Sometimes it just takes a little hands-on research to solve a problem.
A member of Hometalk.com asked this question: How can we remove the rust stain from a fertilizer from our concrete driveway and sidewalk?
I replied: If the product contained pendimethalin pre-emergent, you have a big problem. Pendimethalin (Halts, etc) is widely known to stain anything it contacts
because the chemical itself is a dye. Sunlight doesn’t fade it much on porous concrete. Get a scrub brush and some TSP cleaner and make a thin paste to scrub the area with. Follow with one of the automotive citrus hand cleaners (GOJO, etc).
Note that the label cautions against applying to stone, concrete, clothing or shoes.
Others suggested oxalic acid….but luckily the homeowner answered her own question: We finally improved our rust on the driveway and sidewalk situation to the point when I can say it is resolved. We couldn’t find the oxalic acid. We tried TSP to no avail. Finally, on the recommendation of a hardware store employee, we tried muriatic acid. We were very careful to follow the safety warnings so had no adverse effects. We applied this 3 times, each time increasing the acid to water ratio a bit. We put the mixture into a plastic watering can and gently spread it on the affected areas. Then we let it sit for a day and gently hosed it off, not getting it on the grass. After the third application, my husband used a stiff-bristled broom to work on the worst spots. After hosing it down, it took a couple of days for the areas to fade almost completely. But they did, and we’re happy.