Novel Ceramic-Polymer Composite Membranes for the Separation of Liquid Waste

Abstract

There is a growing need in the areas of hazardous waste treatment, remediation and pollution prevention for new processes capable of selectively separating and removing target organic species from aqueous steams. Membrane separation processes are especially suited for solute removal from dilute solutions. They have the additional advantage of requiring less energy relative to conventional separation technologies (e.g., distillation, extraction and even adsorption processes). The major difficulty with current membranes is the poor longevity of polymeric membranes under harsh conditions (high temperature, harsh solvents and pH conditions) and the lack of selectivity of ceramic membranes. In our previous work (1996 EMSP project), a first generation of novel polymer-ceramic (PolyCer) composite membranes were developed with the goal of overcoming the above difficulties. The proposed PolyCer membranes are fabricated by a surface-graft polymerization process resulting in a molecular layer of polymer chains which are terminally and covalently anchored to the porous membrane support. The polymer imparts the desired membrane selectivity while the ceramic support provides structural integrity. The PolyCer membrane retain its structural integrity and performance even when the polymer phase is exposed to harsh solvent conditions since the polymer chains are covalently bonded to the ceramic support surface. To date, prototype PolyCer membranes were developed for two different membrane separation processes: (a) pervaporation removal of organics from aqueous systems; and (b) ultrafiltration of oil-in-water emulsions. Pervaporation PolyCer membranes were demonstrated for removal of selected organics (TCE, chloroform and MTBE) from water with permeate enrichment factors as high as 300. While the above results have been extremely encouraging, higher enrichment factors (>1000) should be sought for field applications. The above improvement is feasible by increasing the length and surface density of the grafted polymer chains. The required simultaneous increase in surface polymer graft density and chain length is beyond the capability of present free-radical graft polymerization methods. Therefore, it is proposed to develop a new approach to synthesizing the grafted polymer membrane phase via ''living'' free-radical polymerization. This approach should allow controlled growth of the grafted polymer chains while maintaining the advantage of high surface chain density possible with conventional free-radical polymerization. Optimization of the membrane surface layer will be sought by developing fundamental correlation between surface characteristics (e.g., topology, chain length and surface density) and membrane performance. The ability to tailor-design the grafted polymer surface with long polymer chains of a desired surface density is also advantageous in fabricating non-fouling ultrafiltration membranes for colloidal filtration. Using th e same ''living'' free-radical polymerization technology, as for the pervaporation membranes, ultrafiltration ceramic membranes with terminally anchored surface chains, can be produced to repel colloidal species, thus reducing membrane fouling while increasing permeate rejection. As an outcome of the 1996 EEMSP project, it was discovered that, with sufficiently long surface chains, significant increase in PolyCer UF membrane rejection is possible, especially at high tangential velocities. The fabrication, via ''living'' free-radical polymerization, and optimization of such non-fouling UF membranes is another goal of the proposed research. It is expected that this project will results in the demonstration of a commercially viable technology for the ''tailor design'' and optimization of a new class of selective and robust polymer-ceramic (PolyCer) membranes for aqueous waste treatment and water decontamination applications. The proposed PolyCer approach will allow the rapid deployment of ''field-ready'' and task-specific membranes for recovery and recycle for remediation and pollution prevention applications.