Physics in Chemistry – Reverse Osmosis

Physics of Water Purification

Water crisis is an imminent threat to the world populations with raising sea levels, temperatures and other manifestations of climate change. A whopping 2.8 billion people are already victims of it and the disaster is poised to threaten billions more. The role of technology in mitigating the adverse effects of water crisis, lack of access to potable water and the underlying science is worth a detailed discussion.

Reverse osmosis based purification is the technology modern science has to offer in dealing with the potable water crisis. In order to elucidate it further, let’s discuss an experiment. A U-shaped tube is separated into two halves, vertically, by a semi permeable membrane (a membrane that will allow water molecules to pass through it and not salt particles or ions). Now each of the arms of the U-tube is filled with water and salt is added to one of them. When, the U-tube is left undisturbed for a while, one can observe a rise in water levels of the salt-added-half of the U-tube. This process of movement of water from low salt concentration region to the high salt concentration region across a semi-permeable membrane is called as osmosis. But why do water molecules move across the membrane to the other half and what is the force that drives this movement?

Water molecules surrounded by a lot of water molecules (environment with low concentration of solutes or salts) have high potential energy, as discussed in the other article. These molecules tend to reduce their energy and move to an environment that has relatively less number of water molecules i.e., high concentration environment. This movement is associated with a reduction in the potential energy of the water molecules. Such motion of water molecules can be countered by a pressure applied on the surface of water that is present in the high concentration side of the tube. A movement of water across the barrier is associated with a decrease in the volume of air in one of the arms of U-tube. When this air is pushed down by piston, it exerts pressure on the surface of water that opposes any raise in the water level. When weights are placed on the piston, with an increase in the weight added, slowly the direction of movement of water is reversed. The pressure required to just stop the spontaneous movement of water across the membrane is called osmotic pressure. This process of movement of water across the semi permeable membrane from high salt concentration region to low salt concentration region under the influence of applied pressure is called reverse osmosis. As it is obvious the movement of water affected, is opposite to the natural movement of water from low salt concentration region to high salt concentration region and the term reverse osmosis is justified.

It is now simple to conceive how reverse osmosis is associated with water purification. Assume salt water in one of the arms (of U-tube) is replaced with sea water rich in salt and bacteria. Now repeat the experiment by adding weights to the piston. Water molecules move from sea water side to fresh water side and note that our semi-permeable membrane hinders movement of everything but water. Thus one can filter out salts and bacteria in one go.

Reverse osmosis is a simple, energy efficient method that science has to offer in countering the drastic effects of climate change. While it is not taught during our Physics tuition classes, the principles behind are certainly closely related to the field of Physics.