Drinkable fresh water composes 1 percent of Earth's water. To compensate for the increasing demand on fresh water, salt removal from seawater is required to convert some of the 99 percent of Earth's water to drinkable water. Fresh water has less than 1,000 parts per million (ppm) of salt, while ocean water has about 35,000 ppm. The process of salt removal from seawater is known as desalinization. A number of technologies exist that enable the potability of water.
Heat Distillation. This process, in its industrial form, emulates the natural hydrological process whereby sea-water is evaporated, then condensed to freshwater and salt. There are several engineering processes that employ an increase in the pressure to lower the boiling point of the salt water. In order to lower the heating cost of this process, solar energy is employed to evaporate the water. However, the solar plants are small in scale. Recently, nuclear reactors have been suggested to perform dual functions: produce power and desalinate water through the process of cooling the reactor. The world's largest heat distillation plant is in the United Arab Emirates, which produces 300 million cubic meters of water per year. A pressure reduction through a throttle valve is utilized to reduce the energy requirement on the evaporation process. In 2009, Saudi Arabia announced a new plant that will produce 800,000 cubic meter of fresh water daily. Ion Extraction. Chemical and electrical means are applied to remove the ions composing the salt from the water. Electrodialysis is another process that uses electrical current to concentrate ions through a permeable membrane. The cost of electricity to concentrate and or remove the salt is the prohibitive factor of this process. Freezing Desalinization. During the process of forming ice, salt is excluded from the ice cubes. Hence, the freezing process under controlled conditions allows for the removal of salt. Once the salt is isolated, the water can be melted to form fresh drinkable water. Icebergs are composed of freshwater. Hauling icebergs, if the cost of transportation is manageable, can be a good solution to provide freshwater to needed areas. Reverse Osmosis. This process requires pushing the salt water through a fine membrane that does not allow the salt to pass. Reverse osmosis accounts for 44 percent of the total world desalinization capacity. Two improvements have helped reduce the operating costs of reverse osmosis plants: the developments of membranes that can operate efficiently at lower pressures, and the use of energy recovery devices. In order to further reduce the cost of this process, new membranes that selectively pass freshwater and prevent salt to pass at low pressure need to be developed.
Recent studies suggest that nanotechnology can contribute to lowering the cost for desalinization. Singlewall, double-wall, and multi-wall carbon nanotubes have been tried as membranes. Aligned carbon nanotubes are produced using chemical vapor deposition. The most intriguing among all are single-wall carbon nanotubes. While the chemical vapor deposition produces well-aligned carbon nanotubes, the process is limited to fabrication small fabrication samples. Different techniques have been reported to produce aligned carbon nanotubes, among these techniques the use of low and high magnetic fields, electric fields, functional groups, and ultrafine filters.
For the purpose of desalination, carbon nanotubes were aligned on a polymeric matrix or silicon membrane. The carbon nanotubes were aligned along the direction of the water flow. The pore diameters of the carbon nanotubes are two nanometers. The energy needed to force the water through the carbon nanotubes was found to be three times less and several orders of magnitude faster than that of conventional membrane. The process matches that of the reverse osmosis; however, the membrane is made out of carbon nanotubes.
Multi-wall carbon nanotubes were also utilized as electrodes in a flow through capacitor for desalinization process. The results showed that when multi-wall carbon nanotubes were used, the desalinization performance improved over the use of activated carbon as an electrode. According to the principle of flow-through capacitor, the salt removal ability of the electrode is proportional to the electrostatic capacity. The electrostatic capacity of double-layer capacitor depends on the surface area of the electrode. The multi-wall carbon nanotubes contribute to the increase of the surface area. In the flow-through capacitor, salt water is passed through a capacitor at a flow of 10 milliliters per minute. Applying a direct voltage of one volt on the multiwall carbon nanotubes results in removal of salt from the water.
Membranes with nanopores were prepared by insertion of removable nanoparticles with sizes in the range of the desired pores. The nanofiltration membranes have been employed in pretreatment operations in both thermal and membrane seawater desalinization. The employment of nanoflitration membranes resulted in reduction of chemicals used in pretreatment processes and energy requirement. Nanoparticles with biocidal effect have been proposed to eliminate bacterial growth around membranes. Such nanocidal particles include silver-based particles that have efficacy against bacteria at low concentrations, such that it remains within the set toxicity limits.
Carbon Nanotubes, Nanomaterials, Nanosilver, Water Purification.
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