Abstract
A novel ion concentration polarization-based microfluidic device is proposed for continuous extraction of Li(+) from high Mg(2+)/Li(+) ratio brines. With simultaneous application of the cross-channel voltage that drives electroosmotic flow and the cross-membrane voltage that induces ion depletion, Li(+) is concentrated much more than other cations in front of the membrane in the microchannel. The application of external pressure produces a fluid flow that drags a portion of Li(+) (and Na(+)) to flow through the microchannel, while keeping most of Mg(2+) (and K(+)) blocked, thus implementing continuous Li(+) extraction. Two-dimensional numerical simulation using a microchannel of 120 µm length and 4 µm height and a model, highly concentrated brine, shows that the system may produce a continuous flow rate of 1.72 mm/s, extracting 25.6% of Li(+), with a Li(+)/Mg(2+) flux ratio of 2.81×10(3), at a pressure of 100 Pa and cross-membrane voltage of 100 times of thermal voltages (25.8 mV). Fundamental mechanisms of the system are elaborated and effects of the cross-membrane voltage and the external pressure are analyzed. These results and findings provide clear guidance for the understanding and designing of microfluidic devices not only for Li(+) extraction, but also for other ionic or molecular separations.