The good news: Alum is not the same as aluminium. Alum is a common name for 'Aluminium Sulphate' Or Potassium alum. It has been used to purify water for thousands of years.
The bad news: Alum does seem to contain aluminum, which is probably used way more in your favourite restaurant‘s kitchen, and while current literature is still defining mechanisms, pathways, and the reagents that contribute to its accumulation in Alzheimer’s patients. It is generally considered most toxic as an ingestible. So wash your hands after using it.
Potassium alum molecules have a negative ionic charge, making it unable to pass through the cell wall. THEY ARE NOT ABSORBED. (We think)