The fundamental reality is that all boards are composite in construction- Made up of laminating or gluing layers of different materials together with pressure and possibly heat, means that even if one uses a sustainable material, you will probably not be able to separate it and thus recycle it in any way.
Sorry, that is the cold hard fact of boarding :( Want to be Eco - Buy a board, use it and love it for a while, then let someone else use it and love it.