Permselectivity limits of biomimetic desalination membranes

Abstract

Water scarcity and inadequate membrane selectivity have spurred interest in biomimetic desalination membranes, in which biological or synthetic water channels are incorporated in an amphiphilic bilayer. As low channel densities (0.1 to 10%) are required for sufficient water permeability, the amphiphilic bilayer matrix will play a critical role in separation performance. We determine selectivity limits for biomimetic membranes by studying the transport behavior of water, neutral solutes, and ions through the bilayers of lipid and block-copolymer vesicles and projecting performance for varying water channel densities. We report that defect-free biomimetic membranes would have water/salt permselectivities ~10⁸-fold greater than current desalination membranes. In contrast, the solubility-based permeability of lipid and block-copolymer bilayers (extending Overton's rule) will result in poor rejection of hydrophobic solutes. Defect-free biomimetic membranes thus offer great potential for seawater desalination and ultrapure water production, but would perform poorly in wastewater reuse. Potential strategies to limit neutral solute permeation are discussed.

Fig. 1. Projecting performance of biomimetic desalination membranes.

(A) To make a composite biomimetic selective layer, biological or synthetic water channels are incorporated into a fluid-like lipid or amphiphilic BCP bilayer that is supported by a porous substrate. The same materials can form defect-free LUVs, for which intrinsic permeabilities can be measured. (B) Hydraulic water permeability ${\mathit{P}}_{\mathrm{w}}^{\mathrm{H}}$ of a composite selective layer for given channel densities based on published single-channel permeabilities (8, 32, 50). Permeabilities were experimentally determined for all but modified cyclic peptide nanotube (mCPN), for which simulations were used (50). Depending on the channel type, densities of 0.1 to 10% will yield sufficient water permeability. CNTP, carbon nanotube porin; AQP1, aquaporin-1; AQPZ, aquaporin-Z.

Fig. 2. Permeability measurements using lipid and BCP vesicles.

(A) Cryo-EM images of MDM and PB-PEO LUVs prepared by extrusion through 200-nm pores. The vesicles are located near the edge of a carbon-coated copper microscopy grid. Small, irregular structures are artifacts of the ice structure and the carbon coating. From these and more images (fig. S1), the nonpolar core thicknesses of the MDM and PB-PEO bilayers were estimated to be 15 ± 2 and 13 ± 2 nm, respectively. (B) Water and (C) solute permeability measurements using osmolarity differences. Normalized LUV volume was determined via the self-quenching of encapsulated fluorophore upon exposure at 25°C to osmotic gradients induced by (B) relatively impermeable NaCl and (C) relatively permeable solutes. The data are averaged from 3 to 10 time traces and fitted (solid curves) using Eqs. 1 and 2 to determine water and solute permeabilities. LUV volumes change because of water transport that balances the osmotic pressures inside and outside of the vesicles. Solute permeability measurements in (C) are shown for MDM. (D) Acid influx measurements for DOPC at 6.4°C, showing intravesicular acid concentration based on decreased intravesicular pH and the pH sensitivity of encapsulated fluorophore. Data are averaged from 6 to 10 time traces and fitted (solid curves) using Eq. 5 to determine acid permeability. (E) Sodium efflux from LUVs at 25°C, shown as the cumulative change in extravesicular sodium concentration, as determined using a sodium-selective electrode (n = 3). Dashed lines are linear fits used in Eq. 6 to determine the sodium permeability. After 6 days, a surfactant (Triton X-100) was added to solubilize the vesicles and determine the total vesicle volume.

Fig. 3. Permeability trends of lipid bilayers, BCP bilayers, and conventional desalination membranes.

(A) Solubility-based permeability of fluid-like lipid and amphiphilic BCP bilayers. Water and solute permeabilities P (in m/s) were measured at 25°C (n = 3) and are compared with the octanol-water partition coefficient K_ow, a commonly used measure of solute hydrophobicity. Regression lines consider all species except for water because of its anomalously rapid permeation stemming from its small size (21, 51). The strong correlation between solute permeability and hydrophobicity matches and extends Overton’s rule, which was originally formulated for lipid bilayers (21). (B) Solute rejection, defined as 1 − C_permeate/C_feed, for a commercial PA-RO membrane (SW30XLE, Dow), measured at 15.5 bar and 25°C (n = 6). Rejection is size-based, with a molecular weight cutoff of ~80 Da, which leads to (C) a strong dependence of permeability (in m/s) on the solute diffusivity in water D (in m²/s), which decreases with increasing solute size.

Fig. 4. Projected performance of biomimetic membranes in seawater desalination.

Water/salt (top) and water/boron (bottom) permselectivities are calculated from measured bilayer permeabilities (solid symbols), bilayer permeabilities assuming increasing incorporation densities of AQPZ (lines and open symbols), and measured permeabilities (black circles) and manufacturer specifications (gray circles) (5, 52) for commercial PA-RO membranes. AQPZ is shown because of its frequent usage in biomimetic desalination membrane research. A single-channel water permeability of 2.9 × 10⁻¹³ cm³/s was used (table S1), adjusted from recently measured values (1.8 × 10⁻¹³ cm³/s at 5°C) (32) using an activation energy of 4 kcal/mol (42). Black isorejection lines are displayed to indicate the expected performance of the various materials and are calculated for salt (top) and boron (bottom) assuming standard seawater RO test conditions of 32,000–parts per million (ppm) NaCl, 5-ppm boron, and 55.2-bar applied pressure. R, observed rejection.

Fig. 5. Projected performance of biomimetic membranes in wastewater reuse.

(A) Calculated permselectivities for a PB-PEO–based biomimetic selective layer, assuming 3% incorporation of AQPZ. Because PB-PEO is the least permeable material considered, water/solute permselectivities are correspondingly the greatest. Permselectivities that correspond to the displayed approximate rejections (dashed lines) were calculated for 2000-ppm solute and 15.5-bar applied pressure. The approximate hydrophobicity cutoff of log K_ow = −0.2 is depicted by solid green and diagonal red shadings. (B) Physicochemical properties of various micropollutants (species that can adversely affect human or ecological health at low concentrations) that are relevant to wastewater reuse (–40). Nearly all of the micropollutants are relatively hydrophobic and are expected to be poorly rejected by a fluid-like biomimetic membrane. Conversely, almost all have molecular weights that exceed the molecular weight cutoff (80 Da) of the tested PA-RO membrane. Micropollutants shown are listed in table S4.