Human survival is inseparable from water. Although there is a lot of water on the earth, it is a pity that there is very little fresh water that can be used by human beings. When mankind is confused about water shortage, there is no doubt that it will turn its attention to sea water with a much larger total than fresh water. Seawater contains 3.5% of salt compounds. How to remove these compounds from water at low cost has always been the goal of chemists.
At present, there are several common desalination methods:
distillation. Heat the sea water to turn the water into steam, and then condense the steam. Since the salts dissolved in seawater will not come out with steam, the water obtained is distilled water almost pure water. However, both heating and condensation require energy. For every gram of water evaporated, 2.3 kJ of energy is required. Therefore, this method can save the emergency, but it is not a long-term plan.
Ion exchange method. The salts dissolved in water exist in the form of cations and anions. If there is something that can remove these ions, the water will be purified. Ion exchange resin has such ability. Ion exchange resin is a kind of water-insoluble polymer material with network structure. It is like a big tree. There are many branches on it, and there is a pocket on each branch. When the sea water passes through, the cation will exchange the hydrogen ion in the "pocket" of the cation exchange resin, the anion will exchange the hydroxyl ion in the "pocket" of the anion exchange resin, and the hydrogen ion and hydroxyl ion will meet and become water.
However, the ion exchange resin will reach saturation after being used for a period of time. How to make the ion exchange resin sustainable? Because the ion exchange is reversible, it can be renewed by reverse exchange with acid and alkali respectively, so that the ion exchange resin can be reused. However, the treatment capacity of ion exchange method is limited, and a large amount of acid and alkali are needed to "regenerate" the resin. If it is used for seawater desalination, the cost is too high. Therefore, at present, this technology is mainly used in the further purification of water.
Reverse osmosis. Infiltration is a very common natural phenomenon in nature. For example, plants absorb water through the penetration of roots, and the penetration balance is also very important to human life activities. Penetration is achieved by a material called semi permeable membrane. The semi permeable membrane allows water to pass freely, while other chemicals in the water cannot pass through.
If the left side of the semi permeable membrane is pure water and the right side is solution, the water passing through the semi permeable membrane from the pure water side will be much more than the water passing through the semi permeable membrane on the right side of the solution. This is because the saturated vapor pressure (P0) above the pure water is greater than the saturated vapor pressure (P) above the solution. When both pure water and solution are above 1.01 × At a pressure of 105 PA (1 ATM), the power driving the movement of water molecules depends on the saturated vapor pressure. Therefore, the liquid level of pure water drops and the liquid level of solution rises. When it reaches a certain liquid level difference, it will reach equilibrium, which is the phenomenon of infiltration. The epidermis of plant roots is such a semi permeable membrane.
If you pressurize the solution so that the total pressure above the solution is greater than the atmospheric pressure above the pure water, the situation will be reversed. At this time, the power driving water molecules through the semi permeable membrane depends on the pressure difference between the two sides. If the pressure difference is large, more water in the solution will flow to pure water through the semi permeable membrane.
The method of making seawater into fresh water by reverse osmosis is also known as "membrane technology". Its key lies in the semi permeable membrane. This film should have a certain strength, because the first treatment process is to be pressurized; Secondly, it must have 100% selectivity, which can only let water pass through, while any other chemical substance cannot pass through; Finally, it should also have a large throughput to speed up the preparation process. At present, the problems of semi permeable membrane are high cost and short service life. The higher the concentration of the treated solution, the shorter the service life of the membrane. If these two problems can be solved, membrane technology will become the most promising method of seawater desalination.