Hmmm not too sure I'd do that... in case there was chlorine leaching out of the plastic.
But having said that if you want to try to cleanse them, soak them with water and some Seachem Prime or any quality dechlorinator, at several times suggested dose.
Then just for extra caution, I'd let the buckets sit out in the sun for a bit to burn off any residual chlorine.
Then take a good whiff. If it smells at all like chlorine - I wouldn't use it.
Jenn