How does seawater desalination achieve?

Water is closely related to life, although more than 70% of the surface is occupied by seawater, but the proportion of fresh water that can be used directly for humans is low. In order to solve the water shortage, many people will look to the endless sea. Seawater contains 3.5% of the salt compounds, these compounds need to be removed from the sea water, can be used for people, known as desalination, the current commonly used seawater desalination methods are the following:

1. Distillation method
The sea water to heat, so that the water into steam, let the steam condensed down. As the salt dissolved in the sea will not come out with the steam, so the water is almost pure water distilled water. However, whether it is heating or condensation, need to spend energy. Every 1 gram of water will need 2.3 kJ of energy. So this method can be an emergency, but not a long-term plan.

2. Ion exchange method
Salts dissolved in water are present in the form of cations and anions. If there is a thing that can remove these ions, then the water will be purified. Ion exchange resin has such a capacity. The ion exchange resin is a water-insoluble polymer material having a network structure. It is like a tree, there are many branches above, each branch has a pocket. When the sea passes, the cation will exchange the hydrogen ions in the cation exchange resin "pocket", and the anion will exchange the hydroxide ions in the "pocket" of the anion exchange resin, and the hydrogen ions and the hydroxide ions meet Turned into water However, ion exchange resin can be used for some time after the saturation state, how to make the ion exchange resin can be used continuously? Since the ion exchange is reversible, it can be separately with acid and alkali reverse exchange to update, so that can make The ion exchange resin can be used repeatedly. However, the ion exchange process has limited processing capacity and requires a large amount of acid and alkali to make the resin "regenerate". If it is used in a large amount for desalination, the cost is too high. Therefore, this technology is mainly used in the further purification of water.

3. Reverse osmosis method
Penetration is a very common natural phenomenon in nature. For example, the plant is the root of the infiltration to absorb moisture, osmotic balance of life activities is also extremely important. Penetration is achieved by a material called semipermeable membrane. The semipermeable membrane allows the water to pass freely, and other chemicals in the water are passed. If the left side of the semipermeable membrane is pure water, the right side is the solution, then from the pure water side through the semipermeable membrane of water than the right side of the solution through the semipermeable membrane water much more. This is because the saturated vapor pressure above the pure water is greater than the saturated vapor pressure above the solution. When the pure water and the solution are above 1.01 × 105 Pa (1 standard atmospheric pressure) under the pressure to drive the movement of water molecules determined by the saturated vapor pressure. So, pure water side of the liquid drop, the solution side of the liquid rise to a certain level of difference to reach a balance, which is the penetration phenomenon. The root of the plant is such a semipermeable membrane