It is safe to store celery in aluminum foil? Because I cannot find on a regular basis celery, I am trying to preserve what I buy as long as possible.

After trying some methods, the one I found is aluminum foil.
I wash the celery, dry it, put in paper towels, then in aluminum foil, then put in the vegetables area of the freezer.
I managed to have fresh celery for 2 weeks, maximum, keeping in mind I didn't use a metal knife for cutting the edges.

Is it safe to store celery in aluminum foil?
Yes, it's safe.  It's not acidic, and thus won't corrode the foil or cause it to leach in.  

Personally, my favorite was to store celery is in the crisper, in a vented tupperware container, lightly moistened.  This extends lifetime somewhat.