Zeolites / Decontamination / Radioactive waste microfiltration / UltrafiltrationSummary. The purpose of the work was to improve the process for treatment of liquid radioactive waste containing complexing agents, which are generated during the decontamination operations.We performed some experiments using simulated waste solutions like secondary waste from the modified CANDEREM process (Canadian Decontamination and Remediation Process) and secondary waste from the modified CANDECON process (Canadian Decontamination Process).To improve efficiency and economics of the process it was proposed to treat the waste by combining the sorption of radionuclides on natural inorganic sorbents (zeolites) with membrane filtration.Standard procedures are applied to compare the sorption of radionuclides on different sorbent forms-determination of the ion exchange capacity, construction of sorption isotherms, determination of the distribution coefficients, and kinetics experiments.To check the influence of converting the sorbents to various cationic forms on their sorption properties, distribution coefficients of 137 Cs and 57 Co on natural zeolites from local deposits converted to NH 4 + , Na + or H + forms were determined. The results obtained show that the distribution coefficients of 137 Cs on the materials converted to Na + form are higher than for the remaining forms studied [1].The parameters of Langmuir, Freundlich and DubininRadushkevich adsorption isotherms have been determined using sorption data. The Dubinin-Radushkevich model shows better correlation between the theoretical and experimental data for 137 Cs sorption on natural zeolites from local deposits converted to NH 4 + and H + forms than Langmuir and Freundlich equations.Kinetic studies were carried out with various zeolite forms. The sorbents studied are natural zeolites from local deposits (Marsid-Romania).The batch sorption kinetics has been tested for pseudosecond order reaction. The pseudo-second order model fits the experimental data well for all of the systems studied.

The paper presents the experimental tests concerning the treatment by membrane techniques of radioactive aqueous waste. Solutions, which have been treated by using the bench-scale installation, were radioactive simulated secondary wastes from the decontamination process with modified POD. Generally, an increasing of the retention is observed for most of the contaminants in the reverse osmosis experiments with pre-treatment steps. The main reason for taking a chemical treatment approach was to selectively remove soluble contaminants from the waste. In the optimization part of the precipitation step, several precipitation processes were compared. Based on this comparison, mixed [Fe(CN)6]4-/Al3+/Fe2+ was selected as a precipitation process applicable for precipitation of radionuclides and flocculation of suspended solid. Increased efficiencies for cesium radionuclides removal were obtained in natural zeolite adsorption pre-treatment stages and this was due to the fact that volcanic tuff used has a special affinity for this element. Usually, the addition of powdered active charcoal serves as an advanced purifying method used to remove organic compounds and residual radionuclides; thus by analyzing the experimental data (for POD wastes) one can observe a decreasing of about 50% for cobalt isotopes subsequently to the active charcoal adsorption.. The semipermeable membranes were used, which were prepared by the researchers from the Research Center for Macromolecular Materials and Membranes, Bucharest. The process efficiency was monitored by gamma spectrometry.

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