Development of Anti-biofouling Nanocomposite Polypropylene Fibers for Membrane Feed Spacers Membrane technologies offer great promise to meet increasingly stringent regulatory requirements for potable water production. While other technologies can achieve similar treatment objectives, membranes offer notable advantages. Nanofiltration (NF) and reverse osmosis (RO) membranes have made alternative water reclamation (i.e., brackish water and seawater) and wastewater reuse possible solutions to address the growing global scarcity of traditional water sources. Implementation of NF and RO processes in treating traditional water sources can provide a steady-state level of removal that eliminates the need for regeneration of ion exchange resins or granular activated carbon. Moreover, RO can help meet future potable water demands through desalination of seawater and brackish waters. Although NF and RO membranes are generally not intended for disinfection, they provide an additional barrier for virus and bacteria removal, which is essential for indirect potable, wastewater reuse. An optimistic view for the future of membrane technology must be tempered, however, by recognition of the technical and cost issues that remain to be addressed. Of these issues, the fouling of membranes by chemicals and microbes that are rejected continues to demand considerable attention. The focus of this project is microbial fouling, or biofouling, which is the accumulation of microorganisms onto the membrane surface and on the feed spacer as present between the envelopes (i.e. spiral wound elements, in which the membrane is folded over a polypropylene spacer attached to a center tube). Most of the research and development in the area of biofouling prevention has focused on pretreatment of the feed water, improved cleaning solutions, cleaning procedures, and fouling resistant membranes. The goal of this project is to develop anti-biofouling nanocomposite polypropylene (PP) fibers loaded with copper or silver ions. These PP fibers will then be used to make feed spacers for RO spiral wound elements. Copper and silver ions are effective disinfectants and, conventionally, have been added to the water electrolytically or as metal salts to disinfect water. The advantage of using anti-biofouling feed spacers would be to decrease costs associated with chemical additions/storage. Intellectual Merit: The novelty proposed here is to covalently bind metal affinity ligands to PP fibers that can be charged with copper or silver ions to allow for slow release of metals into the feedwater in membrane systems for biofouling control. The immobilized metal affinity chromatograph (IMAC) system was chosen because of ease of functionalization and ability to control the degree of copper/silver binding through modification of the initial metal affinity ligand. The hypothesis proposed here is that PP fibers can be nanostructured with metal affinity ligands specific to copper/silver to lead to biofouling resistant membrane feed spacers. To test the hypothesis, the project is divided into three objectives:
1. Objective A: Functionalize the nanocomposite PP fibers.
2. Objective B: Test the biofouling properties of the nanocomposite PP fibers.
3. Objective C: Determine the effects of the nanocomposite fibers on the membrane.
Broader Impacts: Membranes are capable of separating species as a function of their physical and chemical properties when a driving force is applied, and they enable filtration for removal of colloids, cells and molecules. The major concern associated with the use of membranes is fouling. Membrane fouling adversely affects membrane performance and cost through loss in flux, increase in pressure, and cleaning frequency. Developing a membrane process with low fouling is the ?holy grail? of membrane research. The results of the proposed project will be disseminated through the training of graduate and undergraduate students, peer-reviewed publications and conference presentations, and the education of young underrepresented migrant students on drinking water problems and treatment.
Background: Globally, one in three people endures some form of water scarcity, one-quarter of the world's population lives in areas where water is not plentiful, and existing supplies may be limited in quantity or quality for meeting the increasing demands of population growth and industry expansion. The increasing lack of fresh water has been the driving force for seawater desalination, which is accomplished efficiently and reliably using membrane technologies. Membranes used to filter water come in a variety of sizes, from relatively large pores (microfiltration), to nonporous materials (reverse osmosis). Reverse osmosis membranes are the ones used for desalination since they allow water but not ions to pass through. As water is filtered through the membranes, anything that does not pass through the membrane — solids, organic substances, microorganisms —accumulates on the surface. Most of this can be removed by other, less expensive, forms of pretreatment, which is a process that removes fouling compounds from the water before the water goes through the reverse osmosis membrane. However, a problem arises when bacteria survive pretreatment and then accumulate on the reverse osmosis membrane. Bacteria on the membrane surface excrete a glue-like substance, called extracellular polymeric substance (EPS) that creates a shelter where they breed and multiply and eventually prevent the passage of any water at all through the membrane. Reverse osmosis membranes are created by winding into a spiral; that is, they are enveloped within a polypropylene mesh called a feed spacer (polypropylene, the same #5 plastic used in grocery store containers) to give them mechanical support and provide water flow channels. The goal of this research is to add copper to the polypropylene feed spacer. Adding copper ions to the polypropylene feed spacer increases its charge and hydrophilicity (or attraction to water). This increase of both charge and hydrophilicity is thought to decrease the propensity of cell adhesion, which is the first step in biofilm formation. Additionally, copper ions are known to have antimicrobial actions which are generally attributed to their effect on cell membrane permeability and their ability to generate hydroxyl radicals, which can cause cellular damage imparted via oxidative stress. These properties result in a less hospitable environment for cell adhesion or growth. Results: The goal of this project was to show that copper-charged polypropylene feed spacers could be used to control biofouling. To this end, polypropylene was functionalized with metal affinity ligands that could be charged with the biocidal metal, copper. This modification gave antimicrobial properties to the polypropylene feed spacer and also increased its hydrophilicity. The chelation of copper to the feed spacer was determined to be preferential to metals, chelators and organic matter in both batch and cross flow leaching studies. Thus, environmental impacts from copper leaching when using Cu-charged polypropylene feed spacers would be minimal. More significantly, use of the copper-charged feed spacer led to a consistently lower rate of flux decline during filtration. This increased resistance to fouling, and more specifically, biofouling, was hypothesized to be attributed to the hindrance of cell adhesion to the membrane/feed spacer interface. Fourier transform infrared spectrometer analysis verified the presence of polysaccharides only on the membranes fouled while using the unmodified polypropylene feed spacers. Polysaccharides are known to make up the largest portion of EPS, and are related to cell adhesion during initial stages of biofilm formation. It is believed that the antimicrobial property, as well as the increased hydrophilicity, of the Cu-charged feed spacers aided in hindering cell adhesion and, consequently, biofilm formation and biofouling. Therefore, the use of copper-charged feed spacers have the potential to increase membrane life and decrease chemical cleanings associated with detrimental biofouling of membranes. Scientific Uniqueness: Traditional melt-phase polypropylene grafting often occurs at temperatures over 160°C. The use of polypropylene powder or granules with a reaction temperature of 100-140°C has been shown to yield ~7% grafting. For radical development in polypropylene, other studies have shown that allowing the reaction to occur in supercritical CO2 for 10 hours and 130 bar at 70°C followed by thermal-induced grafting at 120oC yielded ~ 15% grafting. In this study, benzoyal peroxide was used as a radical initiator for the graft polymerization of glycidyl methacrylate (GMA) to the polypropylene surface at a temperature of 80oC, or nearly half of temperatures outlined in the literature, and yielded ~ 30% grafting. The GMA acted as a nano-scale spacer arm to which a chelator, iminodiacetic acid (IDA), was then attached. In the last step, copper was fixed to the chelator, which kept the copper in its electrolytic form. Keeping the copper as a charged species is the uniqueness of this research. The results are disseminated broadly to enhance scientific and technological understanding: This research group has been on the news on WGTE Plugged In Series (www.wgte.org/wgte/watch/item.asp?item_id=2037), and the feature of a public lecture at the Lake Erie Center (www.knowledgestream.org/kstream/index.asp?item_id=2380).
