That isn't what an impurity is. An impurity is simply a moral privation. You're missing something that should be there. You don't have dirty stuff on you, ladies and gentlemen. Evil isn't a dirty, slimy ooze that floats through the universe and gloms onto us and causes us to look filthy before God.
Evil is a description of a privation of good.
We're missing something. We're missing perfect goodness. So, there's nothing to be burned out in purgatory, first of all. In other words, it is beholding the Lord Jesus that transforms us from morally imperfect to moral perfection. The text also says in another place that, when we see Him, we shall be like Him because we will see Him as He is.