Where is the energy involved in osmosis?

All things naturally tend towards equilibrium, moving from high energy to low energy. Osmotic pressure is the force that helps achieve osmotic equilibrium, so it is really just a manifestation of that natural tendency. You don't really need energy to create osmotic pressure, the osmotic pressure will be present until equilibrium is reached. It comes from the higher concentration itself, not some other force. Osmosis is a sort of 'gradient-driven' process, which comes about through entropy.

https://courses.lumenlearning.com/introchem/chapter/osmotic-pressure/ https://simple.wikipedia.org/wiki/Osmosis

Consider a U shaped tube with a membrane in the middle, permeable to water but not to salt. It is partially filled with water. The height of the water surface on both sides of the tube will be the same, as they both feel the same atmospheric pressure.

Now we add salt to the left side of the tube, which fully dissolves (and it can't go through the membrane so it all stays there). We observe that the water level on the left rises and on the right side it falls. So there was a net movement of water molecules from right to left.

This movement went against gravity, as it lifted some of the solution, which is now weighting down on the rest of the liquid, on top of the atmospheric pressure already there. Yet the solution stays risen.

Such net movement, implies an attraction between water and salt. Salt dissolves mostly because of entropic effects (rather than, say, forming more stable bonds). Once dissolved, the water molecules are attracted to the charges of the ions, which are stronger than the mere dipole moment other water molecules have.

It is this attractive force that pushes water to the left, and causes the solution to rise. In other words, this attraction supplies the necessary energy to make the solution rise.