[mikeyyuiok]

al-Salamu `Aleykum,

No it is not allowed to divorce a wife for taking off Niqaab and going back to Hijab.

Brother, I don't know if that is the opinion in the Shafi'i madhhab, but according to the Hanafis, the husband can divorce his wife for any reason, whether it be religious, materialistic, or even idiotic! Of course the latter may be unjust and the sin would be from the injustice.