| In this thesis, macroscopic sorption and dissolution studies as well as surface techniques, primarily Atomic Force Microscopy (AFM), were used to address a very old question in geochemistry and soil science: What are the roles of humic and fulvic acids in the dissolution of iron oxides in acidic soils and streams? Dissolution of hematite by simple organic acids was studied first, to establish a baseline for comparison. Results of in-situ and ex-situ AFM of hematite samples upon exposure to simple organic acids showed that dissolution occurred at specific topographic features on the hematite surface, consistent with a surface-controlled dissolution mechanism. As a first-order approach to understanding the roles of humic and fulvic acids, the sorption to and dissolution of hematite by Suwannee River fulvic acid and a soil humic acid were characterized at pH 3 and 4 and I {dollar}sim{dollar}.008, in batch reaction vessels open to atmospheric O{dollar}sb2{dollar} and in the dark. At 24 hrs reaction time, Fe release was up to two orders of magnitude greater for the humic acid than the fulvic acid. For both acids, amount of Fe released correlated with organic-acid sorption density, suggesting that sorption plays an important role in the dissolution process. Next, the effects of humic and fulvic acids on hematite dissolution by.001 M oxalic acid were studied under the same experimental conditions. At pH 3 and 4, fulvic acid had no substantial effect on hematite dissolution in oxalic acid; fulvic acid was found to inhibit slightly the initial release of Fe relative to the oxalic-acid system alone, but to enhance slightly the (Fe) in solution under conditions approaching steady state. In contrast, the humic acid was found to enhance significantly the amount of Fe released to solution at pH 3, but to have no significant effect at pH 4. It is hypothesized that the different reactivities of the humic and fulvic acids may be related to the greater aromaticity, greater N- and S-concentrations, and greater combined carboxyl plus phenolic functionality of the humic acid. |
