Silica Based Microfiltration Membranes: Derivatization and Functionalization for Dissolved Metal Capture
D. Bhattacharyya (1), S. Ritchie (1), J. Hestekin (1), and L. G. Bachas (2)
(1) Dept. of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506-0046 (2) Dept. of Chemistry, University of Kentucky
Silica based membranes are widely used for microfiltration of particles. However, because of their excellent physical properties such as high internal surface area (80 m2/g), acid resistance, and mechanical strength, their pore surface may also be used as a substrate for attaching polymers containing highly selective functional groups. If these groups are capable of ionic and /or chelation interactions, then dissolved metal species may be captured from aqueous solutions. In this study, anionic polypeptides, such as poly-glutamic and poly-aspartic acid, have been attached to silica based membranes. These membranes have subsequently been used to capture heavy metals, such as Pb, Cd, Cu, from aqueous solutions.
Preparation of the silica based (polyethylene matrix, 70-80% silica) microfiltration (0.1 micron pore size) membranes was accomplished by a two-stage process: derivatization to attach a mono-functional epoxide group and functionalization to attach the poly-amino acids. Derivatization was achieved by silylation of the pretreated membrane with an epoxide-containing trialkoxysilane in xylene. Silylation has been achieved both by diffusion over 24 hours, and by convection over a much smaller time period. Functionalization (in aqueous media), achieved convectively, involved reaction of the terminal amine group of the poly-amino acid with the silica matrix containing epoxide groups. Since poly-glutamic (MW 12-36 K) and poly-aspartic (MW 10-36 K) acids were used, there is only one amine group per molecule, thereby allowing for single point attachment.
Once prepared, the membranes were contacted with heavy-metal laden (25-1,000 mg/L) aqueous solutions at a pH of 5-6. The flux for each convective process was around 0.02 m3/(m2)(hr) at 0.3-1 bar. Metal sorption results were reported on three bases: grams sorbed metal/gram membrane, moles sorbed metal/mole repeat unit (carboxylic acid), and milligrams sorbed metal/cm2 membrane external area. Comparable studies have been performed on poly-amino acid functionalized cellulosic microfiltration membranes. Because of the higher density of silica, our highest achieved sorption of 0.4 g Pb/g membrane is lower than the observed >1 g Pb/g cellulosic membrane. However, since silica based membranes have a high internal surface area (about 10 times that of cellulosic membrane), our highest observed capacity of 4.9 mg/cm2 was about twice that observed for cellulosic membranes. For the silica membranes used a molar sorption ratio of 1 mole Pb/mole COOH was obtained. It should be noted that regeneration studies on Pb sorbed silica membranes have yielded little recovery (at pH 3), while similar Cd regeneration studies on other membranes have shown nearly 100% recovery. This opens the possibility of selective recovery of valuable metals. The metal capture results can be explained in terms of polypeptide helix-coil transition and electrostatic interactions. This project was funded by the US EPA and DoD.