Nitric acid was known to the alchemists as Aqua Fortis. From Wikipedia: Aqua fortis was prepared by mixing either sand, alum, or vitriol, or the last two together, with saltpeter, then distilling it by a hot fire. The gas collected from this condenses into aqua fortis.

We know they've got saltpeter if they can still trade with the Bullet Farm, and boy are they ok for sand. Alum is terribly useful for all kinds of things, and I think can be mined or created by reaction. Oil of vitriol is sulfuric acid, which can be made from any sulfur they happen to have lying around (again, probably a mining operation).

There are of course laboratory methods for making nitric acid, but it's doubtful that the Citadel has the capacity for them yet.



Also, separately, but speaking of water quality: In the movie, we see Immortan Joe, Corpus Colossus, Rictus Erectus, and a couple of imperators up on his stone balcony, which is open to the air. Behind him, also open to the air is a large pool of water with working pumps. WTH? Why wouldn't that be closed off? He clearly knows that breathing dust is not good, why would he be ok drinking it? Also, I've wondered this for a while, why would you bother pumping the water alllllll the way up there? Store it lower down! Water is heavy and pumping it is hard!