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, time warrants, and certificates of obligation of the city. The proportion of ad valorem taxes which are devoted for debt service shall be subject to the limitations imposed by state law. The interest and sinking fund shall be deposited in a separate account and shall not be diverted to or used for any other purpose than to pay the interest and principal on all such bonds, time warrants, and certificates of obligations issued by the City of Longview. The sinking fund maintained for the redemption of any debt may be invested in any interest-bearing security authorized by the laws of this state. Investments of all sinking funds for interest shall mature at least fifteen (15) days prior to the date of payment due on bonds issued by the City of Longview. The Council may invest public funds as authorized by the Public Funds Investment Act of V.T.C.A., Chapter 2256 Government Code, as amended, or as to any lawful investment otherwise specifically permitted by state law.