Grażyna Zakrzewska-Trznadel

Concentration of radioactive components in liquid low-level radioactive waste by membrane distillation

Abstract The paper addresses some aspects of liquid low-level radioactive waste (LLLW) purification. Since the volume of liquid low-level wastes is usually large and the activity is rather low, the radioactive substances separated from the non-active portion have to be concentrated into the small volume for subsequent conditioning and disposal. The need for the improvement of decontamination and minimisation of the costs have led to new specific methods being under examination and development. The method proposed in the paper is membrane distillation. The experimental work described below supports the statement that membrane distillation can be an attractive alternative for liquid radioactive waste treatment. The advantages of membrane distillation over the other processes commonly used for the processing of LLLW are discussed in the paper.

Membrane processes in nuclear technology-application for liquid radioactive waste treatment

Membrane methods can be considered as the most energy-saving separation techniques. Separation abilities of membranes allow the elimination of many non-effective and energy consuming methods and their replacement by new, modern technologies, friendly environment friendly. An application of membrane methods for liquid radioactive wastes treatment requires solving many problems connected with the proper selection of the membranes, membrane modules and other equipment according local conditions: chemical and radiochemical composition of the effluents treated, their activity and total salinity. The installations working in nuclear industry have to fulfil very strict requirements. They ought to be reliable, constructed from special materials defined by separate regulations. Only small number of manufacturers of membrane devices has for their products the certificates of International Atomic Energy Agency. Reverse osmosis (RO) as a method for liquid waste treatment has been examined at laboratory and pilot plant installations. The experience with the process led to design and construction of the industrial plant, 1 m3 capacity, composed of three RO stages. The plant will be included into the system for liquid radioactive wastes purification operating at Institute of Atomic Energy in Swierk near Warsaw, treating the liquid waste from all of Poland. Membrane distillation (MD) can be an alternative for liquid radioactive waste concentration. On the basis of previous laboratory tests a pilot plant for liquid radioactive wastes concentration employing direct contact MD was constructed. Pilot plant experiments showed MD is interesting solution for liquid low-level radioactive waste treatment. As MD is characterised by high retention, large decontamination factors were obtained in separation of radionuclides, which are present in liquid low-level radioactive wastes mainly in an ionic form. The RO and MD plants for liquid low level radioactive wastes treatment are presented in the paper and the evaluation of both methods, as well.

Removal of cobalt ions from aqueous solutions by polymer assisted ultrafiltration using experimental design approach. part 1: optimization of complexation conditions.

The polymer assisted ultrafiltration process combines the selectivity of the chelating agent with the filtration ability of the membrane acting in synergy. Such hybrid process (complexation-ultrafiltration) is influenced by several factors and therefore the application of experimental design for process optimization using a reduced number of experiments is of great importance. The present work deals with the investigation and optimization of cobalt ions removal from aqueous solutions by polymer enhanced ultrafiltration using experimental design and response surface methodological approach. Polyethyleneimine has been used as chelating agent for cobalt complexation and the ultrafiltration experiments were carried out in dead-end operating mode using a flat-sheet membrane made from regenerated cellulose. The aim of this part of experiments was to find optimal conditions for cobalt complexation, i.e. the influence of initial concentration of cobalt in feed solution, polymer/metal ratio and pH of feed solution, on the rejection efficiency and binding capacity of the polymer. In this respect, the central compositional design has been used for planning the experiments and for construction of second-order response surface models applicable for predictions. The analysis of variance has been employed for statistical validation of regression models. The optimum conditions for maximum rejection efficiency of 96.65% has been figured out experimentally by gradient method and was found to be as follows: [Co(2+)](0)=65 mg/L, polymer/metal ratio=5.88 and pH 6.84.

Radioactive solutions treatment by hybrid complexation-UF/NF process

Abstract Application of inorganic membranes was considered for radioactive wastes processing purpose. The results of experiments with ceramic membranes in nanofiltration and ultrafiltration range are presented in the paper. The experiments were performed with non-active and radioactive model solutions and original radioactive waste samples. To achieve high decontamination factors the process was arranged as a “seeded ultrafiltration”, e.g. UF process enhanced by chemical complexation. Such complexing agents as poly(acrylic) acid and poly(acrylic) acid salts of different crosslinking, polyethylenimine and cyanoferrates of transient metals were tested. It was proved that decontamination factors for hybrid process were higher than those for membrane filtration solely. The effectiveness of complexation by each ligand was strongly pH-dependent.

Removal of cobalt ions from aqueous solutions by polymer assisted ultrafiltration using experimental design approach: Part 2: Optimization of hydrodynamic conditions for a crossflow ultrafiltration module with rotating part

Application of shear-enhanced crossflow ultrafiltration for separation of cobalt ions from synthetic wastewaters by prior complexation with polyethyleneimine has been investigated via experimental design approach. The hydrodynamic conditions in the module with tubular metallic membrane have been planned according to full factorial design in order to figure out the main and interaction effects of process factors upon permeate flux and cumulative flux decline. It has been noticed that the turbulent flow induced by rotation of inner cylinder in the module conducts to growth of permeate flux, normalized flux and membrane permeability as well as to decreasing of permeate flux decline. In addition, the rotation has led to self-cleaning effect as a result of the reduction of estimated polymer layer thickness on the membrane surface. The optimal hydrodynamic conditions in the module have been figured out by response surface methodology and overlap contour plot, being as follows: DeltaP=70 kPa, Q(R)=108 L/h and W=2800 rpm. In such conditions the maximal permeate flux and the minimal flux decline has been observed.

Modeling and multi-response optimization of pervaporation of organic aqueous solutions using desirability function approach.

The factorial design of experiments and desirability function approach has been applied for multi-response optimization in pervaporation separation process. Two organic aqueous solutions were considered as model mixtures, water/acetonitrile and water/ethanol mixtures. Two responses have been employed in multi-response optimization of pervaporation, total permeate flux and organic selectivity. The effects of three experimental factors (feed temperature, initial concentration of organic compound in feed solution, and downstream pressure) on the pervaporation responses have been investigated. The experiments were performed according to a 2(3) full factorial experimental design. The factorial models have been obtained from experimental design and validated statistically by analysis of variance (ANOVA). The spatial representations of the response functions were drawn together with the corresponding contour line plots. Factorial models have been used to develop the overall desirability function. In addition, the overlap contour plots were presented to identify the desirability zone and to determine the optimum point. The optimal operating conditions were found to be, in the case of water/acetonitrile mixture, a feed temperature of 55 degrees C, an initial concentration of 6.58% and a downstream pressure of 13.99 kPa, while for water/ethanol mixture a feed temperature of 55 degrees C, an initial concentration of 4.53% and a downstream pressure of 9.57 kPa. Under such optimum conditions it was observed experimentally an improvement of both the total permeate flux and selectivity.

Removal of radionuclides by membrane permeation combined with complexation

Membrane permeation combined with complexation was tested for radioactive wastes processing purpose. The ceramic membranes MEMBRALOX® and CeRAM INSIDE® were used. To enhance the UF separation effect, different soluble polymers and ferrocyanides, binding cobalt and caesium ions were used. The complexing agents were tested in laboratory to establish the best conditions of the process. The experiments showed a significant increase of decontamination factors while the macromolecular compounds were added. The effectiveness of complexation by each ligand is strongly dependent on pH and alkali metals concentration.

Separation of protium/deuterium and oxygen-16/oxygen-18 by membrane distillation

Abstract An attempt to explain the significant isotope effect of deuterium and heavy oxygen observed in the MD process was undertaken. A study of the transport mechanism through porous PTFE membranes showed that membrane mass transfer coefficients can be well estimated from the molecular diffusion model or from the combination of Knudsen and molecular diffusion models. the existence of a diffusion isotope effect in membrane distillation enhances the separation factors for H 2 O HDO and H 2 16 O H 2 18 O enrichment. The discussion of the nature of the isotope effect in MD is presented.

CONCENTRATION OF LOW- AND MEDIUM-LEVEL RADIOACTIVE WASTES WITH THREE-STAGE REVERSE OSMOSIS PILOT PLANT

A study of the application of different membrane processes to nuclear technology has been conducted at the Institute of Nuclear Chemistry and Technology (INCT) for many years. One application of membrane methods involves the purification and concentration of liquid low- and medium-level radioactive wastes (LLLRWs and LMLRWs). The separation of radioactive model solutions as well as LLLRW and LMLRW samples has been examined with various reverse osmotic modules equipped with thin-composite polymeric membranes. On the basis of these investigations, membrane modules were selected for the pilot plant processing of liquid wastes from nuclear centers. A three-stage JP3RO reverse osmosis unit was constructed. The installation is described in this paper, and the results of preliminary pilot plant experiments are presented.

Application of ceramic membranes for hazardous wastes processing: pilot plant experiments with radioactive solutions

Abstract The membrane method combined with complexation was applied for hazardous wastes containing radioactive substance processing. Such complexing agents like soluble chetating polymers and cyanoferrates of transient metals, tested and selected in the laboratory, were used to bind radioactive ions and to enlarge the separated molecule size. The preliminary pilot plant experiments are presented with installation equipped with a ceramic 23-channel module, cut-off 8 kD. The experiments were performed with model solutions and original radioactive wastes. They showed the feasibility of a UF/complexation hybrid method for reduction of long-lived radioisotope concentration in the effluent (permeate). The radioactive substances concentrated in a small volume can be directly fossilized.