PROCESS OF TREATMENT FOR OXIDIZING AN ACIDIC SOLUTION CONTAINING AN IODIDE ION AND AN IRON (II) ION

[Problem to be Solved] A method for producing an iron (III) ion from the acidic solution containing an iodide ion and an iron (II) ion efficiently and stably is provided. [Means for Solving Problem] A method comprising performing the following steps (a)-(b), repeatedly and continuously: (a) a step wherein the iron (II) ion in the acidic solution containing the iodide ion and the iron (II) ion is oxidized into iron (III) ion in a reactor using a microbes immobilizing carrier to which iron oxidizing microbes attached; (b) a step wherein sedimentation of the solution obtained in the step (a) is performed in a sedimentation tank to obtain the solution containing the iron (III) ion and concurrently the sediment of the microbes immobilizing carrier to which said iron oxidizing microbes have attached is recovered and then reintroduced into the reactor in the step (a).