MicroSISAK: continuous liquid–liquid extractions of radionuclides at 0.2 mL/min

Abstract

Continuous liquid–liquid extraction of short-lived radionuclides has traditionally been performed with the SISAK system consisting of static mixers and H-centrifuges for phase separation. SISAK operates at flow rates of typically 1 mL/s. Thus, it produces large volumes of radioactive liquid waste that is difficult to dispose of. Therefore, it has been aimed to develop and use a further miniaturised extraction unit based on microtechnology and precision engineering to reduce the flow rate by at least two orders of magnitude. The accordingly developed MicroSISAK device is a micro membrane extractor in which a micromixer element with 2 × 16 feed channels of 30 µm width followed by a 60 µm high mixing chamber is used for intimately contacting the aqueous and organic phase. Subsequent phase separation is achieved via hydrophobic Teflon membranes with a pore size of 1 µm. The MicroSISAK device has been tested and optimized with radiotracers of the group-4 elements Zr and Hf in the system H₂SO₄/trioctyl amine (TOA) in toluene. At a temperature of 58°C and a flow rate of 0.2 mL/min of both phases, extraction yields of 87 ± 3% were achieved. The transport time from the micromixer to the first Teflon membrane was in this case 3.9 s. It can be shortened to 1.56 s at a flow rate of 0.5 mL/min. Under similar conditions, the extraction yield of ^99mTc milked from a ⁹⁹Mo generator in the system HNO₃/tetraphenyl arsonium chloride (TPAC) in chloroform was 83 ± 3%. In an on-line experiment at the TRIGA Mainz reactor, short-lived Tc isotopes produced in the fission of ²³⁵U with thermal neutrons were transported by a He/KCl gas-jet to the chemistry apparatus, deposited by impaction, dissolved in 0.01 mol/L HNO₃/KBrO₃, and extracted into 10⁻⁴ mol/L TPAC in chloroform in MicroSISAK. The separated phases were transported via capillaries to two separate flow-through cells positioned in front of two Ge detectors. The extraction yield determined as the ratio of the Tc γ-ray activities in both detectors was 76 ± 1%. With this experiment, it was demonstrated that MicroSISAK is in principle ready for an on-line experiment for the chemical characterization of the superheavy element bohrium, element 107. However, the detection of α-particle activities by liquid scintillation counting still needs to be worked out.