The paint on the concrete floor of my walk-in refrigerator has
over the course of many years of use become unpainted in the
middle traffic areas. I want to repaint tbe whole floor. Is
there any special prep work I need to do? What's the best kind
of (nontoxic) paint I can use for this situation?

The BEST solution is poured epoxy flooring, like
http://www.hitechflooring.com/
or http://www.florock.net/

It's tougher than the concrete it goes over, seamless, chemical resistant,
stable down to cryogenic temperatures, self-leveling, non-skid, and you
can run it right up the wall with a rounded cove so that crud doesn't get
stuck in cracks in the corners.

--Goedjn