mussels won't close after cleaning. Always discard?

All good advice above. I did want to re-emphasize the point that it's very rare that mussels have much sand to purge. Even most wild ones.

And another small bit of advice--don't remove the byssal threads (the beard) until just before cooking. Ripping out the threads can lead to death.

And one more thought about lloreen's problem: did you perhaps keep your mussels in a plastic bag between the time you bought them and when you cooked them? That could have killed them too.

For soaking mollusks, you don't need actual sea water. It's the salinity that's important. 1/3 cup salt / gallon will keep them happy.

Up until three years ago I used to live in Hermosa Beach. Just two miles away in Redondo there was Quality Seafoods (it's right under the Pier if you happen to be in the neighborhood). The bivalves are held in tanks of seawater. I could reach in pull out clams, mussels, cockles, whelk etc. that were still breathing. I'm still close enough to good purveyors but I don't have that luxury anymore of grabbing my own. No amount of tapping on the shell will reanimate a dead mussel. Mussels are one of your best choices as they are sustainably raised. But who knows how long this monger had been holding them out of water before you got to the market to purchase them. If there was a lot of sand in them they probably weren't farm raised---they grow on ropes that have been seeded.

Just one more thought. I don't know where you live, but mussels are not generally so expensive that you can't learn to love cooking and eating them. Talk to that fishmonger at your farmer's market, and come back here for more advice.

From 10 am until 5 pm should have been fine if you had just kept them cold and not put them in water. Even then, you could have expected a few gapers. If you really want cold, put fish and shellfish on top of, not in, ice. And definitely not in water. Unless it's seawater and sandier clams than mussels.

No you were probably not ripped off. You drowned those poor bivalves. Mussels are more fragile than a lot of other clam relatives. As pierino says, they should be cooked, if not eaten, on the day you buy them. They do not need any soaking in water, and if they did, you'd want to soak them in seawater. Live and learn. But good that you tossed them. Dead mussels grow a lot of undesirable bacteria.

I bought them at 10am and tried to cook them at 5pm...is that too long?

You did the right thing in tossing them. But you did the wrong thing in holding them overnight. Basically, you killed them. It's bad practice to consume dead (uncooked) shellfish. They begin to break down very, very quickly. Mussels and clams live in a salt water environment. They should be consumed within hours of when you purchase them.

We eat a lot of mussels and shellfish in general...no good comes from eating mussels that didn't close. I've had to throw them away too and that stinks but...not as much as getting sick.