What is the ruling on the money deposited in the bank, which the father placed in investment certificates with usurious interest, after the questioner reaches maturity and becomes responsible for it? Is this money permissible for him, despite his prior knowledge of its prohibition? Is he required to purify it from usurious interest and pay zakat for the past years for which the father did not pay it?

You did well to advise your father to stop investing money in forbidden things. If you become responsible, you must withdraw the money and dispose of the forbidden usurious interest by spending it on the welfare of Muslims and the poor. You are only entitled to your principal, as Allah Almighty says: "And if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged." Any amount exceeding your principal from interest is considered illicit wealth. If the father has not paid Zakat on the money for past years, it must be paid only on the principal, without the forbidden interest.