It shouldn't be castable more than once per turn. Allow it to target anything (creature/player)
You could give the skill a cost and instead of needing x poison, it could generate an Oil Tank which scales with how much poison it sapped. But that makes it a bit easy to spam purification, so the cost would have to be a bit high or you could have it leave 1-2 points of poison (which suits the idea leftover waste)