Abiotic reduction-complexation reactions of iron with natural organic matter at circumneutral pH

While considerable progress has been made in assessing natural organic matter (NOM) interactions with minerals in soils, sediments and natural waters, the mechanisms involved in OM stabilization are not fully understood. Metal complexation by organic matter is an important potential mechanism controlling its solubility, mobility and decomposability, and may depend on the means of OM association, the types of complexes or co-precipitates formed, or the functional groups involved in complexation. Here we used synchrotron-based scanning transmission microscopy (STXM) in combination with near edge x-ray absorption fine structure (NEXAFS) and fourier transform infrared (FTIR) spectroscopy to monitor changes in Fe and C speciation in Fe complexed with sodium alginate or amylose (acid and neutral polysaccharides, respectively). Stock solutions of Fe(III) complexes with sodium alginate and amylose were synthesized from ferric chloride (FeCl₃.6H₂O) at pH 6.5 under ambient conditions using a Fe:C molar ratio of ~1:100. STXM/NEXAFS data of Fe(III)-alginate and Fe(III)-amylose at C K-edge and Fe L-edges revealed changes in C functionalities associated with a changes in Fe complexation or redox state at certain hotspots of the complexes. Specifically, C NEXAFS data show a decrease in the ~288.5 carboxylic acid (COO^-) resonance and increase in lower energy resonances in the ~287.8-288.1 eV range, possibly due to Fe complexation of the amylose or alginate. In the same samples, the relative intensity of peaks from Fe L-edge XANES data also indicate a shift in complexation or redox state. FTIR data of both Fe(III)-alginate and Fe(III)-amylose solutions show frequency shifts of absorptions from 2900 to 2917 cm^-1, 1692 to 1684 cm^-1 and 1390 to 1340 cm^-1, indicating COO^- stretching, C-H stretching and skeletal C-C vibrations. These results indicate a simultaneous reduction and complexation process where binding of O- ligand to Fe(III) leads to the formation of square planar Werner type Fe-OM complexes, with Fe(III) binding to either the hydroxyl groups of the D-glucose units of amylose and/or side chain COO^- of alginate. The results from this molecular level study with well characterized organic compounds inform our mechanistic understanding of OM stabilization via metal interactions in terrestrial ecosystems.