Chemistry- reactions help?

I need help answering this question... A huge pharmaceutical company is polluting the water supply of your town. You know the company uses aqueous iron in its factory and you have a hunch that somehow this iron is finding its way to the groundwater. You have a sample of the groundwater taken from a site near the property of the plant. How could you go about removing iron from the sample of ground water? Name 1 anion you would use and how would you use it.