Sorbent platforms with structures tailored at the molecular through device scales are needed to meet the demand for separation systems that can isolate trace analytes from complex mixtures in an energy-efficient manner. Non-solvent induced phase separation is a common technology used to create thin polymer membrane-based sorbents, which are attractive due to their rapid mass transfer characteristics. However, these membranes suffer from relatively low capacities. While smaller pores and thicker membranes can increase the surface area and number of binding sites, these changes will inevitably decrease the permeability of the membranes. Addressing this tradeoff requires manufacturing methods that provide control over sorbent structures at different length scales. Here, direct-ink-writing 3D printing is combined with a surface-segregation and vapor-induced phase separation (SVIPS) process to create hierarchical sorbents that satisfy this criterion. Composite inks containing polysulfone, polystyrene-block-poly(acrylic acid), and carbon nanotubes were extruded to form woodpile structures with micrometer-scale channels between the printed filaments, resulting in sorbents with high hydraulic permeabilities. Simultaneously, because the printing occurred within a controlled humidity environment, the SVIPS process generates a fully-interconnected network of nanoscale pores within the filaments to form the sorbent matrix. In addition to creating this high nanoporosity, the SVIPS mechanism drives the PAA blocks to the surface of the pore walls, thereby allowing the sorbent chemistry to be further tailored. Sequential carbodiimide coupling reactions were used to introduce branched-polyethylenimine and then terpyridine functionality, which increased the density of binding sites and affinity towards transition metal ions, respectively. The binding capacity increased linearly with the number of layers, which indicated solute capture was achieved throughout the whole sorbent. Remarkably, a 24-layer Terp-functionalized sorbent had a capacity of 30 mmol m–2 towards Cu2+ and exhibited selective recovery of Co2+ from mixtures of cobalt and lithium. Finally, dynamic flowthrough experiments demonstrate the efficient removal of copper. In conclusion, incorporating the 3D printing technique with SVIPS casting enables the creation of sorbents with well-controlled hierarchical structures that achieve high binding capacity, affinity, and permeability simultaneously.

High-affinity heavy metal removal using structured sorbents 3D-printed from carbon nanotube-enriched polymer solutions

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ACS Fall 2022

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