Fe(III) Reduction in the Presence of Catechol in Seawater

Fe(II)-Fe(III) redox behavior has been studied in the presence of catechol under different pH, ionic media, and organic compound concentrations. Catechol undergoes oxidation in oxic conditions producing semiquinone and quinone and reduces Fe(III) in natural solutions including seawater (SW). It is a pH-dependent process. Under darkness, the amount of Fe(II) generated is smaller and is related to less oxidation of catechol. The Fe(II) regeneration is higher at lower pH values both in SW with log k = 1.86 (M⁻¹ s⁻¹) at pH 7.3 and 0.26 (M⁻¹ s⁻¹) at pH 8.0, and in NaCl solutions with log k of 1.54 (M⁻¹ s⁻¹) at pH 7.3 and 0.57 (M⁻¹ s⁻¹) at pH 8.0. At higher pH values, rate constants are higher in NaCl solutions than in SW. This is due to the complexation of Mg(II) present in the media with the semiquinone that inhibits the formation of a second Fe(II) through the reaction of this intermediate with other center Fe(Cat)⁺.