During photosynthesis, the energy from sunlight is used to split water molecules. what happens to the hydrogen ions that are split fron the water molecules

This process of Hydrogen Ions splitting happens during the light dependent reactions.
During this, a reaction called Photolysis occurs which means “splitting by light” and during photolysis, water splits in hydrogen ions, electrons and oxygen.

These hydrogen ions and electrons go on to carry energy onto the next phase of photosynthesis, the light independent reactions.