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.

In membrane-based separation, molecular size differences relative to membrane pore sizes govern mass flux and separation efficiency. In applications requiring complex molecular differentiation, such as in natural gas processing, cascaded pore size distributions in membranes allow different permeate molecules to be separated without a reduction in throughput. Here, we report the decoration of microporous polymer membrane surfaces with molecular fluorine. Molecular fluorine penetrates through the microporous interface and reacts with rigid polymeric backbones, resulting in membrane micropores with multimodal pore size distributions. The fluorine acts as angstromscale apertures that can be controlled for molecular transport. We achieved a highly effective gas separation performance in several industrially relevant hollow-fibrous modular platform with stable responses over 1 year.

Existing polymeric membranes struggle to separate small molecule solvents in the liquid phase due to low selectivity from solvent-induced plasticization and dilation. Mixed matrix membranes (MMMs) can potentially alleviate this issue via diffusion-based separations within rigid framework materials. Previous work from our lab and others has shown that organic solvent reverse osmosis membranes have different responses to transmembrane pressure depending on whether the material is a rigid structure (e.g., a carbon, zeolite, or metal-organic framework) or a swollen polymer. This work combines two Maxwell−Stefan transport models, representing the flexible polymer phase and a rigid microporous filler, with the Maxwell model to predict mixed matrix membrane solvent separation performance as a function of pressure and membrane material properties. The model demonstrates that for every filler perm-selectivity, there is a filler permeability that provides the largest separation factor in the final MMM. This optimum permeability increases with the filler's perm-selectivity. Dual-layer UiO-66/Matrimid hollow fiber MMMs were created to evaluate the model's prediction on the influence of transmembrane pressure on the separation of toluene and mesitylene as a test case. The UiO-66/Matrimid membrane demonstrated a predicted decline in permeance as pressure was increased. The separation factors increased as higher pressures increased the driving force for separation, consistent with the model. UiO-66 was shown to have superior selectivity to Matrimid in toluene/mesitylene; however, we conclude that ultraselective materials are ultimately needed to enable the mixed matrix membrane concept for the most challenging solvent− solvent separations, and open questions remain about polymer−filler pairings for organic solvent reverse osmosis.