I think the platters are made of an aluminum alloy (or at least they were) or glass. They aren't like lead based batteries. The cases are usually aluminum too. I don't think you risk any sort of problem by throwing them away in the trash. Here in Denver, we have curbside recycling of pretty much everything (Al included).
If you're concerned about data, open it up and smash the platters with a hammer. There are companies that dispose of old hard drives (for large businesses are government entities) but I think that's a waste of time for 1 drive.
Or, you can google dod wipe and delete the hard drive using software.