I may be wrong, but I think the "absorbent" part of a disposable diaper is the same thing that plant people put in potting mix to hold water. It is a polymer that grabs and holds any moisture that touches it.
I had one nursery worker (plants, not kids) tell me that she took off the plastic from damaged, unused adult diapers, cut them up and used them to add to potting mixtures. They held lots of water and were very effective in keeping plants happy in our dragon's breath hot summers.