Investigating the Selective Separation Ability of an Optimized Microfluidic Chip with Focus on Metal Ions

In the Reactor Institute of Delft, the process of gaining Molybdenum-99 from the irradiation of low enriched Uranium-235 solution for medical purposes is being investigated. When Molybdenum99 decays the isotope Technetium-99m can be obtained. This isotope is widely used for medical imaging. Due to the half-life time of 6 hours it is not suitable for transport and that is where Molybdenum-99 comes in. This isotope has a half-life time of 66 hours and is therefore much more suitable for transportation. When the low enriched Uranium-235 solution is irradiated a large amount of fission products will arise. Molybdenum-99 should therefore be solely separated from these other fission products. One method of extracting species from an aqueous solution is liquid-liquid extraction on micro scale. This method is bases on the immiscibility of two phases, normally an aqueous phase in which the to be separated specie is dissolved and an organic phase. The small dimensions of the system allow for example for a large interfacial area to volume ratio, short diffusion distance and rapid stabilization of the interface. In previous published work by S. Goyal et al liquid-liquid extraction is used with an extractant present in the organic phase. This extractant is known as HOBO and selectively bounds to copper(II) ions. The work claims an extraction rate of exceeding 95 % for a channel length of 7 millimeters and an average residing time of 0.21 seconds. This research will focus on this variant of liquid-liquid extraction, which is referred to as active liquid-liquid extraction. Investigation is done on the parameters of the channel to find the optimized parameter combination and predict the extraction rate. Next to this also the qualitative influence of the parameters is investigated. It is done by the hand of a model made in COMSOL® which is evaluated by comparing it to known studies and test it with analytical approaches. After validating the model a parametric sweep is made. The length, width and height of the channel as well as the viscosity of the organic phase, the diffusion coefficient in the aqueous phase and the aqueous flow rate are swept for several values. These values are restricted to account for the known limitations on their values. The large data obtained is analyzed to find the optimal setup and to deduce the qualitative influence of the parameters. It is found that a small width and a large height and length are optimal for the dimensions of the extraction channel. Furthermore, a low flow rate, a high diffusion coefficient and a large organic viscosity value relative to the aqueous viscosity show an optimum for the phase parameters. Keeping the limitations on the values in mind the optimal setup will result in an extraction rate of 72 percent. Additionally, it has been deduced that changing the organic viscosity has the least amount of influence on the extraction rate, while changing the width caused the most influence. If the limitations on the length and the width are taken out of consideration an extraction rate of 99 percent for an extraction channel length of 4 cm is found. For the width, an extraction percentage of 90 will be found when the width is narrowed down to 20 μm. Furthermore, it has been demonstrated that the mentioned extraction rate in the work of S. Goyal et al cannot be achieved in the way it has been interpreted in this research. This research found an extraction rate of less than 10 percent which has been justified by an analytical approximation.