Selective permeation up a chemical potential gradient to enable an unusual solvent purification modality

Abstract

The need for energy-efficient recovery of organic solutes from aqueous streams is becoming more urgent as chemical manufacturing transitions toward nonconventional and bio-based feedstocks and processes. In addition to this, many aqueous waste streams contain recalcitrant organic contaminants, such as pharmaceuticals, industrial solvents, and personal care products, that must be removed prior to reuse. We observe that rigid carbon membrane materials can remove and concentrate organic contaminants via an unusual liquid-phase membrane permeation modality. Surprisingly, detailed thermodynamic calculations on the chemical potential of the organic contaminant reveal that the organic species has a higher chemical potential on the permeate side of the membrane than on the feed side of the membrane. This unusual observation challenges conventional membrane transport theory that posits that all permeating species move from high chemical potential states to lower chemical potential states. Based on experimental measurements, we hypothesize that the organic is concentrated in the membrane relative to water via favorable binding interactions between the organic and the carbon membrane. The concentrated organic is then swept through the membrane via the bulk flow of water in a modality known as “sorp-vection.” We highlight via simplified nonequilibrium thermodynamic models that this “uphill” chemical potential permeation of the organic does not result in second-law violations and can be deduced via measurements of the organic and water sorption and diffusion rates into the carbon membrane. Moreover, this work identifies the need to consider such nonidealities when incorporating unique, rigid materials for the separations of aqueous waste streams.

Structure–Transport Relationships of Water–Organic Solvent Co-transport in Carbon Molecular Sieve (CMS) Membranes

“…Despite the molecular size difference between water (2.6 Å) and p -xylene (5.8 Å), the PVDF-CMS membranes exhibited selectivity toward p -xylene and behaved as organic concentrating membranes in water–organic separation. This type of behavior was also recently observed in the liquid phase for the separation of water and dimethylformamide …”

“…A more simplified equation was employed to estimate the fluxes of more complex membrane separation modalities, such as RO and PV, in which the driving forces and boundary conditions are more complicated than those found in vapor permeation. The SD flux equation for a diffusive molar flux (J i ) (eq 6) was derived 23 from a simplified MS expression (eq 5).…”

“…28 fiber membranes. This study utilizes a polymer of intrinsic microporosity-1 (PIM-1) as the precursor due to its known successful fabrication into hollow fiber membranes for liquid separation 13,23,30 and its versatility in adjusting the microstructure through adjustments in the pyrolysis atmosphere. 13 Previous work 22 investigated the transport of water and pxylene vapors in polyvinylidene fluoride (PVDF)-derived CMS membranes using a sorption-diffusion (SD) transport model.…”

“…Carbon molecular sieves (CMS) have shown great potential as high-performance membrane materials in gas and organic solvent separation due to their thermal and chemical stability as well as scalability. − Various types of precursors including polyimides , and cellulose-based polymers , have been investigated for the CMS membranes for gas separations. Recent studies have also highlighted the potential of CMS membranes for treating dissolved organics in water via an organic selective separation. , A critical advantage of CMS as a membrane material is that its pore size distribution (PSD) can be precisely engineered by altering the polymer precursors and pyrolysis conditions. − CMS membranes offer tailored rates of guest molecule mobility within the microporous environment, offering the potential to address specific separation challenges encountered in wastewater remediation. CMS materials are amorphous and carbonaceous, with a rigid microporous structure derived from a pyrolyzed polymeric precursor.…”

“…The processability of the polymer precursor allows CMS membranes to be manufactured in commercially relevant form factors, including hollow fiber membranes. This study utilizes a polymer of intrinsic microporosity-1 (PIM-1) as the precursor due to its known successful fabrication into hollow fiber membranes for liquid separation ,, and its versatility in adjusting the microstructure through adjustments in the pyrolysis atmosphere …”