When "absorb" went to become a noun it dropped it's "b" and picked up a "p" on the way. Nobody knows why it did so. Anyway, long story short, "absorbtion" is a misspelling. Absorption is correct.
Don't confuse "absorption" with "adsorption" which is a scientific term for the process in which atoms/molecules adhere to the surface.
Absorption is known as a process that has a solid entity suck up or swallow up a liquid or chemical solution.
"As soon as I placed it in, the foam absorb the water in the bucket."
Absorbtion is an ignorant spelling for the word absorption. It is often thought that adding the '-tion' suffix changes the structure of any sentence but in this case, its redundant.