Rain collection - Barrel question

[DntBrnDPig]

I have several 55g barrels that I have no clue what was in there previously. Can I clean these things out with something like Purple Power or some other cleaner for use in a rainwater collection system?

I'm not going to be drinking this - its for my raised beds.

 

[TAFKAP]

I wouldn't use it unless it was specifically used for and traceable to food-use only. Too much risk for my taste.