Membrane processes for environmental protection: applications in nuclear technology

Membrane processes are considered as potential methods useful in clean technologies that minimize the use of raw materials, rationalize energy consumption and reduce waste production. They are capable to solve many environmental problems, among them problems related to nuclear technology field. Membrane processes have been already applied for liquid radioactive waste processing in many nuclear centres around the world. Reverse osmosis (RO) was implemented at the Institute of Atomic Energy (IAE) at Świerk for liquid low-level radioactive waste concentration. A 3-stage RO plant supplements the existing waste processing system based on an evaporator giving the possibility of initial concentration of liquid waste or final polishing of the condensate after evaporation. Intensive studies on ultrafiltration (UF) enhanced by sorption on inorganic sorbents or complexation with chelating polymers were carried out. Ceramic membranes made of alumina, titania and zirconia were used in the experiments. Such membranes show a high chemical, temperature and radiation resistance. Thermal process, namely membrane distillation with the use of resistant porous membranes from PTFE was proposed and tested for radioactive waste concentration. The results collected in laboratory and pilot plant experiments allowed to consider the process usable for small installations operated with utilization of cheap energy sources or waste heat. Other methods like liquid membranes and electric processes with ion-exchange membranes as possible applications in nuclear industry are under development. Membrane methods were considered as alternative solutions for reclamation of different materials that can be recycled and reused. Some of them allow minimizing the total energy consumption by various energy recovery systems and utilization of cheap energy sources

Investigation of hydrodynamic behaviour of membranes using radiotracer techniques

The aim of the work was to study membrane devices using short-lived radioisotopes like Ba-137m and Ga-68 as tracers. These radioisotopes were obtained from radionuclide generators: Cs-137/Ba-137m and Ge-68/Ga-68. The first radionuclide, namely Ba-137m with a half-life of 2.55 minutes was applied as a liquid phase tracer for studying hydrodynamic conditions inside the membrane apparatus. The membrane module with ceramic membranes was tested by using Ba-137m. The experiments showed that this radionuclide with a short half-life is a perfect tracer for liquid phase, whereas Ga-68 with longer half-life equal to 68 minutes was considered as a solid phase (bentonite) tracer. Ga-68 was used to gain more knowledge about the phenomena occurring in the membrane boundary layer. After kinetic studies of isotope adsorption into the carrier material, the growth rate of the deposit layer as well as deposit's thickness on the flat-sheet membrane were studied. The influence of such process parameters like pressure, linear velocity of liquid and feed concentration on formation of the bentonite layer on the membrane surface was studied

Response surface modelling and optimization in pervaporation

Both the conventional method of experimentation, in which one of factors is varied maintaining the other factors fixed at constant levels and the statistically designed experimental method, in which all factors are varied simultaneously are carried out for organic removal from water by pervaporation. Binary acetonitrile-water mixtures are considered. The effects of the operating parameters on the pervaporation performance of the membrane system have been investigated. The overall mass transfer coefficients have been determined for different conditions of feed temperature and initial organic concentration. in addition, the activation energy associated to the permeation process has been determined and discussed for each feed organic mixture. Statistical experimental design and response surface methodology, RSM, have been applied to optimize the operational conditions of pervaporation. process in order to maximize the output responses, which are permeate flux ratio and concentration of organic in permeate. The input variables employed for experimental design were the feed temperature, initial concentration of organic in feed and operational downstream pressure. Based on the design of experiment the quadratic response surface models have been developed to link the output responses with the input variables via mathematical relationships. The constructed response models have been tested using the analysis of variance and the canonical analysis. The obtained optimal point by means of Monte Carlo simulation method and desirability function corresponds to a feed temperature of 57.69 degrees C, a feed acetonitrile concentration of 6.96 wt% and a downstream pressure of 28.95 kPa. The maximal values of the permeate flux ratio and the concentration of organic in permeate obtained under optimal process conditions have been confirmed experimentally

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 multiresponse 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 21 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

Response surface methodology for cobalt removal from aqueous solutions using Isparta pumice and zeolite 4A adsorbents

In this study, the adsorption of non-radioactive cobalt ions from aqueous solutions onto Isparta pumice and zeolite 4A sorbents was investigated. Both adsorbent materials have been activated at 873 K for 2 h prior to adsorption experiments in batch mode. The maximum removal efficiencies of 90% and 99% have been obtained experimentally using Isparta pumice and zeolite 4A, respectively. In addition, the experiments with radioactive 60Co were performed to test zeolite ability to remove radioactive compounds. Likewise, the response surface methodology (RSM) has been applied to develop the predictive regression models to describe the adsorption of cobalt and radiocobalt ions onto zeolite 4A and Isparta pumice. The results indicated that zeolite 4A as well as Isparta pumice could be used as the efficient sorption materials for cobalt and radiocobalt ions removal

Extraction of uranium from low - grade Polish ores : dictyonemic shales and sandstones

Leaching studies were performed to develop a suitable method for extraction of uranium from domestic uranium ores: dictyonemic shales and sandstones. The extracting procedure was composed of successive separation steps: crushing and grinding, acid or alkaline leaching, solid-liquid separation, and inductively coupled plasma mass spectrometry (ICP-MS) analyses of post-leaching solutions. The influence of such process parameters as temperature, pressure, particle size of solid material, kind of leaching solution and its concentration, on the recovery efficiency of uranium and accompanying metals were tested. The efficiency of uranium leaching with sulphuric acid solution achieved 81%. Satisfactory results were obtained for the alkaline leaching process. It was found that uranium can be selectively extracted by the alkaline leaching solution. Almost complete extraction of uranium from sandstones was achieved when a mixture of sodium carbonate and sodium hydroxide, with hydrogen peroxide as a leaching solution was used