Multivariate Examination of Fe(II)/Fe(III) Cycling and Consequent Hydroxyl Radical Generation

The introduction of Fe(II)aq into aerated solutions resulted in net Fe(II) oxidation with concomitant, rapid Fe(II)/Fe(IIII) cycling and concurrent generation of reactive oxygen species. The effect of mixtures of naturally occurring solutes on Fe(II)/Fe(III) cycling and the concurrent oxidation of dissolved organics is reported. The experimental strategy was based on a multivariate, microscale, high-throughput approach for evaluating the effect of covarying concentrations of bromide, iodide, Suwannee River natural organic matter (SRNOM), chloride, and total carbonate species. Superoxide and HO• were evaluated at the center point condition of the experimental design with selective scavengers (superoxide dismutase and benzoic acid). The rate of Fe(II) oxidation decreased in the presence of these scavengers, indicating it is a function of oxygen, superoxide, and HO•. HO• generated during Fe(II)/Fe(III) cycling was quantified with the selective probe 1,3-dicyanotetrachlorobenzene (DCTCB). Through the range of experimental conditions of this design, the ratio of the number of moles of HO• produced to the number of moles of Fe(II) consumed varied from 3 to 750, corresponding to approximately 10 to 2200 Fe(II)/Fe(III) cycles, respectively. The implications of these findings with respect to organic oxidation during the aeration of anoxic Fe(II) rich groundwaters are discussed.