Not a clue, the people selling them dont know either. They are in a pile of surplus.
The lids seal air tight though,so some kind of liquid.
Found this info.
<2>HDPE (high density polyethylene) is used in milk, juice and water containers in order to take advantage of its excellent protective barrier properties. Its chemical resistance properties also make it well suited for items such as containers for household chemicals and detergents. Most five gallon food buckets are made from HDPE. Examples: Milk bottles, shopping bags
Guess I should still worry what might have been in them?