Rinse it under a tap several times( don't rub too hard or you'll break the stems ) Really it would be better to buy some fresh or keep some in a bucket of water for a week or so , and keep refreshing the water to purge away anything horrid living on it.
Water lettuce can be used in aquariums, it's long trailing roots look effective underwater, and small fish and fry like to use them as cover. Salvinia Natans is also a good one. It can be rather virulent like duckweed, but as long as you manage it and keep it in check it can be fine. Just grab out handfuls of excess plants and either chuck them in your pond, move to other tanks or give to friends for their tanks, or throw it on the garden to rot down . I have Salvinia in all 3 of my tanks and it goes like the clappers. I have to throw so much away! But it does look good and is good cover for my Gourami who is a little nervous.
You can see the Salvinia from underneath in the second pic in my sig.