(244d) High-Flux, High Capacity Adsorptive Membranes Based on Polysulfone and Block Polymer Composites

In the coming decades, the increasing global population, improved standards of living, and the expansion of irrigated agriculture will make meeting the demand for fresh water a challenge. One critical aspect of this challenge, is ensuring the adequate removal of heavy metal contamination that enters fresh water resources through a variety of mechanisms. The design and implementation of adsorptive membranes that target the capture of heavy metal ions is one potential strategy for helping to ensure a continuous supply of potable drinking water. Herein, we report a high-flux, high capacity adsorptive membrane fabricated from commercially-available polysulfone and block polymer materials. Through the appropriate selection of casting parameters, membranes with a bi-continuous network of pores ~1 µm in diameter were fabricated using a surface-segregation and vapor induced phase separation methodology. Compared with traditional ion-exchange resins, which are ~300 µm in diameter, the membranes exhibit reduced mass transfer limitations due to their uniform pore size and shorter diffusion length. After casting, hydraulic permeability measurements demonstrate that the as-cast parent membranes are pH-responsive with the hydraulic permeability ranging from 3.3×10⁴ L m^-2 h^-1 bar^-1 in a solution at pH 1 to 1.9 × 10⁴ L m^-2 h^-1 bar^-1 in a solution at pH 13. This behavior provides evidence that the poly(acrylic acid) (PAA) moieties of the block polymer segregate to the pore wall during membrane fabrication. Moreover, these PAA brushes allow the pore wall chemistry to be tailored for heavy metal removal via straightforward solid state coupling reactions. Specifically, the covalent attachment of metal ion chelating groups enables the highly efficient purification of simulated ground water or seawater solutions by capturing 99⁺% of the cations dissolved in them. In this effort, to develop an adsorptive membrane capable of efficiently removing metal ions under conditions where the contamination is present at trace concentrations (e.g., < 50 ppm), a tailor-made terpyridine group was incorporated along the pore wall of the membranes. The high metal binding affinity of the terpyridine moiety results in an adsorptive membrane that reaches its saturation capacity (1.2 mmol g^-1) at bulk ion concentrations less than 1 mM. The heavy metal binding performance of these membranes are further examined in a broad spectrum of ions and simulated background electrolyte conditions (e.g., deionized water, ground water or sea water) at different ion concentrations. Using the terpyridine-functionalized membranes, metal ion removals greater than 99% were observed in single stage equilibrium experiments. Additionally, the cation binding performance is coupled with a modest change in the hydraulic permeability for the composite membranes (2.8×10⁴ L m^-2 h^-1 bar^-1). With their readily tunable surface chemistry, these microfiltration membranes are a potential platform for the design and fabrication of membrane devices that can be implemented in advanced separation and purification applications.