Selective Electrolytic Reduction of Fe(III)-edta Complex in Mixed Aqueous Solution of Fe(III)-edta Complex, Hydrogensulfite Ion, and Fe(II)(NO)-edta Complex

Abstract

Electrolytic technique was examined for the reproduction of Fe (II)-edta complex, formed from Fe(III)-edta complex, the oxidation product of Fe(II)-edta complex, whose aqueous solution was used as an absorbing solution for SO₂+NO_x simultaneously in a boiler flue gas. First, the voltammetric study showed that the electrolytic reduction took place in the following order: Fe (III)-edta complex, hydrogensulfite ion and Fe(II) (NO)-edta complex, and that the reduction current of Fe(III)-edta was pH-dependent.
Second? the electrolytic reduction in terns of a battery type electrolyzer, whose cathode and anode chambers were separated by a cation-exchange membrane and, whose anolyte was a dilute sulfuric acid solution brought about the selective reduction of Fe(III)-edta complex at a potential higher than -0.35 V vs. SCE. The stainless steel electrode modified by carbon cloth was found to be a good cathode for the reduction of Fe(III)-edta complex of low concentration, giving rise to the high current density and the high current efficiency. Furthermore, the following relation was found between cathodic current density, D_c (mA/ cm²), and linear velocity of catholyte, U (cm/sec)
D_c cc U^0.53∼0.57
when using the filter-press type electrolyzer, whose electrode and membrane being separated by the distance, 0.3∼5.0 mm. No potential distribution was detected on the cathode surface. Thus, this type of electrolyzer was found to be effective for the purpose of the present investigation.
Finally, the continuous operation of the apparatus, in the presence of SO₂+NO_x for simultaneous removal of SO₂+NO_x by absorption, for electrolysis of the solution, and for release of absorbed SO₂+NO_x revealed that Fe(III)-edta complex was selectively reduced on the carbon cloth modified electrode at a potential of -0.30 V vs. SCE and that the absorbed SO₂+NO_x was quantitatively released as a highly concentrated gas.