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Earlier studies in our laboratory indicated oxidation of humic acid by Fe(VI) but the quenching of fluorescence by a reaction product Fe(III) led to uncertainty about the extent of oxidation. The interactions of Fe(VI)/Fe(III) with the humic acids (HAs) are investigated using synchronous scan fluorescence (SSF) spectroscopy, UV-Vis spectroscopy and DART-MS spectroscopy. In the fluorescence study, there was an immediate decrease in the SSF intensity after addition of both Fe(VI) and Fe(III) to all the HA mixtures, which suggested that there is a reaction between HAs with Fe(VI) as well as Fe(III). These reactions were further investigated quantitatively by UV-vis spectroscopy. The results show that the HAs oxidize immediately after the addition of Fe(VI). Also, HAs react with Fe(III), forming a complex between the functional groups of HA and Fe(III). These interactions were further investigated by with pH change and by DART-MS. The results show a greater number of lower m/z peaks after mixing of HAs with Fe(VI) than before mixing, suggesting that the oxidation of the HAs by Fe(VI) leads to the fragmentation of these HAs.
