Another off the shelf product containing oxalic acid is Zuds. It comes in a liquid slurry or a powder much like a cleanser product. It also has abrasives like cleansers so it may scratch some surfaces. I use around the house occasionally for removing light rust stains from wood, some fabrics, concrete, and countertops. I am a little “rusty” on my chemistry, but I believe oxalic works as a reducing agent that a takes the ferric iron (rust) to soluble ferrous state. It may also have chelating properties like citrates, but I am a not sure of that. Like most everything else in substitution assessments, it has some risks. I found this definition via Google: A dicarboxylic acid, chemically COOH—COOH. Poisonous in large amounts; present especially in spinach, chocolate, rhubarb, and nuts. The toxicity of rhubarb leaves is due to their high content of oxalic acid. High concentrations of oxalates in the urine can form kidney stones; while most of these oxalates are of endogenous metabolic origin, patients with hyperoxaluria are advised to avoid dietary sources of oxalates.
So if you use oxalic acid products around Thanksgiving, you may want to go light on the rhubarb pie – at least a second helping.
The most common way to remove rust stains is with an acid. Hydrofluoric acid is most effective but very dangerous to handle. Commercial rust removers most often contain oxalic acid. Bar Keepers Friend is a scouring powder with oxalic acid. Being an organic acid, oxalic acid is biodegradable and acceptable for sewer discharge. The abrasive solids might be an issue, but only if you were trying to use them on an industrial scale. Small use on an occassional rust stain should not be an issue. I don't know if any commercial products use citric acid, but it too should be capable of removing light rust stains. Once the stain is removed, you want to flush the surface with lots of water (to remove the acid) and if metal, apply a protective oil-based coating. Acid cleaned metal will quickly rust again if you don't take measures to protect it from air and moisture. Hope this helps.