Making Reverse Osmosis Membranes

Su Lv shows off a reverse osmosis membrane cartridge.

Making ultra-thin materials with holes the size of water molecules: While visiting GE's China Technology Center, we got to take a look at reverse osmosis membranes. Reverse osmosis is the most energy-efficient means of removing dissolved substances from water. It's what's used commercially for desalination, the process of producing drinking water from seawater.

The term "membrane" is typically used to mean a thin sheet of some material (in fact, the word "sheet" appears in the definition of the term). But for some of the things GE is using it for, the membranes were thin yet robust tubes, each one capable of supporting the weight of a bowling ball. Despite that toughness, features on the tubes are so fine that they can allow water molecules to pass through but reject many things that are roughly the same size, such as the salt ions found in seawater.

This all raises an obvious question: how do you actually produce anything like that? We decided to look into the process of making reverse osmosis membranes. It quickly became clear that the toughness of the membranes is a key feature. Water purification systems need to survive repeated cleaning cycles and go right back to use. We talked to Sijing Wang in Shanghai, who said that some membranes in the systems GE makes can be used for up to eight years.

That toughness, however, is provided by polymers that are microporous, in that they have features a thousand times larger than what is needed for reverse osmosis. These would do little to help remove salts from water, but they provide structural support for membranes that can. The large pores also ensure that water can easily flow through the system once it has passed through the membrane that acts as a filter.

 A test setup that allows Sijing Wang to see how membranes (held in the rectangular cases, lower right) respond to different types of waste material.

The interactions between that membrane and the water it's purifying help dictate the efficiency of the system. Since the membrane doesn't interact well with water, more water will flow through it when it's thinner. For reverse osmosis membranes, this layer is kept to a micrometer or less in thickness. The pores within it have to be kept small so that the ions of the salts in the water can't pass through a pore without interacting with the membrane, which will repel them. (Technically, the pores aren't small enough to physically block the ions from passing through, but the interactions between ions and the membrane keep them from getting too close to the pore opening.)

So you need to both layer a thin membrane across your support membrane and control the size of the pores that form within it, typically limiting them to less than 10 nanometers.

There are two methods of creating thin membranes. One involves forming a polymer but keeping it dissolved in a solvent that also mixes with water (often an alcohol of some sort). As you increase the fraction of water present, the polymer will eventually precipitate out. There are several ways of doing this. The simplest is to just heat the solution so that the solvent evaporates, which increases the fraction of water until the polymer precipitates. Alternatively, you can place the solution in a humid environment until the fraction of water goes up.

The most common method, however, is to create a viscous, 20 percent polymer solution and dunk it directly in water. The solution is so viscous that it won't mix into the water; instead, water infiltrates it and causes the polymer to drop out of solution. This process is often done on a continuous roll of material that's sent through a vat of water.

In all these instances, the action takes place at the interface between the polymer surface and the environment. As a result, the membrane primarily forms at this interface, creating the very thin barrier needed for reverse osmosis. Polymer deeper in the solution tends to form a larger, more open structure, which allows water to flow freely away from the membrane.

An alternative approach that functions in a similar manner is to use a building block for the polymer that dissolves in some solvents and a chemical activator that dissolves in an immiscible one. The two solvents will form two different layers (much like oil and water), and the building block and activator will only meet each other at the interface. As a result, polymerisation only takes place at this interface, resulting in a very thin layer.

How do you put holes in it? To a certain extent, the process takes care of that itself. As water begins to enter a solvent it's not fully compatible with, it will form tiny droplets that are held together by surface tension. The polymer will form around those droplets, leaving small holes behind. The size of these holes is determined by the speed of the process; the quicker it takes place, the smaller the water droplets will be and the smaller the resulting pores. By varying the solutions being used and the speed of the process, it's possible to have fine control over the pore formation process.

There are also additional layers of control possible. It's possible to include molecules that act as "pore generators" in the solutions, which are then removed when the membrane is rinsed later. Wang said most of the polymers GE uses are made of aromatic polyamine—which means a carbon ring that nitrogens are attached to. These chemicals do allow a certain degree of flexibility, in that they can be different sizes (one or more rings) and have slightly different chemical properties. (They're also carcinogenic before they're polymerised, but they're inert afterward. While water purification systems can be said to "contain a carcinogen," they pose absolutely no threat to human health.)

By adjusting the chemistry of the polymer, as well as the process by which it's formed, it's possible to have very fine control over the membrane that ultimately forms. This allows manufacturers to customise membranes for different tasks and to provide the durability that's needed for multiple years of use.

Sidney Loeb (left) with first RO membrane.

First Demonstration Of Reverse Osmosis: In the late 1940s, researchers began examining ways in which pure water could be extracted from salty water. During the Kennedy administration, saline water conversion was a high priority technology goal-"go to the moon and make the desert bloom" was the slogan. Supported by federal and state funding, a number of researchers quickly advanced the science and technology of sea water conversion, but UCLA made a significant breakthrough in 1959 and became the first to demonstrate a practical process known as reverse osmosis (RO).

At that time, Samuel Yuster and two of his students, Sidney Loeb and Srinivasa Sourirajan, produced a functional synthetic RO membrane from cellulose acetate polymer. The new membrane was capable of rejecting salt and passing fresh water at reasonable flow rates and realistic pressures. The membrane was also durable, and could be cast in a variety of geometric configurations. The impact of this discovery has been felt worldwide, ranging from applications in home demineralizers to "rivers of fresh water" in the Middle East and North Africa, where desalination facilities produce trillions of gallons of pure water every day. About 60 percent of the world's desalination capacity is located on the Arabian peninsula.

The process of osmosis through semipermeable membranes was first observed in 1748 by Jean-Antoine Nollet (pictured). For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949, the University of California at Los Angeles first investigated desalination of seawater using semipermeable membranes. Researchers from both University of California at Los Angeles and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable until the discovery at University of California at Los Angeles by Sidney Loeb and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, of techniques for making asymmetric membranes characterised by an effectively thin "skin" layer supported atop a highly porous and much thicker substrate region of the membrane. John Cadotte, of FilmTec Corporation, discovered that membranes with particularly high flux and low salt passage could be made by interfacial polymerisation of m-phenylene diamine and trimesoyl chloride. Cadotte's patent on this process[4] was the subject of litigation and has since expired. Almost all commercial reverse osmosis membrane are now made by this method. By the end of 2001, about 15,200 desalination plants were in operation or in the planning stages worldwide.

Nanotechnology, Fullerenes & Bucky Balls - Window Cleaning Future?

What does this mean to the window cleaner? Water fed pole membranes & filters will last longer, but at what cost? Nanotechnology has already appeared as window films in both hydrophobic & hydrophillic coatings - and yet we know so little. Today & today only - WOOT are offering Limited Edition Green Buckyballs 216 Piece Magnetic Set – 2 Pack for $29.99 - science in your hands!

Even our water fed poles resemble the carbon nano-tubes.. read on..

Researchers at Duke University in Durham, North Carolina have shown that buckyballs could prevent bacteria and other contaminants from attaching to water pipes and the membranes used to filter water in filtration plants. When bacteria attach to these surfaces, they attract other organics that eventually form a film that both blocks and contaminates the water within. Preventing this buildup using buckyballs could limit or prevent membrane replacements, reducing costs and making water safer at the same time.

A team at Duke's Environmental Molecular Biotechnology Laboratory led by Assistant Professor of Civil and Environmental Engineering Claudia Gunsch and post doctoral fellow So-Ryong Chae, has been experimenting with the buckyballs. Buckyballs, or systems with 60 carbon atoms arranged in a lattice of pentagons and hexagons much like a soccer ball, are the most common nanoparticles, also known as buckminsterfullerenes.

In addition to preventing attachment to the membranes, the buckyballs also appear to inhibit the ability of the bacteria to fuel their activities with oxygen. This reduces the number of bacteria available for attachment, further lessening the impact of the bacteria on water systems. The researchers say they do not fully understand why this is the case, but all experiments so far have yielded consistent results.

The bacterial attachment, called biofouling, is one of the most common problems in membrane-based water filtration plants. The pores in the membranes are so small that they can get clogged very easily, limiting throughput considerably. The difference seen in membranes operating with buckyballs versus those without was significant; treated membranes had about 20 bacteria colonies after three days while untreated membranes had too many colonies to count.

All experiments to date have been conducted with Escherichia coli K12, a strain of bacteria widely used in laboratory settings. The next step is trying the buckyballs with other types of bacteria and with mixed environments of multiple bacteria that more closely mimic real world situations. The longevity of these coatings must also be tested. Additionally, the Duke team plans to build its own miniature filtration plant for more intense testing.

The winning Aquaduct Mobile Filtration vehicle was designed to help people living in developing countries, who often do not have access to clean water. As the rider peddles, the peristaltic pump draws the water from the storage tank and through the filter. The filtered water is then transferred to the clean tank. The clean tank is a closed removable container which can be simply taken into the home

TFOT has previously reported on the Aquaduct Mobile Filtration bicycle that filters water as you pedal it. We have also reported on other technology based on buckyballs and other fullerenes including buckypaper composites with all of the properties of carbon nanotubes, an 80-atom buckyball made of Boron atoms instead of carbon, and ultralong carbon nanotubes developed at the University of Cincinnati.

Buckyballs for water treatment systems: Microscopic coal particles of the fullerene family called buckyballs could help to maintain cleaner water pipes. In the course of time, water membranes and pipelines accumulate bacteria and other microorganisms for water treatment. As bacteria are joining these surfaces attract other organic matter, creating a biological film that grows over time. The results obtained in the laboratory by a team from Duke University show that buckyballs might be able to prevent this coating, known as biofouling. The only alternative to this system is expensive, involves digging up the pipes, and replaces the membranes. Biofouling is one of the problems of higher costs associated with water treatment systems based on membranes. A group of engineers from Duke University says that buckyballs hamper the action of bacteria and other microorganisms accumulated in membranes that treatment plants use to filter water. Due to this property, these nanoparticles could solve one of the most expensive problems in water treatment. These membranes have small quickly coating pores that would be covered quickly. If the half-life time of membranes increases it would cause a significant reduction in the cost of these systems. The membranes treated with buckyballs present a lower bacterial layer than untreated ones.

Buckyballs Could Keep Water Systems Flowing:  Microscopic particles of carbon known as buckyballs may be able to keep the nation's water pipes clear in the same way clot-busting drugs prevent arteries from clogging up. Engineers at Duke University have found that buckyballs hinder the ability of bacteria and other microorganisms to accumulate on the membranes used to filter water in treatment plants. This attribute leads the researchers to believe that coating pipes and membranes with these nanoparticles may prove to be an effective strategy for addressing one of the major problems and costs of treating water.

"Just as plaque can build up inside arteries and reduce the flow of blood, bacteria and other microorganisms can over time attach and accumulate on water treatment membranes and along water pipes," said So-Ryong Chae, post-doctoral fellow in Duke's environmental and civil engineering department. The results of his experiments were published March 5, 2009 in the Journal of Membrane Sciences. "As the bacteria build up on these surfaces, they attract other organic matter, creating a biofilm that slowly builds up over time," Chae continued, "The results of our experiments in the laboratory indicate that buckyballs may be able to prevent this clogging, known as biofouling. The only other options to address biofouling are digging up the pipes and replacing the membranes, which can be expensive and inconvenient."

A buckyball, or C60, is one shape within the family of tiny carbon shapes known as fullerenes. They are named after Richard Buckminster Fuller, the inventor of the geodesic dome, since their shape resembles his famous structure. "Biofouling is viewed as one of the biggest costs associated with membrane-based water treatment systems," said Claudia Gunsch, assistant professor of civil engineering at Duke's Pratt School of Engineering and senior member of the research team. "These membranes have very small pores, so they can get stopped up quickly. If we could increase the time between membrane replacements by 50 percent, for example, that would be a huge cost savings."

According to Chae, the addition of buckyballs to treatment membranes had a two-fold effect. First, treated membranes showed less bacterial attachment than non-treated membranes. After three days, the membranes treated with buckyballs had on average 20 colony forming units, the method by which bacterial colonies are counted. "In contrast, the number of bacterial colonies on the untreated membrane was too numerous to count," Chae said. Chae also found that the presence of the buckyballs inhibited respiration, or the ability of the bacteria to use oxygen to fuel its activities. "As the concentration of buckyballs increased, so did the inhibition of respiration," Chae said. "This respiratory inhibition and anti-attachment suggests that this nanoparticle may be useful as an anti-fouling agent to prevent the biofouling of membranes or other surfaces." Gunsch said the mechanisms involved are not well-understood.

But what are the risks?..

Caution flag raised on buckyballs; harm to the environment possible: Buckyballs, described by some scientists as "the perfect molecule" and a hallmark of Rice University research, may cause more havoc in the environment than researchers originally thought. A team of researchers at Rice and Georgia Tech universities has found that the ultra-tiny, soccer-ball-shaped buckyballs, contrary to what they had thought, do in fact dissolve in water, a finding that suggests they could pose a risk for wildlife and water supplies.

The new results compound concerns raised by earlier studies that found buckyballs can cause brain damage in bass and harm human cells. Discovered nearly two decades ago at Rice, buckyballs are among a handful of new materials, far smaller than human cells or even DNA, driving the nanotechnology revolution. "This doesn't mean that we should put a halt on nanotechnology," said Joseph Hughes, an environmental engineer at Georgia Tech and the study's lead author. "Quite the opposite."

No scientists or government regulators have called for stopping the research and commercialization of nanotechnology, a rapidly expanding field of specialized materials that encompasses everything from novel medical approaches to bulletproof vests. Nor are many likely to call for a ban now.

What the new findings should do, researchers say, is increase pressure on the federal government to further regulate the production and handling of buckyballs and potentially other nanotechnology materials, such as carbon nanotubes. "I don't view this new research as something that's very scary," said Kristen Kulinowski, executive director of Rice's Center for Biological and Environmental Nanotechnology. "But it may highlight the need for caution."

'Buckyballs' have high potential to accumulate in living tissue: Research at Purdue University suggests synthetic carbon molecules called fullerenes, or buckyballs, have a high potential of being accumulated in animal tissue, but the molecules also appear to break down in sunlight, perhaps reducing their possible environmental dangers. Buckyballs may see widespread use in future products and applications, from drug-delivery vehicles for cancer therapy to ultrahard coatings and military armor, chemical sensors and hydrogen-storage technologies for batteries and automotive fuel cells.

"Because of the numerous potential applications, it is important to learn how buckyballs react in the environment and what their possible environmental impacts might be," said Chad Jafvert, a professor of civil engineering at Purdue. The researchers mixed buckyballs in a solution of water and a chemical called octanol, which has properties similar to fatty tissues in animals. Jafvert and doctoral student Pradnya Kulkarni were the first to document how readily buckyballs might be "partitioned," or distributed into water, soil and fatty tissues in wildlife such as fish. Findings indicated buckyballs have a greater chance of partitioning into fatty tissues than the banned pesticide DDT. However, while DDT is toxic to wildlife, buckyballs currently have no documented toxic effects, Jafvert said.

"This work points out the need for a better understanding of where the materials go in the environment," he said. "Our results show they are going to be taken up by fish and other organisms, possibly to toxic levels. This, however, indicates only the potential of buckyballs to bioaccumulate. They could break down in the environment or in an organism once taken up." Researchers do not yet know whether buckyballs will break down in the environment or will be metabolized by animals, which would reduce the risk of accumulating in fatty tissues.