I'm not entirely sure what prime does to "detoxify heavy metals," but I have a feeling that it binds to them and perhaps precips them out of the water. When the prime wears off maybe some of the metals will go back into the water column...I'm not sure.
I don't think prime will break DTPA iron bond. I'm not sure though. I think it only binds to free ions like Fe+2, Mn+2 ect ect