How to remove rust stains from concrete without having to paint it?

We just recently moved into an older house with a front porch, which I love. The only problem is that there is a big round rust stain where a metal planter used to be from the previous owners. My husband has tried many different things to get rid of it and nothing is working. We would like to be able to remove it without having to paint. Any suggestions would be very much appreciated.
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