I stumbled across an article on water treatment for chromium (VI) contaminated effluents. The authors successfully used sodium sulphite solutions at fairly low pH to reduce Cr (VI) to Cr (III), the latter of course being relatively harmless. I haven't seen in my literature any suggestion that gum prints should be first washed in an acidic sulphite bath before further washing, but it seems like good practice to at least investigate whether that would make the bichromate processes more environmentally friendly.
There are a few questions worth asking though.
i) Will the low pH bath negatively affect the gelatin sizing or gum layers?
ii) How much post washing, or what alternative treatment, will be required if multiple layers are to be applied? Residual sulphite in the paper might consume some of the bichromate before UV exposure, rendering the sensitivity lower or affecting the contrast etc of the subsequent layer. If it is uneven, it would be worse.
These are of course only ideas at this stage. I will definitely look into it when I do gum printing again (soon, I hope!), but would welcome the response of regular gum printers.
Other reagents such as ferrous sulphate or ferrous ammonium sulphate may also be used, but since most APUGgers have sulphite stashed somewhere, I thought it would make for a neat practical way to reduce our environmental footprint.