Cesium removal method in radioactive wastewater
Cesium-containing wastewater mainly comes from nuclear fuel processing in the nuclear industry, nuclear fission products in atomic power stations, and research institutes using radioisotopes, etc. The amount of water can reach hundreds of kilograms to tens of thousands of tons. Among them, 137Cs is not only a high-releasing thermal fission product nuclide with a long half-life (T1/2=30a) in the isotope of cesium, but also the main radioactive source of β and γ rays. The proportion of its radioactivity in the total radioactivity of fission products varies with decay. increased over time.
For wastewater containing radioactive elements, no water treatment method can change its inherent radioactive decay characteristics. Extremely low level of radioactive wastewater can be discharged into waters (such as oceans, lakes, rivers) and made harmless by dilution and diffusion; when dealing with low, medium and high levels of radioactive wastewater, the concentrated products of wastewater can be solidified and mixed with the human living environment. Long-term isolation, let it decay naturally. When treating radioactive wastewater, the decontamination factor (DF) and the concentration factor (CF) should be as high as possible. The former refers to the ratio of the original radioactive activity of the wastewater to the residual radioactive activity after treatment, and the latter refers to the original volume of the wastewater compared to the original volume of the wastewater. The volume ratio of the concentrated product after treatment. Removing cesium in radioactive wastewater can reduce high radioactive wastewater to medium and low radioactive wastewater. After further treatment, the wastewater that meets the standard can be discharged or reused, and the concentrated product can be solidified and buried deep for disposal. Cesium can be removed by chemical precipitation method, ion exchange method, evaporation method, extraction method, biological method, etc. The author will focus on the research progress of chemical precipitation method and ion exchange method and some new methods of related research at home and abroad.
1 Chemical precipitation method
The chemical precipitation method is to add a certain precipitant to the solution, so that the metal ions to be removed react with the precipitant to form insoluble compounds and precipitate out, and then the metal ions are removed by solid-liquid separation . The method is based on the solubility product theory. The smaller the solubility product of the compound, the easier it is to form precipitation. Therefore, the selection of a suitable precipitant is the key. Inorganic precipitation products are obtained by using inorganic substances as precipitants, which are beneficial to curing and disposal, and no organic pollution is introduced during the treatment process, which makes inorganic substances the first choice for precipitation agents. Table 1 lists the solubility products (Ksp) of cesium inorganic insoluble compounds at 25°C .
The Ksp of most of the inorganic insoluble substances of cesium is between 10-5 and 10-2, and the Ksp of this order of magnitude is difficult to apply to the chemical precipitation method. The Ksp of Cs3[Co(NO2)6] is much smaller than that of other compounds. In theory, Co(NO2)6- can react with Cs+ to form a precipitate to remove Cs+. However, in the application, the Cs+ concentration in the treated solution should be reduced as small as possible. Calculated by Ksp=[Cs+]3˙[Co(NO2)6-], the required Co(NO2)6- concentration is very high, and there is no The feasibility of the application has not been reported in the literature. Platinum compounds are expensive and are not suitable for precipitants. Therefore, it is extremely unlikely to find a precipitant for cesium in inorganic materials.
D.J.McCabe research shows that Na[B(C6H5)4](NaTPB) can react with Cs+: and the Ksp of CsTPB at 25℃ is 1.0×10-10. According to the calculation of this Ksp, it can be seen that TPB- is completely feasible as a precipitant. Using NaTPB as precipitant to separate and remove cesium in wastewater is the research topic of many scholars. R.A.Peterson et al added the high-level cesium-containing wastewater from the Savannah River in the United States with a radioactivity of 1.85×1010Bq/L and a NaTPB solution of 0.31mol/L to 500mL reaction at the flow rates of 0.73 and 0.27mL/min, respectively. The test results show that the radioactivity of cesium in the effluent can be reduced to below 3.7×104Bq/L. Due to the high radioactivity of the raw water, the DF of this method is greater than 105. S.M.Ponder et al.  injected NaTPB solution into the alkaline simulated wastewater with an initial concentration of Cs+ of 1.4×10-4mol/L in a countercurrent manner, and used a continuous flow process to precipitate and separate cesium in the wastewater, and 99.8% of the cesium could be precipitated. come out. M.F.Debreuille et al.  used NaTPB to separate the cesium precipitate and solidified the precipitated product for disposal. At the same time, the flammable gas such as benzene produced in the reaction was sent to an incinerator for disposal. This technology has been industrially applied in the United States. The temperature is 20~30℃, the residence time is 0.5~2h, the stirring speed is 200~1000r/min, the initial concentration of Cs+ is 1×10-4mol/L, the amount of NaTPB Under the condition of 50% excess, the DF exceeds 1000. E.H.Lee et al.  treated the simulated fission product wastewater with Cs+ mass concentration of (926±20) mg/L with NaTPB. When the pH is 6.3~13.2, and the ratio of the initial concentration of NaTPB to Cs+ is >1, more than 99% of cesium can be precipitated by stirring for 10 minutes, and the temperature (25~50℃) and stirring speed (400~1000r/min) ) had no effect on the amount of precipitation. Using NaTPB to treat cesium in highly radioactive wastewater, the reaction time is short and the precipitation effect is good, but it will produce foam when running in the reactor, this is because TPB- is easily decomposed into benzene, tertiary Phenyl boron, diphenyl boron, phenyl boron, phenol and other flammable and volatile products . These decomposition products make the method a potential safety risk .
2 Ion exchange method
Cesium in solution usually exists as Cs+, so it can be removed by cation exchanger. Among them, organic ion exchangers are easily damaged under high temperature and ionizing radiation, and their applications are limited; while inorganic ion exchangers have strong mechanical, thermal and radiation stability, are easy to operate, and the ion exchange positions of inorganic crystals are more uniform. , so that there is significant selectivity for certain elements. In recent years, more inorganic ion exchangers have been studied, such as natural/artificial zeolite and clay minerals, heteropoly acid salts and composite ion exchange materials, metal ferrocyanide, titanium silicon compounds, etc. .
2.1 Zeolite and clay minerals
Zeolite has the framework structure of aluminosilicate and exchangeable cations, and has strong adsorption and ion exchange performance . The research of E.H.Borai et al.  showed that compared with natural clinoptilolite, natural mordenite and synthetic mordenite, natural chabazite has stronger adsorption capacity and partition coefficient Kd for cesium (Kd can reflect the adsorption capacity in solid, Migration ability and separation efficiency in liquid and two phases). For the 134Cs solution with a radioactivity of 2.28×104Bq/L, when the dosage of natural chabazite is 0.01g/L, its Kd for 134Cs is 4.97×103mL/g. A.M.El-Kamash  used synthetic A-type zeolite as ion exchanger, and adopted two operation modes of sequencing batch type and fixed bed column type to remove cesium in water. The results show that the adsorption of cesium on zeolite is an endothermic process and the reaction proceeds spontaneously; the removal effect is affected by the flow rate of raw water, bed height and initial concentration, and the adsorption rate constant increases with the increase of flow rate. JiaojiaoWu et al.  treated cesium nitrate solution with a mass concentration of 30 μg/L with montmorillonite. When the dosage of montmorillonite was 20 g/L, the adsorption rate of cesium at room temperature exceeded 98%, and the adsorption could be reached within 5 minutes. equilibrium, and the adsorption process can be described by the Langmuir adsorption isotherm. Since the exchange capacity of zeolite and clay minerals is greatly affected by the acidity and salt content of the solution, and the exchange capacity for cesium is low under high salt and strong acidity, this type of ion exchanger is more suitable for treating low acidity and low salt content. of radioactive wastewater.
2.2 Polyvalent metal phosphates and composite ion exchange materials
It has been reported that polyvalent metal phosphates and composite ion exchange materials have high selectivity and strong adsorption capacity for cesium [14,15], and these materials are also a research hotspot. R. Yavari et al.  believed that in the presence of pH<2 and low concentration of NaNO3, titanium phosphate (TMP) had a high affinity for cesium and strontium, but when the concentration of NaNO3 increased from 0 to 1 mol/L, the Kd increased from 104 dropped to below 102mL/g. The adsorption of cesium by TMP is faster than that of strontium, and 80% of cesium can be adsorbed within 10 minutes, and the adsorption equilibrium is reached in 80 minutes, while the adsorption equilibrium of strontium takes more than 100 minutes. S.A.Shady prepared the organic composite ion exchanger resorcinol-formaldehyde (R-F) and the inorganic composite ion exchanger zirconia-ammonium pyrophosphomolybdate (ZMPP) and investigated their effects on Cs, Co, Zn, Eu exchange capacity. The results show that the selective exchange order of R-F and ZMPP for ions is Cs+>Co2+>Eu3+>Zn2+, which is because ions with small radius are more likely to enter the pores of the ion exchanger. At the same pH, the Kd of R-F for cesium was higher than that of ZMPP. When the pH was 7.21, the Kd of R-F was 6.4 × 103 mL/g, while that of ZMPP was 158 mL/g. Y.J.Park et al.  studied the removal effect of ammonium phosphomolybdate-polyacrylonitrile (AMP-PAN) on the removal of Co, Sr and Cs in radioactive washing wastewater from nuclear power plants, and investigated the effect of coexisting ions and surfactants on the removal effect. influences. The results show that the adsorption capacity of AMP-PAN for three elements is Cs>>Co>Sr, the adsorption capacity of cesium can reach 0.61mmol/g, and Na+ and anionic and cationic surfactants can reduce the adsorption capacity. Polyvalent metal phosphates are susceptible to the interference of coexisting Na+, which affects the treatment effect. The organic composite ion exchange material has relatively good cesium removal effect, but its radiation resistance is low, and the subsequent treatment of the concentrated product is more difficult.