It's pretty much inert, so sitting outside isn't going to really do much to it. (barring exposure to pesticides, soap/detergent, petrochemicals, etc.)
I wouldn't even worry about sterilizing it, I doubt it would pick up much harmful to a fish in a terrestrial environment. - just give it a good rinsing to get out any dust/bugs/debris that may have accumulated
I had some outside for five years. A regular rinse and all is good.