Radionuclides are types of atoms that are radioactive. The most common radionuclides in drinking water are radium, radon and uranium. Radionuclides occur naturally as trace elements in rocks and soils as a consequence of the “radioactive decay” of uranium-238 (U-238) and thorium-232 (Th-232). As these rocks weather, the resulting clays and other materials may transmit radionuclides into drinking water. Higher levels of radionuclides tend to be found more often in groundwater, such as from wells, than in surface water, such as lakes and streams.
This decay occurs because radioactive atoms have an excess of energy. When these atoms release or transfer their extra energy, the phenomenon is known as decay. The energy they release is called ionizing radiation, which may be in the form of alpha particles, beta particles, or gamma rays. This energy is transmitted through space or some other medium in waves (e.g., x-rays or gamma rays) or particles (e.g., electrons or neutrons) and is capable of either directly or indirectly removing electrons from atoms, thereby creating ions, which are electrically charged atoms.
Radon-222, radium-226, radium-228, uranium-238, and uranium-234 are ions of the U-238 and Th-232 decay series. They are the most common radionuclides found in groundwater. Other naturally occurring radionuclides tend to be environmentally immobile or have short half-lives, meaning they are far less likely to be found in significant amounts in groundwater.
Most of the radionuclides in drinking water occur naturally at very low levels and are not considered a public health concern. However, radionuclides can also be discharged into drinking water from human activity, such as from active nuclear power plants or other facilities that make or use radioactive substances. Natural sources have been the primary cause of this type of contamination. Maximum contaminant levels in drinking water have been established for a variety of radionuclides. For radium, the MCL has been set at 5 pCi/L (picocuries per liter, a unit of measure for levels of radiation). The MCL for gross alpha radiation is 15 pCi/L, and the maximum limit for gross beta radiation is 50 pCi/L. Radionuclide contamination of drinking water is a significant, emerging issue.
When ionizing radiation strikes a living organism’s cells, it may injure these cells. If radiation affects a significant number of them, the organism may eventually develop cancer. At extremely high doses it may even die. Radon is a naturally occurring radioactive gas that emits ionizing radiation. National and international scientific organizations have concluded that radon causes lung cancer in humans. Ingesting drinking water that contains radon also presents a risk of internal organ cancers, primarily stomach cancer.
Tap water only emits approximately 1% to 2% of the radon found in indoor air. However, breathing radon from this source increases the risk of lung cancer over the course of a lifetime. Radium-226 and radium-228 are natural groundwater contaminants that usually occur in trace quantities. At high exposure levels, radium-226 and radium-228 can cause bone cancer in humans and are believed to cause stomach, lung, and other cancers as well.
Radium-226 and radium-228 are natural groundwater contaminants that usually occur in trace quantities. At high exposure levels, radium-226 and radium-228 can cause bone cancer in humans and are believed to cause stomach, lung, and other cancers as well. Uranium is a naturally occurring radioactive contaminant found in both groundwater and surface water. At high exposure levels, uranium is believed to cause bone cancer and other cancers in humans. EPA also believes that uranium can be toxic to the kidneys.
The U.S. Environmental Protection Agency (EPA) and the U.S. Surgeon General recommend testing indoor air for radon in all homes and apartments located below the third floor.
Water treatment process to remove radionuclide
Whether or not a particular treatment technology effectively removes radionuclides from drinking water depends on the contaminant’s chemical and physical characteristics as well as the water system’s characteristics (e.g., the source water quality and the water system size). Other considerations include cost, service life and co-treatment compatibility.
Ion Exchange: Small systems may readily use IE treatment, which removes approximately 90% of radionuclides. The effluent must be regularly monitored and the IE resin must be frequently regenerated to ensure that breakthrough does not occur. Ion exchange units may be controlled automatically, requiring less of the operator’s time.
Lime Softening: This very common process used treat hard water also can be applied to remove radium from drinking water with 80% to 95% efficiency. Also, adding lime or lime-soda ash to water increases the pH of the water and induces calcium carbonate and magnesium hydroxide precipitation.
Reverse Osmosis: This system can remove many inorganic contaminants very effectively, including heavy metals and radionuclides such as radium and uranium. RO can remove 87% to 98% of radium from drinking water. Similar elimination can be achieved for alpha particle activity and total beta and photon emitter activity. Reverse osmosis can be a cost-effective solution for small systems.
Scientists report Radionuclide removal using graphene oxide in March 2021
RUSSIAN SCIENTISTS FROM MOSCOW STATE University and the Kurchatov Institute, together with colleagues from Sweden and Germany, have increased the sorption properties of graphene by 15 times, improving its ability to remove radionuclides from water. The development was carried out by an international group of specialists, who succeeded in synthesising and characterising graphene oxide with specified defects in the molecular structure. These features improve the sorption properties of the material by a factor of 15.
“We studied the mechanism of uranium sorption on graphene oxide to determine which method of its synthesis makes it possible to obtain the most effective material,” Aleksandr Trigub, a researcher at the Kurchatov Institute, told Russian newspaper Izvestia. The compound was prepared from reduced graphene oxide, using a reaction known as the Hummers method. The authors modified this classical method for producing graphene oxide using an explosive thermal delamination process. As a result, defects appear in the graphene’s carbon structure, which ‘trap’ heavy metal cations (ie those with a positive electric charge).
In conventional graphene oxide, the carbon atoms are structured in flat (two dimensional) ‘sheet’ of connected hexagons, with the oxygen on the surfaces. In a ‘defective’ oxide like the one described above, oxygen acts on graphene in such a way that it creates many irregularities. The sorption of uranium cations occurs due to the fact that carboxyl groups are placed on the oxygen atoms located at the edge of the voids in the structure of defective graphene oxide. Active oxidation of graphene oxide increases the number of carboxyl groups, which increases its sorption properties. The new material was developed specifically to increase the number of defects. Scientists have studied its structure in detail using powerful microscopes, x-rays and spectroscopic methods.
Two-dimensional carbon materials, which include graphene oxide, are increasingly used in industry. Now this topic is one of the most fast-developing in science, and researchers are looking for new applications of such materials, says Pavel Postnikov, associate professor at the Research School of Chemical and Biomedical Technologies of Tomsk Polytechnic University. “Unlike graphene, which is expensive to produce, graphene oxide can be synthesised quite simply,” said Pavel Sorokin, a leading researcher from the Inorganic Nanomaterials Research Laboratory at Russia’s National University of Science and Technology (NUST MISIS).
The proposed method for obtaining the substance is easily scalable and can be applied for mass production of graphene oxide for water purification, said associate professor Anton Konakov, senior researcher at the Department of Theoretical Physics at the Faculty of Physics of Lobachevsky University. The new work is a logical continuation of the authors’ works on the study of the sorption capacity of graphene oxide, in which a high sorption of ions containing radionuclides on edge carboxyl groups was found, said Vyacheslav Almyashev, associate professor of the Department of Physical Chemistry at the St Petersburg Electrotechnical University. “The researchers have developed a technology for creating highly defective graphene oxide to increase the number of sorbing centres, and their previous assumptions were confirmed,” he said.
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