Hello,

I have found some information from the Wisconsin Extension office that you may find useful:

Removing Rust Stains from Concrete

How can rust stains be removed from a concrete floor?

The procedure recommended by the Portland Cement Association is to dissolve 1 lb. of oxalic acid powder in 1 gallon of water. Liberally soak the stains and allow solution to remain in place for two to three hours. Then, rinse well, scrubbing at the same time with a stiff broom. Oxalic acid is sold as a wood bleach in hardware stores and building supply centers. Follow package directions carefully.

There is also this warning about oxalic acid from the New Mexico State Extension office:

Oxalic acid is a bleaching agent that is an effective rust remover. It is poisonous and corrosive. Make sure to keep children and pets away while using this product. Dispose of cloths and brushes used to apply oxalic acid. Otherwise, the acid could be transferred to kitchen utensils and dishes, from which this poisonous substance could be ingested. See the Caution section before using.

Please also note that oxalic acid will burn plants so you do not want to let it splash on to your lawn or any surrounding plantings.  The dilute solution from rinsing is not as problematic.

Let us know if you have any further questions and good luck!