  The development of a selective, membrane-based ion separation technology may
prove useful in a wide range of applications such as water treatment, battery
recycling, ion specific chemical sensors, extraction of valuable metals from
sea water, and bio-medical devices. In this work we show that a flashing
ratchet mechanism can be used for high precision ion separation. The suggested
ratchet-based ion pumps utilize a unique feature of ratchets, the frequency
dependent current reversals, to drive ions with the same charge but different
diffusion coefficients in opposite directions. We show that for the prevalent
ions in water, ions with a relative diffusion coefficient difference as small
as 1% can be separated by driving them in opposite directions with a velocity
difference as high as 1.2 mm/s. Since the pumping properties of the ratchet are
determined by an electric input signal, the proposed ion pumps can pave the way
for simple large-scale, fit-to-purpose selective ion separation systems.
