How to dissolve iron powder?
I did some brief research and I saw that sulfuric acid can dissolve iron:

Fe(s) + H$_2$SO$_4$(aq) → Fe$^{2+}$(aq) + SO$_4$$^{2-}$(aq) + H$_2$(g)

Of course, this would leave sulfate ions and perhaps some precautions would need to be taken because of hydrogen gas production. If you absolutely cant have any sulfate ions, you could precipitate them out by adding barium or silver ions to the solution, but that would require extra work and introduce new ions to the solution. Also note, the iron ions should be aqueous in +2 state but I read that if there is oxygen in the system (which there naturally would be), some iron(II) can oxidize to iron(III).