Conical Nanopores for Efficient Ion Pumping and Desalination.

Previous experimental and theoretical studies have demonstrated that nanofabricated synthetic channels are able to pump ions using oscillating electric fields. We have recently proposed that conical pores with oscillating surface charges are particularly effective for pumping ions due to rectification that arises from their asymmetric structure. In this work, the energy and thermodynamic efficiency associated with salt pumping using the conical pore pump is studied, with emphasis on pumps needed to desalinate seawater. The energy efficiency is found to be as high as 0.60 to 0.83 mol/kJ when the radius of the tip side of the conical pore is two Debye lengths and the pump works with a concentration gradient smaller than 1.5. As a result, the energy consumption needed for seawater desalination with 20% salt rejection is 0.32 kJ/L. In addition, the energy consumption can be further reduced to 0.21 kJ/L (20% salt rejection) if the bias voltage is adaptively altered four times during the pump cycle while salt concentration is reduced. If the bias voltage is adaptively increased to higher values, then salt rejection can be improved to values that are needed to produce fresh water that satisfies standard requirements. Numerical analysis indicates that the energy consumption is 4.9 kJ/L for 98.6% salt rejection, which is smaller than the practical minimum energy requirement for RO-based methods. In addition, the pumping efficiency can be further improved by tuning the pump structure, increasing the surface charge, and employing more adaptive bias voltages. The conical pores are also found to more efficiently counteract the concentration gradient compared to cylindrical counterparts.