Add time:09/01/2019         Source:sciencedirect.com

Both Fe(II) adsorbed onto mineral surfaces and soluble Fe(II) complexes are important natural reductants; however, no research has directly compared their reaction kinetics and mechanisms. In this work, the reduction kinetics of heterogeneous Fe(II)-goethite versus homogeneous Fe(II)-tiron were compared for the first time toward thirteen structurally diverse nitrogen-oxygen containing compounds (NOCs). The reduction of NOCs followed pseudo-first-order reaction kinetics in Fe(II)-goethite, with the rate constant k varying over a wide range from 8.56 h−1 to <0.0001 h−1. In electrochemical experiments where NOCs were physically separated from Fe(II)-goethite, k of carbadox was 3.6 times greater than that of a structurally similar compound that cannot complex with surface Fe(II), while such difference in batch reactors was 531 times. Similar differences in the reactivity of NOCs between the two reactors were reported for Fe(II)-tiron. The good linear cross correlations between the reactivity of Fe(II)-goethite and that of Fe(II)-tiron toward NOCs, nitroaromatics, and polyhalogenated compounds (R2 = 0.76–0.94) can be used to predict the reactivity of other structurally related compounds by both reductants. On the basis of the FTIR and UV spectra, surface complexation was further inferred; both the amino functional groups and the pyridine ring were involved in the complexation, with the ring-N more strongly involved than the ring-O. As opposed to the large differences observed in the reactivity of the two Fe(II) reductants, similar reactivity was obtained when an outer-sphere reductant dithionite was employed. Overall, these results show that complexation between certain NOCs and surface-associated Fe(II) existed which facilitated the reduction reaction.

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