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

The membrane method combined with complexation was applied for hazardous wastes containing radioactive substance processing. Such complexing agents like soluble chelating 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.

The ultrafiltration/complexation hybrid method is able to remove radioactive compounds from radioactive liquid waste and can be used as one stage of radioactive liquid waste processing. A proper selection of the binding agent, process parameters and conditions may result in high decontamination; however, a variety of constituents present in the solution sometimes requires the use of different complexing compounds. Some radioisotopes need special attention, especially if they occur in high concentrations. Considering the specific composition of radioactive waste in Poland (wastes coming mainly from medical and industrial applications with high cesium content), the best way of purification is using two complexing agents: a high-molecular weight chelating polymer and a cyanoferrate of transient metal, which binds the cesium ions. Such a method is efficient and the results of purification are sufficient to reduce the concentration of radioactive substances and other toxic compounds in the effluent and to concentrate them in a small volume, proper for further common treatment techniques. The pilot plant presented in the paper, equipped with a CéRAM INSIDE® 23-channel module with a cut-off of 8 kD, is able to process radioactive liquid waste from chemical laboratories in a medium-size nuclear centre like INCT. For further implementation at the Institute of Atomic Energy, the installation has to be scaled up to reach capacity of approximately 1 m3/h. Present experience with up-scaling is rather promising. The 23-channel module behaved well for long-time operation. However, periodical cleaning (at least once a day) is necessary when macromolecular ligands are used to bind radioactive species. The recommended work regime is as follows: 22 h of operation, then a 2-h cleaning procedure using acidic and alkaline solutions at elevated temperature, alternately with water baths. Additional efforts for the selection of new complexing agents for bonding some specific radioisotopes efficiently will be made in the future. As cyanoferrates are used to bind cesium ions, careful control of cyanides in the effluent is necessary (both analyses — chemical and radiochemical of the effluent — were performed periodically, as well as continuous record of specific conductivity). To avoid the cyanoferrates passing through the membrane, all these sorbents were converted into colloidal form before use, retained completely by the membrane and stable under the experimental conditions (pH ~10). The concentrate of radioactive substances is fossilized in the next stage of radioactive waste processing and stored; therefore, the cyanide concentration in the retentate is not essential. To apply the method of heavy metals removal from other hazardous wastes, other binding ligands instead of cyanoferrates, if environmentally acceptable, can be used. The hybrid process in some cases can be an alternative for RO; however, this is hardly to say that it can completely replace RO. In spite of high pressures applied, RO is a well-elaborated process, eliminating completely (in multi-stage arrangement) radioactive compounds from the effluent. The use of a hybrid UF/complexation process is reasonable when the composite RO membranes cannot stand chemically aggressive or highly radioactive process conditions (e.g., during high-level radioactive waste treatment). The ceramic membranes are recommended in such cases, as well as in the case when cleaning with acidic or alkaline solution in high temperatures is needed. A more selective membrane/complexation process can be also applied when recovery of some valuable components of the solution is considered.