I came across this method of extending the storage life of leafy veg. a few years back when could only buy them twice a month. Sink full of cold water. Add vegs and gently agitate allow any dirt/sand to settle to the bottom. Remove veg to rest on cloth and drain. Know the secret: Drain the sink washing away any dirt/sand. Fill with the hottest water you can. You may need to boil a pot of water and add to the sink. In another sink or big pot fill with the coldest water you can. The next step needs quick timing. Plunge all the vegs into the hot water and IMMEDIATELY remove with a spider or whatever and plunge into the cold water. Remove immediately. You have some paper towels to recieve the dripping veg. Gently pat away the excess water. Leave the veg wet. Now gently wrap up the vegs in paper enough paper towels to absorb some of the water but not all. A couple of dinners worth in each 'tube'. Then into a Zip lock bag then into the veg crisper. B/c you've made a number of tubes, say enough for a couple of days consumption each you don't have to unwrap the entire bunch.
So what's happened? Imagine you are a plant cell. You were brought home and stuck into cold water which caused you to constrict. Then you were plunged into hot water for only a few of seconds. This was long enough for you to 'open up' and relax. At that moment any water you happen to need to absorb you will. Now you have as much water as you can hold. Then another plunge into cold water constricts you again. Now you're full of water. Then into a crispy cold place to await your fate. In a week or more you'll be fresh and crispy. I'v keep lettuce using this method for a month.