The removal of toxic metals from organic and aqueous sources is an important process for both industry and the environment. There are currently few membrane separation techniques that target metal ions. Our research involves trying to produce a polymeric equivalent of crown ethers, which bind readily to metals, and cast them in the form of a membrane. Once suitable membranes are produced they will be used in flowthrough applications as well as in facilitated transport.
The impetus ofthis research is based on industry's need for the removal toxic metals for hydrocarbon condensates as well as from waste water streams. Transition metals such as mercury, arsenic, and vanadium are present in hydrocarbon condensates and have the ability to poison catalysts used in refining. Further support comes from agencies with environmental concerns. Removal of toxic metals such as cadmium, nickel. chromium from ground water and other contaminated sources would prove to be a valuable process.
Crown ether have excellent binding properties with metals including good selectivity. Crown ethers themselves are hard to work with on industrial scales because they are expensive, and toxic. Recovery of all of the crown ethers is very important both from an economic point of view as well as for safety. Attempts have been made to incorporate them into polymeric hosts to avoid the need for recovery from liquids, but they have proven difficult at best. Our solution involves forming crown ether-like substituents on a polymer backbone, in situ. A copolymerization reaction involving a monofunctional and a difunctional acrylate is used to form pseudocrown ethers. By using a templating ion to chelate poly(ethylene glycol) diacrylate, intramolecular cycles can be formed that are connected to a backbone of hydroxyethyl methacrylate. The polymerization must take place diluted with a non-polar solvent. The dilution increases the probability that the poly(ethylene glycol) diacrylate will sell-cyclize versus forming a crosslink. The solvent must be non-polar so that it will not dissolve that templating ion away from the difunctional acrylate. In a concentrated solution the diacrylate would be more likely to crosslink. Some crosslinking is desired for mechanical stability. The poly(ethylene glycol) diacrylate -co- hydroxyethyl methacrylate polymer does not have suitable mechanical properties to form a membrane without the use of a support. For this purpose a high density poly(ethylene) support with 70% porosity is used. The polymerization takes place on the surface of this support and the added copolymer forms inside the pores and on the surface of the membrane. The photoinitiator 2,2-dimethoxy-2acetophenone and ultra violet light is used to polymerize the acrylates.
Additional monomers that contain nitrogen or sulfur, instead of oxygen, will also be investigated because it is know that nitrogen and sulfur analogs to crown ethers have better binding constants for transition metals.
The membranes will be tested in two different systems. The first is simply a dead end filtration. A finite volume of feed is forced completely through the membrane. Sample aliquots are taken from the permeate side and analyzed for metal content using Inductively Coupled Plasma (ICP). Targeted feed concentrations are approximately 50 to 100 PPM, and the desired permeate concentrations are below 1 PPM. The second membrane system is facilitated transport. This technique exploits the fact that the binding constant is a function of pH. The metal ion binding is lower at lower pH. If the feed side of the membrane is maintained at a higher pH than the sweep side, the metal ions could theoretically be pumped against the overall concentration gradient. This would be because the local concentration gradient in the membrane would favor ion transport in that direction.
The ability to produce pseudocrown ether membranes would be an economical and safe means of removing metals from organic and aqueous sources. It would provide a way to avoid catalyst poisoning in the petrochemical industry, as well as offer new methods for environmental clean up and prevention of water contamination.