Re: molluscs?
Slug baits tend to be iron phosphate rather than sulphate. The reason for the cost is that iron phosphate is used in lots of other applications from rust inhibitor to bonding agent and is even used in some types of lithium batteries so it's unlikely to be landfill. Iron sulphate on the other hand doesn't seem particularly toxic to aquatic molluscs (pdf).
Biting the hand that feeds IT
