on brain development from lack of thyroid hormone in fetuses, newborns, and children are not.[62]

Toxic effects of perchlorate have been studied in a survey of industrial plant workers who had been exposed to perchlorate, compared to a control group of other industrial plant workers who had no known exposure to perchlorate. After undergoing multiple tests, workers exposed to perchlorate were found to have a significant systolic blood pressure rise compared to the workers who were not exposed to perchlorate, as well as a significant decreased thyroid function compared to the control workers.[63]

A study involving healthy adult volunteers determined that at levels above 0.007 milligrams per kilogram per day (mg/(kg·d)), perchlorate can temporarily inhibit the thyroid gland's ability to absorb iodine from the bloodstream ("iodide uptake inhibition", thus perchlorate is a known goitrogen).[64] The EPA converted this dose into a reference dose of 0.0007 mg/(kg·d) by dividing this level by the standard intraspecies uncertainty factor of 10. The agency then calculated a "drinking water equivalent level" of 24.5 ppb by assuming a person weighs 70 kg (150 lb) and consumes 2 L (0.44 imp gal; 0.53 US gal) of drinking water per day over a lifetime.[65]

In 2006, a study reported a statistical association between environmental levels of perchlorate and changes in thyroid hormones of women with low iodine. The study authors were careful to point out that hormone levels in all the study subjects remained within normal ranges. The authors also indicated that they did not originally normalize their findings for creatinine, which would have essentially accounted for fluctuations in the concentrations of one-time urine samples like those used in this study.[66] When the Blount research was re-analyzed with the creatinine adjustment made, the study population limited to women of reproductive age, and results not shown in the original analysis, any remaining association between the results and perchlorate intake disappeared.[67] Soon after the revised Blount Study was released, Robert Utiger, a doctor with the Harvard Institute of Medicine, testified before the US Congress and stated: "I continue to believe that that reference dose, 0.007 milligrams per kilo (24.5 ppb), which includes a factor of 10 to protect those who might be more vulnerable, is quite adequate."[68]

At a 2013 presentation of a previously unpublished study, it was suggested that environmental exposure to perchlorate in pregnant women with hypothyroidism may be associated with significant risk of low IQ in their children.[69]

Lung toxicity

Some studies suggest that perchlorate has pulmonary toxic effects as well. Studies have been performed on rabbits where perchlorate has been injected into the trachea. The lung tissue was removed and analyzed, and it was found that perchlorate injected lung tissue showed several adverse effects when compared to the control group that had been intratracheally injected with saline. Adverse effects included inflammatory infiltrates, alveolar collapse, subpleural thickening, and lymphocyte proliferation.[70]

Aplastic anemia

In the early 1960s, potassium perchlorate use to treat Graves disease was implicated in the development of aplastic anemia—a condition where the bone marrow fails to produce new blood cells in sufficient quantity—in thirteen patients, seven of whom died.[71] Subsequent investigations have indicated the connection between administration of potassium perchlorate and development of aplastic anemia to be "equivocable at best", which means that the benefit of treatment, if it is the only known treatment, outweighs the risk, and it appeared a contaminant poisoned the 13.[72]

Regulation in the U.S.

Water

In 1998, perchlorate was included in the EPA Contaminant Candidate List, primarily due to its detection in California drinking water.[73][3]

In 2003, a federal district court in California found that the Comprehensive Environmental Response, Compensation and Liability Act applied, because perchlorate is ignitable, and therefore was a "characteristic" hazardous waste.[74]

In 2003, California's legislature enacted AB 826, the Perchlorate Contamination Prevention Act of 2003, requiring California's Department of Toxic Substances Control (DTSC) to adopt regulations specifying best management practices for perchlorate and perchlorate-containing substances. On December 31, 2005, the "Perchlorate Best Management Practices" were adopted and became operative on July 1, 2006.[75]

In early 2006, EPA issued a "Cleanup Guidance" and recommended a Drinking Water Equivalent Level (DWEL) for perchlorate of 24.5 µg/L. Both DWEL and Cleanup Guidance were based on a 2005 review of the existing research by the National Academy of Science (NAS).[76]

Lacking a federal standard, several states in the U.S. subsequently enacted their own drinking water standard for perchlorate including Massachusetts in 2006 and California in 2007. Other states, including Arizona, Maryland, Nevada, New Mexico, New York, and Texas have established non-enforceable, advisory levels for perchlorate.

It was not until 2008, that EPA issued an interim drinking water health advisory for perchlorate and with it a guidance and analysis concerning the impacts on the environment and drinking water.[77] California also issued guidance regarding perchlorate use.[78] Both the Department of Defense and some environmental groups voiced questions about the NAS report, but no credible science has emerged to challenge the NAS findings.

In February 2008, U.S. Food and Drug Administration reported that U.S. toddlers on average are being exposed to more than half of EPA's safe dose from food alone.[79] In March 2009, a Centers for Disease Control study found 15 brands of infant formula contaminated with perchlorate. Combined with existing perchlorate drinking water contamination, infants could be at risk for perchlorate exposure above the levels considered safe by EPA.[80]

On February 11, 2011, EPA determined that perchlorate meets the Safe Drinking Water Act criteria for regulation as a contaminant.[77][81] The agency found that perchlorate may have an adverse effect on the health of persons and is known to occur in public water systems with a frequency and at levels that it presents a public health concern. Since then EPA has continued to determine what level of contamination is appropriate. The EPA prepared extensive responses to submitted public comments.[82]

In 2016, the Natural Resources Defense Council filed a lawsuit to accelerate EPA's regulation of perchlorate. A federal district court in New York issued a consent decree requiring EPA to issue a proposed rule in October 2018, and a final rule in December 2019.[83]

Other

FDA approved perchlorate use in food packaging in 2005.

References

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  2. ↑ Draft Toxicological Profile for Perchlorates, Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services, September, 2005.
  3. 1 2 3 4 5 Kucharzyk, Katarzyna (2009). "Development of drinking water standards for perchlorate in the United States". Journal of Environmental Management. Elsevier B.V. 91: 303–310. doi:10.1016/j.jenvman.2009.09.023.
  4. 1 2 Helmut Vogt, Jan Balej, John E. Bennett, Peter Wintzer, Saeed Akbar Sheikh, Patrizio Gallone "Chlorine Oxides and Chlorine Oxygen Acids" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH. doi:10.1002/14356007.a06_483
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  6. ↑ Kathleen Sellers, Katherine Weeks, William R. Alsop, Stephen R. Clough, Marilyn Hoyt, Barbara Pugh, Joseph Robb. Perchlorate: Environmental Problems and Solutions, 2007, p 9. Taylor & Francis Group, LLC.
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  16. ↑ Kounaves S. P.; et al. (2014). "Identification of the Perchlorate Parent Salts at the Phoenix Mars Landing Site and Possible Implications". Icarus. 232: 226–231. Bibcode:2014Icar..232..226K. doi:10.1016/j.icarus.2014.01.016.
  17. ↑ Chevrier, V. C., Hanley, J., and Altheide, T.S. (2009). "Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars". Geophysical Research Letters. 36 (10): L10202. Bibcode:2009GeoRL..3610202C. doi:10.1029/2009GL037497.
  18. ↑ Miller, Glen. "Photooxidation of chloride to perchlorate in the presence of desert soils and titanium dioxide". American Chemical Society. March 29, 2006
  19. ↑ Schuttlefield Jennifer D.; Sambur Justin B.; Gelwicks Melissa; Eggleston Carrick M.; Parkinson B. A. (2011). "Photooxidation of Chloride by Oxide Minerals: Implications for Perchlorate on Mars". J. Am. Chem. Soc. 133: 17521–17523. doi:10.1021/ja2064878. PMID 21961793.
  20. ↑ Carrier B. L.; Kounaves S. P. (2015). "The Origin of Perchlorates in the Martian Soil". Geophys. Res. Lett. 42: 3746–3754. Bibcode:2015GeoRL..42.3739C. doi:10.1002/2015GL064290.
  21. ↑ Kounaves S. P.; Carrier B. L.; O'Neil G. D.; Stroble S. T. & Clair M. W. (2014). "Evidence of Martian Perchlorate, Chlorate, and Nitrate in Mars Meteorite EETA79001: Implications for Oxidants and Organics". Icarus. 229: 206–213. Bibcode:2014Icar..229..206K. doi:10.1016/j.icarus.2013.11.012.
  22. ↑ Adam Mann. "Look What We Found on Mars - Curiosity Rover Serves Up Awesome Science". Slate (magazine). 26 September 2013.
  23. ↑ Chang, Kenneth (1 October 2013). "Hitting Pay Dirt on Mars". New York Times. Retrieved 2 October 2013.
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  26. ↑ Mars covered in toxic chemicals that can wipe out living organisms, tests reveal. Ian Sample, The Guardian. 6 July 2017.
  27. ↑ Webster, Guy; Agle, DC; Brown, Dwayne; Cantillo, Laurie (28 September 2015). "NASA Confirms Evidence That Liquid Water Flows on Today's Mars". Retrieved 28 September 2015.
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