You can send your used metal/synthetic strings to the ‘D’addario String Recycling Center’ and earn points in their ‘players circle’ program if you’re based in the US. Unfortunately, equivalent organisations do not yet exist in other parts of the world.  Alternatively, you can donate your old strings to a musical charity like ‘Open Strings’ or the ‘Rachel Barton Pine Foundation’ so musicians with little access to necessary materials can benefit.