Current firearms will suit your needs fine.
Gunpowder hasn't been a common propellant in over a century. Modern 'smokeless' propellants burn much more cleanly, leaving minimal byproducts, and most importantly they can be fairly easily synthesized. Even if this society has previously abolished weaponry (utopia, right?), It won't be hard to start production from scratch.
Primers might need some tweaking. The most common primers in current use are mercury fulminate, which are toxic. Now, there's so little of it in a loaded round that it's not a significant problem, but you want clean weapons, so an alternative impact-sensitive explosive or, perhaps, something more exotic like a piezoelectric igniter will do.
The cases themselves are either brass or steel, easily collected and reused. Brass cases can be run through a resizing die and reloaded (usually) around five times before becoming too weak to reuse, whereupon they are melted down and reformed. Steel casings are generally non-reloadable, but can be reprocessed.
The guns themselves are steel, aluminum, and typically either wood or polymer. Current polymers may be non-viable if petroleum is not available, but newer environmentally-friendly plastics can be used instead, and steel and aluminum are easy to reclaim.
If you take a look at the weapons produced in the Khyber Pass, you will see a culture that has already embraced a policy of maximum recycling in regards to their weaponry.