It shall be the duty of the Council to levy an annual tax sufficient to pay the interest on and provide the necessary sinking fund required by law on all outstanding general obligation bonds of the City. The interest and sinking fund shall not be diverted to or used for any other purpose than to pay the interest and principal on all such bonds issued by the City. The interest and sinking fund maintained for the redemption of any debt may be invested in accordance with policies adopted by the Council and as provided by state law.