Capewind,
Nearly every plastic barrel is made of HDPE or High Density PolyEthylene. They purposely use this plastic because it is structurally strong and it is also chemical resistant. So, the chemicals which were contained within would not have permeated the plastic so it is going to be relatively easy to clean out. I would simply rinse the heck out of it with fresh tap water first. If the chemical was petroleum based, a residue might stick to the side walls, but add some warm water and some Amway L.O.C. and scrub, scour or pressure wash it really good several times and I think it will be perfectly acceptable. After that, if you desire, you can fill it with clean tap water and pour in a couple gallons of white vinegar and let it sit in the sunlight for a couple of days. Then rinse again.
I am sure you will have no problems cleaning that barrel out this way. The Amway L.O.C. does wonders! I also use LOC in my tow behind yard tank sprayer as a wetting agent for herbicides and pesticides and what not. It makes "water wetter" by breaking down the surface tension. This aides the chemicals you use or want to remove by dissolving them better into the water. Crystal White dish detergent works about the same, if you can find it. Other dish detergents work the same way too, but many have additives that you don't really welcome or are more expensive. L.O.C. works the best IMO.
Catfishnut