New method for the desalination of seawater
New processes for seawater desalination are gaining importance. Production of drinking water and process water in arid areas without sufficient natural surface water or groundwater deposits and water generation on ships or remote islands are of high economic importance. The most widely used process is reverse osmosis that is based on a semipermeable membrane that only allows pure water to pass with the help of pressure and retains salt-containing water. Other methods are multi-stage flash evaporation (MSF), electrodialysis or freezing processes. All conventional methods, however, are expensive, complicated, and energy-intensive. The new process is based on the use of hydrogels, polymers that are insoluble in water and whose polymer chains are linked in a three-dimensional network. Thanks to their hydrophilic components, they can take up water as efficiently as a sponge. This allows the superabsorber to reach an uptake volume of more than 100 g water per g polymer. The new method of seawater desalination uses a networked polyacrylic acid gel as superabsorber. It contains ionic groups. Due to this own load, the superabsorber can reduce the uptake of foreign ions in seawater and, hence, will only take up water that contains far less salt than the surrounding solution. Recovery of the salt-depleted water from the gel is achieved by mechanical pressure. For this purpose, a special hydraulic press was developed for the separation of water from gel using a sieve and collection tanks (s. Figure 1). Thus, none of the two substances is lost during recovery. The pressed-out aqueous phase contains half of the salt only. With a few repetitions of recovery, potable water is generated. Further development of material properties might even improve the results.
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