If you put a spoonful of honey into a glass of pure water, what will happen? Of course, you cannot separate them anymore and you get a glass of a mixture of honey and water. This phenomenon is called diffusion. There is something different between diffusion and osmosis. Diffusion is the movement of atoms or molecules from a region where the concentration is higher to a region where the concentration is lower. Osmosis does the same and it is one special type of diffusion. In osmosis, molecules that will move are water and there is a semipermeable membrane to separate two different kinds of concentration of water. The membrane only allows water to pass through. Therefore, the water will pass through it from one side of higher concentration to another side of lower concentration. Other larger molecules (like urea, bacteria, glucose) or ions (like Na+, Ca2+, Cl-) cannot pass through the membrane.
Diffusion, as well as osmosis are thermodynamically favorable and don’t stop unless equilibrium is reached. Osmosis is very special, because it can be slowed, fastened, stopped. Plus, there is enough pressure applied to one side of higher concentration. Therefore, water molecules will pass through the membrane from the lower concentration side to the higher concentration side.
Reverse osmosis cannot occur naturally, because this goes against nature. The applied pressure makes water molecules go through the membrane from the lower concentration side to higher concentration side, which is contrary to the concentration gradient. This method is perfect for using for purifying water. If there is a semipermeable membrane between two kinds of concentration water. One side is fresh water and another side is contaminated water. Naturally, the fresh water will pass through the membrane to dilute the contained water. However, we can apply enough pressure on the side of contaminated water and make the diffusion process reversed. Water molecules move from the side of contaminated water to the side of fresh water. Fresh water becomes more and more. This process can allow us to purify our water, because it can separate pollutants from water.
On the market, more and more water filtration method to filter water. It even can be used to desalinate seawater. Compared to other types of water filtration methods, reverse osmosis is regarded on of the most effective and economical ways to purify water.
Pros and cons compared with other types of filtration methods:
- More effective
- Can remove more types of contaminants
- Can offer plenty of filtered water in a short time
- Costly – the initial cost is a little high and its filters need to be replaced on a regular basis.