Functionalized membranes and environmental applications

The development of separation processes with reduced energy consumption and minimal environmental impact is critical for sustainable operation. Membrane processes provide a highly flexible separation technique for selective solute separation/concentration, and permeate recycling and reuse (Bhattacharyya et al. 2003; Ho et al. 1992; Bhattacharyya and Shah 2005). The special features for membrane processes that make them attractive for industrial applications are their compactness, ease of fabrication, operation, and modular design (Meyer et al. 2006). Although membrane processes such as, Reverse Osmosis, Nanofiltration, Ultrafiltration, and Microfiltration have provided many successful applications ranging from highquality water production to material recovery, but incorporation of biological aspects in membranes should add immense value in the area of separations and material capture. The questions are, Can we make membranes which can capture toxic metals at high capacity? Can we add functional moiety in pores which can go through conformation changes and results in tunable separations? Can we make layer-by-assembly in pores to immobilize proteins and enzymes? Can we synthesize nanosized metals in membranes to detoxify toxic organics at room temperature? The answers to all of these are yes if we can integrate life science field with synthetic membranes to make tunable and functionalized materials. Membranes functionalized with appropriate macromolecules (Decher 1997; Hollman and Bhattacharyya 2002, 2004; Ritchie et al. 1999; Ito et al. 2000; Smuleac et al. 2005) can indeed provide applications ranging from tunable flux and separations, toxic metal capture, to nanoparticle synthesis for toxic organic dechlorination. If the selected macromolecule is a biomolecule, such as, polypeptide (for e.g., poly-glutamic or poly-aspartic acid) then in addition to creating a highly charged field in the membrane pores, conformational changes (such as, helix-coil) can be utilized to conduct tunable nanofiltration type separations or metal capture at high capacity with macroporous membranes at low pressures. The metal sorption results indicated not only high capacity but also rapid uptake rate. Metal sorption with polypeptide functionalized membranes is extremely high (>1 g metal/g for Pb) compared with conventional ion exchange (50–100 mg/g). Helix-coil formation ability of the attached polypeptides can also be used for selective metal regeneration. We have extended the approach to create pore-assembled charged multilayers for nanofiltration-type separations. In addition, one can fabricate charged multilayer assemblies inside of the membrane pores for subsequent incorporation of enzymes for biocatalysis. Traditionally, microfiltration (MF) membrane separations have been performed on the basis of size exclusion and are typically used for filtration of suspended solids, bacteria, viruses, etc. However, microfiltration membranes (e.g., cellulosics, silica, polysulfone, polycarbonate) can be functionalized with a variety of reagents. Depending on the types of functionalized groups (such as, chain length, charge of groups, biomolecule, etc.) and number of layers, these microfiltration membranes could be used in applications ranging from metal (or oxyanions) separation to biocatalysis. The conformation properties dependence on pH and provide tunable separation properties (Hollman and D. Bhattacharyya (&) Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA e-mail: db@engr.uky.edu