Uranium acid mine drainage

Last updated June 21, 2024

Uranium acid mine drainage refers to acidic water released from a uranium mining site using processes like underground mining and in-situ leaching.^[1] Underground, the ores are not as reactive due to isolation from atmospheric oxygen and water. When uranium ores are mined, the ores are crushed into a powdery substance, thus increasing surface area to easily extract uranium. The ores, along with nearby rocks, may also contain sulfides. Once exposed to the atmosphere, the powdered tailings react with atmospheric oxygen and water. After uranium extraction, sulfide minerals in uranium tailings facilitates the release of uranium radionuclides into the environment, which can undergo further radioactive decay while lowering the pH of a solution.^[2]

Uranium chemistry

Uranium may exist naturally as U⁺⁶ in ores but also forms the water-soluble uranyl ion UO₂⁺² when uranium tailings are oxidized by atmospheric oxygen in the following reaction.^[2]

U⁺⁶ + O₂ → UO₂⁺²

The solubility of uranium increases under similar oxidizing conditions when it forms uranyl carbonate complexes in the following reaction.^[2]

U⁺⁶ + O₂ + 2CO₃²⁻→ [UO₂(CO₃)₂]²⁺

Extraction of uranium from the ore may occur under acid or alkaline leaching processes using sulfuric acid and sodium carbonate respectively. If leached with sulfuric acid, uranyl forms a soluble uranyl sulfate complex in the following reaction.^[2] Hydrogen ions in solution react with water to produce hydronium ions which lowers a solution's pH making it more acidic.

UO₂ + 3H₂SO₄ + 1/2 O₂ → [UO₂(SO₄)₃]⁴⁻ + H₂O + 4H⁺

H⁺_(aq) + H₂O_(l) → H₃O⁺_(aq)

During in-situ leaching uranyl reacts with iron, a common natural oxidant, to produce uranyl trioxide which is further oxidized then leached using alkaline sodium carbonate in the following reactions.^[2]

UO₂ + 2Fe³⁺ → UO₂⁺² + 2Fe²⁺

UO₂ + 1/2 O₂ → UO₃

UO₃ + 3Na₂CO₃ + H₂O → [UO₂(CO₃)₃]⁴⁺ + 4Na⁺ + 2NaOH

When considering the formation secondary uranium minerals, as discussed in the case study section below, the pH of the solution that contains uranophane is one of determining factors of how much of the uranophane is in mineral form or in the form of its ions. Shown in figure 2, from a study performed by Tatiana Shvareva et al. in 2011, is the dissolution of uranophane in pH of 3 (Figure 3b) and pH of 4 (Figure 3a). The graphs demonstrate that in a more acidic environment, the concentrations of Ca, U, and Si are more likely to be more abundant in more basic environments where it is more likely that they will form minerals.^[3] This is more likely to happen when the acidic mine drainage is released into rivers or large water deposits and they become diluted to a pH closer to that of water.^[4]

The enthalpies of formation (from elements and from oxide species) and Gibbs free energies of formation (from elements) of the uranium minerals boltwoodite, Na-boltwoodite, and uranophane are shown in Table 1. Solubility constants (dissociation of minerals to ions) of the same minerals, determined using a bomb calorimeter in a study by Shvareva, Tatiana et al. in 2011, are shown in Table 2. The Gibbs free energies of formation show that the process, when the reactions from the individual elements to the oxides are taken into account, is spontaneous. The enthalpies of formation, when only considering the reaction from the oxides to the mineral, suggest a relatively high probability for their Gibbs free energy of formation values to also be spontaneous.^[3]

	Δ_{f, ox}H (kJ/mol)	Δ_{f, el}H (kJ/mol)	Δ_{f, el}G (kJ/mol)
Boltwoodite: K(UO₂)(HSiO₄):HO	-251.2 ± 5.9	-2766.8 ± 6.5	-2758.6 ± 3.5
Na-boltwoodite: Na(UO₂)(HSiO₄):H₂O	-215.8 ± 6.0	-2948.8 ± 6.6	-2725.2 ± 2.6
Uranophane: 1/2[Ca(UO₂)₂(HSiO₄):5H₂O]	-161.1 ± 5.4	-3399.5 ± 5.8	-3099.3 ± 5.6

Table 1. The enthalpy of formation (from oxide to mineral), enthalpy of formation (from individual elements to mineral), and Gibbs free energy (from individual elements to mineral) of boltwoodite, Na-boltwoodite, and uranophane.^[3]

	log Ksp ± 2σ	Mass action equations
Boltwoodite: K(UO₂)(HSiO₄):H₂O	4.12 (-0.48/+0.30)	K_sp = ${\frac {[\alpha _{K^{+}}][\alpha _{{UO_{2}}^{2+}}][\alpha _{{H_{2}}S{O_{4}}}]}{[\alpha _{H^{+}}]}}$
Na-boltwoodite: Na(UO₂)(HSiO₄):H₂O	6.07 (-0.16/+0.26)	K_sp = ${\frac {[\alpha _{Na^{+}}][\alpha _{{U{O_{2}}}^{2+}}][\alpha _{{H_{2}}{S{0_{4}}}}]}{[\alpha _{H^{+}}]}}$
Uranophane: 1/2[Ca(UO₂)₂(HSiO₄):5H₂O]	10.82 (-0.62/+0.29)	K_sp = ${\frac {[\alpha _{Ca^{2+}}][\alpha _{{UO_{2}}^{2+}}][\alpha _{{H_{2}}S{O_{4}}}]}{[\alpha _{H^{+}}]}}$

Table 2. Solubility constants and mass action equations for boltwoodite, Na-boltwoodite, and uranophane.^[3]

Uranium acid mine drainage case study

Two uranium mines in northern Portugal, Quinta do Bispo and Cunha Baixa, have been inactive since 1991. Acidic water is pumped out of the mines for neutralization and precipitation of radionuclides using calcium hydroxide.^[5] Studies in 2002 found that there were high concentrations of soluble and suspended uranium radionuclides in river water samples near the mines.^[5] Castelo river reached suspended uranium isotope concentrations of -72 kBq/kg which is roughly 170x higher than normal concentrations in the Mondego River but returned to normal after 7 km.^[5] The mine waters of Quinta do Bispo and Cunha Baixa had low pH values at 2.67 and 3.48 with U-238 concentrations of 92,000 mBq/L and 2,200 mBq/L, respectively.^[5]

Results from studies done in 2002 showed a significant negative correlation between both dissolved uranium radionuclides and hydrogen ions with pH in mine waters.^[5] Sorption of dissolved uranium radionuclides in rivers combine with nearby rock sediments can form minerals like uranophane.^[5] The chemistry and findings in this case is essentially representative of other uranium mines in the world.

Uranium radionuclides in the environment

A uranium radionuclide is a radioactive isotope. Radioactivity is natural in the environment, however uranium radionuclides can lead to radioactive decay. In the case of uranium mines, these radionuclides can leach into the water and cause the radioactivity to be carried elsewhere, as well as form precipitates that can be harmful to the environment. The uranium radionuclides can eventually be carried to fruits and vegetables via contaminated waters. Sulfuric acid, oxidation, and alkaline leaching are processes of how radionuclides make their way into the environment. When uranium decays it also produces the isotopes ²²⁶Ra and ²²²Rn, which may be environmentally harmful due to the fact that radon is present as an inert gas and therefore, might enter into the soil or atmosphere. Radon then can emit alpha particles and gamma radiation.^[6] The three different radioactive isotopes of uranium are uranium-238, uranium-235, and uranium-234. Each has a different half-life which determines the isotope's decay rate.^[7] When uranium-235 combines with other molecules it creates a chemical reaction that can cause detrimental effects to water. Even though isotope formation occurs naturally, when combined with other elements it can cause the pH of water to become more acidic as discussed previously.^[8]

Related Research Articles

The actinide or actinoid series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.

<span class="mw-page-title-main">Uranium</span> Chemical element with atomic number 92 (U)

Uranium is a chemical element; it has 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 radioactively decays, usually by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

<span class="mw-page-title-main">Alkaline earth metal</span> Group of chemical elements

The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The elements have very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

<span class="mw-page-title-main">Uranium hexafluoride</span> Chemical compound

Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula UF₆. Uranium hexafluoride is a volatile and toxic white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF₃ that resists any further reaction from the compound. UF₆ is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.

<span class="mw-page-title-main">Yellowcake</span> Uranium concentrate powder

Yellowcake is a type of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. It is a step in the processing of uranium after it has been mined but before fuel fabrication or uranium enrichment. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. Typically, yellowcakes are obtained through the milling and chemical processing of uranium ore, forming a coarse powder that has a pungent odor, is insoluble in water, and contains about 80% uranium oxide, which melts at approximately 2880 °C.

<span class="mw-page-title-main">Autunite</span>

Autunite (hydrated calcium uranyl phosphate), with formula Ca(UO₂)₂(PO₄)₂·10–12H₂O, is a yellow-greenish fluorescent phosphate mineral with a hardness of 2–2+1⁄2. Autunite crystallizes in the orthorhombic system and often occurs as tabular square crystals, commonly in small crusts or in fan-like masses. Due to the moderate uranium content of 48.27% it is radioactive and also used as uranium ore. Autunite fluoresces bright green to lime green under UV light. The mineral is also called calco-uranite, but this name is rarely used and effectively outdated.

<span class="mw-page-title-main">Torbernite</span> Copper uranyl phosphate mineral

Torbernite, also known as chalcolite, is a relatively common mineral with the chemical formula Cu[(UO₂)(PO₄)]₂(H₂O)₁₂. It is a radioactive, hydrated green copper uranyl phosphate, found in granites and other uranium-bearing deposits as a secondary mineral. The chemical formula of torbernite is similar to that of autunite in which a Cu²⁺ cation replaces a Ca²⁺ cation. Torbernite tends to dehydrate to metatorbernite with the sum formula Cu[(UO₂)(PO₄)]₂(H₂O)₈.

Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes. Much of radiochemistry deals with the use of radioactivity to study ordinary chemical reactions. This is very different from radiation chemistry where the radiation levels are kept too low to influence the chemistry.

Uranyl nitrate is a water-soluble yellow uranium salt with the formula UO₂(NO₃)₂ · n H₂O. The hexa-, tri-, and dihydrates are known. The compound is mainly of interest because it is an intermediate in the preparation of nuclear fuels. In the nuclear industry, it is commonly referred to as yellow salt.

Uranium(IV) sulfate (U(SO₄)₂) is a water-soluble salt of uranium. It is a very toxic compound. Uranium sulfate minerals commonly are widespread around uranium bearing mine sites, where they usually form during the evaporation of acid sulfate-rich mine tailings which have been leached by oxygen-bearing waters. Uranium sulfate is a transitional compound in the production of uranium hexafluoride. It was also used to fuel aqueous homogeneous reactors.

The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO²⁺
₂. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing.

Uranium trioxide (UO₃), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO₃, is a yellow-orange powder.

Uranyl sulfate describes a family of inorganic compounds with the formula UO₂SO₄(H₂O)_n. These salts consist of sulfate, the uranyl ion, and water. They are lemon-yellow solids. Uranyl sulfates are intermediates in some extraction methods used for uranium ores. These compounds can also take the form of an anhydrous salt.

Uranyl peroxide or uranium peroxide hydrate (UO₄·nH₂O) is a pale-yellow, soluble peroxide of uranium. It is found to be present at one stage of the enriched uranium fuel cycle and in yellowcake prepared via the in situ leaching and resin ion exchange system. This compound, also expressed as UO₃·(H₂O₂)·(H₂O), is very similar to uranium trioxide hydrate UO₃·nH₂O. The dissolution behaviour of both compounds are very sensitive to the hydration state (n can vary between 0 and 4). One main characteristic of uranium peroxide is that it consists of small needles with an average AMAD of about 1.1 μm.

Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications. It usually forms in the +4 and +6 oxidation states, although it can also form in other oxidation states.

<span class="mw-page-title-main">Boltwoodite</span> Hydrated potassium uranyl silicate mineral

Boltwoodite is a hydrated uranyl silicate mineral with formula (K_0.56Na_0.42)[(UO₂)(SiO₃OH)]·1.5(H₂O), distinct in crystal structure from sodium boltwoodite, which has an orthorhombic structure rather than monoclinic. It is formed from the oxidation and alteration of primary uranium ores. It takes the form of a crust on some sandstones that bear uranium. These crusts tend to be yellowish with a silky or vitreous luster.

<span class="mw-page-title-main">Actinide chemistry</span> Branch of nuclear chemistry

Actinide chemistry is one of the main branches of nuclear chemistry that investigates the processes and molecular systems of the actinides. The actinides derive their name from the group 3 element actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. The actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

Bioremediation of radioactive waste or bioremediation of radionuclides is an application of bioremediation based on the use of biological agents bacteria, plants and fungi to catalyze chemical reactions that allow the decontamination of sites affected by radionuclides. These radioactive particles are by-products generated as a result of activities related to nuclear energy and constitute a pollution and a radiotoxicity problem due to its unstable nature of ionizing radiation emissions.

Gauthierite is a very rare mineral with the idealised chemical sum formula KPb[(UO₂)₇O₅(OH)₇]·8H₂O. It is a radioactive, hydrated orange-coloured lead potassium uranyl oxide hydroxide. It was found by analysing old mineral specimens, and is only known from one locality, the Shinkolobwe Mine in the Democratic Republic of the Congo. The mineral was named in honour of Gilbert Gauthier, a Belgian collector of uranium minerals, who provided a sample to one of the co-authors of the study that first identified it in 2017.

References

↑ Virginia, Committee on Uranium Mining in; Resources, Committee on Earth; Council, National Research (2011-12-19). Potential Environmental Effects of Uranium Mining, Processing, and Reclamation. National Academies Press (US).
1 2 3 4 5 Abdelouas, A. (2006-12-01). "Uranium Mill Tailings: Geochemistry, Mineralogy, and Environmental Impact". Elements. 2 (6): 335–341. doi:10.2113/gselements.2.6.335. ISSN 1811-5209.
1 2 3 4 Shvareva, Tatiana Y.; Mazeina, Lena; Gorman-Lewis, Drew; Burns, Peter C.; Szymanowski, Jennifer E.S.; Fein, Jeremy B.; Navrotsky, Alexandra (2011). "Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility". Geochimica et Cosmochimica Acta. 75 (18): 5269–5282. Bibcode:2011GeCoA..75.5269S. doi:10.1016/j.gca.2011.06.041.
↑ Pereira, Wagner de Souza; Kelecom, Alphonse Germaine Albert Charles; Silva, Ademir Xavier da; Carmo, Alessander Sá do; Júnior, Delcy de Azavedo Py (2018). "Assessment of uranium release to the environment from a disabled uranium mine in Brazil". Journal of Environmental Radioactivity. 188: 18–22. doi:10.1016/j.jenvrad.2017.11.012. PMID 29153863. S2CID 4215263.
1 2 3 4 5 6 P., Carvalho, Fernando; M., Oliveira, João; Isabel, Faria (2009). "Alpha Emitting Radionuclides in Drainage from Quinta do Bispo and Cunha Baixa Uranium Mines (Portugal) and Associated Radiotoxicological Risk". Bulletin of Environmental Contamination and Toxicology. 83 (5): 668–673. doi:10.1007/s00128-009-9808-3. ISSN 0007-4861. PMID 19590808. S2CID 38648832.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Mir, Feroz A.; Rather, Sajad A. (2015-04-01). "Measurement of radioactive nuclides present in soil samples of district Ganderbal of Kashmir Province for radiation safety purposes". Journal of Radiation Research and Applied Sciences. 8 (2): 155–159. doi: 10.1016/j.jrras.2014.03.006 .
↑ Chang, Gray. "Calculation of Radioactive Decay Rate of 1.00 gram of Natural Uranium in kBq". ataridogdaze.com. Retrieved 2017-11-28.
↑ Carvalho, Fernando P.; Oliveira, João M.; Faria, Isabel (November 2009). "Alpha emitting radionuclides in drainage from Quinta do Bispo and Cunha Baixa uranium mines (Portugal) and associated radiotoxicological risk". Bulletin of Environmental Contamination and Toxicology. 83 (5): 668–673. doi:10.1007/s00128-009-9808-3. ISSN 1432-0800. PMID 19590808. S2CID 38648832.

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[:1-1] Virginia, Committee on Uranium Mining in; Resources, Committee on Earth; Council, National Research (2011-12-19). Potential Environmental Effects of Uranium Mining, Processing, and Reclamation. National Academies Press (US).

[:0-2] 1 2 3 4 5 Abdelouas, A. (2006-12-01). "Uranium Mill Tailings: Geochemistry, Mineralogy, and Environmental Impact". Elements. 2 (6): 335–341. doi:10.2113/gselements.2.6.335. ISSN 1811-5209.

[:3-3] 1 2 3 4 Shvareva, Tatiana Y.; Mazeina, Lena; Gorman-Lewis, Drew; Burns, Peter C.; Szymanowski, Jennifer E.S.; Fein, Jeremy B.; Navrotsky, Alexandra (2011). "Thermodynamic characterization of boltwoodite and uranophane: Enthalpy of formation and aqueous solubility". Geochimica et Cosmochimica Acta. 75 (18): 5269–5282. Bibcode:2011GeCoA..75.5269S. doi:10.1016/j.gca.2011.06.041.

[4] Pereira, Wagner de Souza; Kelecom, Alphonse Germaine Albert Charles; Silva, Ademir Xavier da; Carmo, Alessander Sá do; Júnior, Delcy de Azavedo Py (2018). "Assessment of uranium release to the environment from a disabled uranium mine in Brazil". Journal of Environmental Radioactivity. 188: 18–22. doi:10.1016/j.jenvrad.2017.11.012. PMID 29153863. S2CID 4215263.

[:2-5] 1 2 3 4 5 6 P., Carvalho, Fernando; M., Oliveira, João; Isabel, Faria (2009). "Alpha Emitting Radionuclides in Drainage from Quinta do Bispo and Cunha Baixa Uranium Mines (Portugal) and Associated Radiotoxicological Risk". Bulletin of Environmental Contamination and Toxicology. 83 (5): 668–673. doi:10.1007/s00128-009-9808-3. ISSN 0007-4861. PMID 19590808. S2CID 38648832.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[6] Mir, Feroz A.; Rather, Sajad A. (2015-04-01). "Measurement of radioactive nuclides present in soil samples of district Ganderbal of Kashmir Province for radiation safety purposes". Journal of Radiation Research and Applied Sciences. 8 (2): 155–159. doi: 10.1016/j.jrras.2014.03.006 .

[7] Chang, Gray. "Calculation of Radioactive Decay Rate of 1.00 gram of Natural Uranium in kBq". ataridogdaze.com. Retrieved 2017-11-28.

[8] Carvalho, Fernando P.; Oliveira, João M.; Faria, Isabel (November 2009). "Alpha emitting radionuclides in drainage from Quinta do Bispo and Cunha Baixa uranium mines (Portugal) and associated radiotoxicological risk". Bulletin of Environmental Contamination and Toxicology. 83 (5): 668–673. doi:10.1007/s00128-009-9808-3. ISSN 1432-0800. PMID 19590808. S2CID 38648832.

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