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Nanotechnology Helps to Make Saltwater Drinkable

Researchers are investigating nanomaterials for use in making salt water drinkable. Today, the desalination of salt water is a very expensive process, but nanotechnology could change that.

While there is lots of water around, only a small portion of it is drinkable. According to the USGS (United States Geological Survey), our oceans contain about 96.5 percent of all the water on Earth, but obviously saltwater isn’t drinkable. Then there are problems in the supply and quality of water.

The problems range from the population of cities being too large for the amount of water available in rivers and lakes, to rural villages where the available water is polluted but they have no water treatment facilities. (The United Nations estimates that 1.1 billion people do not have access to water treatment facilities.)

Given these problems, the idea of making saltwater drinkable is appealing. However, the cost of doing so is currently very high. For communities near seawater, the good news is that nanotechnology is helping to make desalination of saltwater more economical.

The natural movement of water molecules into saltwater to equalize the concentration of contaminates is called osmosis. The process of forcing water molecules from saltwater to freshwater is the reverse of this process and therefore is called reverse osmosis.

The reverse osmosis process is used in many desalination plants. In this system, you place saltwater under pressure, which forces it through a membrane. The membrane is generally a polymer with many nanosize pores, and is a material that allows water through the pores but stops salts and bacteria.

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Reverse osmosis systems require pumps to maintain sufficient pressure to force the water through the membrane as well as cleaning procedures to clean bacteria that grows on the saltwater side of the membrane, referred to as fouling.

Researchers are investigating the use of nanomaterials to reduce the pressure needed to force water through the membrane and to reduce the capability of bacteria to grow on the membrane.

Make water flow more easily through the membrane

We may be able to benefit from carbon nanotubes in the membranes that are used in reverse osmosis to help with the process of desalination. For example, a company called NanOasis is working on membranes that contain a very dense polymer film with carbon nanotube pores.

Because the inside of carbon nanotubes is very smooth, water is transported through them more easily. And, while the nano pores allow water to flow through, they stop salt ions, making this method perfect for desalination. This method could reduce the energy required for desalination by 30 to 50 percent.

Reduce bacteria in reverse osmosis

A company called NanoH2O adds nanoparticles to their membrane to optimize properties such as surface roughness and charge. The company has been able to reduce the chance of bacteria adhering to the membrane. Because bacteria on the membrane can reduce the amount of water passing through, reducing the bacteria on the membrane means that you don’t have to shut down the system for cleaning as often.

Explore capacitive deionization

A desalinization method called capacitive deionization has the potential to become more cost effective than reverse osmosis. A capacitive deionization cell contains two electrodes, one positively charged and one negatively charged.

The electrodes are charged because salt consists of negative and positive ions. Because opposite charges attract, negatively charged ions are attracted to the positively charged electrode and positively charged ions are attracted to the negatively charged electrode. For example, if you dissolve regular table salt (sodium chloride) in water, the sodium and chloride separate to form positively charged sodium ions and negatively charged chloride ions.

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As seawater runs through the cell, the salt ions attach to the electrodes and deionized water leaves the other end of the cell. This technique doesn’t require high pressure to push the water through the membrane, as in reverse osmosis, so it would be less expensive and use less power.

Researchers are developing electrodes made with nanomaterials to increase the electrode surface area, which should increase the speed at which a cell can remove salt ions from seawater. One interesting technique is the use of electrodes constructed from graphene flakes, which researchers at the University of South Australia have demonstrated. Researchers from around the world are also attempting to develop low-cost capacitive deionization systems using nanostructured electrodes.

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