It doesn't appear that pyrene is incredibly dangerous. I would assume that it would be soluable in any organic solvent and that would include acetone which would be particularly useful since the left over mix would probably be soluable in water and be completely washed away.

I am not suggesting that pyrene is benign, but neither is acetone or gasoline or methyl ethyl ketone or other paint thinners. My approach would be to slosh some acetone in there; dump it out in an unused area and slosh some water in there and repeat.

Hmm... A coal tar derivative... made of 4 benzene rings... Toxic to the kidneys and liver.... Potentially carcinogenic...

I think I would try to find another barrel....

Whatever you do - Don't use it for Potable (Drinking) water, water for animals or for irrigating crops.. If you are going to use it in a boiler to heat your house or something like that - it would probably be OK....

If you want a barrel for storing Potable (drinking) water - you need to buy a barrel specifically rated for potable water.

Thanks