Anthony P. Straub

Pressure-driven distillation (PD) is a membrane-based water treatment process that uses applied pressure to drive vapor transport through a porous hydrophobic membrane. PD offers potential advantages in selectivity and chemical resilience, but its performance with feedwaters containing foulants remains poorly understood. In this study, we examine the organic fouling behavior of PD, isolate the effect of pressure on flux decline, and explore potential fouling mitigation strategies. We find the severity of organic fouling is highly dependent on foulant properties, with more hydrophobic humic acid causing the most severe flux decline and bovine serum albumin proteins causing moderate fouling. Sodium alginate, a hydrophilic gel-forming polysaccharide, showed the least severe fouling despite higher surface coverage. Even after severe flux decline, PD membranes do not show signs of wetting or a loss of rejection. We find that the applied pressure in PD exacerbates fouling by comparing performance in PD to osmotic distillation, an unpressurized analogous process. Finally, we evaluate two fouling mitigation strategies: cleaning with sodium hydroxide, which restores nearly full flux, and continuous direct ozonation, which partially suppresses fouling. Overall, this work provides insights into organic fouling behavior in PD and demonstrates effective fouling mitigation strategies.

The integration of catalysts onto the surface of membranes enables simultaneous physical separation and catalytic transformation of constituents in a feed stream, facilitating improved contaminant removal and fouling mitigation. Distillation membranes are a particularly attractive platform for catalytic membranes because they reject nonvolatile species and exhibit exceptional resistance to oxidative and radical-driven degradation. However, imparting catalytic functionality onto hydrophobic, porous distillation membranes has proven challenging since the membranes used are chemically inert and difficult to modify. Furthermore, catalysts on the membrane surface can decrease hydrophobicity and increase the membrane's susceptibility to pore wetting and failure. In this work, we create a catalytic distillation membrane by coating a polytetrafluoroethylene membrane surface with titanium dioxide (TiO₂) via plasma-assisted atomic layer deposition (ALD). By precisely tuning the ALD parameters, we demonstrate localized growth of TiO₂ near (within approximately 1 μm) the surface of polytetrafluoroethylene membranes, forming a catalytically active interface while preserving the underlying hydrophobic pore structure. Localized growth of TiO₂ is confirmed by electron microscopy and spectroscopy techniques, and membranes coated with 500 cycles of ALD show pressure tolerance up to 12.8 bar and higher than 95% salt rejection in pressure-driven distillation. Photocatalytic activity is demonstrated via the degradation of methylene blue dye under UV irradiation, where increasing TiO₂ loading leads to an enhancement in dye degradation. These results establish a general strategy for integrating catalytic functionality into chemically inert, hydrophobic membranes without compromising distillation performance, providing a pathway toward multifunctional membranes that couple advanced oxidation with membrane separation for water treatment.

The structural diversity and complex transport behavior of per- and polyfluoroalkyl substances (PFAS) complicate a universal characterization of their removal in membrane systems. This study compiles 2353 data points from the literature on PFAS rejection by nanofiltration and reverse osmosis membranes, spanning a broad range of PFAS, membranes, feedwater compositions, and operating conditions. Using machine learning, this data set is modeled to evaluate how solute, membrane, and solution properties jointly influence PFAS removal. Of the 13 experimental system descriptors analyzed, membrane water permeance and PFAS molecular volume demonstrated the strongest main effect on rejection, emphasizing the significance of steric exclusion. The effects of background ions and dissolved organic matter were highly condition-dependent, exhibiting nonmonotonic behavior governed by competing mechanisms. Low concentrations of organic matter and ions enhanced PFAS rejection, consistent with complexation that increases apparent solute size, while higher concentrations reduced PFAS rejection, indicating that charge shielding and concentration polarization increasingly drive transport behavior. Overall, this analysis provides a unified, data-driven framework for interpreting previously inconsistent findings across studies, identifies critical gaps in existing experimental data, and highlights opportunities to guide targeted membrane design and treatment strategies.

Efficient gas separation membranes are essential for carbon capture, biogas upgrading, and hydrogen purification. Inspired by how plants absorb CO ₂ through water, we present a membrane platform that uses liquid water as the selective layer. Hydrophilic sub-100-nm pores stabilize water through strong capillary forces, enabling operation at feed pressures above 72 bar under dry and humid conditions. Selectivity is governed by gas solubility in water, while permeance is tuned by adjusting the water layer thickness. Reducing this thickness below 200 nm yields CO ₂ permeances up to 11,600 gas permeation units with CO ₂ :N ₂ , CO ₂ :CH ₄ , and CO ₂ :H ₂ selectivities of 40, 26, and 31, respectively, surpassing the performance of state-of-the-art membranes. Operation is sustained for over a week without water loss, and performance scales using commercially available porous polymer supports under mixed-gas crossflow conditions. Water’s dissolution-based transport avoids saturation and reaction-rate limits, enabling a robust, high-performance, and environmentally benign gas separation platform.

222 nm irradiation, emitted from krypton chloride excimer (KrCl*) lamps, of waters containing nitrate has previously been demonstrated to lead to de facto advanced oxidation, due to the high absorbance and hydroxyl radical quantum yield of nitrate at 222 nm. Because nitrate is widespread in environmental waters, including nitrified wastewaters and wastewater-impacted surface water, this raises the possibility of UV advanced oxidation, which harnesses in situ nitrate for radical generation when KrCl* lamps are used as the UV source. In turn, this process is effective directly in wastewater effluent without the addition of chemicals. In this study, we compare UV222 treatment, without the addition of an external radical promoter, to UV254 with 10 mg/L hydrogen peroxide (H₂O₂) for the removal of 1,4-dioxane in three effluent waters containing nitrate at 2.1-8.9 mg-N/L. UV222 leads to 2.8-3.9 faster 1,4-dioxane degradation, on an incident fluence basis, resulting in an electrical consumption that is within 0.89-1.3 times that of UV254/H₂O₂, despite low-pressure mercury vapor lamps having at least 3.5× higher electrical efficiency than KrCl* lamps. When the cost of hydrogen peroxide dosing and quenching is considered, UV222 treatment leads to a projected cost savings of 12.4-21.8% compared to UV254/H₂O₂. Furthermore, despite the low UV transmittance (UVT) of these effluent waters at 222 nm (3^•- and NO₂^•, can contribute significantly to the degradation of micropollutants such as pharmaceuticals and consumer products.

We developed pressure-resistant omniphobic membranes that enable stable distillation of low-surface-tension liquids at applied pressures exceeding 15 bar. Membranes were synthesized by grafting re-entrant nanostructures onto porous alumina membranes, followed by hydrophobic modification. The membranes exhibited a high liquid entry pressure of 36.2 bar with water and withstood an applied pressure up to 15.5 bar with a low-surface-tension 15 wt % ethanol-water mixture. Simulations revealed that the enhanced wetting resistance is due to the presence of re-entrant structures, which facilitated a 220% increase in wetting pressure for the low-surface-tension liquid compared to a control membrane with cylindrical pores. We further demonstrated stable pressure-driven distillation of low-surface-tension liquids, achieving higher than 97% salt rejection. This work is the first demonstration of distillation membranes operating with low-surface-tension liquids under high applied pressures and provides critical validation of wettability theory under extreme pressures.