Hydroquinone-Mediated Redox Cycling of Iron and Concomitant Oxidation of Hydroquinone in Oxic Waters under Acidic Conditions: Comparison with Iron-Natural Organic Matter Interactions

作者：Jiang Chao; Garg Shikha; Waite T David^*

来源：Environmental Science & Technology, 2015, 49(24): 14076-14084.

DOI：10.1021/acs.est.5b03189

Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3-5 in the air-saturated and partially deoxygenated solution are examined here. Our results show that 1,4-hydroquinone reduces Fe(III) in acidic conditions, generating semiquinone radicals (Q(center dot-)) that can oxidize Fe(II) back to Fe(III). The oxidation rate of Fe(II) by Q(center dot-)-increases with increase in pH due to the spedation change of Q(center dot-) with its deprotonated form (Q(center dot-)) oxidizing Fe(II) more rapidly than the protonated form (HQ(center dot)). Although the oxygenation of Fe(II) is negligible at pH < 5, O-2 still plays an important role in iron redox transformation by rapidly oxidizing Q(center dot-) to form benzoquinone (Q). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron-Suwannee River fulvic acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution are rapidly oxidized by dioxygen, while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen, with the result that steady-state semiquinone concentrations in SRFA solutions are 2-3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in the steady-state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states, with iron turnover frequencies in solutions containing SRFA being 10-20 times higher than those observed in solutions of 1,4-hydroquinone.

• 出版日期2015-12-15