(313c) Designing High-Performance Block Polymer-Based Membrane Adsorbers for Water Purification

Authors: 
Boudouris, B. W., Purdue University
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. Here, we will discuss two block polymer design strategies to address this opportunity. In the first example, we describe the synthesis of a high molecular weight triblock polymer, polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) using a reversible addition-fragmentation chain transfer (RAFT) polymerization scheme. In this way, we are able to generate macromolecules with tunable molecular weights and block polymer compositions while retaining relative narrow molecular weight distributions. Then, these A-B-C triblock polymers are cast into nanoporous thin films, using standard industrial methodologies, to produce nanostructured membranes. Importantly, the self-assembly and non-solvent induced phase separation (SNIPS) casting method is utilized. This results in a membrane with well-defined, nanoscale pores at the upper surface of the membrane that quickly taper into microscale pores in a rapid manner. This tailored structure affords a membrane with both high flux and high selectivity. Moreover, we demonstrate that the PDMA moiety that lines the pore walls of this nanoporous film can be hydrolyzed to poly(acrylic acid) (PAA) in a ready manner. Once complete, the PAA functionality allows for the removal of the copper cations of copper(II) chloride from solution. This removal occurs through a specific chemical interaction between the PAA chains and the Cu2+ species; therefore, the adsorption of the species is rather large (4.1 mmol Cu2+ g-1 membrane). Furthermore, we demonstrate that this specific chemical interaction allows for the selective removal of Cu2+ ions from a mixture of these cations and Ni2+ cations despite the relatively similar size of the two species in solution. In the second example, the self-assembly and vapor-induced phase separation (SVIPS) casting procedure is utilized to create 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 bicontinuous network of pores ~1 µm in diameter were created. 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 these membranes, hydraulic permeability measurements demonstrate that the as-cast parent membranes exhibit pH-responsive hydraulic permeability values. This behavior provides evidence that the 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. 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. These efforts demonstrates that the appropriate design of block polymers can lead to high-performance membrane adsorbers that serve as platform separation materials for high-fidelity water purification applications.