Is there a method to separate iron from mercury?
As iron is one of few metals not forming amalgams,


  Almost all metals can form amalgams with mercury, the notable exceptions being iron, platinum, tungsten, and tantalum.


..it should preferably contaminate the mercury surface.

I would try, perhaps repeated,  dropping mercury, e.g via punched filtration paper to sulphuric acid bath.

It can be combined with  agitating mercury - acid phase border.

Important can be the origin of impurity and the intended purpose of purified mercury. There is possible the only reasonable way would be distillation, mentioned in the other answer, respecting the warning.