How do you purge freshwater mussels?

I got some freshwater mussels (I don’t know what the name is in English) from the beach. They are brownish with two little white dots on the bottom, or can be yellowish. See the picture below.

I didn’t read enough and tried to purge them in saltwater, and half of them are slightly open, one completely. I think I killed them all. Just to be safe I’ll chuck them and get new ones, but next time, what should I do? Should I purge them in normal tap water?