IN A NUTSHELL: A new type of antifouling coatings for prevention of organic and biological fouling of water filtration membranes used in reverse osmosis, ultra- and/or nano-filtration is being developed. These, dendritic polymers-based coatings for the first time successfully combine three main features known to effectively deter such contaminants: high surface hydrophilicity, brush-like surface topology and the presence of nano-encapsulated, non-leaching chemical repellants. The ongoing research program is focused on the coatings for commercially available aromatic polyamide membranes which have excellent separation properties, including salt rejection and permeating water flux. Sea water desalination and purification of brackish water are of particular interest considering the level of importance and urgency that providing enough drinking water for the growing World’s population is expected to have in the not too distant future. 

 
THE PROBLEM: During the past two decades, reverse osmosis (RO) has become the main technology for the production of drinking and ultrapure water for the electronics and pharmaceutical industry. At present, over 3 billion gallons of water are produced this way every day, and its importance is expected to further increase in the future because desalination by RO has become the technology of choice to satisfy the requirements for drinking water that will be imposed by the anticipated population growth in the first half of the 21st century, particularly in the poorest and the most remote areas of the World. The heart of every RO system is the semipermeable membrane which permits passage of pure water while rejecting dispersed and dissolved solids, and both technological and economical aspects of the process are directly affected by its properties and performance. At present, there is a variety of commercially available semipermeable membranes that possess excellent separation properties (i.e., % solids rejection and permeate flux), but their fouling by contaminants in the feed water represents a major problem from both technological and economical perspective. For example, some of the most successful commercially available RO membranes are those from composite structures with aromatic polyamide separationlayers which readily attract organic and biological species that quickly build up an additional filtration barrier that reduces the flux of permeate water through the membrane. To compensate for this flux decline, various actions must be taken, including increasing the operating pressure by as much as 50% in some cases, interrupting production for cleaning the membranes by frequent chlorination, and eventually replacing the entire membrane elements sooner than desired. Of these, just the costs of energy required by the high-pressure pumps to remedy the fouling-induced flux decline often amounts to ca. 30% of the total operating expense and become the main factor that controls the economics of the water production by RO processes. As a consequence, there is a fast growing need for effective prevention of membrane fouling in RO processes, and among the methods under consideration, surface modification of existing, commercially available membranes by antifouling coatings has attracted considerable research attention.

THE STATE OF TECHNOLOGY: The key requirement for a successful antifouling RO membrane coating is to eliminate the fouling while retaining rejection characteristics and permeate fluxes comparable to those of unmodified, non-fouled membranes. However, additional benefits also include: avoidance of expensive and difficult development of completely new types of membranes, and a cost-effective solution to the problem, considering that only a very thin coating (of the order of hundreds of nanometers) would likely satisfy the purpose. In addition, the application of such coatings could generally be realized by a relatively simple procedure added to the end of the existing membrane fabrication process so that no major engineering changes are necessary. While biofouling remains a very complex and yet not completely understood problem, it has been established that toward the elimination of fouling by organic and biological species, such as humic acid, weed killers, gasoline additives, industrial oils and chemicals, bacteria, etc., some of the best results can be obtained by synthetic polymers that provide: (a) increased hydrophilicity to attract water but repel contaminants, and (b) dynamic, brush-like topologies which can undergo cilia-like Brownian motions on membrane surfaces and create a few nanometers thick layer of ultrapure water at the membrane-feed interface to prevent contaminants from settling. In addition, we have recently also shown that exceptional antifouling effectiveness can be achieved by dendritic polymer-based honeycomb-like coatings that encapsulate repelling agents capable of deterring the foulants even while entrapped in the coating’s cells unable to leach into the aquatic environment (see “Environmentally Benign Antifouling Coatings for Marine Applications”).

OUR NOVELTY: In this new approach to the membrane biofouling problem, we combine for the first time in a single nano-structured type of polymer coating all of the above described highly desirable membrane properties, including:  (i) a very high surface hydrophilicity, (ii) a dynamic, polymer brush surface topology, and (iii) a 3D honeycomb-like architecture capable of encapsulating very effective but non-toxic antifouling deterrents. In addition, our technology also enables preparation of such coatings in the form of rather loose network structures designed to provide sufficient mechanical integrity but not to interfere with the permeate water flux through the membrane. All of these highly desirable features are enabled by the utilization of our own dendritic polymer nanotechnology which has matured to commercial production and which permits precise tailoring of the targeted supramolecular structures by utilizing the extremely high functionality of dendrimers and hyperbranched polymers.