Sforno on Numbers 31:23/ספורנו על במדבר ל״א:כ״ג

1וטהר, it will have become decontaminated from the negative vibes attached to it as residue of idolaters.

2אך במי נדה יתחטא, even making it red hot does not automatically remove its impurity, even if it looks as if new. It requires the waters and ash of the red heifer, i.e. מי נדה