This question once came up about Terramin clay too. It might be just a natural ingredient that combines ok with the rest of the ingredients. Sorry I can't remember where that link is.
Also try the FAQ at top of page.

Bentonite is an ADSORBER, which means that it pulls toxins from the body. It is not ABSORBED into the body. There have been several studies done that confirm that ingesting Bentonite results in no increase in aluminum in the body. It provide some links, but I'm short on time. You can google Bentonite and aluminum and that should pull up the info.