By this - exile of Israel (the “sending away,” Isa 27:8).
purged - expiated [Horsley].
all the fruit - This is the whole benefit designed to be brought about by the chastisement; namely, the removal of his (Israel’s) sin (namely, object of idolatry; Deu 9:21; Hos 10:8).
when he - Jehovah; at the destruction of Jerusalem by Nebuchadnezzar, His instrument. The Jews ever since have abhorred idolatry (compare Isa 17:8).