Cavitation Strengthen The Edta Degradation In Simulated Radioactive Waste Water Treatment Applications

The chemical decontaminating agents, such as ethylenediaminetetraacetic acid (EDTA), are widely used in the cleaning of equipment in nuclear and non-nuclear industries. The radioactive waste water generated from chemical decontamination of the contaminated equipment of nuclear facilities must be treated to remove the radionuclides for safe disposal.The complexing agents such as EDTA can chelate with most of metals, such as Pb, Sr, forming stable complexes and making difficulties for the removal of these metals from waste water. It is therefore necessary to destroy the complexes existing in waste water prior to the removal of the nuclides.In this paper, advanced oxidation processes are studied for the degradation of organic complexing agents (e.g. EDTA) from the simulated low-level radioactive waste water. The degradation processes using ozone (O₃), ultrasonic/ozone (US/O₃), ultrasonic/H₂O₂ (US/ H₂O₂) and swirting/H₂O₂ are performed. Various factors affecting the degradation process are discussed and the degradation mechanisms are briefly analyzed.EDTA is rather stable and difficult to be degradated. Under the O₃ system, the degradation rate of EDTA increased with the decrease of pH value and increase of O₃ flow rate, but the change of degradation rate tended to be insignificant when pH>7. The increase of the initial concentration of EDTA was not helpful to the degradation efficiency, but the mass of EDTA being decomposed from the unit volume of solution was increased. Higher solution temperature lowers the degradation rate owing to the lowering of ozone solubility in water, but the degree of decrease was lower than that of ozone solubility in water.The degradation rate was very low by solely using ultrasonic, which can however enhance the EDTA degradation by O₃. Under the system of US/O₃, degradation efficiency was increased with the decrease of pH and initial concentration of EDTA. It also increased at higher O₃ flow rate and ultrasonic intensity. Higher solution temperature was also not helpful for decomposition. S ynergetic effect was observed in the US/O₃ system for EDTA degradation. This may be explained by the two ways: (1) Atomic oxygen was produced by sonochemical O₃ thermolysis, atomic oxygen then reacted with water vapor in cavitation bubbles and hydroxyl radical produced. (2) The solubility of O₃ in water was increased because of sonication. Determining the fluorescence intensity of terephthalic acid indicated that EDTA was mainly oxidized by hydroxyl radical in the system of O₃ and US/O₃. More hydroxyl radicals were produced in US/O₃ system than that in ultrasonic and O₃ systems.Through the combination of gas chromatograph with mass spectrometer (GC-MS) and ion chromatograph (IC), the intermediates of EDTA degradation were found to be acetic acid, propanoic acid, butanoic acid, 2- methyl butanoic acid, etc.Under the swirling system, the swirling flow state and pressure distribution in the swirling chamber were calculated by using software Ansys CFX first, which can simulate the state of liquid. It was indicated that, when the injection ports width of swirling chamber was 0.2mm, the length of the swirling chamber 100mm and inner diameter 10mm, the maximal low pressure region could be formed. Besides this, the crash board can also affect the formation of cavitation. The best position of crash board was found at 35mm under the outlet.The degradation of Cu-EDTA through swirling/H₂O₂ was studied. It was indicated that swirling/H₂O₂ could be performed in a wide pH range, which is a much flexible pH condition compared to Fenton reagent. But efficiency of decomposition was decreased with the increase of pH owing to the increased numbers of radicals capturers in high pH solution. By increasing the surface area of low pressure, higher temperature was helpful to forming of cavitation, and it was different from ultrasonic cavitation.Determining of the fluorescence intensity indicated that the oxidation by swirling and swirling/H₂O₂ were all through hydroxyl radicals. The numbers of radicals were too limited by swirling solely, swirling cavitation should be used in combination with other methods.The removal of Sr from simulated liquid radioactive waste containing Sr-EDTA was studied. It was indicated that the percentage of Sr removed was 4.3% without any degradation treatment. After degradation treatment by O₃ and US/O₃ for 1 hour, the Sr removal rate was increased to 9.6% and 13.8% respectively. Correspondingly, in the system of swirling/H202, 28.6% of Sr removal was found after 1 hour.