Al-Aḥzāb (الأحزاب, "The Clans, The Coalition, The Combined Forces") is the 33rd chapter (sūrah) of the Qur'an with 73 verses (āyāt). The sūrah takes its name from the mention of the parties (al-aḥzāb), or confederates (an alliance among the Quraysh and other tribes), who fought the Muslims at the Battle of the Trench (5/627), also known as the Battle of the Parties and as the Siege of Madinah.
'Allamah Tabataba'i mentions the view in this link. It is not the dominant view of Shi'i scholars today, but I think it makes sense.
God knows best
The Verse 33 - 33 in Sura Al Ahzaab mentioned many things about the wives of the Prophet (Peace Be Upon Him and His Holy Progeny) but the specific part of the purification has been explained by the Prophet himself when he clearly said that: These are my Progeny who are been purified. My purified Progeny are Ali (a.s.), Fatima (s.a.), Hasan (a.s.) and Husain (a.s.). No one knows the meaning of Quranic verses more than the Prophet, because the Prophet (Peace Be Upon Him and His Holy Progeny) is been asked by Allah and he is the responsible of explaining the meaning of Quranic Verses to the Ummah, that is why he explained exactly who are meant in the Verse 33 – 33 and he did not allow anyone from his wives to join when he put the Kisa (the Cloth) on Ali (a.s.), Fathema (s.a.), Hasan (a.s.) and Husain (a.s.) Only with not allowing any of his wives even Umme Salama who was very pious wife but she was not allowed to join them. This is a very clear evidence that the Verse does not talk about the wives but only the purified Progeny of Ahlulbayt who are Ali (a.s.), Fathema (s.a.), Hasan (a.s.) and Husain (a.s.).