Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution.

Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute-surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute-surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil-water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute-surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity-suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute-surface interactions.

Evidence for multiple removal pathways in low-temperature (200-400 °C) thermal treatment of pentachlorophenol-laden soils.

Polychlorinated aromatic compounds (PCACs) pose significant remediation challenges, since their high soil affinities preclude mobile-phase partitioning and subsurface extraction. To enhance partitioning and desorption, subsurface temperatures are raised using a technique called thermal conduction heating-soil vapor extraction (TCH-SVE). While this technique improves PCAC partitioning, it can also promote several degradation reactions under temperatures typical of low-temperature TCH-SVE (200-400 °C). While these reactions are labile, the extent to which they occur in flow-through TCH-SVE is unclear. The current research used bench-scale, flow-through TCH-SVE to assess relative importance of three removal pathways: (1) target volatilization, (2) reductive dechlorination, and (3) oxidation via OH-addition. Pentachlorophenol was used as a representative PCAC, and pathway contributions, extents, and regioselectivity were examined as a function of temperature (225-375 °C) and gas-phase oxygen content (air vs. nitrogen). Across treatments, OH-addition and dechlorination occurred in parallel and accounted for significantly more removal than PCP volatilization. OH-addition byproducts had highest yields (regardless of oxygen content) and were consistent with surface-mediated OH production and ring addition. OH-addition increased with temperature while volatilization and dechlorination decreased. Notable exceptions occurred between 225 and 325 °C (where dechlorination dropped 10-fold) and 325 and 375 °C (where OH-addition fell 75%), signifying major mechanism shifts in these intervals.

Water Treatment: Are Membranes the Panacea?

Alongside the rising global water demand, continued stress on current water supplies has sparked interest in using nontraditional source waters for energy, agriculture, industry, and domestic needs. Membrane technologies have emerged as one of the most promising approaches to achieve water security, but implementation of membrane processes for increasingly complex waters remains a challenge. The technical feasibility of membrane processes replacing conventional treatment of alternative water supplies (e.g., wastewater, seawater, and produced water) is considered in the context of typical and emerging water quality goals. This review considers the effectiveness of current technologies (both conventional and membrane based), as well as the potential for recent advancements in membrane research to achieve these water quality goals. We envision the future of water treatment to integrate advanced membranes (e.g., mixed-matrix membranes, block copolymers) into smart treatment trains that achieve several goals, including fit-for-purpose water generation, resource recovery, and energy conservation.