Ultrafast ion sieving using nanoporous polymeric membranes

The great potential of nanoporous membranes for water filtration and chemical separation has been challenged by the trade-off between selectivity and permeability. Here we report on nanoporous polymer membranes with an excellent balance between selectivity and permeability of ions. Our membranes are fabricated by irradiating 2-μm-thick polyethylene terephthalate Lumirror® films with GeV heavy ions followed by ultraviolet exposure. These membranes show a high transport rate of K+ ions of up to 14 mol h−1 m−2 and a selectivity of alkali metal ions over heavy metal ions of >500. Combining transport experiments and molecular dynamics simulations with a polymeric nanopore model, we demonstrate that the high permeability is attributable to the presence of nanopores with a radius of ~0.5 nm and a density of up to 5 × 1010 cm−2, and the selectivity is ascribed to the interaction between the partially dehydrated ions and the negatively charged nanopore wall.

chamber is filled with deionized (DI) water (right). b. I-V curve of ionic conductance across the PET Lumirror ® films (irradiated with 5×10 10 Bi ions cm -2 and 4-hour UV radiation) measured with 0.1 M KCl solution in both chambers. No notable rectification effect was observed as the curve is nearly symmetric. It shows a typical activated shape, i.e., the slope increases as V increases, suggesting the transport is related to barriers such as the interaction between the ions and the pore wall or dehydration. c. Ion conductance (circles) across the PET Lumirror ® film (0.1 M KCl solution and applied voltage 1 V) as a function of UV exposure time. The dashed line is a guide for the eye. d. Ionic transport rates (red squares) across the PET Lumirror ® film (0.1 M KCl solution, 10 V applied voltage) as a function of irradiation fluence. For UV-treated samples, the conductance increases linearly over the entire range from 5×10 9 to 5×10 10 ions cm -2 . e. Ionic transport rates versus the KCl electrolyte concentration (applied voltage 10 V) following a power law relationship (exponent factor=1.07, R 2 =0.986). f. Selectivity of K + /Mg 2+ as a function of pH value. The transport rates of Mg 2+ ions and K + ions through the PET Lumirror ® films were measured with feed solutions of MgCl2 (1 M) and KCl (1 M) at an applied voltage of 5 V. The films used in the measurements were irradiated with 1.4 GeV Bi ions at a fluence of 5×10 10 ions cm -2 and subsequently exposed to UV radiation for 4 hours. g. Evaporation rates of water during the transport measurements. The dashed line is a linear fit of the water height reduction (R 2 =0.997) measured by means of a capillary inserted into the conductivity cell. The water evaporation rate, determined from the slope, is about 0.456 mm h -1 , which is corresponding to 25.33 mol h -1 m -2 .  & The films used in the measurements were irradiated with 1.4 GeV Bi ions at a fluence of 1×10 10 ions cm -2 and subsequently exposed to 4-hour UV radiation.

Supplementary Table 4. Comparison of UV absorbance and absorption coefficients of PET Lumirror ® films and PET Hostaphan ® films irradiated with different ion fluences at the UV wavelength of 365 nm.
PET  18 0.4417 0.49280 -0.75 & σ and ε are two key parameters to calculate the Lennard-Jones interaction, q is the charge of an ion.

Supplementary Note 1 Water transport and evaporation measurements through the nanoporous PET
Lumirror ® membranes.
The water transport experiments were performed using a sealed permeation measurement apparatus (Supplementary Fig. 1a). The 0.01 M KCl solution and deionized water were injected into the feed and permeate chamber, respectively. Then two capillaries (length=100 mm, inner radius r=0.5 mm, Huaxi Medical University Instrument Factory) were sealed in the two chambers separately. An external bias voltage of 10 V was applied between the two chambers, and the change of the liquid surface in the capillary was recorded as a function of time. As a control, the water evaporation rate was measured in the same way with no applied voltage. The measurements were conducted at room temperature.  (Fig. 2d). The slopes of the lines represent respective transport rates, i.e., the average amount of water and ions transported per square meter per hour separately, or the water and ion transport rates. Thus, the average ratio of the transported water molecules over ions is n(H2O) / n(ion) =6.3.

Supplementary Note 2 UV absorption coefficient calculation
The absorbance was recorded in the UV-visible spectrum measurements. The absorption coefficient α is calculated with the formula 19