A friend recently mentioned that his work office is right beside another business that does maintenance for household swimming pools, and that they generate a large quantity of waste HDPE buckets that they receive calcium hypochlorite in for the chlorination of pools.
The smaller size that they receive are roughly 6.75 gallons in volume, with a diameter commensurate with a 5 gallon bucket and a plastic flippy handle. The larger buckets are roughly 12 gallons in volume, much larger in diameter and height, and have either two or four handholds molded into the area below the rim.
I am not sure for how long they need to be aired out before they are safe to use for plant-type of purposes, but after a few days sitting open in the sunlight I detect no chlorine scent lingering.