What is the ruling on the portion of zakat that was paid on usurious interest throughout the past ten years, and is it permissible for the zakat payer to recover an equivalent amount from the usurious interest he will dispose of and add it to his original capital?

What is required concerning the presented issue is to pay Zakat (definition) on the principal amount of the deposit. As for the usurious interest, it is ill-gotten wealth that is neither owned nor subject to Zakat. Rather, one must dispose of it by spending it on the general welfare of Muslims. Since you disbursed an amount from your own money with the intention of Zakat, and it later became clear that you were not obligated to disburse it, you are entitled to count that amount towards the Zakat on your wealth for the coming year or to donate it. The Zakat you spent in error will be counted as part of the total Zakat obligatory upon you in the coming year. All interest is forbidden wealth that must be spent on the poor, the needy, orphans, and the like.