At 13:00 local time on 24 August 2023, Japan’s Fukushima Daiichi nuclear power plant initiated the discharge of nuclear contaminated water into the sea. It aroused widespread concern in the world. China’s Ambassador to Japan Wu Jianghao lodged a solemn protest to the Japanese government over the discharge of Fukushima nuclear polluted water into the sea; China’s General Administration of Customs: A complete suspension of imports of Japanese aquatic products; South Korean Prime Minister: South Korea will maintain the import ban on Japanese Fukushima aquatic products unchanged.
Nuclear sewage, as the name implies, is sewage containing radioactivity. Such as highly radioactive waste water produced in nuclear leakage accidents, or cooling water in direct contact with nuclear fuel. Experts say that nuclear wastewater is not equal to nuclear wastewater. Nuclear sewage is even more harmful, containing 64 kinds of nuclear radioactive substances including tritium.
Nuclear sewage contains many radioactive elements, such as uranium, plutonium, caesium, strontium, iodine and cobalt. Some of them have long half-lives, such as the half-life of uranium-238, which is 4.5 billion years, and the half-life of plutonium-239, which is 24,000 years. These radioactive elements pose serious hazards to the human body and the environment, such as carcinogenicity, teratogenicity and mutagenicity. Depending on different sources and circumstances, the concentrations and proportions of various radioactive elements in nuclear effluent vary, but usually far exceed international standards and safety limits.
If nuclear sewage is discharged into the environment without proper treatment, it will have a serious impact on the ecosystem and human health. Radioactive substances can be transmitted through water, soil, air, food chains, etc., resulting in increased radiation doses in organisms, causing various diseases and gene mutations. Therefore, the treatment and disposal of nuclear sewage must follow strict safety standards and norms to avoid any possible leakage and accidents.
The nuclear wastewater treatment technologies commonly used in various countries are as follows:
- Chemical precipitation method
The chemical precipitation method is a method of coprecipitating the precipitating agent and the trace amount of radionuclides in the wastewater. Hydroxides, carbonates, phosphates and other compounds of radionuclides in wastewater are mostly insoluble and thus can be removed during treatment. The purpose of chemical treatment is to transfer and concentrate the radionuclides in the wastewater to a small volume of sludge, so that the deposited wastewater has little radioactivity remaining and can meet the discharge standards.
The advantages of this method are low cost, good removal effect of logarithmic radionuclides, and the ability to treat non-radioactive components and their concentrations and fluidized wastewater with considerable fluidization. The treatment facilities and technologies used have quite mature experience.
At present, precipitants such as iron salts, aluminum salts, phosphates, and soda are the most commonly used. In order to promote the coagulation process, coagulation aids such as clay, activated silica, and polymer electrolytes are added. For cesium, ruthenium, iodine and other concentrated radioactive nuclides that are difficult to remove, special chemical precipitants should be used. For example, cesium can be removed by coprecipitation of ferric ferrocyanide and copper ferrocyanide. Some people use insoluble starch xanthate to treat metal-containing radioactive wastewater. The treatment effect is good, the applicability is wide, and the radioactive removal rate is >90%. It is an ion exchange flocculant with excellent performance. Because there is no residual sulfide when treating wastewater, it is more suitable for wastewater treatment.
- Ion exchange method
Many radionuclides are ionized in water, especially radioactive wastewater after chemical precipitation treatment. Due to the removal of suspended and colloidal radionuclides, what remains is almost ionic species, most of which are cations. Moreover, radionuclides exist in small amounts in water, so they are very suitable for ion exchange treatment, and in the absence of interference from non-radioactive ions, ion exchange can work effectively for a long time. Most cation exchange resins have high removal capacity and large exchange capacity for radioactive strontium. The phenolic cation resin can effectively remove radioactive cesium, and the microporous cation resin can not only remove radioactive cations, but also remove zirconium, niobium, cobalt in the form of colloids, and ruthenium in the form of complexes through adsorption. However, this law has a fatal weakness. When the content of radioactive nuclides or non-radioactive ions in the waste liquid is high, the resin bed will quickly penetrate and become invalid. Usually, the resin used to treat radioactive wastewater is not regenerated, so once it fails, it should be replaced immediately.
The ion exchange method uses ion exchange resin, which is suitable for waste liquid with low salt content. When the salt content is high, the cost of treatment with ion exchange resin is higher than that of selective process. This is mainly due to the low selectivity of the resin which has a great correlation with radionuclides. In the purification of radioactive wastewater, the use of electrodialysis can increase the utilization efficiency of ion exchange process.
- Adsorption method
Adsorption is an effective method for removing heavy metal ions from water by using porous solid substances to adsorb and remove heavy metal ions. The key technology of adsorption method is the choice of adsorbent. Commonly used adsorbents are activated carbon, zeolite, kaolin, bentonite, clay, etc. Among them, zeolite is cheap, safe and easy to get. Compared with other inorganic adsorbents, zeolite has greater adsorption capacity and better purification effect. The purification ability of zeolite is 10 times higher than that of other inorganic adsorbents, so it is a very competitive water treatment agent. It is often used as an adsorbent in water treatment processes, and it also functions as an ion exchanger and a filter agent.
Activated carbon has strong adsorption capacity and high removal rate, but the regeneration efficiency of activated carbon is low, and the water quality of the treated water is difficult to meet the reuse requirements. The price is expensive and the application is limited. In recent years, a variety of adsorbent materials with adsorption capacity have been gradually developed. Relevant studies have shown that chitosan and its derivatives are good adsorbents for heavy metal ions. After crosslinking, chitosan resin can be reused many times without significantly reducing the adsorption capacity. Using modified sepiolite to treat heavy metal wastewater has good adsorption capacity for Co and Ag, and the heavy metal content in the treated wastewater is significantly lower than the comprehensive sewage discharge standard.
- Evaporation and concentration
The evaporative concentration method has a high concentration factor and purification factor, and is mostly used to treat medium and high-level radioactive wastewater. The working principle of the evaporation method is: the radioactive waste water is sent to the evaporation device, and at the same time, the heating steam is introduced to evaporate the water into water vapor, while the radionuclides remain in the water. The condensed water formed during the evaporation process is discharged or reused, and the concentrated liquid is further solidified. The evaporation concentration method is not suitable for treating waste water containing volatile nuclides and foaming easily; it consumes a lot of heat energy and has high operating costs; at the same time, potential threats such as corrosion, scaling, and explosion must be considered during design and operation. In order to improve steam utilization and reduce operating costs, countries have been sparing no effort in the development of new evaporators, such as vapor compression evaporators, thin film evaporators, vacuum evaporators and other new evaporators have achieved remarkable results.
- Membrane separation technology
Membrane technology is a relatively efficient, economical and reliable method for treating radioactive wastewater. Because membrane separation technology has the characteristics of good effluent quality, no phase change of materials, and low energy consumption, membrane technology has been actively researched.
Membrane technology used abroad are: microfiltration, ultrafiltration, nanofiltration, water-soluble polymer – membrane filtration, reverse osmosis (RO), electrodialysis, membrane distillation, electrochemical ion exchange, liquid membrane, ferrite adsorption filtration membrane separation and anion-exchange paper membrane and other methods.
Melamine foam, a kind of nanoscale stable three-dimensional net organic filtration substrate, its pore size up to 1nm, pore capacity up to 0.297cc/g, pore opening rate up to 98%. It can achieve efficient adsorption and filtration of nuclear sewage, even most kinds of adsorbents be added with.
- Biological treatment
Biological treatment methods include phytoremediation and microbial methods. Phytoremediation refers to a new in-situ treatment technology that uses green plants and their rhizosphere indigenous microorganisms to work together to remove pollutants in the environment.
From the existing research results, the applicable types of bioremediation technologies mainly include constructed wetland technology, rhizosphere filtration technology, plant extraction technology, plant solidification technology, and plant evaporation technology. The test results show that almost all the uranium in the water body can be enriched in the roots of plants.
Microbiological treatment of low-radioactive wastewater is a new technology that began to be studied in the 1960s. There have been some studies on the removal of uranium in radioactive wastewater by this method at home and abroad, but most of them are in the experimental research stage at present.
With the development of biotechnology and the in-depth study of the interaction mechanism between microorganisms and metals, people gradually realized that the use of microorganisms to treat radioactive wastewater pollution is a very promising method. Microbial cells are used as biological treatment agents to absorb, enrich and recover radionuclides such as uranium in aqueous solution, which has high efficiency, low cost and low energy consumption. Moreover, there is no secondary pollutant, which can achieve the reduction goal of radioactive waste and create favorable conditions for the regeneration or geological disposal of nuclides.
- Magneto-molecule method
The United States Electric Power Research Institute (EPRI) has developed the Mag-Mole-clue method for reducing the number of radioactive wastes such as strontium, cesium and cobalt generated. The method is based on a protein called ferritin, which is modified to use magnetic molecules to selectively bind contaminants, which are then removed from solution by magnets, and then the bound metals are recovered by backwashing the magnetic filter bed. Ferritin is a highly conserved multifunctional multiunit protein commonly found in living organisms, which has the special characteristics of resistance to dilute acids (pH<2.0), dilute bases (pH= 12.0), and higher temperatures (invariant to 70～75°C water temperature). With the deepening of ferritin research, great progress has been made in the study of new functions in vitro using the nano space of its protein shell. In vitro studies have shown that ferritin has the ability to store heavy metal ions in vitro. In addition, previous studies have focused on the mechanism of iron storage and release from ferritin using other heavy metal ions as probes that compete with iron ions. In contrast, recent studies have shown that this property of ferritin to capture metal ions and to be resistant to reversal can be exploited to construct a ferritin reactor and use it in the field for continuous monitoring of the degree of contamination of flowing water by heavy metal ions. Under specific conditions in vitro, some metal nuclei, such as FeS nuclei, CdS nuclei, Mn3O4 nuclei, Fe3O4 magnetic iron nuclei and uranium nuclei of radioactive materials, have been successfully assembled into the nano space of the ferritin protein shells.
- Inert curing method
Pennsylvania State University and Savannah River National Laboratory have developed a new method for processing certain low-level radioactive waste liquids into a solidified form for safe disposal. This new process utilizes a low-temperature (< 90°C) coagulation method to stabilize highly alkaline, low-activity radioactive waste liquid, that is, to convert the waste liquid into an inert solidified body. The scientists called the final cured body a “hydro ceramic” (a bisque-fired porous ceramic). They say that the final solidified body is very hard, stable and long-lasting, and can fix radionuclides in its zeolite structure. This preparation process is similar to the formation process of rocks in nature.
- Zero-valent iron percolation reaction wall technology
The permeable reactive barrier (PRB) is a method emerging in developed countries such as Europe and the United States for in-situ removal of pollutant components in polluted groundwater. PRBs are generally installed in underground aquifers, perpendicular to the direction of groundwater flow. When the polluted groundwater flows through the reaction wall under the action of its own hydraulic gradient, the pollutants will be removed by physical and chemical reactions with the reaction materials in the wall, so as to achieve the purpose of pollution restoration.
This is a passive restoration technique that requires little maintenance and is inexpensive. As an important branch of PRB technology, Fe0-PRB technology has been researched and developed in many countries and in many aspects of groundwater pollution treatment. Encouraging results have been achieved in the study of the reaction mechanism, the structure and installation of PRBs, and the study of new active materials. China’s scholars have begun to study the active percolation wall technology represented by zero-valent iron for the remediation (treatment) of uranium tailings radioactive wastewater, and the current research has achieved certain results.
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