Do your own due diligence!! (homework) I have gotten some IBCs from
local manufacturers that have held various things:
vegetable oils used for plastics molding (probably safe)
agave syrup (natural sweetener, definitely safe)
sodium lauryl/laureth sulfate (not so great, harmful to aquatic life) seller represented as "a derivative of coconut oil" since that's the odor one smells from it (and technically it is). Not very useful to me as any part of building an aquaponics system... It might rinse away with LOTS of water/pressure washing/soap, but the investment in time is better spent finding suitable IBCs in the first place. I'll use them for thermal mass in a
greenhouse.
Here's what
https://en.wikipedia.org/wiki/Sodium_laureth_sulfate says about it.
Use your good judgement. If buying directly from a user of the "product" and it is a food business, it's likely a known quantity. If buying from a reseller, check and be sure, identify the actual contents... also make sure you are given what you are paying for (the right ones, not the "right here" ones). If someone isn't forthright, or they aren't sure, I'd pass on it, You don't need to inherit a disposal problem.