Can a polypropylene 55 gallon drum previously filled with chemicals used in the chrome plating process (possibly Nickel Sulfate) ever really be "cleaned out" for re-use? Can polypropylene ever absorb and retain such chemicals, or are these drums practical for this use because they do not?
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polypropylene is hydrocarbon and does not adsorb polar and ionic compounds. So, several treatments with mild hydrochloric acid followed with several treatments with clean water should work. However, I woldn't store drinking water there without proven test. For example, leave water there for several weeks and then perform analysis of the water for metal ions. If all concentrations will be within safe levels, it should be ok.
Many of the chemicals used in metal plating are extremely poisonous (cyanide), cause severe allergies (Ni), and are carcinogenic (Cr(VI)).
I'd consider it as extremely unprofessional (besides violating many laws and regulations), to re-use anything that has been in contact with these materials for any purpose that requires a 'clean' container, regardless of the theoretical possibility of cleaning everything off.
To use such a drum for food or drinking water would be criminal negligence.
I'd only consider it ok if it was thoroughly cleaned (my myself) and only used by myself as a trashcan in my machine shop outside of my home.
I would even disallow to use it as a trashcan in the machine shop at work. Why risk it?
Even if PP does not absorb ions in principle, there is always the real world:
There are hundreds of accidents where contaminated containers have been improperly cleaned and re-purposed. Please don't do it, even if it is actually safe.
Even if the drums can be cleaned, imagine anything happening, like some kid being admitted to the ER with a Nickel allergy.
You'd be in a tough position proving that no one got injured from your unprofessionally cleaned drums. The people who used the drums in the first place will have all the paperwork to 'prove' that it wasn't them.
Btw. we had an accident with a metal plating bath that was not properly cleaned and injured a technician who had do do some modifications to it.
It might not hurt anyone, but it's better not to do it.