EXAFS study on the reactions between iron and fulvic acid in acid aqueous solutions

• Iron(III) competes with trace metals for binding sites on organic ligands. We used X-ray absorption fine structure (EXAFS) spectroscopy to determine the binding mode and oxidation state of iron in solutions initially containing only iron(I I I) and fulvic acid at pHs 2 and 4. EXAFS spectra were recorded at different times after sample preparation. Iron was octahedrally configured with inner-sphere Fe-O interactions at 1.98-2.10 A, depending on the oxidation state of iron. Iron(III) formed complexes with fulvic acid within 15 min. Iron(III) was reduced to iron(II) with time at pH 2, whereas no significant reduction occurred at pH4. No signs of dimeric/trimeric hydrolysis products were found in any of the solution samples (<0.45 mu m). However, the isolated precipitate of the pH 2 sample (>0.45 mu m) showed Fe center dot center dot center dot Fe distances, indicating the presence of tightly packed iron(III) trimers and/or clusters of corner-sharing octahedra. It is suggested that the binding mode of iron(Ill) to fulvic acid at low pH may be phase-dependent: in solution mononuclear complexes predominate, whereas in the solid phase hydrolyzed polynuclear iron(III) complexes form, even at very low pH values. The observed pH dependence of iron(III) reduction was consistent with expected results based on thermodynamic calculations for model ligands.

Nyckelord

ray-absorption spectroscopy

body distribution-functions

natural

organic-matter

humic substances

condensed matter

redox properties

ionic-strength

fresh-waters

reduction

binding

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